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2023 Volume 39 Issue 2
2023, 39(2): 211103
doi: 10.3866/PKU.WHXB202111037
Abstract:
Fuel cells are essential energy conversion devices for future renewable energy structures. Mainstream proton exchange membrane fuel cells (PEMFCs) generally exhibit satisfactory performance despite requiring noble metal catalysts to be stable in acidic environments. Alkaline polymer electrolyte fuel cells (APEFCs), in contrast, offer the benefit of employing non-noble metal catalysts in fuel cells, but their overall performance and especially their long-term stability require further improvement. A critical component within APEFCs is the membrane electrode assembly (MEA), which comprises a hydroxide ion conductive polymer membrane, a cathode, and an anode (including a catalyst layer and a gas diffusion layer). MEA is where electrochemical reactions occur; thus, it plays a crucial role in determining fuel cell performance. Herein, the fabrication of a cone-shaped array on the surface of an alkaline polymer electrolyte membrane for improving the overall device performance is presented. The cone array was prepared using a sacrificial anodic aluminum oxide (AAO) template, and the array side of the polymer electrolyte was used as the cathode to construct the MEA, denoted as A-MEA. The control sample with no cone arrays on the polymer electrolyte surface is denoted as P-MEA. The Pt loadings on both the anode and cathode sides were approximately 0.2 mg∙cm−2. APEFCs with A-MEA and P-MEA were separately assembled and tested in an 850e Fuel Cell Test System at a cell temperature of 80 ℃. Fully humidified hydrogen and oxygen were both supplied at a flow rate of 1000 mL·min−1. The back pressure for both the anode and the cathode was 0.2 MPa. As a result, the APEFC with A-MEA exhibited a higher peak power density than that of the APEFC with P-MEA (1.48 vs. 1.04 W∙cm−2). The enhanced electrochemical performance of the APEFC with A-MEA was ascribed to the array-structured cathode, which improved the hydrophilicity of the polymer electrolyte membrane and increased the utilization efficiency of the catalyst. The hydrophilicity of the polymer electrolyte membrane with cone arrays was confirmed using contact angle measurements. The contact angles of the membranes with and without cone arrays were ~0° and 70.8°, respectively. The hydrophilic membrane promotes the electrode reaction at the cathode side. The electrochemically active surface area (ECSA) was also measured using cyclic voltammetry (CV) between 0.08 and 1 V (vs. reversible hydrogen electrode, RHE) at a scan rate of 20 mV∙s-1, using fully humidified H2 and N2. A flow rate of 1000 mL∙min−1 and back pressure of 0 MPa were employed. Results revealed that the ECSA of the cathode without the array was smaller than that of the array-structured cathode (21.17 vs. 24.89 m2∙g−1), indicating that the array structure improved the catalyst utilization efficiency compared to that of the control sample. This study provides an effective strategy for the structural design and optimization of the MEAs in APEFCs. ![]()
Fuel cells are essential energy conversion devices for future renewable energy structures. Mainstream proton exchange membrane fuel cells (PEMFCs) generally exhibit satisfactory performance despite requiring noble metal catalysts to be stable in acidic environments. Alkaline polymer electrolyte fuel cells (APEFCs), in contrast, offer the benefit of employing non-noble metal catalysts in fuel cells, but their overall performance and especially their long-term stability require further improvement. A critical component within APEFCs is the membrane electrode assembly (MEA), which comprises a hydroxide ion conductive polymer membrane, a cathode, and an anode (including a catalyst layer and a gas diffusion layer). MEA is where electrochemical reactions occur; thus, it plays a crucial role in determining fuel cell performance. Herein, the fabrication of a cone-shaped array on the surface of an alkaline polymer electrolyte membrane for improving the overall device performance is presented. The cone array was prepared using a sacrificial anodic aluminum oxide (AAO) template, and the array side of the polymer electrolyte was used as the cathode to construct the MEA, denoted as A-MEA. The control sample with no cone arrays on the polymer electrolyte surface is denoted as P-MEA. The Pt loadings on both the anode and cathode sides were approximately 0.2 mg∙cm−2. APEFCs with A-MEA and P-MEA were separately assembled and tested in an 850e Fuel Cell Test System at a cell temperature of 80 ℃. Fully humidified hydrogen and oxygen were both supplied at a flow rate of 1000 mL·min−1. The back pressure for both the anode and the cathode was 0.2 MPa. As a result, the APEFC with A-MEA exhibited a higher peak power density than that of the APEFC with P-MEA (1.48 vs. 1.04 W∙cm−2). The enhanced electrochemical performance of the APEFC with A-MEA was ascribed to the array-structured cathode, which improved the hydrophilicity of the polymer electrolyte membrane and increased the utilization efficiency of the catalyst. The hydrophilicity of the polymer electrolyte membrane with cone arrays was confirmed using contact angle measurements. The contact angles of the membranes with and without cone arrays were ~0° and 70.8°, respectively. The hydrophilic membrane promotes the electrode reaction at the cathode side. The electrochemically active surface area (ECSA) was also measured using cyclic voltammetry (CV) between 0.08 and 1 V (vs. reversible hydrogen electrode, RHE) at a scan rate of 20 mV∙s-1, using fully humidified H2 and N2. A flow rate of 1000 mL∙min−1 and back pressure of 0 MPa were employed. Results revealed that the ECSA of the cathode without the array was smaller than that of the array-structured cathode (21.17 vs. 24.89 m2∙g−1), indicating that the array structure improved the catalyst utilization efficiency compared to that of the control sample. This study provides an effective strategy for the structural design and optimization of the MEAs in APEFCs.
2023, 39(2): 220304
doi: 10.3866/PKU.WHXB202203043
Abstract:
The use of high-capacity ternary cathode materials for high-energy batteries can cause thermal runaway of lithium-ion batteries (LIBs), hindering their safe use and further development. Therefore, improving the energy density of LIBs while maintaining their safety is essential. Current collectors (CCs), which serve as the electron carrier during the electrochemical process, do not contribute to capacity and are regarded as "dead weight" to the cells. The use of composite CCs, which have a sandwich structure where a thin metal (e.g., Al and Cu) layer is deposited on both sides of polymer films, can reduce the weight of CCs owing to the use of the low-density insulating substrate and improve the safety of LIBs (evaluated by the nail penetration test). However, due to the weak interfacial adhesion between the substrate and metal coating layer, the composite CCs may easily delaminate in electrolytes during high-temperature immersion, which could not meet the requirement for the long-term stability. Herein, we introduced an oxide strengthening layer between the substrate (polyethylene terephthalate, PET) and Al layer. The objective of strengthening layer is to increase the interface binding force between the metal and polymer substrate by enhancing the mechanical interlocking effect between the layers and forming a stable chemical bond at the interface. This increased interface binding force effectively improved the electrolyte compatibility of composite CCs even at a high temperature of 85 ℃. Based on the results of atomic force microscopy and X-ray photoelectron spectroscopy, we proposed a mechanism for the enhancement of both mechanical interlocking and chemical bonding. Additionally, the composite CCs possessed good mechanical properties that ensure their compatibility with conventional battery fabrication technologies. LIBs using composite CCs exhibited a comparable electrochemical performance to that of aluminum-CC-based (Al CCs) cells, but better performance in nail penetration test. After 280 cycles at 0.2 C, the cell showed high-capacity retention. Al-CC-based cells and PET-AlOx-Al-CC based cells remain 80.55% and 80.9% capacity retention respectively, which indicates the comparable performance. This shows that the composite CCs technology is fully adapted to the existing battery manufacturing technology, and has little influence on the electrochemical performance of LIBs. Specifically, cells with PET-AlOx-Al CCs easily passed the nail penetration test under 100% state of charge without an obvious temperature rise. Furthermore, the voltage of the punctured batteries remained at ~4 V and could still be charged and discharged. The composite CCs successfully prevented the internal short circuit and markedly improved the safety of LIBs during the nail penetration test. Our findings provide theoretical guidance and solutions for the industrialization of composite CCs.![]()
The use of high-capacity ternary cathode materials for high-energy batteries can cause thermal runaway of lithium-ion batteries (LIBs), hindering their safe use and further development. Therefore, improving the energy density of LIBs while maintaining their safety is essential. Current collectors (CCs), which serve as the electron carrier during the electrochemical process, do not contribute to capacity and are regarded as "dead weight" to the cells. The use of composite CCs, which have a sandwich structure where a thin metal (e.g., Al and Cu) layer is deposited on both sides of polymer films, can reduce the weight of CCs owing to the use of the low-density insulating substrate and improve the safety of LIBs (evaluated by the nail penetration test). However, due to the weak interfacial adhesion between the substrate and metal coating layer, the composite CCs may easily delaminate in electrolytes during high-temperature immersion, which could not meet the requirement for the long-term stability. Herein, we introduced an oxide strengthening layer between the substrate (polyethylene terephthalate, PET) and Al layer. The objective of strengthening layer is to increase the interface binding force between the metal and polymer substrate by enhancing the mechanical interlocking effect between the layers and forming a stable chemical bond at the interface. This increased interface binding force effectively improved the electrolyte compatibility of composite CCs even at a high temperature of 85 ℃. Based on the results of atomic force microscopy and X-ray photoelectron spectroscopy, we proposed a mechanism for the enhancement of both mechanical interlocking and chemical bonding. Additionally, the composite CCs possessed good mechanical properties that ensure their compatibility with conventional battery fabrication technologies. LIBs using composite CCs exhibited a comparable electrochemical performance to that of aluminum-CC-based (Al CCs) cells, but better performance in nail penetration test. After 280 cycles at 0.2 C, the cell showed high-capacity retention. Al-CC-based cells and PET-AlOx-Al-CC based cells remain 80.55% and 80.9% capacity retention respectively, which indicates the comparable performance. This shows that the composite CCs technology is fully adapted to the existing battery manufacturing technology, and has little influence on the electrochemical performance of LIBs. Specifically, cells with PET-AlOx-Al CCs easily passed the nail penetration test under 100% state of charge without an obvious temperature rise. Furthermore, the voltage of the punctured batteries remained at ~4 V and could still be charged and discharged. The composite CCs successfully prevented the internal short circuit and markedly improved the safety of LIBs during the nail penetration test. Our findings provide theoretical guidance and solutions for the industrialization of composite CCs.
2023, 39(2): 220703
doi: 10.3866/PKU.WHXB202207035
Abstract:
Operando spectroscopic characterization is effective for examining electrocatalytic reaction mechanisms. However, most operando characterization techniques currently used are based on (quasi-)steady-state spectroscopy, which often cannot directly measure transient changes occurring on the micro-millisecond time scale. Herein, an operando electrochemical UV-Vis absorption spectroscopy with 3 μs time resolution was realized by introducing bias pulses and synchronizing the bias pulse and spectral signals. Comparing the time-dependence curves of the bias pulse, collected spectral curve, and controlling voltage, a good time consistence for the three signals was observed, demonstrating the time-resolved ability of the novel apparatus. More importantly, two oxidation reactions, water oxidation reaction and hole sacrifice reagent oxidation reaction, showed distinct dynamics, verifying the reliability of the time-resolved kinetics. The water oxidation kinetics on a ferrihydrite (Fh) electrocatalyst were studied by this novel operando spectroscopic system. Different water oxidation steps were decoupled by analyzing the accumulation and decay dynamics of the operando time-resolved UV-Vis absorption data with various pulse widths and magnitudes of applied bias. A long bias pulse with width above 1s enabled the continuous accumulation of reaction intermediates in Fh electrocatalyst to reach a quasi-equilibrium state with electron extraction into the external circuit. In addition, a fast decay for water oxidation was observed after the applied bias was turned off. Importantly, when a short bias pulse with tens of ms width was applied, an abnormal intermediate accumulation process was observed after the applied bias was shut off, revealing a spontaneous species transformation process. These results confirm the validity of this novel method for examining species transformation kinetics at a fast timescale. The formation, transformation, and reaction kinetics of water oxidation reaction intermediates were directly studied on a µs to s time scale. Therefore, operando electrochemical UV-Vis absorption spectroscopy with µs time resolution can promote the understanding of various electrocatalytic reaction mechanisms and be used to guide the design and synthesis of novel high-efficiency electrocatalysts.![]()
Operando spectroscopic characterization is effective for examining electrocatalytic reaction mechanisms. However, most operando characterization techniques currently used are based on (quasi-)steady-state spectroscopy, which often cannot directly measure transient changes occurring on the micro-millisecond time scale. Herein, an operando electrochemical UV-Vis absorption spectroscopy with 3 μs time resolution was realized by introducing bias pulses and synchronizing the bias pulse and spectral signals. Comparing the time-dependence curves of the bias pulse, collected spectral curve, and controlling voltage, a good time consistence for the three signals was observed, demonstrating the time-resolved ability of the novel apparatus. More importantly, two oxidation reactions, water oxidation reaction and hole sacrifice reagent oxidation reaction, showed distinct dynamics, verifying the reliability of the time-resolved kinetics. The water oxidation kinetics on a ferrihydrite (Fh) electrocatalyst were studied by this novel operando spectroscopic system. Different water oxidation steps were decoupled by analyzing the accumulation and decay dynamics of the operando time-resolved UV-Vis absorption data with various pulse widths and magnitudes of applied bias. A long bias pulse with width above 1s enabled the continuous accumulation of reaction intermediates in Fh electrocatalyst to reach a quasi-equilibrium state with electron extraction into the external circuit. In addition, a fast decay for water oxidation was observed after the applied bias was turned off. Importantly, when a short bias pulse with tens of ms width was applied, an abnormal intermediate accumulation process was observed after the applied bias was shut off, revealing a spontaneous species transformation process. These results confirm the validity of this novel method for examining species transformation kinetics at a fast timescale. The formation, transformation, and reaction kinetics of water oxidation reaction intermediates were directly studied on a µs to s time scale. Therefore, operando electrochemical UV-Vis absorption spectroscopy with µs time resolution can promote the understanding of various electrocatalytic reaction mechanisms and be used to guide the design and synthesis of novel high-efficiency electrocatalysts.
2023, 39(2): 220304
doi: 10.3866/PKU.WHXB202203048
Abstract:
Solar cells, which are excellent alternatives to traditional fossil fuels, can efficiently convert sunlight into electricity. The intensive development of high-performance photovoltaic materials plays an important role in environmental protection and the utilization of renewable energy. Organic–inorganic hybrid perovskite materials, with a formula of ABX3 (A = methylammonium (MA) or formamidinium (FA); B = Pb or Sn; X = Cl, I, or Br), have exhibited remarkable commercial prospects in high-performance photovoltaic devices owing to their long carrier diffusion length, excellent light absorption properties, high charge carrier mobility, and weak exciton binding energy. Recently, perovskite solar cells, fabricated using halide perovskite materials as light-absorbing layers, have achieved remarkable results; their certified power conversion efficiency has continuously improved and reached 25.7%. However, high-performance devices are usually fabricated using spin-coating methods with active areas below 0.1 cm2. Hence, long-term research goals include achieving a large-scale uniform preparation of high-quality photoactive layers. The current one-step preparation of perovskite films involves the nucleation-crystalline growth process of perovskite. Auxiliary processes, such as using an anti-solvent, are often required to increase the nucleation rate and density of the film, which is not suitable for industrial large-area preparation. Additionally, the large-area preparation of perovskite films by spin-coating will result in different film thicknesses in the center and edge regions of the film due to an uneven centrifugal force. This will cause intense carrier recombination in the thicker area of the film and weak light absorption in the thinner area, which will reduce the performance of the device. To address these problems, the development of a large-area fabrication method for high-performance perovskite light-absorbing layers is essential. In this study, a two-step sequential blade-coating strategy was developed to prepare the FA-based perovskite layer. In general, PbI2 easily forms a dense film; therefore, formamidinium iodide (FAI) cannot deeply penetrate to completely react with PbI2. The PbI2 residue is therefore detrimental to charge transportation. To fabricate the desired porous PbI2 film, tetrahydrothiophene 1-oxide (THTO) was introduced into the PbI2 precursor solution. By forming PbI2·THTO complexes, PbI2 crystallization is controlled, resulting in the formation of vertically packed PbI2 flaky crystals. These crystals provide nanochannels for easy FAI penetration. The 5 cm × 5 cm modules fabricated through this strategy achieved a high efficiency of 18.65% with excellent stability. This indicates that the two-step sequential blade-coating strategy has considerable potential for scaling up the production of perovskite solar cells.![]()
Solar cells, which are excellent alternatives to traditional fossil fuels, can efficiently convert sunlight into electricity. The intensive development of high-performance photovoltaic materials plays an important role in environmental protection and the utilization of renewable energy. Organic–inorganic hybrid perovskite materials, with a formula of ABX3 (A = methylammonium (MA) or formamidinium (FA); B = Pb or Sn; X = Cl, I, or Br), have exhibited remarkable commercial prospects in high-performance photovoltaic devices owing to their long carrier diffusion length, excellent light absorption properties, high charge carrier mobility, and weak exciton binding energy. Recently, perovskite solar cells, fabricated using halide perovskite materials as light-absorbing layers, have achieved remarkable results; their certified power conversion efficiency has continuously improved and reached 25.7%. However, high-performance devices are usually fabricated using spin-coating methods with active areas below 0.1 cm2. Hence, long-term research goals include achieving a large-scale uniform preparation of high-quality photoactive layers. The current one-step preparation of perovskite films involves the nucleation-crystalline growth process of perovskite. Auxiliary processes, such as using an anti-solvent, are often required to increase the nucleation rate and density of the film, which is not suitable for industrial large-area preparation. Additionally, the large-area preparation of perovskite films by spin-coating will result in different film thicknesses in the center and edge regions of the film due to an uneven centrifugal force. This will cause intense carrier recombination in the thicker area of the film and weak light absorption in the thinner area, which will reduce the performance of the device. To address these problems, the development of a large-area fabrication method for high-performance perovskite light-absorbing layers is essential. In this study, a two-step sequential blade-coating strategy was developed to prepare the FA-based perovskite layer. In general, PbI2 easily forms a dense film; therefore, formamidinium iodide (FAI) cannot deeply penetrate to completely react with PbI2. The PbI2 residue is therefore detrimental to charge transportation. To fabricate the desired porous PbI2 film, tetrahydrothiophene 1-oxide (THTO) was introduced into the PbI2 precursor solution. By forming PbI2·THTO complexes, PbI2 crystallization is controlled, resulting in the formation of vertically packed PbI2 flaky crystals. These crystals provide nanochannels for easy FAI penetration. The 5 cm × 5 cm modules fabricated through this strategy achieved a high efficiency of 18.65% with excellent stability. This indicates that the two-step sequential blade-coating strategy has considerable potential for scaling up the production of perovskite solar cells.
2023, 39(2): 220702
doi: 10.3866/PKU.WHXB202207026
Abstract:
The ever-increasing carbon dioxide (CO2) emissions caused by excessive fossil fuel consumption induce environmental issues such as global warming. To overcome this, the electrocatalytic CO2 reduction (ECR) under ambient conditions offers an appealing approach for converting CO2 to value-added chemicals and realizing a closed carbon loop. Among the ECR products, ethylene (C2H4), an important building block for plastics and other chemicals, has attracted considerable attention owing to its compatibility with existing infrastructure and the promising substitution of industrial steam cracking. In recent years, numerous efforts have been devoted to developing highly active and selective catalysts for converting CO2 to C2H4, with most studies having focused on Cu-based materials. Despite the significant advancements made to date, the development of the ECR-to-C2H4 process is still hindered by the lack of suitable catalysts that can effectively activate CO2 and strengthen the surface binding of *CO and *COH species. In this study, an amorphous copper oxide (CuOx) nanofilm that is rich in oxygen vacancies was prepared via a facile vacuum evaporation method for the efficient electrocatalytic conversion of CO2 to C2H4. It was expected that the nano-scale electrode thickness would greatly accelerate charge- and mass-transfer during CO2 electrolysis. Moreover, the introduction of oxygen vacancies favored the adsorption of CO2 and intermediates. As a result, in a typical H-cell, the synthesized defective catalyst delivered a maximum Faradaic efficiency of 85 ± 3% at −1.3 V versus the reversible hydrogen electrode and maintained a stable C2H4 selectivity over 48 h in a 0.1 M potassium bicarbonate solution. Interestingly, the performance observed with the synthesized electrocatalyst in this study is comparable with that of state-of-the-art Cu-based ECR catalysts. Additional structural and chemical characterizations confirmed the robust nature of the as-prepared catalyst. Moreover, when the catalyst was utilized in a membrane electrode assembly cell, it achieved a maximum C2H4 partial current density of approximately 115.4 mA∙cm−2 at a cell voltage of −1.95 V and Faradaic efficiency of 78 ± 2% at a cell voltage of −1.75 V. Furthermore, theoretical and experimental analyses revealed that oxygen defects not only favored CO2 adsorption but also enabled strong affinities for *CO and *COH intermediates, which synergistically contributed to a high selectivity for C2H4 formation. We believe that our present work will motivate the exploration of amorphous Cu-based materials for achieving efficient CO2-to-C2H4 electrolysis and be a guide towards fundamentally understanding the mechanism of catalytic CO2 reduction.![]()
The ever-increasing carbon dioxide (CO2) emissions caused by excessive fossil fuel consumption induce environmental issues such as global warming. To overcome this, the electrocatalytic CO2 reduction (ECR) under ambient conditions offers an appealing approach for converting CO2 to value-added chemicals and realizing a closed carbon loop. Among the ECR products, ethylene (C2H4), an important building block for plastics and other chemicals, has attracted considerable attention owing to its compatibility with existing infrastructure and the promising substitution of industrial steam cracking. In recent years, numerous efforts have been devoted to developing highly active and selective catalysts for converting CO2 to C2H4, with most studies having focused on Cu-based materials. Despite the significant advancements made to date, the development of the ECR-to-C2H4 process is still hindered by the lack of suitable catalysts that can effectively activate CO2 and strengthen the surface binding of *CO and *COH species. In this study, an amorphous copper oxide (CuOx) nanofilm that is rich in oxygen vacancies was prepared via a facile vacuum evaporation method for the efficient electrocatalytic conversion of CO2 to C2H4. It was expected that the nano-scale electrode thickness would greatly accelerate charge- and mass-transfer during CO2 electrolysis. Moreover, the introduction of oxygen vacancies favored the adsorption of CO2 and intermediates. As a result, in a typical H-cell, the synthesized defective catalyst delivered a maximum Faradaic efficiency of 85 ± 3% at −1.3 V versus the reversible hydrogen electrode and maintained a stable C2H4 selectivity over 48 h in a 0.1 M potassium bicarbonate solution. Interestingly, the performance observed with the synthesized electrocatalyst in this study is comparable with that of state-of-the-art Cu-based ECR catalysts. Additional structural and chemical characterizations confirmed the robust nature of the as-prepared catalyst. Moreover, when the catalyst was utilized in a membrane electrode assembly cell, it achieved a maximum C2H4 partial current density of approximately 115.4 mA∙cm−2 at a cell voltage of −1.95 V and Faradaic efficiency of 78 ± 2% at a cell voltage of −1.75 V. Furthermore, theoretical and experimental analyses revealed that oxygen defects not only favored CO2 adsorption but also enabled strong affinities for *CO and *COH intermediates, which synergistically contributed to a high selectivity for C2H4 formation. We believe that our present work will motivate the exploration of amorphous Cu-based materials for achieving efficient CO2-to-C2H4 electrolysis and be a guide towards fundamentally understanding the mechanism of catalytic CO2 reduction.
2023, 39(2): 220502
doi: 10.3866/PKU.WHXB202205028
Abstract:
Hydrogen is considered as a desirable clean energy source for supporting human life in the future. Electrochemical water splitting is a promising method for generating carbon-free hydrogen. However, the relatively high overpotential of anodic oxygen evolution reaction (OER) is the main obstacle hindering the widespread popularity of water electrocatalysis technology. Recently, urea oxidation reaction (UOR) has gained significant attention as a potential alternative to OER for hydrogen production since the equilibrium potential of UOR is 0.86 V lower than that of OER. Transition metal-based layered double hydroxides (TM-LDHs) have been explored as promising UOR electrocatalysts, with the advantages of diversified metal species, stable two-dimensional layered structure and exchangeability of interlayer anions. To date, most studies have focused on TM-LDHs of late transition metals (e.g., Ni, Co, and Fe). In this work, by combining early and late transition metals, CoV-LDHs nanosheets were fabricated via a simple one-step coprecipitation method as high-performance UOR electrocatalysts. Additionally, cobalt hydroxide (Co(OH)2), with a similar lamellar structure, was synthesized via the same method. When compared with Co(OH)2, CoV-LDHs nanosheets exhibited better UOR performance owing to the following advantages: 1) The nanosheet structure of the as-fabricated CoV-LDHs electrocatalyst exposed a high number of active sites for the electrocatalytic conversion of urea. 2) The introduction of V enhanced the wettability of the CoV-LDHs electrocatalyst; thus, increasing its intrinsic electrocatalytic kinetics. 3) The d-electron compensation effect between Co (3d74s2) and V (3d34s2) was conducive to promoting the adsorption of urea. Therefore, the CoV-LDHs electrocatalyst exhibited a low electrochemical potential (1.52 V vs. the reversible hydrogen electrode, RHE) to achieve a current density of 10 mA∙cm−2 in 1 mol∙L−1 of potassium hydroxide containing 0.33 mol∙L−1 urea, which was 70 mV less than that of Co(OH)2. The Tafel slope value of the CoV-LDHs electrocatalyst (99.9 mV∙dec−1) was lower than that of Co(OH)2 (115.9 mV∙dec−1), indicating faster UOR kinetics over the CoV-LDHs electrocatalyst. Furthermore, the CoV-LDHs electrocatalyst displayed high stability, with a negligible potential increase after a 10-h chronopotentiometry test by maintaining the current density of 10 mA∙cm−2. In conclusion, the present work not only shows that the d-electron compensation effect between early and late transition metals could adjust the local electronic structure of TM-LDHs to improve the UOR efficiency, but also provides a feasible route to design dedicated nanostructured TM-LDHs as high-performance UOR electrocatalysts.![]()
Hydrogen is considered as a desirable clean energy source for supporting human life in the future. Electrochemical water splitting is a promising method for generating carbon-free hydrogen. However, the relatively high overpotential of anodic oxygen evolution reaction (OER) is the main obstacle hindering the widespread popularity of water electrocatalysis technology. Recently, urea oxidation reaction (UOR) has gained significant attention as a potential alternative to OER for hydrogen production since the equilibrium potential of UOR is 0.86 V lower than that of OER. Transition metal-based layered double hydroxides (TM-LDHs) have been explored as promising UOR electrocatalysts, with the advantages of diversified metal species, stable two-dimensional layered structure and exchangeability of interlayer anions. To date, most studies have focused on TM-LDHs of late transition metals (e.g., Ni, Co, and Fe). In this work, by combining early and late transition metals, CoV-LDHs nanosheets were fabricated via a simple one-step coprecipitation method as high-performance UOR electrocatalysts. Additionally, cobalt hydroxide (Co(OH)2), with a similar lamellar structure, was synthesized via the same method. When compared with Co(OH)2, CoV-LDHs nanosheets exhibited better UOR performance owing to the following advantages: 1) The nanosheet structure of the as-fabricated CoV-LDHs electrocatalyst exposed a high number of active sites for the electrocatalytic conversion of urea. 2) The introduction of V enhanced the wettability of the CoV-LDHs electrocatalyst; thus, increasing its intrinsic electrocatalytic kinetics. 3) The d-electron compensation effect between Co (3d74s2) and V (3d34s2) was conducive to promoting the adsorption of urea. Therefore, the CoV-LDHs electrocatalyst exhibited a low electrochemical potential (1.52 V vs. the reversible hydrogen electrode, RHE) to achieve a current density of 10 mA∙cm−2 in 1 mol∙L−1 of potassium hydroxide containing 0.33 mol∙L−1 urea, which was 70 mV less than that of Co(OH)2. The Tafel slope value of the CoV-LDHs electrocatalyst (99.9 mV∙dec−1) was lower than that of Co(OH)2 (115.9 mV∙dec−1), indicating faster UOR kinetics over the CoV-LDHs electrocatalyst. Furthermore, the CoV-LDHs electrocatalyst displayed high stability, with a negligible potential increase after a 10-h chronopotentiometry test by maintaining the current density of 10 mA∙cm−2. In conclusion, the present work not only shows that the d-electron compensation effect between early and late transition metals could adjust the local electronic structure of TM-LDHs to improve the UOR efficiency, but also provides a feasible route to design dedicated nanostructured TM-LDHs as high-performance UOR electrocatalysts.
2023, 39(2): 220504
doi: 10.3866/PKU.WHXB202205045
Abstract:
The ever-worsening world-wide energy crisis and environmental issues are encouraging the development of green and renewable energy. Thus, rechargeable batteries are being developed and employed for energy storage and conversion in various electronic equipment. When compared with metal lithium batteries, aqueous rechargeable batteries have gained significant attention due to their advantages of high safety, low cost, and environmental friendliness. Among the various known rechargeable batteries (Li+, Na+, K+, NH4+, Mg2+, Ca2+, and Al3+), aqueous zinc-ion batteries (ZIBs) are considered as promising energy storage devices because the zinc electrode exhibits high capacity (820 mAh∙g−1) and low potential (−0.76 V vs. Standard hydrogen electrode (SHE)). To date, various ZIBs cathode materials with excellent performance have been developed, such as manganese- and vanadium-based oxides, Prussian blue and its analogues, and organic compounds. Unfortunately, some of these materials, especially manganese- and vanadium-based oxides, suffer from critical structural collapse, dissolution, and cathode/electrolyte interfacial side reactions, which lead to low Coulombic efficiency and poor cycle performance. The poor cycle performance is one of the main obstacles hindering the large-scale application of manganese- and vanadium-based oxides. Therefore, the structural design of cathodes and electrolyte regulation strategies have been extensively investigated to solve these problems and improve electrochemical performance. In comparison, electrolyte regulation is an important and effective strategy for improving the performance of ZIBs cathodes. It is well known that a strong interaction force exists between Zn2+ and H2O, therefore, Zn2+ can coordinate with six H2O molecules to form [Zn(H2O)6]2+ in the dilute aqueous electrolyte, while forming numerous hydrogen bonds between the H2O molecules. The Zn2+-solvation structure and hydrogen bonds can be destructed and restructured by changing the anion, and using highly concentrated electrolyte and/or organic solvent, thereby decreasing the number of H2O molecules in the solvated structure and the activity of free water. Furthermore, additives can change the pH value of the aqueous electrolyte and build a dissolution equilibrium between the cathode and electrolyte. Hence, an appropriate electrolyte regulation strategy can broaden the electrochemical stability window of electrolytes, improve the working potential, suppress the occurrence of interfacial side reactions, and prevent the dissolution of the active materials, thereby improving the electrochemical performance of ZIBs. Herein, we review the possible electrolyte regulation strategies for enhancing the electrochemical performance of ZIBs cathodes and classify regulation strategy into two main categories: 1) Solute (including different zinc salts, additive, and water-in-salt) and 2) Solvent (composite of organic/inorganic hybrid electrolytes). We then discuss the advantages and challenges of each strategy, and finally predict the possible future direction of electrolyte development.![]()
The ever-worsening world-wide energy crisis and environmental issues are encouraging the development of green and renewable energy. Thus, rechargeable batteries are being developed and employed for energy storage and conversion in various electronic equipment. When compared with metal lithium batteries, aqueous rechargeable batteries have gained significant attention due to their advantages of high safety, low cost, and environmental friendliness. Among the various known rechargeable batteries (Li+, Na+, K+, NH4+, Mg2+, Ca2+, and Al3+), aqueous zinc-ion batteries (ZIBs) are considered as promising energy storage devices because the zinc electrode exhibits high capacity (820 mAh∙g−1) and low potential (−0.76 V vs. Standard hydrogen electrode (SHE)). To date, various ZIBs cathode materials with excellent performance have been developed, such as manganese- and vanadium-based oxides, Prussian blue and its analogues, and organic compounds. Unfortunately, some of these materials, especially manganese- and vanadium-based oxides, suffer from critical structural collapse, dissolution, and cathode/electrolyte interfacial side reactions, which lead to low Coulombic efficiency and poor cycle performance. The poor cycle performance is one of the main obstacles hindering the large-scale application of manganese- and vanadium-based oxides. Therefore, the structural design of cathodes and electrolyte regulation strategies have been extensively investigated to solve these problems and improve electrochemical performance. In comparison, electrolyte regulation is an important and effective strategy for improving the performance of ZIBs cathodes. It is well known that a strong interaction force exists between Zn2+ and H2O, therefore, Zn2+ can coordinate with six H2O molecules to form [Zn(H2O)6]2+ in the dilute aqueous electrolyte, while forming numerous hydrogen bonds between the H2O molecules. The Zn2+-solvation structure and hydrogen bonds can be destructed and restructured by changing the anion, and using highly concentrated electrolyte and/or organic solvent, thereby decreasing the number of H2O molecules in the solvated structure and the activity of free water. Furthermore, additives can change the pH value of the aqueous electrolyte and build a dissolution equilibrium between the cathode and electrolyte. Hence, an appropriate electrolyte regulation strategy can broaden the electrochemical stability window of electrolytes, improve the working potential, suppress the occurrence of interfacial side reactions, and prevent the dissolution of the active materials, thereby improving the electrochemical performance of ZIBs. Herein, we review the possible electrolyte regulation strategies for enhancing the electrochemical performance of ZIBs cathodes and classify regulation strategy into two main categories: 1) Solute (including different zinc salts, additive, and water-in-salt) and 2) Solvent (composite of organic/inorganic hybrid electrolytes). We then discuss the advantages and challenges of each strategy, and finally predict the possible future direction of electrolyte development.
2023, 39(2): 220505
doi: 10.3866/PKU.WHXB202205050
Abstract:
Using renewable energy sources such as wind, solar, and tidal power is one of the most effective ways to address the global energy crisis and the ensuing environmental issues. As essential complementary components to renewable energy, high-performance energy storage devices and systems are urgently required. Since the 1990s, the global battery market has been dominated by lithium-ion batteries (LIBs) owing to their high energy density and long cycle life. They have been widely used in portable electronics, and more recently, in electric vehicles. However, lithium resources are limited and unevenly distributed; therefore, the manufacturing costs of LIBs are still high. There is also increasing concern about their operational safety. Thus, it is crucial to develop next-generation battery technologies with lower costs and higher safety. In recent years, magnesium-ion batteries (MIBs) have attracted increasing attention as one of the most promising multivalent ion batteries. The use of magnesium is encouraged owing to its good air stability, lower reduction potential (−2.356 V vs. standard hydrogen electrode), higher volumetric specific capacity (3833 mAh∙cm−3), and dendrite-free deposition upon cycling. Moreover, magnesium reserves (2.3%) are 1045 times more than those of lithium (0.0022%), because of which, MIBs are considerably less expensive than LIBs. The development of MIBs has, however, encountered a few challenges arising from the comprising cathodes, electrolytes, and anodes. Mg2+ ions with smaller radii and higher charge densities have strong Coulomb interactions with electrode materials, which leads to sluggish kinetics and high diffusion barriers during de-/intercalation. Contemporary electrolytes generally have poor chemical compatibility with cathodes of MIBs, narrow electrochemical windows, and high deposition overpotential, which limits the development of high-voltage MIBs. Moreover, Mg tends to react with organic solvents (especially carbonates and nitriles), forming passivation layers on the surfaces, which increase the interfacial resistance and lead to battery irreversibility. Therefore, material design and technological innovation are crucial for developing commercially viable MIBs. This review focuses on recent advances on MIB cathode materials. First, we present a brief description of the characteristics of MIBs and discuss their strengths and drawbacks. Then, we overview three types of cathode materials, namely, intercalation-type cathodes, conversion-type cathodes, and organic cathodes, followed by a summary of their limitations and recent efforts for addressing the above-mentioned challenges. We conclude with perspectives for future research directions.![]()
Using renewable energy sources such as wind, solar, and tidal power is one of the most effective ways to address the global energy crisis and the ensuing environmental issues. As essential complementary components to renewable energy, high-performance energy storage devices and systems are urgently required. Since the 1990s, the global battery market has been dominated by lithium-ion batteries (LIBs) owing to their high energy density and long cycle life. They have been widely used in portable electronics, and more recently, in electric vehicles. However, lithium resources are limited and unevenly distributed; therefore, the manufacturing costs of LIBs are still high. There is also increasing concern about their operational safety. Thus, it is crucial to develop next-generation battery technologies with lower costs and higher safety. In recent years, magnesium-ion batteries (MIBs) have attracted increasing attention as one of the most promising multivalent ion batteries. The use of magnesium is encouraged owing to its good air stability, lower reduction potential (−2.356 V vs. standard hydrogen electrode), higher volumetric specific capacity (3833 mAh∙cm−3), and dendrite-free deposition upon cycling. Moreover, magnesium reserves (2.3%) are 1045 times more than those of lithium (0.0022%), because of which, MIBs are considerably less expensive than LIBs. The development of MIBs has, however, encountered a few challenges arising from the comprising cathodes, electrolytes, and anodes. Mg2+ ions with smaller radii and higher charge densities have strong Coulomb interactions with electrode materials, which leads to sluggish kinetics and high diffusion barriers during de-/intercalation. Contemporary electrolytes generally have poor chemical compatibility with cathodes of MIBs, narrow electrochemical windows, and high deposition overpotential, which limits the development of high-voltage MIBs. Moreover, Mg tends to react with organic solvents (especially carbonates and nitriles), forming passivation layers on the surfaces, which increase the interfacial resistance and lead to battery irreversibility. Therefore, material design and technological innovation are crucial for developing commercially viable MIBs. This review focuses on recent advances on MIB cathode materials. First, we present a brief description of the characteristics of MIBs and discuss their strengths and drawbacks. Then, we overview three types of cathode materials, namely, intercalation-type cathodes, conversion-type cathodes, and organic cathodes, followed by a summary of their limitations and recent efforts for addressing the above-mentioned challenges. We conclude with perspectives for future research directions.
2023, 39(2): 220900
doi: 10.3866/PKU.WHXB202209001
Abstract:
Green hydrogen is obtained by electrochemical water splitting using electricity converted from renewable energy sources. When green hydrogen undergoes combustion, it produces only water, leading to zero CO2 emissions from the source, which is important for the global energy transition. The sluggish kinetics of the hydrogen evolution reaction (HER)/oxygen evolution reaction (OER) in alkaline media have hindered an enhancement in hydrogen production from electrochemical water splitting. A detailed understanding of the alkaline reaction kinetics is important to accomplish the global mission of carbon neutrality. This review presents the theoretical kinetics for the HER and OER in alkaline media using different designed electrocatalysts, and discusses their corresponding reaction mechanisms. Subsequently, current design concepts and generalities on catalysts for water electrolysis are discussed. Enhancements in the OER activity for alkaline water electrolysis can be achieved through strategies that are classified into two major categories. In the first category, the exposure of numerous active sites is achieved by engineering the morphology and obtaining a high surface area. In the second category, the intrinsic activity of the catalyst toward the OER is enhanced by heteroatomic participation, vacancy formation, and the use of heterogeneous media. Advanced characterization techniques and in-situ testing techniques have confirmed the presence of complex oxidation media for the OER, which have a significant impact on the catalyst structure and local coordination. Research on the active sites of the catalyst, high concentrations of active species, and the design of highly efficient reaction media is required to further drive catalyst development for the OER. The evaluation of electrocatalysts exhibiting high performance at high current densities to produce green hydrogen is crucial for their implementation in industrial applications. Currently, large-scale synthesis a key technology to obtain industrial electrodes. Meanwhile, the construction of superaerophobic electrodes and three-dimensional electrodes facilitates the design of high-performance industrial catalytic electrodes. Subsequently, three different electrolytic cells that are typically used to obtain green hydrogen at the industrial scale are presented. The limitations to the design of electrolytic cells and the related solutions are also discussed. In-depth investigations on the design of either industrial electrocatalysts, commercial membranes, or electrolyzers can improve the understanding of industrial design principles to be applied to obtain industrial electrolyzers with increased efficiency, safety, and practicality. Finally, recent developments on electrocatalysts for water splitting and their limitations for industrial applications are presented to provide new perspectives and guidelines on the preparation of next-generation electrolytic catalysts. ![]()
Green hydrogen is obtained by electrochemical water splitting using electricity converted from renewable energy sources. When green hydrogen undergoes combustion, it produces only water, leading to zero CO2 emissions from the source, which is important for the global energy transition. The sluggish kinetics of the hydrogen evolution reaction (HER)/oxygen evolution reaction (OER) in alkaline media have hindered an enhancement in hydrogen production from electrochemical water splitting. A detailed understanding of the alkaline reaction kinetics is important to accomplish the global mission of carbon neutrality. This review presents the theoretical kinetics for the HER and OER in alkaline media using different designed electrocatalysts, and discusses their corresponding reaction mechanisms. Subsequently, current design concepts and generalities on catalysts for water electrolysis are discussed. Enhancements in the OER activity for alkaline water electrolysis can be achieved through strategies that are classified into two major categories. In the first category, the exposure of numerous active sites is achieved by engineering the morphology and obtaining a high surface area. In the second category, the intrinsic activity of the catalyst toward the OER is enhanced by heteroatomic participation, vacancy formation, and the use of heterogeneous media. Advanced characterization techniques and in-situ testing techniques have confirmed the presence of complex oxidation media for the OER, which have a significant impact on the catalyst structure and local coordination. Research on the active sites of the catalyst, high concentrations of active species, and the design of highly efficient reaction media is required to further drive catalyst development for the OER. The evaluation of electrocatalysts exhibiting high performance at high current densities to produce green hydrogen is crucial for their implementation in industrial applications. Currently, large-scale synthesis a key technology to obtain industrial electrodes. Meanwhile, the construction of superaerophobic electrodes and three-dimensional electrodes facilitates the design of high-performance industrial catalytic electrodes. Subsequently, three different electrolytic cells that are typically used to obtain green hydrogen at the industrial scale are presented. The limitations to the design of electrolytic cells and the related solutions are also discussed. In-depth investigations on the design of either industrial electrocatalysts, commercial membranes, or electrolyzers can improve the understanding of industrial design principles to be applied to obtain industrial electrolyzers with increased efficiency, safety, and practicality. Finally, recent developments on electrocatalysts for water splitting and their limitations for industrial applications are presented to provide new perspectives and guidelines on the preparation of next-generation electrolytic catalysts.
