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2024 Volume 40 Issue 2
2024, 40(2): 230400
doi: 10.3866/PKU.WHXB202304001
Abstract:
Hydrogen peroxide (H2O2) is an important chemical and has been extensively used in various industrial and manufacturing applications, such as wastewater treatment, sterilization, energy storage, and oxidation of small molecules. With increasing demand in various fields, the global hydrogen peroxide market is expected to grow to $8.9 billion by 2031. Currently, over 90% of H2O2 is industrially synthesized by the anthraquinone process, which requires complex infrastructure and expensive catalysts. Additionally, the anthraquinone process is energy intensive and leads to increased levels of environmental pollution. Although the direct synthetic process, which involves mixing hydrogen and oxygen, can achieve high atomic utilization, its development is limited due to explosion risk and high cost. Thus, there is a pressing need for a safe, cost-effective, and efficient industrial method for the production of H2O2. The electrochemical synthesis of H2O2 via a two-electron oxygen reduction reaction (2e- ORR) has emerged as an attractive method for the decentralized production of H2O2, which could effectively address the issues associated with the indirect anthraquinone and direct synthetic processes. However, sluggish reaction kinetics and poor selectivity decrease the energy efficiency of electrochemical H2O2 synthesis. In this regard, developing electrocatalysts with high 2e- ORR selectivity is vital for the efficient production of H2O2. In the past decades, extensive efforts have been devoted to developing effective 2e- ORR electrocatalysts such as noble metals/alloys, carbon-based materials, single-atom catalysts, and molecular complexes. However, the reported catalysts still have unsatisfactory catalytic performances. Therefore, there is still a long way to realize the large-scale production of H2O2 via electrochemical 2e- ORR pathway. In this perspective, we systematically summarize recent developments regarding the direct production of H2O2 through electrochemical two-electron oxygen reaction. First, the fundamental aspects of electrochemical 2e- ORR are discussed, including their reaction mechanisms, possible reaction pathways, testing techniques and performance figures of merit. This introduction is followed by detailed discussions on the different types of 2e- ORR electrocatalysts, with an emphasis on the underlying material design principles used to promote reaction activity, selectivity, and stability. Subsequently, the applications of electrosynthetic hydrogen peroxide in various fields are briefly described, including pollutant degradation, water sterilization, energy storage, and small-molecule synthesis. Finally, potential future directions and prospects in 2e- ORR catalysts for electrochemically producing H2O2 are examined.
Hydrogen peroxide (H2O2) is an important chemical and has been extensively used in various industrial and manufacturing applications, such as wastewater treatment, sterilization, energy storage, and oxidation of small molecules. With increasing demand in various fields, the global hydrogen peroxide market is expected to grow to $8.9 billion by 2031. Currently, over 90% of H2O2 is industrially synthesized by the anthraquinone process, which requires complex infrastructure and expensive catalysts. Additionally, the anthraquinone process is energy intensive and leads to increased levels of environmental pollution. Although the direct synthetic process, which involves mixing hydrogen and oxygen, can achieve high atomic utilization, its development is limited due to explosion risk and high cost. Thus, there is a pressing need for a safe, cost-effective, and efficient industrial method for the production of H2O2. The electrochemical synthesis of H2O2 via a two-electron oxygen reduction reaction (2e- ORR) has emerged as an attractive method for the decentralized production of H2O2, which could effectively address the issues associated with the indirect anthraquinone and direct synthetic processes. However, sluggish reaction kinetics and poor selectivity decrease the energy efficiency of electrochemical H2O2 synthesis. In this regard, developing electrocatalysts with high 2e- ORR selectivity is vital for the efficient production of H2O2. In the past decades, extensive efforts have been devoted to developing effective 2e- ORR electrocatalysts such as noble metals/alloys, carbon-based materials, single-atom catalysts, and molecular complexes. However, the reported catalysts still have unsatisfactory catalytic performances. Therefore, there is still a long way to realize the large-scale production of H2O2 via electrochemical 2e- ORR pathway. In this perspective, we systematically summarize recent developments regarding the direct production of H2O2 through electrochemical two-electron oxygen reaction. First, the fundamental aspects of electrochemical 2e- ORR are discussed, including their reaction mechanisms, possible reaction pathways, testing techniques and performance figures of merit. This introduction is followed by detailed discussions on the different types of 2e- ORR electrocatalysts, with an emphasis on the underlying material design principles used to promote reaction activity, selectivity, and stability. Subsequently, the applications of electrosynthetic hydrogen peroxide in various fields are briefly described, including pollutant degradation, water sterilization, energy storage, and small-molecule synthesis. Finally, potential future directions and prospects in 2e- ORR catalysts for electrochemically producing H2O2 are examined.
2024, 40(2): 230305
doi: 10.3866/PKU.WHXB202303059
Abstract:
In recent decades, the oxygen evolution reaction (OER) has attracted significant attention because of its critical role in energy storage and conversion technologies. This reaction requires highly efficient catalysts such as IrO2 and RuO2 to accelerate its slow reaction rate. Among existing developed low-cost materials, NiFe layered double hydroxides (NiFe LDH) have demonstrated great potential for use in OERs in alkaline electrolytes with low overpotential (200–300 mV at 10 mA∙cm-2). Extensive efforts have been devoted to developing efficient electrocatalysts based on NiFe LDHs; Further reducing their overpotential can be a challenging task. To overcome this bottleneck, it is necessary to clearly identify the catalytic mechanism and active sites and finding new solutions to obtain catalysts with ultra-low overpotential. Through this review, we thoroughly examined the structure, composition, and development history of NiFe LDHs. Despite the extensive investigation of the catalytic active sites and mechanism, it still remains elusive and controversial. Herein, existing studies that have aimed to elucidate the catalytic sites are presented and comprehensively analyzed, providing an insightful understanding of the catalytic mechanism and active sites of NiFe LDHs. Additionally, various strategies, such as heteroatom doping and the introduction of vacancies, have been proposed to enhance the catalytic activities of these materials. Considering the electronic and geometrical structures of NiFe LDHs, this review summarizes and categorizes activity enhancement methods based on different enhancement mechanisms, offering new insights and directions for developing high-performance NiFe LDH-based catalysts. Furthermore, despite being crucial to the practical use of the catalyst, catalyst stability is often overlooked, especially under technological conditions such as high current densities. Recent works have suggested that NiFe LDH-based catalysts suffer severe activity fading under high current densities after a short period of operation. It is important to update recent research on the stability of these catalysts. This review emphasizes the stability issues of NiFe LDH-based catalysts to draw more attention toward research and analyses related to the decay mechanisms of these catalysts. We have summarized and discussed the recent strategies that have been proposed to reduce the stability problem developed based on these decay mechanisms. Finally, the review concludes with a discussion of possible directions for producing NiFe LDHs with extraordinary catalytic activities and stabilities.
In recent decades, the oxygen evolution reaction (OER) has attracted significant attention because of its critical role in energy storage and conversion technologies. This reaction requires highly efficient catalysts such as IrO2 and RuO2 to accelerate its slow reaction rate. Among existing developed low-cost materials, NiFe layered double hydroxides (NiFe LDH) have demonstrated great potential for use in OERs in alkaline electrolytes with low overpotential (200–300 mV at 10 mA∙cm-2). Extensive efforts have been devoted to developing efficient electrocatalysts based on NiFe LDHs; Further reducing their overpotential can be a challenging task. To overcome this bottleneck, it is necessary to clearly identify the catalytic mechanism and active sites and finding new solutions to obtain catalysts with ultra-low overpotential. Through this review, we thoroughly examined the structure, composition, and development history of NiFe LDHs. Despite the extensive investigation of the catalytic active sites and mechanism, it still remains elusive and controversial. Herein, existing studies that have aimed to elucidate the catalytic sites are presented and comprehensively analyzed, providing an insightful understanding of the catalytic mechanism and active sites of NiFe LDHs. Additionally, various strategies, such as heteroatom doping and the introduction of vacancies, have been proposed to enhance the catalytic activities of these materials. Considering the electronic and geometrical structures of NiFe LDHs, this review summarizes and categorizes activity enhancement methods based on different enhancement mechanisms, offering new insights and directions for developing high-performance NiFe LDH-based catalysts. Furthermore, despite being crucial to the practical use of the catalyst, catalyst stability is often overlooked, especially under technological conditions such as high current densities. Recent works have suggested that NiFe LDH-based catalysts suffer severe activity fading under high current densities after a short period of operation. It is important to update recent research on the stability of these catalysts. This review emphasizes the stability issues of NiFe LDH-based catalysts to draw more attention toward research and analyses related to the decay mechanisms of these catalysts. We have summarized and discussed the recent strategies that have been proposed to reduce the stability problem developed based on these decay mechanisms. Finally, the review concludes with a discussion of possible directions for producing NiFe LDHs with extraordinary catalytic activities and stabilities.
2024, 40(2): 230306
doi: 10.3866/PKU.WHXB202303060
Abstract:
With the development of modern society, the demand for energy is increasing. Consequently, the efficient utilization of renewable energy has become the primary concern in the energy sector. Secondary batteries can accomplish energy storage through efficient electrical/chemical energy conversion, thereby providing an effective solution for the utilization of renewable energy. Lithium-ion batteries have been the most widely used secondary battery systems, owing to their high energy densities and long lifetimes. Nevertheless, traditional inorganic cathode materials have recently encountered problems such as increasing manufacturing costs, lithium supply-chain constraints, and safety issues. Meanwhile, organic electrode materials (OEMs) have emerged as promising electrode candidates for secondary batteries owing to several advantages, such as their low costs, abundant resources, environmental friendliness, and structural designability. In recent decades, considerable efforts have been dedicated to OEM research. To date, commonly used OEMs include carbonyl polymers, conductive polymers, nitrile compounds, organic sulfides, organic free radical compounds, imine compounds, and Azo compounds. OEMs have been used in various metal ion battery systems, including lithium-, sodium-, aluminum-, zinc-, magnesium-, potassium-, and calcium-based batteries. However, the commercialization of OEMs still encounters several challenges, mainly owing to their low conductivity, high solubility, and low discharge potential. The low intrinsic conductivity of OEMs leads to difficulties in ion diffusion, while their high solubility in organic electrolytes inevitably reduces cyclic stability. Moreover, the low discharge potential of OEMs decreases energy density and rate performance. In view of the technical restrictions affecting OEMs, researchers have focused on modifications and optimizations of the structure, preparation strategies, and sizes of OEMs. In this paper, we review the development history and applications of OEMs and systemically summarize their classification, reaction mechanisms, and primary challenges. In addition, we thoroughly report on OEM modification strategies. By shaping their molecular structures, such as either by substituent introduction, conjugated structure formation, or small molecule polymerization, the solubility of OEMs can be reduced, and their discharge potential can be enhanced. The conductivity of OEMs can be improved significantly by combining them with conductive carbon materials. Nano-sized optimization and electrode–electrolyte coupling can also significantly improve their cycle stability and rate performance. Additionally, the electrochemical performance of OEMs can be improved by optimizing preparation processes and determining the best technological parameters. Finally, we envision future research paths of OEM modification, which could provide a future reference in OEM design and research.
With the development of modern society, the demand for energy is increasing. Consequently, the efficient utilization of renewable energy has become the primary concern in the energy sector. Secondary batteries can accomplish energy storage through efficient electrical/chemical energy conversion, thereby providing an effective solution for the utilization of renewable energy. Lithium-ion batteries have been the most widely used secondary battery systems, owing to their high energy densities and long lifetimes. Nevertheless, traditional inorganic cathode materials have recently encountered problems such as increasing manufacturing costs, lithium supply-chain constraints, and safety issues. Meanwhile, organic electrode materials (OEMs) have emerged as promising electrode candidates for secondary batteries owing to several advantages, such as their low costs, abundant resources, environmental friendliness, and structural designability. In recent decades, considerable efforts have been dedicated to OEM research. To date, commonly used OEMs include carbonyl polymers, conductive polymers, nitrile compounds, organic sulfides, organic free radical compounds, imine compounds, and Azo compounds. OEMs have been used in various metal ion battery systems, including lithium-, sodium-, aluminum-, zinc-, magnesium-, potassium-, and calcium-based batteries. However, the commercialization of OEMs still encounters several challenges, mainly owing to their low conductivity, high solubility, and low discharge potential. The low intrinsic conductivity of OEMs leads to difficulties in ion diffusion, while their high solubility in organic electrolytes inevitably reduces cyclic stability. Moreover, the low discharge potential of OEMs decreases energy density and rate performance. In view of the technical restrictions affecting OEMs, researchers have focused on modifications and optimizations of the structure, preparation strategies, and sizes of OEMs. In this paper, we review the development history and applications of OEMs and systemically summarize their classification, reaction mechanisms, and primary challenges. In addition, we thoroughly report on OEM modification strategies. By shaping their molecular structures, such as either by substituent introduction, conjugated structure formation, or small molecule polymerization, the solubility of OEMs can be reduced, and their discharge potential can be enhanced. The conductivity of OEMs can be improved significantly by combining them with conductive carbon materials. Nano-sized optimization and electrode–electrolyte coupling can also significantly improve their cycle stability and rate performance. Additionally, the electrochemical performance of OEMs can be improved by optimizing preparation processes and determining the best technological parameters. Finally, we envision future research paths of OEM modification, which could provide a future reference in OEM design and research.
2024, 40(2): 230306
doi: 10.3866/PKU.WHXB202303061
Abstract:
Lithium-sulfur (Li-S) batteries have emerged as promising candidates for next-generation secondary power batteries given that they exhibit extremely high discharge specific capacity (1672 mAh·g-1) when sulfur is used as the positive electrode. Despite the potential of Li-S batteries for commercial applications, two significant issues need to be addressed: the shuttle effect of dissolved high-order lithium polysulfides (Li2Sn, 4 ≤ n ≤ 8) during charge/discharge processes and the slow redox kinetics of sulfur species. Fortunately, the introduction of electrochemical catalysis is an effective strategy to mitigate the above problems. In the context of electrochemical catalysis, in this paper we discuss the existence forms of polysulfides and draw clear conclusions. Specifically, in ether electrolyte systems, the dominant form of polysulfide is the neutral molecule, while a smaller proportion exists as anions and cations. In addition, we also propose the corresponding solutions for different forms of polysulfides. Unlike previous reports, we analyze the conversion mechanism of polysulfides from two perspectives: adsorption-catalysis and reactive intermediates. In terms of the strength of the interaction force between the substrate materials and polysulfides, adsorption-catalysis can be classified into physisorption-catalysis and chemisorption-catalysis. The differences between both types are analyzed and discussed in-depth. Additionally, the reactive intermediates are further classified into sulfur free radicals, thiosulfates, and organosulfur molecules based on different electrochemical reaction pathways. The mechanisms involved in the reactions of these intermediates are subsequently analyzed in detail. We also evaluate different strategies and list the types of catalysts that may correspond to each mechanism. Finally, the quantitative evaluation method of catalytic performance is also summarized, which paves a new way for the design of high-efficiency electrocatalysts in Li-S batteries. The nucleation transformation ratio (NTR) is a quantitative measure we developed to assess the catalytic properties of materials. When the reaction is ideal, the NTR should be equal to 3. A calculated NTR close to 3 indicates that the reaction from Li2S6 to Li2S4 occurs rapidly, suggesting that the material is highly catalytic to polysulfide nucleation. This quantitative approach enables researchers to determine the adsorption and catalytic effects of cathode materials on polysulfides, allowing the study of lithium-sulfur battery cathode materials to move from qualitative description to quantitative evaluation with specific factors. As a result, we can move from a qualitative description of lithium-sulfur battery cathode materials to their quantitative evaluation.
Lithium-sulfur (Li-S) batteries have emerged as promising candidates for next-generation secondary power batteries given that they exhibit extremely high discharge specific capacity (1672 mAh·g-1) when sulfur is used as the positive electrode. Despite the potential of Li-S batteries for commercial applications, two significant issues need to be addressed: the shuttle effect of dissolved high-order lithium polysulfides (Li2Sn, 4 ≤ n ≤ 8) during charge/discharge processes and the slow redox kinetics of sulfur species. Fortunately, the introduction of electrochemical catalysis is an effective strategy to mitigate the above problems. In the context of electrochemical catalysis, in this paper we discuss the existence forms of polysulfides and draw clear conclusions. Specifically, in ether electrolyte systems, the dominant form of polysulfide is the neutral molecule, while a smaller proportion exists as anions and cations. In addition, we also propose the corresponding solutions for different forms of polysulfides. Unlike previous reports, we analyze the conversion mechanism of polysulfides from two perspectives: adsorption-catalysis and reactive intermediates. In terms of the strength of the interaction force between the substrate materials and polysulfides, adsorption-catalysis can be classified into physisorption-catalysis and chemisorption-catalysis. The differences between both types are analyzed and discussed in-depth. Additionally, the reactive intermediates are further classified into sulfur free radicals, thiosulfates, and organosulfur molecules based on different electrochemical reaction pathways. The mechanisms involved in the reactions of these intermediates are subsequently analyzed in detail. We also evaluate different strategies and list the types of catalysts that may correspond to each mechanism. Finally, the quantitative evaluation method of catalytic performance is also summarized, which paves a new way for the design of high-efficiency electrocatalysts in Li-S batteries. The nucleation transformation ratio (NTR) is a quantitative measure we developed to assess the catalytic properties of materials. When the reaction is ideal, the NTR should be equal to 3. A calculated NTR close to 3 indicates that the reaction from Li2S6 to Li2S4 occurs rapidly, suggesting that the material is highly catalytic to polysulfide nucleation. This quantitative approach enables researchers to determine the adsorption and catalytic effects of cathode materials on polysulfides, allowing the study of lithium-sulfur battery cathode materials to move from qualitative description to quantitative evaluation with specific factors. As a result, we can move from a qualitative description of lithium-sulfur battery cathode materials to their quantitative evaluation.
2024, 40(2): 230402
doi: 10.3866/PKU.WHXB202304026
Abstract:
As technology and society have continued to develop, the demand for energy storage solutions has increased significantly. Indeed, the development of low-cost, low-carbon, environmentally friendly energy conversion and storage systems is required to address the environmental and ecological problems faced by society. Due to their fast charging and discharging speeds, long cycle life and environmentally friendly characteristics, supercapacitors are widely used in many fields, especially in wind power generation systems, communication and transportation. Among all kinds of electrode materials, silicon carbide (SiC) nanomaterials and SiC-derived carbon (SiC-CDC) materials present long life, high power density, and uncomplicated working mechanisms, which hold significant promise as electrode materials for supercapacitors. So far, various strategies and approaches for controlling the microstructure of SiC nanomaterials and SiC-CDC materials have been developed to achieve further improvement from preparation methods to electrochemical properties. As such, this review systematically introduces the common preparation methods of SiC nanomaterials and SiC-CDC, including the template method, chemical vapor deposition (CVD) method, high temperature halogen etching method and high temperature thermal decomposition process for preparing SiC-CDC. Furthermore, the advantages and disadvantages of different preparation methods are discussed. Additionally, the review covers the progress in employing SiC nanomaterials and SiC-CDC materials as supercapacitor electrode materials in detail. However, despite this progress, the commercial application of SiC nanomaterials and SiC-CDC materials as supercapacitor electrodes has been restricted by some problems, in particular their limited conductivity and poor wettability. More importantly, the low energy density of supercapacitors is still a major problem. Thus, current methods and developmental trends of the strategies to improve electrochemical performance such as “highly conductive carbon material composite”, “heteroatomic doping”, “pseudocapacitance composites”, “multi-stage pore structure design”, “chemical activation” are further analyzed with regards to the current challenges. For example, the introduction of heteroatoms and functional group molecules for reactions into SiC and SiC-CDC materials can inhibit the agglomeration of materials (such as particles and nanosheets), improve their conductivity and wettability, and enhance their specific capacitance. Finally, the challenges and opportunities in the application of SiC nanomaterials and their derived carbons in the field of energy storage for supercapacitors are summarized and prospected. As current preparation methods are limited to the laboratory scale, the combination and improvement of different preparation methods and the development of large-scale and low-cost preparation technology are still the directions of the next efforts. This comprehensive review is expected to further advance the research of SiC nanomaterials and SiC-CDC materials.
As technology and society have continued to develop, the demand for energy storage solutions has increased significantly. Indeed, the development of low-cost, low-carbon, environmentally friendly energy conversion and storage systems is required to address the environmental and ecological problems faced by society. Due to their fast charging and discharging speeds, long cycle life and environmentally friendly characteristics, supercapacitors are widely used in many fields, especially in wind power generation systems, communication and transportation. Among all kinds of electrode materials, silicon carbide (SiC) nanomaterials and SiC-derived carbon (SiC-CDC) materials present long life, high power density, and uncomplicated working mechanisms, which hold significant promise as electrode materials for supercapacitors. So far, various strategies and approaches for controlling the microstructure of SiC nanomaterials and SiC-CDC materials have been developed to achieve further improvement from preparation methods to electrochemical properties. As such, this review systematically introduces the common preparation methods of SiC nanomaterials and SiC-CDC, including the template method, chemical vapor deposition (CVD) method, high temperature halogen etching method and high temperature thermal decomposition process for preparing SiC-CDC. Furthermore, the advantages and disadvantages of different preparation methods are discussed. Additionally, the review covers the progress in employing SiC nanomaterials and SiC-CDC materials as supercapacitor electrode materials in detail. However, despite this progress, the commercial application of SiC nanomaterials and SiC-CDC materials as supercapacitor electrodes has been restricted by some problems, in particular their limited conductivity and poor wettability. More importantly, the low energy density of supercapacitors is still a major problem. Thus, current methods and developmental trends of the strategies to improve electrochemical performance such as “highly conductive carbon material composite”, “heteroatomic doping”, “pseudocapacitance composites”, “multi-stage pore structure design”, “chemical activation” are further analyzed with regards to the current challenges. For example, the introduction of heteroatoms and functional group molecules for reactions into SiC and SiC-CDC materials can inhibit the agglomeration of materials (such as particles and nanosheets), improve their conductivity and wettability, and enhance their specific capacitance. Finally, the challenges and opportunities in the application of SiC nanomaterials and their derived carbons in the field of energy storage for supercapacitors are summarized and prospected. As current preparation methods are limited to the laboratory scale, the combination and improvement of different preparation methods and the development of large-scale and low-cost preparation technology are still the directions of the next efforts. This comprehensive review is expected to further advance the research of SiC nanomaterials and SiC-CDC materials.
2024, 40(2): 230403
doi: 10.3866/PKU.WHXB202304035
Abstract:
Owing to the growing consumption of non-renewable resources and increased environmental pollution, significant attention has been directed toward developing renewable and environmentally friendly energy sources. Hydrogen has emerged as a clean energy carrier and is considered an ideal chemical for power generation via fuel cells. Using renewable energy to power hydrogen production is an attractive prospect, and hydrogen production through photoelectrochemical water splitting is considered a promising area of interest; consequently, significant research is being conducted on rationally designed photoelectrodes. Generally, a photocathode for hydrogen evolution must have a conduction band that is more negative than the reduction potential of hydrogen. Numerous photocathode materials have been developed based on this premise; these include p-Si, InP, and GaN. Compared with other photocathode materials, Cu-based compounds are advantageous owing to their low preparation costs and diverse chemical states. For example, Cu2O is a non-toxic p-type metal oxide semiconductor material with an appropriate band structure for water splitting and a direct band gap of 1.9–2.2 eV. Furthermore, the production of Cu2O is facile, and the required materials are abundant; thus, it has attracted significant interest as a material for photocathodes. However, Cu2O suffers from rapid recombination of photogenerated carriers and severe photo-corrosion, leading to unsatisfactory efficiency and poor stability. Intrinsically, the poor photo-stability of Cu2O can be attributed to the location of the redox potential of Cu2O within its bandgap, owing to which photoelectrons tend to preferentially reduce Cu2O to Cu rather than reduce water to reduction. Therefore, Cu2O itself is not an ideal hydrogen evolution catalyst. Thus, co-catalysts are necessary to improve its hydrogen evolution activity and photostability. In addition to co-catalysts, combining Cu2O with tailored n-type semiconductors to generate built-in electric fields of p–n junctions has attracted extensive attention owing to its ability of increasing the separation of photogenerated carriers. Similarly, applying a hole transfer layer on the substrate can promote photocarrier separation. Furthermore, considering that water is indispensable for Cu2O reduction, one effective approach to improve the stability of Cu2O is the addition of a protective/passivation layer to isolate Cu2O from water in aqueous electrolytes. In this review, we provide a brief overview of the mechanism of photoelectrochemical water splitting and the band structure of Cu2O; preparation methods of Cu2O photocathodes; strategies to improve the efficiency and stability of Cu2O photocathodes, including the construction of p–n junctions, integration with co-catalysts, and modifications using hole transport layers; advanced photoelectrochemical characterization techniques; and a discussion regarding the direction of future photocathode research.
Owing to the growing consumption of non-renewable resources and increased environmental pollution, significant attention has been directed toward developing renewable and environmentally friendly energy sources. Hydrogen has emerged as a clean energy carrier and is considered an ideal chemical for power generation via fuel cells. Using renewable energy to power hydrogen production is an attractive prospect, and hydrogen production through photoelectrochemical water splitting is considered a promising area of interest; consequently, significant research is being conducted on rationally designed photoelectrodes. Generally, a photocathode for hydrogen evolution must have a conduction band that is more negative than the reduction potential of hydrogen. Numerous photocathode materials have been developed based on this premise; these include p-Si, InP, and GaN. Compared with other photocathode materials, Cu-based compounds are advantageous owing to their low preparation costs and diverse chemical states. For example, Cu2O is a non-toxic p-type metal oxide semiconductor material with an appropriate band structure for water splitting and a direct band gap of 1.9–2.2 eV. Furthermore, the production of Cu2O is facile, and the required materials are abundant; thus, it has attracted significant interest as a material for photocathodes. However, Cu2O suffers from rapid recombination of photogenerated carriers and severe photo-corrosion, leading to unsatisfactory efficiency and poor stability. Intrinsically, the poor photo-stability of Cu2O can be attributed to the location of the redox potential of Cu2O within its bandgap, owing to which photoelectrons tend to preferentially reduce Cu2O to Cu rather than reduce water to reduction. Therefore, Cu2O itself is not an ideal hydrogen evolution catalyst. Thus, co-catalysts are necessary to improve its hydrogen evolution activity and photostability. In addition to co-catalysts, combining Cu2O with tailored n-type semiconductors to generate built-in electric fields of p–n junctions has attracted extensive attention owing to its ability of increasing the separation of photogenerated carriers. Similarly, applying a hole transfer layer on the substrate can promote photocarrier separation. Furthermore, considering that water is indispensable for Cu2O reduction, one effective approach to improve the stability of Cu2O is the addition of a protective/passivation layer to isolate Cu2O from water in aqueous electrolytes. In this review, we provide a brief overview of the mechanism of photoelectrochemical water splitting and the band structure of Cu2O; preparation methods of Cu2O photocathodes; strategies to improve the efficiency and stability of Cu2O photocathodes, including the construction of p–n junctions, integration with co-catalysts, and modifications using hole transport layers; advanced photoelectrochemical characterization techniques; and a discussion regarding the direction of future photocathode research.
2024, 40(2): 230300
doi: 10.3866/PKU.WHXB202303006
Abstract:
With increasing global energy demand and stricter environmental protection requirements, energy storage technology has become a research hotspot in the global energy field. New types of energy storage devices continue to emerge owing to the continuous development of cost-effective energy storage technology. Among them, potassium-ion batteries have received widespread attention as a new type of alkali metal ion battery because of their high capacity and low cost and are considered one of the future development directions. However, the research on potassium-ion batteries is still in its infancy, with many challenges to overcome regarding practical applications. A key factor affecting the performance of potassium-ion batteries is the anode material, as it not only affects the manufacturing costs but also directly affects the power density and energy density of the battery. Traditional anode materials for lithium-ion batteries cannot meet the requirements of potassium-ion batteries. Therefore, developing high-performance anode materials suitable for potassium-ion batteries is an important research direction at present. The charge and discharge rate and cycling life of potassium-ion batteries also need further improvements. Currently, the low-rate performance, short cycle life, and unsatisfactory practical capacities limit their practical application and commercialization. However, the future of potassium-ion batteries remains promising. Upon resolving the aforementioned issues, potassium-ion batteries will have diverse application prospects, such as electric vehicles, energy storage stations, and smart grids, providing important support for solving energy problems. Therefore, the research and development of potassium-ion batteries are an important direction in the global energy field. Current research efforts are primarily focused on exploring novel anode materials with exceptional ratability and cyclability. In this regard, we synthesized a new type of anode material based on bismuth telluride (Bi2Te3) and experimentally studied its applicability in potassium-ion batteries. The performance of Bi2Te3 anode for potassium-ion batteries has been limited by its structural instability and slow electrochemical reaction kinetics. In this study, rod-like Bi2Te3 was grown on accordion-like MXene, followed by P-doping to obtain a high-performance P-Bi2Te3/MXene superstructure. This novel anode had abundant Te vacancies and good self-auto adjustable function, providing excellent cycling stability (323.1 mAh·g-1 after 200 cycles at 0.2 A·g-1) and outstanding rate capability (67.1 mAh·g-1 at 20 A·g-1). Kinetic analysis and ex situ characterization indicate that the superstructure exhibits superior pseudocapacitive properties, high electrical conductivity, favorable diffusion capability, and reversible insertion and conversion reaction mechanism.
With increasing global energy demand and stricter environmental protection requirements, energy storage technology has become a research hotspot in the global energy field. New types of energy storage devices continue to emerge owing to the continuous development of cost-effective energy storage technology. Among them, potassium-ion batteries have received widespread attention as a new type of alkali metal ion battery because of their high capacity and low cost and are considered one of the future development directions. However, the research on potassium-ion batteries is still in its infancy, with many challenges to overcome regarding practical applications. A key factor affecting the performance of potassium-ion batteries is the anode material, as it not only affects the manufacturing costs but also directly affects the power density and energy density of the battery. Traditional anode materials for lithium-ion batteries cannot meet the requirements of potassium-ion batteries. Therefore, developing high-performance anode materials suitable for potassium-ion batteries is an important research direction at present. The charge and discharge rate and cycling life of potassium-ion batteries also need further improvements. Currently, the low-rate performance, short cycle life, and unsatisfactory practical capacities limit their practical application and commercialization. However, the future of potassium-ion batteries remains promising. Upon resolving the aforementioned issues, potassium-ion batteries will have diverse application prospects, such as electric vehicles, energy storage stations, and smart grids, providing important support for solving energy problems. Therefore, the research and development of potassium-ion batteries are an important direction in the global energy field. Current research efforts are primarily focused on exploring novel anode materials with exceptional ratability and cyclability. In this regard, we synthesized a new type of anode material based on bismuth telluride (Bi2Te3) and experimentally studied its applicability in potassium-ion batteries. The performance of Bi2Te3 anode for potassium-ion batteries has been limited by its structural instability and slow electrochemical reaction kinetics. In this study, rod-like Bi2Te3 was grown on accordion-like MXene, followed by P-doping to obtain a high-performance P-Bi2Te3/MXene superstructure. This novel anode had abundant Te vacancies and good self-auto adjustable function, providing excellent cycling stability (323.1 mAh·g-1 after 200 cycles at 0.2 A·g-1) and outstanding rate capability (67.1 mAh·g-1 at 20 A·g-1). Kinetic analysis and ex situ characterization indicate that the superstructure exhibits superior pseudocapacitive properties, high electrical conductivity, favorable diffusion capability, and reversible insertion and conversion reaction mechanism.
2024, 40(2): 230302
doi: 10.3866/PKU.WHXB202303021
Abstract:
Graphite has been extensively employed as commercial anode material in Li-ion batteries due to its high abundance, low cost, and negative electrode potential. Furthermore, it has demonstrated significant potential for use in K-ion batteries. However, distinct structural damage caused by the larger radius of K-ion (0.138 nm) compared to that of Li-ion (0.076 nm) leads to obvious capacity decay and unstable cycle life. It is crucial to improve the cycling stability of graphite in potassium ion batteries (PIBs). Herein, we design a stable interface of graphite anode by graphene coating with a simple and efficient microwave method. According to X-ray photoelectron spectroscopy (XPS), microwave reduction can effectively remove the oxygen group of graphene oxide (GO) within 10 s. The graphene coating can buffer the volume expansion of the graphite to suppress structural collapse; it can also accelerate electronic transmission to improve rate performance. As a result, the graphene-coating graphite anode, named GCG, exhibits super cycling stability with a capacity of 262 mAh∙g-1after 3000 cycles at a current density of 0.2 A∙g-1, which means it can operate smoothly for one year. In contrast, at the same current density, graphite exhibits capacity fading to less than 150 mAh∙g-1 after 150 cycles. Moreover, compared to graphite, GCG demonstrates better rate performance achieving a capacity of 161.2 mAh∙g-1 at 500 mA∙g-1. Further electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT) tests show that GCG exhibits faster electrical conductivity and ion diffusion compared to graphite. Raman spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) images after cycling verify that the graphene buffer interface benefits the integrity of the electrode structure and improves the stability of the solid electrolyte interphase (SEI). Compared to graphite, the GCG anode exhibits better performance, as follows: 1) The graphene coating inhibits exfoliation of graphite during cycling, solving the problem of graphite anode’ short cycling life, and 2) the graphene protective layer improves the ion diffusion rate, resulting in better rate performance of the GCG. In addition, this approach offers the advantages of simple operation and low cost, hopefully enabling large-scale applications of potassium-ion batteries.
Graphite has been extensively employed as commercial anode material in Li-ion batteries due to its high abundance, low cost, and negative electrode potential. Furthermore, it has demonstrated significant potential for use in K-ion batteries. However, distinct structural damage caused by the larger radius of K-ion (0.138 nm) compared to that of Li-ion (0.076 nm) leads to obvious capacity decay and unstable cycle life. It is crucial to improve the cycling stability of graphite in potassium ion batteries (PIBs). Herein, we design a stable interface of graphite anode by graphene coating with a simple and efficient microwave method. According to X-ray photoelectron spectroscopy (XPS), microwave reduction can effectively remove the oxygen group of graphene oxide (GO) within 10 s. The graphene coating can buffer the volume expansion of the graphite to suppress structural collapse; it can also accelerate electronic transmission to improve rate performance. As a result, the graphene-coating graphite anode, named GCG, exhibits super cycling stability with a capacity of 262 mAh∙g-1after 3000 cycles at a current density of 0.2 A∙g-1, which means it can operate smoothly for one year. In contrast, at the same current density, graphite exhibits capacity fading to less than 150 mAh∙g-1 after 150 cycles. Moreover, compared to graphite, GCG demonstrates better rate performance achieving a capacity of 161.2 mAh∙g-1 at 500 mA∙g-1. Further electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT) tests show that GCG exhibits faster electrical conductivity and ion diffusion compared to graphite. Raman spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) images after cycling verify that the graphene buffer interface benefits the integrity of the electrode structure and improves the stability of the solid electrolyte interphase (SEI). Compared to graphite, the GCG anode exhibits better performance, as follows: 1) The graphene coating inhibits exfoliation of graphite during cycling, solving the problem of graphite anode’ short cycling life, and 2) the graphene protective layer improves the ion diffusion rate, resulting in better rate performance of the GCG. In addition, this approach offers the advantages of simple operation and low cost, hopefully enabling large-scale applications of potassium-ion batteries.
2024, 40(2): 230304
doi: 10.3866/PKU.WHXB202303041
Abstract:
As the most advanced battery technology to date, lithium-ion battery has occupied the main battery markets for electric vehicles and grid scale energy storage systems. However, the limited lithium reserves as well as the high price raise concerns about the sustainability of lithium-ion battery. Although sodium-ion battery is proposed as a good supplement to lithium-ion battery, expensive and flammable electrolyte components, harsh assembly environments and potential safety hazards have limited the rapid development to a certain extent. The organic electrolyte was replaced with an aqueous solution to construct a new type of aqueous sodium ion battery capacitor (ASIBC). It is of great significance for next-generation energy storage system owing to its low cost, high power, and inherent safety. However, applicable ASIBC system is rarely reported so far. Here, a rechargeable alkaline sodium ion battery capacitors constructed by using Na0.44MnO2 cathode, activated carbon (AC) anode, 6 mol∙L-1 NaOH electrolyte, and cheap stainless-steel current collector. Because of high overcharge tolerance of Na0.44MnO2 cathode in alkaline electrolyte, the shortcomings of the half-sodium Na0.44MnO2 cathode and low initial Coulombic efficiency of AC anode can be resolved by in situ overcharging pre-activation process during first charging. The available capacity of Na0.44MnO2 in half cell largely increased from ~40 mAh∙g-1 (neutral electrolyte) to 77.3 mAh∙g-1 (alkaline electrolyte) due to broadened Na+ intercalation potential region. Thus, the AC||Na0.44MnO2 ASIBC delivers outstanding electrochemical properties with a high energy density of 26.6 Wh∙kg-1 at a power density of 85 W∙kg-1 and long cycling stability with a capacity retention of 89% after 10,000 cycles. The advantages of the alkaline electrolyte for the AC||Na0.44MnO2 ASIBC can be concluded as follows: (1) through the in situ electrochemical pre-activation process, the overcharging oxygen evolution reaction during first charging process can balance the adverse effects of the half-sodium Na0.44MnO2 cathode and low initial Coulombic efficiency of AC anode on the energy density of full cell; (2) the overcharging self-protection function can promote the generated oxygen to be eliminated at anode during overcharging, which improves the system safety; (3) the low-cost materials in alkaline environment can be scaled up to construct AC||Na0.44MnO2 ASIBC. In addition, the AC||Na0.44MnO2 ASIBC also possesses wide operating temperature range, achieving satisfied electrochemical performance at a high temperature of 50 ℃ and a low temperature of -20 ℃. Considering the merits of low-cost, high safety, no toxicity and environment-friendly, we believe the AC||Na0.44MnO2 rechargeable alkaline sodium-ion battery capacitors have the potential to be applied to large-scale energy storage.
As the most advanced battery technology to date, lithium-ion battery has occupied the main battery markets for electric vehicles and grid scale energy storage systems. However, the limited lithium reserves as well as the high price raise concerns about the sustainability of lithium-ion battery. Although sodium-ion battery is proposed as a good supplement to lithium-ion battery, expensive and flammable electrolyte components, harsh assembly environments and potential safety hazards have limited the rapid development to a certain extent. The organic electrolyte was replaced with an aqueous solution to construct a new type of aqueous sodium ion battery capacitor (ASIBC). It is of great significance for next-generation energy storage system owing to its low cost, high power, and inherent safety. However, applicable ASIBC system is rarely reported so far. Here, a rechargeable alkaline sodium ion battery capacitors constructed by using Na0.44MnO2 cathode, activated carbon (AC) anode, 6 mol∙L-1 NaOH electrolyte, and cheap stainless-steel current collector. Because of high overcharge tolerance of Na0.44MnO2 cathode in alkaline electrolyte, the shortcomings of the half-sodium Na0.44MnO2 cathode and low initial Coulombic efficiency of AC anode can be resolved by in situ overcharging pre-activation process during first charging. The available capacity of Na0.44MnO2 in half cell largely increased from ~40 mAh∙g-1 (neutral electrolyte) to 77.3 mAh∙g-1 (alkaline electrolyte) due to broadened Na+ intercalation potential region. Thus, the AC||Na0.44MnO2 ASIBC delivers outstanding electrochemical properties with a high energy density of 26.6 Wh∙kg-1 at a power density of 85 W∙kg-1 and long cycling stability with a capacity retention of 89% after 10,000 cycles. The advantages of the alkaline electrolyte for the AC||Na0.44MnO2 ASIBC can be concluded as follows: (1) through the in situ electrochemical pre-activation process, the overcharging oxygen evolution reaction during first charging process can balance the adverse effects of the half-sodium Na0.44MnO2 cathode and low initial Coulombic efficiency of AC anode on the energy density of full cell; (2) the overcharging self-protection function can promote the generated oxygen to be eliminated at anode during overcharging, which improves the system safety; (3) the low-cost materials in alkaline environment can be scaled up to construct AC||Na0.44MnO2 ASIBC. In addition, the AC||Na0.44MnO2 ASIBC also possesses wide operating temperature range, achieving satisfied electrochemical performance at a high temperature of 50 ℃ and a low temperature of -20 ℃. Considering the merits of low-cost, high safety, no toxicity and environment-friendly, we believe the AC||Na0.44MnO2 rechargeable alkaline sodium-ion battery capacitors have the potential to be applied to large-scale energy storage.
2024, 40(2): 230904
doi: 10.3866/PKU.WHXB202309046
Abstract:
Owing to their remarkable miniaturization and integration capabilities, all-solid-state thin-film lithium-ion batteries are quite appropriate as the on-chip power for microsystems, such as implantable medical devices, micro-electro-mechanical systems and integrated circuits. The performance of the all-solid-state thin-film lithium-ion batteries is greatly determined by the anode film. Metal Li is usually adopted as the anode material, however, the issues, including Li dendrite growth and poor thermal stability, hinder its applications in the high-temperature and high-safety fields, such as industrial and military. Therefore, various anode materials have been investigated in recent years. Unfortunately, few anode materials can achieve high specific capacity and good stability simultaneously. Due to its relatively high specific capacity and good electrochemical stability, LiNbO3 has been widely used as a coating layer in the batteries and has been demonstrated to effectively suppress side reactions at the electrode|electrolyte interface. However, there is still lack of deep understanding of the electrochemical characteristics of LiNbO3; also, no previous work has been performed to explore the applications of LiNbO3 in the all-solid-state thin-film lithium-ion batteries. In this work, the electrochemical characteristics of LiNbO3as a new anode material are carefully investigated. It is found that the LiNbO3anode has relatively high specific capacity (410.2 mAh∙g-1), high rate capability (80.9 mAh∙g-1 at 30C), good cycling stability (100% capacity retention over 2000 cycles at 1C) and high ionic conductivity (4.5×10-8 S∙cm–1 at room temperature). Moreover, an all-solid-state thin-film lithium-ion battery with a Pt current collector|NCM523 cathode|LiPON electrolyte|LiNbO3 anode|Pt current collector configuration is also prepared. This full battery presents good performance in terms of its relatively high area capacity (16.3 μAh∙cm-2 at a current density of 0.5 μA∙cm-2), good rate characteristic (1.9 μAh∙cm-2 even at a high current density of 30 μA∙cm-2) and good stability (86.4% capacity retention after 300 cycles). Particularly, the retained capacity remains as high as 95.6% even when this full battery operates continuously at 100 ℃ for ~200 h, demonstrating its good thermal stability. As confirmed by both the electrochemical and micro characterization, the LiPON|LiNbO3interface is quite stable under both the repeated charge/discharge cycling and high temperature operation, which contributes to the good performance of this full battery even under high temperatures. For comparison, the LiPON|Li interface degrades significantly under high temperatures, thus resulting in poor performance of the corresponding full battery. This work is helpful to develop a new anode film and all-solid-state thin-film lithium-ion battery which is suitable for the industrial and military applications.
Owing to their remarkable miniaturization and integration capabilities, all-solid-state thin-film lithium-ion batteries are quite appropriate as the on-chip power for microsystems, such as implantable medical devices, micro-electro-mechanical systems and integrated circuits. The performance of the all-solid-state thin-film lithium-ion batteries is greatly determined by the anode film. Metal Li is usually adopted as the anode material, however, the issues, including Li dendrite growth and poor thermal stability, hinder its applications in the high-temperature and high-safety fields, such as industrial and military. Therefore, various anode materials have been investigated in recent years. Unfortunately, few anode materials can achieve high specific capacity and good stability simultaneously. Due to its relatively high specific capacity and good electrochemical stability, LiNbO3 has been widely used as a coating layer in the batteries and has been demonstrated to effectively suppress side reactions at the electrode|electrolyte interface. However, there is still lack of deep understanding of the electrochemical characteristics of LiNbO3; also, no previous work has been performed to explore the applications of LiNbO3 in the all-solid-state thin-film lithium-ion batteries. In this work, the electrochemical characteristics of LiNbO3as a new anode material are carefully investigated. It is found that the LiNbO3anode has relatively high specific capacity (410.2 mAh∙g-1), high rate capability (80.9 mAh∙g-1 at 30C), good cycling stability (100% capacity retention over 2000 cycles at 1C) and high ionic conductivity (4.5×10-8 S∙cm–1 at room temperature). Moreover, an all-solid-state thin-film lithium-ion battery with a Pt current collector|NCM523 cathode|LiPON electrolyte|LiNbO3 anode|Pt current collector configuration is also prepared. This full battery presents good performance in terms of its relatively high area capacity (16.3 μAh∙cm-2 at a current density of 0.5 μA∙cm-2), good rate characteristic (1.9 μAh∙cm-2 even at a high current density of 30 μA∙cm-2) and good stability (86.4% capacity retention after 300 cycles). Particularly, the retained capacity remains as high as 95.6% even when this full battery operates continuously at 100 ℃ for ~200 h, demonstrating its good thermal stability. As confirmed by both the electrochemical and micro characterization, the LiPON|LiNbO3interface is quite stable under both the repeated charge/discharge cycling and high temperature operation, which contributes to the good performance of this full battery even under high temperatures. For comparison, the LiPON|Li interface degrades significantly under high temperatures, thus resulting in poor performance of the corresponding full battery. This work is helpful to develop a new anode film and all-solid-state thin-film lithium-ion battery which is suitable for the industrial and military applications.
2024, 40(2): 230400
doi: 10.3866/PKU.WHXB202304006
Abstract:
Photoelectrochemical water splitting using semiconductor materials is one of the most promising methods for converting solar energy into chemical energy. Among the commonly used semiconductors, p-type CuBi2O4 is considered one of the most suitable photocathode materials and can allow a theoretical photocurrent density of about 20 mA·cm-2 for photoelectrochemical water splitting. However, due to severe charge carrier recombination, the obtained photocurrent density is much lower than the theoretical value. Highly efficient photoelectrochemical performance relies on fast charge carrier separation and transport, and prompt reaction kinetics. In this study, we report the development of a polyoxometalate-modified CuBi2O4/Mg-CuBi2O4 homojunction photocathode to improve both the bulk and interfacial charge carrier transport in the photocathode. For the bulk of the photocathode, the built-in electric field originating from the CuBi2O4/Mg-CuBi2O4 homojunction promotes the migration of photo-excited electrons on the conduction band from pure CuBi2O4 to Mg-doped CuBi2O4. Additionally, the electric field facilitates the transfer of holes from the valence band of Mg-doped CuBi2O4 to pure CuBi2O4. This directional transfer of both photo-excited electrons and holes plays a significant role in promoting separation and suppressing the recombination of the charge carriers. On the surface of the photocathode, the reduced polyoxometalate co-catalyst Ag6[P2W18O62] (AgP2W18) was used as a proton sponge to accelerate surface reaction kinetics and suppress carrier recombination. These synergistic effects improved the photo-generated charge carrier transfer and reaction kinetics. As a result, the novel photocathode displayed excellent photoelectrochemical properties, and the photocurrent density was observed to be -0.64 mA·cm-2 at 0.3 V vs. RHE, which is better than that of -0.39 mA·cm-2 for a pure photocathode. Furthermore, the novel photocathode had an applied bias photon-to-current efficiency (ABPE) higher than 0.19% at 0.3 V vs. RHE. In contrast, the pure photocathode had an ABPE of ~0.12% under the same conditions. Additionally, when H2O2 was used as an electron scavenger, the photocurrent density was -3 mA·cm-2 at 0.3 V vs. RHE, which is an improvement of approximately 1.5 times compared to the pure photocathode. Furthermore, the charge separation and charge injection efficiency of the novel photocathode were significantly improved compared with the pure photocathode. The experimental results conclusively indicate that the formation of the CuBi2O4/Mg-CuBi2O4 homojunction and AgP2W18 modification played a significant role in the improved performance of the CuBi2O4 photocathode. The performance of the novel photocathode was comparable with the results reported in previous studies, demonstrating its promising potential in real applications.
Photoelectrochemical water splitting using semiconductor materials is one of the most promising methods for converting solar energy into chemical energy. Among the commonly used semiconductors, p-type CuBi2O4 is considered one of the most suitable photocathode materials and can allow a theoretical photocurrent density of about 20 mA·cm-2 for photoelectrochemical water splitting. However, due to severe charge carrier recombination, the obtained photocurrent density is much lower than the theoretical value. Highly efficient photoelectrochemical performance relies on fast charge carrier separation and transport, and prompt reaction kinetics. In this study, we report the development of a polyoxometalate-modified CuBi2O4/Mg-CuBi2O4 homojunction photocathode to improve both the bulk and interfacial charge carrier transport in the photocathode. For the bulk of the photocathode, the built-in electric field originating from the CuBi2O4/Mg-CuBi2O4 homojunction promotes the migration of photo-excited electrons on the conduction band from pure CuBi2O4 to Mg-doped CuBi2O4. Additionally, the electric field facilitates the transfer of holes from the valence band of Mg-doped CuBi2O4 to pure CuBi2O4. This directional transfer of both photo-excited electrons and holes plays a significant role in promoting separation and suppressing the recombination of the charge carriers. On the surface of the photocathode, the reduced polyoxometalate co-catalyst Ag6[P2W18O62] (AgP2W18) was used as a proton sponge to accelerate surface reaction kinetics and suppress carrier recombination. These synergistic effects improved the photo-generated charge carrier transfer and reaction kinetics. As a result, the novel photocathode displayed excellent photoelectrochemical properties, and the photocurrent density was observed to be -0.64 mA·cm-2 at 0.3 V vs. RHE, which is better than that of -0.39 mA·cm-2 for a pure photocathode. Furthermore, the novel photocathode had an applied bias photon-to-current efficiency (ABPE) higher than 0.19% at 0.3 V vs. RHE. In contrast, the pure photocathode had an ABPE of ~0.12% under the same conditions. Additionally, when H2O2 was used as an electron scavenger, the photocurrent density was -3 mA·cm-2 at 0.3 V vs. RHE, which is an improvement of approximately 1.5 times compared to the pure photocathode. Furthermore, the charge separation and charge injection efficiency of the novel photocathode were significantly improved compared with the pure photocathode. The experimental results conclusively indicate that the formation of the CuBi2O4/Mg-CuBi2O4 homojunction and AgP2W18 modification played a significant role in the improved performance of the CuBi2O4 photocathode. The performance of the novel photocathode was comparable with the results reported in previous studies, demonstrating its promising potential in real applications.
2024, 40(2): 230402
doi: 10.3866/PKU.WHXB202304021
Abstract:
Rechargeable zinc-air batteries (ZABs) have been extensively investigated owing to their high power density and environmental friendliness. However, the slow kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) processes limit their practical application. Currently, IrO2 and RuO2 are considered the optimal OER electrocatalysts, and Pt/C is the most effective ORR electrocatalyst. However, the practical application of Pt, Ir, and Ru in ZABs is severely limited owing to their low natural abundance and high cost. Therefore, the fabrication of inexpensive and high-performance bifunctional catalysts is essential for the development of rechargeable ZABs. Transition-metal alloys have a high electrical conductivity and low energy barrier for the reaction of oxygen, and thus they are considered promising ORR electrocatalysts. Transition-metal nitride-transition-metal alloy core-shell nanostructures can be fabricated to improve the bifunctional electrocatalytic activity. In this study, a bifunctional electrocatalyst with Fe0.64Ni0.36@Fe3NiN core-shell structures encapsulated in N-doped carbon nanotubes (Fe0.64Ni0.36@Fe3NiN/NCNT) was designed for highly efficient rechargeable ZABs. Fe0.64Ni0.36@Fe3NiN/NCNT was synthesized by pyrolyzing the nickel-iron-layered double hydroxide (NiFe-LDH) precursor, followed by ammonia etching of the Fe0.64Ni0.36 alloy. The core-shell structure produced more ORR/OER active sites. The Fe0.64Ni0.36 core exhibited high electrical conductivity, which facilitates charge transfer. The Fe3NiN shell enhanced the OER performance and improved the bifunctional performance. Moreover, the NCNT structures not only efficiently enhanced the mass transfer efficiency and intrinsic electrical conductivity, but also provided a large electrochemical active surface area. The high anticorrosion property of the Fe3NiN shell effectively protected the Fe0.64Ni0.36 core, which consequently enhanced electrocatalyst stability during the electrochemical processes. The protective carbon layer and the superior chemical stability of the Fe3NiN shell resulted in the ultrahigh stability of Fe0.64Ni0.36@Fe3NiN/NCNT. The catalyst exhibited an excellent bifunctional oxygen electrocatalytic performance, with a half-wave potential of 0.88 V for the ORR and low OER overpotential of 380 mV at 10 mA∙cm-2. Moreover, the catalyst exhibited electrochemical stability (92.8% current retention after 8 h). In addition, the Fe0.64Ni0.36@Fe3NiN/NCNT-based ZAB exhibited a higher peak power density (214 mW·cm-2) than the ZABs based on Pt/C+IrO2 (155 mW·cm-2) and Fe0.64Ni0.36/NCNT (89 mW·cm-2). Moreover, the Fe0.64Ni0.36@Fe3NiN/NCNT-based ZAB delivered a high capacity of 781 mAh·g-1, while the ZABs based on Fe0.64Ni0.36/NCNT and Pt/C+IrO2 reached capacities of 688 and 739 mAh·g-1, respectively. Furthermore, the Fe0.64Ni0.36@Fe3NiN/NCNT-based ZAB exhibited ultralong cycling stability (cycle life > 1100 h), which exceeded those of Pt/C (50 h) and Fe0.64Ni0.36/NCNT (450 h). We propose that this study will facilitate the design of novel catalysts for highly stable and efficient ZABs.
Rechargeable zinc-air batteries (ZABs) have been extensively investigated owing to their high power density and environmental friendliness. However, the slow kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) processes limit their practical application. Currently, IrO2 and RuO2 are considered the optimal OER electrocatalysts, and Pt/C is the most effective ORR electrocatalyst. However, the practical application of Pt, Ir, and Ru in ZABs is severely limited owing to their low natural abundance and high cost. Therefore, the fabrication of inexpensive and high-performance bifunctional catalysts is essential for the development of rechargeable ZABs. Transition-metal alloys have a high electrical conductivity and low energy barrier for the reaction of oxygen, and thus they are considered promising ORR electrocatalysts. Transition-metal nitride-transition-metal alloy core-shell nanostructures can be fabricated to improve the bifunctional electrocatalytic activity. In this study, a bifunctional electrocatalyst with Fe0.64Ni0.36@Fe3NiN core-shell structures encapsulated in N-doped carbon nanotubes (Fe0.64Ni0.36@Fe3NiN/NCNT) was designed for highly efficient rechargeable ZABs. Fe0.64Ni0.36@Fe3NiN/NCNT was synthesized by pyrolyzing the nickel-iron-layered double hydroxide (NiFe-LDH) precursor, followed by ammonia etching of the Fe0.64Ni0.36 alloy. The core-shell structure produced more ORR/OER active sites. The Fe0.64Ni0.36 core exhibited high electrical conductivity, which facilitates charge transfer. The Fe3NiN shell enhanced the OER performance and improved the bifunctional performance. Moreover, the NCNT structures not only efficiently enhanced the mass transfer efficiency and intrinsic electrical conductivity, but also provided a large electrochemical active surface area. The high anticorrosion property of the Fe3NiN shell effectively protected the Fe0.64Ni0.36 core, which consequently enhanced electrocatalyst stability during the electrochemical processes. The protective carbon layer and the superior chemical stability of the Fe3NiN shell resulted in the ultrahigh stability of Fe0.64Ni0.36@Fe3NiN/NCNT. The catalyst exhibited an excellent bifunctional oxygen electrocatalytic performance, with a half-wave potential of 0.88 V for the ORR and low OER overpotential of 380 mV at 10 mA∙cm-2. Moreover, the catalyst exhibited electrochemical stability (92.8% current retention after 8 h). In addition, the Fe0.64Ni0.36@Fe3NiN/NCNT-based ZAB exhibited a higher peak power density (214 mW·cm-2) than the ZABs based on Pt/C+IrO2 (155 mW·cm-2) and Fe0.64Ni0.36/NCNT (89 mW·cm-2). Moreover, the Fe0.64Ni0.36@Fe3NiN/NCNT-based ZAB delivered a high capacity of 781 mAh·g-1, while the ZABs based on Fe0.64Ni0.36/NCNT and Pt/C+IrO2 reached capacities of 688 and 739 mAh·g-1, respectively. Furthermore, the Fe0.64Ni0.36@Fe3NiN/NCNT-based ZAB exhibited ultralong cycling stability (cycle life > 1100 h), which exceeded those of Pt/C (50 h) and Fe0.64Ni0.36/NCNT (450 h). We propose that this study will facilitate the design of novel catalysts for highly stable and efficient ZABs.
