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2022 Volume 38 Issue 2
2022, 38(2): 200604
doi: 10.3866/PKU.WHXB202006046
Abstract:
Graphene fiber material is one type of macroscopically one-dimensional materials assembled by graphene building blocks or coating graphene on other fibrous building blocks. The typical graphene fiber materials can be classified into graphene fiber and graphene-coated hybrid fiber based on their different building blocks. This type of materials exhibits superior tensile strength, excellent electrical and thermal conductivities, making them favorable for applications in flexible energy storage devices, electromagnetic shielding and wearable electronics. Recently, the chemical vapor deposition (CVD) method, conventionally used for fabricating film-like graphene, has been widely applied to the synthesis of graphene fiber materials. For preparing graphene fiber, the use of CVD method can prevent the complicated and time-consuming reducing treatment of graphene oxide (GO), which is well known as an imperative step in the commonly used wet spinning method. For preparing graphene-coated hybrid fiber, the CVD method can achieve an efficient modulation of graphene quality, and ensure a strong adhesion between graphene and fibrous substrates. In this review, we summarized the CVD methods for fabricating graphene fiber materials, including graphene-assembled graphene fiber and graphene-coated hybrid fiber, and introduced their excellent mechanical, electrical, thermal and optical properties along with their broad applications in intelligent sensors, optoelectronic devices, and flexible electrodes. Furthermore, the challenges in synthesizing CVD-fabricated graphene fiber materials were also analyzed. This review can be briefly divided into three parts: (1) Synthesis of graphene fibers: Up to now, the CVD method is a feasible and effective way to synthesize graphene with high crystallinity. The CVD strategies for fabricating graphene fibers mainly consist of the template method, the secondary growth method, and the film-scrolling method, which can simplify the fabrication process and efficiently modulate graphene quality. (2) Synthesis of graphene glass fibers: Similar to graphene growth directly on non-catalytic glass surfaces, CVD method can also be applied to synthesize graphene on glass fibers. By modifying the experimental parameters (carbon source, pressure, temperature, etc.), high-quality graphene films with controllable thickness can be uniformly coated on glass fibers. Meanwhile, the as-fabricated graphene glass fiber can be further used as a high-performance flexible electrode, electro-optic modulator, or electrocatalyst. (3) Synthesis of graphene metal fibers: Graphene can be controllably grown on metal fibers using the CVD method. Compared to the bare metal fiber, the fabricated graphene metal fiber exhibited enhanced electrical and thermal conductivities as well as better chemical stability, which can expand its applications in ultra-thin electronics and high-power circuits. ![]()
Graphene 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.
2022, 38(2): 200706
doi: 10.3866/PKU.WHXB202007068
Abstract:
Graphene wafers have emerged in response to the increasing demand of wafer-scale two-dimensional materials and high-performance on-chip devices in the field of integrated circuits, microelectromechanical systems, and sensors. Wafer-scale graphene films display great application potential, owing to their atomic layer thickness, excellent thermal and electrical conductivity, and compatibility with wafer-processing technology. Therefore, batch production of graphene wafers is exceedingly crucial. The chemical vapor deposition (CVD) method has attracted attention for the production of high-quality graphene wafers with high controllability, high compatibility, and low cost. Mature CVD manufacturing has been widely employed in the semiconductor industry to date, aiding in the industrialization of CVD graphene wafers and creating opportunities for graphene to enter the new era of nanoelectronics. The quality of graphene wafers has a significant impact on the subsequent device fabrication; hence, considerable efforts have been made to date to realize precise control over domain size, structural defects, and layer thickness during synthesis. In this study, we summarize the recent progress made in wafer-scale CVD graphene synthesis. Initially, we introduce the quality requirements of graphene wafers targeting various application scenarios, and propose the classification of graphene wafers. Single crystallinity is considered to be a key requirement for the graphene used in high-performance electronics and optoelectronics. We then review the recent CVD-derived graphene wafers with regard to substrate types (metal/nonmetal), highlighting the constrictions in graphene quality and corresponding synthetic solutions. Batch synthesis of graphene wafers is further discussed. The significant role of flow dynamics in the up-scaling process is emphasized, followed by relevant experimental instances based on computational fluid dynamics simulations. Finally, strategies for obtaining graphene wafers are overviewed, with the proposal of future perspectives. This study focuses on three areas: (1) Application requirements for the quality of graphene wafers, including target substrate types and as-grown graphene features (chemical stability and electrical and thermal properties), (2) CVD strategies of graphene wafers: As for the growth scenarios on metal substrates, controllable preparation of bilayer/multilayer graphene and the elimination of structural defects remain challenging. With respect to the synthesis over nonmetal wafers, concrete examples highlighting the epitaxial growth on a crystalline substrate and tailorable growth on a surface-reconstructed substrate are summarized. (3) Batch synthesis of graphene wafers: CVD routes for scalable production are explored. ![]()
Graphene 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.
2022, 38(2): 200709
doi: 10.3866/PKU.WHXB202007093
Abstract:
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 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.
2022, 38(2): 201208
doi: 10.3866/PKU.WHXB202012085
Abstract:
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.![]()
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.
2022, 38(2): 210100
doi: 10.3866/PKU.WHXB202101001
Abstract:
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. ![]()
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.
2022, 38(2): 210100
doi: 10.3866/PKU.WHXB202101009
Abstract:
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 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.
2022, 38(2): 201105
doi: 10.3866/PKU.WHXB202011050
Abstract:
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. ![]()
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.
2022, 38(2): 201206
doi: 10.3866/PKU.WHXB202012062
Abstract:
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. ![]()
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.
2022, 38(2): 201208
doi: 10.3866/PKU.WHXB202012088
Abstract:
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.![]()
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.
2022, 38(2): 210306
doi: 10.3866/PKU.WHXB202103060
Abstract:
