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.

Converting CO₂ into value-added products via sustainable energy, such as electrical energy, has several advantages. First, it is one of the most promising routes to close the carbon loop and plays a crucial role in significantly reducing the CO₂ concentration in the atmosphere. Second, it can utilize CO₂ as a valuable industry reactant that can store energy by converting electrical energy to chemical energy. Although the CO₂ reduction reaction has been studied for more than three decades, the sluggish kinetics remain a bottleneck, which requires a highly efficient catalyst. However, none of the reported catalysts meets the requirements for any practical application due to low activity and poor selectivity. To rationally design a more efficient CO₂ reduction catalyst, understanding the reaction mechanism is crucial. Although it is challenging to experimentally capture and characterize the reactive intermediates, atomic modeling serves as an alternative for providing an understanding of the elementary reactions on a microscale. Significant progress has been made in understanding the reaction mechanism using multiscale simulations. In this study, important progress in revealing the reaction mechanism of CO₂ reduction using computational simulation in recent years is summarized. First, the advances in simulation methods for electrochemical reactions are introduced, and the advantages and disadvantages of various methods are compared. Second, the detailed reaction mechanism of CO₂ reduction to various major products, such as CO, CH₄, and C₂H₄, and minor products, such as ethanol and acetate, are disused. Different results obtained from different approximations are compared, while a mechanism that can better explain the existing experimental results is recommended. Third, the operando technique, such as ambient pressure X-ray photoelectron spectroscopy, is disused. The operando analysis results are direct evidence to validate the theoretically proposed reaction pathway. In turn, the theoretical predictions can help resolve the experimental spectrum, which is usually too complex to refer to a reference system. The combination of theory and operando experiments should be one of the most promising directions in determining the reaction mechanism. Fourth, novel synthesis strategies are discussed. These new ideas are beneficial for simplifying the synthesis process or increasing the diversity of products. Finally, the recent progress in the application of machine learning to big data for CO₂ reduction is discussed. These new powerful tools may play a crucial role in reaction mechanism studies. Overall, in the study of electrochemical reaction mechanism, theoretical simulation can provide the reaction details and energy information of elementary reactions at the atomic level. Therefore, in the study of electrochemical reaction mechanism of carbon dioxide, the microscopic mechanism that the experiment cannot provide is supplemented. On the one hand, it explains the existing experimental phenomena; however, on the other hand, it provides new insights for the study of reaction mechanism. On this basis, the use of new research paradigms, such as high-throughput computing and machine learning, provides new ideas for a rational design for accelerating material development.

Since 2009, organic-inorganic halide perovskites have been widely studied in the field of optoelectric materials due to their unique optical and electrical properties. Pb-based halide perovskite solar cells (PSCs), in particular, currently have a record efficiency of 25.2%, thus showing strong potential in commercialization. However, the market prospects of PSCs have been hampered by the toxicity of lead-based materials. Therefore the seeking of less toxic and environmentally friendly elements that can replace Pb is of great interest. Tin-based perovskites are the most promising choice at present due to its similar electronic configuration as Pb, and can even have more superior semiconductor properties. As a rising star of lead-free perovskite solar cells, tin-based PSCs have drawn much attention and made promising progress during the past few years. However, it is still challenging to obtain efficient and stable tin-based PSCs because of the low defects formation energy and the oxidation of bivalent tin. Among all Pb-free perovskite materials that show photovoltaic performance, formamidinium tin tri-iodide (FASnI₃) based PSCs are the most promising because of the suitable band gap, low exciton bind energy, and high carrier mobility. The main drawbacks of tin-based perovskite material are its instability because of the easy oxidation of Sn²⁺ into Sn⁴⁺ and high dark current which arises from high p-type carrier concentration. The latter originates from the low formation energy of Sn vacancies. Many strategies have been developed to overcome these problems and promote the performance of tin-based PSCs. On one type of pursuit to avoid the oxidation of Sn²⁺, reduction additives (e.g., SnF₂, pyrazine, hydrazine vapor, hydroxybenzene sulfonic acid or its salt, and π-conjugated polymer) and solvent-free processing have been introduced and shown to be effective up to a point. In another type, Cs or Br alloying and construction of low-dimensional structures in tin-based perovskite have also been shown to be promising. In this review, the optical and electrical properties of tin-based perovskite are systematically discussed. And then, the film fabrication methods and different device architectures of tin-based PSCs are summarized. Finally, the current challenges and a future outlook for tin-based PSCs are discussed.

In less than a decade, metal halide perovskites (MHPs) have been demonstrated as promising solar cell materials because the photoelectric conversion efficiency (PCE) of the representative material CH₃NH₃PbI₃ rapidly increased from 3.8% in 2009 to 25.2% in 2009. However, defects play crucial roles in the rapid development of perovskite solar cells (PSCs) because they can influence the photovoltaic parameters of PSCs, such as the open circuit voltage, short-circuit current density, fill factor, and PCE. Among a series of superior optoelectronic properties, defect tolerance, i.e., the dominate defects are shallow and do not act as strong nonradiative recombination centers, is considered to be a unique property of MHPs, which is responsible for its surprisingly high PCE. Currently, the growth of PCE has gradually slowed, which is due to low concentrations of deep detrimental defects that can influence the performances of PSCs. To further improve the PCE and stability of PSCs, it is necessary to eliminate the impact of these minor detrimental defects in perovskites, including point defects, grain boundaries (GBs), surfaces, and interfaces, because nonradiative recombination centers seriously affect device performance, such as carrier generation and transport. Owing to its defect tolerance, most intrinsic point defects, such as V_I and V_MA, form shallow level traps in CH₃NH₃PbI₃. The structural and electronic characteristics of the charged point defect V_I- are similar to those of the unknown donor center in a tetrahedral semiconductor. It is a harmful defect caused by a large atomic displacement and can be passivated to strengthen chemical bonds and prevent atom migration by the addition of Br atoms. Owing to the ionic nature of MHPs and high ion migration speed, there are a large number of deep detrimental defects that can migrate to the interfaces under an electric field and influence the performance of PSCs. In addition, the ionic nature of MHPs results in surface/interface dangling bonds terminated with cations or anions; thus, deep defects can be passivated through Coulomb interactions between charged ions and passivators. Hence, the de-active deep-level traps resulting from charged defects can be passivated via coordinate bonding or ionic bonding. Usually, surface-terminated anions or cations can be passivated by corresponding cations or anions through ionic bonding, and Lewis acids or bases can be passivated through coordinated bonding. In this review, we not only briefly summarize recent research progress in defect tolerance, including the soft phonon mode and polaron effect, but also strategies for defect passivation, including ionic bonding with cations or anions and coordinated bonding with Lewis acids or bases.

In recent years, lead-halide perovskites, one of the most competitive material types in the field of semiconductors, has attracted widespread attention because of its easy preparation, low cost, and high performance. Lead-halide perovskites are a type of material with an ABX₃ structure, in which A is an organic or inorganic monovalent cation, B is a divalent cation, and X is a halogen ion. Among them, the B-site ion and X-site ion form an octahedron, with the B-site ion occupying the center and the X-site ion located at the apex of the octahedron. This type of octahedron can undergo lattice changes such as rotation or tilt through the replacement of different halogen anions, which affects the material band gap. The octahedron is located in the center of a cube, which is composed of A-site ions. These structures constitute the basic unit of the perovskite. Compared with the widely used Ⅱ-Ⅵ or Ⅲ-Ⅴ semiconductor nanocrystalline materials, perovskite nanocrystals have great application potential owing to their superior optoelectronic performance. However, their stability problem restricts further development, making them unable to compete in commercial applications. Studies on the stability of perovskite materials began in 2009. It was discovered through experiments that perovskite materials would undergo irreversible degradation under the action of liquid polar solvents, which confirmed that humidity and air are important factors in perovskite degradation. With further research, the problem of illumination has also come to the surface. It was found through experiment that, when oxygen and humidity were excluded, the light condition could also have a certain negative impact on perovskite materials, and subsequently perform a certain repair effect. Research in this area can lay a foundation for the preparation of high-stability perovskite materials and devices, adjust the structure and performance of perovskite by lighting technology (especially laser irradiation), and expand its comprehensive application in the field of optoelectronics. This article focuses on the changes in perovskites under laser irradiation and the related applications. First, it reviews the unstable changes and micro-mechanisms that laser-irradiation induces in lead-halide perovskites, including accelerated degradation, repair of defects, segregation, phase transitions, and changes in the grain size. Second, based on these mechanisms, it explains how researchers have recently used laser-irradiation technology to control the performance of perovskite films and devices. In addition, it also introduces the application of the laser direct writing process in the fields of perovskite patterning and photoelectric detection. Finally, this paper summarizes the changes induced by the laser-irradiation illumination and applications of laser-irradiated lead-halide perovskites.

Halide perovskites are ionic semiconductors with outstanding features, such as high defect tolerance, long carrier diffusion length, strong photoluminescence, narrow emission line width, solution processability, and low cost of fabrication. These advantages render them promising candidates for photovoltaics, lasers, displays, and photodetectors. Theoretical and experimental studies have demonstrated that the optical properties of perovskite materials can be strongly affected by their crystal size and dimension. Owing to their ionic nature and low formation energy, perovskites can be synthesized via precipitation. This process typically involves in situ transformation of the precursors with solvent evaporation and/or solvent mixing. It is well known that the physical/chemical properties of solvents play a vital role in determining the size and dimension of the resultant products. Therefore, elucidating the effects of the solvent on perovskite crystallization is crucial for improving the performance of perovskite-based devices. Moreover, the coordination effects between perovskite precursors and solvents are a dominant parameter that influence the crystallization process because the dissolution of perovskite precursors is strongly correlated with the coordination between the perovskite precursors and solvents. Herein, this minireview summarizes recent research advances in comprehending the perovskite precursor-solvent interactions with a focus on the coordination effects. In particular, we have endeavored to discuss the influence of coordination effects on the formation of polycrystalline thin films, quantum dots, and single crystals. It was found that the formation of perovskite-solvent intermediates in coordinated solvents retard the nucleation and growth of perovskite crystals; this proves beneficial for the fabrication of high-quality micrometer-sized perovskite polycrystalline films. Meanwhile, the preformed intermediates contribute to undesired impurities and defects in single crystals and quantum dots. These insights are exceedingly helpful for the crystallization control of perovskites, thus enabling better device performance and enhanced stability. Finally, the minireview discusses the challenges facing perovskite crystallization, along with a short perspective for future studies.

Metal halide perovskites are considered as promising candidates for lighting applications owing to their excellent optoelectronic properties, such as high electron/hole mobility, high photoluminescence quantum yield, high color purity, and facile color tunability. In recent years, perovskite light-emitting diodes (LEDs) have developed rapidly, and their external quantum efficiencies (EQEs) have exceeded 20% for green and red emissions. However, the EQEs and stabilities of blue (particularly deep-blue) perovskite LEDs are still inferior to the green and red counterparts, which severely restricts the application of perovskite LEDs in high-performance and wide color gamut displays as well as white light illumination. Therefore, summarizing the development of blue perovskite LEDs and discussing the opportunities and challenges associated with their future applications will help to guide the further development of the entire perovskite LED field. In this review, according to the emission color, we divide the blue perovskite LEDs into three parts for a better discussion, i.e., the emissions in the sky-blue, pure-blue, and deep-blue regions. We introduce their developed history and discuss the basic strategies to achieve blue emission. There are three typical methods to obtain perovskite emitters with blue emission, i.e., (1) composition engineering, (2) dimensional engineering, and (3) synthesis of perovskite nanocrystals and quantum dots. For composition engineering, changing ions in perovskite ABX₃ structure can easily tune the perovskite emission color, particularly while changing the anions in "X" position. Therefore, modulating the ratio between the X-site anions of Br^- and Cl^- can cause perovskites to emit blue photons ranging from 420 to 490 nm, which almost covers the entire blue spectrum. For dimensional engineering, perovskite materials can form a series of low-dimensional structures (layered structures) with the insertion of organic ligands between the perovskite frameworks. This type of low-dimensional perovskite material typically exhibits better lighting properties than those exhibited by its three-dimensional counterpart owing to its unique charge or energy transfer process of charge carriers. Blue perovskite nanocrystals and quantum dots with high photoluminescence quantum yields are excellent candidates for realizing high-performance pure-blue and deep-blue devices because they can easily incorporate Cl^- in their crystals, which is considerably limited in perovskite thin films owing to the poor solubility of inorganic chloride sources in polar solvents. Furthermore, we discuss several challenges associated with blue perovskite LEDs, such as the inferior device performance in the pure-blue and deep-blue regions, difficulty in hole injection, electroluminescence (EL) instability of mixed halide perovskite systems, and lagged operation lifetime, and introduce potential solutions accordingly. Note that the challenges faced by blue perovskite LEDs are also the opportunities for research in this area. Therefore, this review is of a great reference value for the next evolution of blue perovskite LEDs.

Organic-inorganic metal halide perovskite-based tandem solar cells have attracted significant research attention in recent years. The power conversion efficiency of perovskite-based tandem can efficiently meet the requirements of practical applications; however, their instability limits their commercialization. The most commonly used wide-bandgap perovskites suitable for top sub-cells, which are based on I/Br alloying at X site, often suffer from severe phase segregation. When exposed to light illumination, a smaller bandgap phase appears and acts as a carrier trap, leading to a reduction in the quasi-Fermi level splitting and large V_OC deficit. The narrow-bandgap perovskites suitable for bottom sub-cells, which are based on Sn/Pb alloying at B sites, always face atmospheric instability. When exposed to air, Sn²⁺ is rapidly oxidized to Sn⁴⁺, which can shorten the carrier diffusion length and result in a drop in efficiency. Herein, we summarize the recent advances in perovskite-based tandem solar cells from the viewpoint of stability. We analyzed the stability data of highly efficient perovskite-based tandems reported so far, such as perovskite/silicon, perovskite/perovskite, and perovskite/copper indium gallium selenide (CIGS) tandems. We found that the key to improve the perovskite-based tandems is to improve the stability of the perovskite sub-cells. Then, we systematically analyzed the phase and atmospheric instability of wide- and narrow-bandgap perovskite, respectively, providing some reasonable strategies to tackle the instability. Compositional engineering, crystallinity optimization, and employing other perovskites with wide bandgaps are effective means to avoid phase instability of the I/Br alloying perovskite. Introducing the reducing additives, improving the film morphology, and forming a 2D/3D structure can help in improving the atmospheric stability of Sn-Pb narrow bandgap perovskites. Furthermore, we review the intrinsic instability of perovskite and corresponding improvement methods, which are inevitable in future tandem solar cells. By reducing the methylamine (MA) content in perovskite component and suppressing ion migration, the long-term operational stability is greatly enhanced. Finally, we briefly summarize the instability issues related to the interconnecting layer. In addition to the optimization of perovskite-based tandem devices, encapsulation also plays a crucial role in improving stability against environmental stressors. Studies based on improving the stability of perovskite-based tandems are still in the early stage. However, with a deeper understanding of the stability of perovskite sub-cells and the interconnecting layer, the commercialization of perovskite-based tandems, especially perovskite/silicon tandem devices, is promising to be achieved in the near future.

In the past few decades, lithium-ion batteries (LIBs) have dominated the market of rechargeable batteries and are extensively applied in the field of electronic devices (e.g., mobile phones and computers). However, lack of lithium resources, high cost of lithium as well as toxic and flammable organic electrolytes significantly hinder further development and large-scale application of LIBs. Therefore, it is necessary to develop next-generation green rechargeable batteries to replace LIBs. Recently, aqueous zinc-ion batteries (AZIBs) have been considered as energy storage devices with substantial development prospects for future large-scale storage systems owing to their high safety performance, low production cost, abundant zinc resources, and environmental friendliness. Typically, we use zinc metal as the anode with neutral or weakly acidic aqueous electrolyte (pH: 3.6–6.0). However, cathode materials have high requirements for AZIBs while considering the charge effect of multivalent metal ions. Currently, one of the research emphases is to develop suitable zinc ion intercalation cathode materials with stable structures and high capacities. Among all types of cathode materials, vanadium-based compounds have the advantages of low cost and high reversible capacity. Additionally, their structure is variable, mainly including layered, tunneled and natrium super ionic conductor (NASICON) structure. Therefore, vanadium-based compounds have clear application possibility in AZIBs. However, there are still several significant problems. In particular, vanadium-based compounds generally have poor conductivity and low voltage platform. Electrochemical performance can be significantly improved mainly by pre-inserting metal ions or water molecules, optimizing the electrolyte, and controlling morphology of nanomaterials (nanosheets, nanospheres, etc.). In addition, the zinc storage mechanism in vanadium-based compounds is more complicated and controversial, including Zn²⁺ intercalation/deintercalation mechanism, co-insertion mechanism, and conversion reaction mechanism. Moreover, different materials usually exhibit different electrochemical properties and energy storage mechanisms. In this review, we comprehensively describe the energy storage mechanisms of vanadium-based compounds and discuss the application as well as development status of vanadium-based materials in AZIBs. Further, several strategies for improving their performance are proposed, including structural design (e.g., pre-insertion of metal ions or water molecules), morphology control (e.g., carbon coating), and electrolyte optimization (e.g., adjustment of composition and concentration). In particular, pre-insertion of metal ions or water molecules in the original structure can effectively solve these problems of low ion diffusion rate, poor conductivity, and structural instability, thereby achieving excellent electrochemical performance. Moreover, the application of a high-concentration electrolyte is a simple and effective strategy that can not only significantly widen the electrochemical stability window of the aqueous electrolyte but also suppress the dissolution of vanadium, thereby effectively improving energy density and cycling stability for AZIBs. Accordingly, the future development direction of AZIBs and their vanadium-based cathode materials is further prospected, aiming at designing high-performance electrode materials for AZIBs.

The growing demand for electric vehicles, communication devices, and grid-scale energy storage systems urgently calls for the development of rechargeable batteries. Although lithium-ion batteries have dominated the new energy market for decades, there are challenges limiting their development, such as the high cost of lithium, as well as the toxicity and flammability of the organic electrolyte. In recent years, aqueous zinc-ion batteries (ZIBs) have gained much attention due to their advantages of high safety, high capacity, low cost, and nontoxicity. Materials based on multivalent vanadium and manganese have shown great potential for application as cathodes that are compatible with the metallic zinc anode in ZIBs. However, the commercialization of ZIBs has been hindered by the choice of cathodes, since the cathode materials show unsatisfactory energy densities and suffer from severe structural collapse, dissolution of the electrode components, sluggish reaction kinetics and detrimental side reactions during cycling. This stalemate was broken when a Zn²⁺/H₂O co-inserted V₂O₅ (Zn_0.25V₂O₅·nH₂O) material was first reported in 2016, and it showed much higher cycling stability and capacity than those of V₂O₅. The Zn²⁺ and water molecules pre-intercalated into the interlayer served as pillars to maintain the crystal structure and increase the interplanar spacing, leading to high structural stability and fast Zn²⁺ diffusion. Since then, several guest ions (Li⁺, Na⁺, K⁺, Ca²⁺, NH₄⁺, PO₄^3-, N^3-, etc.) and molecules (H₂O, polyethylene dioxythiophene (PEDOT), polyaniline (PANI, etc.) have been widely used to improve the electrochemical performance of aqueous ZIB cathodes, especially with manganese-based and vanadium-based materials. It is demonstrated that pre-intercalation of the guest ions or molecules can effectively optimize the electronic structure, regulate the interplanar spacing, and improve the reaction kinetics of the host. The local coordination structure of the host with pre-intercalated guest ions/molecules directly influences the zinc-ion storage performance. For example, sodium vanadates with a tunneled structure generally show better cycling stability than those with a layered structure due to their stronger Na-O bonds, since the O atoms on their layer surfaces are only single-connected. Manganese dissolution could be greatly suppressed by intercalation of the large potassium ions into tunneled α-MnO₂, where solid K-O bonds act as pillars to be connected with Mn polyhedrons, and thus strengthen the structure. New mechanisms underlying reduction/displacement reactions could also be revealed in vanadates upon the introduction of Ag⁺ and Cu²⁺. Thus, we believe that guest pre-intercalation is a promising method for optimizing the zinc-ion storage performance of the appropriate cathodes and is worthy for further exploration. Here we have reviewed the recent advances in manganese-based and vanadium-based cathodes via the guest pre-intercalation strategy, discussed the related advantages and challenges. The future research direction for these two kinds of aqueous ZIB cathodes is also prospected.

Semitransparent organic solar cells (ST-OSCs) have attracted attention for use in building integrated photovoltaics because of their large range tunability in colors, transparency, and high efficiency. However, the development of semitransparent devices based on fullerene acceptors remained almost stagnant in the early period. This was due to the weak absorption of fullerene small molecules in the visible and near-infrared regions as well as the large non-radiative energy loss, resulting in drastic open-circuit voltage loss. In addition, the energy level and chemical structure of fullerene molecules cannot be easily regulated, and the strong aggregation characteristics of fullerenes greatly limit the development of OSCs. In contrast, the designability of the chemical structures and controllability of the energy levels of non-fullerene electron acceptors has encouraged researchers to explore high-performance organic solar cells while and simultaneously stimulating the development of ST-OSCs. In this review, the recent progress in non-fullerene small molecule acceptors for ST-OSCs is summarized. The article focuses on ST-OSCs from the aspects of device structures and active layers. In view of the semitransparent device structure, except for replacing the traditional electrodes with semitransparent electrodes, researchers have introduced suitable interface layers to regulate the absorption and reflection of sunlight. The interface layers mainly contain a reflective layer (evaporated on the top electrode to reflect near-infrared light); an anti-reflection layer (located below ITO (indium tin oxide)) to mitigate light reflection at the air-glass interface and thus enhance the absorption of sunlight); and an optical outcoupling layer (simultaneously increasing reflection and transmission). From the active layer, it is mainly divided into two categories. First, researchers have optimized the photovoltaic performance of semitransparent devices from the perspective of molecular structures, mainly by broadening the absorption window of non-fullerene small molecule acceptors, thus improving the crystallinity and charge mobility of small molecules, and regulating the stacking behavior and orientation of molecules in the films. Second, regarding the active layer processing, much effort has been undertaken to optimize the light absorption, morphology, and charge carrier transport channels of blended films.

Although traditional graphite anodes ensure the cycling stability and safety of lithium-ion batteries, the inherent drawbacks, particularly low theoretical specific capacity (372 mAh·g^-1) and Li-free character, of such anodes limit their applications in high energy density battery systems, especially in lithium-sulfur and lithium-air batteries. Lithium metal has been considered as one of the best next-generation anode materials due to its extremely high theoretical specific capacity (3860 mAh·g^-1) and low redox potential (-3.04 V vs. the standard hydrogen electrode). The first generation of commercial rechargeable lithium metal batteries were developed by Moli Energy in the late 1980s and were not widely used due to several problems, including low coulombic efficiency, poor cycle stability, and safety hazards. These problems associated with the Li metal anode are mainly caused by lithium dendrite growth, electrode volume changes, and interface instability. During the charge and discharge processes, Li deposition is not uniform across the electrode surface. Due to the low surface energy and high migration energy of Li metal, dendrites are preferentially formed during Li deposition. These dendrites proceed to grow with successive battery cycling, penetrate the separator, and eventually reach the cathode, thereby causing short circuits and thermal runaway. Additionally, the growth of the lithium dendrite is inherently correlated with the reaction interface structure, and dendrite growth results in inhomogeneity of the SEI (solid electrolyte interface) which is inevitably formed on the Li metal surfaces. Moreover, the volume change of lithium metal anodes is of importance, particularly during battery cycling and Li stripping/deposition processes which make the SEI layers considerably unstable. SEI layers usually cannot withstand the mechanical deformation caused by volume changes; such layers continuously break and repair during cycling and consume large amounts of the electrolyte. Additionally, some Li dendrites could break and become wrapped by SEI layers to form electrically isolated "dead" Li, which results in the loss of active Li in the Li metal anode. All these factors are responsible for the failure of Li metal anodes. Herein, recent investigations on the failure mechanisms of lithium metal anodes are reviewed and summarized, including the formation of SEI layers on the surface of Li metal anodes, the behavior and mechanism of lithium dendrite growth, and the mechanism of "dead" lithium formation. Additionally, some advanced characterization techniques for investigating lithium metal anodes are introduced, including in situ tools, cryo-electron microscopy, neutron depth analysis technology, and solid state nuclear magnetic resonance technology. These techniques enable researchers to gain in-depth insights into the failure mechanisms of Li metal anodes.

Improvement in the energy density of conventional lithium-ion batteries (LIBs), based on the intercalation-extraction chemistry of graphite and transition metal layered oxides, has apparently lagged behind the advances in consumer electronics and electric vehicles. Secondary Li-metal batteries (LMBs), utilizing metallic Li as the anode material, have incomparable advantages in terms of energy density due to their high specific capacity (3860 mAh·g^-1) and low redox potential (-3.04 V vs. standard hydrogen electrode) of Li metal. Irrespective of whether Li anodes are coupled with intercalation-type cathodes (e.g. LiFePO₄, LiCoO₂, LiNi_xCo_yMn_zO₂, etc.) or conversion-type cathodes (S, O₂), the energy density of LMBs is much higher than that of traditional LIBs, which should solve the range concern of electric vehicles. However, the intrinsically high reactivity between metallic Li and organic electrolytes could induce the formation of a solid electrolyte interface (SEI). The heterogeneous SEI, consisting of a flexible organic outer layer and a brittle inorganic inner layer, suffers from repeated rupture and regeneration due to infinite volume expansions associated with Li deposition and dissolution reactions. Meanwhile, Li is preferentially deposited on the "hot sites" and is stripped from the root of sediments, resulting in uncontrolled dendrite growth during charging and formation of electrochemically isolated Li ("dead" Li) during discharging. Thus, the Columbic efficiency of Li metal full cells is greatly limited by interfacial side effects and continuous loss of active Li, especially in conventional carbonate-based electrolyte, viz. 1 mol·L^-1 LiPF₆-EC/DEC (ethylene carbonate/diethyl carbonate), which impedes the large-scale employment of Li metal batteries. Recently, novel electrolytes with high or localized-high salt concentrations have attracted considerable attention because of their unique physiochemical properties and excellent electrochemical performance. In high-concentration electrolytes, the reduction in the population of free solvent molecules inhibits irreversible electrolyte decomposition at the electrode-electrolyte interface. In localized-high-concentration electrolytes, the introduction of a dilute reagent retains the desired solvation structure, while improving the physicochemical properties (conductivity and viscosity) of the electrolyte. Herein, we systemically review the latest progress in high-concentration and localized-high-concentration electrolytes for use in Li metal batteries. The solvation chemistry structure, physicochemical properties, and interfacial-stabilizing mechanisms are analyzed in detail, and special attention is devoted to their superior interfacial compatibility with Li metal anodes. Finally, we briefly clarify the current problems associated with the research of high-concentration and localized-high-concentration electrolytes from the viewpoints of basic scientific research and practical applications, and some possible solutions are provided to further pave the way to practical Li metal batteries.

In the early 1990s, Sony launched the first commercial lithium ion battery (LIB), which achieved great success in energy storage systems. The current commercially used insertion anode, graphite, is approaching its capacity limit (~372 mAh·g^-1), and is inadequate to satisfy the ever-increasing energy demand for power grids and large-scale energy storage systems. In order to address this challenge, lithium metal anodes have been the focus of considerable research effort in recent years, and are regarded as the most promising anode materials because of their extremely high theoretical capacity (3860 mAh·g^-1), lowest electrode potential (-3.04 V vs. standard hydrogen electrode), and low density (0.534 g·cm^-3). For example, the theoretical energy densities of lithium-sulfur batteries and lithium-air batteries are as high as 2567 and 3505 Wh·kg^-1, respectively. However, the uncontrollable dendrite growth during cycling leads to low coulombic efficiency and puncture of the separator, causing a short circuit or even explosion of the battery, thereby seriously hindering the development of the lithium metal anode. Many solutions have been proposed to inhibit dendrite growth, including the use of electrolyte additives, solid electrolytes, and artificial protective films. During charging and discharging, the solid electrolyte interphase (SEI) plays an important role in lithium metal anodes. However, the infinite volume changes of the electrode during plating/stripping processes result in breakage of the SEI film, which continuously consumes the electrolyte and lithium metal. Designing an artificial interface on the surface of lithium metal anodes has been considered as a simple and efficient strategy to control lithium deposition behavior, and is achieved by precoating a protective layer on the surface of lithium metal. An ideal artificial protective film should possess high ionic conductivity, chemical stability, and excellent mechanical strength, in order to prevent side reactions between lithium metal and the electrolyte and realize dendrite-free lithium metal anodes with a long cycle life and high coulombic efficiencies. In this paper, the research progress on artificial protective films for lithium metal anodes in recent years is reviewed. Further, the structural characteristics and preparation methods of various protective films are introduced in detail, including polymer protective films, inorganic protective films, organic-inorganic composite protective films, and alloy protective films. The mechanisms of various protective films toward the suppression of dendrite growth are summarized. Existing challenges and future research directions are also proposed, which together provide a reference for promoting the use of lithium metal in high-energy batteries.

Lithium-metal anode batteries have the potential to serve as next-generation, high energy density batteries with high specific capacity and low electrode potential. However, due to the high reactivity of lithium, complex interfacial reactions and uncontrollable dendrite growth obstruct their application. These lithium-metal anode interfacial reactions are often accompanied by the organic electrolyte spontaneously decomposing and combustible gas subsequently escaping, which is a safety concern. It also affects the form of the solid electrolyte interphase (SEI), which is important for stabilizing the interface between the Li-metal anode and electrolyte. Uncontrollable Li dendrite growth could penetrate the separator or electrolyte, creating the risk of a short circuit. Therefore, it is necessary to optimize the lithium nucleation and deposition processes. Solid state electrolytes (SSEs) have also attracted attention for improving the energy density and safety of Li-ion batteries; however, problems such as poor ionic conductivity still exist. Computational simulations, such as molecular dynamics (MD) simulations and first-principles calculations based on density function theory (DFT), can help elucidate reaction mechanisms, explore electrode materials, and optimize battery design. In this review, we summarize the theoretical perspective gained from computational simulation studies of lithium-metal anodes. This review is organized into four sections: interfacial reactions, SEIs, lithium nucleation, and SSEs. We first explore organic-electrolyte interfacial reaction mechanisms that were revealed through MD simulations and how electrolyte additives, electrolyte concentration, operating temperature affect them. For SEI, DFT can provide an in-depth understanding of the surface chemical reaction, surface morphology, electrochemical properties, and kinetic characteristics of SEI. We review the developments in SEI transmission mechanisms and SEI materials' properties alteration by lithium metal. We further explore artificial SEI design requirements and compare the performances of artificial SEIs, including double-layer, fluorine-, and sulfur-SEIs. Lithium dendrite growth as a result of lithium nucleation and deposition is then discussed, focusing on computational studies that evaluated how doped graphene, 3D carbon fibers, porous metals, and other matrix materials regulated these processes and inhibited dendrite growth. Computational simulations evaluating transport phenomena and interface reactions between SSEs and lithium-metal anodes are then explored, followed by ideas for further design optimization. Finally, potential research directions and perspectives in this field are proposed and discussed.

The applications of lithium-ion batteries have been limited because their energy density can no longer meet the requirements of an emerging energy society. Lithium metal batteries (LMBs) are being considered as potential candidate for next-generation energy storage systems owing to the high theoretical specific capacity and low electrochemical potential of lithium metal. However, the commercialization of LMB is limited due to several challenges, such as uncontrollable formation of dendrites, unstable solid electrolyte interface, and infinite anode volume change, which can lead to grievous catastrophe. In this study, several typical mechanisms of lithium dendrite formation and growth are summarized. The results suggest that a smaller current density, greater Li⁺ transference number, higher mechanical strength of the electrolyte, and a more homogeneous distribution of Li⁺ on the substrate are conducive to the uniform deposition morphology of lithium metal. In view of these results, combined with the researches on LMBs conducted in recent years, composite anodes can be summarized into three level from internal to external. (ⅰ) Internal composite of lithium metal anode: the scaffolds composited with lithium metal are classified as non-conductive (NC), electron-conductive (EC), ion-conductive (IC), and mixed ion and electron-conductive (MIEC) scaffolds. Composited with NC scaffolds, the tip effect can be weakened through the interaction between polar functional groups and Li⁺. The composite of lithium metal and EC scaffolds can effectively reduce the local current density, while IC scaffolds can increase the ion flux. However, the performance of LMBs may be hindered by the insulation of electrons or Li⁺ at high rates. In comparison, MIEC scaffolds can provide fast ion/electron transfer channels for the deposition or dissolution of lithium metal, which is beneficial for the electrochemical performance of LMBs even at high rates. (ⅱ) Internal composite of LMB: Compared with liquid electrolytes, solid-state electrolytes (SSEs) and quasi-solid-state electrolytes are much safer. However, their interfacial contact with lithium metal anodes has been seriously criticized. Lithium metal anodes can be composited with SSEs or quasi-solid-state electrolytes to optimize the interface contact performance and reduce the interface resistance, thereby promoting the development of solid-state batteries. (ⅲ) Composite of internal environment and external operating conditions: Composited with external physical fields, such as electric fields, magnetic fields, and temperature fields, the distribution of Li⁺ can be homogeneous and the initial nucleation process can be regulated. Overall, this review summarizes several composite anodes that have greatly optimized the performance of LMBs and highlights the potential of multi-level composites for applications in lithium metal anodes.

The ever-increasing demand for high-energy Li-ion batteries has acted as a powerful stimulus for the development of Li metal as an anode material. Li metal has long been regarded as a "Holy Grail" in Li-ion batteries due to its high discharge capacity (3860 mAh·g^-1) and low electric potential (-3.04 V). However, the formation of unstable solid electrolyte interphases (SEIs) and Li dendrites, as well as the resultant safety issues initiated by catastrophic dendrite growth, have greatly impeded further application. High ion conductivity, surface electron insulation, and favorable mechanical strength are essential properties for an ideal SEI film, which can allow for uniform Li deposition, providing a fast transfer path for Li ions and suppressing Li dendrite growth. Therefore, designing a functional SEI layer is an effective strategy to solve the problems encountered with Li metal anodes. So far, a variety of inorganic, organic, and inorganic/organic hybird SEI layers have been designed and fabricated. Inorganic SEIs are characterized by mechanical strength and ion conductivity; organic SEIs are flexible and have electron insulation properties. Inorganic/organic composite SEIs show favorable ion conductivity derived from the inorganic components, electron insulation properties originating from the organic components, and mechanical strength benefiting from the reinforcing effect between the inorganic and organic components. Oxides, metal sulfides, lithium nitride (Li₃N) and its derivates, lithium halide (LiX, X = F, Cl), two-dimensional (2D) layered structure materials, lithium phosphate and "Janus" composite are the representative examples of inorganic SEIs. The design principle of various SEI layers is based on the inhibition of Li dendrite formation and growth. Therefore, it is a prerequisite to better understand the relevant intrinsic mechanisms. Despite past investigations, further studies are still required to fully elucidate the related mechanisms by providing more broadly accepted evidence combined with theoretical calculations and offer reliable guidance for the design of multifunctional SEI layers, boosting the performance of Li metal anodes. In this review, on the basis of the mechanisms underlying Li dendrite formation and growth, strategies for constructing various functional SEI films, highlights in structure and property of the films, and their effects on the performance of Li metal anodes are summarized. Moreover, some challenges encountered with the practical applications of Li metal anodes and the future direction for the development of Li metal anodes are addressed. This review can reveal possible strategies for the commercialization of high-energy, safe and stable Li-ion batteries.

Lithium (Li) metal is considered as the most promising anode material for high-energy-density batteries owing to its ultra-high theoretical capacity (3860 mAh·g^-1) and the lowest negative electrochemical potential (-3.040 V versus standard hydrogen electrode). However, the unstable solid electrolyte interphase (SEI) layers, uncontrollable dendrite growth, and huge volume changes during the plating/stripping processes significantly limit the practical applications of Li metal anodes. Since the unstable SEI layers can promote the nucleation and growth of Li dendrites, they play a crucial role in the decay process of Li metal anodes. The fracture and regeneration of SEI layers continuously consume electrolytes and Li metal anodes during plating/ stripping processes, and the accumulation of SEI layers can increase the interface impedance. Therefore, building artificial interphase layers is one of the most effective strategies to construct a stable SEI, reduce dendrite growth, accommodate large volume changes, and thus obtain excellent cycling performance. In this review, artificial interphase layers have been summarized into three parts based on the conductive properties of interphase, including artificial SEI layers (electronically insulating while ionically conducting), mixed ionic and electronic conductor interphase layers, and nanostructured interphase passivation layers (both ionically and electronically insulating). Artificial SEI layers with high ionic conductivity and low electronic conductivity can be classified into inorganic, organic, and organic/inorganic complex SEI according to the composition of artificial SEI layers. The artificial inorganic SEI layers with a high Young's modulus can suppress the dendrite growth. The artificial organic SEI layers with flexible features can accommodate large interface fluctuations and improve the interphase wettability. The artificial organic/inorganic complex SEI layers with a rigid-flexible structure can restrain dendrite growth and buffer volume change. The mixed ionic and electronic conductor layers possess high ionic conductivity and high Young's modulus, which are beneficial for enhancing the interphase stability and reducing dendrite growth. The artificial alloy mixed conductor layers can improve the Li diffusion coefficient and reduce Li nucleation overpotential, guiding uniform Li plating/stripping. Furthermore, the artificial mixed conductor layers comprising inorganic and organic matter have commendable flexibility and excellent interface compatibility, thereby enhancing the interphase stability and reducing dendrite growth. The nanostructured interphase passivation layers with high chemical stability can deliver Li ion through a confined electrolyte in a uniform porous structure, thereby achieving homogeneous Li plating/stripping. In addition, the structure-effective relationship of artificial interphase layers has been analyzed, and methods for improving the performance of artificial interphase layers, such as physical and chemical stability, ion transportation, interface strength and flexibility, and interfacial compatibility, have been discussed in this review. Finally, we present the main challenge and perspectives of artificial interphase layers.

The attention towards lithium (Li) metal anode (LMA) has been rekindled in recent years as it can augment the energy density of Li batteries due to its high theoretical specific capacity (3860 mAh·g^-1) and low electrochemical potential (-3.04 V versus standard hydrogen electrode), especially when paired with Li-free cathodes such as Li-oxygen and Li-sulfur. However, severe interfacial instability and safety concerns on rechargeable LMA, associated with Li dendrite formation, continuous side reactions, and infinite volume changes, extremely hinder its commercialization. Numerous strategies have been employed to modify LMA for realizing a uniform distribution of the Li ion flux through interface and dendrite-free Li deposits during repeated Li plating/stripping, which leads to a better cycling performance; however, to the best of our knowledge, a clear understanding of the Li deposition/dissolution behavior and the nucleation growth mechanism of Li dendrites is still lacking, which is conducive to more efficient modification studies on LMA. Therefore, it is critical to achieve considerable progress in the development of advanced characterization techniques. However, the high reactivity of Li metal, which leads to complexity of products and diversity in morphology, causes many difficulties in the characterization of in situ spectroscopy. Recently, some promising characterization techniques have been introduced to further investigate the evolution of LMA during cycling, such as cryo-electron microscopy, solid-state nuclear magnetic resonance technology, and neutron depth profiling (NDP) technique. Because of its high-penetration characteristics, quantitative and nondestructive merits, and highly selective sensitivity to ⁶Li via the capture reaction with neutrons, the NDP technique shows a broad application prospect for obtaining real-time information of the electrochemical behavior of Li in Li metal batteries. The NDP results contain a wealth of information about time and space for Li. Accordingly, not only can the real-time distribution and migration of Li ions be detected, but also changes in the active sites of Li deposition/dissolution can be analyzed to understand the formation principle of Li dendrites and the failure mechanism of Li metal batteries. In addition, the NDP technique has shown its potential in the diagnosis and prediction of short circuit in Li metal batteries, which is confirmed through voltage curves. This review first briefly introduces the principle of the NDP technique and the methods for improving its space/time resolution; second, it summarizes the recent use of the NDP technique in the research on LMAs based on liquid or solid cell systems. Finally, it provides a prospect for the future development of NDP technique.

Conventional lithium-ion batteries with graphite anode have gradually ceased satisfying demand for the rapid development of modern electric commodities, such as portable electronic devices and electric vehicles. Therefore, metallic lithium is considered the ultimate alternative anode material for future high-energy-density lithium batteries because of its excellent properties, including the highest theoretical capacity and lowest potential of available materials as well as its low density. However, research on lithium metal anodes in traditional liquid batteries has encountered impediments. Numerous studies have shown that lithium dendrites, dead lithium, solid electrolyte interphase problems, and the correlating safety hazards are the main hindrances to the practical application of liquid-based lithium metal batteries. For solid-state batteries, the challenges of lithium metal anodes continue to grow. Studies on the mechanical, thermal, chemical, and electrochemical stability of solid-state electrolyte and lithium metal anode indicate that, unlike early recognition, solid-state lithium metal batteries remain far from commercialization. Unexpected issues like lithium growth along crystal boundaries, mixed-conductivity interphase generation, and interfacial contact losses have emerged that complicate the solid-state lithium metal battery. To achieve practically applicable lithium metal anodes, it is necessary to deepen our understanding of the basic scientific issues. This review systematically discusses the electrode behaviors of lithium metal and the corresponding electrode characterization techniques at multiple spatial scales. First, the basic science and technology issues of lithium metal anodes at different scales are reviewed. Lithium electrodeposition behaviors from the atomic to the macroscale are divided into ion transportation, deposition, nucleation, crystallization, expansion and growth. Various issues are also categorized among different characteristic scales. Second, advanced characterization techniques for all spatial scales are reviewed in light of recent works. Finally, the technical characteristics of various characterization techniques from the atomic to macroscale are analyzed. Features and possible directions of improvement of various characterization techniques used to examine lithium metal anodes in solid-state batteries are highlighted. In situ observation has become a common requirement for battery characterization as it can connect macroscale phenomena to microscale mechanisms. Meanwhile, non-damaging detection techniques have faced growing demand because of the urgent need to understand the complete actual reactions at the bulk and interfaces of solid-state electrolytes and lithium anodes. The combination of techniques for different scales should provide comprehensive information to characterize lithium metal anodes and identify reasonable mechanisms for their behaviors.

The Li metal anode is considered the most promising anode for next-generation high energy density batteries owing to its high theoretical capacity and low electrode potential. The development of batteries with high energy density is essential to meet the growing demand for energy storage devices in the modern world. However, the Li metal anode has operational problems. The high activity of Li causes dendritic growth during the cycling process, which leads to the cracking of the SEI (solid-electrolyte interphase), increased side reactions, and formation of dead Li. Furthermore, if the growth of Li dendrites is left uncontrolled, it can penetrate the separator and create a short-circuit accompanied by thermal runaway. Additionally, the complete utilization of active Li is challenged by the infinite volume expansion of the Li anode. To improve the application scope of Li metal batteries, it is imperative to develop advanced strategies for inhibiting Li dendritic growth, enhancing the stability of the SEI, reducing the accumulation of dead Li, and buffering the volume expansion. Understanding the mechanisms and models of Li nucleation and growth provides insight into solving these problems. This review summarizes some of the important models of Li nucleation and growth such as the surface nucleating model, charge-induced model, SEI model, and deposition/dissolution model. These models aid comprehension of the Li nucleation and growth process under various conditions. This review also discusses the strategies explored in the literature for improving the electrodes (such as three-dimensional (3D) matrix), electrolyte, SEI, and separator to realize uniform deposition of Li and improved utilization of Li. The 3D matrix strategy for improving the electrode design explores various matrices including graphene-based, carbon fiber-based, porous metal-based, and powder-based for buffering the volume expansion and reducing the local current density. To improve the electrolyte, concentrated lithium salts and functional additives are employed to stabilize the SEI and inhibit dendritic growth by regulating the chemical composition of SEI and inducing the deposition of Li. With respect to improving the design of the SEI, strategies for the construction of inorganic or organic components with high ionic conductivity and stable structure are explored for even distribution of Li ions and to avoid SEI rupture. This can reduce electrolyte consumption and dead Li formation. The modification of the separator by functional nanocarbon layer can control the direction of dendritic growth, thereby preventing the penetration of dendrites into the separator and achieving a uniform Li deposition layer. Finally, all solid state Li metal batteries (ASSLMBs) are discussed that utilize ceramic and polymer electrolytes owing to high safety of the solid state electrolyte. Therefore, reducing the interfacial resistance and suppressing dendritic growth between the Li anode and the electrolyte is key for the practical applications of ASSLMBs. Overall, this review provides a summary and outlook for promoting the practical applications of Li metal batteries.

Lithium metal batteries (LMBs) are representative systems for high-energy-density batteries. The design of LMBs with high capacity and high cycle stability is imperative. However, the development of LMBs is hindered by typical interface-related problems such as lithium dendrite growth, incompatible separator interfaces, and unstable cathode interfaces because of the inhomogeneous ionic flux and composition distribution. The intrinsic instability significantly hinders electron/ion transfer at the interface, causing serious issues such as dendrite growth, volume changes, low coulombic efficiency, dead lithium, interface deterioration, capacity degradation, and loss of safety. Metal-organic frameworks (MOFs) are organic-inorganic hybrid materials with a stable highly porous structure, which can allow for highly efficient gas adsorption, separation and purification, catalysis, etc., in addition to facilitating their application in nanomedicine and other fields. In recent years, MOFs have attracted much attention in the field of LMBs as a possible solution to the typical interface problems abovementioned. The porous structure and open metal sites (OMs) of MOFs provide an excellent interface structure for uniform and high ionic conductivity. As additional bonus, the stable structure provides high mechanical strength with different functional groups and metal sites, resulting in significant versatility of functionality for interface stabilization. MOFs are usually synthesized by hydrothermal/solvothermal, microwave-assisted, electrochemical, and spray-drying methods. The excellent properties of MOFs have prompted researchers to pursue their rational design and modification. Much progress has been made in this direction, and exemplary investigations have been performed to solve the abovementioned interfacial problems encountered with LMBs. Consequently, metallic lithium deposition frameworks, artificial solid electrolyte interface films, electrolyte additives, separator materials, cathode materials for lithium-sulfur batteries, and lithium-air batteries have been developed. However, there is a long way to go before the commercialization of batteries based on MOF materials. In practical, more complex electrochemical reactions occur at the lithium-metal interface, and the operating conditions (temperature, over charging/discharging, external stress, etc.) vary widely. Moreover, MOFs as electrode materials have intrinsic drawbacks, including structural collapse, pore blockage, and low inherent conductivity during the cycles. Based on these interfacial challenges, in LMBs, it introduces the structural characterization and optimization of MOFs and the key chemical components that determines the MOFs of structure (central atom, organic ligand, etc.). Subsequently, we summarized the growth mechanism of lithium dendrites and discussed the applications of MOFs and their derivatives to battery cathodes, separators, anodes, and electrolytes.The manuscript contents would be a guide to solve the problem of unstable interfaces in LMBs by the use of MOFs. Furthermore, the prospects and rational design of MOF-based materials are discussed.

Lithium ion batteries have been widely used in the fields of portable energy storage devices and electric vehicles due to their high energy density and high safety, and have a profound impact on modern society. However, the frequent occurrence of battery fire and explosion accidents has caused widespread concern of thermal runaway and thermal safety issues. Many reviews have reported the measures to mitigate thermal runaway of lithium ion batteries. Due to the use of graphite with a smaller capacity as the negative electrode material, the specific energy of lithium ion batteries has approached the theoretical limit, and there is an urgent need to develop more efficient electrode materials to meet the growing demand of the energy storage market. Lithium metal anode has smaller density, higher theoretical capacity and lower potential, which is the ideal anode material for the next generation of high energy density battery system. However, the high reactivity of lithium will cause uncontrollable lithium dendrite growth during the cycle, which may penetrate the separator and cause internal short circuit of the battery, and then cause thermal runaway, fire and even explosion. Therefore, the thermal runaway of lithium metal batteries are more complicated and serious, which hinders the commercial application of batteries. Aiming at the thermal runaway problem of lithium metal batteries, this article first introduces the causes of thermal runaway, which are mainly uncontrollable exothermic reactions caused by internal short circuits. The basic process of thermal runaway is divided into three stages. By analyzing the three characteristic temperatures and heating rate, it is proved that improving the thermal stability of electrolyte and seperator can alleviate thermal runaway. Then we investigated the influence of thermodynamics on the nucleation and growth of lithium dendrites, revealing the dual effects of temperature, and proving that a uniform thermal field is beneficial to obtain uniform lithium deposition and improve battery cycle performance and safety. Secondly, a variety of strategies to improve battery thermal safety are reviewed at the material level. In terms of liquid electrolytes, the development of non-flammable electrolyte systems includes the use of flame-retardant electrolytes and ionic liquid electrolytes with lower flammability. In addition, high-concentration electrolyte and local high-concentration electrolyte can change the solvation structure of lithium ions, and improve safety by reducing the number of free solvent molecules. In terms of separators, high thermal stability separators and thermal response separators with thermal shutdown function have been developed. The flame-retardant separators can release flame retardants to inhibit combustion. In addition, the new intelligent separators have dendrite detection, early warning and elimination functions, which effectively improve the safety and cycle life of the battery. In terms of solid electrolytes, thermally responsive polymer electrolytes have been developed to avoid thermal runaway through the strain function of polymer materials. Finally, further research on the thermal runaway of lithium metal batteries in the future is prospected.

Li is highly attractive anode material for next-generation high-energy-density batteries, such as Li-air, Li-sulfur, and solid-state Li-based systems because of its exceedingly low electrode potential (-3.04 V vs the standard hydrogen electrode) and ultra-high theoretical capacity (3860 mAh-g^-1). However, Li metal anodes and Li-based batteries are plagued by issues, including unstable solid electrolyte interface (SEI), dead Li formation, and uncontrollable dendritic growth. These limitations result in low cycling stability and could induce short circuits, thermal runaway, and safety hazards. In recent years, a variety of efficient strategies have been proposed to alleviate the challenges faced by Li anodes. For example, the design of Li-free anodes (with Li supplied from the lithiated cathode) or Li-composite anodes has attracted significant attention. Their population can be ascribed to the use of non-excessive Li metal that could be potentially safer and easier to produce. In Li-free and Li-composite anodes, the initial nucleation sites play a crucial role in influencing the subsequent Li electroplating behavior. Stable, homogenous Li electrodeposition is crucial for improving Coulomb efficiency and inhibiting dendrite formation. Moreover, it is also desirable to explore the nucleation and growth mechanism of Li metal on substrates or current collectors. Therefore, in this article, we aim to provide an overview of the mechanism of Li nucleation and strategies to enhance Li metal batteries via substrate modification. The mechanisms of Li nucleation are discussed in terms of nucleation-driven forces and the relation between nuclei size/distribution and overpotential/current density. Heterogeneous nucleation and Chazalviel space charge models are introduced to describe the deposition behaviors of Li in the initial nucleation stage. In the heterogeneous nucleation process, the formation of Li nuclei and its kinetics depend on the nucleation barrier, which correlates with the properties of substrates, such as their crystal structure, lattice matching, facets, and defects. The space charge model can be applied to low-concentration electrolytes or rapid Li deposition, where the decrease in ion concentration on the electrode surface induces a localized space charge and polarized electric field. This subsequently affects the microstructure and morphology of the deposited Li. After discussing the nucleation mechanism and substrate effect, strategies to stabilize nucleation and suppress dendrite are highlighted, such as three-dimensional frameworks, heterogeneous crystal nuclei, Li storage buffer layers, electric field effects, and lattice matching engineering. Information gained from the perspective of Li nucleation and the substrate effect might enlighten the development of strategies to upgrade metallic Li anodes for application in Li-based batteries.

Optogenetics transforms specific types of neurons through genetic engineering to achieve the cell membrane expression of photosensitive channel protein. When a specific wavelength of light irradiates the photosensitive channel protein, the cell is either excited or inhibited. Optogenetics provides a precise and fast method to control the activity of individual neurons for neuroscience research, which has gained increasing attention as a means of neural regulation. To realize the photogenetic regulation of neurons, light should be introduced into the brain safely and efficiently. Thus, specialized photoelectric devices are needed. Optrode plays a significant role in the application of optogenetics tools, which is the technical basis for the application of optogenetics. Optrode is a kind of implantable neural interface device. It can introduce light into the brain to regulate neural activity and record the changes of neural electrical signals under the control of lights. As the research of optogenetic technology continues, More and more optrodes are being developed and applied in the study of neuroscience and diseases, such as neural circuit, cognition and memory, epilepsy, and sensory function damage. The combination of optrode with optogenetic technologies provides various developmental modes in terms of material selection, device structure, light supply method, and integrated ways. The difficulty in fabricating optrodes lies in performing light stimulation and electrical signal recording without causing the immune rejection of the test animal and affecting its normal physiological activities simultaneously. In this study, based on structural characteristics and manufacturing process, optrodes are classified into two categories: waveguide-based and micro-light emitting diode-based. Subsequently, based on manufacturing process and light supply method, waveguide-based optrodes are further divided into optical fiber-optrode, optical waveguide-optrode based on MENS technology, and LD/LED waveguide-optrode. Similarly, micro-light emitting diode-based optrodes are divided into hard μLED optrode and soft μLED optrode. The advantages and disadvantages of different types of optrodes, as well as the evolution direction, are reviewed and summarized. Additionally, problems with existing optrodes, such as signal quality, biocompatibility, and device reliability, are discussed. Further, the ideal form of the device is presented as possessing the following characteristics: μLED and recording electrode integrated on flexible substrate, small size, high spatial resolution, high biocompatibility, wireless energy supply, wireless data transmission, etc. As optrode technologies are continuously updated, in the application of optogenetic technologies, research on brain neural circuit and functional structure will be better studied, and various nerve diseases will be gradually tamed.

A human brain is composed of a large number of interconnected neurons forming a neural network. To study the functional mechanism of the neural network, it is necessary to record the activity of individual neurons over a large area simultaneously. Brain-computer interface (BCI) refers to the connection established between the human/animal brain and computers/other electronic devices, which enables direct interaction between the brain and external devices. It plays an important role in understanding, protecting, and simulating the brain, especially in helping patients with neurological disorders to restore their impaired motor and sensory functions. Neural electrodes are electrophysiological devices that form the core of BCI, which convert neuronal electrical signals (carried by ions) into general electrical signals (carried by electrons). They can record or interfere with the state of neural activity. The Utah Electrode Array (UEA) designed by the University of Utah is a mainstream neural electrode fabricated by bulk micromachining. Its unique three-dimensional needle-like structure enables each electrode to obtain high spatiotemporal resolution and good insulation between each other. After implantation, the tip of each electrode affects only a small group of neurons around it even allowing to record the action potential of a single neuron. The availability of a large number of electrodes, high quality of signals, and long service life has made UEA the first choice for collecting neuronal signals. Moreover, UEA is the only implantable neural electrode that can record signals in the human cerebral cortex. This article mainly serves as an introduction to the construction, manufacturing process, and functioning of UEA, with a focus on the research progress in fabricating high-density electrode arrays, wireless neural interfaces, and optrode arrays using silicon, glass, and metal as that material of construction. We also discuss the surface modification techniques that can be used to reduce the electrode impedance, minimize the rejection by brain tissue, and improve the corrosion resistance of the electrode. In addition, we summarize the clinical applications where patients can control external devices and get sensory feedback by implanting UEA. Furthermore, we discuss the challenges faced by existing electrodes such as the difficulty in increasing electrode density, poor response of integrated wireless neural interface, and the problems of biocompatibility. To achieve stability and durability of the electrode, advancements in both material science and manufacturing technology are required. We hope that this review can broaden the scope of ideas for the development of UEA. The realization of a fully implantable neural microsystem can contribute to an improved understanding of the functional mechanisms of the neural network and treatment of neurological diseases.

Optogenetics is a neuromodulation technology that combines light control technology with genetic technology, thus allowing the selective activation and inhibition of the electrical activity in specific types of neurons with millisecond time resolution. Over the past several years, optogenetics has become a powerful tool for understanding the organization and functions of neural circuits, and it holds great promise to treat neurological disorders. To date, the excitation wavelengths of commonly employed opsins in optogenetics are located in the visible spectrum. This poses a serious limitation for neural activity regulation because the intense absorption and scattering of visible light by tissues lead to the loss of excitation light energy and also cause tissue heating. To regulate the activity of neurons in deep brain regions, it is necessary to implant optical fibers or optoelectronic devices into target brain areas, which however can induce severe tissue damage. Non- or minimally-invasive remote control technologies that can manipulate neural activity have been highly desirable in neuroscience research. Upconversion nanoparticles (UCNPs) can emit light with a short wavelength and high frequency upon excitation by light with a long wavelength and low frequency. Therefore, UCNPs can convert low-frequency near-infrared (NIR) light into high-frequency visible light for the activation of light-sensitive proteins, thus indirectly realizing the NIR optogenetic system. Because NIR light has a large tissue penetration depth, UCNP-mediated optogenetics has attracted significant interest for deep-tissue neuromodulation. However, in UCNP-mediated in vivo optogenetic experiments, as the up-conversion efficiency of UCNPs is low, it is generally necessary to apply high-power NIR light to obtain up-converted fluorescence with energy high enough to activate a photosensitive protein. High-power NIR light can cause thermal damage to tissues, which seriously restricts the applications of UCNPs in optogenetic technology. Therefore, the exploration of strategies to increase the up-conversion efficiency, fluorescence intensity, and biocompatibility of UCNPs is of great significance to their wide applications in optogenetic systems. This review summarizes recent developments and challenges in UCNP-mediated optogenetics for deep-brain neuromodulation. We firstly discuss the correspondence between the parameters of UCNPs and employed opsins in optogenetic experiments, which mainly include excitation wavelengths, emission wavelengths, and luminescent lifetimes. Thereafter, we introduce the methods to enhance the conversion efficiency of UCNPs, including optimizing the structure of UCNPs and modifying the organic dyes in UCNPs. In addition, we also discuss the future opportunities in combining UCNP-mediated optogenetics with flexible microelectrode technology for the long-term detection and regulation of neural activity in the case of minimal injury.

Neural interfaces have contributed significantly to our understanding of brain functions as well as the development of neural prosthetics. An ideal neural interface should create a seamless and reliable link between the nervous system and external electronics for long periods of time. Implantable electronics that are capable of recording and stimulating neuronal activities have been widely applied for the study of neural circuits or the treatment of neurodegenerative diseases. However, the relatively large cross-sectional footprints of conventional electronics can cause acute tissue damage during implantation. In addition, the mechanical mismatch between conventional rigid electronics and soft brain tissue has been shown to induce chronic tissue inflammatory responses, leading to signal degradation during long-term studies. Thus, it is essential to develop new strategies to overcome these existing challenges and construct more stable neural interfaces. Owing to their unique physical and chemical properties, one-dimensional (1D) and two-dimensional (2D) nanomaterials constitute promising candidates for next-generation neural interfaces. In particular, novel electronics based on 1D and 2D nanomaterials, including carbon nanotubes (CNTs), silicon nanowires (SiNWs), and graphene (GR), have been demonstrated for neural interfaces with improved performance. This review discusses recent developments in neural interfaces enabled by 1D and 2D nanomaterials and their electronics. The ability of CNTs to promote neuronal growth and electrical activity has been proven, demonstrating the feasibility of using CNTs as conducting layers or as modifying layers for electronics. Owing to their good mechanical, electrical and biological properties, CNTs-based electronics have been demonstrated for neural recording and stimulation, neurotransmitter detection, and controlled drug release. Different from CNTs-based electronics, SiNWs-based field effect transistors (FETs) and microelectrode arrays have been successfully demonstrated for intracellular recording of action potentials through penetration into neural cells. Significantly, SiNWs FETs can detect neural activity at the level of individual axons and dendrites with a high signal-to-noise ratio. Their ability to record multiplexed intracellular signals renders SiNWs-based electronics superior to traditional intracellular recording techniques such as patch-clamp recording. Besides, SiNWs have been explored for optically controlled nongenetic neuromodulation due to their tunable electrical and optical properties. As the star of the 2D nanomaterials family, GR has been applied as biomimetic substrates for neural regeneration. Transparent GR-based electronics combining electrophysiological measurements, optogenetics, two-photon microscopy with multicellular calcium imaging have been applied for the construction of multimodal neural interfaces. Finally, we provide an overview of the challenges and future perspectives of nanomaterial-based neural interfaces.

Nervous system injury can disrupt communications between neurons, leading to loss of basic nerve functions and even paralysis. The clinical prognosis of nervous system injury is usually poor. This adversely affects the physical and mental health of patients and their families, and causes serious economic losses to the society. Due to slow and incomplete healing, the regenerative capacity of the nervous system is limited. Despite development of various biomedical treatment options such as, stem cell transplantation, neurotrophic factors and scaffold application, it is still very difficult to achieve adequate therapeutic effects that can benefit clinical practice. It is worth noting that nervous system components are closely related to electric fields (EFs), and a fundamental property of neurons is plasticity in response to endogenous and exogenous electrical stimulations. Electrical stimulation has been applied by researchers to induce nerve repair. This review summarizes the progress in research on EFs on neurons and applications of EFs in the treatment of peripheral nerve system and central nerve system injuries, focusing on the methods and effects of electrical stimulation. Research using direct, alternating, and pulsed EFs, with various parameters, has all demonstrated its positive effects on nerve healing and motor function recovery. Research on nanogenerators (NGs), a novel electrical stimulation technology that can convert mechanical energy into electrical energy, has achieved great progress in recent years. In biomedicine, NGs can collect the mechanical energy of human motion and convert it into electrical stimulations without requiring an external power supply, which can lead to significant innovations in electrical stimulation therapy. This review also discusses the recent applications of NGs in the treatment of nervous system diseases. NGs can be used to fabricate miniature, ultra-thin, flexible, and biodegradable healthcare devices according to different application scenarios such as in vivo or in vitro. NGs have enabled specific applications in deep brain stimulation, peripheral nerve stimulation, muscle stimulation, and sensory substitution to restore nervous system function. In order to apply electrical stimulation therapy in the clinical setting and improve the quality of life of patients with neurological injuries, further research into stimulation devices and their settings and parameters is highly desirable.

The human brain comprises over 100 billion neurons that communicate with each other via electrical activities called action potentials. Sensory perception, cognition, and behavior all emerge from these activities. Neuroengineering is a developing interdisciplinary field that employs knowledge from neurobiology, electrical and electronic engineering, materials science and engineering, computer science, and many others. Neuroengineering aims to develop tools for understanding the mechanism of brain function at the circuit level, and to further the development of neuromodulation strategy and neuroprosthetics for motor, sensory, and mental rehabilitation from disabilities and illnesses.

For high spatial and temporal resolution interfacing with neurons in the brain, implantable multielectrode arrays (MEAs) are a key member of the family of neuroengineering devices, which are designed and fabricated for in vivo electrophysiology, deep brain stimulation, and brain-computer interfaces (BCIs). On the one hand, action potential recording from MEAs can indicate the subject's mental state and movement intentions, thus enabling the BCI technology to control external motor restoration devices such as robotic arms. On the other hand, neural stimulation electrodes can modulate abnormal neural activity and treat disorders like Parkinson's disease, epilepsy, and depression. The physical and chemical properties of the electrodes, nanofabrication of arrays, and electrode–tissue interface materials are all important research subjects in translational neuroscience studies, and the utilization of nanomaterials and nanodevices continuously improves neural electrode technologies.

At present, neural interface technology is confronting numerous challenges and opportunities, especially for in vivo neural circuit analysis, neuroelectronic medicine, and functional neuromodulation. The development of neural interface devices eagerly demands super-high-density, mesoscopic recording, minimal invasion, biosignal stability, and wireless interfacing. Achievement of these next-generation neural interface technology capabilities requires collaboration between neuroscientists, neurosurgeons, material scientists, microelectronic engineers, and many others.

As a powerful tool for monitoring and modulating neural activities, implantable neural electrodes constitute the basis for a wide range of applications, including fundamental studies of brain circuits and functions, treatment of various neurological diseases, and realization of brain-machine interfaces. However, conventional neural electrodes have the issue of mechanical mismatch with soft neural tissues, which can result in tissue inflammation and gliosis, thus causing degradation of function over chronic implantation. Furthermore, implantable neural electrodes, especially depth electrodes, can only carry out limited data sampling within predefined anatomical regions, making it challenging to perform large-area brain mapping. With excellent electrical, mechanical, and chemical properties, carbon-based nanomaterials, including graphene and carbon nanotubes (CNTs), have been used as materials of implantable neural electrodes in recent years. Electrodes made from graphene and CNT fibers exhibit low electrochemical impedance, benefiting from the porous microstructure of the fibers. This enables a much smaller size of neural electrode. Together with the low Young's modulus of the fibers, this small size results in very soft electrodes. Soft neural electrodes made from graphene and CNT fibers show a much-reduced inflammatory response and enable stable chronic in vivo action potential recording for 4-5 months. Combining different modalities of neural interfacing, including electrophysiological measurement, optical imaging/stimulation, and magnetic resonance imaging (MRI), could leverage the spatial and temporal resolution advantages of different techniques, thus providing new insights into how neural circuits process information. Transparent neural electrode arrays made from graphene or CNTs enable simultaneous calcium imaging through the transparent electrodes, from which concurrent electrical recording is taken, thus providing complementary cellular information in addition to high-temporal-resolution electrical recording. Transparent neural electrodes from carbon-based nanomaterials can record well-defined neuronal response signals with negligible light-induced artifacts from cortical surfaces under optogenetic stimulation. Graphene and CNT-based materials were used to fabricate MRI-compatible neural electrodes with negligible artifacts under high field MRI. Simultaneous deep brain stimulation (DBS) and functional magnetic resonance imaging (fMRI) with graphene fiber electrodes in the subthalamic nucleus (STN) in Parkinsonian rats revealed robust blood oxygenation level dependent responses along the basal ganglia-thalamocortical network in a frequency-dependent manner, with responses from some regions not previously detectable. This review introduces the recent development and application of neural electrode technologies based on graphene and CNTs. We also discuss biological safety issues and challenges faced by neural electrodes made from carbon nanomaterials. The use of carbon-based nanomaterials for the fabrication of various soft and multi-modality compatible neural electrodes will provide a powerful platform for both fundamental and translational neuroscience research.