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.

Owing to the continuous increase in energy consumption and the growing depletion of traditional fossil fuels, the development of renewable energy is becoming increasingly urgent. Renewable energy has come to the fore, represented by geothermal energy and solar energy. However, the application of these energy sources is highly susceptible to weather, season, location, and time. Thus, these alternative energies are unstable, random, fluctuating, intermittent, and inefficient. The development of energy storage technologies can efficiently solve these problems, storing and releasing energy when needed. Among the key materials used in various energy-storage technologies, phase-change materials (PCMs) are strong candidates for smart thermal energy management and portable thermal energy sectors. As most innate PCMs face issues of low thermal conductivity, environmental pollution, and leakage over their melting point, encapsulating PCMs into supporting materials is necessary. However, these supporting materials face significant challenges in their application. First, skeleton materials should be resistant to the PCM volume changes during melting and solidification processes to achieve suitable structural stability. Second, skeleton materials should also have high thermal conductivity and a low leakage rate. Graphene aerogel (GA) has proven to be an effective supporting skeleton to improve the shape-stability of PCMs; however, the leakage caused by the phase transition and the brittleness of the network structure is a primary problem restricting its application. Skeleton materials play a crucial role in the performance of PCMs. Herein, we propose a double-pulse plating reinforcement strategy for fabricating copper@graphene aerogel (Cu@GA) as a skeleton material for phase change energy. In this design, individual nanosheets of the GA were uniformly covered and interlinked by copper particles. The Cu@GA interlinked networks ensure suitable thermal conductivity and a robust framework, beneficial for phase change heat transfer and leak-suppression performance. In addition, we prepared a PCM composite with high structural stability and low leakage rate by encapsulating octadecylamine (ODA) in Cu@GA through vacuum impregnation to ensure homogeneous ODA dispersion in the Cu@GA porous structure. The influence of different skeletons on the PCM composite leakage rate was investigated by comparing the weight change of the PCM composite. Benefiting from these structural features, the optimized composite phase change material (CPCM) Cu@GA/ODA showed a reduced leakage rate of 19.82% (w, mass fraction) compared to 80.31% (w) of GA/ODA and 72.99% (w) of GOA/ODA after 20 heat storage and release cycles. The cycled skeleton morphology was investigated using scanning electron microscopy to determine the origin of this influence. The skeleton integrity of Cu@GA/ODA was well maintained, while the three-dimensional network structures of GOA/ODA and GA/ODA showed shrinkage or collapse. Thus, the copper coating increased the skeleton's microstructural stability, conducive to high structural stability and reducing the leakage rate of the PCM composite. This study paves the way for the construction of ideal metal-coating GA composites with an excellent comprehensive performance for future phase change energy storage, porous microwave absorption, and energy storage applications.

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.

Currently, water shortage is a globally prevalent issue, with approximately 1.5 billion people in over 80 countries in the world are facing a shortage of fresh water. Among them, 300 million people in 26 countries face daily water shortages. It is estimated that by 2025, billions of people will suffer due water shortage. The desalination of seawater and other water treatment technologies have been widely investigated to solve this problem. Recently, a lot of study have been carried out on the production of clean water via solar evaporation with new materials and technologies. Under the condition of illumination, the light-absorbing material converts solar energy directly into heat energy to realize rapid and large amount of water evaporation, after condensation, clean water was obtained. It is important that this technology can effectively remove salt, bacteria, and other pollutants from raw water, and the quality of the obtained water fully meets the drinking water quality standard set by the World Health Organization. This is an efficient, green, and low-cost method for solving the shortage of water resources. Three-dimensional (3D) graphene materials have excellent physical and chemical properties, high photothermal conversion efficiency, high solar absorption rate, rich internal micro- and nano-channels, good water transmission channels, and large surface water evaporation area; in addition, they can achieve an ultra-high water evaporation rate under solar irradiation. These properties are highly significant in the research and practical applications of photothermal water treatment. In this study, the research progress of 3D-graphene is discussed with regard to the following three aspects. 1) The main preparation method of 3D-graphene was investigated. The advantages and disadvantages of different preparation methods, such as self-assembly, template, and chemical vapor deposition methods were summarized and compared. It can provide reference for readers to choose the preparation method of 3D-graphene; 2) The basic principle of photothermal water evaporation is introduced in detail. The research progress of photothermal water evaporation was summarized based on pure graphene, graphene/polymer composites, and graphene/metal oxide composites. The evaporation properties of different materials were compared. The development, fabrication, and performance of small photothermal conversion devices are briefly introduced; 3) The water treatment of graphene photothermal water evaporation was investigated, and its limitations were analyzed and summarized. Consequently, the challenges faced by photothermal evaporation in theoretical research and the problems to be solved in practical production applications are finally prospected. This review is a valuable reference for the development of 3D-graphene materials and solar-thermal steam generation and water treatment.

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.

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

Dimethyl furan-2, 5-dicarboxylate (DMFDCA) is a valuable biomass-derived chemical that is an ideal alternative to fossil-derived terephthalic acid as a monomer for polymers. The one-step oxidation of 5-hydroxymethylfurfural (HMF) to DMFDCA is of practical significance. It not only shortens the reaction pathway but also avoids the separation process of intermediates; thus, reducing cost. In this work, non-noble bimetallic catalysts supported on N-doped porous carbon (CoMn@NC) were synthesized via a one-step co-pyrolysis procedure using different pyrolysis temperatures and proportions of metal precursors and additives. We employed the prepared CoMn@NC catalysts in the aerobic oxidation of HMF under mild reaction conditions to obtain DMFDCA. High-yield DMFDCA was obtained by screening the prepared catalysts and optimizing the reaction conditions, including the strength and amount of the base, as well as the reaction temperature. The optimized yield of DMFDCA was 85% over the Co₃Mn₂@NC-800 catalyst after 12 h at 50 ℃ using ambient-pressure oxygen. The physicochemical properties of the catalysts were determined using a variety of characterization techniques, the factors affecting the performance of each catalyst were investigated, and the relationship between the physicochemical properties and performance of the prepared catalysts was elucidated. A porous structure with a high surface area had a positive effect on mass transfer efficiency. Cobalt nanoparticles (NPs) and atomically dispersed Mn were coordinated to N-doped carbon to form M―N_x (where M = Co or Mn). Based on the Mott-Schottky effect, there was significant electron transfer between each metal and the N-doped carbon, additionally, the metal NPs supplied electrons to the carbon atoms. The electron-deficient metal site in the pyridinic N-rich carbon was beneficial for the activation of HMF and oxygen. The activation of oxygen produced reactive oxygen species (such as superoxide radical anions) to ensure high selectivity to DMFDCA through dehydrogenative oxidation of the hemiacetal intermediate and hydroxymethyl group of 5-hydroxymethyl-2-methyl-furoate. The existence of disordered and defective carbons increased the number of active sites. Subsequently, we performed a series of control experiments. Based on our current experimental results and previous studies, we propose a simple mechanism for the aerobic oxidation of HMF to DMFDCA. The catalyst was stable, its performance decreased slightly after two cycles, and it was tolerant to SCN⁻ ions and resistant against N or S poisoning. Furthermore, the use of this catalytic system can be expanded to various substituted aromatic alcohols, such as benzyl alcohols with different substituents, furfuryl alcohol, and heterocyclic alcohols. Simultaneously, the product type was further extended from methyl esters to ethyl esters with a high yield when the substrate reacted with ethanol. In conclusion, this catalytic system can be applied in the production of carboxylic esters for polymers.

Lignin is a natural aromatic polymer that accounts for nearly 30% of lignocellulose and is considered the only renewable aromatic (re)source for producing aromatic chemicals or liquid fuels via the cleavage of C―O ether bonds and C―C bonds. Thus far, the majority of investigations involving the production of valuable compounds via lignin hydrogenolysis have focused on the cleavage of relatively labile C―O bonds only, which restricts the efficiency of hydrogenolysis. Therefore, in this work, a bifunctional Pt/NbPWO catalyst was synthesized using hydrothermal and wet impregnation methods. It was found that aromatic monomers with a yield of 18.02% could be obtained by breaking the C―O and C―C bonds in alkali lignin. This reaction was applicable to breaking the key C―C bonds when the C―O ether bonds were broken in lignin polymers. The hydrogenolysis mechanism most likely involves the abundant Brønsted acid and Lewis acid sites on the catalyst that facilitate C―C bond activation. Additionally, the synergy between the support and Pt species in the Pt/NbPWO catalyst was primarily emphasized.

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

Because fossil fuels are continuously depleted, valorization of biomass into valuable liquid products and chemicals is of great significance yet it remains challenging. Among many biomass-derived products, lactic acid is one of the most important renewable monomers for preparing the degradable polymer polylactic acid. The use of raw biomass to produce lactic acid through catalytic conversion is an attractive approach. In this work, the catalytic reaction performance and mechanism of different Lewis acids (Y³⁺, Sc³⁺, and Al³⁺) for the production of lactic acid from cellulose were investigated in detail by isotopic nuclear magnetic resonance (NMR) and mass spectrometry. The production of lactic acid from cellulose includes tandem and competing reactions. The order of catalytic activity for the one-pot conversion of cellulose into lactic acid is as follows: Y³⁺ > Al³⁺ > Sc³⁺. The main tandem reactions involve the hydrolysis of cellulose into glucose, the isomerization of glucose into fructose (the order of catalytic activity, the same below: Y³⁺ > Al³⁺, Y³⁺ > Sc³⁺), the cleavage of fructose via a retro-aldol reaction to glyceraldehyde (GLY) and 1, 3-dihydroxyacetone (DHA) (Sc³⁺ > Y³⁺ > Al³⁺), and the conversion of DHA or GLY to the final product lactic acid (Al³⁺ > Y³⁺ > Sc³⁺). It was found that the process of glucose isomerization to fructose was the key step to the final selectivity of the tandem reaction of cellulose conversion to lactic acid, and it was clarified that the production of lactic acid from DHA underwent a keto-enol (K-E) tautomerization process rather than a classical 1, 2-shift process. First, DHA was transformed into GLY via the isomerization process, then the adjacent hydroxyl group of GLY was removed in the form of water to produce an α, β-unsaturated species. After that, the α, β-unsaturated species underwent K-E tautomerization to generate unsaturated aldehyde-ketone intermediates. Meanwhile, a molecule of water was added to aldehyde-ketone intermediates to obtain a diol product, the hydrogen atom at the methine position was transferred and the lactic acid was finally obtained through the K-E tautomerization process. The in-depth understanding of the reaction mechanism presented in this work will help to design more selective catalysts for cellulose conversion into value-added oxygen-containing small molecule chemicals.

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

Selective hydrogenation is a vital class of reaction. Various unsaturated functional groups in organic compounds, such as aromatic rings, alkynyl (C≡C), carbonyl (C＝O), nitro (-NO₂), and alkenyl (C＝C) groups, are typical targets in selective hydrogenation. Therefore, selectivity is a key indicator of the efficiency of a designed hydrogenation reaction. 5-(Hydroxymethyl)furfural (HMF) is an important platform compound in the context of biomass conversion, and recently, the hydrogenation of HMF to produce fuels and other valuable chemicals has received significant attention. Controlling the selectivity of HMF hydrogenation is paramount because of the different reducible functional groups (C＝O, C-OH, and C＝C) in HMF. Moreover, the exploration of new routes for hydrogenating HMF to valuable chemicals is becoming attractive. 5-Methylfurfural (MF) is also an important organic compound; thus, the selective hydrogenation of HMF to MF is an essential synthetic route. However, this reaction has challenging thermodynamic and kinetic aspects, making it difficult to realize. Herein, we propose a strategy to design a highly efficient catalytic system for selective hydrogenation by exploiting the synergy between steric hindrance and hydrogen spillover. The design and preparation of the Pt@PVP/Nb₂O₅ catalyst (PVP = polyvinyl pyrrolidone; Nb₂O₅ = niobium(V) oxide) were also conducted. Surprisingly, HMF could be converted to MF with 92% selectivity at 100% HMF conversion. The reaction pathway was revealed through the combination of control experiments and density functional theory calculations. Although PVP blocked HMF from accessing the surface of Pt, hydrogen (H₂) could be activated on the surface of Pt due to its small molecular size, and the activated H₂ could migrate to the surface of Nb₂O₅ through a phenomenon called H₂ spillover. The Lewis acidic surface of Nb₂O₅ could not adsorb the C＝O group but could adsorb and activate the C-OH group of HMF; therefore, when HMF was adsorbed on Nb₂O₅, the C-OH groups were hydrogenated by the spilled over H₂ to form MF. The high selectivity of this reaction was realized because of the unique combination of steric effects, hydrogen spillover, and tuning of the electronic states of the Pt and Nb₂O₅ surfaces. This new route for producing MF has great potential for practical application owing to its discovered advantages. We believe that this novel strategy can be used to design catalysts for other selective hydrogenation reactions. Furthermore, this study demonstrates a significant breakthrough in selective hydrogenation, which will be of interest to researchers working on the utilization of biomass, organic synthesis, catalysis, and other related fields.

Glycerol is a versatile platform compound that is formed in considerable amounts as a by-product of biodiesel production. The catalytic selective hydrogenolysis of glycerol to 1, 3-propanediol (1, 3-PDO) provides a sustainable route for the synthesis of this important diol. In this study, a series of platinum-tungsten oxide (Pt-WO_x) catalysts with different WO_x surface densities dispersed on titanium(Ⅳ) oxide, zirconium(Ⅳ) oxide, and aluminum oxide supports were prepared and evaluated for the glycerol hydrogenolysis to 1, 3-PDO. The highest reaction activity and 1, 3-PDO selectivity were achieved at a WO_x density of approximately 1.5–2.0 W·nm⁻², with all three support materials. Such a strong dependence on the surface density of WO_x revealed the critical role of the dispersed WO_x domains in the hydrogenolysis of glycerol to 1, 3-PDO. The infrared spectra for carbon monoxide adsorption confirmed the electron transfer and strong interaction between the Pt particles and WO_x domains. These phenomena were hypothesized to contribute to the superior selectivity to 1, 3-PDO over 1, 2-PDO of the supported Pt-WO_x catalysts when compared with the corresponding supported Pt catalysts. The realized superior 1, 3-PDO selectivity was consistent with its higher stability on the Pt-WO_x catalysts, as reflected by the lower reaction rate constant of 1, 3-PDO than those of 1, 2-PDO and glycerol obtained in their hydrogenolysis reactions. There existed a volcano-type dependence of the glycerol reaction activity on the hydrogen partial pressure. Such a dependence, together with the measured ratio (1 : 2) of the secondary to the primary C−H bonds in 1, 3-PDO in the presence of deuterium and deuterium oxide (replacing hydrogen and water, respectively), indicated that the glycerol hydrogenolysis proceeds by the kinetically relevant dehydrogenation of glycerol to the glyceraldehyde intermediate, followed by the dehydration and hydrogenation of glyceraldehyde to 1, 3-PDO over the Pt-WO_x catalysts.

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

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

From the industrial perspective, poly(3-hexylthiophene) (P3HT) is one of the most attractive donor materials in organic photovoltaics. The large bandgap in P3HT makes it particularly promising for efficient indoor light harvesting, a unique advantage of organic photovoltaic (PV) devices, and this has started to gain considerable attention in the field of PV technology. In addition, the up-scalability and long material stability associated with the simple chemical structure make P3HT one of the most promising materials for the mass production of organic solar cells. However, the solar cells based on P3HT has a low power conversion efficiency (PCE), which is less than 11%, mainly due to significant voltage losses. In this study, we identified the origin of the high quantum efficiency and voltage losses in the P3HT: non-fullerene based solar cells, and we proposed a strategy to reduce the losses. More specifically, we observed that: 1) the non-radiative decay rate of the charge transfer (CT) states formed at the donor–acceptor interfaces was much higher for the P3HT: non-fullerene solar cells than that for the P3HT: fullerene solar cells, which was the main reason for the more severely limited photovoltage; 2) the origin of the high non-radiative decay rate in the P3HT: non-fullerene solar cell could be ascribed to the short packing distance between the P3HT and non-fullerene acceptor molecules at the donor–acceptor interfaces (DA distance), which is a rarely studied interfacial structural property, highly important in determining the decay rate of CT states; 3) the lower voltage loss in the state-of-the-art P3HT solar cell based on the 2, 2'-((12, 13-bis(2-butyldecyl)-3, 9-diundecyl-12, 13-dihydro-[1, 2, 5]-thiadiazolo[3, 4-e]thieno[2'', 3'': 4', 5']thieno[2', 3': 4, 5]p-yrolo[3, 2-g]thieno[2', 3': 4, 5]thieno[3, 2-b]indole-2, 10-diyl)bis(methanelylidene))bis(5, 6-dichloro-1H-indene-1, 3(2H)-dion-e) (ZY-4Cl) acceptor could be associated with the better alignment of the energy levels of the active materials and the longer DA distance, compared to those based on the commonly used acceptors. However, the DA distance was still very short, limiting the device voltage. Thus, improving the performance of the P3HT based solar cells requires a further increase in the DA distance. Our findings are expected to pave the way for breaking the performance bottleneck of the P3HT based solar cells.

In view of the continuously worsening environmental problems, fossil fuels will not be able to support the development of human life in the future. Hence, it is of great importance to work on the efficient utilization of cleaner energy resources. In this case, cheap, reliable, and eco-friendly grid-scale energy storage systems can play a key role in optimizing our energy usage. When compared with lithium-ion and lead-acid batteries, the excellent safety, environmental benignity, and low toxicity of aqueous Zn-based batteries make them competitive in the context of large-scale energy storage. Among the various Zn-based batteries, due to a high open-circuit voltage and excellent rate performance, Zn-Ni batteries have great potential in practical applications. Nevertheless, the intrinsic obstacles associated with the use of Zn anodes in alkaline electrolytes, such as dendrite, shape change, passivation, and corrosion, limit their commercial application. Hence, we have focused our current efforts on inhibiting the corrosion and dissolution of Zn species. Based on a previous study from our research group, the failure of the Zn-Ni battery was caused by the shape change of the Zn anode, which stemmed from the dissolution of Zn and uneven current distribution on the anode. Therefore, for the current study, we selected K₃[Fe(CN)₆] as an electrolyte additive that would help minimize the corrosion and dissolution of the Zn anode. In the alkaline electrolyte, [Fe(CN)₆]^3– was reduced to [Fe(CN)₆]^4– by the metallic Zn present in the Zn-Ni battery. Owing to its low solubility in the electrolyte, K₄[Fe(CN)₆] adhered to the active Zn anode, thereby inhibiting the aggregation and corrosion of Zn. Ultimately, the shape change of the anode was effectively eliminated, which improved the cycling life of the Zn-Ni battery by more than three times (i.e., from 124 cycles to more than 423 cycles). As for capacity retention, the Zn-Ni battery with the pristine electrolyte only exhibited 40% capacity retention after 85 cycles, while the Zn-Ni battery with the modified electrolyte (i.e., containing K₃[Fe(CN)₆]) showed 72% capacity retention. Moreover, unlike conventional organic additives that increase electrode polarization, the addition of K₃[Fe(CN)₆] not only significantly reduced the charge-transfer resistance in a simplified three-electrode system, but also improved the discharge capacity and rate performance of the Zn-Ni battery. Importantly, considering that this strategy was easy to achieve and minimized additional costs, K₃[Fe(CN)₆], as an electrolyte additive with almost no negative effect, has tremendous potential in commercial Zn-Ni batteries.

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

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

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

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

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

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

Compared to traditional sensor device arrays, optical fiber systems capable of wide-range detection are gradually emerging as strong candidates for distributed monitoring owing to their simplified structure. However, the working mechanism of optical fiber sensors limits their use to the detection of physical parameters such as refractive index and is an obstacle for the detection of small doses of molecules by optical fiber systems. Several researchers have focused on this aspect to endow sensitivity to these optical fibers for gas or liquid molecules. By deliberately destroying the fiber structure, strong interactions between the evanescent field of optical fibers and the target materials, such as microfibers, D-shaped fiber, etc. can be achieved. Assisted by the surface plasmon resonance techniques, such configurations can exhibit highly enhanced sensitivity to a change in the refractive index caused by gas or liquid molecules. Two-dimensional materials are an excellent candidate as coating materials due to their high specific surface area, which also guarantees a large sensing response and simultaneously minimizes any side effects by suppressing the propagating mode of optical fibers. However, owing to the obstacles in optical fiber engineering and device fabrication, the abovementioned functional 2D sensors are still limited to sample-scale fabrication, and their mass-production has not yet been realized. An all-fiber distributed sensing system with high single-spot sensitivity is still difficult to fabricate. Here, we propose a new configuration of a grid-distributed environmental optical fiber sensing by introducing low-pressure chemical vapor deposition (LPCVD)-grown graphene photonic crystal fiber (PCF) into the optical fiber sensing system. We successfully synthesized monolayer and/or bilayer graphene in the air holes of PCF. By fusing the graphene PCF (Gr-PCF) to a single mode optical fiber, we fabricated an all-optical-fiber sensing system. Preliminary experiments suggest that Gr-PCF can selectively detect NO₂ gas at ppb-level and exhibit ionic sensitivity in liquids. The ability to detect NO₂ gas is attributed to the graphene layer's interaction among light-mode and adsorbed molecules: adsorption-induced additional hole-doping caused a shift in the Fermi level of graphene and eventually modulated its light absorption, leading to changes in the light intensity signals. We believe that the sensor can be extended to other kinds of gases and liquids, considering the affinity of graphene toward various molecules. In view of practical optical sensors, our design is compatible with the time domain or wavelength domain multiplexing techniques of optical fiber communication systems. Because CVD-based synthesis can be used to realize mass production, the design proposed herein shall be one of the answers to the distributed optical fiber environmental sensors.

Carbon quantum dots (CQDs) have attracted extensive interest due to their strong fluorescence as well as inexpensive and plentiful resources for manufacture. There are numerous published reports on the preparation of CQDs and direct applications based on their photoluminescence. Successive chemical modification of CQDs in an appropriate manner might expand the application scope of CQDs and transform them into practical fine chemicals. The various functional groups on the surface of CQDs allow for efficient chemical modification while imparting them with hydrophilicity. Covalent linking of hydrophobic hydrocarbon chains to CQDs would lead to the formation of novel surfactants. Here, a technique for preparing CQD-based cationic surfactants is depicted in detail. This was rare to be reported according to recent publishes. First, a mixture of ethylenediamine tetraacetic acid and ethylenediamine in the presence of hydrogen peroxide in an aqueous medium was pyrolyzed at 180 ℃ for 60 min. The resulting CQDs are represented as OX-CQDs. Then, the OX-CQDs were subjected to quaternization with 1-chlorododecane for obtaining the cationic surfactant (OX-CQDs-C₁₂H₂₅). The OX-CQDs-C₁₂H₂₅ surfactant effectively decreased the surface tension of water from 72.0 to 26.7 mN∙m⁻¹ at the critical micelle concentration of 5.0 mg∙mL⁻¹, thus demonstrating superior performance over several new Gemini cationic surfactants. The OX-CQDs-C₁₂H₂₅ surfactant also decreased the contact angles of water considerably. However, when longer alkyl chains such as －C₁₄H₂₉ or －C₁₆H₃₃ were attached to the CQDs, the corresponding surfactant was less effective in decreasing the surface tension of water. Calculations based on the Gibbs absorption isothermal equation revealed that two more －C₁₂H₂₅ chains were bonded with a carbon quantum dot averagely, implying that the as-prepared CQD-cationic surfactant belonged to the category of Gemini surfactants. Quaternization with 1-chlorododecane also led to a notable enhancement in the antibacterial activity for Escherichia coli as compared with that of unmodified CQDs. The antibacterial percentage approached 100% even the solution was diluted to 0.41 mg∙mL⁻¹, which was much lower than the critical micelle concentration. The fluorescence quantum yield of OX-CQDs-C₁₂H₂₅ reached 6.44%. Experimental results revealed that hydrogen peroxide played a positive role in improving the surface activity and fluorescence quantum yield of OX-CQDs-C₁₂H₂₅. The surface activity, antibiosis, and fluorescence endowed the versatilities of OX-CQDs-C₁₂H₂₅. This novel, economical technique for synthesizing cationic surfactants eliminates the need for introducing hydrophilic groups. The hydrothermal approach for preparing CQDs satisfies the demand for green chemical synthesis. From this aspect, our technique provides efficient access to synthesizing cationic surfactants.

Morphology regulation and the improvement of carrier separation efficiency are important strategies for the preparation of photocatalysts with excellent performance. MoS_x with a three-dimensional (3D) nanoflower morphology formed by nanosheet stacking was prepared by a simple hydrothermal method, and MoS_x/In₂O₃ with good hydrogen evolution activity was obtained by coupling with In₂O₃. The preparation of the three-dimensional nanoflower morphology combined with the construction of an S-scheme heterojunction improves the electron accumulation at the active site for hydrogen evolution reaction. The UV diffuse reflection test showed that the issue of poor light absorption of In₂O₃ was improved. The rapid separation and transfer of electrons were effectively confirmed by characterization methods such as fluorescence spectroscopy and electrochemical tests. The most intuitively manifestation of the performance improvement of the composite material is that the optimal hydrogen evolution rate reached 6704.2 μmol∙g⁻¹∙h⁻¹, which is 1.8 times that of pure MoS_x. Therefore, in this study, a new idea for the development of molybdenum-based sulfides for photocatalytic hydrogen production is provided.

Electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF) is considered one of the most environment friendly, economical, and efficient methods for synthesizing 2,5-furandicarboxylic acid (FDCA), which is a promising bio-based precursor of polyethylene 2,5-furandicarboxylate. In this study, we synthesized PtRuAgCoNi high-entropy alloy nanoparticles, with an average diameter of approximately 9 nm, using a solvothermal method. The synthesized nanoparticles displayed a core-shell microstructure, in which Co, Ru, Ag, and Ni were distributed over the entire core-shell microstructure of each nanoparticle, while Pt was mainly concentrated in the shell structure. A two-step method, including small-molecule substitution and low-temperature calcination, was used to remove the surfactant from the synthesized nanoparticles without changing the structure and composition of the nanoparticles. After being deposited on a carbon support, the high-entropy alloy nanoparticles, with or without surfactants, exhibited better catalytic performance in the electrocatalytic oxidation of HMF to FDCA than the commercial Pt/C catalyst. The removal of surfactants after calcination at 185 ℃ can further improve electrocatalytic performance, suggesting promising application prospects of high-entropy alloy nanoparticles in electrocatalysis and green chemistry.

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