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

The gradual increase of CO₂ concentration in the atmosphere is believed to have a profound impact on the global climate and environment. To address this issue, strategies toward effective CO₂ conversion have been developed. As one of the most available strategies, the CO₂ electrochemical reduction approach is particularly attractive because the required energy can be supplied from renewable sources such as solar energy. Electrochemical reduction of CO₂ to chemical feedstocks offers a promising strategy for mitigating CO₂ emissions from anthropogenic activities; however, a critical challenge for this approach is to develop effective electrocatalysts with ultrahigh activity and selectivity. Herein, we report the facile synthesis of a highly efficient and stable atomically isolated nickel catalyst supported by ultrathin nitrogenated carbon nanosheets (Ni-N-C) for CO₂ reduction through pyrolysis of Ni-doped metal-organic frameworks (MOFs) and dicyandiamide (DCDA). MOFs are crystalline and assembled by metal-containing nodes and organic linkers, which have a large specific surface area, tunable pore size and porosity, and highly dispersed unsaturated metal centers. Thus, Ni-doped MOFs were chosen as the precursors to endow Ni-N-C with a porous carbon structure and nickel ions. The nitrogen in Ni-N-C came from DCDA, which acts as the active site to anchor nickel ions. Because of the porous structure and numerous nitrogen sites, the Ni content of Ni-N-C was as high as 7.77% (w). There were two types of nickel ion-containing structures, including Ni⁺-N-C and Ni²⁺-N-C. The structure transformation of the Ni⁺-N-C species from the initial Ni²⁺ (Ni-MOF) was achieved by pyrolysis, and the ratio of Ni⁺ and Ni²⁺ varied with the pyrolysis temperature. Compared to other Ni-N-C prepared at other temperatures, Ni-N-C-800 contained more Ni⁺-N-C species that possessed the optimum *CO binding energy and thus boosted the CO desorption and evolution rate, thereby exhibiting higher CO Faradaic efficiency (FE) up to 94.6% at -0.9 V (vs. the reversible hydrogen electrode, RHE) in 0.1 mol·L^-1 KHCO₃. In addition, it has been found that the rate of CO formation on the Ni-N-C-800 electrode relies on the electrolyte concentration. With the optimal electrolyte concentration, the Ni-N-C-800 electrode achieved a superior Faraday efficiency of > 90% for CO over a wide potential range of -0.77 to -1.07 V (vs. RHE) and displayed a CO FE as high as 97.9% with a current density of 12.6 mA·cm^-2 at -0.77 V (vs. RHE) in 0.5 mol·L^-1 KHCO₃. After testing at -0.77 V for 12 h, the Ni-N-C-800 electrode maintained a CO FE of approximately 95%, indicating superior long-term stability. We believe that this study will contribute to the design and synthesis of highly catalytically active atomically dispersed monovalent metal sites for metal-N-C catalysts.

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

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.

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

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

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

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.

Graphene-wrapped natural spherical graphite (G/SG) composites were prepared using the encapsulation–carbonization approach. The morphology and structure of the composites were characterized by scanning electron microscopy and X-ray diffraction analysis. The electrochemical performance of the composites with different graphene contents as anode materials for lithium-ion batteries was investigated by various electrochemical techniques. In the absence of acetylene black (AB), the G/SG composites were found to exhibit high specific capacity with high first-cycle coulombic efficiency, good cycling stability, and high rate performance. Compared with the natural spherical graphite (SG) electrode, the G/SG composite electrode with 1% graphene exhibited higher reversible capacity after 50 cycles; this capacity performance was equal to that of the SG + 10%AB electrode. Moreover, when the addition of 2.5% graphene, the composite electrode exhibited higher initial charge capacity and reversible capacity during 50 cycles than the SG+10%AB electrode. The significant improvement of the electrochemical performance of the G/SG composite electrodes could be attributed to graphene wrapping. The graphene shell enhances the structural integrity of the natural SG particles during the lithiation and delithiation processes, further improving the cycling stability of the composites. Moreover, the bridging of adjacent SG particles allows the formation of a highly conductive network for electron transfer among SG particles. Graphene in the composites serves as not only an active material but also a conductive agent and promotes the improvement of electrochemical performance. When 5%AB was added, the reversible capacity of the 5%G/SG electrodes significantly increased from 381.1 to 404.5 mAh·g^-1 after 50 cycles at a rate of 50 mA·g^-1 and from 82.5 to 101.9 mAh·g^-1 at 1 A·g^-1, suggesting that AB addition improves the performance of the G/SG composite electrodes. AB particles connect to G/SG particles through point contact type and fill the gaps between G/SG. A more effective conductive network is synergistically formed via graphene-AB connection. Although graphene wrapping and AB addition improve the performance of natural graphite electrodes, such as through increase in electrical conductivity and enhancement of Li-storage performance, including improvement of reversible capacity, rate performance, and cycling stability, electrode density typically decreases with graphene or AB addition, which should consider the balance between the gravimetric and volumetric capacities of graphite anode materials in practical applications. These results have great significance for expanding the commercial application scope of natural graphite. Our work provides new understanding and insight into the electrochemical behavior of natural SG electrodes in lithium-ion batteries and is helpful for the fabrication of high-performance anode materials.

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 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.

The intercalation of potassium in graphite provides high energy density owing to the low potential of 0.24 V vs. K/K⁺, thereby making it a promising anode material for potassium ion batteries. However, the high volume expansion (60%) of graphite after potassium intercalation induces significant stress and electrode pulverization. Additionally, the sluggish kinetics of potassium insertion undermine the rate capability of electrodes. Using few-layer exfoliated graphite (EG) as a negative electrode material effectively relieves expansion-induced stress. Unfortunately, the close stacking of ultra-thin two-dimensional EG impedes ion transport. Furthermore, EG with smooth surfaces lacks active sites to adsorb K⁺, which is unfavorable for intercalation reactions. To address these problems, in this study, we designed an rGO/EG/rGO sandwich that coats EG with reduced graphene oxide (rGO). This complex material has two main advantages: (1) its 3D network can effectively prevent EG from stacking and buffer the volumetric variation of EG to improve the cyclic stability of the electrode, and (2) the loose structure and rich functional groups of rGO can also enhance the kinetic of potassium intercalation. Through hydrothermal reduction, GO was coated onto the EG surface and cross-linked to form a 3D network, by which EG stacking could be effectively mitigated. The rGO : EG ratio was precisely controlled by modulating the amount of reactant GO and EG. Transmission electron microscopy and scanning electron microscopy images showed that the rGO was uniformly coated on the EG surface to form a sandwich structure. X-ray diffraction patterns and Raman spectra demonstrated that rGO was physically adsorbed on the EG surface without notable chemical interactions. The EG structure was retained to ensure that its characteristic electrochemical properties were unaffected. Cyclic voltammetry and galvanostatic cycling tests were performed on the complex material with various rGO : EG ratios, exhibiting that rGO : EG = 1 : 1 (w/w) was optimal with a specific capacity of 443 mAh·g^-1 at 50 mA·g^-1. Even when operated at a high current density of 800 mA·g^-1, a specific capacity of 190 mAh·g^-1 was achieved, retaining 42.9% of the low-rate capacity, far exceeding those of pristine EG (14.2%) and rGO (27.2%). These results demonstrate that the rGO coating indeed enhanced the kinetics of potassium intercalation and efficiently improved the capacity and rate capability compared to pristine EG. We hope this work sheds light on novel approaches to improving potassium intercalation mechanisms in graphite.

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

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.