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2021 Volume 37 Issue 7
2021, 37(7): 200703
doi: 10.3866/PKU.WHXB202007031
Abstract:
Asymmetric manganese cluster, the active center of photosystem II (PSII) in nature, is hydrogen-bonded to surrounding amino acid residues and water molecules. This phenomenon is of great inspiration significance for developing and studying artificial Mn-based oxygen evolution reaction (OER) catalysts. Herein, we prepared manganese phosphate nanosheets through intercalation of ethylenediamine ions and water molecules ((EDAI)(H2O)MnPi) using a simple co-precipitation method. (EDAI)(H2O)MnPi is also hydrogen-bonded to interlayer ethylenediamine ions and water molecules, forming a hydrogen-bonding network. The morphology of the (EDAI)(H2O)MnPi sample was characterized by scanning electron microscopy (SEM) and transmission electron microscopy. The thickness of (EDAI)(H2O)MnPi was characterized by atomic force microscopy. The composition and structure of (EDAI)(H2O)MnPi were characterized by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy. For control studies, manganese phosphate (EDAI)MnPi and (H2O)MnPi samples were also synthesized. The structure and morphology of (EDAI)MnPi and (H2O)MnPi samples were characterized by XRD and SEM. The difference between (EDAI)(H2O)MnPi, (EDAI)MnPi, and (H2O)MnPi were further characterized by thermal gravimetric analysis and derivative thermogravimetric analysis. Electrocatalytic properties of the (EDAI)(H2O)MnPi, (EDAI)MnPi, and (H2O)MnPi for OER were studied in 0.05 mol∙L−1 pH = 7 phosphate buffered saline solution, through linear sweep voltammetry, electrochemical impedance spectroscopy, and controlled potential electrolysis (CPE) test. The electrochemical surface area (ECSA) analyses of (EDAI)(H2O)MnPi, (EDAI)MnPi, and (H2O)MnPi samples were recorded by charging currents in the non-Faradaic potential region at different scan rates. Considering the different ECSAs of different materials, the water oxidation activities of three materials were normalized by ECSA. Compared with counterparts of (EDAI)MnPi (610 mV) and (H2O)MnPi (580 mV), manganese phosphate nanosheets (EDAI)(H2O)MnPi exhibited a lower overpotential of 520 mV when driving a current density of 1 mA∙cm−2 in neutral conditions. The CPE experiment revealed that (EDAI)(H2O)MnPi remained active for at least 10 h. Manganese phosphate nanosheets containing a rich, extensive, and continuous hydrogen bond network exhibited improved OER performance in neutral conditions. The hydrogen-bonding network in manganese phosphate nanosheets has similar functions to the hydrogen-bonding network in PSII, which could accelerate the transfer rate of protons and facilitate electrocatalytic water oxidation. This study may provide guidance for the design of water oxidation catalysts with rich hydrogen bond network. ![]()
Asymmetric manganese cluster, the active center of photosystem II (PSII) in nature, is hydrogen-bonded to surrounding amino acid residues and water molecules. This phenomenon is of great inspiration significance for developing and studying artificial Mn-based oxygen evolution reaction (OER) catalysts. Herein, we prepared manganese phosphate nanosheets through intercalation of ethylenediamine ions and water molecules ((EDAI)(H2O)MnPi) using a simple co-precipitation method. (EDAI)(H2O)MnPi is also hydrogen-bonded to interlayer ethylenediamine ions and water molecules, forming a hydrogen-bonding network. The morphology of the (EDAI)(H2O)MnPi sample was characterized by scanning electron microscopy (SEM) and transmission electron microscopy. The thickness of (EDAI)(H2O)MnPi was characterized by atomic force microscopy. The composition and structure of (EDAI)(H2O)MnPi were characterized by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy. For control studies, manganese phosphate (EDAI)MnPi and (H2O)MnPi samples were also synthesized. The structure and morphology of (EDAI)MnPi and (H2O)MnPi samples were characterized by XRD and SEM. The difference between (EDAI)(H2O)MnPi, (EDAI)MnPi, and (H2O)MnPi were further characterized by thermal gravimetric analysis and derivative thermogravimetric analysis. Electrocatalytic properties of the (EDAI)(H2O)MnPi, (EDAI)MnPi, and (H2O)MnPi for OER were studied in 0.05 mol∙L−1 pH = 7 phosphate buffered saline solution, through linear sweep voltammetry, electrochemical impedance spectroscopy, and controlled potential electrolysis (CPE) test. The electrochemical surface area (ECSA) analyses of (EDAI)(H2O)MnPi, (EDAI)MnPi, and (H2O)MnPi samples were recorded by charging currents in the non-Faradaic potential region at different scan rates. Considering the different ECSAs of different materials, the water oxidation activities of three materials were normalized by ECSA. Compared with counterparts of (EDAI)MnPi (610 mV) and (H2O)MnPi (580 mV), manganese phosphate nanosheets (EDAI)(H2O)MnPi exhibited a lower overpotential of 520 mV when driving a current density of 1 mA∙cm−2 in neutral conditions. The CPE experiment revealed that (EDAI)(H2O)MnPi remained active for at least 10 h. Manganese phosphate nanosheets containing a rich, extensive, and continuous hydrogen bond network exhibited improved OER performance in neutral conditions. The hydrogen-bonding network in manganese phosphate nanosheets has similar functions to the hydrogen-bonding network in PSII, which could accelerate the transfer rate of protons and facilitate electrocatalytic water oxidation. This study may provide guidance for the design of water oxidation catalysts with rich hydrogen bond network.
2021, 37(7): 200707
doi: 10.3866/PKU.WHXB202007079
Abstract:
With the rapid development of human society, clean energy forms are imperative to sustain the normal operations of various mechanical and electrical facilities under a cozy environment. Hydrogen is considered among the most promising clean energy sources for the future. Recently, electrochemical water splitting has been considered as one of the most efficient approaches to harvest hydrogen energy, which generates only non-pollutant water on combustion. However, the sluggish anodic oxygen evolution reaction significantly restricts the efficiency of water splitting and requires a relatively high cell voltage to drive the electrolysis. Therefore, seeking a thermodynamically favorable anodic reaction to replace the sluggish oxygen evolution reaction by utilizing highly active bifunctional electrocatalysts for the anodic reaction and hydrogen evolution are crucial for achieving energy-efficient hydrogen production for industrial applications. Nevertheless, it is known that the oxygen evolution reaction can be replaced with other useful and thermodynamically favorable reactions to reduce the electrolysis voltage for realizing energy-efficient hydrogen production. Therefore, in this study, we present a bifunctional nickel nanoparticle-embedded carbon (Ni@C) prism-like microrod electrocatalyst synthesized via a two-step method involving the synthesis of a precursor metal-organic framework-74 and subsequent carbonization treatment for methanol oxidation and hydrogen evolution. The interfacial structure consisting of a nickel and carbon skeleton was realized via in situ carbonization. However, the dispersed nickel nanoparticles do not easily aggregate owing to the partition by the surrounding carbon as it would sufficiently expose the active Ni sites to the electrolytes, ensuring fast charge transfer between the catalyst and electrolytes by accelerating the electrochemical kinetics. In the anodic methanol oxidation, the products were detected as carbon dioxide and formate with faradaic efficiencies of 36.2% and 62.5%, respectively, at an applied potential of 1.55 V. Meanwhile, the Ni@C microrod catalyst demonstrated high activity and durability (2.7% current decay after 12 h of continuous operation) toward methanol oxidation, which demonstrates that methanol oxidation precedes oxidation under voltage forces. Notably, the bifunctional catalyst not only exhibits excellent performance toward methanol oxidation but also yields a low overpotential of 155 mV to drive 10 mA∙cm−2 toward hydrogen evolution in 1.0 mol∙L−1 KOH aqueous solution with 0.5 mol∙L−1 methanol at room temperature, which guarantees the hydrogen production efficiency. More importantly, the constructed two-electrode electrolyzer produced a current density of 10 mA∙cm−2 at a low cell voltage of 1.6 V, which decreased by 240 mV after replacing the oxygen evolution reaction with methanol oxidation. ![]()
With the rapid development of human society, clean energy forms are imperative to sustain the normal operations of various mechanical and electrical facilities under a cozy environment. Hydrogen is considered among the most promising clean energy sources for the future. Recently, electrochemical water splitting has been considered as one of the most efficient approaches to harvest hydrogen energy, which generates only non-pollutant water on combustion. However, the sluggish anodic oxygen evolution reaction significantly restricts the efficiency of water splitting and requires a relatively high cell voltage to drive the electrolysis. Therefore, seeking a thermodynamically favorable anodic reaction to replace the sluggish oxygen evolution reaction by utilizing highly active bifunctional electrocatalysts for the anodic reaction and hydrogen evolution are crucial for achieving energy-efficient hydrogen production for industrial applications. Nevertheless, it is known that the oxygen evolution reaction can be replaced with other useful and thermodynamically favorable reactions to reduce the electrolysis voltage for realizing energy-efficient hydrogen production. Therefore, in this study, we present a bifunctional nickel nanoparticle-embedded carbon (Ni@C) prism-like microrod electrocatalyst synthesized via a two-step method involving the synthesis of a precursor metal-organic framework-74 and subsequent carbonization treatment for methanol oxidation and hydrogen evolution. The interfacial structure consisting of a nickel and carbon skeleton was realized via in situ carbonization. However, the dispersed nickel nanoparticles do not easily aggregate owing to the partition by the surrounding carbon as it would sufficiently expose the active Ni sites to the electrolytes, ensuring fast charge transfer between the catalyst and electrolytes by accelerating the electrochemical kinetics. In the anodic methanol oxidation, the products were detected as carbon dioxide and formate with faradaic efficiencies of 36.2% and 62.5%, respectively, at an applied potential of 1.55 V. Meanwhile, the Ni@C microrod catalyst demonstrated high activity and durability (2.7% current decay after 12 h of continuous operation) toward methanol oxidation, which demonstrates that methanol oxidation precedes oxidation under voltage forces. Notably, the bifunctional catalyst not only exhibits excellent performance toward methanol oxidation but also yields a low overpotential of 155 mV to drive 10 mA∙cm−2 toward hydrogen evolution in 1.0 mol∙L−1 KOH aqueous solution with 0.5 mol∙L−1 methanol at room temperature, which guarantees the hydrogen production efficiency. More importantly, the constructed two-electrode electrolyzer produced a current density of 10 mA∙cm−2 at a low cell voltage of 1.6 V, which decreased by 240 mV after replacing the oxygen evolution reaction with methanol oxidation.
2021, 37(7): 200905
doi: 10.3866/PKU.WHXB202009051
Abstract:
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The existing energy and environmental issues are the primary issues that restrict the continual development of the mankind. Cost-effective energy storage and conversion devices have attracted significant attention. Rechargeable zinc-air batteries (ZABs) are widely studied because they are portable, possess high power density, and are environmentally friendly. However, the slow kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) limit their practical application. It is crucial to develop dual-functional electrocatalysts with excellent electrocatalytic performance, low price, simple operation, and outstanding stability. Therefore, transition metals and carbon-based materials should be combined. Although Co2P has been widely reported as an efficient OER catalyst, there are few studies based on the ORR activity. Herein, a facile pyrolysis of cobalt salt, phytic acid, and k-carrageenan aerogel was carried out on Co2P nanoparticles within P-doped porous carbon (Co2P-PCA-800), showing enhanced ORR activity. The resulting composite (Co2P-PCA-800) with a three-dimensional (3D) hierarchical porous architecture exhibited outstanding ORR activity with a high half-wave potential (E1/2) of approximately 0.84 V, which is comparable to that of Pt/C. Simultaneously, we fabricated phosphorus-doped porous carbon (PCA) and cobalt-doped porous carbon (Co-CA) to compare the effect of structural morphology on the catalytic performance. Studies have found that a regular interconnected porous structure can be beneficial for mass transfer and can ensure uniform distribution of ion current, thereby resulting in increased number of effective active sites. The outstanding ORR activity mainly results from the synergistic effect of the 3D honeycomb hierarchical porous structure and positively charged Co2P nanoparticles encapsulated in P-doped carbon. In addition, the 3D honeycomb porous carbon structure not only facilitates mass transfer and accelerates electron transfer but also protects the cobalt phosphide. Finally, we assembled a rechargeable ZAB with Co2P-PCA-800 as the air cathode catalyst. Compared with precious metal catalysts, the catalyst has considerable charge-discharge performance and energy density as well as higher specific capacity and better cycle stability. We believe that this study will provide a significant direction for solving energy and environmental issues.
2021, 37(7): 200905
doi: 10.3866/PKU.WHXB202009054
Abstract:
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In recent years, electrochemical water splitting that involves the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) has attracted widespread attention because the process is clean, environmentally friendly, and the generated oxygen/hydrogen gas can be converted into electricity in a fuel cell. However, the HER and OER kinetics are sluggish and require highly efficient electrocatalysts for enhancing the reaction rate. Currently, precious metals such as Pt and RuO2 have shown excellent HER and OER performance, respectively, but their practical applications are limited by their scarcity and high price. Therefore, the use of 3d transition metals, such as iron (Fe) and nickel (Ni), as electrocatalysts has attracted significant attention. To obtain a catalytic performance similar to that of precious metals, several methods have been explored, such as alloying 3d transition metals with precious metals, compositing a variety of transition metals, or requiring good carbon-based materials as a supporter. Metal-organic framework (MOF)-derived nanomaterials have emerged as some of the most promising non-precious metal bifunctional electrocatalysts. The MOF structure consists of metal-based units and special organic ligands, and after annealing, metal atoms can be converted into unsaturated metal-based active sites, and the organic ligands can be converted into carbon support. This could accelerate the charge transfer efficiency and can be beneficial for achieving excellent HER and OER performance. However, bifunctional electrocatalysts derived from nickel (Ni)-based MOFs have not been studied intensively, and their catalytic activity and stability remain to be improved. Herein, a Ni-MOF precursor was synthesized via a liquid-phase coordination reaction using Ni2+ and benzene-1, 3, 5-tricarboxylic acid. The obtained Ni-MOF samples were annealed under H2/Ar atmosphere at 600, 700, and 800 ℃, named Ni/C-H2-600, Ni/C-H2-700, Ni/C-Ar-700, and Ni/C-H2-800, respectively. During high-temperature annealing treatment, Ni nanoparticles were grown in situ on a rod-shaped carbon substrate to form novel hybrid architecture Ni/C nanoparticles used as a high-performance bifunctional electrocatalyst for overall water splitting. The HER overpotential of Ni/C-H2-700 was 120 mV at a current density of 10 mA∙cm−2, which is much lower than that of Ni/C-H2-600 (250 mV), Ni/C-H2-800 (348 mV), and Ni/C-Ar-700 (275 mV). Ni/C-H2-700 required an OER overpotential of 350 mV to achieve a current density of 10 mA∙cm−2, which is lower than that of Ni/C-H2-600 (370 mV), Ni/C-H2-800 (430 mV), and Ni/C-Ar-700 (380 mV). Furthermore, Ni/C-H2-700 showed the idea of HER and OER durability. Presumably, good structural properties and the abundant surface area of the carbon substrate elevated HER/OER activity owing to the synergistic advantages of accessible active sites and enhanced electronic conductivity.
2021, 37(7): 200908
doi: 10.3866/PKU.WHXB202009082
Abstract:
In the 21st century, hydrogen energy is a novel energy source. Its use is expected to mitigate the problems of environmental pollution and global warming caused by the excessive use of conventional fossil fuels. The hydrogen evolution reaction (HER) for water splitting has attracted considerable attention because of its environmental friendliness. To improve electrocatalyst performance and reduce operation cost, carbon-based metal hybrid materials exhibiting high efficiency and catalytic activity have been developed. Among them, carbon dots (CDs) have garnered significant research attention and have been widely applied in biosensing, bioimaging, and energy conversion/storage because of their facile synthesis, biocompatibility, tunable photoluminescence, excellent stability, and good electronic properties. CDs are widely used as carriers in the construction of electrocatalysts prepared from carbon-based metal hybrid materials. At present, it is believed that CDs exhibit excellent confinement effects, which can effectively inhibit the growth and agglomeration of metal nanoparticles, thereby preparing well-distributed carbon-based metal hybrid materials with a uniform and controllable size. However, the formation process of the small-molecule raw materials of CDs has not been elucidated. In this study, CDs and small-molecule raw materials from synthetic CDs were used as precursors to prepare nitrogen-doped CD-supported ruthenium nanoparticle (Ru@CDs) and small-molecule-supported ruthenium nanoparticle (Ru@Molecule) hybrid materials, respectively. The interaction between the small molecules and Ru in the process of CD formation and the effect on HER performance were explored. Moreover, we prepared different carriers such as metal organic frameworks(MOF), carbon nanotubes (CNTs), and graphene (GO)-supported ruthenium nanoparticle hybrid materials. Among them, Ru@CDs exhibited controllable size and excellent dispersibility and exhibited outstanding HER activity and good stability. Ru@CDs were found to require a low overpotential of 22 mV to reach a current density of 10 mA·cm−2. Moreover, we observed the presence of an intermediate state between the molecules and CDs and demonstrated that the intermediate state exhibits no confinement effect. Furthermore, we found that with increasing calcination temperature, the intermediate state gradually changes to CDs. The unique spatial confinement between CDs and metal ions is key to the formation of monodisperse Ru nanoparticles. Our results confirmed that Ru@CDs serve as excellent HER catalyst supports. This work not only reveals the effect of the unique spatial confinement of CDs on the supported metals and their promoting effect on electrocatalytic activity but also provides guides the future development of CD-based metal hybrid electrocatalysts. ![]()
In the 21st century, hydrogen energy is a novel energy source. Its use is expected to mitigate the problems of environmental pollution and global warming caused by the excessive use of conventional fossil fuels. The hydrogen evolution reaction (HER) for water splitting has attracted considerable attention because of its environmental friendliness. To improve electrocatalyst performance and reduce operation cost, carbon-based metal hybrid materials exhibiting high efficiency and catalytic activity have been developed. Among them, carbon dots (CDs) have garnered significant research attention and have been widely applied in biosensing, bioimaging, and energy conversion/storage because of their facile synthesis, biocompatibility, tunable photoluminescence, excellent stability, and good electronic properties. CDs are widely used as carriers in the construction of electrocatalysts prepared from carbon-based metal hybrid materials. At present, it is believed that CDs exhibit excellent confinement effects, which can effectively inhibit the growth and agglomeration of metal nanoparticles, thereby preparing well-distributed carbon-based metal hybrid materials with a uniform and controllable size. However, the formation process of the small-molecule raw materials of CDs has not been elucidated. In this study, CDs and small-molecule raw materials from synthetic CDs were used as precursors to prepare nitrogen-doped CD-supported ruthenium nanoparticle (Ru@CDs) and small-molecule-supported ruthenium nanoparticle (Ru@Molecule) hybrid materials, respectively. The interaction between the small molecules and Ru in the process of CD formation and the effect on HER performance were explored. Moreover, we prepared different carriers such as metal organic frameworks(MOF), carbon nanotubes (CNTs), and graphene (GO)-supported ruthenium nanoparticle hybrid materials. Among them, Ru@CDs exhibited controllable size and excellent dispersibility and exhibited outstanding HER activity and good stability. Ru@CDs were found to require a low overpotential of 22 mV to reach a current density of 10 mA·cm−2. Moreover, we observed the presence of an intermediate state between the molecules and CDs and demonstrated that the intermediate state exhibits no confinement effect. Furthermore, we found that with increasing calcination temperature, the intermediate state gradually changes to CDs. The unique spatial confinement between CDs and metal ions is key to the formation of monodisperse Ru nanoparticles. Our results confirmed that Ru@CDs serve as excellent HER catalyst supports. This work not only reveals the effect of the unique spatial confinement of CDs on the supported metals and their promoting effect on electrocatalytic activity but also provides guides the future development of CD-based metal hybrid electrocatalysts.
2021, 37(7): 200910
doi: 10.3866/PKU.WHXB202009107
Abstract:
The continuous consumption and excessive use of fossil fuels promote the exploration of new energy conversion technologies. Meanwhile, the increase in the supply of ethane encourages the development of industrial technology for the production of ethylene chemical raw materials. Compared with traditional fossil fuel energy conversion equipment, solid oxide ethane cogeneration fuel cells are an efficient energy processing device. Through selective oxidation of fuel gas on the anode, the endothermic process of ethane dehydrogenation is converted into an exothermic oxidation reaction, which has a greater driving force for reaction thermodynamics, and simultaneously produces clean electricity and value-added chemicals without CO2 emissions. The anode material used for the proton conductor ethane fuel cells needs to operate stably and efficiently under hydrocarbon fuel. Consequently, excellent catalytic activity, low polarization resistance, and anti-coking stability are essential. In this work, CeO2 was uniformly impregnated into the surface of the porous cubic perovskite Pr0.4Sr0.6Co0.2Fe0.7Mo0.1O3−δ anode by wet impregnation, and then calcined and reduced to obtain a CeO2/RP-PSCFM@CoFe composite anode embedded with nanoparticles, which was successfully used in electrolyte-supported proton conductor fuel cells. CeO2 has a high ionic conductivity and transport capacity, which accelerates the transfer rate of protons on the anode and improves the catalytic reaction and transport process. Moreover, uniformly dispersed CeO2 can effectively increase the three-phase interface of the anode reaction and increase the range of reaction activity. The peak power densities before and after wet impregnation reached 172 and 253 mW·cm−2, respectively, at 750 ℃. When switching to ethane as the fuel, the peak power densities reached 136 and 183 mW·cm−2, respectively. The polarization resistance of the impregnated single cell was significantly reduced, and the catalytic activity improved. Moreover, there was no attenuation for 10 h in the long-term test. Inversely, the current density increased with the continuous reduction of the composite anode. Product analysis revealed that the yield of ethylene increased from 23.52% at 650 ℃ to 34.09% at 750 ℃, and the ethylene selectivity exceeded 94%. These results clearly show that the impregnated anode exhibited excellent catalytic activity and anti-coking ability in hydrocarbon fuels at high temperatures. Combining CoFe nanoparticles with CeO2 enhanced the electronic conductance and ionic conductance of the electrode, improved the transmission of electric energy and the efficient conversion of chemicals, thus successfully producing the cogeneration of electric energy and ethylene. ![]()
The continuous consumption and excessive use of fossil fuels promote the exploration of new energy conversion technologies. Meanwhile, the increase in the supply of ethane encourages the development of industrial technology for the production of ethylene chemical raw materials. Compared with traditional fossil fuel energy conversion equipment, solid oxide ethane cogeneration fuel cells are an efficient energy processing device. Through selective oxidation of fuel gas on the anode, the endothermic process of ethane dehydrogenation is converted into an exothermic oxidation reaction, which has a greater driving force for reaction thermodynamics, and simultaneously produces clean electricity and value-added chemicals without CO2 emissions. The anode material used for the proton conductor ethane fuel cells needs to operate stably and efficiently under hydrocarbon fuel. Consequently, excellent catalytic activity, low polarization resistance, and anti-coking stability are essential. In this work, CeO2 was uniformly impregnated into the surface of the porous cubic perovskite Pr0.4Sr0.6Co0.2Fe0.7Mo0.1O3−δ anode by wet impregnation, and then calcined and reduced to obtain a CeO2/RP-PSCFM@CoFe composite anode embedded with nanoparticles, which was successfully used in electrolyte-supported proton conductor fuel cells. CeO2 has a high ionic conductivity and transport capacity, which accelerates the transfer rate of protons on the anode and improves the catalytic reaction and transport process. Moreover, uniformly dispersed CeO2 can effectively increase the three-phase interface of the anode reaction and increase the range of reaction activity. The peak power densities before and after wet impregnation reached 172 and 253 mW·cm−2, respectively, at 750 ℃. When switching to ethane as the fuel, the peak power densities reached 136 and 183 mW·cm−2, respectively. The polarization resistance of the impregnated single cell was significantly reduced, and the catalytic activity improved. Moreover, there was no attenuation for 10 h in the long-term test. Inversely, the current density increased with the continuous reduction of the composite anode. Product analysis revealed that the yield of ethylene increased from 23.52% at 650 ℃ to 34.09% at 750 ℃, and the ethylene selectivity exceeded 94%. These results clearly show that the impregnated anode exhibited excellent catalytic activity and anti-coking ability in hydrocarbon fuels at high temperatures. Combining CoFe nanoparticles with CeO2 enhanced the electronic conductance and ionic conductance of the electrode, improved the transmission of electric energy and the efficient conversion of chemicals, thus successfully producing the cogeneration of electric energy and ethylene.
2021, 37(7): 200903
doi: 10.3866/PKU.WHXB202009033
Abstract:
The CO2 level in the atmosphere has been increasing since the industrial revolution owing to anthropogenic activities. The increased CO2 level has led to global warming and also has detrimental effects on human beings. Reducing the CO2 level in the atmosphere is urgent for balancing the carbon cycle. In this regard, reduction in CO2 emission and CO2 storage and usage are the main strategies. Among these, CO2 usage has been extensively explored, because it can reduce the CO2 level and simultaneously provide opportunities for the development in catalysts and industries to convert CO2 as a carbon source for preparing valuable products. However, transformation of CO2 to other chemicals is challenging owing to its thermodynamic and kinetic stabilities. Among the CO2 utilization techniques, electrochemical CO2 reduction (ECR) is a promising alternative because it is generally conducted under ambient conditions, and water is used as the economical hydrogen source. Moreover, ECR offers a potential route to store electrical energy from renewable sources in the form of chemical energy, through generation of CO2 reduction products. To improve the energy efficiency and viability of ECR, it is important to decrease the operational overpotential and maintain large current densities and high product selectivities; the development of efficient electrocatalysts is a critical aspect in this regard. To date, many kinds of materials have been designed and studied for application in ECR. Among these materials, metal oxide-based materials exhibit excellent performance as electrocatalysts for ECR and are attracting increasing attention in recent years. Investigation of the mechanism of reactions that involve metallic electrocatalysts has revealed the function of trace amount of oxidized metal species—it has been suggested that the presence of metal oxides and metal-oxygen bonds facilitates the activation of CO2 and the subsequent formation and stabilization of the reaction intermediates, thereby resulting in high efficiency and selectivity of the ECR. Although the stability of metal oxides is a concern as they are prone to reduction under a cathodic potential, the catalytic performance of metal oxide-based catalysts can be maintained through careful designing of the morphology and structure of the materials. In addition, introducing other metal species to metal oxides and fabricating composites of metal oxides and other materials are effective strategies to achieve enhanced performance in ECR. In this review, we summarize the recent progress in the use of metal oxide-based materials as electrocatalysts and their application in ECR. The critical role, stability, and structure-performance relationship of the metal oxide-based materials for ECR are highlighted in the discussion. In the final part, we propose the future prospects for the development of metal oxide-based electrocatalysts for ECR. ![]()
The CO2 level in the atmosphere has been increasing since the industrial revolution owing to anthropogenic activities. The increased CO2 level has led to global warming and also has detrimental effects on human beings. Reducing the CO2 level in the atmosphere is urgent for balancing the carbon cycle. In this regard, reduction in CO2 emission and CO2 storage and usage are the main strategies. Among these, CO2 usage has been extensively explored, because it can reduce the CO2 level and simultaneously provide opportunities for the development in catalysts and industries to convert CO2 as a carbon source for preparing valuable products. However, transformation of CO2 to other chemicals is challenging owing to its thermodynamic and kinetic stabilities. Among the CO2 utilization techniques, electrochemical CO2 reduction (ECR) is a promising alternative because it is generally conducted under ambient conditions, and water is used as the economical hydrogen source. Moreover, ECR offers a potential route to store electrical energy from renewable sources in the form of chemical energy, through generation of CO2 reduction products. To improve the energy efficiency and viability of ECR, it is important to decrease the operational overpotential and maintain large current densities and high product selectivities; the development of efficient electrocatalysts is a critical aspect in this regard. To date, many kinds of materials have been designed and studied for application in ECR. Among these materials, metal oxide-based materials exhibit excellent performance as electrocatalysts for ECR and are attracting increasing attention in recent years. Investigation of the mechanism of reactions that involve metallic electrocatalysts has revealed the function of trace amount of oxidized metal species—it has been suggested that the presence of metal oxides and metal-oxygen bonds facilitates the activation of CO2 and the subsequent formation and stabilization of the reaction intermediates, thereby resulting in high efficiency and selectivity of the ECR. Although the stability of metal oxides is a concern as they are prone to reduction under a cathodic potential, the catalytic performance of metal oxide-based catalysts can be maintained through careful designing of the morphology and structure of the materials. In addition, introducing other metal species to metal oxides and fabricating composites of metal oxides and other materials are effective strategies to achieve enhanced performance in ECR. In this review, we summarize the recent progress in the use of metal oxide-based materials as electrocatalysts and their application in ECR. The critical role, stability, and structure-performance relationship of the metal oxide-based materials for ECR are highlighted in the discussion. In the final part, we propose the future prospects for the development of metal oxide-based electrocatalysts for ECR.
2021, 37(7): 200904
doi: 10.3866/PKU.WHXB202009043
Abstract:
NH3 plays an important role in modern society as an essential building block in the manufacture of fertilizers, aqueous ammonia, plastics, explosives, and dyes. Additionally, it is regarded as a green alternative fuel, owing to its carbon-free nature, large hydrogen capacity, high energy density, and easy transportation. The Haber-Bosch process plays a dominant role in global NH3 synthesis; however, it involves high pressure and temperature and employs N2 and H2 as feeding gases, thus suffering from high energy consumption and substantial CO2 emission. As a promising alternative to the Haber-Bosch process, electrochemical N2 reduction enables sustainable and environmentally benign NH3 synthesis under ambient conditions. Moreover, its applied potential is compatible with intermittent solar, wind, and other renewable energies. However, efficient electrocatalysts are required to drive N2-to-NH3 conversion because of the extremely inert N≡N bond. To date, significant efforts have been made to explore high-performance catalysts with high efficiency and selectivity. Generally, noble-metal catalysts exhibit efficient performance for the NRR, but their scarcity and high cost limit their large-scale application. Therefore, considerable attention has been focused on earth-abundant transition-metal (TM) catalysts that can use empty or unoccupied orbitals to accept the lone-pair electrons of N2, while donating the abundant d-orbital electrons to the antibonding orbitals of N2. However, these catalysts may release metal ions, leading to environmental pollution. Most of these TM electrocatalysts may also favor the formation of TM—H bonds, facilitating the hydrogen evolution reaction (HER) during the electrocatalytic reaction. Recent years have seen a surge in the exploration of metal-free catalysts (MFCs). MFCs mainly include carbon-based catalysts (CBCs) and some boron-based and phosphorus-based catalysts. Generally, CBCs exhibit a porous structure and high surface area, which are favorable for exposing more active sites and providing rich accessible channels for mass/electron transfer. Moreover, the Lewis acid sites of most metal-free compounds could accept the lone-pair electron of N2 and adsorb N2 molecules by forming nonmetal—N bonds, further widening their potential for electrocatalytic NRR. Compared with metal-based catalysts, the occupied orbitals of metal-free catalysts can only form covalent bonds or conjugated π bonds, hindering electron donation from the electrocatalyst to N2 and molecular activation. In this review, we summarize the recent progress in the design and development of metal-free electrocatalysts (MFCs) for the ambient NRR, including carbon-based catalysts, boron-based catalysts, and phosphorus-based catalysts. In particular, heteroatom doping (N, O, S, B, P, F, and co-dopants), organic polymers, carbon nitride, and defect engineering are highlighted. We also discuss strategies to boost NRR performance and provide an outlook on the development perspectives of MFCs. ![]()
NH3 plays an important role in modern society as an essential building block in the manufacture of fertilizers, aqueous ammonia, plastics, explosives, and dyes. Additionally, it is regarded as a green alternative fuel, owing to its carbon-free nature, large hydrogen capacity, high energy density, and easy transportation. The Haber-Bosch process plays a dominant role in global NH3 synthesis; however, it involves high pressure and temperature and employs N2 and H2 as feeding gases, thus suffering from high energy consumption and substantial CO2 emission. As a promising alternative to the Haber-Bosch process, electrochemical N2 reduction enables sustainable and environmentally benign NH3 synthesis under ambient conditions. Moreover, its applied potential is compatible with intermittent solar, wind, and other renewable energies. However, efficient electrocatalysts are required to drive N2-to-NH3 conversion because of the extremely inert N≡N bond. To date, significant efforts have been made to explore high-performance catalysts with high efficiency and selectivity. Generally, noble-metal catalysts exhibit efficient performance for the NRR, but their scarcity and high cost limit their large-scale application. Therefore, considerable attention has been focused on earth-abundant transition-metal (TM) catalysts that can use empty or unoccupied orbitals to accept the lone-pair electrons of N2, while donating the abundant d-orbital electrons to the antibonding orbitals of N2. However, these catalysts may release metal ions, leading to environmental pollution. Most of these TM electrocatalysts may also favor the formation of TM—H bonds, facilitating the hydrogen evolution reaction (HER) during the electrocatalytic reaction. Recent years have seen a surge in the exploration of metal-free catalysts (MFCs). MFCs mainly include carbon-based catalysts (CBCs) and some boron-based and phosphorus-based catalysts. Generally, CBCs exhibit a porous structure and high surface area, which are favorable for exposing more active sites and providing rich accessible channels for mass/electron transfer. Moreover, the Lewis acid sites of most metal-free compounds could accept the lone-pair electron of N2 and adsorb N2 molecules by forming nonmetal—N bonds, further widening their potential for electrocatalytic NRR. Compared with metal-based catalysts, the occupied orbitals of metal-free catalysts can only form covalent bonds or conjugated π bonds, hindering electron donation from the electrocatalyst to N2 and molecular activation. In this review, we summarize the recent progress in the design and development of metal-free electrocatalysts (MFCs) for the ambient NRR, including carbon-based catalysts, boron-based catalysts, and phosphorus-based catalysts. In particular, heteroatom doping (N, O, S, B, P, F, and co-dopants), organic polymers, carbon nitride, and defect engineering are highlighted. We also discuss strategies to boost NRR performance and provide an outlook on the development perspectives of MFCs.
2021, 37(7): 200907
doi: 10.3866/PKU.WHXB202009074
Abstract:
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As one of the most promising hydrogen production technologies, electrochemical water splitting is an effective measure for solving environmental pollution and energy crises. However, the slow kinetics and high overpotential of the oxygen evolution reaction (OER) are the primary deterrents for improving the efficiency of water splitting devices. Iridium- and ruthenium-based noble metal catalysts are extremely expensive, which limits the industrial-scale development of this technology. Therefore, the development of oxygen evolution catalysts with high activity, excellent stability, and low costs is significantly important for water splitting technologies. Nickel-based materials meet the requirements of high abundance, cost-effectiveness, and high activity. In recent years, nickel-based metal organic frameworks (Ni-based MOFs) have attracted increasing research attention owing to their diverse and tunable topological structures and large specific surface areas. Furthermore, the mesoporous three-dimensional structure of MOFs can promote the diffusion of reactants, rendering them excellent candidates for catalytic applications. In order to utilize the advantages of Ni-MOFs more efficiently, the following methods are usually used to improve their catalytic performance. Owing to their unique properties, metal nodes can be replaced without affecting the MOF skeleton. As iron series metals, Co and Fe doping show unique catalytic activity and structural stability due to the synergistic effect between metal centers. Further, Ni-MOFs can simultaneously be used as precursors for oxidation, phosphating, or vulcanization to obtain Ni-MOF derivatives with different components. Among them, high-temperature carbonization treatment can make use of abundant organic ligands of Ni-MOFs to form a partially graphitized carbon-based framework, thereby augmenting conductivity, preventing the aggregation and corrosion of transition metals, and improving the overall support strength. The catalytic performance of oxygen production can be further improved by directly growing the Ni-MOFs on the substrate and introducing other active substances or conductive materials. Herein, the latest developments of Ni-based MOFs and their derivatives have been reviewed with regard to their utilization in OER catalysis, including nickel oxides, nickel hydroxides, nickel phosphides, nickel sulfides, and carbon composite materials. First, the mechanism and measurement criteria of the OER are briefly introduced. Second, the structures of several typical Ni-based MOFs (MOF-74, MILs, PBAs, and ZIFs) and their preparation methods are described. Subsequently, recent advances in the application of Ni-based MOFs and their derivatives in the OER are discussed, with an emphasis on materials design strategies and catalytic mechanisms. Finally, the main challenges and opportunities in this field are proposed.
Conductive Metal-Organic Frameworks for Electrocatalysis:Achievements, Challenges, and Opportunities
2021, 37(7): 201002
doi: 10.3866/PKU.WHXB202010025
Abstract:
To fulfill the demands of green and sustainable energy, the production of novel catalysts for different energy conversion processes is critical. Owing to the intriguing advantages of the intrinsic active species, tunable crystal structure, remarkable chemical and physical properties, and good stability, metal-organic frameworks (MOFs) have been extensively investigated in various electrochemical energy conversions, such as the CO2 reduction reaction, N2 reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, and oxygen reduction reaction. More importantly, it is feasible to change the chemical environments, pore sizes, and porosity of MOFs, which will theoretically facilitate the diffusion of reactants across the open porous networks, thereby improving the electrocatalytic performance. However, owing to the high energy barriers of charge transfer and limited free charge carriers, most MOFs show poor electrical conductivity, thus limiting their diverse applications. As reported previously, MOFs were used as a porous substrate to confine the growth of nanoparticles or co-doped electrocatalysts after annealing. The conductive MOFs can combine the advantages of conventional MOFs with electronic conductivity, which significantly enhance the electrocatalytic performance. In addition, conductive MOFs can achieve conductivity via electronic or ionic routes without post-annealing treatment, thereby extending their potential applications. Different synthesis strategies have recently been developed to endow MOFs with electrical conductivity, such as post-synthesis modification, guest molecule introduction, and composite formatting. The performance of conductive MOFs can even outperform those of commercial RuO2 catalysts or Pt-group catalysts. However, it is difficult to endow most MOFs with high conductivity. This review summarizes the mechanisms of constructing conductive MOFs, such as redox hopping, through-bond pathways, through-space pathways, extended conjugation, and guest-promoted transport. Synthetic methods, including hydro/solvothermal synthesis and interface-assisted synthesis, are introduced. Recent advances in the use of conductive MOFs as heterogeneous catalysts in electrocatalysis have been comprehensively elucidated. It has been reported that conductive MOFs can demonstrate considerable catalytic activity, selectivity, and stability in different electrochemical reactions, revealing the immense potential for future displacement of Pt-group catalysts. Finally, the challenges and opportunities of conductive MOFs in electrocatalysis are discussed. Based on systematic synthesis strategies, more conductive MOFs can be constructed for electrocatalytic reactions. In addition, the morphology and structure of conductive MOFs, which can change the electrochemical accessibility between substrates and MOFs, are also crucial for catalysis, and thus, they should be extensively studied in the future. It is believed that a breakthrough for high-performance conductive MOF-based electrocatalysts could be achieved. ![]()
To fulfill the demands of green and sustainable energy, the production of novel catalysts for different energy conversion processes is critical. Owing to the intriguing advantages of the intrinsic active species, tunable crystal structure, remarkable chemical and physical properties, and good stability, metal-organic frameworks (MOFs) have been extensively investigated in various electrochemical energy conversions, such as the CO2 reduction reaction, N2 reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, and oxygen reduction reaction. More importantly, it is feasible to change the chemical environments, pore sizes, and porosity of MOFs, which will theoretically facilitate the diffusion of reactants across the open porous networks, thereby improving the electrocatalytic performance. However, owing to the high energy barriers of charge transfer and limited free charge carriers, most MOFs show poor electrical conductivity, thus limiting their diverse applications. As reported previously, MOFs were used as a porous substrate to confine the growth of nanoparticles or co-doped electrocatalysts after annealing. The conductive MOFs can combine the advantages of conventional MOFs with electronic conductivity, which significantly enhance the electrocatalytic performance. In addition, conductive MOFs can achieve conductivity via electronic or ionic routes without post-annealing treatment, thereby extending their potential applications. Different synthesis strategies have recently been developed to endow MOFs with electrical conductivity, such as post-synthesis modification, guest molecule introduction, and composite formatting. The performance of conductive MOFs can even outperform those of commercial RuO2 catalysts or Pt-group catalysts. However, it is difficult to endow most MOFs with high conductivity. This review summarizes the mechanisms of constructing conductive MOFs, such as redox hopping, through-bond pathways, through-space pathways, extended conjugation, and guest-promoted transport. Synthetic methods, including hydro/solvothermal synthesis and interface-assisted synthesis, are introduced. Recent advances in the use of conductive MOFs as heterogeneous catalysts in electrocatalysis have been comprehensively elucidated. It has been reported that conductive MOFs can demonstrate considerable catalytic activity, selectivity, and stability in different electrochemical reactions, revealing the immense potential for future displacement of Pt-group catalysts. Finally, the challenges and opportunities of conductive MOFs in electrocatalysis are discussed. Based on systematic synthesis strategies, more conductive MOFs can be constructed for electrocatalytic reactions. In addition, the morphology and structure of conductive MOFs, which can change the electrochemical accessibility between substrates and MOFs, are also crucial for catalysis, and thus, they should be extensively studied in the future. It is believed that a breakthrough for high-performance conductive MOF-based electrocatalysts could be achieved.
2021, 37(7): 200907
doi: 10.3866/PKU.WHXB202009077
Abstract:
The implementation of clean energy techniques, including clean hydrogen generation, use of solar-driven photovoltaic hybrid systems, photochemical heat generation as well as thermoelectric conversion, is crucial for the sustainable development of our society. Among these promising techniques, electrocatalysis has received significant attention for its ability to facilitate clean energy conversion because it promotes a higher rate of reaction and efficiency for the associated chemical transformations. Noble-metal-based electrocatalysts typically show high activity for electrochemical conversion processes. However, their scarcity and high cost limit their applications in electrocatalytic devices. To overcome this limitation, binary catalysts prepared by alloying with transition metals can be used. However, optimization of the activity of the binary catalysts is considerably limited because of the presence of the miscibility gap in the phase diagram of binary alloys. The activity of binary electrocatalysts can be attributed to the adsorption energy of molecules and intermediates on the surface. High-entropy alloys (HEAs), which consist of diverse elements in a single NP, typically exhibit better physical and/or chemical properties than their single-element counterparts, because of their tunable composition and inherent surface complexity. Further, HEAs can improve the performance of binary electrocatalysts because they exhibit a near-continuous distribution of adsorption energy. Recently, HEAs have gained considerable attention for their application in electrocatalytic reactions. This review summarizes recent research advances in HEA nanostructures and their application in the field of electrocatalysis. First, we introduce the concept, structure, and four core effects of HEAs. We believe that this part will provide the basic information about HEAs. Next, we discuss the reported top-down and bottom-up synthesis strategies, emphasizing on the carbothermal shock method, nanodroplet-mediated electrodeposition, fast moving bed pyrolysis, polyol process, and dealloying. Other methods such as combinatorial co-sputtering, ultrashort-pulsed laser ablation, ultrasonication-assisted wet chemistry, and scanning-probe block copolymer lithography are also highlighted. Among these methods, wet chemistry has been reported to be effective for the formation of nano-scale HEAs because it facilitates the concurrent reduction of all metal precursors to form solid-solution alloys. Next, we present the theoretical investigation of HEA nanocatalysts, including their thermodynamics, kinetic stability, and adsorption energy tuning for optimizing their catalytic activity and selectivity. To elucidate the structure–property relationship in HEAs, we summarize the research progress related to electrocatalytic reactions promoted by HEA nanocatalysts, including the oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, methanol oxidation reaction, and CO2 reduction reaction. Finally, we discuss the challenges and various strategies toward the development of HEAs. ![]()
The implementation of clean energy techniques, including clean hydrogen generation, use of solar-driven photovoltaic hybrid systems, photochemical heat generation as well as thermoelectric conversion, is crucial for the sustainable development of our society. Among these promising techniques, electrocatalysis has received significant attention for its ability to facilitate clean energy conversion because it promotes a higher rate of reaction and efficiency for the associated chemical transformations. Noble-metal-based electrocatalysts typically show high activity for electrochemical conversion processes. However, their scarcity and high cost limit their applications in electrocatalytic devices. To overcome this limitation, binary catalysts prepared by alloying with transition metals can be used. However, optimization of the activity of the binary catalysts is considerably limited because of the presence of the miscibility gap in the phase diagram of binary alloys. The activity of binary electrocatalysts can be attributed to the adsorption energy of molecules and intermediates on the surface. High-entropy alloys (HEAs), which consist of diverse elements in a single NP, typically exhibit better physical and/or chemical properties than their single-element counterparts, because of their tunable composition and inherent surface complexity. Further, HEAs can improve the performance of binary electrocatalysts because they exhibit a near-continuous distribution of adsorption energy. Recently, HEAs have gained considerable attention for their application in electrocatalytic reactions. This review summarizes recent research advances in HEA nanostructures and their application in the field of electrocatalysis. First, we introduce the concept, structure, and four core effects of HEAs. We believe that this part will provide the basic information about HEAs. Next, we discuss the reported top-down and bottom-up synthesis strategies, emphasizing on the carbothermal shock method, nanodroplet-mediated electrodeposition, fast moving bed pyrolysis, polyol process, and dealloying. Other methods such as combinatorial co-sputtering, ultrashort-pulsed laser ablation, ultrasonication-assisted wet chemistry, and scanning-probe block copolymer lithography are also highlighted. Among these methods, wet chemistry has been reported to be effective for the formation of nano-scale HEAs because it facilitates the concurrent reduction of all metal precursors to form solid-solution alloys. Next, we present the theoretical investigation of HEA nanocatalysts, including their thermodynamics, kinetic stability, and adsorption energy tuning for optimizing their catalytic activity and selectivity. To elucidate the structure–property relationship in HEAs, we summarize the research progress related to electrocatalytic reactions promoted by HEA nanocatalysts, including the oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, methanol oxidation reaction, and CO2 reduction reaction. Finally, we discuss the challenges and various strategies toward the development of HEAs.
2021, 37(7): 200909
doi: 10.3866/PKU.WHXB202009099
Abstract:
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Currently, because of the worldwide over-exploitation and consumption of fossil fuels, energy crisis and environmental pollution are becoming more prominent. Hence, the production and utilization of clean energy such as hydrogen are crucial. As significant electrochemical reactions in energy conversion devices, the oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and oxygen reduction reaction (ORR) have garnered considerable attention. However, the sluggish kinetics of these reactions, especially of the OER and ORR because of the multiple electron transfer steps, and the inevitable usage of noble metal catalysts (such as those based on Pt for HER/ORR and Ru/Ir for HER/OER) are the bottlenecks to realizing energy conversion devices, including overall water-splitting electrolyzers, fuel cells, and metal-air batteries. Therefore, the development of efficient non-precious metal catalysts is imperative. Transition metal nitrides (TMNs) have been recently studied and shown to exhibit high catalytic activity because of their ability to alter the electronic structure of host metals, specifically the downshift of the d-band center, the contraction of the filled state, and the broadening of the unfilled state. This high activity is attributed to the optimization of the adsorption energy between metals and adsorbates. In addition, metallic bonding in TMNs increases the conductivity of the catalysts. Thus, in this review, we focus on the latest developments in TMNs and their application as high-activity and high-stability electrocatalysts for water splitting and in fuel cells and zinc-air batteries. First, the origin of the high activity of TMNs is explained with the help of the d-band theory. The effect of nitrogen on TMNs, such as in terms of the location in the crystal structure, is briefly discussed. The preparation strategies for TMNs, including physical and chemical methods as well as the modification techniques such as doping, changing carrier properties, and defect construction, are outlined. Next, we summarize the applications of TMNs as an electrocatalyst for the HER, OER, and ORR. At the same time, to explain the bifunctional catalytic activity of TMNs, we discuss the modification strategies for single-metal-based nitrides, such as doping with other highly active atoms to adjust the electronic structure and increase the catalytic activities as well as using coupling materials with different catalytic selectivities to construct heterostructures. Finally, we discuss the challenges and development approaches for realizing the electrocatalytic applications of TMNs, such as through further improvement in catalytic activity, and for facilitating in-depth understanding of electrocatalytic processes through in situ characterization to reveal the electrocatalytic mechanism of TMNs. Undoubtedly, this review will promote the application of TMNs in the field of electrocatalysis.
2021, 37(7): 201004
doi: 10.3866/PKU.WHXB202010043
Abstract:
2021, 37(7): 201101
doi: 10.3866/PKU.WHXB202011015
Abstract:
2021, 37(7): 201006
doi: 10.3866/PKU.WHXB202010068
Abstract:
2021, 37(7): 201101
doi: 10.3866/PKU.WHXB202011013
Abstract:
2021, 37(7): 201107
doi: 10.3866/PKU.WHXB202011077
Abstract:
2021, 37(7): 201205
doi: 10.3866/PKU.WHXB202012052
Abstract:
