Sodium-ion battery is one of the most promising and feasible energy storage candidates. However, compared to the lithium ion, the larger ionic radius and higher molecular mass of the sodium ion lead to inferior electrochemical performance of sodium-ion batteries. Therefore, achieving the rational design and construction of high-performance electrode materials is a key point and remains a great challenge for sodium-ion batteries. In this work, we focus on the transport properties of sodium ions and electrons and discuss design strategies of sodium electrodes from the perspective of solid-state ionics. First, for the bulk sodium electrode materials, investigating their transport properties, such as ionic conductivity, electronic conductivity, and diffusion coefficient, is a prerequisite for electrode design. Although there are various methods of measuring the diffusion coefficient, separately achieving the intrinsic ionic and electronic conductivity of the pure materials is highly important. Doping and carbon-coating are the most useful approaches to improve the specific transport properties of the investigated materials. Building defect chemistry models based on measured transport properties and relevant defect chemistry theory is crucial but remains a great challenge for the design of sodium electrodes. Second, for the nano sodium electrodes, size effects can be applied to design and construct electrodes from a nanoionics perspective. Thermodynamically, the equilibrium shape and equilibrium voltage change with a reduction in the particle size and facilitate the discovery of new electroactive electrode materials. Kinetically, according to τ~L²/D (where τ is diffusion time, L is particle radius, and D is diffusion coefficient), a smaller particle size leads to better kinetic behavior (higher rate performance) and also improves the diffusion coefficient in some cases. In terms of sodium transport and storage mechanisms, size effects result in the transition from a two-phase to a single-phase mechanism, an increase in the interfacial storage and surface reaction, as well as a variation of the sodium storage mechanism in pores, further leading to variation of the discharge voltage plateau. Finally, whether for bulk or nano-electrode materials, constructing efficient electrochemical circuits by the optimization of the phases and dimensionalities based on the transport properties of electrode materials is significant in achieving the rational design of sodium electrode materials and optimizing the electrochemical performance of sodium-ion batteries. We believe that this study will serve as a useful guide for the development of sodium electrode materials and will certainly contribute to the commercialization of sodium-ion batteries.

The resolution limit of scanning transmission electron microscopy (STEM) has now reached atomic resolution. Further, owing to its flexible multi-channel imaging and powerful spectral characterization abilities, STEM has shown immense promise for microscale characterization in materials sciences, life sciences, and other fields. However, the traditional STEM detector is limited by its single-pixel integral detection mechanism, due to which it can only collect scattered electrons at a specific angle. This not only results in a loss of angle-resolved information of the scattered electrons, but also reduces the dose efficiency of the incident electrons. Therefore, it is imperative that new imaging techniques are developed to achieve high-throughput, high-electron-dose-efficiency imaging. Recent advances in electron direct detection techniques and detectors with partitioned or pixelated configurations, as well as the rapidly increasing computing power and disk storage, have contributed to the rapid development of four-dimensional STEM (4D-STEM) technology. Uniquely, 4D-STEM allows one to acquire structural information associated with scattered electrons. During the acquisition of 4D-STEM data, the convergent electron beam performs two-dimensional scanning on the sample plane, while a pixelated array detector with a high frame rate, wide dynamic range, and high signal-to-noise ratio collects two-dimensional diffraction data in the far field. Because these diffraction data are angle-resolved, they can be used for conventional STEM imaging as well as phase contrast imaging at the leading edge. For example, electron ptychography is used to reconstruct the sample object function from a series of diffraction patterns measured at different spatial locations. In addition, 4D-STEM technology can be explored to obtain more information about the internal structure of materials, providing opportunities for the multi-scale characterization of materials. This paper introduces the basic principles of 4D-STEM imaging and summarizes a series of 4D-STEM applications ranging from the microstructural characterization of materials to the analysis of their physicochemical properties. Typical applications include virtual detector imaging as well as measurements of micro-electromagnetic fields, micro-crystal orientations, micro-strain distributions, and the local specimen thickness. In addition, electron ptychography imaging technology realized using 4D-STEM data is highly promising for low-electron-dose applications owing to its high utilization efficiency of scattered electrons. The application of 4D-STEM technology in low-electron-dose applications is discussed. Overall, with the rapid development of electron detectors and post-processing analysis software for 4D-STEM data, it is believed that the novel 4D-STEM technology will eventually completely replace traditional STEM technology.

Lithium-ion batteries (LIBs) have attracted considerable attention owing to their high energy density and long cycle life. However, lithium resources have become scarcer with the rapid development of electric vehicles and smart grid technologies. Considering the inexpensive and abundant supply of sodium, sodium-ion batteries (SIBs) are expected to replace LIBs for large-scale energy storage systems. However, the development of high-energy SIBs is usually limited by the poor initial Coulombic efficiency (ICE) of the anode materials, although a series of advanced sodium storage electrode materials have been reported. This is because active sodium ions are all provided by the cathode material in a full cell. The low ICE of the anode indicates that numerous active sodium ions are irreversibly consumed during the first cycle, reducing the reversible capacity and shortening the cycle life of the full cell. The significant loss of active sodium ions is attributed to the formation of a solid electrolyte interface (SEI) on the anode side and irreversible sodium capture by defect sites and surface functional groups on the anode material. Consequently, excessive cathode material is required in the full cell, which significantly reduces the utilization rate of the cathode material and the energy density of the full cell. Furthermore, many reported cathode materials, such as Fe₂S, are sodium-deficient and cannot be directly matched with anodes, limiting the selection of electrode materials. Presodiation technology is considered the most direct and effective method to solve the state-matching problem of cathode and anode materials by compensating for active sodium-ion loss and increasing the energy density, which are crucial for the commercial application of SIBs. The aim is to eliminate the irreversible capacity loss during the first cycle by incorporating additional active sodium ions to the electrode material in advance. This review comprehensively summarizes the latest research progress on various presodiation strategies, including short circuit with sodium metal, electrochemical presodiation, sodium metal addition, chemical presodiation, and cathode sacrificial additives. The advantages and challenges of existing methods are thoroughly analyzed and discussed from the perspective of their reaction mechanism, safety, compatibility, efficiency, and scalability. Emphasis is placed on the state-of-the-art advancements in chemical presodiation and cathode sacrificial additives, which are considered the two most promising methods for commercial applications. The unresolved scientific problems and technical difficulties are further discussed from a practical perspective. This review may provide guidance for the investigation of advanced presodiation technology and promote further development of high-energy SIBs.

The ever-worsening world-wide energy crisis and environmental issues are encouraging the development of green and renewable energy. Thus, rechargeable batteries are being developed and employed for energy storage and conversion in various electronic equipment. When compared with metal lithium batteries, aqueous rechargeable batteries have gained significant attention due to their advantages of high safety, low cost, and environmental friendliness. Among the various known rechargeable batteries (Li⁺, Na⁺, K⁺, NH₄⁺, Mg²⁺, Ca²⁺, and Al³⁺), aqueous zinc-ion batteries (ZIBs) are considered as promising energy storage devices because the zinc electrode exhibits high capacity (820 mAh∙g⁻¹) and low potential (−0.76 V vs. Standard hydrogen electrode (SHE)). To date, various ZIBs cathode materials with excellent performance have been developed, such as manganese- and vanadium-based oxides, Prussian blue and its analogues, and organic compounds. Unfortunately, some of these materials, especially manganese- and vanadium-based oxides, suffer from critical structural collapse, dissolution, and cathode/electrolyte interfacial side reactions, which lead to low Coulombic efficiency and poor cycle performance. The poor cycle performance is one of the main obstacles hindering the large-scale application of manganese- and vanadium-based oxides. Therefore, the structural design of cathodes and electrolyte regulation strategies have been extensively investigated to solve these problems and improve electrochemical performance. In comparison, electrolyte regulation is an important and effective strategy for improving the performance of ZIBs cathodes. It is well known that a strong interaction force exists between Zn²⁺ and H₂O, therefore, Zn²⁺ can coordinate with six H₂O molecules to form [Zn(H₂O)₆]²⁺ in the dilute aqueous electrolyte, while forming numerous hydrogen bonds between the H₂O molecules. The Zn²⁺-solvation structure and hydrogen bonds can be destructed and restructured by changing the anion, and using highly concentrated electrolyte and/or organic solvent, thereby decreasing the number of H₂O molecules in the solvated structure and the activity of free water. Furthermore, additives can change the pH value of the aqueous electrolyte and build a dissolution equilibrium between the cathode and electrolyte. Hence, an appropriate electrolyte regulation strategy can broaden the electrochemical stability window of electrolytes, improve the working potential, suppress the occurrence of interfacial side reactions, and prevent the dissolution of the active materials, thereby improving the electrochemical performance of ZIBs. Herein, we review the possible electrolyte regulation strategies for enhancing the electrochemical performance of ZIBs cathodes and classify regulation strategy into two main categories: 1) Solute (including different zinc salts, additive, and water-in-salt) and 2) Solvent (composite of organic/inorganic hybrid electrolytes). We then discuss the advantages and challenges of each strategy, and finally predict the possible future direction of electrolyte development.

Using renewable energy sources such as wind, solar, and tidal power is one of the most effective ways to address the global energy crisis and the ensuing environmental issues. As essential complementary components to renewable energy, high-performance energy storage devices and systems are urgently required. Since the 1990s, the global battery market has been dominated by lithium-ion batteries (LIBs) owing to their high energy density and long cycle life. They have been widely used in portable electronics, and more recently, in electric vehicles. However, lithium resources are limited and unevenly distributed; therefore, the manufacturing costs of LIBs are still high. There is also increasing concern about their operational safety. Thus, it is crucial to develop next-generation battery technologies with lower costs and higher safety. In recent years, magnesium-ion batteries (MIBs) have attracted increasing attention as one of the most promising multivalent ion batteries. The use of magnesium is encouraged owing to its good air stability, lower reduction potential (−2.356 V vs. standard hydrogen electrode), higher volumetric specific capacity (3833 mAh∙cm⁻³), and dendrite-free deposition upon cycling. Moreover, magnesium reserves (2.3%) are 1045 times more than those of lithium (0.0022%), because of which, MIBs are considerably less expensive than LIBs. The development of MIBs has, however, encountered a few challenges arising from the comprising cathodes, electrolytes, and anodes. Mg²⁺ ions with smaller radii and higher charge densities have strong Coulomb interactions with electrode materials, which leads to sluggish kinetics and high diffusion barriers during de-/intercalation. Contemporary electrolytes generally have poor chemical compatibility with cathodes of MIBs, narrow electrochemical windows, and high deposition overpotential, which limits the development of high-voltage MIBs. Moreover, Mg tends to react with organic solvents (especially carbonates and nitriles), forming passivation layers on the surfaces, which increase the interfacial resistance and lead to battery irreversibility. Therefore, material design and technological innovation are crucial for developing commercially viable MIBs. This review focuses on recent advances on MIB cathode materials. First, we present a brief description of the characteristics of MIBs and discuss their strengths and drawbacks. Then, we overview three types of cathode materials, namely, intercalation-type cathodes, conversion-type cathodes, and organic cathodes, followed by a summary of their limitations and recent efforts for addressing the above-mentioned challenges. We conclude with perspectives for future research directions.

Green hydrogen is obtained by electrochemical water splitting using electricity converted from renewable energy sources. When green hydrogen undergoes combustion, it produces only water, leading to zero CO₂ emissions from the source, which is important for the global energy transition. The sluggish kinetics of the hydrogen evolution reaction (HER)/oxygen evolution reaction (OER) in alkaline media have hindered an enhancement in hydrogen production from electrochemical water splitting. A detailed understanding of the alkaline reaction kinetics is important to accomplish the global mission of carbon neutrality. This review presents the theoretical kinetics for the HER and OER in alkaline media using different designed electrocatalysts, and discusses their corresponding reaction mechanisms. Subsequently, current design concepts and generalities on catalysts for water electrolysis are discussed. Enhancements in the OER activity for alkaline water electrolysis can be achieved through strategies that are classified into two major categories. In the first category, the exposure of numerous active sites is achieved by engineering the morphology and obtaining a high surface area. In the second category, the intrinsic activity of the catalyst toward the OER is enhanced by heteroatomic participation, vacancy formation, and the use of heterogeneous media. Advanced characterization techniques and in-situ testing techniques have confirmed the presence of complex oxidation media for the OER, which have a significant impact on the catalyst structure and local coordination. Research on the active sites of the catalyst, high concentrations of active species, and the design of highly efficient reaction media is required to further drive catalyst development for the OER. The evaluation of electrocatalysts exhibiting high performance at high current densities to produce green hydrogen is crucial for their implementation in industrial applications. Currently, large-scale synthesis a key technology to obtain industrial electrodes. Meanwhile, the construction of superaerophobic electrodes and three-dimensional electrodes facilitates the design of high-performance industrial catalytic electrodes. Subsequently, three different electrolytic cells that are typically used to obtain green hydrogen at the industrial scale are presented. The limitations to the design of electrolytic cells and the related solutions are also discussed. In-depth investigations on the design of either industrial electrocatalysts, commercial membranes, or electrolyzers can improve the understanding of industrial design principles to be applied to obtain industrial electrolyzers with increased efficiency, safety, and practicality. Finally, recent developments on electrocatalysts for water splitting and their limitations for industrial applications are presented to provide new perspectives and guidelines on the preparation of next-generation electrolytic catalysts.

In recent years, flexible and wearable electronic devices have attracted increasing research, industrial, and consumer attention. In particular, flexible zinc-air batteries (ZABs) are expected to become a promising power supply source for next-generation electronic products, especially the flexible and wearable ones, because of their high theoretical energy density, high specific capacity, high safety, and adaptability to uneven surfaces like human body. In the research field of flexible ZABs, a steady progress has been observed, and various ZAB preparation methods have been recently proposed. In this review, the main achievements and limitations of the recent research related to flexible ZABs are described. Firstly, the importance and applications of ZABs are discussed, followed by the working principle and configuration of typical ZABs. In the main text, the recent development of gel electrolytes, anodes, and cathodes is reviewed in detail. Currently, one of the most important limitations in the preparation of high-performance ZABs is the selection or preparation of a suitable gel electrolyte. A good gel electrolyte should have the ability of high-water holding capacity, high and low temperature resistance, high CO₂-tolerance, excellent ionic conductivity, and good mechanical ductility. Several gel electrolytes with various functions have been developed. However, novel gel electrolytes with multifunctional properties have not been developed. In addition, interfaces between the gel electrolyte and air cathode and those between the gel electrolyte and metal anode must be investigated in detail for ZAB performance improvement. Till now, only the effects of physical compression on the electrolyte-air cathode and electrolyte-metal anode interfaces have been adopted and investigated. Moreover, the air cathode and metal anode must exhibit high flexibility to expand the application scope of ZABs as flexible power supplies. Carbon cloth has been typically used as the substrate of the air cathode; however, carbon corrosion occurs under high potential, which needs to be overcome. Meanwhile, the use of nickel mesh or copper foam as the substrate for the cathode will make the flexible ZABs too rigid and not bendable. For the metal anode, mostly zinc sheet or zinc spring have been used to meet the demand of flexibility. However, if novel strategies for the development of doped zinc anodes are investigated, such as those based on the utilization of zinc powder-metal combination, ZAB performance will be significantly improved. If the above-mentioned limitations are overcome, flexible ZABs will not be limited to laboratory use, and can be widely applied in commercial wearable electronic products. Furthermore, the challenges and future perspectives of ZABs are discussed in this review.

Utilizing a direct chemical vapor deposition approach to synthesize graphene on insulating substrates has received enormous attention to date in both scientific and technological realms. In contrast to the graphene growth on metal substrates, the catalytic inertness of insulators toward feedstock decomposition and the high energy barrier for carbon fragment migration on the insulating surface result in not only high density of grain boundaries but also a low growth rate. Thus-obtained graphene film is usually accompanied by massive defects and limited crystal quality, which adversely affect the physical integrity and electrical performance of the fabricated graphene-based device. In this respect, various strategies have been adopted to modify the direct growth processes of graphene, e.g., sacrificial metal catalysis approach, self-terminating confinement approach and near-equilibrium growth approach. Among these mentioned above, the gaseous-promotor-assisted growth methodology has proven to be a beneficial way in enhancing crystal quality and augmenting the growth rate of graphene. For the gaseous-promotor-assisted chemical vapor deposition route, the gaseous promotor can not only regulate the composition/content of active carbon species in the gas-phase reaction process but also promote the surface migration and growth reactions. In this contribution, we review the recent advances in gaseous-promotor-assisted direct growth of graphene with high crystallinity, optimized uniformity, and enhanced growth rate on insulating substrates. First of all, we provide a systematic description of the growth behavior of graphene on insulators, including both the surface and gas-phase reactions combined with elementary steps during the growth process. We then summarize developed strategies aiming to achieve the direct growth of high-quality graphene via the assistance of gaseous promotors, with special emphasis on the effects and mechanisms of the growth process. The types of promotors commonly used in the gaseous-promotor-assisted strategy can be divided into metallic and non-metallic vapor species. These gaseous promotors can play influence on the feedstock decomposition, graphene nucleation, and enhance the enlargement and merging of individual domains. The corresponding mechanisms of the strategy can be classified into three parts: (1) The existence of highly concentrated metallic vapor species can promote thermal decomposition of carbon feedstock, which is the key to the growth of high-quality graphene; (2) The introduction of oxygen-containing species can effectively reduce the nucleation density, etch the amorphous carbon, leading to a high-quality, uniform growth of graphene film. In addition, hydroxylation of substrate through oxygen-containing species weakens the binding energy between the graphene edge and surface of the substrate, facilitating carbon fragment migration to evolve uniform monolayer graphene film; (3) The appearance of silicon and fluorine species reduces the growth kinetic barrier for carbon feedstock migrating onto the graphene edge to form the honeycomb lattice, which ensures the ultrafast growth of graphene on insulating substrates. Finally, we describe existed challenges and present future perspectives on the direct growth of high-quality graphene on insulating substrates to stimulate more efforts devoted to direct graphene growth and ultimate applications. We hope this review can propel in-depth comprehension of the direct growth of graphene on insulators by gaseous-promotor-assisted strategy, and pave the way for the development and applications of graphene materials.

Graphdiyne (GDY) bearing sp- and sp²-hybridized carbon networks, which is usually artificially synthesized via the in situ homocoupling reaction of hexaethylbenzene on copper foil, is an emerging two-dimensional (2D) carbon allotrope. During preparation, well-defined GDY structures including nanowires, nanowalls, and nanotubes are obtained. Such materials with varying morphologies have been shown to possess promising electronic, chemical, magnetic, and mechanical properties, rendering them applicable in various domains including energy storage, catalysis, and field emission. In addition, replacing hexaethylbenzene with other aryne derivatives under similar synthesis conditions has resulted in the generation of various GDY derivatives. Thus, a series of GDY derivatives with specific structures and controllable sizes have been readily prepared in recent years. Aryne precursors typically contain polycyclic aromatic carbocycles, heteroarenes (e.g., N, B, S, P, Si, Ge, and Ga). The intrinsic GDY has also been doped with metal elements (e.g., Hg, Ag, and Au). Chemical synthetic strategies such as Glaser coupling, Glaser-Hay coupling, and Eglinton coupling are also described. The structural design of various precursors has been effectively tailored to the constitution of the local carbon framework of GDY-based materials, which has enabled the realization of the targeted performance in terms of the electronic conductivity, band gap, mobility, cavity size, and charge separation. For example, three-dimensional (3D) carbyne riched nanospheres formed by the extended coupling of spatially rigid-structured spirobifluorene have provided abundant storage spaces and convenient multi-directional transmission paths for metal ions. The use of hetero-doped GDY has enabled the effective optimization of the thermal stability and mechanical, electronic, and optical properties. Metal element-based GDY, referred to as "metalated" GDY, could serve as efficient bifunctional catalysts possessing favorable transport properties to facilitate the diffusion of small molecules. By extension, such materials can be used more broadly in electrochemical energy storage, electrocatalysis, optoelectronics, nonlinear optics, oil-water separation, and numerous other fields. In this review, we have summarized the design, synthesis, and structural characterization of various GDY derivatives through the recently demonstrated substitution of various aryne precursors for hexaethylbenzene, while examining the functional relationships between the desired optoelectronic properties of GDY derivatives and their defined nanostructures and morphologies. In addition, important prospective applications of GDY derivatives have been described. These observations may motivate the construction of novel polar and electron-rich GDY derivatives with unique properties that can address practical challenges encountered in various devices.

In recent years, colloidal quantum wells (CQWs), also known as semiconductor nanoplatelets, have become the new kind of promising optoelectronic material because of their excellent optoelectronic properties, such as high color purity, high photoluminescence quantum efficiency, and adjustable color emissions. As a significant application of CQWs, light-emitting diodes based on CQWs (or CQW-LEDs) possess a number of advantages, such as an extremely narrow spectrum, excellent color purity, high efficiency, solution-processed fabrication, and good compatibility with flexible electronics. CQW-LEDs demonstrate an important application prospect in the fields of next-generation display and solid-state lighting, and therefore, attract significant attention from academic and industrial settings. In this review, some basic concepts of CQW-LEDs are first introduced (e.g., the design of CQW materials, employment of device structures, and understanding of emission mechanisms), which are expected to help with understanding this new type of LEDs. Thereafter, from the perspective of CQW emitting material types, the recent research progress in the development of CQW-LEDs based on core-only CQWs, core/crown CQWs, core/shell CQWs, complex-heterojunction-based CQWs, and impurity-doped CQWs is presented. The properties of various CQWs are also compared. In this section, by combining the recent work from our research group, the design strategies of high-performance CQW-LEDs are discussed in detail, including the analyses of material selection, device structure, working mechanism, and luminescence process. In the next section, the integrated applications of CQW-LEDs are illustrated, such as their use in LiFi-type communication, furthermore, their preparation as flexible optoelectronic materials is also reported. Finally, the present challenges (e.g., low efficiencies, short lifetimes, sub-optimal device engineering, and a narrow emission color region) and future development opportunities (e.g., flexible displays, flexible lighting, and CQW-LEDs with low-cost printing fabrication processes) of CQW-LEDs are discussed. Although the performance of CQW-LEDs still lags behind other kinds of state-of-the-art soft-material-based LEDs (e.g., organic LEDs, colloidal quantum dot LEDs, and perovskite LEDs), it has been gradually enhanced in the last eight years. Upon overcoming the current challenges, the prospect for the mass production of CQW-LEDs will be undoubtedly feasible. Thus, this review is not only an important reference that discusses the evolution of CQW-LEDs, it also provides insightful ideas for the development of materials for other optoelectronic applications (e.g., solar cells, lasers, photodetectors, sensors, X-ray imaging, and light communication).

The storage and utilization of energy is one of the important topics in the development of science and technology, especially regarding secondary batteries, which are efficient electrical-/chemical-energy-converting devices. This review systematically introduces the important research progress made in the context of secondary batteries, starting from their development and leading to the introduction of related basic theoretical knowledge. The current research on secondary batteries that are based on different systems and related key materials is discussed in detail, and includes lithium-ion batteries, sodium-ion batteries, potassium-ion batteries, magnesium-ion batteries, zinc-ion batteries, calcium-ion batteries, aluminum-ion batteries, fluorine-ion batteries, chloride-ion batteries, dual-ion batteries, lithium-sulfur (Se) batteries, sodium-sulfur (Se) batteries, potassium-sulfur (Se) batteries, polyvalent metal-sulfur-based batteries, lithium-oxygen batteries, sodium-oxygen batteries, potassium-oxygen batteries, aqueous metal-ion batteries, polyvalent metal-oxygen batteries, lithium-bromine (or lithium-iodine) batteries, photo-assisted batteries, flexible batteries, organic batteries, and metal-carbon dioxide batteries. Additionally, the common characterization techniques for electrode reaction processes in battery research are also introduced, including cryo-electron microscopy, transmission electron microscopy, synchrotron radiation, in situ spectroscopic techniques, and magnetic characterization. This paper will help researchers to systematically understand secondary battery technology and provide good guidance for future research on secondary batteries.

Because of the advantages of high safety, environment-friendliness, affordability, and ease of processing, aqueous rechargeable zinc batteries (ARZBs) are promising candidates for next-generation large-scale energy storage systems. In recent years, various cathode materials based on vanadium/manganese/cobalt oxides, Prussian blue analogs, and organic compounds have been reported. Among them, manganese dioxide (MnO₂) is widely used in ARZBs due to their outstanding advantages of low toxicity, eco-friendliness, and high capacity (616 mAh∙g⁻¹ based on two-electron transfer). However, the diversity of the crystal structures of MnO₂ and the unpredictability of the electrochemical reaction make it difficult to investigate the specific internal storage mechanism, which impedes further development of the optimal modification strategies. To date, the main recognized energy storage mechanisms are (de)intercalation and dissolution-deposition mechanisms. In the traditional (de)intercalation mechanism, the predominant issues related to MnO₂ during the cycling process include Mn dissolution, irreversible phase transformation, structural collapse, and sluggish ion diffusion kinetics. On the other hand, the detailed reaction path for the dissolution-deposition mechanism, which was developed in recent years, remains controversial. In addition, the incomplete dissolution-deposition of MnO₂ and the highly acidic environment inevitably leads to corrosion and hydrogen evolution of the zinc anode, as well as low Coulombic efficiency. Accordingly, optimization strategies for different reaction mechanisms have been proposed to make zinc-manganese batteries more competitive. For the (de)intercalation mechanism, modification of composite materials and nanostructure optimization strategies can be adopted to inhibit the dissolution of MnO₂ and increase the number of highly active reaction sites, thus enhancing the electrochemical performance. Moreover, the guest pre-intercalation strategy can help optimize the crystal structure of MnO₂, preventing the collapse of the internal structure during cycling. Besides, defect engineering and element doping strategies focus on regulating the distribution of the electronic structure for affecting the properties of MnO₂, resulting in lowering the energy barrier of zinc insertion. For the dissolution-deposition mechanism, the introduction of a neutral acetate and a halide mediator can effectively facilitate the dissolution-deposition of MnO₂. Meanwhile, metal element catalysis can accelerate the reaction kinetics of the MnO₂ dissolution-deposition, so that high-rate performance can be achieved. Furthermore, the decoupling battery system can separate the cathodic and anodic electrolytes to restrain the hydrogen and oxygen evolution reactions and enhance the potential difference. The flow battery system can effectively eliminate the influence of concentration polarization and stabilize the ion concentration in the electrolytes, thus leading to a large capacity (> 100 mAh). Undoubtedly, MnO₂ as a high-capacity, high-voltage cathode material has broad development prospects for ARZBs. Here, we systematically summarize the crystal structures and reaction mechanisms of MnO₂. We also discuss the optimization strategies toward advanced MnO₂ cathode materials for resolving the highlighted issues in zinc-manganese batteries, which are expected to provide research directions for the design and development of high-performance ARZBs.

Industrialization undoubtedly boosts economic development and improves the standard of living; however, it also leads to some serious problems, including the energy crisis, environmental pollution, and global warming. These problems are associated with or caused by the high carbon dioxide (CO₂) and sulfur dioxide (SO₂) emissions from the burning of fossil fuels such as coal, oil, and gas. Photocatalysis is considered one of the most promising technologies for eliminating these problems because of the possibility of converting CO₂ into hydrocarbon fuels and other valuable chemicals using solar energy, hydrogen (H₂) production from water (H₂O) electrolysis, and degradation of pollutants. Among the various photocatalysts, silicon carbide (SiC) has great potential in the fields of photocatalysis, photoelectrocatalysis, and electrocatalysis because of its good electrical properties and photoelectrochemistry. This review is divided into six sections: introduction, fundamentals of nanostructured SiC, synthesis methods for obtaining nanostructured SiC photocatalysts, strategies for improving the activity of nanostructured SiC photocatalysts, applications of nanostructured SiC photocatalysts, and conclusions and prospects. The fundamentals of nanostructured SiC include its physicochemical characteristics. It possesses a range of unique physical properties, such as extreme hardness, high mechanical stability at high temperatures, a low thermal expansion coefficient, wide bandgap, and superior thermal conductivity. It also possesses exceptional chemical characteristics, such as high oxidation and corrosion resistance. The synthesis methods for obtaining nanostructured SiC have been systematically summarized as follows: Template growth, sol-gel, organic precursor pyrolysis, solvothermal synthesis, arc discharge, carbon thermal reduction, and electrospinning. These synthesis methods require high temperatures, and the reaction mechanism involves SiC formation via the reaction between carbon and silicon oxide. In the section of the review involving the strategies for improving the activity of nanostructured SiC photocatalysts, seven strategies are discussed, viz., element doping, construction of Z-scheme (or S-scheme) systems, supported co-catalysts, visible photosensitization, construction of semiconductor heterojunctions, supported carbon materials, and construction of nanostructures. All of these strategies, except element doping and visible photosensitization, concentrate on enhancing the separation of holes and electrons, while suppressing their recombination, thus improving the photocatalytic performance of the nanostructured SiC photocatalysts. Regarding the element doping and visible photosensitization strategies, element doping can narrow the bandgap of SiC, which generates more holes and electrons to improve photocatalytic activity. On the other hand, the principle of visible photosensitization is that photo-induced electrons move from photosensitizers to the conduction band of SiC to participate in the reaction, thus enhancing the photocatalytic performance. In the section on the applications of nanostructured SiC, photocatalytic H₂ production, pollutant degradation, CO₂ reduction, photoelectrocatalytic, and electrocatalytic applications will be discussed. The mechanism of a photocatalytic reaction requires the SiC photocatalyst to produce photo-induced electrons and holes during irradiation, which participate in the photocatalytic reaction. For example, photo-induced electrons can transform protons into H₂, as well as CO₂ into methane, methanol, or formic acid. Furthermore, photo-induced holes can convert organic waste into H₂O and CO₂. For photoelectrocatalytic and electrocatalytic applications, SiC is used as a catalyst under high temperatures and highly acidic or basic environments because of its remarkable physicochemical characteristics, including low thermal expansion, superior thermal conductivity, and high oxidation and corrosion resistance. The last section of the review will reveal the major obstacles impeding the industrial application of nanostructured SiC photocatalysts, such as insufficient visible absorption, slow reaction kinetics, and hard fabrication, as well as provide some ideas on how to overcome these obstacles.

Na-ion batteries (SIBs) are promising alternatives for Li-ion batteries owing to the natural abundance of sodium resources and similar energy storage mechanisms. Although significant progress has been achieved in research on SIBs, there remain several challenges to be addressed. One of the major challenges in the construction of high-performance SIBs is the development of suitable anode materials with a large reversible capacity, high cycling stability, and good rate performance. Alloying anode materials mainly composed of elements from Groups IVA and VA, as well as their alloys, have attracted widespread attention because of their low working voltage, high cost-effectiveness, and large theoretical capacity. Alloying-type anode materials can be alloyed with metallic Na to achieve large reversible capacities, ensuring a high energy density. Antimony is a promising anode material for SIBs owing to its high theoretical specific capacity (660 mAh·g⁻¹, corresponding to the full sodiation Na₃Sb alloy), small degree of electrode polarization (~0.25 V), appropriate Na⁺ deintercalation potential (0.5–0.75 V), low price, and environmental friendliness. However, an important challenge for using Sb-based anode materials is that the high specific capacity is accompanied by large volume changes during cycling. Such changes lead to the pulverization of the active materials and their falling off from the collector, which significantly limit their large-scale application in the field of sodium-ion batteries. Therefore, mitigating the volume expansion issue of Sb-based anode materials in the charge-discharge process is very important for the design of high-performance SIBs. In recent years, researchers have attempted to address this issue by designing special structures to prepare various composites, and substantial progress has been achieved in improving the electrochemical performance of SIBs. In this review, the relationship between the structure and properties of Sb-based materials and their applications in SIBs are presented and discussed in detail. The latest research progress on using Sb-based anode materials for SIBs in redox reaction mechanisms along with their morphology design, structure-performance relationship, etc. have been reviewed. The main objective of this review is to explore the determining factors of the performance of Sb-based anode materials to propose suitable modification strategies for improving their reversible capacity and cycle stability. Finally, future developments, challenges, and prospects of Sb-based anode materials for SIBs are discussed. Despite several challenges, Sb-based materials are very promising anode materials for SIBs with alloying reaction mechanisms. To further improve the large-scale application of Sb-based anode materials, it is necessary to optimize the binder, electrode structure, and electrolyte composition. The combination of in-depth studies on the electrochemical reaction mechanisms and advanced characterization technologies is important for the development and construction of advanced Sb-based anode materials for SIBs. Finally, to achieve extensive large-scale applications, it is necessary to further explore environmentally friendly, low-cost, and controllable synthetic technologies to prepare high-performance Sb-based anode materials. This review provides specific perspectives for the construction and optimization of Sb-based anode materials and suggests scope for future work on Sb-based anode materials, thereby promoting the rapid development and practical application of SIBs.

Driven by the excessive environmental pollution caused by the over-use of non-renewable fossil-derived energy, renewable energy and electrochemical energy storage devices have made great progress in the past decades. Electrochemical energy storage devices, such as lithium-ion batteries, have the advantages of high capacity, long life cycle, and good safety performance; therefore, they have been used in various applications. For example, economical and environment-friendly electric vehicles have recently taken up increasing market share. However, when compared with vehicles propelled using fossil-derived energy, the slow charging speed of electric vehicles has always restricted their further promotion. The realization of rapid charging for electric vehicles can alleviate the high-pressure usage of charging piles as well as increase the application and market share of electric vehicles. Therefore, it is important to develop high-performance lithium-ion batteries with rapid charge and discharge capacities. The fast-charging capacity of lithium-ion batteries is limited by the slow migration of lithium ions in the electrode and the electrode/electrolyte interface. Therefore, the key to developing fast-charging lithium-ion batteries lies in the successful design of suitable electrode materials. Because of its low cost and excellent electrochemical performance, graphite has been widely used to develop the cathode of lithium-ion batteries. However, the migration of lithium ions in graphite is slow, resulting in large polarization during the high-current charge and discharge processes. In addition, the low lithium intercalation potential of graphite leads to lithium precipitation during fast charging, which can decrease the electrochemical performance and cause potential safety hazards. Therefore, graphite must be improved to meet the needs of such fast-charging devices. In this article, we systematically introduce the research progress made in recent years within the scope of rapid-charging improvement of graphite(-based) cathodes and then highlight the modification strategies for graphite with the goal of achieving functional coating, desired morphological and structural design, optimized electrolyte properties, and an improved charging protocol. Additionally, this article evaluates the advantages and disadvantages of the modification strategies as well as their application prospects. The scheme of functional coating for modifying graphite must simplify the process and improve production efficiency to meet the needs of industrial development. Morphology design should ensure satisfactory initial Coulomb efficiency, while the improvement of the electrolyte properties and optimization of the charging protocol need to consider the commercialization costs. Finally, this paper proposes further evaluation of the effects of the modification strategies based on soft-pack or cylindrical batteries to strengthen the commercialization prospect of the modification strategies.

Lithium metal batteries, which use lithium metal as the anode, have attracted tremendous research interest in recent years, owing to their high energy density and potential for future energy storage applications. Despite their advantages such as high energy density, the safety concerns and short lifespan significantly impede their practical applications in transportation and electronic devices. Tremendous efforts have been devoted to overcoming these problems, including materials design, interface modification, and electrolyte engineering. Among these strategies, electrolyte regulation plays a key role in improving the efficiency, stability, and safety of lithium metal anodes. As an important class of electrolyte components, fluorinated solvents, which can decompose to form LiF-rich interphase layers on both anode and cathode, have been proven to enhance the stability of lithium metal anodes and improve the oxidative stability of the electrolytes. Meanwhile, the spatial structure of fluorinated solvents, such as the number and sites of fluorine atoms, can influence the physicochemical properties of the electrolytes and the compositions/structure of the solid-electrolyte interphase, which eventually dictates the cycling performance of Li metal batteries. Recently, many fluorinated solvents with different molecular structures have been designed to regulate the solvation structure of electrolytes, and these solvents exhibit novel electrochemical properties in lithium metal batteries. However, there are few comprehensive reviews that summarize the fluorinated solvents used in Li metal batteries and discuss their functions in electrolytes and their physicochemical properties. This review summarizes the novel fluorinated solvents used in lithium metal batteries in recent years, which have been classified into three parts: diluents, traditional solvents, and novel molecules, based on their functions in the electrolytes. In every part, the understanding of the interactions between fluorinated solvents and Li ions, the decomposition mechanism of fluorinated solvents at the interface of the electrode, the functions of fluorinated solvents in the electrolytes, and the structure-activity relationship between the fluorinated solvents and battery performance have been comprehensively summarized and discussed. Moreover, the advantages and disadvantages of fluorinated solvents have been discussed, and the importance of precisely controlling the number of fluorine atoms and the structure of fluorinated solvents has been emphasized. At the end of this review, a perspective for designing new fluorinated solvents has been proposed. We believe that this review can provide insights on designing novel fluorinated solvents for high-performance Li metal batteries.

Rechargeable potassium-ion batteries (PIBs), with their low cost and the abundant K reserves, have been promising candidates for energy storage and conversion. Among all anode materials for PIBs, metal sulfides (MSs) show superiority owing to their high theoretical capacity and variety of material species. Nevertheless, the battery performance of MSs is hindered by many factors such as poor conductivity, low ion diffusivity, sluggish interfacial/surface transfer kinetics, and drastic volume changes. In this review, the electrochemical reaction mechanisms, challenges, and synthesis methods of MSs for PIBs are summarized and discussed. In particular, the most common synthesis methods of MSs for PIBs are highlighted, including template synthesis, hydro/solvothermal synthesis, solid-phase chemical synthesis, electrospinning synthesis, and ion-exchange synthesis. During the potassium storage process, the two-dimensional layered MSs follow the intercalation/extraction mechanism, and the MSs with inactive metal undergo the conversion reaction, whereas the metal-active MSs follow the conversion-alloying reaction mechanism. Given the inherent properties of MSs and the reactions they undergo during cycling, when used as anodes for PIBs, such materials experience a series of problems, including poor ion-/electron-transport kinetics, structural instability, and loss of active material caused by the dissolution of discharged polysulfide products and the occurrence of side reactions. These problems can be solved by optimizing the methods for synthesizing MSs with an ideal composition and structure. The template method can precisely prepare porous or hollow-structured materials, the hydro/solvothermal method can alter the thickness or size of the material by adjusting certain synthesis parameters, and the one-dimensional-structured material obtained via electrospinning often has a large specific surface area, all of which can shorten the transport pathway for potassium ions, thereby improving the performance of the battery. The ion-exchange method affords difficult-to-synthesize MSs via anion- or cation-exchange, in which the product inherits the structure of the starting material. The solid-phase synthesis method makes it possible to combine MSs with other materials. Combinations with materials such as carbon or other MSs helps to provide sufficient buffer space for the volume expansion of MSs during cycling, while promoting electron transport and improving the potassium-storage properties of the anodes. Therefore, this review aims to highlight the current defects of MS anodes and explore the construction of their ideal architecture for high-performance PIBs by optimizing the synthesis methods. Ultimately, we propose the possible future advancement of MSs for PIBs.

Through the combustion of fossil fuels and other human activities, large amounts of CO₂ gas have been emitted into the atmosphere, causing many environmental problems, such as the greenhouse effect and global warming. Thus, developing and utilizing renewable clean energy is crucial to reduce CO₂ emission and achieve carbon neutrality. The electrochemical CO₂ reduction reaction (CO₂RR) has been considered as an effective approach to obtain high value-added chemicals and fuels, which can store intermittent renewable energy and achieve the artificial carbon cycle. In addition, due to its multiple advantages, such as mild reaction conditions, tunable products, and simple implementation, electrochemical CO₂RR has attracted extensive attention. Electrochemical CO₂RR involves multiple electron–proton transfer steps to obtain multitudinous products, such as C₁ products (CO, HCOOH, CH₄, etc.) and C₂ products (C₂H₄, C₂H₅OH, etc.). The intermediates, among which ^*CO is usually identified as the key intermediate, and reaction pathways of different products intersect, resulting in an extremely complex reaction mechanism. Currently, copper has been widely proven to be the only metal catalyst that can efficiently reduce CO₂ to hydrocarbons and oxygenates due to its suitable adsorption energy for ^*CO. However, the low product selectivity, poor stability, and high overpotential of pure Cu hinder its use for the production of industrial-grade multi-carbon products. Tandem catalysts with multiple types of active sites can sequentially reduce CO₂ molecules into desired products. When loaded onto a co-catalyst that can efficiently convert CO₂ to ^*CO (such as Au and Ag), Cu acts as an electron donor owing to its high electrochemical potential. ^*CO species generated from the substrate can spillover onto the surface of electron-poor Cu due to the stronger adsorption and be further reduced to C₂₊ products. The use of Cu-based tandem catalysts for electrochemical CO₂RR is a promising strategy for improving the performance of CO₂RR and thus, has become a research hotspot in recent years. In this review, we first introduce the reaction routes and tandem mechanisms of electrochemical CO₂RR. Then, we systematically summarize the recent research progress of Cu-based tandem catalysts for electrochemical CO₂RR, including Cu-based metallic materials (alloys, heterojunction, and core-shell structures) as well as Cu-based framework materials, carbon materials, and polymer-modified materials. Importantly, the preparation methods of various Cu-based tandem catalysts and their structure–activity relationship in CO₂RR are discussed and analyzed in detail. Finally, the challenges and opportunities of the rational design and controllable synthesis of advanced tandem catalysts for electrochemical CO₂RR are proposed.

The combustion of fossil fuels increases atmospheric carbon dioxide (CO₂) concentrations, leading to adverse impacts on the planetary radiation balance and, consequently, on the climate. Fossil fuel utilization has contributed to a marked rise in global temperatures, now at least 1.2 ℃ above 'pre-industrial' levels. To meet the 2015 Paris Agreement target of 1.5 ℃ above pre-industrial levels, considerable efforts are required to efficiently capture and utilize CO₂. Among the different strategies developed for converting CO₂, electrochemical CO₂ reduction (ECR) to valuable chemicals using renewable energy is expected to revolutionize the manufacture of sustainable "green" chemicals, thereby achieving a closed anthropogenic carbon cycle. However, CO₂ is a thermodynamically stable and kinetically inert molecule that requires high electrical energy to bend the linear O＝C＝O bond by attacking the C atom. To facilitate the ECR with good energy efficiency, it is essential to lower the reaction overpotential as well as maintain a high current density and desirable product selectivity; therefore, the design and development of advanced electrocatalysts are crucial. A plethora of heterogeneous and homogeneous materials has been explored in the ECR. Among these materials, single-atom catalysts (SACs) have been the focus of most extensive research in the context of ECR. A SAC with isolated metal atoms dispersed on a supporting host exhibits a unique electronic structure, well-defined coordination environment, and an extremely high atom utilization maximum; thus, SACs have emerged as promising materials over the last two decades. Single-atom catalysis has covered the periodic table from d-block and ds-block metals to p-block metals. The types of support materials for SACs, ranging from metal oxides to tailored carbon materials, have also expanded. The adsorption strength and catalytic activity of SACs can be effectively tuned by modulating the central metal and local coordination structure of the SACs. In this article, we discuss the progress made to date in the field of single-atom catalysis for promoting ECR. We provide a comprehensive review of state-of-the-art SACs for the ECR in terms of product distribution, selectivity, partial current density, and performance stability. Special attention is paid to the modification of SACs to improve the ECR efficiency. This includes tailoring the coordination of the heteroatom, constructing bimetallic sites, engineering the morphologies and surface defects of supports, and regulating surface functional groups. The correlation of the coordination structure of SACs and metal-support interactions with ECR performance is analyzed. Finally, development opportunities and challenges for the application of SACs in the ECR, especially to form multi-carbon products, are presented.

Sustainable fuels and chemicals are receiving unprecedented attention worldwide in the context of achieving global carbon neutrality. Biomass, as the only natural and sustainable carbon-based source, shows great potential in addressing our current environmental/energy problems and in creating a carbon-neutral society. Lignocellulosic biomass is made up of basic structural units containing C―O/C―C bonds, and the catalytic cleavage of these C―O/C―C bonds is the key for biomass valorization; thus, garnering considerable attention in the past decade. This viewpoint begins with a brief report on the current status of catalytic activation/cleavage of C―O/C―C bonds during biomass conversion, and then goes on to discuss the key challenges experienced and possible strategies that can be implemented using cooperative catalysis. Our goal is not to provide a comprehensive overview of the activation/cleavage of the C―O/C―C bonds in biomass, but rather to highlight the core questions and challenges related to this process and the requirements for future investigations. We selected several representative C―O/C―C bonds in carbohydrates and lignin to discuss their catalytic mechanism in terms of total/selective bond cleavage, and then present our own insights for future studies. Therefore, this article mainly discusses the following two aspects: (1) The activation and cleavage of C―O bonds, which includes total and selective C―O bond cleavage in furan-based fuel precursors and lignin. When aiming to produce liquid fuels, including alkanes and arenes from biomass, the total cleavage of C―O bonds is essential. During the hydrodeoxygenation (HDO) of furan-based fuel precursors, various C―O bonds need to be cleaved, especially the C―O bond of each tetrahydrofuran ring, which has the highest bond energy. When compared with the total HDO of fuel precursors, the removal of the phenolic hydroxyl groups in lignin to produce arenes is more challenging because of the competition between the over-hydrogenation of the benzene rings and the cleavage of phenolic C―O bonds. The selective or partial cleavage of C―O/C―C bonds to form highly functionalized chemicals has recently attracted great interest and is believed to be a dynamic future research avenue. For example, the production of phenol from lignin or lignin-model compounds, through the selective removal of methoxy groups and para-side-chain groups, while preserving the phenolic hydroxyl groups, has been extensively explored in the past few years. (2) The other important aspect of this article is the cleavage of the C―C bonds in carbohydrates and lignin. The cleavage of carbohydrate C―C bonds occurs via retro-aldol condensation, which produces propylene glycol, ethylene glycol, ethanol, and lactic acid. The cleavage of C―C bonds in lignin is challenging because the bond energy of the C―C bonds is generally higher than that of the C―O bonds in lignin. Therefore, in this section, we discuss the cleavage of the strongest 5―5' bond in lignin. Finally, some subjective perspectives and future directions are provided, also highlighting several major challenges in this field.

High-melting hydrocarbon waxes (melting point: > 80 ℃), consisting of saturated alkanes with carbon numbers greater than 40, exhibit unique features including high melting points, high stability, low penetration, high viscosity, as well as good wear resistance and hardness. These features make high-melting waxes suitable for use in foods, cosmetics, materials processing, electronic machinery, national defense, aviation, medical fields, etc. Considering the fast growth of technology and the electronics industry, the world's economy relies on the production and utilization of high-quality high-melting waxes. However, most waxes in the world's current markets are prepared from mineral oils, and such commercial waxes have melting points in the range of 50–70 ℃. Considering the rapid consumption of high-melting waxes and specialty waxes, their supply insufficiency is anticipated to exceed 700000 t. High-melting waxes are divided into polyethylene (PE) wax and Fischer-Tropsch synthesis (FTS) wax, based on synthesis methodology. PE wax can be obtained via the polymerization of ethylene and can also be prepared via the thermal or catalytic cracking of plastics. PE cracking to form waxes, with the advantage of low cost, can effectively solve the problem of "white pollution" and make use of existing catalytic cracking units. However, this process results in high energy consumption to achieve waste polymer depolymerization and exhibits some drawbacks, such as a wide carbon number distribution and high impurity content in the obtained PE waxes. However, there are some new methods for synthesizing PE waxes, such as cross alkane metathesis. The FTS, which uses carbon monoxide and hydrogen as raw materials, realizes the synthesis of waxes through carbon chain growth. Although the high-melting FTS waxes display excellent performance and the technology is gradually maturing, FTS waxes with different melting points are produced by rectification of products with various carbon chain lengths. Nonetheless, PE and FTS waxes are widely used in various industries because of their excellent properties. However, their synthesis is based on petroleum and coal-derived chemical products. Biomass-derived waxes have a narrow melting range due to their precise carbon chain growth process. Based on different application demands, small biomass platform molecules can be functionalized to fabricate biomass-derived waxes with special functions. More importantly, the biomass-based synthesis route is sustainable and in-line with the global values for mitigating carbon dioxide emissions and achieving carbon neutrality. This review discusses the recent advances in the synthesis techniques for high-melting waxes, including PE waxes, FTS waxes, and biomass-derived waxes. Furthermore, the catalysts and reaction mechanisms involved in the synthesis of high-melting waxes are discussed in detail. Finally, the perspectives and trends of high-melting waxes are reviewed to promote the emergence of new processes and technical routes.

Biomass, as a renewable carbon resource in nature, has been considered as an ideal starting feedstock to produce various valuable chemicals, fuels, and materials, and thus, can help build a sustainable chemical industry. Because cellulose is one of the richest components in lignocellulosic biomass, the efficient transformation of cellulose plays a crucial role in biomass utilization. However, there are many oxygen-containing groups in cellulose, and therefore, the selective removal of particular functional groups from cellulose becomes an essential step in the synthesis of the chemicals or fuels that can meet the requirements set by current chemical industries. In the past decades, several efficient catalytic systems have been developed to selectively split the C―O bonds inside cellulose and its derivatives, thereby producing various valuable chemicals. In this review article, we highlight recent progress made in the selective deoxygenation of cellulose and its derived key platforms such as glucose and 5-hydroxymethyl furfural (HMF) into ethanol, dimethyl furfural (DMF), 1, 6-hexanediol (1, 6-HD), and adipic acid. The selection of these reactions is primarily because these chemicals are of great significance in chemical industries. More importantly, the formation of these chemicals represents the cleavage of different C―O bonds in biomass molecules. For instance, the synthesis of ethanol requires cleaving of only one C―O bond and two C―C bonds of the glucose unit inside cellulose. If two or more C―O bonds in the sugar or sugar acids are cleaved, olefins, oxygen-reduced sugars, and adipic acid will be attained. HMF has a furan ring linked by hydroxyl/carbonyl groups, and hence, either a furanic compound (e.g., DMF) or linear products (e.g., 1, 6-HD and adipic acid) can be synthesized by selective removal of hydroxyl/carbonyl oxygen or ring oxygen atoms. This article focuses on the selective cleavage of particular C―O bonds via heterogeneous catalysis. Efficient catalytic systems using hydrogenolysis and/or deoxydehydration strategies for these transformations are discussed. Moreover, the functions of typical catalysts and reaction mechanisms are presented to obtain insight into the C―O bond cleavage in these biomass molecules. Additionally, other factors such as reaction conditions that also influence the deoxygenation performance are analyzed. We hope that these knowledge gained on the catalytic deoxygenation of cellulose and its derived platforms will promote the rational design of effective strategies or catalysts in the future utilization of lignocellulosic biomass.

Fibers and textiles with good flexibility, air permeability, and mechanical properties are indispensable materials in our daily lives. With the rapid development of flexible electronics, fabricating fibers and textiles that exhibit intelligent characteristics has become an attractive research topic. On the basis of the characteristics of common fibers or textiles, intelligent fibers and textiles may exhibit unique functions such as sensing, feedback, response, self-diagnosis, self-repair, and self-regulation. The development of intelligent fibers and textiles is closely related with the development of material science. Many materials, including metals, artificial polymers and natural biopolymers can be used for fabricating intelligent fibers and textiles. Compared with metals and artificial polymers, natural biopolymers have advantages of green source, biosafety, biodegradability and lightweight. Among natural biopolymers, natural silks, especially that from Bombyx mori, can be obtained in large amounts and have been used for clothes for thousands of years. Silkworm silk has exceptional mechanical properties, attractive luster, good biocompatibility and biodegradability. Therefore, silk materials are considered to be one of most promising candidates for intelligent fibers and textiles. In this review, we firstly introduce the hierarchical structures and basic properties of natural silk fibers. The exceptional mechanical properties of silk fibers can be ascribed to their unique hierarchical structures (from polypeptide chains, secondary structures to macroscopic fibers). The approaches to fabricate regenerated silk materials are briefly reviewed. The basic properties of silk materials, including the mechanical properties, biocompatibility, biodegradability, optical properties, piezoelectric properties, and thermal stability are presented. Then, the application of silk materials in various intelligent fibers and textiles, including fiber-based sensors, actuators, optical devices, energy harvesting and storage devices, are reviewed. Silk fibers can be functionalized and made into strain sensors, pressure sensors, and humidity sensors for applications in health monitoring. They can also transform into electrically conductive materials through high-temperature carbonization and then be fabricated into high-performance sensors or other functional devices. Silk-based actuators have been fabricated based on the response of silk to water or other molecules. Besides, silk-based fluorescence fibers and optical fibers were developed. Silk fibers have also been used in wearable energy devices by designing and fabricating piezoelectric nanogenerators, triboelectric nanogenerators, super-capacitors and batteries. The preparation methods, performance, and working mechanisms of those silk-based intelligent fibers and textiles are discussed in details. Finally, the persisting challenges and future opportunities of silk-based intelligent fibers and textiles are discussed. We believe that silk-based materials have great potential for intelligent fibers and textiles. The further development of this field will be accelerated by the continued development of material science and related techniques.

Graphene fiber, a macroscopic one-dimensional material formed by assembling elementary graphene flakes, has emerged in response to the increasing demand for multifunctional or even smart fibers. Based on the astonishing properties of graphene building blocks, graphene fiber presents a series of attractive features, such as superior mechanical strength and electronic conductivity, light-weight, and efficient thermal conductivity. As a result, graphene fiber exhibits broad prospects for application in ultralight cables for aerospace, wearable energy storage devices, biosensors, and neuroelectronics. graphene fiber may provide a critical breakthrough for realizing multi-functional fibers or even smart textiles. Since it was first prepared in 2011, numerous fabrication techniques have been developed to assemble graphene fiber, such as wet spinning, space-confined hydrothermal assembly, film twisting approaches, and template-assisted chemical vapor deposition. Among various graphene fiber preparation approaches, wet spinning has great application potential, as it affords the best mechanical strength and electrical conductivity of the prepared graphene fiber, along with great compatibility with the commercialized wet spinning technique. Therefore, the wet spinning approach has attracted extensive attention for batch production of high-performance graphene fiber. Herein, we introduce the pivotal steps of the wet spinning preparation of graphene fiber, with focus on summarizing the detailed strategies for enhancing the fiber properties and we also discuss the relationship between the structure and assembly approaches. The wet spinning technology for assembling graphene fiber includes a series of critical steps, such as preparation of the spinning liquid, bath coagulation, spinneret design, and post-treatment process. These procedures may have a significant influence on the micro-, meso-, and macro-structure of the final prepared graphene fiber. We also discuss the fundamental relationship between the typical properties of graphene fibers and their hierarchical structures, such as the in-planar structure of graphene sheets, aggregation structure of graphene flakes, and the macrostructure or morphology of graphene fiber. The recent advances in graphene fiber-based smart fibers and fabric applications are also analyzed, highlighting possible strategies for promoting structural-functional integrated applications. Finally, the current challenges and possible approaches for further improving the mechanical and electric properties of graphene fiber are presented. This review can be briefly divided into three parts: (1) details of the wet spinning process and its specific influence on the structural features of graphene fiber, (2) characteristics of current graphene fibers and promising strategies for enhancing the properties, (3) latest studies of graphene fiber applications and perspectives for future application.

Carbon nanotube fiber (CNTF) comprises continuous yarn-like macro aggregates with a large amount of carbon nanotubes and bundles thereof. CNTFs have excellent properties, such as high strength, toughness, and conductivity, because of which, they have broad prospects in several fields, such as structure-function integrated composite materials, fibrous energy devices, artificial muscle, and lightweight conductive wire. After two decades of development, breakthroughs have been made in continuous preparation technology, performance enhancement, and application exploration of CNTF materials. In this review, the development history of CNTF materials is summarized, and various continuous preparation technologies of CNTFs, including wet spinning, array spinning, and floating catalyst chemical vapor deposition (FCCVD) direct spinning, are described and compared. The wet spinning technology for fabricating CNTFs can be easily scaled due to its similarity to the conventional wet spinning technology used for fabricating high-performance fibers, while the obtained CNTFs have relatively high conductivity. The main challenges in wet spinning are the mass preparation and appropriate dispersion of high-quality carbon nanotubes (CNTs) with large aspect ratios. The array spinning technology can produce CNTFs with high purity and controllable structures, and its challenges are the relatively low preparation efficiency and high cost, because of which, it is challenging to meet the needs of large-scale applications. The FCCVD direct spinning technology can continuously produce CNTFs with relatively high strengths and at low cost, and it is easily adaptable for large-scale fabrication. The main drawbacks of CNTFs obtained from direct spinning are the relatively high impurity content and nonuniform CNT structures. Since CNTFs were first reported in 2000, one of the major challenges has been transferring the excellent properties of individual CNTs to the macroscopic assemblies of CNTs. To answer this question, the correlation between the structures and properties of CNTFs is discussed in detail, and contemporary techniques used for the enhancement of mechanical and electrical properties of CNTFs are reviewed. Based on the fiber fracture mechanism of slippage between CNTs, typical mechanical performance enhancement techniques include manipulating the CNT structures (namely wall number, diameter, aspect ratio, and collapse state), aligning the CNT along the fiber axis, enhancing the packing density and the interaction between CNTs, and combining with other reinforcing materials. The electrical performance of CNTFs is attributed to a 3D hopping electron transport mechanism in CNTFs. Conductivity enhancement techniques mainly include improving the assembly structure of CNTFs, using conductive materials as fillers between the CNTs, oxidative p-doping, and combining with metallic conductors. Finally, the main challenges in terms of performance enhancement and large-scale fabrication are discussed, and the development directions of CNTF materials are proposed.

The development of new types of artificial muscles is of utmost importance as traditional actuators based on mechanical drive systems no longer meet the stringent requirements of flexibility, high efficiency, and multi-stimuli responses in advanced functional fields, such as soft and biomimetic robots, sensors, artificial intelligent control, and artificial intelligence. Carbonene materials refer to carbon materials composed of all carbon atoms with sp² hybridization, mainly including carbon nanotubes and graphene. Owing to their exceptional properties such as light weight, excellent mechanical performance, high conductivity, flexibility, and large specific surface area, carbonene materials demonstrate significant application potential in artificial muscles, thereby promoting the rapid development of corresponding fields. Herein, the recent progress of the application of carbonene materials in artificial muscles is summarized to provide a comprehensive understanding of the preparation, properties, and applications of artificial muscles composed of carbonene materials. First, carbonene artificial muscles integrating response, actuation, and structure are introduced. As carbonene materials are unique building blocks that can be readily assembled into macroscopic materials with various structures, fibrous and membranous artificial muscles based on carbonene materials are discussed in detail. Carbonene fiber actuators demonstrate diverse actuation performances when fabricated with different structures. Bending actuation typically occurs when carbonene artificial muscles with asymmetric structures are subjected to external stimulation. The untwisting of carbonene artificial muscle fibers with twisted structures causes torsional and tensile actuation, which can be attributed to the volume expansion induced by external stimuli. Furthermore, coiled structures achieved by twisting a fiber until it is fully coiled can enhance the actuation stroke. Thus, the actuation of artificial muscle fibers made of carbonene materials can be classified into bending, rotation, and contraction actuations. Second, carbonene materials have long been considered as a functional component in composite materials for specific applications owing to their excellent physical and chemical properties. Therefore, the application of carbonene materials as an additional component to other artificial muscle materials (such as smart hydrogels, dielectric elastomers, and conducting polymers) is reviewed. By employing carbonene materials, artificial muscle materials exhibit improved electrical and mechanical properties, thereby leading to superior actuation performances. In addition, integrating carbonene materials into artificial muscles can endow the muscles with programmable actuation and sensing functions. Finally, the challenges faced in the application of artificial muscles based on carbonene materials and the future application of carbonene artificial muscles with multi-functional actuation performance are briefly discussed.

The development of high-performance polymer fibers is one of the main focus areas for the global polymer fiber industry. To ensure the advancement of important industries such as national aerospace, the performance of existing fibers should be improved, while new fibers that combine various properties and functions should also be developed. Carbonene materials, mainly comprising graphene and carbon nanotubes, exhibit excellent mechanical, electrical, thermal, and other properties; thus, they are considered ideal modifiers for high-performance polymer fibers. Herein, carbonene materials modified high-performance polymer fibers are reviewed to provide a comprehensive overview of their preparation, properties, and applications. Firstly, the preparation methods for these fibers, such as the dispersion of carbonene materials and polymer fiber modification methods, will be discussed. The dispersion methods employed for carbonene materials include mechanical mixing as well as covalent and non-covalent functionalization. Although mechanical mixing is relatively straightforward, functionalization typically provides better dispersion. To obtain well-dispersed carbonene materials, these methods should be combined. Polymer fiber modification methods include mixing, in situ polymerization, and coating. Although mixing can be performed during compounding of carbonene materials as well as a wide range of polymers, in situ polymerization generates stronger connections between carbonene materials and polymers, thus resulting in better properties compared to that obtained from mixing. Employing coating as a modification method offers the advantage of improving the surface properties as well as the possibility to introduce additional functionalities to the high-performance polymer fibers. Therefore, during preparation, the structure and function design of carbonene materials modified high-performance polymer fibers should be considered when the compounding method is selected. Subsequent discussions on the properties associated with these fibers will primarily focus on mechanical, electrical, and thermal properties. As carbonene materials can support loads and promote polymer crystallization and molecular chain orientation, it will contribute to improved mechanical properties. In addition, carbonene materials can develop conductive paths in the polymer fiber, thereby improving the electrical properties. These conductive networks further contribute to reducing segment motions in polymer molecular chains at a high temperature, thereby improving the thermal conductivity and thermostability of the materials. Through the addition of carbonene materials, new functions, such as UV resistance, resistance to photo-degradation, and improved surface affinity, can also be introduced. Finally, applications of carbonene materials modified high-performance polymer fibers will be addressed. These include potential applications as structural, heat-resistant, and wear-resistant materials that can be expected to exhibit superior performance when compared to conventional high-performance polymer fibers. Furthermore, additional functions that can be introduced to these modified fibers should make them ideally suited for applications in supercapacitors, sensors, electromagnetic shields, and artificial muscles. To conclude, existing challenges and potential future developments in carbonene materials modified high-performance polymer fibers will be discussed. The excellent properties associated with the modified fibers, as well as continuous development of materials and techniques should ensure their future applications in numerous fields.

Flexible and wearable electronics can integrate multiple functions, such as sensing, actuation, and wireless communication, showing great potential for application in flexible displays, health monitoring, human-computer interaction, and other fields. Energy devices to supply power are an important part of wearable electronics. Traditional energy devices have a relatively rigid plate structure, and their poor mechanical flexibility, low breathability and moisture conductivity make them difficult to adapt to the needs of wearability. These problems have severely limited the development and application of wearable devices, and there is therefore an urgent need to develop flexible, lightweight, high-performance wearable energy devices. Fiber-based energy devices have several obvious advantages. First, the diameter of these devices usually ranges from micrometers to millimeters, which makes them small in size and light in weight. Then, their outstanding flexibility endows them with wearable comfort and stable performance under mechanical deformation. Third, fibers can be woven or knitted into deformable textiles with excellent wearability and breathability. Because of these advantages, fiber-based energy devices have attracted considerable attention. Traditional fiber-based energy devices usually use polymer fibers covered by metal wires as electrodes, but these have inherent defects, such as poor chemical stability, inferior matching with active materials, and a lack of mechanical flexibility, that hinder their application in wearable devices. Carbonene materials are low-dimensional all-carbon materials composed of sp²-hybridized carbon atoms, including carbon nanotubes and graphene, which have the advantages of low density, good mechanical properties, excellent electrical and thermal conductivity, and high stability. "Carbonene fibers" mainly refers to high-performance fiber-like macroscopic assemblies composed of carbonene materials, and includes carbon nanotube fibers, graphene fibers, and graphene/carbon nanotube composite fibers. Carbonene fibers can effectively transfer the excellent performance of carbonene materials at the micro scale to the macro scale, showing high conductivity, strength, flexibility, stability, and ease of manufacture, making them widely used in research on advanced energy devices. In recent years, researchers have developed a variety of carbonene fiber-based energy devices. This paper reviews recent progress in the application of carbonene fibers in energy devices, including energy conversion and energy storage devices such as solar cells, moisture actuators and moisture power generators, thermoelectric generators, supercapacitors, and electrochemical cells. The preparation methods and wearable applications of carbonene fiber-based energy devices are emphasized. Discussion of the development prospects and challenges of energy storage/conversion devices based on carbonene fibers is included, and it is expected that this will provide valuable ideas for the future development of high-performance fiber-based wearable energy devices.

With the rapid advancement of intelligent microelectronics and the "Internet of Things" sensing microsystems with miniaturized and wearable properties, the development of novel fiber-based functional materials for application in flexible and microscale electrochemical energy storage devices has become an important strategic direction. However, this imparts higher requirements on the properties of fiber functional materials for use in flexible energy storage devices, including high bendability, stretchability, foldability, high strength, excellent interfacial stability, and high energy storage density. Based on its unique structure, excellent conductivity, and favorable mechanical and electrochemical properties, graphene-based fibers are expected to be a novel flexible functional material with high performance. To date, various strategies have been developed to control the microstructure to achieve further improvements in graphene fibers, from preparation methods to fundamental properties. In this review, a systematic summary of the recent advances in the preparation methods of graphene-based fibers is presented, including the limited hydrothermal synthesis, chemical vapor deposition (CVD), dry spinning, and wet spinning methods, and each method is discussed in terms of its advantages and disadvantages. Subsequently, strategies to improve the mechanical strength, electrical conductivity, and thermal conductivity of graphene fibers are highlighted, including the regulation of basic materials, improvement of the preparation process, and controlling subsequent processing. Recent research on the application of graphene fiber in energy storage and conversion is also summarized. Based on the exceptional electrical conductivity and pore structure of graphene fibers, it has significant application prospects in the field of electrochemical energy storage devices, such as supercapacitors, metal-ion batteries, and solar cells. Moreover, graphene fibers have a wide range of applications in phase change fibers and thermoelectric generators owing to their excellent thermal conductivity. This review summarizes and discusses the preparation of the basic constituent units of graphene fibers, development of novel graphene fibers, interfaces between graphene fibers and active materials, packaging strategies and safety issues of graphene fiber-based electrochemical energy storage devices, and current evaluation criteria for graphene fiber performance. Finally, the ongoing challenges and future prospects of graphene fibers for advanced energy conversion and storage systems are presented.

Technological advances such as electronic information and the Internet of Things have increased the daily use and demand for wearable electronic devices and intelligent fabrics. This has led to an unprecedented development of functional fibers, the properties of which are largely determined by their basic building blocks. Transitional metal carbon/nitrogen compounds (MXenes) are an emerging class of two-dimensional materials that have been widely used in many wearable devices owing to their high electrical conductivity, excellent processability, tunable surface properties, and outstanding mechanical strength. In this paper, we summarize the various synthetic methods for MXenes materials. Moreover, we also compare the characteristics of the different preparation techniques and elaborate the mechanical, electrical, optical, and chemical stability properties of the materials. This paper primarily focuses on the surface terminal groups of MXenes and the effect they have on different properties. At present, various methods have been developed for the preparation of MXenes-functionalized fibers, including pasting MXenes on the surface of matrix fibers by coating and producing solid fibers from a slurry containing MXenes by wet spinning or electrospinning. Among them, wet spinning has been the most widely adopted method, and is very promising for the large-scale production of MXenes-functionalized fibers. This paper also summarizes the properties of functional fibers obtained by various preparation methods. Furthermore, functional fibers prepared by different processes have been applied several fields, including flexible energy storage devices, wearable sensors, wires for electrical signal transmission and conversion, and integration of multifunctional intelligent fabrics. Great progress has been made in the research of supercapacitors and sensors with MXenes-functionalized fibers as electrodes which are anticipated to be integrated into intelligent textiles. This paper summarizes the potential applications of MXenes-functionalized fibers and reports on the challenges that must be addressed before practical applications can be realized. Firstly, a fluorine-free preparation of MXenes materials must be achieved whilst improving yields. Secondly, tunability of the functional groups on the surface of MXenes materials must be attained. Lastly, an improvement to the long-term chemical stability of MXenes in the environment should be accomplished. While efficiently obtaining high-quality MXenes materials, it is equally important to develop new MXenes-functionalized fiber preparation techniques. Furthermore, the potential applications of MXenes-functionalized fibers could be broadened by developing new fiber weaving processes. We finally summarize the potential applications of intelligent fabrics based on MXenes-functionalized fibers. Whilst challenges remain, MXenes are an emerging family of two-dimensional materials with many attractive properties and many potential applications worth exploring.

The gradual popularization of new energy technologies has led to rapid development in the field of electric transportation. At present, the demand for high-power density batteries is increasing and next-generation higher-energy battery chemistries aimed at replacing current lithium-ion batteries are emerging. The lithium-air batteries (LABs) are thought to be the ultimate energy conversion and storage system, because of their highest theoretical specific energy compared with other known battery systems. Current LABs are operated with pure O₂ provided by weighty O₂ cylinders instead of the breathing air, and this configuration would greatly undermine LAB's energy density and practicality. However, when the breathing air is used as O₂ feed for LABs, CO₂, as an inevitable impurity therein, usually leads to severe parasitic reactions and can easily deteriorate the performance of LABs. Specifically, Li₂O₂ will react with CO₂ to form Li₂CO₃ on the cathode surface. Compared with the desired discharge product Li₂O₂, the Li₂CO₃ is an insulating solid, which will accumulate and finally passivate the electrode surface leading to the "sudden death" phenomenon of LABs. Moreover, Li₂CO₃ is hard to decompose and a high overpotential is required to charge LABs containing Li₂CO₃ compounds, which not only degrades energy efficiency but also decomposes other battery components (e.g., cathode materials and electrolytes). In recent years, researchers have proposed many strategies to alleviate the negative effects brought about by Li₂CO₃, such as catalyst engineering, electrolyte design, and so on, in which O₂ selective permeable membranes are worth noting. This review summarizes the recent progresses on the understanding of the CO₂-related chemistry and electrochemistry in LABs and describes the various strategies to mitigate and even avoid the negative effects of CO₂. The perspective of CO₂ separation technology using selective permeable membranes/filters in the context of LABs is also discussed.

The electrocatalytic carbon dioxide reduction reaction (E-CO₂RR) has attracted attention in recent years for its ability to effectively alleviate the environmental problems caused by the rapid increase of CO₂ in the atmosphere and transform CO₂ into high value-added fuels or chemicals (e.g., CO, HCOOH, CH₄, CH₃OH, C₂H₄, C₂H₅OH, etc.) under mild conditions. In addition, clean energy sources, such as solar and wind energy, can provide electrical energy for the electrochemical CO₂ conversion technology used in large-scale industrial applications. One limitation of the E-CO₂RR is that CO₂ is a thermodynamically stable linear molecule with a slow kinetic reaction rate. In addition, the E-CO₂RR involves complex processes, such as gas diffusion and multi-electron transfer, making its selectivity problematic. Therefore, constructing highly efficient and stable catalytic electrodes has become a core research topic in the field of E-CO₂RR. Unfortunately, the traditional method of coating electrodes with binders (e.g., Nafion, polyvinylidene fluoride, and polytetrafluoroethylene) usually results in a low utilization ratio of active sites due to the easy aggregation of the catalysts themselves. This could result in the severe embedding of active sites and limited mass transfer. Moreover, the dissolution of the catalyst layer during the electrocatalytic process also reduces the activity and stability of the electrodes, making it difficult to reuse. Therefore, it is necessary to regulate the electrode reaction interface to improve the utilization ratio of active sites. The integrated electrodes, where the catalyst is grown directly on the current collector, can avoid the use of binders to facilitate the exposure of active sites and transfer of electrons. The integrated structure can also enhance the bonding strength between the active material and current collector and improve the cycling stability of the electrodes. Meanwhile, the micro-environment (e.g., pH, concentration of CO₂, and intermediates) at the three-phase interface can be effectively controlled on the integrated electrodes, which can enhance the performance of the E-CO₂RR. In recent years, encouraging progress has been achieved in the study of the E-CO₂RR. However, current reviews of the E-CO₂RR mainly focus on the regulation of the intrinsic activity of catalysts; discussions and reviews from the perspective of the electrodes are rarely reported. This article reviews the latest research of the integrated electrodes for the E-CO₂RR with a focus on the application of different types of integrated electrodes (e.g., metal, alloy, metal oxide, metal sulfide/phosphide, and metal single atom). It also analyzes the effects of morphology, surface, and interface regulation on the electrocatalytic performance of the E-CO₂RR. Finally, it highlights the challenges that still exist in this field and discusses the future development of the integrated electrodes.

Owing to the rapid development of scientific technology, the demand for energy storage equipment is increasing in modern society. Among the current energy storage devices, lithium-ion batteries (LIBs) have been widely used in portable electronics, handy electric tools, medical electronics, and other fields owing to their high energy density, high power density, long lifespan, low self-discharge rate, wide operating temperature range, and environmental friendliness. However, in recent years, with rapid development in various technological fields, such as mobile electronics and electric vehicles, the demand for batteries with much higher energy densities than the current ones has been increasing. Hence, the development of LIBs with a high energy density, prolonged cycle life, and high safety has become a focal interest in this field. To achieve the above objectives, it is important to strategically use novel anode materials with relatively high specific capacities. At present, artificial graphite is commonly used as an anode material for commercialized traditional LIBs, which can only deliver a practical capacity of 360–365 mAh·g^-1. Therefore, LIBs using graphite anodes have limited room for improvement in energy density. In the past two decades, considerable efforts have been devoted to silicon-based anode materials, which belong to the same family as carbon. To date, common silicon anode materials primarily include nano-silicon (nano-Si), silicon monoxide (SiO), suboxidized SiO (SiO_x), and amorphous silicon metal alloy (amorphous SiM). Among them, SiO has attracted the most attention for use as a negative electrode material for LIBs. As an anode for lithium-ion batteries (LIBs), silicon monoxide (SiO) has a high specific capacity (~2043 mAh·g^-1) and suitable charge (delithiation) potential (< 0.5 V). In addition, with the abundance of its raw material resource, low manufacturing cost, and environmental friendliness, SiO is considered a promising candidate for next-generation high-energy-density LIBs. Based on the testing of existing commercialized SiO materials, the reversible specific capacity of pure SiO can reach 1300–1700 mAh·g^-1. However, when acting as the anode for LIBs, SiO undergoes a severe volume change (~200%) during the lithiation/delithiation process, which can result in severe pulverization and detachment of the anode material. Meanwhile, lithium silicate and lithium oxide are irreversibly formed during the initial discharge–charge cycle. Moreover, the electrical conductivity of SiO is relatively low (6.7 × 10^-4 S·cm^-1). These shortcomings seriously impact the interfacial stability and electrochemical performance of SiO-based LIBs, leading to a low initial Coulombic efficiency and poor long-term cycling stability, which has significantly restricted its commercial application. In recent years, substantial efforts have been made on structural optimization and interfacial modification of SiO anodes. However, there is still a lack of a more comprehensive summary of these important developments. Therefore, this review aims to introduce the research work in this area for readers interested in this emerging field and to summarize in detail the research work on the performance optimization of SiO in recent years. Based on the structural characteristics of the SiO anode material, this review expounds the main challenges facing the material, and then summarizes the structural and interfacial modification strategies from the perspectives of SiO structure optimization, SiO/carbon composites, and SiO/metal composites. The methods and their features in all the studies are concisely introduced, the electrochemical performances are demonstrated, and their correlations are compared and discussed. Finally, we propose the development of the structural and interfacial optimization of the SiO anode in the future.

The storage and conversion of renewable energy through electrocatalysis is of considerable significance for improving the energy structure, protecting the ecological environment, and achieving the national strategy of carbon peaking and carbon neutrality. The development of low-cost and high-efficiency electrocatalysts has become a major scientific challenge worldwide. Microorganisms are widely found in nature and are characterized by their rich structure, composition and metabolism. These properties facilitate their use as intelligent templates for electrocatalyst structures and as sources of non-metallic elements such as carbon, phosphorus, sulfur, as well as metallic elements. The use of microorganisms in electrocatalyst production has become a new trend owing to the advantages of non-toxicity, reproducible production, and ease of scaling up. Thus, this paper reviews the development of microbial "intelligence" guided preparation of electrocatalysts and their current applications in the fields of hydrogen evolution reactions, oxygen evolution reactions, oxygen reduction reactions, carbon dioxide reductions and lithium batteries. In order to achieve the function of "intelligent" guidance of microorganisms, four aspects need to be addressed: (1) the selection of suitable microbial species and the culture and activation conditions, which significantly helps in tailoring the microbial properties for specific applications; (2) the exploration of microbial species that can accumulate metal species from their living environment and thus produce metal nanoparticles, which will help obtain nanocomposites with desired properties; (3) the selection of compounds with good catalytic properties, high stability, and compatibility with microbial substrates; and (4) the development of highly controllable nanocatalysts through modern molecular biology and genetic engineering to regulate microbial life processes such as metabolic proliferation and apoptosis. With the resolution of these issues, we believe that the application of microbial intelligent templates guided electrocatalysts can be further extended to other electrocatalytic reactions such as ethanol oxidation reactions (EOR), nitrogen reduction reactions (NRR), and to other applications in fields such as electronics, sensing, imaging, and biomedicine. The goal of this review is to promote a deeper understanding of the correlations among microbial metabolism, catalyst micro-nano structures and structure-activity relationships. Furthermore, the challenges associated with such materials and the prospects for future development are discussed herein.

Hydrogen (H₂) is an important component in the framework of carbon-neutral energy, and the scalable production of H₂ from seawater electrolysis offers a feasible route to address global energy challenges. With abundant seawater reserves, seawater electrolysis, especially when powered by renewable electricity sources, has great prospects. However, chloride ions (Cl^-) in seawater can participate in the anodic reaction and accelerate the corrosion of electrode materials during electrolysis. Although the oxygen evolution reaction (OER) is thermodynamically favorable, the chlorine evolution reaction is highly competitive because fewer electrons are involved (2e^-). These two problems are compounded by the dearth of corrosion-resistant electrode materials, which hinders the practical applications of seawater electrolysis. Therefore, intensive research efforts have been devoted to optimizing electrode materials using fundamental theories for practical applications. This review summarizes the recent progress in advanced electrode materials with an emphasis on their selectivity and anti-corrosivity. Practical materials with improved selectivity for oxygen generation, such as mixed metal oxides, Ni/Fe/Co-based composites, and manganese oxide (MnO_x)-coated heterostructures, are reviewed in detail. Theoretically, alkaline environments (pH > 7.5) are preferred for OER as a constant potential gap (480 mV) exists in the high pH region. Nevertheless, corrosion of both the cathode and anode from ubiquitous Cl^- is inevitable. Only a few materials with good corrosion resistance are capable of sustained operation in seawater systems; these include metal titanium and carbon-based materials. The corrosion process is usually accompanied by the formation of a passivated layer on the surface, but the aggressive penetration of Cl^- can damage the whole electrode. Therefore, the selective inhibition of Cl^- transport in the presence of a robust layer is critical to prevent continuous corrosion. Advances in anti-corrosion engineering, which encompasses inherently anti-corrosive materials, extrinsically protective coating, and in situ generated resistive species, are systematically discussed. Rational design can impart the material with good catalytic activity, stability, and corrosion resistance. Finally, we propose the following opportunities for future research: 1) screening of selective and anti-corrosive materials; 2) mechanism of competitive reactions and corrosion; 3) evaluation of anti-corrosive materials; 4) industrial-scale electrolysis with high current density; 5) optimization of experimental conditions; and 6) development of integrated electrolyzer devices. This review provides insights for the development of strategies aimed at tackling chlorine-related issues in seawater electrolysis.

The design functions of lithium-ion batteries are tailored to meet the needs of specific applications. It is crucial to obtain an in-depth understanding of the design, preparation/ modification, and characterization of the separator because structural modifications of the separator can effectively modulate the ion diffusion and dendrite growth, thereby optimizing the electrochemical performance and high safety of the battery. Moreover, the development and utilization of various characterization techniques are critical and essential in bridging the intrinsic properties of separators and their impacts on the electrochemical performance, which guide the functional modification of the separators. In this review, we systematically summarized the recent progress in the separator modification approaches, primarily focusing on its effects on the batteries' electrochemical performance and the related characterization techniques. Herein, we provide a brief introduction on the separators' classification that mainly includes (modified) microporous membranes, nonwoven mats, and composite membranes; thereafter, we discuss the basic requirements that facilitate the use of membranes as separators, such as good wettability with electrolyte, high permeability for ions, and several intrinsic properties including good thermal stability, electronic insulation, excellent (electro)chemical stability, high mechanical strength, and appropriate thickness/porosity. We then highlight the factors that affect the batteries' performance from the viewpoints of ion diffusion, dendrite growth, and safety, along with the modification approaches. Specifically, the separator should possess high ionic conductivity and uniform ion transmission, which can be achieved by adjusting its composition and through surface modifications. The severe dendrite growth, especially in lithium-metal batteries, could be inhibited by controlling the pore structures, increasing affinity between separator and metal anode, constructing artificial solid electrolyte interphase (SEI), adopting high strength separator, as well as smart design of the separator. The safety issue, which is a major concern that limits battery applications, could be mitigated by increasing the separator's mechanical strength, thermal stability, and shutting the batteries down below thermal runaway temperature through various functionalization approaches. More importantly, the characterizations of the separators' structure, and their mechanical, thermal, and electrochemical properties are systematically summarized, including scanning electron microscope (SEM)/atomic force microscope (AFM) for surface morphology observation, focused ion beam scanning electron microscopic (FIB-SEM)/X-ray tomography (X-ray CT) for 3D structure detection, mercury intrusion porosimetry (MIP)/Brunauer-Emmett-Teller (BET)/Gurley number measurement for pore structure analysis, contact angle and climbing behavior of electrolyte in separators for wettability measurements, characterizations of the separator's tensile behavior, puncture behavior and compression behavior, thermo-gravimetric analysis (TGA)/differential scanning calorimetry (DSC)/infrared thermography (FLIR) for thermal properties test, and the electrochemical methods for determining the separator's electrochemical stability, ionic conductivity, internal resistance, lithium-ion transference number, cycle/rate performance, as well as self-discharge characteristic. These characterizations provide theoretical and practical basis for the rational design of functional separators and optimization of the electrochemical performance of lithium-ion batteries. Finally, we provide the perspectives on several related issues that need to be further explored in this research field.

Sustainable freshwater supply is a grave challenge to the society because of the severe water scarcity and global pollution. Seawater is an inexhaustible source of industrial and potable water. The relevant desalination technologies with a high market share include reverse osmosis and thermal distillation, which are energy-intensive. Capacitive deionization (CDI) is a desalination technology that is gaining extensive attention because of its low energy consumption and low chemical intensity. In CDI, charged species are removed from the aqueous environment via applying a voltage onto the anode and cathode. For desalination, Na⁺ and Cl^- ions are removed by the cathode and anode, respectively. With the boom in electrode materials for rechargeable batteries, the Na⁺ removal electrode (cathode) has evolved from a carbon-based electrode to a faradaic electrode, and the desalination performance of CDI has also been significantly enhanced. A conventional carbon-based electrode captures ions in the electrical double layer (EDL) and suffers from low charge efficiency, thus being unsuitable for use in water with high salinity. On the other hand, a faradaic electrode stores Na⁺ ions through a reversible redox process or intercalation, leading to high desalination capacity.However, the Cl^- removal electrode (anode) has not yet seen notable development. Most research groups employ activated carbon to remove Cl^-, and therefore, summarizing Cl^- storage electrodes for CDI is necessary to guide the design of electrode systems with better desalination performance. First, this review outlines the evolution of CDI configuration based on the electrode materials, suggesting that the anode and cathode are of equal importance in CDI. Second, a systematic summary of the anode materials used in CDI and a comparison of the characteristics of different electrodes, including those based on Ag/AgCl, Bi/BiOCl, 2-dimensional (2D) materials (layered double hydroxide (LDH) and MXene), redox polymers, and electrolytes, are presented. Then, the underlying mechanism for Cl^- storage is refined. Similar to the case of Na⁺ storage, traditional carbon electrodes store Cl- via electrosorption based on the EDL. Ag/AgCl and Bi/BiOCl remove Cl^- through a conversion reaction, i.e., phase transformation during the reaction with Cl^-. 2D materials store Cl^- in the space between adjacent layers, a process referred as ion intercalation, with layered double hydroxide (LDH) and MXene showing higher Cl^- storage potential. Redox polymers and electrolytes allow for Cl^- storage via redox reactions. Among all the materials mentioned above, Bi/BiOCl and LDH are the most promising for the construction of CDI anodes because of their high capacity and low cost. Finally, to spur the development of novel anodes for CDI, the electrodes applied in a chlorine ion battery are introduced. This is the first paper to comb through reports on the development of anode materials for CDI, thus laying the theoretical foundation for future materials design.

Metal oxide semiconductor (MOS) gas sensors have been widely used in military and scientific research, as well as various industries; this is because of the unique advantages of MOS gas sensors including their small size, low power consumption, high sensitivity, and good silicon chip compatibility. However, the poor selectivity of MOS sensors has restricted their potential application in the Internet of Things (IoT) era. In this paper, progress in the research addressing the selectivity issues of MOS sensors is reviewed, and three strategies for selective MOS sensors, and performance improvements of MOS, e-nose, and thermal modulation, are introduced. Research on the performance improvements of MOS-sensitive materials provides an important guarantee for fast and accurate identification of trace gas molecules. The e-nose system adopts an array of sensors with distinct surface chemical properties; more "features" of volatile organic compound (VOC) molecules can be extracted by enlarging the number of sensor arrays, providing a "many-to-one" or "many-to-many" approach to discriminate VOC gas molecules via pattern recognition/machine learning algorithms. For thermal modulation technology, the working temperature of the sensor is intentionally swept during one measurement cycle, and the dynamic response signals of the sensor to different VOC gases under a given temperature mode are tested. Combined with signal processing and pattern recognition/machine learning, the "one-to-many" recognition of VOC gas molecules is realized by a single MOS sensor. Principal component analysis (PCA), linear discriminant analysis (LDA), and neural network (NN) pattern recognition/machine learning algorithms are compared in this review. Among them, the LDA algorithm based on supervised learning can be used as a signal dimension reduction or pattern recognition method. It is mainly applicable to the gas identification and classification of small datasets of VOC gas molecules. LDA is superior to PCA (based on unsupervised learning) in identifying and classifying VOC gas molecules. Compared with the LDA algorithm, an artificial neural network (ANN) based on the back-propagation algorithm, as a highly robust machine learning classification model, has the potential to process large datasets and realize the classification and identification of multiple kinds of VOC gases. Finally, the deep learning algorithm of convolutional neural networks (CNNs), with the performance of data dimension reduction, feature extraction, and robust identification, is expected to be applied in the field of VOC gas identification. Based on the performance improvement of MOS, a combination of multiple modulation methods and array technology, as well as the latest developments of deep learning algorithms in the artificial intelligence (AI) field, will greatly enhance the VOC molecular recognition capability of nonselective MOS sensors.

With the increasing demand for safe high energy density energy storage systems, solid-state lithium metal batteries have attracted extensive attention. The solid electrolyte, which is expected to replace the traditional liquid organic electrolyte core in solid-state lithium metal batteries because of its excellent mechanical properties and non-flammability. Lithium-ion solid-state electrolytes can be categorized into two broad types: inorganic electrolytes and polymer electrolytes. Inorganic solid electrolytes have the advantages of high room-temperature ionic conductivity, wide electrochemical window, and high mechanical strength. However, their high brittleness, high solid-solid interface contact resistance, complex preparation process, and high cost make future development and practical applications challenging. In contrast to inorganic electrolytes, polymer electrolytes are easy to process and exhibit better flexibility and easy formation of a good, stable interface with lithium metal. However, solid polymer electrolytes still exhibit insufficient ionic conductivity at room temperatures compared with polymer solid electrolytes. Therefore, neither the inorganic electrolytes nor the polymer electrolytes alone can meet the requirements of high-performance solid-state lithium metal batteries. Recently, dispersing ceramic fillers (especially fast lithium-ion conductors) in a polymer matrix to integrate with composite polymer electrolytes has been developed as an effective strategy for enhancing room-temperature ionic conductivity, mechanical properties, and thermal stability of solid polymer electrolytes. Inorganic fillers do not only reduce the polymer matrix crystallization but also improve the lithium-ion conductivity by promoting the dissociation of lithium salts. The Lewis acid-base groups and oxygen vacancy at the surface of inorganic fillers can increase the migration number of lithium ions. Nevertheless, the effect of the percolation structure of inorganic fillers on the conductivity of organic-inorganic composite electrolytes should be discussed. It is believed that the organic-inorganic interface is the main reason for the significantly enhanced lithium-ion conductivity of composite electrolytes based on the percolation theory. In this paper, from the perspective of percolation structure design, we summarize the progress on high lithium-ion conductive organic-inorganic composite electrolytes with different dimensional-structured inorganic fillers. From one-dimensional filler to three-dimensional filler, the ionic conductivity of a composite electrolyte can be significantly influenced by the rational design and optimization of the percolation structure and orientation of the inorganic filler. Vertically aligned inorganic fillers provide optimal ion transport pathways in the polymer matrix, significantly improving the lithium-ion conductivity of the composite electrolytes. Furthermore, the advantages and disadvantages of the different percolation structures are compared and discussed objectively. Finally, future development trends of organic-inorganic composite electrolytes are discussed.

Graphene fiber material is one type of macroscopically one-dimensional materials assembled by graphene building blocks or coating graphene on other fibrous building blocks. The typical graphene fiber materials can be classified into graphene fiber and graphene-coated hybrid fiber based on their different building blocks. This type of materials exhibits superior tensile strength, excellent electrical and thermal conductivities, making them favorable for applications in flexible energy storage devices, electromagnetic shielding and wearable electronics. Recently, the chemical vapor deposition (CVD) method, conventionally used for fabricating film-like graphene, has been widely applied to the synthesis of graphene fiber materials. For preparing graphene fiber, the use of CVD method can prevent the complicated and time-consuming reducing treatment of graphene oxide (GO), which is well known as an imperative step in the commonly used wet spinning method. For preparing graphene-coated hybrid fiber, the CVD method can achieve an efficient modulation of graphene quality, and ensure a strong adhesion between graphene and fibrous substrates. In this review, we summarized the CVD methods for fabricating graphene fiber materials, including graphene-assembled graphene fiber and graphene-coated hybrid fiber, and introduced their excellent mechanical, electrical, thermal and optical properties along with their broad applications in intelligent sensors, optoelectronic devices, and flexible electrodes. Furthermore, the challenges in synthesizing CVD-fabricated graphene fiber materials were also analyzed. This review can be briefly divided into three parts: (1) Synthesis of graphene fibers: Up to now, the CVD method is a feasible and effective way to synthesize graphene with high crystallinity. The CVD strategies for fabricating graphene fibers mainly consist of the template method, the secondary growth method, and the film-scrolling method, which can simplify the fabrication process and efficiently modulate graphene quality. (2) Synthesis of graphene glass fibers: Similar to graphene growth directly on non-catalytic glass surfaces, CVD method can also be applied to synthesize graphene on glass fibers. By modifying the experimental parameters (carbon source, pressure, temperature, etc.), high-quality graphene films with controllable thickness can be uniformly coated on glass fibers. Meanwhile, the as-fabricated graphene glass fiber can be further used as a high-performance flexible electrode, electro-optic modulator, or electrocatalyst. (3) Synthesis of graphene metal fibers: Graphene can be controllably grown on metal fibers using the CVD method. Compared to the bare metal fiber, the fabricated graphene metal fiber exhibited enhanced electrical and thermal conductivities as well as better chemical stability, which can expand its applications in ultra-thin electronics and high-power circuits.

Graphene wafers have emerged in response to the increasing demand of wafer-scale two-dimensional materials and high-performance on-chip devices in the field of integrated circuits, microelectromechanical systems, and sensors. Wafer-scale graphene films display great application potential, owing to their atomic layer thickness, excellent thermal and electrical conductivity, and compatibility with wafer-processing technology. Therefore, batch production of graphene wafers is exceedingly crucial. The chemical vapor deposition (CVD) method has attracted attention for the production of high-quality graphene wafers with high controllability, high compatibility, and low cost. Mature CVD manufacturing has been widely employed in the semiconductor industry to date, aiding in the industrialization of CVD graphene wafers and creating opportunities for graphene to enter the new era of nanoelectronics. The quality of graphene wafers has a significant impact on the subsequent device fabrication; hence, considerable efforts have been made to date to realize precise control over domain size, structural defects, and layer thickness during synthesis. In this study, we summarize the recent progress made in wafer-scale CVD graphene synthesis. Initially, we introduce the quality requirements of graphene wafers targeting various application scenarios, and propose the classification of graphene wafers. Single crystallinity is considered to be a key requirement for the graphene used in high-performance electronics and optoelectronics. We then review the recent CVD-derived graphene wafers with regard to substrate types (metal/nonmetal), highlighting the constrictions in graphene quality and corresponding synthetic solutions. Batch synthesis of graphene wafers is further discussed. The significant role of flow dynamics in the up-scaling process is emphasized, followed by relevant experimental instances based on computational fluid dynamics simulations. Finally, strategies for obtaining graphene wafers are overviewed, with the proposal of future perspectives. This study focuses on three areas: (1) Application requirements for the quality of graphene wafers, including target substrate types and as-grown graphene features (chemical stability and electrical and thermal properties), (2) CVD strategies of graphene wafers: As for the growth scenarios on metal substrates, controllable preparation of bilayer/multilayer graphene and the elimination of structural defects remain challenging. With respect to the synthesis over nonmetal wafers, concrete examples highlighting the epitaxial growth on a crystalline substrate and tailorable growth on a surface-reconstructed substrate are summarized. (3) Batch synthesis of graphene wafers: CVD routes for scalable production are explored.

Graphene fiber is a macroscopic carbonaceous fiber composed of microscopic graphene sheets, and has attracted extensive attention. Graphene building blocks form a highly ordered structure, resulting in fibers with the same properties as graphene, such as superior mechanical and electrical performance, low weight, excellent flexibility, and ease of functionalization. Moreover, graphene fibers are compatible with traditional textile technologies, facilitating the development of wearable electronics, flexible energy devices, and smart textiles. Graphene fibers were first prepared in 2011 by wet spinning of graphene oxide (GO) solution, which was dispersed in water. Various fabrication methods have been developed to assemble graphene sheets into fibers since then and different strategies have been proposed to optimize their structure and performance. Graphene fibers have applications in numerous fields, including conductors, sensors, actuators, smart textiles, and flexible energy devices. This review aims to provide a comprehensive picture of the preparation approaches, properties, and applications of graphene fibers. Firstly, the preparation processes, unique structures, and properties of three typical carbonaceous fibers-arbon fibers, carbon nanotube (CNT) fibers, and graphene fibers-re compared. It can be seen that graphene fibers possess the unique structures, such as the large grain sizes and highly aligned structure, endowing them with the outstanding properties. Then a variety of fabrication techniques have been summarized, including wet spinning, dry spinning, dry-jet wet spinning, space-confined hydrothermal assembly, film conversion approach, and template-assisted chemical vapor deposition (CVD). Wet spinning is a common method to fabricate high-performance graphene fibers and is promising for the large-scale production of graphene fibers. Besides, various strategies for improving the mechanical, electrical, and thermal properties of graphene fibers are introduced in detail, including well-chosen graphene building blocks, optimized fabrication processes, and high-temperature treatments. Although the electrical and thermal transport properties of typical graphene fibers are better than those of carbon fibers, the strength and modulus of graphene fibers are inferior. Therefore, the enhancement of the mechanical properties of graphene fibers by optimizing the composition of precursors, controlling and adjusting the assembly processes, and exploring feasible post-treatment procedures are essential. Meanwhile, the review outlines the applications of graphene fibers in high-performance conductors, functional fabrics, flexible sensors, actuators, fiber-shaped supercapacitors and batteries. Finally, the persisting challenges and the future scope of graphene fibers are discussed. We believe that graphene fibers will become a new structural and functional material that can be applied in numerous fields in the future, aided by the continuous development of materials and techniques.

With the rapid development of the functional applications of portable and wearable electronic products (such as curved smartphones, smartwatches, laptops, and electronic skins), there is an urgent need to fabricate flexible, lightweight, and highly efficient energy storage devices that can provide sufficient power support. Flexible supercapacitors with high power density, high charging/discharging rates, wide operating temperature ranges, low maintenance consumption, and a long cycling lifespan can be integrated with smart wearable electronic products to provide power support. The conventional preparation method for the electrodes of flexible supercapacitors involves directly coating the active materials on flexible substrates. However, inactive materials such as the substrates and binders occupy a large volume and contribute notably to the weight of flexible electrodes, which is unsuitable for highly integrated flexible electronic devices. Owing to its unique characteristics, including large theoretical specific surface area, high electrical conductivity, excellent mechanical flexibility, good chemical stability, and ease of film processing, graphene has been widely used as an electrode material for flexible supercapacitors. The graphene film is a macrostructure with graphene nanosheets as the main structural units. As opposed to conventional flexible electrodes containing non-electrochemical active components such as collectors, conductive agents, and binders, graphene film electrodes are considered highly promising electrode materials for flexible supercapacitors because of their light weight and robust mechanical properties. However, the inevitable aggregation of graphene during electrode preparation creates ''dead volume'' in the film electrodes, where the electrolyte cannot reach, further limiting the specific capacitance. In this review, we review the recent research on graphene films used for flexible supercapacitors, with emphasis on the assembling methods for graphene films, regulation of the graphene units, and their electrochemical performance. First, simple preparation methods for graphene films are introduced: vacuum-assisted self-assembly, blade coating, pressing aerogel, wet spinning, and interfacial self-assembly. Second, two major strategies for structural control and surface modification of the graphene units are described in detail: (1) structural control can transform the two-dimensional graphene nanosheets into defect graphene, which not only weakens the van der Waals force and π–π bond interactions between the nanosheets, but also leads to the formation of three-dimensional conductive networks and ion transport channels during the assembly process; (2) surface modification, which can suppress the agglomeration of graphene nanosheets by introducing heteroatoms and reactive functional group molecules, while improving their electrical conductivity and wettability, and introducing pseudocapacitance. Finally, the persisting challenges and future development of the commercial applications of graphene films are discussed.

Lithium-sulfur batteries are considered to be one of the most promising new-generation energy storage devices, owing to their ultra-high theoretical energy density and the merits of sulfur cathodes, which include natural abundance, low cost, and no toxicity. However, the commercial application of lithium-sulfur batteries is still subject to various intractable challenges. First, the insulation of sulfur and its solid discharge products (Li₂S₂/Li₂S) leads to low utilization of the active materials. Second, the cathode suffers from an 80% volume expansion after the discharge process, which adversely affects its structural stability. Finally, intermediary lithium polysulfides can easily dissolve into the electrolyte, which can trigger the "shuttle effect." This results in the loss of active materials, fast capacity fading, and low Coulombic efficiency. Graphene has garnered significant interest as a host material to accommodate sulfur for high-performance lithium-sulfur battery. A graphene host featuring a high specific surface area, excellent conductivity, and excellent mechanical stability can ensure a good electrical contact between the sulfur species and the current collector and withstand the volumetric strain of the electrode during cycling. Unfortunately, lithium polysulfides are still prone to escape from cathodes owing to the open two-dimensional (2D) plane structure of graphene sheets. To address this issue, various graphene-based materials with unique structures and chemical compositions have been trialed as sulfur hosts. In this review, we summarize research progress regarding three-dimensional (3D) graphene, graphene with modified surface chemistry, graphene-based composites, and graphene-based flexible materials as sulfur hosts for lithium-sulfur batteries. Furthermore, we analyze the challenges of applying graphene host materials in high-performance lithium-sulfur batteries. This review is mainly divided into four parts: (1) 3D graphene materials as sulfur hosts: the interconnected 3D porous network structure assembled from 2D graphene sheets provides a half-enclosed cavity to accommodate sulfur and its discharge products, which can inhibit the diffusion of lithium polysulfides to a certain extent. (2) Graphene materials with modified surface chemistry as sulfur hosts: hydrophilic surface functional groups and doped non-metal or metal heteroatoms on graphene can chemically adsorb polar lithium polysulfides. (3) Graphene-based composites as sulfur hosts: in various graphene-based composites, graphene usually functions as a conductive and flexible substrate. Other components, such as other types of carbon or metal compounds, can play an important role in restricting lithium polysulfides and propelling their reaction kinetics. (4) Flexible graphene-sulfur electrodes: the excellent flexibility and conductivity of graphene endowed it and its composites with a broad range of prospective applications regarding flexible lithium-sulfur batteries.

With the excessive exploitation and utilization of conventional fossil fuels such as coal, petroleum, and natural gas, the concentration of carbon dioxide (CO₂) in the atmosphere has increased significantly, leading to serious greenhouse effect. The electrocatalytic conversion of CO₂ to liquid fuels and value-added chemicals is one of ideal strategies, considering the atomic economy and artificial carbon circle. Moreover, this process can be driven by renewable energy (solar, wind, tidal power, etc.), thus achieving efficient clean-energy utilization. Electrocatalytic CO₂ reduction (ECR) can be carried out under ambient conditions, yielding diverse products such as C₁ (carbon monoxide, methane, methanol, formic acid/formate), C₂ (ethane, ethanol, ethylene, acetic acid), and C₂₊ (propyl alcohol, acetone, etc.). However, it faces some challenging problems such as high overpotential on electrodes, the poor selectivity of C₂ and C₂₊ products, the severely competitive hydrogen evolution reaction and the stability in the practice. The rational design and construction of highly active electrocatalysts with low cost, high selectivity, and robust stability are key to these issues. Recently, graphene-based materials have attracted significant attention owing to the following attributes: (1) robust stability in electrochemical environments; (2) tailorable atomic and electronic structures, leading to tuned catalytic activity; (3) adjustable dimensions and hierarchical porous structure, large surface area, and number of active sites; and (4) an excellent conductivity coupled with active, well-defined materials, synergistically enhancing the electrocatalytic activity in the ECR. In this review, recent progress in graphene-based electrocatalysts for ECR is summarized. First, ECR fundamentals, such as reaction routes, products, electrolyzers (e.g., H-cell electrolyzers, flow-cell electrolyzers, and membrane electrode assembly cells), electrolytes (e.g., inorganic electrolytes, organic electrolytes, and solid-state electrolytes), and evaluation parameters of ECR performance (e.g., faradaic efficiency, onset potential, overpotential, current density, Tafel slope, and stability) are briefly introduced. The methods for making graphene-based catalysts for ECR are outlined and discussed in detail, including in situ or post-treatment doping, surface functionalization, microwave-assisted synthesis, chemical vapor deposition, and static self-assembly. The relationships between the graphene structures, including the point/line defects, the surface functional groups (e.g., -COOH, -OH, C-O-C, C＝O, C≡O), heteroatom-doping configurations (e.g., pyridinic N, graphitic N, and pyrrolic N, and oxidized pyridinic N), metal single-atom species (e.g., Fe, Zn, Ni, Cu, Co, Sn, Mo, In, Bi), surface/interface properties, and catalytic performance are highlighted, shedding light on the design principles for efficient yet stable carbon-based catalysts for ECR. Finally, the opportunities and perspectives of graphene-based catalysts for ECR are outlined.

The development of large-scale and controlled graphene production lays the foundation for macroscopic assembly. Among the diverse assembly strategies, modulating the interlayer interaction of graphene nanosheets is of vital importance because it determines the mechanical, electrical, thermal, and permeation properties of the macroscopic objects. Depending on the nature and strength of the interlayer interaction, covalent and noncovalent bondings, such as hydrogen bonding, ionic interaction, π-π interaction, and van der Waals force, are classified as two main types of interlayer connection methods, which solely or synergistically link the individual graphene nanosheets for practical macroscopic materials. Among them, the covalent bonding within the interlayer space renders graphene assembly adjusted interlayer distance, strong interlayer interaction, a rich diversity of functionalities, and potential atomic configuration reconstruction, which has attracted considerable research attention. Compared with other noncovalent assembly methods, covalent connections are stronger and thus more stable; however, there are some issues that remain. First, the covalent modification of the graphene surface depends on the defects and/or functional groups, which becomes difficult for graphene films free of surface imperfections. Second, the covalent connection partly alters the sp² hybrid carbon atoms to sp³, resulting in a deteriorated electrical conductivity. Thus, the electrical properties of the macroscopic assembly are far inferior to those of the constituent nanosheets, thereby restricting their applications. Lastly, covalent bonding is naturally rigid, rendering high modulus and strength to the graphene assembly while impairing the toughness. As in certain applications, both high strength and toughness are required; thus, a balanced covalent and noncovalent interaction is required. In this review, we discuss the recent progress in the construction method, properties, and applications of the interlayer covalently connected graphene materials. In the construction method, graphene is classified according to the synthesis method as oxidation-reduction and chemical vapor deposition method, wherein the latter represents graphene without abundant surface bonding sites and is hard to be covalently connected. For the former graphene produced by the oxidation-reduction method, the paper and fiber assembly forms are discussed. Then, the influence of covalent bonding on the mechanical and electrical properties is studied. Note that both the enhancement and potential impairments caused by covalent bonding are addressed. Finally, the applications in electrical devices, energy storage, and ion separation are summarized. The interlayer covalently connected macroscopic graphene material unifies the exceptional properties of graphene and the advantages of assembly strategy and will find applications in related fields. Moreover, it will also inspire the assembly of other graphene-like two-dimensional materials for a richer diversity of applications.

Graphene has attracted great attention owing to its excellent physical and chemical properties and potential applications. Presently, we can grow large-scale single-crystal graphene on transition metal substrates, especially Cu(111) or CuNi(111) surfaces, using the chemical vapor deposition (CVD) method. To optimize graphene synthesis for large-scale production, understanding the growth mechanism at the atomic scale is critical. Herein, we summarize the theoretical studies on the roles of the metal substrate in graphene CVD growth and the related mechanisms. Firstly, the metal substrate catalyzes the carbon feedstock decomposition. The dissociation of CH₄, absorption, and diffusion of active carbon species on various metal surfaces are discussed. Secondly, the substrate facilitates graphene nucleation with controllable nucleation density. The dissociation and diffusion of carbon atoms on the CuNi alloy surface with different Ni compositions are revealed. The metal substrate also catalyzes the growth of graphene by incorporating C atoms from the substrate into the edge of graphene and repairing possible defects. On the most used Cu(111), each armchair site on the edge of graphene is intended to be passivated by a Cu atom and lowers the barrier of incorporating C atoms into the graphene edge. The potential route of healing the defects during graphene CVD growth is summarized. Moreover, the substrate controls the orientation of the epitaxial graphene. The graphene edge-catalyst interaction is strong and is responsible for the orientation determination of a small graphene island in the early nucleation stage. There are three modes for graphene growth on metal substrate, i.e. embedded mode, step-attached mode and on-terrace mode, and the preferred growth modes are not all alike but vary from metal to metal. On a soft metal like Cu(111), graphene tends to grow in step-attached or embedded modes and therefore has a fixed orientation relative to the metal crystal lattice. Finally, the formation of wrinkles and step bunches in graphene because of the difference in thermal expansion coefficients between graphene and the metal substrate is discussed. The large friction force and strong interaction between graphene and the substrate make it energetically unfavorable for the formation of wrinkles. Different from the formation of wrinkles, the main driving force behind metal surface step-bunching in CVD graphene growth, even in the absence of a compression strain is revealed. Although significant effort is still required to adequately understand graphene catalytic growth, these theoretical studies offer guidelines for experimental designs. Furthermore, we provide the key issues to be explored in the future.

Graphene has attracted enormous interest in both academic and industrial fields, owing to its unique, extraordinary properties and significant potential applications. Various methods have been developed to synthesize high-quality graphene, among which chemical vapor deposition (CVD) has emerged as the most encouraging for scalable graphene film production with promising quality, controllability, and uniformity. However, a gap still exists between ideal graphene, having remarkable properties, and the currently available CVD-derived graphene films. To close this gap, numerous studies in the past decade have been devoted to decreasing defect density, grain boundaries, and wrinkles, and increasing the controllability of layer thickness and doping of graphene. Significant recent advances in this regard were the discovery of the inevitable contamination of graphene surface during high-temperature CVD growth and the synthesis of superclean graphene, representing a new growth frontier in CVD graphene research. Surface contamination of graphene is a major hurdle in probing its intrinsic properties, and strongly hinders its applications, for instance, in electrical and photonic devices. In this review, we aim to provide comprehensive knowledge on the inevitable contamination of CVD graphene and current synthesis strategies for preparing superclean graphene films, and an outlook for the future mass production of high-quality superclean graphene films. First, we focus on surface contamination formation, e.g. amorphous carbon, during the high-temperature CVD growth process of graphene. After introducing evidence to confirm the origin of surface contamination, the formation mechanism of the amorphous carbon is thoroughly discussed. Meanwhile, the influence of the intrinsic cleanness of graphene on the peeling and transfer quality is also revealed. Second, we summarize the state-of-the-art superclean growth strategies and classify them into direct-growth approaches and post-growth treatment approaches. For the former, modification of the CVD gas-phase reactions, for example, using metal-vapor-assisted methods or cold-wall CVD, is effective in inhibiting the formation of amorphous carbon. For the latter, both chemical and physical cleaning methods are employed to eliminate amorphous carbon without damaging the graphene, e.g. selective etching of as-formed amorphous carbon using CO₂, and removal of amorphous carbon from the graphene surface using a lint roller based on interfacial force control. Third, we summarize the outstanding electrical, optical, and thermal properties of superclean graphene. Superclean graphene exhibits high carrier mobility, low contact resistance, high transparency, and high thermal conductivity, further highlighting the significance of superclean graphene growth. Finally, future opportunities and challenges for the industrial production of high-quality superclean graphene are discussed.

Graphene has shown great promise in the development of next-generation electronic and optoelectronic devices owing to its atomic thickness and extraordinary electrical/optical/thermal/mechanical properties. Surface charge transfer doping is an important strategy to modulate graphene's electrical and optical properties. Compared with other doping methods, surface charge transfer doping shows distinct advantages in several aspects such as the minimized negative impact on the carrier mobility without disrupting the graphene lattice, wide range and precise control over the doping concentration, and highly efficient treatment processes without using high-temperature or ion implantation. Therefore, it is necessary to develop strong and stable surface charge transfer dopants to improve the electrical and optical performances of graphene, advancing its potential application in electronics and optoelectronics. For more than a decade, efforts has been devoted to developing diverse surface charge transfer p- and n-type dopants, including acids, gases, transition metals, alkali metals, metal chlorides, metal oxides, organics containing electron-donating/withdrawing groups, ferroelectric organics, and carbon-based materials, which serve as a wide range of ways to modulate the properties of graphene. Recently, remarkable progress has been made in realizing heavy and stable doping by surface charge transfer. In this review, we summarize the research status of surface charge transfer doping for graphene and its application in electronic and optoelectronic devices by focusing on the doping strength and stability. Initially, we survey the typical surface charge transfer doping mechanisms and widely used characterization measures, discussing their advantages and limitations. We then review the recent progress in the development of strong p- and n-type surface charge transfer dopants for graphene. For example, heavy p- and n-doping in graphene has been achieved by intercalation doping with metal chlorides and alkali metals, respectively. A large-area graphene film with stable p-doping was also realized. Of particular interest, organics are promising materials for developing emerging dopants with high structural tunability and diverse functions. We also introduce novel stable dopants and effective strategies for improving the ambient/thermal/solvent stability of typical dopants. Then, we devote a manuscript section to advances in high-performance optoelectronic devices using doped graphene electrodes with superior performances, focusing on graphene-based touch screens, organic light-emitting diodes, and organic photovoltaics. In this area, graphene-based flexible light-emitting devices have demonstrated advantages over typical tin-doped indium oxide (ITO) devices in terms of overall efficiencies. Finally, we discuss the challenges faced in developing state-of-the-art surface charge transfer dopants with future perspectives.

The advancements in the development of intelligent systems have resulted in an increase in the number, density, and distribution range of sensors. Traditional energy supply methods cannot meet the demands of the complex and variable sensor systems. However, the emergence of self-powered sensing devices that generate energy from their surroundings has provided a solution to this problem. Graphene, which has both an excellent sensing performance and wide range of applications in energy devices, facilitates the design of self-powered sensing systems. In recent years, several graphene-based self-powered sensors have been developed to overcome the design limitations of sensing systems. In this review, these sensors are divided into five categories according to their different energy conversion methods. (1) Self-powered by the electrochemical effect. The traditional electrochemical battery can be designed as a flexible structure that is responsive to external stimuli, including pressure, deformation, humidity, light, and temperature. It is an effective, stable, self-driving sensor, with working life determined by the amount of oxidizing/reducing agent present and the reaction rate. Flexible electrochemical cells with a high strain sensitivity ((I/I₀)/ε = 124) and stretchability (2000%) have been achieved. (2) Self-powered by the photovoltaic effect. Graphene can form a Schottky junction when coupled with various semiconducting materials, such as Si, GaAs, MoS₂, and some of their nanostructures. In these heterostructures, the van der Waals interface exhibits a Schottky barrier, which can separate photogenerated electron-hole pairs without external bias. Graphene-based Schottky junctions have been widely used as self-powered photodetectors with extremely high responsivities (~149 A·W^-1). (3) Self-powered by the triboelectric effect. The contact and separation of two surfaces can result in the separation of charges due to the difference in electron affinities of the materials. This results in an induced electrostatic force between the electrodes, thereby driving the flow of electrons in an external circuit. Triboelectric nanogenerators can realize self-driving touch/pressure sensing and are used for several applications, including touch screens, neural finger skin, and electronic skin. (4) Self-powered by the hydrovoltaic effect. Graphene can interact with water at the solid-liquid interface and generate an electrical signal. Therefore, graphene-based hydrovoltaic devices can constitute very simple self-driving sensors that are efficient in determining fluid flow, solution concentration, and humidity, among others. (5) Self-powered by other effects, such as the thermoelectric effect, piezoelectric effect, or pyroelectric effect. Although the electrical signals generated by these effects are relatively weak, they can be used for some special applications, such as temperature or infrared sensors. Finally, we discuss the future developments, challenges, and prospects of graphene-based self-powered sensing devices and systems.

Since its emergence in 2004, graphene has attracted enormous attention because of its unique and fantastic properties, which signals the birth of two-dimensional (2D) nanomaterials. The strictly atomic-layered 2D structure endows graphene with unconventional optical, electronic, magnetic, and mechanical properties. Owing to these extraordinary features, graphene has exhibited great potential in various fields, such as biology, medicine, chemistry, physics, and the environment. Notably, when graphene is used in these fields, it is always functionalized to facilitate its manipulation or meet the different area demands. After functionalization, the properties of graphene, such as its composition, size, shape, and structure, are modified, leading to changes in its electronic structure, surface chemistry, solubility, and mechanical and chemical properties. Functionalization of graphene can be achieved through various approaches, including chemical oxidation, doping, covalent and non-covalent modification, and hybridization with other materials, yielding various products (i.e., graphene oxide, nano graphene, graphene nanoribbons (GNRs), graphene nanomeshes, and graphene-polymer hybrids). However, these resulting products have not been systematically classified or strictly defined until now; although they have been classified as covalent and non-covalent functionalized graphene, graphene-based polymer composites, and graphene-based composites. Systematic classification and exact definition will benefit research on functionalizing graphene. In this review, based on research on functionalization of graphene, we propose a systematic classification of the products from graphene functionalization, their corresponding definitions, and preparation strategies, which are illustrated by representative examples. All the products from graphene functionalization are defined as functionalized graphene materials, which fall into two categories: functionalized graphene and functionalized graphene composite. Functionalized graphene is the product of modifying graphene by tuning its composition, framework, dimension, and morphology, and functionalized graphene composites are hybrids of graphene (or functionalized graphene) with other materials, including small molecules, polymers, metals, inorganic compounds, and carbon nanotubes (CNTs). Functionalized graphene materials are prepared through two strategies: "top-down" and "bottom-up, " each of which has its advantages and shortcomings and includes many corresponding preparation methods. The selection of preparation strategies depends on the application requirements, as different applications require different types of graphene. Both strategies are elucidated with detailed examples through an extensive analysis of the literature. Finally, the major challenges and perspectives of functionalized graphene materials are discussed. This review presents the proposed systematic classification and exact definition of functionalized graphene materials, which can enhance their development. It is believed that functionalized graphene materials will achieve significant progress in the future.

With the irreversible trend of miniaturization and the pursuit of a high power density in electronic devices, heat dissipation has become crucial for designing next-generation electronic products. Graphene, which has the highest thermal conductivity among all discovered solid materials, has attracted attention from both academia and the industry. As a two-dimensional material with atom-scale thickness, graphene is suitable for investigating the phonon transport behavior at reduced dimensions. The mass production technique of graphene makes it a promising material for thermal management in consumer electronics, information technology, medical devices, and new energy automobiles. In this review, we summarize the recent progress on the thermal conduction of graphene. In the first part, we introduce the thermal conductivity measurement methods for graphene, including the optothermal Raman method, suspended-pad method, and time-domain thermoreflectance (TDTR) method. The thermal measurement of graphene with high accuracy is key to understanding the heat transfer mechanism of graphene; however, it is still a significant challenge. Despite the development of measurement methods, the thermal measurement of suspended single-layer graphene is limited by the graphene transfer technique, estimation of the thermal contact resistance, sensitivity to the in-plane thermal conductivity in the thermal model, and other factors. In the second part, we discuss the theoretical study of the thermal conductivity of graphene via first principle calculations and molecular dynamics simulation. The "selection rule" of phonon scattering explains the thickness-dependent thermal conductivity of few-layer graphene, and the understanding of the contribution of phonon modes to the thermal conductivity of graphene has been updated recently by taking multiple-phonon scattering into consideration. The size effect on the thermal conductivity of graphene is discussed in this section for a better understanding of the phonon transport behavior of graphene. In the third part, we conclude with the thermal management applications of graphene, including a highly thermally conductive graphene film, graphene fiber, and graphene-enhanced thermal interface materials. For graphene films, which are the pioneering thermal management applications in industrial use, we focus on the challenge of fabricating highly thermally conductive graphene films with large thicknesses and propose possible technical methods. For graphene-enhanced thermal interface materials, we summarize the main factors affecting the thermal properties and discuss the tradeoff between the high thermal conductivity of graphene flakes and the dispersibility of graphene in the polymer matrix. It was demonstrated that a 3D thermal conductive network is essential for efficient heat dissipation in graphene-based composites. Finally, a summary of opportunities and challenges in the thermal study of graphene is presented at the end of the review. Research on the thermal properties of graphene has made immense progress since the discovery of the thermal conductivity of graphene more than a decade ago, and will continue in order to address the urgent requirements of thermal management.

Chemical vapor deposition (CVD) is considered as the most promising method for the mass production of high-quality graphene films owing to its fine controllability, uniformity, and scalability. In the past decade, significant efforts have been devoted to exploring new strategies for growing graphene with improved quality. During the high-temperature CVD growth process of graphene, besides the surface reactions, gas-phase reactions play an important role in the growth of graphene, especially for the decomposition of hydrocarbons. However, the effect of gas-phase reactions on the CVD growth of graphene has not been analyzed previously. To fill this gap, it is essential to systematically analyze the relationship between gas-phase reactions and the growth of graphene films. In this review article, we aim to provide comprehensive knowledge of the gas-phase reactions occurring in the CVD system during graphene growth and to summarize the typical strategies for improving the quality of graphene by modulating gas-phase reactions. After briefly introducing the elementary steps and basic concept of graphene growth, we focus on the gas-phase dynamics and reactions in the CVD system, which influence the decomposition of hydrocarbons, nucleation of graphene, and lateral growth of graphene nuclei, as well as the merging of adjacent graphene domains. Then, a systematic description of the mass transport process in gas phase is provided, including confirmation of the states of gas flow under different CVD conditions and introduction to the boundary layer, which is crucial for graphene growth. Furthermore, we discuss the possible reaction paths of carbon sources in the gas phase and the corresponding active carbon species existing in the boundary layer, based on which the main impact factors of gas-phase reactions are discussed. Representative strategies for obtaining graphene films with improved quality by modulating gas-phase reactions are summarized. Gas-phase reactions affect the crystallinity, cleanness, domain size, layer number, and growth rate of graphene grown on both metal and non-metal substrates. Therefore, we will separately review the detailed strategies, corresponding mechanisms, key parameters, and latest status regarding the quality improvement of graphene. Finally, a brief summary and proposals for future research are provided. This review can be divided into two parts: (1) gas-phase reactions occurring in the high-temperature CVD system, including the mass transport process and the reaction paths of hydrocarbons; and (2) the synthesis of high-quality graphene film via modulation of the gas-phase reaction, in order to improve the crystallinity, cleanness, domain size, layer number, and growth rate of graphene.

The availability of renewable energy resources (e.g., solar, wind, and tides) is crucial for promoting sustainable development and alleviating environmental issues. However, the intermittent nature of renewable energy requires the application of grid-level electrical-energy storage (EES) technologies to achieve a continuous supply of electricity. As is well known, lithium-ion batteries (LIBs) with high energy density dominate the rechargeable battery market. When faced with the requirements of large-scale power stations, high cost, and limited availability of raw materials, these become serious issues in the application of LIBs. In contrast, sodium-ion batteries (SIBs), which share similar operation mechanisms with LIBs, are considered to be more suitable for grid-level storage due to easy accessibility and geographically available reserves of sodium raw material, with significant improvements in its processing technology made recently. Nevertheless, limited energy density and unsatisfactory cycling life hinder the commercialization of SIBs significantly, which necessitates the use of novel electrode materials with high specific capacities and extended durability. Compared with the accelerated development of cathodes, graphite, on the anode side, as a commercialized anode for LIBs fails to store Na-ions owing to unfavorable thermodynamics. Hence, discovering and designing novel anode materials for SIBs have become a significant challenge. Among different anode materials, phosphorus-based (including phosphides) anodes have been recognized as one of the most promising materials because of their high theoretical capacity (2596 mAh·g^-1 for phosphorus) and the abundance of phosphorus resources. Nonetheless, phosphorus-based anodes exhibit low conductivity and large volume expansion, resulting in inferior cycling performance and rating property. Therefore, various strategies, including nanosizing, morphology control, and carbon (non-carbon) modification, have been adopted to improve the performance of phosphorus-based anodes. In this review, the current progress on phosphorus-based anodes for SIBs are summarized. The Na-storage mechanisms of phosphorus-based materials are briefly discussed. Next, strategies for overcoming the disadvantages of phosphorus-based anodes are discussed extensively, including the size and morphology adjustment as well as the carbon (non-carbon) modification. Specifically, the carbon modification not only increases the conductivity but also decreases the volume expansion. Finally, the challenges and perspective of phosphorus-based anodes for SIBs are proposed. In this review paper, the development of suitable anode materials that can help to accelerate the commercialization of SIBs is highlighted.

The past decades have witnessed an increasing interest in molecular electronics aiming to assemble functional circuits using single molecules. Researchers from various disciplines have devoted considerable attention in the design and construction of single-molecule junctions and sophisticated functional devices, accompanied by the discovery and utilization of numerous novel quantum phenomena. Many new breakthroughs benefit from the utilization of various stimulus response methods to tune the charge transport in molecular devices, such as light, temperature, magnetic field, pH, and mechanical force. Electrostatic field has superb but distinct abilities to modulate the charge transport in molecular devices. First, like in other electronic devices, electrostatic fields act on single-molecule devices as a noninvasive means. However, unlike in these traditional electronic devices, the voltage applied in the extremely tiny single-molecule devices would generate a large electrostatic field, which could provide the necessary conditions for regulating charge transport and catalyzing single-molecule-scale chemical reactions. This review focuses on the recent advances made in tuning charge transport by electrostatic field in the single-molecule devices. In the second section, we introduce and compare two break junction techniques commonly used to construct molecular junctions: the scanning tunneling microscopy break junction (STMBJ) technique and the mechanically controllable break junction (MCBJ) technique; furthermore, the three-electrode systems based on these two break junction techniques are also introduced. These techniques laid the foundation for various new techniques in tuning charge transport in molecular junctions based on electrostatic field. In the third section, the applications of electrostatic field are introduced, including controlling the molecular-electrode interfaces, varying molecule configurations and conformations, catalyzing single-molecule-scale chemical reactions, switching molecule spin states, changing molecule redox states and shifting the energy levels of the electrodes and molecules. Finally, we discussed the shortcomings of the applications electrostatic field in single-molecule devices. Including the low stability of single-molecule devices under strong electrostatic field, and the introduction of electrostatic field will increase the difficulty of understanding the charge transport mechanism in single-molecule devices. In addition, we point out that electrostatic field modulation of single-molecule charge transport is expected to be further developed in the following aspects: Firstly, multi-stimulus response molecule devices could be built by combining electrostatic field with other stimulus. Secondly, electrostatic field could be used to catalyze more types of chemical reactions, even control the configurations and conformations of products. Thirdly, electrostatic field can be used to design fullerene-based switching molecular diodes that proper for application in random-access memories and memristors.

Owing to the serious energy crisis and environmental problems caused by fossil energy consumption, development of high-energy-density batteries is becoming increasingly significant to satisfy the rapidly growing social demands. Lithium-ion batteries have received widespread attention because of their high energy densities and environmental friendliness. At present, they are widely used in portable electronic devices and electric vehicles. However, security aspects need to be addressed urgently. Substantial advances in liquid electrolyte-based lithium-ion batteries have become a performance bottleneck in the recent years. Traditional lithium-ion batteries use organic liquids as electrolytes, but the flammability and corrosion of these electrolytes considerably limit their development. Continuous growth of lithium dendrites can pierce the separator, leading to electrolyte leakage and combustion, which is a serious safety hazard. Replacement of organic electrolytes with solid-state electrolytes is one of the promising solutions for the development of next-generation energy storage devices, because they have high energy densities and are safe. Solid electrolytes can remarkably alleviate the safety hazards involved in the use of traditional liquid-based lithium-ion batteries. In addition, the composite of solid-state electrolytes and lithium metal is expected to result in a higher energy density. However, due to the lack of fluidity of the solid electrolytes, problems such as limited solid-solid contact area and increased impedance at the interface when solid-state electrolytes are in contact with electrodes must be solved. The localized and buried interface is a major drawback that restricts the electrochemical performance and practical applications of the solid-state batteries. Fabrication of a stable interface between the electrodes and solid-state electrolyte is the main challenge in the development of solid-state lithium metal batteries. All these aspects are critical to the electrochemical performance and safety of the solid-state batteries. Current research mainly focuses on addressing the problems related to the solid-solid interface in solid-state batteries and improving the electrochemical performance of such batteries. In this review, we comprehensively summarize the challenges in the fabrication of solid-state batteries, including poor chemical and electrochemical compatibilities and mechanical instability. Research progress on the improvement strategies for interface problems and the advanced characterization methods for the interface problems are discussed in detail. Meanwhile, we also propose a prospect for the future development of solid-state batteries to guide the rational designing of next-generation high-energy solid-state batteries. There are many critical problems in solid-state batteries that must be fully understood. With further research, all-solid-state batteries are expected to replace the traditional liquid-based lithium-ion batteries and become an important system for a safe and reliable energy storage.

Lithium-ion batteries have achieved tremendous success in the fields of portable mobile devices, electric vehicles, and large-scale energy storage owing to their high working voltage, high energy density, and long-term lifespan. However, lithium-ion batteries are ultimately unable to satisfy increasing industrial demands due to the shortage and rising cost of lithium resources. Sodium is another alkali metal that has similar physical and chemical properties to those of lithium, but is more abundant. Therefore, sodium-ion batteries (SIBs) are promising candidates for next-generation energy storage devices. Nevertheless, SIBs generally exhibit inferior electrochemical reaction kinetics, cycling performance, and energy density to those of lithium-ion batteries owing to the larger ion radius and higher standard potential of Na+ compared to those of Li+. To address these issues, significant effort has been made toward developing electrode materials with large sodiation/desodiation channels, robust structural stability, and high theoretical capacity. As electrode performance is closely related to its architecture, constructing an advanced electrode structure is crucial for achieving high-performance SIBs. Conventional electrodes are generally prepared by mixing a slurry of active materials, conductive carbon, and binders, followed by casting on a metal current collector. Electrodes prepared this way are subject to shape deformation, causing the active materials to easily peel off the current collector during charge/discharge processes. This leads to rapid capacity decay and short cycle life. Moreover, binders and other additives increase the weight and volume of the electrodes, which reduces the overall energy density of the batteries. Therefore, binder-free, three-dimensional (3D) array electrodes with satisfactory electronic conductivity and low ion-path tortuosity have been proposed. In addition to solving the aforementioned issues, this type of electrode significantly reduces contact resistance through the strong adhesion between the array and the substrate. Furthermore, electrolyte infiltration is greatly facilitated by the abundant interspacing between individual nanostructures, which promotes fast electron transport and shortens ion diffusion, thus enabling the electrode reaction. The array structure can also readily accommodate substantial volume variations that occur during repeated sodiation/desodiation processes and release the generated stress. Therefore, it is of great interest to explore binder-free array electrodes for sodium-ion storage applications. This review summarizes the recent advances in various 3D array anode materials for SIBs, including elemental anodes, transition metal oxides, sulfides, phosphides, and titanates. The preparation methods, structure/morphology characteristics, and electrochemical performance of various array anodes are discussed, and future opportunities and challenges from employing array electrodes in SIBs are proposed.

Since their discovery, two-dimensional (2D) materials have attracted significant research attention owing to their excellent and controllable physical and chemical properties. These materials have emerged rapidly as important material system owing to their unique properties such as electricity, optics, quantum properties, and catalytic properties. 2D materials are mostly bonded by strong ionic or covalent bonds within the layers, and the layers are stacked together by van der Waals forces, thereby making it possible to peel off 2D materials with few or single layers. The weak interaction between the layers of 2D materials also enables the use of van der Waals gaps for regulating the electronic structure of the system and further optimizing the material properties. The introduction of guest atoms can significantly change the interlayer spacing of the original material and coupling strength between the layers. Also, interaction between the guest and host atom also has the potential to change the electronic structure of the original material, thereby affecting the material properties. For example, the electron structure of a host can be modified by interlayer guest atoms, and characteristics such as carrier concentration, optical transmittance, conductivity, and band gap can be tuned. Organic cations intercalated between the layers of 2D materials can produce stable superlattices, which have great potential for developing new electronic and optoelectronic devices. This method enables the modulation of the electrical, magnetic, and optical properties of the original materials, thereby establishing a family of 2D materials with widely adjustable electrical and optical properties. It is also possible to introduce some new properties to the 2D materials, such as magnetic properties and catalytic properties, by the intercalation of guest atoms. Interlayer storage, represented by lithium-ion batteries, is also an important application of 2D van der Waals gap utilization in energy storage, which has also attracted significant research attention. Herein, we review the studies conducted in recent years from the following aspects: (1) changing the layer spacing to change the interlayer coupling; (2) introducing the interaction between guest and host atoms to change the physico-chemical properties of raw materials; (3) introducing the guest substances to obtain new properties; and (4) interlayer energy storage. We systematically describe various interlayer optimization methods of 2D van der Waals gaps and their effects on the physical and chemical properties of synthetic materials, and suggest the direction of further development and utilization of 2D van der Waals gaps.

Since their commercialization in 1991, lithium-ion batteries (LIBs), one of the greatest inventions in history, have profoundly reshaped lifestyles owing to their high energy density, long lifespan, and reliable and safe operation. The ever-increasing use of portable electronics, electric vehicles, and large-scale energy storage has consistently promoted the development of LIBs with higher energy density, reliable and safe operation, faster charging, and lower cost. To meet these stringent requirements, researchers have developed advanced electrode materials and electrolytes, wherein the electrode materials play a key role in improving the energy density of the battery and electrolytes play an important role in enhancing the cycling stability of batteries. In addition, further improvements in the current LIBs and reviving lithium metal batteries have received intensive interest. The electrode/electrolyte interface is formed on the electrode surface during the initial charging/discharging stage, whose ionic conductivity and electronic insulation ensure rapid transport of lithium ions and isolating the unsolicited side reactions caused by electrons, respectively. In a working battery, the stability or properties of the interface play a crucial role in maintaining the integrity of the electrode structure, thereby stabilizing the cycling performance and prolonging the service lifespan to meet the sustainable energy demand for the public. Generally, the interface formed on the anode and cathode is called the solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI) respectively, and SEI and CEI are collectively known as the electrode electrolyte interphase. Research on SEI has made remarkable progress; however, the structure, component, and accurate regulation strategy of SEI are still at the initial stage due to the stability and complexity of SEI and the limited research methods at the nanoscale. To improve the performance and lifespan of working batteries, the formation, evolution, and modification of the interface should be paid particular attention. Herein, the latest researches focused on the SEI are reviewed, including the formation mechanism, which discusses two key factors affecting the formation of the electrode/electrolyte film, i.e., the ion characteristic adsorption on the electrode surface and the solvated coordinate structure, evolution, and description that contains the interface layer structure, wherein the mosaic model and the layered structure are the two mainstream views of the SEI structure, and the chemical composition of SEI as well as the possible conduction mechanism of lithium ions, including desolvation and subsequent diffusion across the polycrystalline SEI. The regulation strategies of the interface layer are discussed in detail, and the future prospects of SEI are presented.

In addition to their extensive commercial application in electronic devices such as cell phones and laptops, lithium-ion batteries (LIBs) are most suitable to fulfill the energy storage requirements of electric vehicles because of their recognized safety, portability, and high energy density. Cathodes are the most important part of LIBs, and various cathode materials have been widely investigated over the past decades. Polaron formation has been attracting increasing attention in the research of cathode materials, as it limits electron conduction. In particular, polarons are responsible for low electronic conductivity in cathode materials like olivine phosphate. Polaron is a typical crystal defect caused by the integrated motion of lattice distortion and its trapping electrons. Research on the mechanism of polaron formation will provide theoretical guidance for the design of high-electronic-conductivity cathode materials and improvement of the electrochemical performance of LIBs. Theoretical calculation is a direct and important method to study polaron formation in a specific crystal material, because the presence of polarons and their formation mechanisms can be effectively verified through this method. In this article, we first introduce the basic physical concept of polarons and their dynamical model according to the Marcus and Emin-Holstein-Austin-Mott theories. A comparison of the general properties of large and small polarons, summarized in this chapter, reveals that small polaron formation more likely occurs in cathode materials. Moreover, the theoretical characterization, electrical impact, control and challenges of polarons are reviewed. Although a universal necessary and suitable condition for the theoretical characterization of polarons has not yet been found, we still propose three criteria that are proven to be feasible and practical for the theoretical identification of polarons when applied in combination. Experimental characterizations are also introduced briefly for reference, because the comparison with the experiment is suggested to be necessary and mandatory. The electrical impact caused by polarons results in low electronic conductivity, which has been broadly reported in layered, olivine, and spinel cathode materials. Doping can weaken the influence of polarons and, thus, significantly enhance the electronic conductivity, thereby becoming the most prevalent strategy for tuning polarons. Although theoretical calculations have been widely and effectively conducted in the study of polarons, some challenges may still be faced because of the intrinsic shortcomings of the traditional density functional theory, which need to be addressed. Finally, further research on polarons from the perspective of basic theory and practical applications is prospected.

Lithium ion batteries (LIBs) have broad applications in a wide variety of a fields pertaining to energy storage devices. In line with the increasing demand in emerging areas such as long-range electric vehicles and smart grids, there is a continuous effort to achieve high energy by maximizing the reversible capacity of electrode materials, particularly cathode materials. However, in recent years, with the continuous enhancement of battery energy density, safety issues have increasingly attracted the attention of researchers, becoming a non-negligible factor in determining whether the electric vehicle industry has a foothold. The key issue in the development of battery systems with high specific energies is the intrinsic instability of the cathode, with the accompanying question of safety. The failure mechanism and stability of high-specific-capacity cathode materials for the next generation of LIBs, including nickel-rich cathodes, high-voltage spinel cathodes, and lithium-rich layered cathodes, have attracted extensive research attention. Systematic studies related to the intrinsic physical and chemical properties of different cathodes are crucial to elucidate the instability mechanisms of positive active materials. Factors that these studies must address include the stability under extended electrochemical cycles with respect to dissolution of metal ions in LiPF₆-based electrolytes due to HF corrosion of the electrode; cation mixing due to the similarity in radius between Li⁺ and Ni²⁺; oxygen evolution when the cathode is charged to a high voltage; the origin of cracks generated during repeated charge/discharge processes arising from the anisotropy of the cell parameters; and electrolyte decomposition when traces of water are present. Regulating the surface nanostructure and bulk crystal lattice of electrode materials is an effective way to meet the demand for cathode materials with high energy density and outstanding stability. Surface modification treatment of positive active materials can slow side reactions and the loss of active material, thereby extending the life of the cathode material and improving the safety of the battery. This review is targeted at the failure mechanisms related to the electrochemical cycle, and a synthetic strategy to ameliorate the properties of cathode surface locations, with the electrochemical performance optimized by accurate surface control. From the perspective of the main stability and safety issues of high-energy cathode materials during the electrochemical cycle, a detailed discussion is presented on the current understanding of the mechanism of performance failure. It is crucial to seek out favorable strategies in response to the failures. Considering the surface structure of the cathode in relation to the stability issue, a newly developed protocol, known as surface-localized doping, which can exist in different states to modify the surface properties of high-energy cathodes, is discussed as a means of ensuring significantly improved stability and safety. Finally, we envision the future challenges and possible research directions related to the stability control of next-generation high-energy cathode materials.

The development of highly efficient and low-cost electrocatalysts is important for both hydrogen- and carbon-based energy technologies. The electronic structure and coordination features, particularly the coordination environment and the amount of low-coordination atoms, of the catalyst are key factors that determine their catalytic activity and stability in a particular reaction. The regulation and rational design of catalytic materials at the molecular and atomic levels are crucial to achieving precise chemical synthesis at the atomic scale. Recently, significant efforts have been made to engineer coordination features and electronic structures by reducing the particle size, tuning the composition of the edges, and exposing specific planes of crystals. Among these representative strategies, the methods based on the confinement effect are most effective for achieving precise chemical synthesis with atomic precision at the molecular and atomic levels. Under molecular or atomic scale confinement, the physicochemical properties are largely altered, and the chemical reactions as well as the catalytic process are completely changed. The unique spatial and dimensional properties of the confinement regulate the molecular structure, atomic arrangement, electron transfer, and other properties of matter in space. It not only adjusts the coordination environments to control the formation mechanism of active centers, but also influences the structural and electronic properties of electrocatalysts. Therefore, the adsorption of catalytic intermediates is altered, and consequently, the catalytic activity and selectivity are changed. In a confined reaction, usually in suitable nano-reactors, the physicochemical properties of reaction products, such as the state of matter, solubility, dielectric constant, and molecular orbital, are finely modulated. Thus, the catalysts produced by confinement significantly differ from those produced in an open system. For example, atomic-layered metals with low coordination can be produced in a two-dimensional confined space. The nitrogen configurations of nitrogen-doped graphene can also be regulated in two-dimensional or three-dimensional confined systems. Herein, the confinement-induced methods, specifically the method used for atomic regulation, are reviewed, such as the control of molecular configuration, the modification of the coordination structure, and the alteration of charge transfer. Applications in the field of fuel cells and material energy conversion are also reviewed. In the next stage, it is important to conduct in-depth investigations of the constructed confinement environment by selecting different substrates for the regulation and rational design of confined catalytic materials. The investigation of the derived properties of the catalyst after release from the confinement is crucial for the development of uncommon catalytic properties.

Research on two-dimensional (2D) materials has been explosively increasing in last seventeen years in varying subjects including condensed matter physics, electronic engineering, materials science, and chemistry since the mechanical exfoliation of graphene in 2004. Starting from graphene, 2D materials now have become a big family with numerous members and diverse categories. The unique structural features and physicochemical properties of 2D materials make them one class of the most appealing candidates for a wide range of potential applications. In particular, we have seen some major breakthroughs made in the field of 2D materials in last five years not only in developing novel synthetic methods and exploring new structures/properties but also in identifying innovative applications and pushing forward commercialisation. In this review, we provide a critical summary on the recent progress made in the field of 2D materials with a particular focus on last five years. After a brief background introduction, we first discuss the major synthetic methods for 2D materials, including the mechanical exfoliation, liquid exfoliation, vapor phase deposition, and wet-chemical synthesis as well as phase engineering of 2D materials belonging to the field of phase engineering of nanomaterials (PEN). We then introduce the superconducting/optical/magnetic properties and chirality of 2D materials along with newly emerging magic angle 2D superlattices. Following that, the promising applications of 2D materials in electronics, optoelectronics, catalysis, energy storage, solar cells, biomedicine, sensors, environments, etc. are described sequentially. Thereafter, we present the theoretic calculations and simulations of 2D materials. Finally, after concluding the current progress, we provide some personal discussions on the existing challenges and future outlooks in this rapidly developing field.

The rapid development of industrialization has resulted in severe environmental problems. A comprehensive assessment of air quality is urgently required all around the world. Among various technologies used in gas molecule detection, including Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, mass spectroscopy (MS), electrochemical sensors, and metal oxide semiconductor (MOS) gas sensors, MOS gas sensors possess the advantages of small dimension, low power consumption, high sensitivity, low production cost, and excellent silicon chip compatibility. MOS sensors hold great promise for future Internet of Things (IoT) sensors, which will have a profound impact on indoor and outdoor air quality monitoring. The development of nanotechnology has significantly enhanced the development of MOS gas sensors. Among various nanostructures like nanoparticles, nanosheets and nanowires, the emergence of quasi-one-dimensional (q1D) nanowires/nanorods/nanofibers, with unique q1D geometry (facilitating fast carrier transport) and large surface-to-volume ratio, potentially act as ideal sensing channels for MOS sensors with extremely small dimension, and good stability and sensitivity. These structures have thus been the focus of extensive research. Among the various MOS nanomaterials available, tungsten oxide (WO_3-x, 0 ≤ x < 1) nanowires feature the characteristic properties (multiple oxidation states, rich substoichiometric oxides with distinct properties, photo/electrochromism, (photo)catalytic properties, etc.), and unique q1D geometry (single-crystalline pathway for fast carrier transport, large surface-to-volume ratio, etc.). WO_3-x nanowires have broad applications in smart windows, energy conversation & storage, and gas sensing devices, and have thus become a focus of attention. In this paper, the fundamental properties of tungsten oxide, synthesis methods and growth mechanism of tungsten oxide nanowires are reviewed. Among various (vapor-liquid-solid (VLS), vapor-solid (VS) and thermal oxidation) growth methods, the thermal oxidation method enables an in situ integration of WO_3-x nanowires on predefined electrodes (so-called bridged nanowire devices) via the oxidation of lithographically patterned W film at relatively low growth temperature (~500 ℃) because of interfacial strain, defects and oxygen on the surface of the W film. The novel bridged nanowire-based sensor devices outperform traditional lateral nanowire devices in terms of larger exposure area, low power consumption via self-heating, and greater convenience in device processing. Recent progress in bridged WO_3-x nanowire devices and sensitive NO_x molecule detection under low power consumption have also been reviewed. Power consumption of as low as a few milliwatts was achieved, and the detection limit of NO₂ was reduced to 0.3 ppb (1 ppb = 1 × 10^-9, volume fraction). In situ formed bridged WO_3-x nanowire devices potentially satisfy the strict requirements of IoT sensors (small dimension, low power consumption, high integration, low cost, high sensitivity, and selectivity), and hold great promises for future IoT sensors.

Recently, the problems of environmental pollution and energy shortages arise along with the rapid development of the economy, and gradually becoming significant challenges faced by society. To realize truly sustainable development, novel environment-friendly clean energy technologies need to be developed. The fuel cell is a chemical device that can directly convert the chemical energy of a fuel and an oxidant into electrical energy via an electrochemical reaction. The electrochemical reaction is usually clean and complete, and rarely produces harmful substances. Therefore, fuel cells are considered to be one of the most promising clean energy technologies. Fuel cells can be classified based on their electrolytes: alkaline fuel cells (AFC), phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC), solid oxide fuel cells (SOFC), and proton exchange membrane fuel cells (PEMFC). Although there have been many studies regarding the materials and reactions of fuel cells, direct spectroscopic evidence to understand the reaction mechanisms in the electrodes is lacking. Raman spectroscopy, as a non-destructive molecular spectroscopy technique with ultra-high sensitivity, is suitable for studying fuel cell materials. Over the past decade, the development of surface-enhanced Raman spectroscopy (SERS) and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) has overcome the material and morphology limitations of traditional Raman spectroscopy. The extraordinary progress in SHINERS has enabled researchers to acquire high-quality Raman spectra for many types of materials instead of only on the surface of noble metals such as Au, Ag, and Cu. This strategy can also be applied to trace intermediate reactants on electrodes to fully understand the reaction mechanism of the fuel cell. Although many kinds of characterization methods including X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and X-ray diffraction (XRD) also exhibit excellent sensitivity for studying electrode reactions, they require material pretreatments and long-duration experiments. Compared with the abovementioned methods, SERS and SHINERS show better performance for in situ experiments, which will aid in the rational design of catalysts and electrode materials with higher efficiency. This article provides an overview of the basic concepts of fuel cells, as well as SERS and SHINERS. In addition, the application of Raman spectroscopy, SERS, and SHINERS in fuel cell development is discussed along with future prospects.

Hydrogen oxygen fuel cells and water electrolysis serves as two important systems for realizing the recycling of hydrogen energy, which involves two crucial electrochemical reactions, the hydrogen oxidation reaction (HOR) and the hydrogen evolution reaction (HER). The kinetics of HOR/HER in alkaline media is 2 or 3 orders of magnitude slower than that in acidic media, which is the main bottleneck that hinders the development of alkaline membrane fuel cells and alkaline water electrolysis. Thus, clarifying the underlying difference of HOR/HER activity in alkaline and acid electrolytes, and exploring the alkaline HOR/HER mechanism are the significant challenges for widely commercial application of low temperature alkaline energy conversion devices. Here, this paper briefly reviews the related explanations and controversies about the alkaline HOR/HER mechanism in recent years, including bifunctional mechanism, hydrogen binding energy (HBE) theory and electronic effect. The bifunctional mechanism emphasizes the influence of water dissociation and OH adsorption on HER and HOR, respectively, which possesses guiding significance for designing and fabricating composite catalysts. The HBE theory stresses that H_ad is the key reaction intermediate of HOR/HER, and other external factors, such as electrode potential, pH, ions and so on, affect the HOR/HER mechanism and kinetics by disturbing HBE. HBE is widely considered to be the only activity descriptor of HOR/HER. The electronic effect emphasizes the role of catalysts' composition and reaction intermediates in regulating electronic structure of active sites and changing HOR/HER mechanism. It provides an effective strategy to construct active sites and optimize catalytic activity. In addition, we summarize the theoretical simulation methods of electrochemical interface and their applications in exploring HOR/HER mechanism. In-depth theoretical simulation of HOR/HER mechanism requires the establishment of a more reasonable explicit solvation model on electrode/electrolyte interface and the combination of density functional theory (DFT), ab initio molecular dynamics (AIMD), and microkinetic model, to calculate the electronic structure and the dynamic processes of electrode/electrolyte interface, such as bond breaking and formation, solvent recombination, and proton migration in the electric double layer during reaction process, and then to analyze the HOR/HER mechanism and reaction kinetics under different electrode potentials and electrolytes. The present review is helpful for understanding the ongoing developments of HOR/HER mechanism. And the combination of experiment and theoretical calculation can be employed to explore the pH-dependence of HOR/HER deeply, and design novel HOR/HER catalysts with high activity and stability.

Fuel cells have attracted much attention because of their high specific energy and low environmental load, but their commercial application is limited by the poor performance and high cost of the relevant electrode catalysts. The oxygen reduction reaction (ORR) is the key cathodic reaction in a fuel cell, and it plays an important role in the chemical energy conversion. However, the slow reaction kinetics, large reaction energy barrier, and low selectivity deteriorate the energy efficiency of fuel cells. Thus, rational design of low-cost electrocatalysts that show good activity is highly desirable for improving the performance of fuel cells. Although noble-metal-based electrocatalysts (e.g., Pt/C) show excellent catalytic activity for the ORR, their limited resources, high price, and low stability caused by the migration and agglomeration of nanoparticles on the surface of carbon supports have hindered their extensive application. Because of their excellent electrical conductivity and stability, carbon-based materials are widely used as substrates for electrode materials in the ORR. Heteroatom (e.g., nitrogen, phosphorus, sulfur)-doped carbon materials can influence the adsorption state of oxygen molecules and intermediates by changing the charge distribution of adjacent carbon atoms because of the difference in electronegativity and atomic radius between the heteroatoms and carbon atoms, thus promoting the ORR activity. Optimization of the structure and surface properties of carbon-based electrocatalysts has helped accelerate the four-electron reaction and reduce the overpotential in the ORR. Therefore, non-noble metal and heteroatom-doped carbon-based catalysts exhibit improved ORR activity. The dispersion of non-noble metals on carbon materials via the interaction of metal atoms with the neighboring nitrogen atoms or other heteroatoms produces high-density active sites in the carbon support, thus leading to high atomic utilization and significantly improving the electrocatalytic activity owing to the synergistic effect. This review focuses on the applications of carbon-based electrocatalysts in fuel cells, summarizing the design strategies and electrocatalytic activities of heteroatom-doped carbon-based catalysts with non-noble metals toward improving their ORR activity. Furthermore, the latest research progress in the field of carbon-based catalysts used as cathode catalysts in proton exchange membrane fuel cells (PEMFCs) and anion-exchange membrane fuel cells (AEMFCs) is integrated, and the direction of future development is addressed.

Although platinum (Pt)-based catalysts are suffering from high costs and limited reserves, they are still irreplaceable in a short period of time in terms of catalytic performance. Structural optimization, composition regulation and carrier modification are the common strategies to improve the activity and stability of Pt-based catalyst. Strikingly, the morphological evolution of Pt-based electrocatalyst into nanoframes (NFs) have attracted wide attention to reduce the Pt consumption and improve the electrocatalytic activity simultaneously. Contrary to Pt-based solid nanocrystalline materials, Pt-based NFs have many advantages in higher atomic utilization, open space structure and larger specific surface area, which facilitate electron transfer, mass transport and weaken surface adsorption by more unsaturated coordination sites. Here we introduce the detailed preparation strategies of Pt-based NFs with different etching methods (oxidative etching, chemical etching, galvanic replacement and carbon monoxide etching), crystal structure evolution and formation mechanism, efficient applications for oxygen reduction reaction (ORR), methanol oxidation reaction (MOR) and ethanol oxidation reaction (EOR) in direct alcohol fuel cells (DAFCs). Based on the high-efficiency atom utilization, open space structure and diverse alloy composition, Pt-based NFs exhibit superior activity, stability and anti-poisoning than commercial counterparts in the application of DAFCs. The current challenges and future development of Pt-based NFs are prospected on the type of NFs materials, synthesis and etching methods, crystal control and catalytic performance. We propose a series of improvement mechanisms of Pt-based NFs, such as small size effect, high-energy facets, Pt-skin construction and Pt-C integration, thereby weakening the molecule absorption, increasing the Pt utilization, strengthening the intrinsic stability, and alleviating the metal dissolution and support corrosion. Additionally, the scale-up synthesis of catalytic materials, membrane electrodes assembly, and development of the start-stop system and the circulation system design are essential for the commercial application of Pt-based NFs and industrial manufacturing of DAFCs. More importantly, the reaction mechanism, active site distribution and dynamic changes in the catalytic material during the catalytic reaction are crucial to further explain the maintenance and evolution of catalytic performance, which will open a window to elucidate the improvement mechanism of the catalyst in the fuel cell reactions. This review work would promote continuous upgradations and understandings on Pt-based NFs in the future development of DAFCs.

High-temperature polymer exchange membrane fuel cells (HT-PEMFCs), promising and sustainable energy conversion devices, have received considerable attention ascribed to their high energy conversion efficiency and zero emission. Different from the traditional Nafion PEMFCs, the working temperature ranks from 120 to 250 ℃ for HT-PEMFCs; as a result, HT-PEFMCs show impressive merits, such as theoretically higher kinetics, simple water/heat management and better tolerance toward impurities in hydrogen fuel; especially the elimination of flooding issue in fuel cells. Moreover, the working temperature matches well with the temperature for hydrogen generation from methanol reforming revealing that the generated heat from HT-PEMFCs can be utilized for methanol reforming to generate hydrogen; in this case, hydrogen tank can be replaced by methanol reforming system for HT-PEMFCs leading to a higher safety. Similar to traditional Nafion PEMFCs, polymer electrolyte membrane (PEM) associated with two electrodes representing for anode and cathode compose the membrane electrode assembly (MEA). Electrocatalyst as heart of HT-PEMFCs significantly affects the output of fuel cells, especially the cathodic electrocatalyst since the oxygen reduction reaction (ORR) kinetics is substantially sluggish than hydrogen oxidation reaction (HOR). Phosphoric acid doped polybenzimidazole (PA-PBI) is the state-of-the-art PEM for HT-PEMFCs; while, due to the low interaction between PA and PBI, PA leaching to the catalyst layer is normally observed during the long-term operation resulting in blocking of active sites to reduce three-phase boundary (TPB); besides, oxygen dissolution/diffusion in PA is much lower compared to Nafion, thereby, lower fuel cell performance is customarily recorded than Nafion PEMFCs. Thus, construction of high-performance ORR electrocatalyst with exceptional tolerance toward phosphate and increasing of oxygen concentration at TPB are highly desirable to realize the commercialization of HT-PEMFCs. Additionally, the stability of electrocatalyst should be significantly considered because the coalescence of platinum (Pt) nanoparticles as well as carbon corrosion is accelerated at high working temperature. In this review, we have summarized the recently reported Pt, non-Pt and meta-free electrocatalysts in HT-PEMFCs application. Surficial modification, alloying effect as well as substrate effect have been invited to construct high-performance Pt electrocatalyst in phosphoric acid electrolyte since the adsorption of phosphate on Pt is alleviated by surface coating and modulation of electronic configuration of Pt. Due to the comparably lower interaction with phosphate than Pt and considerable catalytic activity toward ORR, non-Pt and metal-free electrocatalyst have also been systematically investigated as HT-PEMFCs cathodic electrocatalyst. Finally, the perspectives and challenges in HT-PEMFCs have been discussed.

Proton exchange membrane fuel cells (PEMFCs) generate electricity from hydrogen, powering a range of applications while emitting nothing but water. Therefore, PEMFCs are regarded as an environmentally friendly alternative to internal combustion engines for the future. Nevertheless, the high cost and scarcity of platinum (Pt) sources prevent the widespread adoption of fuel cells. With the development of fuel cell manufacturing technology, current Pt utilization has increased to a relatively high level of 0.2 g·kW^-1 in PEMFCs. However, according to the PGM market report from Johnson Matthey (2020), current Pt utilization in fuel cells is still too low to meet the need for its large-scale application in the automotive industry, unless the Pt utilization can be further reduced to an ultra-low level (0.01 g·kW^-1). Therefore, higher Pt mass activity and higher Pt utilization must be realized in membrane electrode assemblies (MEA) to achieve ultra-low Pt loadings and a reduced Pt usage. Many key variables affect the performance of MEA, such as the activity of electrocatalysts, conductivity and distribution of ionomers, gas diffusion in carbon papers, and the thickness of the proton exchange membrane. For example, a wide variety of highly promising catalysts have been developed, such as shape-controlled Pt nanocrystals, Pt alloy/de-alloys, core-shells, the synergetic effect of active supports, single atom/single-atom layer catalysts for improving the utilization of Pt, and anti-poisoning catalysts. However, the super-high activity of a Pt catalyst is elusive in a real fuel cell because of the lack of a fundamental understanding of the reaction interface structure and mass transfer properties in real cells. For instance, the recently developed Pt-Ni nanoframes that exhibited an extremely high mass activity of 5.7 A·mg^-1 for the oxygen reduction reaction (ORR) in a liquid half-cell only showed about one-tenth the activity in a real fuel cell (0.76 A·mg^-1 Pt at 0.90 V). To achieve widespread adoption of Pt in fuel cells, we urgently need to explore new combinations of electrocatalysts, ionomers, gas diffusion layers, and proton exchange membranes. Taking into account all these factors, recent advances have enhanced the performance of MEA, such as a neural-network-like catalyst structure for higher Pt utilization, a highly order-structured with vertically aligned carbon nanotubes as a highly ordered catalyst layer that exhibits higher mass transfer efficiency, a novel anti-flooding electrode, a higher oxygen permeability and ionic conductivity ionomer, and an ultrathin MEA with low Pt loading that exhibits higher fuel cell output efficiency. This review mainly focuses on the recent progress in fuel cell cathode performance at ultra-low Pt loadings. To achieve the ultimate goal of Pt utilization (0.01 g·kW^-1), further efforts to accelerate this progress are urgently needed, including improving catalytic performance by using highly active and stable supports, decreasing the gas diffusion resistance, enhancing the water management in the catalytic layer, improving the anti-poisoning property, and establishing an integrated ultra-thin and low platinum film electrode.

Bipolar plates (BPs) are one of the key components of proton exchange membrane fuel cell (PEMFC) stacks. To ensure that such a stack operates stably, a BP needs to meet exhibit electrical conductivity, heat conduction, H₂ airtight, flexural strength, and durability. Based on these requirements, the BP should also be as thin as possible to reduce the overall cost of PEMFCs, while improving their volumetric energy density. A composite bipolar plate (CBP) exhibits the advantages of a low production cost, low processing difficulty, and corrosion resistance; it is produced using polymers and graphite as the main materials. Moreover, channel structures can be formed directly after a compression molding process. However, the trade-off that exists between electrical conductivity and flexural strength is a major challenge. The electrical conductivity of a CBP is realized through the network formed by graphite materials. Therefore, it not only depends on the filler concentration, but also on the network structure. At the same time, microstructures such as accumulation polymers and graphite/resin interface are directly related to the gas tightness and flexural strength of CBP. This review summarizes the conductive fillers and polymers that are commonly used for fabricating CBPs. The universal modification methods for both (fillers and polymers) are discussed, and a brief description of the conductive theoretical model has also been included. In addition, the advanced production technology of CBP is summarized, which includes the organization of the conductive network, elimination of the polymer on the plate surface, and preparation technology of the layered plates. The relationship between the production process and the performance of the plate was also analyzed. Some studies indicate that the conductive network can be optimized by combining kinds of carbon-based filler or electric field inducing, which could significantly promote the electrical conductivity of CBP. Flexural strength and H₂ permeation rates were increased by introducing carbon-based materials such as carbon fabric and graphite foil. The modification of the filler and polymer could facilitate their bonding with each other, which reduces agglomeration and increases the performance. It is worth noting that the structure had a notable influence on the performance of CBP, which was reflected in the filler/polymer interface or the hybrid layer structure. Based on this results, some ideas have been provided as the next steps that can be taken for the optimization and production of a CBP. We believe that the optimization of the CBP structure will be the key point for its future research.

Fuel cells are clean, efficient energy conversion devices that produce electricity from chemical energy stored within fuels. The development of fuel cells has significantly progressed over the past decades. Specifically, polymer electrolyte fuel cells, which are representative of proton exchange membrane fuel cells (PEMFCs), exhibit high efficiency, high power density, and quick start-up times. However, the high cost of PEMFCs, partially from the Pt-based catalysts they employ, hinders their diverse applicability. Hydroxide exchange membrane fuel cells (HEMFCs), which are also known as alkaline polymer electrolyte fuel cells (APEFCs), alkaline anion-exchange membrane fuel cells (AAEMFCs), anion exchange membrane fuel cells (AEMFCs), or alkaline membrane fuel cells (AMFCs), have attracted much attention because of their capability to use non-Pt electrocatalysts and inexpensive bipolar plates. The HEMFCs are structurally similar to PEMFCs but they use a polymer electrolyte that conducts hydroxide ions, thus providing an alkaline environment. However, the relatively sluggish kinetics of the hydrogen oxidation reaction (HOR) inhibit the practical application of HEMFCs. The anode catalyst loading needed for HEMFCs to achieve high cell performance is larger than that required for other fuel cells, which substantially increases the cost of HEMFCs. Therefore, low-cost, highly active, and stable HOR catalysts in the alkaline condition are greatly desired. Here, we review the recent achievements in developing such HOR catalysts. First, plausible HOR mechanisms are explored and HOR activity descriptors are summarized. The HOR processes are mainly controlled by the binding energy between hydrogen and the catalysts, but they may also be influenced by OH adsorption, interfacial water adsorption, and the potential of zero (free) charge. Next, experimental methods used to elevate HOR activities are introduced, followed by HOR catalysts reported in the literature, including Pt-, Ir-, Pd-, Ru-, and Ni-based catalysts, among others. HEMFC performances when employing various anode catalysts are then summarized, where HOR catalysts with platinum-group metals exhibited the highest HEMFC performance. Although the Ni-based HOR catalyst activity was higher than those of other non-precious metal-based catalysts, they showed unsatisfactory performance in HEMFCs. We further analyzed HEMFC performances while considering anode catalyst cost, where we found that this cost can be reduced by using recently developed, non-Pt HOR catalysts, especially Ru-based catalysts. In fact, an HEMFC using a Ru-based HOR catalyst showed an anode catalyst cost-based performance similar to that of PEMFCs, making the HEMFC promising for use in practical applications. Finally, we proposed routes for developing future HOR catalysts for HEMFCs.

Proton exchange membrane fuel cells (PEMFCs) are considered as one of the most promising energy conversion devices owing to their high power density, high energy conversion efficiency, environment-friendly merit, and low operating temperature. In the cathodic oxygen reduction reaction and anodic small-molecule oxidation reactions, Pt shows excellent catalytic activity. However, several factors limit the practical application of Pt nanoparticles in fuel cells, such as the high price of Pt, easy agglomeration during long-term cycling, and limited electrocatalytic performance. Alloying Pt with 3d-transition metal produces ligand and strain effects, which reduces the center of Pt-d band and weakens the binding strength of oxygen species, thereby improving the catalytic activity and reducing the cost. However, the performance of fuel cells degrades seriously because the transition metals tend to dissolve in acidic electrolytes. The disordered alloy transformed into ordered intermetallic nanoparticles can prevent the dissolution of transition metals. Ordered intermetallics have highly ordered atomic arrangements and strong Pt(5d)-M(3d) orbital interactions, which result in excellent stability in both acidic and alkaline electrolytes. Ordered intermetallic nanoparticles have attracted significant attention owing to their excellent electrocatalytic activity and stability, which can be attributed to controllable composition and structure. Pd has a similar electronic structure and lattice parameters to Pt, and has thus attracted significant attention. Several Pd-based ordered intermetallics have been synthesized, and they exhibit sufficient catalytic performance. This review discusses the recent progress in noble metal-based ordered intermetallic electrocatalysts based on the research status of our group over the years. First, the structural characteristics and characterization methods of ordered intermetallic nanoparticles are introduced, exhibiting approaches to distinguish ordered and disordered phases. Then, the controllable preparation of ordered nanoparticles is highlighted, including thermal annealing and direct liquid phase synthesis. The migration and interdiffusion of atoms in the ordering process is very difficult. High-temperature thermal annealing is the most commonly used method for preparing intermetallics, which can precisely control the composition and atomic ordered arrangement. However, thermal annealing can only produce thermodynamically stable spherical nanoparticles. Supports and coating layers are usually employed to prevent agglomeration of nanoparticles at high temperatures. Finally, the applications of ordered intermetallic nanoparticles in fuel cell electrocatalysts are reviewed, including the oxygen reduction reaction (ORR), hydrogen oxidation reaction (HOR), formic acid oxidation reaction (FAOR), methanol oxidation reaction (MOR), and ethanol oxidation reaction (EOR). In addition, the current challenges and future development directions of the catalysts are discussed and discussed to provide new ideas for the development of fuel cell electrocatalysts.

Proton-exchange membrane fuel cells (PEMFCs) directly transform chemical energy into electrical energy with high energy density and zero carbon emissions, thereby offering a clean energy alternative for fossil fuels and vehicle electrification. However, the existing PEMFCs rely on Pt-based catalysts, especially at the cathode side wherein the sluggish oxygen reduction reaction (ORR) takes place, resulting in high cost and limiting their commercial applications. Therefore, there is a strong interest in developing platinum group metal-free (PGM-free) PEMFCs. Although impressive advancements have been made since metal-nitrogen-carbon (M-N-C) catalysts have been developed as promising candidates for low-cost cathode catalysts, PGM-free PEMFCs still suffer from insufficient activity and durability. Owing to the intricate structure of the tri-phase interface and mass transport limitation, the M-N-C catalysts with high ORR activity in rotating disk electrode (RDE) tests still suffer from unexpected problems such as showing low activity and undesired rapid degradation process in real fuel cell conditions. Therefore, a comprehensive understanding of the active sites and influences of the M-N-C catalyst structure and cathode structure on the PEMFC performance will promote the development of PGM-free PEMFCs. Herein, with an aim to increase the activity and durability of PEMFCs based on M-N-C catalysts, we summarize the recent progress in understanding the active sites of M-N-C catalysts and the relationships between the structures of catalysts/catalyst layers and device performances. At the catalyst level, multiple delicately designed synthetic strategies suggest that attractive device performances can be obtained by tailoring the intrinsic activity and density of the catalyst active sites while engineering the porosity of catalysts to improve the utilization of active sites. Additionally, integrating the catalyst ink into the cathode catalyst layers in PGM-free PEMFC is pivotal for transforming the impressive ORR performance of catalysts in the RDE test to fuel cell performance. Accordingly, the recent advances in the enhancement of mass transfer and charge transport to achieve remarkable fuel cell performance were also included by rationally designing ionomer contents, catalyst morphology, and fabrication process of cathodic catalyst layers. Moreover, durability is the Achilles heel of PEMFCs with M-N-C catalysts, which is currently far behind the commercial requirements. The possible degradation mechanisms and the recent progress in seeking the corresponding solutions are also discussed in this review, including the decomposition of metal species, protonation of nitrogen sites, corrosion of carbon support, and micropore flooding. Based on these insights, the perspective is proposed by articulating open challenges and opportunities in materials innovations and device engineering with an aim to achieve practical M-N-C based PEMFCs.

High-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) have the unique advantages of fast electrode reaction kinetics, high CO tolerance, and simple water and thermal management at their operating temperature (140–200 ℃), which are considered as one of the important research directions of PEMFCs. Membrane electrode assemblies (MEAs), as the core component of HT-PEMFCs, are usually fabricated by sandwiching phosphoric acid (PA)-doped polymer membrane (HT-PEM) between two electrodes. Technically, high PA content is required in HT-PEMs to ensure fast proton conduction, since PA acts as a proton transport carrier, while a high content of PA decreases the interaction among polymer molecules, thus enhancing the movement of the polymer molecules and leading to a decrease in the mechanical strength of the polymer membranes. In addition, PA is driven into catalyst layers owing to capillary force caused by micropore structures, crack connectivity, and accessibility. The PA content in the electrodes is also affected by the hydrophilic/hydrophobic characteristics of the catalyst layers and the surface tension of the acid when it is in close contact with the catalyst layers. Furthermore, PA plays an important role in the construction of electrochemical triple-phase boundaries to promote electrochemical reactions in the catalyst layers. Simultaneously, as a liquid or "free molecule", the migration of PA may be accelerated by the current and the water produced, owing to the formation of charged phosphates or hydronium ions. This process encourages the redistribution of PA within the catalyst layers, and results in acid flooding of the catalytic layers and adsorption on the surface of the platinum catalyst, leading to increased mass transfer resistance for the gas reaction and reduced catalyst activity. Moreover, the increase in supplied absolute flow rate and the temperature elevation in the HT-PEMFC process could accelerate the evaporation of PA from the electrolyte membrane, resulting in a decrease in the stability of HT-PEMFC and corrosion of the metal end plate. Therefore, it is crucial to regulate the distribution and migration of PA in MEAs for the construction of HT-PEMFCs with high performance and stability. Hence, this paper reviews the research status of PA distribution in HT-PEM electrodes in recent years, and summarizes the corresponding regulations and optimization strategies as well as its future development trend.

Proton exchange membrane fuel cells (PEMFCs) have attracted significant attention owing to their high conversion efficiency, high power density, and low pollution. Their performance is mainly governed by the oxygen reduction reaction (ORR) occurring at the cathode. Owing to the sluggish kinetics of ORR, a large amount of electrocatalysts, i.e., platinum (Pt), is required to accelerate the reaction rate and improve the performance of PEMFCs for practical applications. The use of Pt electrocatalysts inevitably increases the cost, thereby hindering the commercialization of PEMFCs. In addition, the activity and stability of the commercial Pt/C catalyst are still insufficient. Therefore, advanced electrocatalysts with high activity, good stability, and low cost are urgently needed. To date, some theoretical models, especially d-band center theory, have been proposed and guided the search for next-generation electrocatalysts with higher ORR activity. Based on these theories, several strategies and catalysts, especially Pt-based alloy catalysts, have been developed to accelerate ORR and improve the fuel cell performance. For instance, Pt–Ni octahedral nanoparticles (NPs) electrocatalysts have achieved remarkable ORR activity, with one order of magnitude higher activity than that of commercial Pt/C. However, PEMFCs are usually operated at a high voltage (0.6–0.8 V) and an acidic electrolyte, where the transition metals (M) are easily oxidized and etched away. The electronic effect induced by the introduction of M would be eliminated due to the dissolution of transition metals and the agglomeration of NPs, leading to the decay of ORR activity. Therefore, the long-term stability of oxygen reduction catalysts and fuel cells remains highly challenging. It is crucial to design an efficient and highly stable ORR catalyst to promote the application of PEMFCs. Aiming to the stability issues of fuel cell cathode catalysts, the current review summarizes the principles, strategies, and approaches for improving the stability of Pt-based catalysts. First, we introduce thermodynamic and kinetic principles that affect the stability of catalysts. Thermodynamic (such as cohesive energy, alloy formation energy, and segregation energy) and kinetic parameters (such as vacancy formation and diffusion barrier) regarding the structural stability of catalysts significantly affect the metal dissolution and atomic diffusion processes. In addition, these parameters seem to be associated with chemical bond energy to some extent, which could be employed as a descriptor for the stability of catalysts. Later, we outline some representative strategies and methods for improving catalyst stability, namely elemental doping, atomic arrangement engineering, chemical or physical confinement, and supporting material design. Finally, a brief summary and future research perspectives are provided.

Layered graphitic carbon nitride (g-C₃N₄) is a typical polymeric semiconductor with an sp² π-conjugated system having great potential in energy conversion, environmental purification, materials science, etc., owing to its unique physicochemical and electrical properties. However, bulk g-C₃N₄ obtained by calcination suffers from a low specific surface area, rapid charge carrier recombination, and poor dispersion in aqueous solutions, which limit its practical applications. Controlling the size of g-C₃N₄ (e.g., preparing g-C₃N₄ nanosheets) can effectively solve the above problems. Compared with the bulk material, g-C₃N₄ nanosheets have a larger specific surface area, richer active sites, and a larger band gap due to the quantum confinement effect. As g-C₃N₄ has a layered structure with strong in-plane C-N covalent bonds and weak van der Waals forces between the layers, g-C₃N₄ nanosheets can be prepared by exfoliating bulk g-C₃N₄. Alternatively, g-C₃N₄ nanosheets can otherwise be obtained through the anisotropic assembly of organic precursors. Nevertheless, some of these methods have various limitations, such as high energy consumption, are time consuming, and have low yield. Accordingly, developing green and cost-effective exfoliation and preparation strategies for g-C₃N₄ nanosheets is necessary. Herein, the research progress of the exfoliation and preparation strategies (including the thermal oxidation etching process, the ultrasound-assisted route, the chemical exfoliation, the mechanical method, and the template method) for two-dimensional C₃N₄ nanosheets are introduced. Their features are systematically analyzed and the perspectives and challenges in the preparation of g-C₃N₄ nanosheets are discussed. This study emphasizes the following: (1) The preparation method of g-C₃N₄ nanosheets should be properly selected according to the practical application needs. Additionally, various strategies (such as chemical method and ultrasonic method) can be combined to exfoliate nanosheets from bulk g-C₃N₄; (2) More reasonable nano- or even subnanostructured g-C₃N₄ nanosheets should be continuously explored; (3) Novel modification strategies, such as defective engineering, heterojunction construction, and surface functional group regulation, should be introduced to improve the reactivity and selectivity of the g-C₃N₄ nanosheets; (4) The application of in situ characterization techniques (such as in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), electron spin resonance (ESR) spectroscopy, and Raman spectroscopy) should also be strengthened to monitor the detailed catalytic process and investigate the g-C₃N₄ nanosheet structure-efficiency relationship. (5) To gain a deeper understanding of the relationship between the macroscopic properties and the microscopic structure, the combination of theoretical calculations and experimental results should be strengthened, which will be beneficial for exploiting high-quality g-C₃N₄ nanosheets.

The use of fossil fuels has caused serious environmental problems such as air pollution and the greenhouse effect. Moreover, because fossil fuels are a non-renewable energy source, they cannot meet the continuously increasing demand for energy. Therefore, the development of clean and renewable energy sources is necessitated. Hydrogen energy is a clean, non-polluting renewable energy source that can ease the energy pressure of the whole society. The sunlight received by the Earth is 1.7× 10¹⁴ J in 1 s, which far exceeds the total energy consumption of humans in one year. Therefore, conversion of solar energy to valuable hydrogen energy is of significance for reducing the dependence on fossil fuels. Since Fujishima and Honda first reported on TiO₂ in 1972, it has been discovered that semiconductors can generate clean, pollution-free hydrogen through water splitting driven by electricity or light. Hydrogen generated through this approach can not only replace fossil fuels but also provide environmentally friendly renewable hydrogen energy, which has attracted considerable attention. Photoelectrochemical (PEC) water splitting can use solar energy to produce clean, sustainable hydrogen energy. Because the oxygen evolution reaction (OER) over a photoanode is sluggish, the overall energy conversion efficiency is considerably low, limiting the practical application of PEC water splitting. A cocatalyst is, thus, necessary to improve PEC water splitting performance. So far, the synthesis of first-row transition-metal-based (e.g., Fe, Co, Ni, and Mn) cocatalysts has been intensively studied. Iron is earth-abundant and less toxic than other transition metals, making it a good cocatalyst. In addition, iron-based compounds exhibit the properties of a semiconductor/metal and have unique electronic structures, which can improve electrical conductivity and water adsorption. Various iron-based catalysts with high activity have been designed to improve the efficiency of PEC water oxidation. This article briefly summarizes the research progress related to the structure, synthesis, and application of iron oxyhydroxides, iron-based layered double hydroxides, and iron-based perovskites and discusses the evaluation of the performance of these cocatalysts toward photoelectrochemical water oxidation.

Two-dimensional photocatalytic materials have potential applications in the fields of environmental purification and energy conversion owing to their rich surface active sites, unique geometric structures, adjustable electronic structures, and good photocatalytic activities. At present, the main two-dimensional photocatalytic materials include metal oxides, metal composite oxides, metal hydroxides, metal sulfides, bismuth-based materials, and non-metallic photocatalytic materials. The absorption of photons in bulk materials or nanoparticles is often limited by the transmittance and reflection at the grain boundary, while the two-dimensional structure can provide a large specific surface area and abundant surface low-coordination atoms to obtain more UV visible light. In addition, the smaller atomic thickness of two-dimensional photocatalytic materials can shorten the carrier migration distance. Thus, in two-dimensional photocatalytic materials, the carriers generated in the interior migrate to the surface faster than that in the bulk materials, which can reduce the recombination of photogenerated carriers and facilitate the photocatalytic reaction. For the surface redox reaction, the two-dimensional structure can provide more abundant surface-active sites to accelerate the reaction process. Additionally, when the thickness is reduced to the atomic scale, the escape energy of atoms is relatively small, thereby increasing the surface defects, which is helpful for the adsorption and activation of target molecules. Thus, the synthesis methods and performance enhancement strategies of two-dimensional photocatalytic materials have been developed rapidly. The former strategies mainly focus on the adjustment of morphology and geometric structure characteristics, which cannot fully meet the design requirements of efficient and stable photocatalysts. The photocatalytic performance and stability can be improved by surface design to construct abundant active sites and adjust the electronic structure. Research on the reaction mechanism of photocatalysis can help us understand the demand for photocatalytic structure characteristics in different reactions, thereby guiding the design of photocatalysts. In this paper, the advances in surface design and electronic structure regulation strategies of two-dimensional photocatalytic materials are reviewed from three aspects: light absorption; charge separation; and active sites, including element doping, heterojunction design, defect construction, single atom modification, and plasmonic metal loading. The effects on the reaction mechanism for typical air pollutant purification by regulating the electronic structure of two-dimensional photocatalytic materials are summarized. Finally, the problems and challenges associated with the development of two-dimensional photocatalytic materials are analyzed and discussed.

Since the pioneering work on polychlorinated biphenyl photodegradation by Carey in 1976, photocatalytic technology has emerged as a promising and sustainable strategy to overcome the significant challenges posed by energy crisis and environmental pollution. In photocatalysis, sunlight, which is an inexhaustible source of energy, is utilized to generate strongly active species on the surface of the photocatalyst for triggering photo-redox reactions toward the successful removal of environmental pollutants, or for water splitting. The photocatalytic performance is related to the photoabsorption, photoinduced carrier separation, and redox ability of the semiconductor employed as the photocatalyst. Apart from traditional and noble metal oxide semiconductors such as P25, bismuth-based compounds, and Pt-based compounds, 2D g-C₃N₄ is now identified to have enormous potential in photocatalysis owing to the special π-π conjugated bond in its structure. However, some inherent drawbacks of the conventional g-C₃N₄, including the insufficient visible-light absorption ability, fast recombination of photogenerated electron-hole pairs, and low quantum efficiency, decrease its photocatalytic activity and limit its application. To date, various strategies such as heterojunction fabrication, special morphology design, and element doping have been adopted to tune the physicochemical properties of g-C₃N₄. Recent studies have highlighted the potential of defect engineering for boosting the light harvesting, charge separation, and adsorption efficiency of g-C₃N₄ by tailoring the local surface microstructure, electronic structure, and carrier concentration. In this review, we summarize cutting-edge achievements related to g-C₃N₄ modified with classified non-external-caused defects (carbon vacancies, nitrogen vacancies, etc.) and external-caused defects (doping and functionalization) for optimizing the photocatalytic performance in water splitting, removal of contaminants in the gas phase and wastewater, nitrogen fixation, etc. The distinctive roles of various defects in the g-C₃N₄ skeleton in the photocatalytic process are also summarized. Moreover, the practical application of 2D g-C₃N₄ in air pollution control is highlighted. Finally, the ongoing challenges and perspectives of defective g-C₃N₄ are presented. The overarching aim of this article is to provide a useful scaffold for future research and application studies on defect-modulated g-C₃N₄.

The CO₂ level in the atmosphere has been increasing since the industrial revolution owing to anthropogenic activities. The increased CO₂ level has led to global warming and also has detrimental effects on human beings. Reducing the CO₂ level in the atmosphere is urgent for balancing the carbon cycle. In this regard, reduction in CO₂ emission and CO₂ storage and usage are the main strategies. Among these, CO₂ usage has been extensively explored, because it can reduce the CO₂ level and simultaneously provide opportunities for the development in catalysts and industries to convert CO₂ as a carbon source for preparing valuable products. However, transformation of CO₂ to other chemicals is challenging owing to its thermodynamic and kinetic stabilities. Among the CO₂ utilization techniques, electrochemical CO₂ reduction (ECR) is a promising alternative because it is generally conducted under ambient conditions, and water is used as the economical hydrogen source. Moreover, ECR offers a potential route to store electrical energy from renewable sources in the form of chemical energy, through generation of CO₂ reduction products. To improve the energy efficiency and viability of ECR, it is important to decrease the operational overpotential and maintain large current densities and high product selectivities; the development of efficient electrocatalysts is a critical aspect in this regard. To date, many kinds of materials have been designed and studied for application in ECR. Among these materials, metal oxide-based materials exhibit excellent performance as electrocatalysts for ECR and are attracting increasing attention in recent years. Investigation of the mechanism of reactions that involve metallic electrocatalysts has revealed the function of trace amount of oxidized metal species—it has been suggested that the presence of metal oxides and metal-oxygen bonds facilitates the activation of CO₂ and the subsequent formation and stabilization of the reaction intermediates, thereby resulting in high efficiency and selectivity of the ECR. Although the stability of metal oxides is a concern as they are prone to reduction under a cathodic potential, the catalytic performance of metal oxide-based catalysts can be maintained through careful designing of the morphology and structure of the materials. In addition, introducing other metal species to metal oxides and fabricating composites of metal oxides and other materials are effective strategies to achieve enhanced performance in ECR. In this review, we summarize the recent progress in the use of metal oxide-based materials as electrocatalysts and their application in ECR. The critical role, stability, and structure-performance relationship of the metal oxide-based materials for ECR are highlighted in the discussion. In the final part, we propose the future prospects for the development of metal oxide-based electrocatalysts for ECR.

NH₃ plays an important role in modern society as an essential building block in the manufacture of fertilizers, aqueous ammonia, plastics, explosives, and dyes. Additionally, it is regarded as a green alternative fuel, owing to its carbon-free nature, large hydrogen capacity, high energy density, and easy transportation. The Haber-Bosch process plays a dominant role in global NH₃ synthesis; however, it involves high pressure and temperature and employs N₂ and H₂ as feeding gases, thus suffering from high energy consumption and substantial CO₂ emission. As a promising alternative to the Haber-Bosch process, electrochemical N₂ reduction enables sustainable and environmentally benign NH₃ synthesis under ambient conditions. Moreover, its applied potential is compatible with intermittent solar, wind, and other renewable energies. However, efficient electrocatalysts are required to drive N₂-to-NH₃ conversion because of the extremely inert N≡N bond. To date, significant efforts have been made to explore high-performance catalysts with high efficiency and selectivity. Generally, noble-metal catalysts exhibit efficient performance for the NRR, but their scarcity and high cost limit their large-scale application. Therefore, considerable attention has been focused on earth-abundant transition-metal (TM) catalysts that can use empty or unoccupied orbitals to accept the lone-pair electrons of N₂, while donating the abundant d-orbital electrons to the antibonding orbitals of N₂. However, these catalysts may release metal ions, leading to environmental pollution. Most of these TM electrocatalysts may also favor the formation of TM—H bonds, facilitating the hydrogen evolution reaction (HER) during the electrocatalytic reaction. Recent years have seen a surge in the exploration of metal-free catalysts (MFCs). MFCs mainly include carbon-based catalysts (CBCs) and some boron-based and phosphorus-based catalysts. Generally, CBCs exhibit a porous structure and high surface area, which are favorable for exposing more active sites and providing rich accessible channels for mass/electron transfer. Moreover, the Lewis acid sites of most metal-free compounds could accept the lone-pair electron of N₂ and adsorb N₂ molecules by forming nonmetal—N bonds, further widening their potential for electrocatalytic NRR. Compared with metal-based catalysts, the occupied orbitals of metal-free catalysts can only form covalent bonds or conjugated π bonds, hindering electron donation from the electrocatalyst to N₂ and molecular activation. In this review, we summarize the recent progress in the design and development of metal-free electrocatalysts (MFCs) for the ambient NRR, including carbon-based catalysts, boron-based catalysts, and phosphorus-based catalysts. In particular, heteroatom doping (N, O, S, B, P, F, and co-dopants), organic polymers, carbon nitride, and defect engineering are highlighted. We also discuss strategies to boost NRR performance and provide an outlook on the development perspectives of MFCs.

As one of the most promising hydrogen production technologies, electrochemical water splitting is an effective measure for solving environmental pollution and energy crises. However, the slow kinetics and high overpotential of the oxygen evolution reaction (OER) are the primary deterrents for improving the efficiency of water splitting devices. Iridium- and ruthenium-based noble metal catalysts are extremely expensive, which limits the industrial-scale development of this technology. Therefore, the development of oxygen evolution catalysts with high activity, excellent stability, and low costs is significantly important for water splitting technologies. Nickel-based materials meet the requirements of high abundance, cost-effectiveness, and high activity. In recent years, nickel-based metal organic frameworks (Ni-based MOFs) have attracted increasing research attention owing to their diverse and tunable topological structures and large specific surface areas. Furthermore, the mesoporous three-dimensional structure of MOFs can promote the diffusion of reactants, rendering them excellent candidates for catalytic applications. In order to utilize the advantages of Ni-MOFs more efficiently, the following methods are usually used to improve their catalytic performance. Owing to their unique properties, metal nodes can be replaced without affecting the MOF skeleton. As iron series metals, Co and Fe doping show unique catalytic activity and structural stability due to the synergistic effect between metal centers. Further, Ni-MOFs can simultaneously be used as precursors for oxidation, phosphating, or vulcanization to obtain Ni-MOF derivatives with different components. Among them, high-temperature carbonization treatment can make use of abundant organic ligands of Ni-MOFs to form a partially graphitized carbon-based framework, thereby augmenting conductivity, preventing the aggregation and corrosion of transition metals, and improving the overall support strength. The catalytic performance of oxygen production can be further improved by directly growing the Ni-MOFs on the substrate and introducing other active substances or conductive materials. Herein, the latest developments of Ni-based MOFs and their derivatives have been reviewed with regard to their utilization in OER catalysis, including nickel oxides, nickel hydroxides, nickel phosphides, nickel sulfides, and carbon composite materials. First, the mechanism and measurement criteria of the OER are briefly introduced. Second, the structures of several typical Ni-based MOFs (MOF-74, MILs, PBAs, and ZIFs) and their preparation methods are described. Subsequently, recent advances in the application of Ni-based MOFs and their derivatives in the OER are discussed, with an emphasis on materials design strategies and catalytic mechanisms. Finally, the main challenges and opportunities in this field are proposed.

The implementation of clean energy techniques, including clean hydrogen generation, use of solar-driven photovoltaic hybrid systems, photochemical heat generation as well as thermoelectric conversion, is crucial for the sustainable development of our society. Among these promising techniques, electrocatalysis has received significant attention for its ability to facilitate clean energy conversion because it promotes a higher rate of reaction and efficiency for the associated chemical transformations. Noble-metal-based electrocatalysts typically show high activity for electrochemical conversion processes. However, their scarcity and high cost limit their applications in electrocatalytic devices. To overcome this limitation, binary catalysts prepared by alloying with transition metals can be used. However, optimization of the activity of the binary catalysts is considerably limited because of the presence of the miscibility gap in the phase diagram of binary alloys. The activity of binary electrocatalysts can be attributed to the adsorption energy of molecules and intermediates on the surface. High-entropy alloys (HEAs), which consist of diverse elements in a single NP, typically exhibit better physical and/or chemical properties than their single-element counterparts, because of their tunable composition and inherent surface complexity. Further, HEAs can improve the performance of binary electrocatalysts because they exhibit a near-continuous distribution of adsorption energy. Recently, HEAs have gained considerable attention for their application in electrocatalytic reactions. This review summarizes recent research advances in HEA nanostructures and their application in the field of electrocatalysis. First, we introduce the concept, structure, and four core effects of HEAs. We believe that this part will provide the basic information about HEAs. Next, we discuss the reported top-down and bottom-up synthesis strategies, emphasizing on the carbothermal shock method, nanodroplet-mediated electrodeposition, fast moving bed pyrolysis, polyol process, and dealloying. Other methods such as combinatorial co-sputtering, ultrashort-pulsed laser ablation, ultrasonication-assisted wet chemistry, and scanning-probe block copolymer lithography are also highlighted. Among these methods, wet chemistry has been reported to be effective for the formation of nano-scale HEAs because it facilitates the concurrent reduction of all metal precursors to form solid-solution alloys. Next, we present the theoretical investigation of HEA nanocatalysts, including their thermodynamics, kinetic stability, and adsorption energy tuning for optimizing their catalytic activity and selectivity. To elucidate the structure–property relationship in HEAs, we summarize the research progress related to electrocatalytic reactions promoted by HEA nanocatalysts, including the oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, methanol oxidation reaction, and CO₂ reduction reaction. Finally, we discuss the challenges and various strategies toward the development of HEAs.

Currently, because of the worldwide over-exploitation and consumption of fossil fuels, energy crisis and environmental pollution are becoming more prominent. Hence, the production and utilization of clean energy such as hydrogen are crucial. As significant electrochemical reactions in energy conversion devices, the oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and oxygen reduction reaction (ORR) have garnered considerable attention. However, the sluggish kinetics of these reactions, especially of the OER and ORR because of the multiple electron transfer steps, and the inevitable usage of noble metal catalysts (such as those based on Pt for HER/ORR and Ru/Ir for HER/OER) are the bottlenecks to realizing energy conversion devices, including overall water-splitting electrolyzers, fuel cells, and metal-air batteries. Therefore, the development of efficient non-precious metal catalysts is imperative. Transition metal nitrides (TMNs) have been recently studied and shown to exhibit high catalytic activity because of their ability to alter the electronic structure of host metals, specifically the downshift of the d-band center, the contraction of the filled state, and the broadening of the unfilled state. This high activity is attributed to the optimization of the adsorption energy between metals and adsorbates. In addition, metallic bonding in TMNs increases the conductivity of the catalysts. Thus, in this review, we focus on the latest developments in TMNs and their application as high-activity and high-stability electrocatalysts for water splitting and in fuel cells and zinc-air batteries. First, the origin of the high activity of TMNs is explained with the help of the d-band theory. The effect of nitrogen on TMNs, such as in terms of the location in the crystal structure, is briefly discussed. The preparation strategies for TMNs, including physical and chemical methods as well as the modification techniques such as doping, changing carrier properties, and defect construction, are outlined. Next, we summarize the applications of TMNs as an electrocatalyst for the HER, OER, and ORR. At the same time, to explain the bifunctional catalytic activity of TMNs, we discuss the modification strategies for single-metal-based nitrides, such as doping with other highly active atoms to adjust the electronic structure and increase the catalytic activities as well as using coupling materials with different catalytic selectivities to construct heterostructures. Finally, we discuss the challenges and development approaches for realizing the electrocatalytic applications of TMNs, such as through further improvement in catalytic activity, and for facilitating in-depth understanding of electrocatalytic processes through in situ characterization to reveal the electrocatalytic mechanism of TMNs. Undoubtedly, this review will promote the application of TMNs in the field of electrocatalysis.

To fulfill the demands of green and sustainable energy, the production of novel catalysts for different energy conversion processes is critical. Owing to the intriguing advantages of the intrinsic active species, tunable crystal structure, remarkable chemical and physical properties, and good stability, metal-organic frameworks (MOFs) have been extensively investigated in various electrochemical energy conversions, such as the CO₂ reduction reaction, N₂ reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, and oxygen reduction reaction. More importantly, it is feasible to change the chemical environments, pore sizes, and porosity of MOFs, which will theoretically facilitate the diffusion of reactants across the open porous networks, thereby improving the electrocatalytic performance. However, owing to the high energy barriers of charge transfer and limited free charge carriers, most MOFs show poor electrical conductivity, thus limiting their diverse applications. As reported previously, MOFs were used as a porous substrate to confine the growth of nanoparticles or co-doped electrocatalysts after annealing. The conductive MOFs can combine the advantages of conventional MOFs with electronic conductivity, which significantly enhance the electrocatalytic performance. In addition, conductive MOFs can achieve conductivity via electronic or ionic routes without post-annealing treatment, thereby extending their potential applications. Different synthesis strategies have recently been developed to endow MOFs with electrical conductivity, such as post-synthesis modification, guest molecule introduction, and composite formatting. The performance of conductive MOFs can even outperform those of commercial RuO₂ catalysts or Pt-group catalysts. However, it is difficult to endow most MOFs with high conductivity. This review summarizes the mechanisms of constructing conductive MOFs, such as redox hopping, through-bond pathways, through-space pathways, extended conjugation, and guest-promoted transport. Synthetic methods, including hydro/solvothermal synthesis and interface-assisted synthesis, are introduced. Recent advances in the use of conductive MOFs as heterogeneous catalysts in electrocatalysis have been comprehensively elucidated. It has been reported that conductive MOFs can demonstrate considerable catalytic activity, selectivity, and stability in different electrochemical reactions, revealing the immense potential for future displacement of Pt-group catalysts. Finally, the challenges and opportunities of conductive MOFs in electrocatalysis are discussed. Based on systematic synthesis strategies, more conductive MOFs can be constructed for electrocatalytic reactions. In addition, the morphology and structure of conductive MOFs, which can change the electrochemical accessibility between substrates and MOFs, are also crucial for catalysis, and thus, they should be extensively studied in the future. It is believed that a breakthrough for high-performance conductive MOF-based electrocatalysts could be achieved.

Titania (TiO₂) has been among the most widely investigated and used metal oxides over the past years, as it has various functional applications. Extensive research into TiO₂ and industrial interest in this material have been triggered by its high abundance, excellent corrosion resistance, and low cost. To improve the activity of TiO₂ in heterogeneous catalytic reactions, noble metals are used to accelerate the reactions. However, in the case of nanoparticles supported on TiO₂, the active sites are usually limited to the peripheral sites of the noble metal particles or at the interface between the particle and the support. Thus, highly dispersed single metal atoms are desired for the effective utilization of precious noble metals. The study of oxide-supported isolated atoms, the so-called single-atom catalysts (SACs), was pioneered by Zhang's group. The high dispersion of precious noble metals results helps reduce the cost associated with catalyst preparation. Because of the presence of active centers as single atoms, the deactivation of metal atoms during the reaction, e.g., by coking for large agglomerates, is retarded. The unique coordination environment of the noble metal center provides special sites for the reaction, consequently increasing the selectivity of the reaction, including the enantioselectivity and stereoselectivity. Hence, supported SACs can bridge homogenous and heterogeneous reactions in solution as they provide selective reaction sites and are recyclable. Moreover, owing to the high site homogeneity of the isolated metal atoms, SACs are ideal models for establishing the structure-activity relationships. The present review provides an overview of recent works on the synthesis, characterization, and photocatalytic applications of SACs (Pt₁, Pd₁, Ir₁, Rh₁, Cu₁, Ru₁) supported on TiO₂. The preparation of single atoms on TiO₂ includes the creation of surface defective sites, surface modification, stabilization by high-temperature shockwave treatment, and metal-ligand self-assembly. Conventional characterization methods are categorized as microscopic imaging and spectroscopic methods, such as aberration-corrected scanning transmission electron microscopy (STEM), scanning tunneling microscopy (STM), extended X-ray absorption fine structure analysis (EXAFS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). We attempted to address the critical factors that lead to the stabilization of single-metal atoms on TiO₂, and elucidate the mechanism underlying the photocatalytic hydrogen evolution and CO₂ reduction. Although many fascinating applications of TiO₂-supported SACs in photocatalysis could only be addressed superficially and in a referencing manner, we hope to provide interested readers with guidelines based on the wide literature, and more specifically, to provide a comprehensive overview of TiO₂-supported SACs.

Solar energy is the largest renewable energy source in the world and the primary energy source of wind energy, tidal energy, biomass energy, and fossil fuel. Photocatalysis technology is a sunlight-driven chemical reaction process on the surface of photocatalysts that can generate H₂ from water, decompose organic contaminants, and reduce CO₂ into organic fuels. As a metal-free polymeric material, graphite-like carbon nitride (g-C₃N₄) has attracted significant attention because of its special band structure, easy fabrication, and low costs. However, some bottlenecks still limit its photocatalytic performance. To date, numerous strategies have been employed to optimize the photoelectric properties of g-C₃N₄, such as element doping, functional group modification, and construction of heterojunctions. Remarkably, these modification strategies are strongly associated with the surface behavior of g-C₃N₄, which plays a key role in efficient photocatalytic performance. In this review, we endeavor to provide a comprehensive summary of g-C₃N₄-based photocatalysts prepared through typical surface modification strategies (surface functionalization and construction of heterojunctions) and elaborate their special light-excitation and response mechanism, photo-generated carrier transfer route, and surface catalytic reaction in detail under visible-light irradiation. Moreover, the potential applications of the surface-modified g-C₃N₄-based photocatalysts for photocatalytic H₂ generation and reduction of CO₂ into fuels are summarized. Finally, based on the current research, the key challenges that should be further studied and overcome are highlighted. The following are the objectives that future studies need to focus on: (1) Although considerable effort has been made to develop a surface modification strategy for g-C₃N₄, its photocatalytic efficiency is still too low to meet industrial application standards. The currently obtained solar-to‑hydrogen (STH) conversion efficiency of g-C₃N₄ for H₂ generation is approximately 2%, which is considerably lower than the commercial standards of 10%. Thus, the regulation of the surface/textural properties and electronic band structure of g-C₃N₄ should be further elucidated to improve its photocatalytic performance. (2) Significant challenges remain in the design and construction of g-C₃N₄-based S-scheme heterojunction photocatalysts by facile, low-cost, and reliable methods. To overcome the limitations of conventional heterojunctions thoroughly, a promising S-scheme heterojunction photocatalytic system was recently reported. The study further clarifies the charge transfer route and mechanism during the catalytic process. Thus, the rational design and synthesis of g-C₃N₄-based S-scheme heterojunctions will attract extensive scientific interest in the next few years in this field. (3) First-principle calculation is an effective strategy to study the optical, electrical, magnetic, and other physicochemical properties of surface strategy modified g-C₃N₄, providing important information to reveal the charge transfer path and intrinsic catalytic mechanism. As a result, density functional theory (DFT) computation will be paid increasing attention and widely applied in surface-modified g-C₃N₄-based photocatalysts.

Recently, extensive studies have been carried out to synthesize spherical microassemblies with hollow interiors and specific surface functionalizations, which usually exhibit fascinating enhanced or emerging properties and have promising applications in catalysis, photocatalysis, energy conversion and storage, biomedical applications, etc. With particular emphasis on the results obtained mainly by the authors' research group, this review provides a brief summary of the recent progress on the fabrication and potential photocatalytic applications of fluorinated TiO₂ porous hollow microspheres(F-TiO₂ PHMs). The synthesis strategies for F-TiO₂ PHMs include a simplified two-step templating method and template-free method based on the fluoride-mediated self-transformation(FMST) mechanism. Compared to the two-step templating method, the template formation, coating, and removal steps for the FMST method are programmatically proceeded in "black-box"-like one-pot reactions without additional manual steps. The four underlying steps involved in the fabrication of F-TiO₂ PHMs through the FMST pathway, nucleation, self-assembly, surface recrystallization, and self-transformation, are presented. By controlling these four steps in the FMST pathway, F-TiO₂ PHMs can be successfully fabricated with a high yield by a simple one-pot hydrothermal treatment. The multi-level microstructural characteristics(including the interior cavity and hierarchical porosity) and compositions of hollow TiO₂ microspheres as well as the primary building blocks can be well tailored. The unique superstructures of the F-TiO₂ PHM photocatalysts provide advantages for photocatalytic applications by improving the light harvesting, mass transfer, and membrane antifouling. In addition, the in situ-introduced surface fluorine species during the formation of F-TiO₂ PHMs provide significant surface fluorination effects, which are not only favorable for the adsorption and activation of reactant molecules, but also beneficial for surface trapping and interfacial transfer of photo-excited electrons and holes. Moreover, the porous hollow superstructures exhibit considerably better compatibility and tolerance to guest modifications, and thus the photocatalytic performances of F-TiO₂ PHMs can be increased by synergetic host and guest modifications, such as ion doping, group functionalization, and nanoparticle loading. The light-harvesting range and intensity can be increased, the charge recombination can be reduced, mass transfer and adsorption can be promoted, and the surface reactivity can be tuned by introducing specific surface functionalities or nanoparticular cocatalysts. Consequently, the entire photocatalytic process can be systematically modulated to optimize the overall photocatalytic performance. The as-prepared F-TiO₂ PHMs typically integrate the merits of interior cavity, hierarchical porosity, and surface fluorination and are open to synergetic host-guest modifications, which provides abundant compositional/structural parameters and specific physicochemical properties for systematically modulating the interconnected photocatalytic processes and promising potential photocatalytic applications.

Inspired by the photosynthesis of green plants, various artificial photosynthetic systems have been proposed to solve the energy shortage and environmental problems. Water photosplitting, carbon dioxide photoreduction, and nitrogen photofixation are the main systems that are used to produce solar fuels such as hydrogen, methane, or ammonia. Although conducting artificial photosynthesis using man-made semiconducting materials is an ideal and potential approach to obtain solar energy, constructing an efficient photosynthetic system capable of producing solar fuels at a scale and cost that can compete with fossil fuels remains challenging. Therefore, exploiting the efficient and low-cost photocatalysts is crucial for boosting the three main photocatalytic processes (light-harvesting, surface/interface catalytic reactions, and charge generation and separation) of artificial photosynthetic systems. Among the various photocatalysts developed, the Z-scheme heterojunction composite system can increase the light-harvesting ability and remarkably suppress charge carrier recombination; it can also promote surface/interface catalytic reactions by preserving the strong reductive/oxidative capacity of the photoexcited electrons/holes, and therefore, it has attracted considerable attention. The continuing progress of Z-scheme nanostructured heterojunctions, which convert solar energy into chemical energy through photocatalytic processes, has witnessed the importance of these heterojunctions in further improving the overall efficiency of photocatalytic reaction systems for producing solar fuels. This review summarizes the progress of Z-scheme heterojunctions as photocatalysts and the advantages of using the direct Z-scheme heterojunctions over the traditional type Ⅱ, all-solid-state Z-schemel, and liquid-phase Z-scheme ones. The basic principle and corresponding mechanism of the two-step excitation are illustrated. In particular, applications of various types of Z-scheme nanostructured materials (inorganic, organic, and inorganic-organic hybrid materials) in photocatalytic energy conversion and different controlling/engineering strategies (such as extending the spectral absorption region, promoting charge transfer/separation and surface chemical modification) for enhancing the photocatalytic efficiency in the last five years are highlighted. Additionally, characterization methods (such as sacrificial reagent experiment, metal loading, radical trapping testing, in situ X-ray photoelectron spectroscopy, photocatalytic reduction experiments, Kelvin probe force microscopy, surface photovoltage spectroscopy, transient absorption spectroscopy, and theoretical calculation) of the Z-scheme photocatalytic mechanism, and the assessment criteria and methods of the photocatalytic performance are discussed. Finally, the challenges associated with Z-scheme heterojunctions and the possible growing trend are presented. We believe that this review will provide a new understanding of the breakthrough direction of photocatalytic performance and provide guidance for designing and constructing novel Z-scheme photocatalysts.

At present, more than 80% of the world's energy demand is fulfilled by the burning of fossil fuels, which has caused the production of a large amount of greenhouse gases, leading to global warming and damage to the environment. The high consumption of fossil fuels every year causes the energy crisis to become increasingly serious. Finding a sustainable and pollution-free energy source is therefore essential. Among all forms of energy sources, solar energy is preferred because of its cleanliness and inexhaustible availability. The energy provided by one year of sunlight is more than 100 times the total energy in known fossil fuel reserves worldwide; however, the extent of solar energy currently used by mankind each year is minute; thus developments in solar energy are imperative. To address the urgent need for a renewable energy supply and to solve environmental problems, a variety of technologies in the field of photocatalysis have been developed. Photocatalytic technology has attracted significant attention because of its superior ability to convert clean solar energy into chemical fuels. Among the photocatalytic materials emerging in an endless stream, perovskite oxide, with the general formula of ABO₃, has great potential in the fields of solar cells and photocatalysis as each site can be replaced by a variety of cations. Furthermore, owing to its unique properties such as high activity, robust stability, and facile structure adjustment, perovskite oxide photocatalysts have been widely used in water decomposition, carbon dioxide reduction and conversion, and nitrogen fixation. In terms of carbon dioxide reduction, oxide perovskites can achieve precise band gap and band edge tuning owing to its long charge diffusion length and flexibility in composition. For the development and utilization of solar energy in the environmental field, perovskite oxide and its derivatives (layered perovskite oxide) are used as photocatalysts for water decomposition and environmental remediation. In terms of nitrogen fixation, the conventional Haber-Bosh process for ammonia synthesis, which has been widely used in the past, requires high temperature and high energy. Therefore, we summarize the recent advances in perovskite oxide photocatalysts for nitrogen fixation from the aspect of activating the adsorbed N₂ by weakening the N $ \equiv $N triple bond, promoting charge separation, and accelerating the charge transfer to the active sites to realize the photochemical reaction. Overall, this review article presents the structure and synthesis of perovskite oxide photocatalysis, focusing on the application of photocatalysis in water splitting, carbon dioxide reduction, and nitrogen fixation. This review concludes by presenting the current challenges and future prospects of perovskite oxide photocatalysts.

The efficient utilization of carbon dioxide (CO₂) as a C1 feedstock is of great significance for green and sustainable development. Therefore, the efficient chemical conversion of CO₂ into value-added products has recently attracted a lot of research attention in recent years. The transformation of CO₂ generally requires high-energy substrates, specific catalysts, and harsh reaction conditions due to its high thermodynamic stability and kinetic inertness. Consequently, several efforts have been dedicated toward the development of high-performance catalysts and new reaction routes for CO₂ conversion over the last few decades. To date, many routes of convert CO₂ into value-added chemicals have been proposed, together with the development of heterogeneous and homogeneous catalysts. Among the advanced catalysts reported to date, ionic liquids (ILs) have been widely investigated and show great potential for the efficient, selective, and economical conversion of CO₂ into highly valuable products under mild conditions, even under ambient conditions. Some task-specific ILs have been designed with unique functional groups (e.g., —OH, —SO₃H, —NH₂, —COOH, and —C≡N), which can act as the solvent, absorbent, activating agent, catalyst, or cocatalyst to realize the transformation of CO₂ under metal-free and mild conditions. In addition, a variety of catalytic systems composed of ILs and metal catalysts have also been reported for the transformation of CO₂, in which the combination of the IL and metal catalyst is responsible for CO₂ conversion with high efficiency. In this review article, we summarize the recent advances in IL-mediated CO₂ transformation into chemicals prepared via C—O, C—N, C—S, C—H, and C—C bond forming processes. ILs that can chemically capture CO₂ with high capacity are first introduced, which can activate CO₂ via the formation of IL-based carbonates or carbamates, thus realizing the transformation of CO₂ under metal-free and mild conditions. Recent progress in IL-mediated CO₂ transformations to form carbonates and various kinds of N- and S-containing compounds (e.g., oxazolidinones, ureas, benzimidazolones, formamides, methylamines, benzothiazoles, and other chemicals) as well as CO₂ hydrogenation to give formic acid, methane, acetic acid, low-carbon alcohols, and hydrocarbons has been summarized in this review with a focus on the reaction routes, catalytic systems, and reaction mechanism. In these reactions, ILs can simultaneously activate the substrate via strong H-bonding in addition to activating CO₂, and the cooperative effects among the ionic and molecular species and metal catalysts accomplish the reactions of CO₂ with various kinds of substrates to afford a wide range of value-added chemicals. Finally, the shortcomings and perspectives of ILs are discussed. In short, IL-mediated CO₂ transformations provide green and effective routes for the synthesis of high-value chemicals, which may have great potential for a wide range of applications.

Nowadays, more than 85% of the energy is generated by fossil fuels. The excessive utilization of finite fossil fuels has resulted in the crises of energy shortage and global warming caused by greenhouse gas emissions. Researchers have conceived several means for trying to solve these problems, among which the sunlight-driven CO₂ reduction is viewed as a sustainable process that utilizes CO₂ as the raw material to produce chemical fuels, including CO, formate, and CH₄; this method not only realizes the conversion and storage of intermittent solar energy, but also decreases the CO₂ concentration in the atmosphere and alleviates global warming. However, photochemical CO₂ reduction usually undergoes a sluggish process due to the inertness of CO₂. Moreover, the selectivity of the CO₂ reduction reaction is also challenged by the hydrogen evolution reaction, which exhibits faster reaction kinetics. In this context, the rational design and synthesis of efficient and selective catalysts for photochemical CO₂ reduction are major challenges.

Recently, non-noble metal Co(Ⅱ) complexes as molecular catalysts have shown excellent catalytic performances in photocatalytic CO₂ reduction. During the past several decades, significant progress has been achieved in improving the applicability of Co(Ⅱ) complexes for photocatalytic CO₂ reduction. In this review, we systematically report the latest research progress on the use of Co(Ⅱ) complexes in photocatalytic CO₂ reduction. To describe the progress of this research, we characterized the Co(Ⅱ)-based molecular catalysts into four categories according to ligand types, namely: (1) macrocyclic ligands, (2) polypyridine ligands, (3) porphyrin and porphyrin-like ligands, and (4) nonplanar N₄ ligands. The progress of the research on the heterogeneity of Co(Ⅱ) molecular complexes used for photochemical CO₂ reduction was also introduced and discussed. Furthermore, the effects of catalyst structures on catalytic efficiency, selectivity, and stability were particularly summarized and discussed, with the aim of revealing and building the relationship between catalyst structures and catalytic performances to guide the future design and synthesis of Co(Ⅱ) molecular complexes with excellent catalytic performance. Finally, the current challenges and problems in photocatalytic CO₂ reduction were summarized, and several suggestions for designing efficient Co(Ⅱ)-based molecular catalysts for photocatalytic CO₂ reduction were put forward. The trends for the future development of Co(Ⅱ) complex molecular catalysts were also investigated with regard to photocatalytic CO₂ reduction.

Burning of fossil fuels increases CO₂ concentration in the atmosphere, resulting in a series of climate- and environment-related concerns such as global warming, sea-level rise, and melting of glaciers. Therefore, utilization of renewable energy to reduce the CO₂ concentration, in order to realize a sustainable development, is urgent. Capturing and utilizing CO₂, a greenhouse gas, can not only address these concerns but also alleviate the current scenario of energy shortage. Thermal catalytic CO₂ hydrogenation offers various pathways with high conversion efficiencies to produce fuels and industrial chemicals including CO, HCOOH, CH₃OH, and CH₄. However, CO₂ is chemically inert due to the highly stable C＝O bond. Thus, harsh reaction conditions such as high temperature and pressure are required for CO₂ hydrogenation.

Electrocatalytic CO₂ reduction using renewable electricity and water is a promising alternative to thermocatalysis. This technology can not only store and transport the intermittent solar or wind energy but can also use water as the proton source instead of H₂, which is indispensable for thermal CO₂ hydrogenation. Electrochemical CO₂ reduction under ambient conditions is a proton-coupled electron transfer process. The key to promote the electrochemical reduction of CO₂ is to develop highly selective and active catalysts with high stability. Among various CO₂ electrocatalysts, copper-based catalysts have attracted significant attention and have been extensively investigated, since they exhibit good selectivity and efficiency for the reduction of CO₂ to hydrocarbons and alcohols. A broad range of products, up to 16 different gases and liquids, can be obtained in the CO₂ electroconversion on copper. Copper is the only metal that has a negative adsorption energy for *CO and a positive adsorption energy for *H. Thus, it has a unique property of generating > 2e⁻ transfer products. However, selectivity of the target product is still low, especially for high value-added C₂₊ species (C₂H₄, C₂H₅OH, CH₃COOH, CH₃CHO, n-C₃H₇OH, etc.).

The selectivity of various products on copper-based catalysts could be enhanced by surface engineering techniques such as tuning the morphologies, particle sizes, surface facets, strains levels, and atomic coordination. Electrolyte engineering could also aid in CO₂ electroreduction. Therefore, improving the selectivity of C₂₊ products by modifying copper-based catalysts could be a hot research topic. In addition, C-C coupling is a key step in forming C₂₊ products, though the C₂₊ product formation pathway is complex, and the mechanisms are still unclear. Considering these, this paper mainly reviews the research progress in copper-based catalysts producing C₂₊ species in the last five years. It also discusses the possible reaction mechanisms and the factors that affect the product selectivities. In the end, further research directions are proposed.

The acceleration of industrialization and the continuous upgradation of consumption structure has increased the atmospheric content of CO₂ far beyond the past levels, leading to a serious global environmental problem. Photocatalytic reduction of CO₂ is one of the most promising methods to solve the problem of rising atmospheric CO₂ content. The core of this technology is to develop efficient, environment-friendly, and affordable photocatalysts. A photocatalyst is a semiconductor that can absorb photons from sunlight and produce electron-hole pairs to initiate a redox reaction. Owing to their low specific surface areas, significant electron-hole recombination, and less surface-active sites, bulk photocatalysts are not satisfactory. Ultrathin layered materials have shown great potential for photocatalytic CO₂ reduction owing to their characteristics of large specific surface area, a large number of low-coordination surface atoms, short transfer distance from the inside to the catalyst surface, along with other advantages. Photoexcited electrons only need to cover a short distance to transfer to the nanowafer surface, and the speed of migrating electrons on the nanowafer surface is much higher than that in the layers or in the bulk catalyst. The ultrathin structure leads to significant coordinative unsaturation and even vacancy defects in the lattice structure of the atoms; while the former can be used as active sites for CO₂ adsorption and reaction, the latter can improve the separation of the electron-hole pair. This review summarizes the latest developments in ultrathin layered photocatalysts for CO₂ reduction. First, the photocatalytic reduction mechanism of CO₂ is introduced briefly, and the factors governing product selectivity are explained. Second, the existing catalysts, such as g-C₃N₄, black phosphorus (BP), graphene oxide (GO), metal oxide, transition metal dichalcogenides (TMDCs), perovskite, BiOX (X = Cl, Br, I), layered double hydroxide (LDH), 2D-MOF, MXene, and two-dimensional honeycomb-like Ge―Si alloy compounds (gersiloxenes), are classified. In addition, the prevalent preparation methods are summarized, including mechanical stripping, gas stripping, liquid stripping, chemical etching, chemical vapor deposition (CVD), template method, self-assembly of surfactant, and the intermediate precursor method of lamellar Bi-oleate complex. Finally, we introduced the strategy of improving photocatalyst performance on the premise of maintaining its layered structure, including the factors of thickness adjustment, doping, structural defects, composite, etc. The future opportunities and challenges of ultrathin layered photocatalysts for the reduction of carbon dioxide have also been proposed.

Industrial revolution has led to increased combustion of fossil fuels. Consequently, large amounts of CO₂ are emitted to the atmosphere, throwing the carbon cycle out of balance. Currently, the most effective method to reduce the CO₂ concentration is direct CO₂ capture from the atmosphere and pumping of the captured CO₂ deep underground or into the mid-ocean. The transformation of CO₂ into high-value chemicals is an attractive yet challenging task. In recent years, there has been much interest in the development of CO₂ utilization technologies based on electrochemical CO₂ reduction, photochemical CO₂ reduction, and thermal CO₂ reduction, and CO₂ valorization has emerged as a hot research topic. In electrochemical CO₂ reduction, the cathodic reaction is the reduction of CO₂ to value-added chemicals. The anodic reaction should be the oxygen evolution reaction, and water is the only renewable and scalable source of electrons and protons in this reaction. There is a plethora of research on the use of various metals to catalyze this reaction. Among these, Cu-based materials have been demonstrated to show unique catalytic activity and stability for the electrochemical conversion of CO₂ to valuable fuels and chemicals. Moreover, the solar-driven conversion of CO₂ into value-added chemical fuels has attracted great attention, and much effort is being devoted to develop novel catalysts for the photoreduction of CO₂, especially by mimicking the natural photosynthetic process. The key step in the photocatalytic process is the efficient generation of electron-hole pairs and separation of these charge carriers. The efficient separation of photoinduced charge carriers plays a crucial role in the final catalytic activity. Compared with CO₂ reduction via electrocatalysis and photocatalysis, thermal reduction is more attractive because of its potential large-scale application in the industry. Heterogeneous nanomaterials show excellent activity in the electrocatalytic, photocatalytic, and thermal catalytic conversion of CO₂. However, nanostructured materials have drawbacks on the investigation of the intrinsic activity of the active sites. In recent years, single-site catalysts have become popular because they allow for maximum utilization of the metal centers, show specific catalytic performance, and facilitate easy elucidation of the catalytic mechanism at the molecular level. Accordingly, numerous single-site catalysts were developed for CO₂ reduction to produce value-added chemicals such as CO, CH₄, CH₃OH, formate, and C₂₊ products. Value-added chemicals have also been synthesized with the aid of amines and epoxides. This review summarizes recent state-of-the-art single-site catalysts and their application as heterogeneous catalysts for the electroreduction, photoreduction, and thermal reduction of CO₂. In the discussion, we will highlight the structure-activity relationships for the catalytic conversion of CO₂ with single-site catalysts.

The efficient utilization of the greenhouse gas CO₂ as a C1 feedstock can effectively reduce its emission and create economic value. Hence, the efficient chemical conversion of CO₂ has been receiving intense attention. Due to the extremely low energy level of the CO₂ molecule, the high energy barrier is the primary challenge for the chemical conversion of CO₂. The chemical conversion of CO₂ is mainly carried out through non-reductive transformation in industrial. Yet, the new route of chemical synthesis based on CO₂ reductive transformation is an interesting topic to expand its resource utilization. In this context, homogeneous reductive carbonylation is a hot topic for the utilization of CO₂ via reductive transformation. In this process, the metal hydride intermediate derived from the activation of the hydrogen source is crucial to the CO₂ reduction. Hydrogen, a clean source with high atom economy, can be used as a reducing agent for the reductive conversion of inert CO₂ through carbonylation, to construct C―O, C―N, and C―C bonds and to synthesize aldehyde/alcohol, carboxylic acid, ester, amide, and other chemicals. These expand the scope of CO₂ high-value utilization and show great potential application in terms of resource utilization and environmental protection. This CO₂ utilization process is thought to involve cascading catalytic reactions of CO₂ reduction and carbonylation. The catalytic systems require the corresponding catalysts to efficiently promote each step and effectively inhibit undesired side reactions. Recently, considerable progress has been made in the homogeneous reductive carbonylation of CO₂ with H₂. However, this kind of reaction is mostly of the cascade type, and hence, requires harsh conditions and noble metal catalysts. The chemoselectivity is low because of the multiple competing reactions. In addition, due to the steric hindrance and electronic effects of the substrate, there are limitations on the types of substrates that can be employed. With the development of new characterization techniques and theoretical calculations, some progress has been made in revealing the reaction mechanism and in the activation of the carbon-oxygen bonds of CO₂. Therefore, there is an urgent need to develop a more efficient catalytic system that requires mild conditions for reductive carbonylation. In this review, we provide an overview of the groundbreaking studies and the recent breakthroughs that have demonstrated the potential of metal catalysts to utilize the combination of CO₂ and H₂ as a C1 synthon, including olefin carbonylation, amine carbonylation, and alcohol/ether carbonylation, while highlighting the effect of different types of metal catalysts on the reaction. We conclude with a perspective on the future prospects of the homogeneous reductive carbonylation of CO₂ with H₂, providing readers a snapshot of this rapidly evolving field.

Carbon dioxide (CO₂) is one of the main greenhouse gases in the atmosphere. The conversion of CO₂ into solar fuels (CO, HCOOH, CH₄, CH₃OH, etc.) using artificial photosynthetic systems is an ideal way to utilize CO₂ as a resource and reduce CO₂ emissions. A typical artificial photosynthetic system is composed of three key components: a photosensitizer (PS) to harvest visible light, a catalyst (C) to catalyze CO₂ or protons into carbon-based fuels or H₂, respectively, and a sacrificial electron donor (SED) to consume the holes generated in the PS. In most cases, the PS and catalyst are two different components of a system. However, some components that possess both light harvesting and redox catalysis functionalities, e.g., nano-semiconductors, are referred to as photocatalysts. During photocatalysis, the PS is typically excited by photons to generate excited electrons. The excited electrons in the PS are transferred to the catalyst to generate a reduced catalyst. The reduced catalyst is used as an active intermediate to perform CO₂ binding and transformation. The PS can be recovered through a reaction with the SED. Nano-semiconductors have been used as photosensitizers and/or photocatalysts in photocatalytic CO₂ reduction systems owing to their excellent photophysical and photochemical properties and photostability. CdS and CdSe nano-semiconductors, such as quantum dots, nanorods, and nanosheets, have been widely used in the construction of photocatalytic CO₂ reduction systems. Systems based on CdS or CdSe nano-semiconductors can be classified into three categories. The first category is systems based on CdS or CdSe photocatalysts. In these systems, CdS or CdSe nano-semiconductors function as photocatalysts to catalyze CO₂ reduction without a co-catalyst under visible-light irradiation. The CO₂ reduction reaction occurs at the surface of the CdS or CdSe nano-semiconductors. The second category is systems based on CdS or CdSe composite photocatalysts. CdS or CdSe nano-semiconductors are combined with functional materials, such as reduced graphene oxide or TiO₂, to prepare composite photocatalysts. These composite photocatalysts are expected to improve the lifetime of the charge separation state and inhibit the photocorrosion of the nano-semiconductors during photocatalysis. The third category is hybrid systems containing a CdS nano-semiconductor and molecular catalysts, such as nickel and cobalt complexes and iron porphyrin. In these hybrid systems, CdS functions as a photosensitizer and the CO₂ reduction reaction occurs at the molecular catalyst. This review article introduces the construction of artificial photosynthetic systems and the photocatalytic mechanism of nano-semiconductors, and summarizes the representative works in the three aforementioned categories of systems. Finally, the challenges of nano-semiconductors for photocatalytic CO₂ reduction are discussed.

The electrocatalytic CO₂ reduction reaction (CO₂RR) driven by renewable energy is an efficient approach to achieve the conversion and utilization of CO₂. In this context, CO₂RR has become an emerging research focus in the field of electrocatalysis over the past decade. While a large number of nanostructured catalysts have been developed to accelerate CO₂RR, the tradeoff between activity and selectivity usually renders the overall electrocatalytic performance very poor. Beyond catalyst design, rationally designing electrolyzers is also of substantial importance for improving the CO₂RR performance and achieving its scale-up for practical applications. To a large extent, the electrolyzer configuration determines the local reaction environment near an electrode by affecting the process conditions, thereby resulting in remarkably different electrocatalytic performances. To be techno-economically viable, the performance of CO₂ electrolyzers is expected to be at least comparable to that of the current state-of-the-art proton exchange membrane (PEM) water electrolyzers, with regard to their activity, selectivity, and stability. Researchers have made great progress in the development of CO₂ electrolyzers over the past few years, but they are also facing many issues and challenges. This review aims to provide an in-depth analysis of the research progress and status of current CO₂ electrolyzers including H-cell, flow-cell, and membrane electrode assembly cell (MEA-cell) electrolyzers. Herein, operation at industrial current densities (> 200 mA∙cm⁻²) is set as a basis when these electrolyzers are discussed and compared in terms of the four main figures of merit (current density, Faradic efficiency, energy efficiency and stability) that describe the CO₂RR performance of an electrolyzer. The advantages and drawbacks of each electrolyzer are discussed and highlighted with emphasis on the key achievements reported to date. Compared to conventional H-cell electrolyzers that work well in mechanistic studies, the newly developed electrolyzers using gas diffusion electrodes, both flow-cell and MEA-cell electrolyzers, are able to break the limitation of CO₂ solubility in water and acquire industrial current densities. Although flow-cell electrolyzers have achieved current densities exceeding 1 A∙cm⁻², they suffer from low energy efficiencies because of the significant iR drop and poor stability owing to the use of alkaline electrolytes. These issues can be overcome in the case of zero-gap MEA-cell electrolyzers with ion exchange membranes being as solid electrolytes. The anion exchange membrane (AEM)-based CO₂ electrolyzers are at the center of the current research, as they demonstrate promising activity and selectivity toward specific CO₂RR products and exhibit excellent stability for over thousands of hours in few cases. Meanwhile, the crossover of CO₂ and liquid products from the cathode to the anode through the membrane tends to lower the utilization efficiency of the CO₂ supplied to the AEM electrolyzers. MEA-cell electrolyzers using cation exchange membranes and bipolar membranes have also been explored; however, neither of them have shown satisfactory CO₂RR performance. The development of new polymer electrolyte membranes and ionomers would help address these problems. While issues and challenges still exist, MEA-cell electrolyzers hold the greatest promise for practical applications. As concluding remarks, research strategies and opportunities for the future have been proposed to accelerate the development of CO₂RR technology for practical applications and to deepen the mechanistic understanding behind improved performance. This review provides new insights into rational electrolyzer design and guidelines for researchers in this field.

Ever-increasing energy demands due to rapid industrialization and urban population growth have drastically reduced petroleum reserves and increased greenhouse-gas production, and the latter has consequently contributed to climate change and environmental damage. Therefore, it is highly desirable to produce fuels and chemicals from non-petroleum feedstocks and to reduce the atmospheric concentrations of greenhouse gases. One solution has involved using carbon dioxide (CO₂), a main greenhouse gas, as a C1 feedstock for producing industrial fuels and chemicals. However, this requires high energy input from reductants or reactants with relatively high free energy (e.g., H₂ gas) because CO₂ is a highly oxidized, thermodynamically stable form of carbon. H₂ can be generated through water photolysis, making it an ideal reductant for hydrogenating CO₂ to CO. In situ generation of CO such as this has been developed for various carbonylation reactions that produce high value-added chemicals and avoid deriving CO from fossil fuels. This is beneficial because CO is toxic, and when extracted from fossil fuels it requires tedious separation and transportation. This combination of CO₂ and H₂ allows for functional molecules to be synthesized as entries into the chemical industry value chain and would generate a carbon footprint much lower than that of conventional petrochemical pathways. Based on this, CO₂/H₂ carbonylations using homogeneous transition metal-based catalysts have attracted increasing attention. Through this process, alkenes have been converted to alcohols, carboxylic acids, amines, and aldehydes. Heterogeneous catalysis has also provided an innovative approach for the carbonylation of alkenes with CO₂/H₂. Based on these alkene carbonylations, the scope of CO₂/H₂ carbonylations has been expanded to include aryl halides, methanol, and methanol derivatives, which give the corresponding aryl aldehyde, acetic acid, and ethanol products. These carbonylations revealed indirect CO₂-HCOOH-CO pathways and direct CO₂ insertion pathways. The use of this process is ever-increasing and has expanded the scope of CO₂ utilization to produce novel, high value-added or bulk chemicals, and has promoted sustainable chemistry. This review summarizes the recent advances in transition-metal-catalyzed carbonylations with CO₂/H₂ and discusses the perspectives and challenges of further research.