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2023 Volume 39 Issue 5
2023, 39(5): 221002
doi: 10.3866/PKU.WHXB202210027
Abstract:
In recent years, aqueous zinc-ion batteries (AZIBs) have received considerable interest as a novel and promising alternative energy storage technology. Owing to their particular structural traits and physicochemical qualities, MXenes as cathodes impart significant beneficial properties to AZIBs, such as readily modifiable two-dimensional (2D) structure, high electrical conductivity, desirable chemical composition, and controllable surface chemical properties. This review includes a comprehensive discussion on the progression of MXenes in AZIBs in relation to the most sophisticated structural design and performance optimization methodologies available for the construction of cathodes and anodes. MXenes may be utilized directly as an active material or a precursor of an active material in cathodes to achieve a long cycle life and high rate performance because of their contribution, which is summarized as follow: (1) MXenes with a 2D layered structure and high conductivity can be employed as a conductive substrate in combination with manganese and vanadium oxides to enhance the cycle and rate performance of composite materials; (2) zinc ion transport kinetics is accelerated in manganese and vanadium oxide composites when 3D MXenes are used as a substrate; (3) MXenes allow excellent electrolyte penetration owing to the presence of abundant hydrophilic functional groups, which may enhance the electrochemical response of composite electrode materials; (4) MXene derivatives contain a broad range of surface functional groups and exhibit high activity and a wide voltage window; (5) MXenes possess remarkable mechanical flexibility, allowing for the production of flexible wearable AZIBs. Moreover, MXenes can be employed as a 2D/3D host, zincophilic seed matrix, and zinc interface protection layer to retard zinc metal corrosion and dendrite formation when zinc metal is used as the anode because of the following advantages: (1) MXenes have a 2D structure and multi-functional surface, allow excellent water dispersion, and can be processed into various porous skeletons; (2) MXenes exhibit excellent electrical conductance and ion diffusion, allowing for rapid electrochemical kinetics during zinc plating/stripping; (3) lattice size compatibility between MXenes and zinc metal allows zinc metal to nucleate and deposit evenly; (4) the abundant functional groups on the MXene surface may serve as zincophilic and nucleation sites to promote the homogeneous nucleation and deposition of zinc. The review also highlights the electrochemical deposition (for zinc foil) and physical mixing techniques for using MXenes as a host to encapsulate zinc (for zinc powder). Moreover, the discussion is directed to the use of MXenes as an electrolyte additive for AZIBs and as an inorganic filler for solid electrolytes to prevent dendrite formation and corrosion issues in zinc anodes. Finally, the challenges and prospects of using MXenes in AZIBs are presented.![]()
In recent years, aqueous zinc-ion batteries (AZIBs) have received considerable interest as a novel and promising alternative energy storage technology. Owing to their particular structural traits and physicochemical qualities, MXenes as cathodes impart significant beneficial properties to AZIBs, such as readily modifiable two-dimensional (2D) structure, high electrical conductivity, desirable chemical composition, and controllable surface chemical properties. This review includes a comprehensive discussion on the progression of MXenes in AZIBs in relation to the most sophisticated structural design and performance optimization methodologies available for the construction of cathodes and anodes. MXenes may be utilized directly as an active material or a precursor of an active material in cathodes to achieve a long cycle life and high rate performance because of their contribution, which is summarized as follow: (1) MXenes with a 2D layered structure and high conductivity can be employed as a conductive substrate in combination with manganese and vanadium oxides to enhance the cycle and rate performance of composite materials; (2) zinc ion transport kinetics is accelerated in manganese and vanadium oxide composites when 3D MXenes are used as a substrate; (3) MXenes allow excellent electrolyte penetration owing to the presence of abundant hydrophilic functional groups, which may enhance the electrochemical response of composite electrode materials; (4) MXene derivatives contain a broad range of surface functional groups and exhibit high activity and a wide voltage window; (5) MXenes possess remarkable mechanical flexibility, allowing for the production of flexible wearable AZIBs. Moreover, MXenes can be employed as a 2D/3D host, zincophilic seed matrix, and zinc interface protection layer to retard zinc metal corrosion and dendrite formation when zinc metal is used as the anode because of the following advantages: (1) MXenes have a 2D structure and multi-functional surface, allow excellent water dispersion, and can be processed into various porous skeletons; (2) MXenes exhibit excellent electrical conductance and ion diffusion, allowing for rapid electrochemical kinetics during zinc plating/stripping; (3) lattice size compatibility between MXenes and zinc metal allows zinc metal to nucleate and deposit evenly; (4) the abundant functional groups on the MXene surface may serve as zincophilic and nucleation sites to promote the homogeneous nucleation and deposition of zinc. The review also highlights the electrochemical deposition (for zinc foil) and physical mixing techniques for using MXenes as a host to encapsulate zinc (for zinc powder). Moreover, the discussion is directed to the use of MXenes as an electrolyte additive for AZIBs and as an inorganic filler for solid electrolytes to prevent dendrite formation and corrosion issues in zinc anodes. Finally, the challenges and prospects of using MXenes in AZIBs are presented.
2023, 39(5): 221100
doi: 10.3866/PKU.WHXB202211005
Abstract:
Aqueous electrochemical energy storage (EES) devices have inherent advantages, such as high safety, environmental-friendliness, and low cost, exhibiting significant potential for application in future smart grids, portable/wearable electronics, and other fields. However, the low thermodynamic decomposition voltage of water (1.23 V) results in a narrow electrochemical stability window (ESW) of the aqueous electrolyte, limiting the selection of electrode materials. Therefore, aqueous alkali metal-ion batteries (AABs) have a low operating voltage and energy density. Considering the diverse application of AABs, the operation of AABs under extreme temperature conditions faces critical challenges. At a low temperature, the electrolyte freezes easily owing to the high freezing point of water (0 ℃); the ionic conductivity of the electrolyte decreases significantly, and the charge/discharge polarization increases. Therefore, AABs generally have a low capacity, poor rate performance, and low energy/power densities, and are unable to operate normally. At a high temperature, the water activity improves, and the side reaction of water decomposition intensifies. Hence, the cycle performance of AABs deteriorates, and the battery exhibits safety issues, such as expansion and thermal runaway. In recent years, significant research has been conducted to overcome the shortcomings of the aqueous EES, inspiring further research and development of future high-performance aqueous EESs. Reducing the water activity and increasing the hydrogen evolution reaction (HER) or oxygen evolution reaction (OER) overpotential are effective strategies to widen the ESW of aqueous electrolytes, which are mostly realized by utilizing high concentration salts, additives, and co-solvents. Using salt additives or organic co-solvents to break the intermolecular hydrogen bonds of water and reduce the interfacial charge transfer resistance are effective strategies to improve the low-temperature performance of AABs. Additionally, salt additives/co-solvents with high thermal stability can form strong hydrogen bonds with water, effectively improving the water retention and reducing the water activity, which ensure the enhanced electrochemical performance of the AABs at high temperatures. This review systematically summarizes the research progress of electrolyte design for AABs with a high voltage/wide operating temperature range. From the perspective of thermodynamics and kinetics, various strategies to widen the ESW and operating temperature range of the electrolyte as well as the relevant mechanisms are introduced. Potential concepts for designing high-voltage aqueous electrolytes with operation ability at a wide temperature range are proposed, and the development direction of high-performance AABs is presented.![]()
Aqueous electrochemical energy storage (EES) devices have inherent advantages, such as high safety, environmental-friendliness, and low cost, exhibiting significant potential for application in future smart grids, portable/wearable electronics, and other fields. However, the low thermodynamic decomposition voltage of water (1.23 V) results in a narrow electrochemical stability window (ESW) of the aqueous electrolyte, limiting the selection of electrode materials. Therefore, aqueous alkali metal-ion batteries (AABs) have a low operating voltage and energy density. Considering the diverse application of AABs, the operation of AABs under extreme temperature conditions faces critical challenges. At a low temperature, the electrolyte freezes easily owing to the high freezing point of water (0 ℃); the ionic conductivity of the electrolyte decreases significantly, and the charge/discharge polarization increases. Therefore, AABs generally have a low capacity, poor rate performance, and low energy/power densities, and are unable to operate normally. At a high temperature, the water activity improves, and the side reaction of water decomposition intensifies. Hence, the cycle performance of AABs deteriorates, and the battery exhibits safety issues, such as expansion and thermal runaway. In recent years, significant research has been conducted to overcome the shortcomings of the aqueous EES, inspiring further research and development of future high-performance aqueous EESs. Reducing the water activity and increasing the hydrogen evolution reaction (HER) or oxygen evolution reaction (OER) overpotential are effective strategies to widen the ESW of aqueous electrolytes, which are mostly realized by utilizing high concentration salts, additives, and co-solvents. Using salt additives or organic co-solvents to break the intermolecular hydrogen bonds of water and reduce the interfacial charge transfer resistance are effective strategies to improve the low-temperature performance of AABs. Additionally, salt additives/co-solvents with high thermal stability can form strong hydrogen bonds with water, effectively improving the water retention and reducing the water activity, which ensure the enhanced electrochemical performance of the AABs at high temperatures. This review systematically summarizes the research progress of electrolyte design for AABs with a high voltage/wide operating temperature range. From the perspective of thermodynamics and kinetics, various strategies to widen the ESW and operating temperature range of the electrolyte as well as the relevant mechanisms are introduced. Potential concepts for designing high-voltage aqueous electrolytes with operation ability at a wide temperature range are proposed, and the development direction of high-performance AABs is presented.
2023, 39(5): 221101
doi: 10.3866/PKU.WHXB202211010
Abstract:
Photocatalytic water splitting is a green technology for sustainable hydrogen evolution. To improve photon-to-electron conversion efficiency, the design and development of efficient, stable, and full-spectrum responsive photocatalysts has attracted increasing attention. Many different classes of materials can be used to harness solar photons for photocatalysis, each having their advantages and drawbacks. Compared to inorganic semiconductors, organic semiconductors are rich in π electrons and can be readily modified, allowing for facile control of the optic (absorption region and intensity) and electronic (energy structure) properties, as well as mechanistic pathways. However, photogenerated charge carriers cannot be effectively employed owing to subpar charge carrier transport properties, which arise from the low concentration and low mobility of free charge carriers in organic semiconductors. Appropriate changes in the molecular structure of the organic semiconductors can allow for sunlight utilization across the full visible region and even the infrared region. By controlling the nature of stacking, organic photocatalysts with different compositions, dimension (0, 1, 2, 3), size, and crystallographic orientation can be harnessed to increase sunlight utilization and charge separation efficiencies. By optimizing these properties, the overall photoelectric conversion efficiency and hydrogen production efficiency can be improved. However, the mechanisms of redox reactions in organic semiconductor photocatalytic systems remain unclear owing to the complex nature of the processes and difficulties in study design. Herein, the physical and chemical processes of organic semiconductors are discussed from the perspective of light harvesting, photoexcited charge separation, and surface reactions. The preparation methods of organic semiconductor nanostructures are summarized and the progressive development of organic nanostructures for photocatalytic hydrogen evolution is systematically reviewed. Typical organic semiconductor materials, including perylene diimide, porphyrin, phthalocyanine, fullerenes, graphitic carbon nitride (g-C3N4), and other conjugated polymers, are highlighted. Moreover, modification strategies for optimizing optical and electrical properties at the molecular or aggregate level are discussed. Element doping or substitution and group functionalization at the molecular level as well as control over morphologies, components, and dimensions at the aggregate level are reviewed to clarify structure/property relationships and further guide photocatalyst design. All the strategies discussed herein focus on enhancing hole and electron separation while suppressing their recombination, thereby improving the photocatalytic performance in evolution hydrogen. Finally, the key challenges and prospects of organic nanomaterials for photocatalytic evolution hydrogen are presented. We particularly focus on the construction of a system to evaluate the reasonable loading of co-catalysts, photocatalyst morphology regulation, and combined in situ characterization and density functional theory calculations in the context of photocatalytic hydrogen production. ![]()
Photocatalytic water splitting is a green technology for sustainable hydrogen evolution. To improve photon-to-electron conversion efficiency, the design and development of efficient, stable, and full-spectrum responsive photocatalysts has attracted increasing attention. Many different classes of materials can be used to harness solar photons for photocatalysis, each having their advantages and drawbacks. Compared to inorganic semiconductors, organic semiconductors are rich in π electrons and can be readily modified, allowing for facile control of the optic (absorption region and intensity) and electronic (energy structure) properties, as well as mechanistic pathways. However, photogenerated charge carriers cannot be effectively employed owing to subpar charge carrier transport properties, which arise from the low concentration and low mobility of free charge carriers in organic semiconductors. Appropriate changes in the molecular structure of the organic semiconductors can allow for sunlight utilization across the full visible region and even the infrared region. By controlling the nature of stacking, organic photocatalysts with different compositions, dimension (0, 1, 2, 3), size, and crystallographic orientation can be harnessed to increase sunlight utilization and charge separation efficiencies. By optimizing these properties, the overall photoelectric conversion efficiency and hydrogen production efficiency can be improved. However, the mechanisms of redox reactions in organic semiconductor photocatalytic systems remain unclear owing to the complex nature of the processes and difficulties in study design. Herein, the physical and chemical processes of organic semiconductors are discussed from the perspective of light harvesting, photoexcited charge separation, and surface reactions. The preparation methods of organic semiconductor nanostructures are summarized and the progressive development of organic nanostructures for photocatalytic hydrogen evolution is systematically reviewed. Typical organic semiconductor materials, including perylene diimide, porphyrin, phthalocyanine, fullerenes, graphitic carbon nitride (g-C3N4), and other conjugated polymers, are highlighted. Moreover, modification strategies for optimizing optical and electrical properties at the molecular or aggregate level are discussed. Element doping or substitution and group functionalization at the molecular level as well as control over morphologies, components, and dimensions at the aggregate level are reviewed to clarify structure/property relationships and further guide photocatalyst design. All the strategies discussed herein focus on enhancing hole and electron separation while suppressing their recombination, thereby improving the photocatalytic performance in evolution hydrogen. Finally, the key challenges and prospects of organic nanomaterials for photocatalytic evolution hydrogen are presented. We particularly focus on the construction of a system to evaluate the reasonable loading of co-catalysts, photocatalyst morphology regulation, and combined in situ characterization and density functional theory calculations in the context of photocatalytic hydrogen production.
2023, 39(5): 221102
doi: 10.3866/PKU.WHXB202211025
Abstract:
Perovskite materials have considerable potential in medical sensors. This is because the diverse element substitution of the perovskite ABX3 composition brings rich physical and chemical properties for perovskite materials, including photoelectric conversion, all-optical conversion, and electro-optical conversion. By modifying the A-site, B-site, or X-site elements, the bandgap width of perovskite materials can be adjusted. Moreover, the absorption spectrum, photoelectric conversion electrical signal, and all-optical conversion luminescence spectrum can be regulated in perovskite materials. Perovskite materials also have the advantages of easy fabrication, excellent biocompatibility after modification, variable chemical valence states of constituent elements, and adjustable morphology. Therefore, perovskite materials are expected to be used in medical sensors with different operation mechanisms, such as photoelectric sensors, all-optical conversion sensors, electrocatalytic sensors, physicochemically loading sensors, and surface plasmon resonance (SPR) sensors. Based on the photoelectric conversion mechanism, perovskite medical sensors can detect metabolic substances, cancer-related substances and drugs in three ways: hindering charge transfer, trapping charges, and changing the number of photo-induced carriers. Furthermore, perovskite photoelectric medical sensors exhibit an ultrasensitive detection performance, even reaching 10−3 fmol·L−1. Based on the all-optical conversion mechanism, metabolite substances and drugs are detected by perovskite all-optical conversion medical sensors via electron/hole transfer, perovskite material degradation, or perovskite material phase transition. Perovskite all-optical conversion medical sensors can be used to detect medical substances based on precise measurement using the photoluminescence spectrum and direct estimation based on the visible color changes. Based on the variable chemical valence states of constituent elements for perovskite materials, metabolite substances, neurotransmitters, cancer-related substances, and drugs are detected by the perovskite electrocatalytic medical sensors via oxidation reaction or reductive reaction. These have variable electrochemical measurement methods for medical substances, such as cyclic voltammetry, amperometry, and differential pulse voltammetry. They can not only simultaneously detect multiple substances but also are biocompatible. Based on the physicochemical loading and SPR mechanisms, metabolite substances and cancer-related substances are detected. Perovskite physicochemically loading medical sensors can detect both liquid and gaseous substances by utilizing the electrical conductivity or adsorbability of perovskite materials, and the detection of perovskite SPR medical sensors will not damage medical substances. In conclusion, owing to the different operation mechanisms of perovskite medical sensors, they exhibit high sensitivity and precision for detecting a wide range of medical substances, which meets the diverse requirements of medical detection. Thus, perovskite medical sensors pave the way for future multidisciplinary integration and development between the medicine and engineering fields. ![]()
Perovskite materials have considerable potential in medical sensors. This is because the diverse element substitution of the perovskite ABX3 composition brings rich physical and chemical properties for perovskite materials, including photoelectric conversion, all-optical conversion, and electro-optical conversion. By modifying the A-site, B-site, or X-site elements, the bandgap width of perovskite materials can be adjusted. Moreover, the absorption spectrum, photoelectric conversion electrical signal, and all-optical conversion luminescence spectrum can be regulated in perovskite materials. Perovskite materials also have the advantages of easy fabrication, excellent biocompatibility after modification, variable chemical valence states of constituent elements, and adjustable morphology. Therefore, perovskite materials are expected to be used in medical sensors with different operation mechanisms, such as photoelectric sensors, all-optical conversion sensors, electrocatalytic sensors, physicochemically loading sensors, and surface plasmon resonance (SPR) sensors. Based on the photoelectric conversion mechanism, perovskite medical sensors can detect metabolic substances, cancer-related substances and drugs in three ways: hindering charge transfer, trapping charges, and changing the number of photo-induced carriers. Furthermore, perovskite photoelectric medical sensors exhibit an ultrasensitive detection performance, even reaching 10−3 fmol·L−1. Based on the all-optical conversion mechanism, metabolite substances and drugs are detected by perovskite all-optical conversion medical sensors via electron/hole transfer, perovskite material degradation, or perovskite material phase transition. Perovskite all-optical conversion medical sensors can be used to detect medical substances based on precise measurement using the photoluminescence spectrum and direct estimation based on the visible color changes. Based on the variable chemical valence states of constituent elements for perovskite materials, metabolite substances, neurotransmitters, cancer-related substances, and drugs are detected by the perovskite electrocatalytic medical sensors via oxidation reaction or reductive reaction. These have variable electrochemical measurement methods for medical substances, such as cyclic voltammetry, amperometry, and differential pulse voltammetry. They can not only simultaneously detect multiple substances but also are biocompatible. Based on the physicochemical loading and SPR mechanisms, metabolite substances and cancer-related substances are detected. Perovskite physicochemically loading medical sensors can detect both liquid and gaseous substances by utilizing the electrical conductivity or adsorbability of perovskite materials, and the detection of perovskite SPR medical sensors will not damage medical substances. In conclusion, owing to the different operation mechanisms of perovskite medical sensors, they exhibit high sensitivity and precision for detecting a wide range of medical substances, which meets the diverse requirements of medical detection. Thus, perovskite medical sensors pave the way for future multidisciplinary integration and development between the medicine and engineering fields.
2023, 39(5): 221200
doi: 10.3866/PKU.WHXB202212005
Abstract:
Sulfur has been considered as an ideal cathode of lithium sulfur batteries (LSBs) owing to its high theoretical energy density (2600 Wh∙Kg-1), excellent discharge capacity (1672 mAh∙g-1), and low cost. During sulfur reduction and oxidation processes, nevertheless, the sluggish redox reaction kinetics of the sulfur cathode and severe shuttle effect of soluble lithium polysulfides intermediates significantly result in poor battery performance. It has been demonstrated that a sulfur host with high adsorption energy and excellent catalytic activity/selectivity can effectively enhance the cycle stability and rate capability of LSBs. As a result, a variety of hosts, such as metal compounds, heterojunctions, defect matrices, and single metal atom catalysts, have been widely developed. Interestingly, single metal atom catalysts with a unique electronic structure, low metal content, theoretical 100% atom utilization efficiency, and high catalytic performance can effectively promote the conversion of different lithium polysulfides intermediates and provide abundant absorption sites for sulfur-contained species, thereby optimizing the redox reaction kinetics of the sulfur cathode and shuttle behavior of the soluble lithium polysulfides. Various single metal atom catalysts, mainly including iron, cobalt, nickel, zinc, tungsten, vanadium, molybdenum, and manganese, have been developed via atomic bonding, spatial confinement, and defect engineering strategies to solve the key challenges of sulfur cathode since single metal atom catalysts were for the first time to be utilized as catalytic agents for LSBs. In this review, the interaction among support materials in single metal atom catalysts, atomically dispersed metal catalytic sites, and the sulfur cathode were addressed in detail, providing a basis for the development of high-performance single metal atom catalysts. Furthermore, advanced characterization techniques such as in situ Raman spectroscopy, X-ray absorption spectroscopy, cyclic voltammograms, and electrochemical impedance spectroscopy, were employed to investigate the catalytic effect of single metal atom catalysts. Notably, the effects of the coordination environment on the catalytic activity and selectivity of single metal atom catalysts were systematically discussed. Simultaneously, the catalytic mechanism of single metal atom catalysts with different metal/nonmetallic atoms and coordination configurations was elucidated using theoretical calculations. In addition, some significant challenges of single metal atom catalyst in LSBs were proposed. It is believed that this review will provide a novel insight into the optimization of atomic catalysts with high activity and catalytic selectivity toward long-lifespan LSBs. ![]()
Sulfur has been considered as an ideal cathode of lithium sulfur batteries (LSBs) owing to its high theoretical energy density (2600 Wh∙Kg-1), excellent discharge capacity (1672 mAh∙g-1), and low cost. During sulfur reduction and oxidation processes, nevertheless, the sluggish redox reaction kinetics of the sulfur cathode and severe shuttle effect of soluble lithium polysulfides intermediates significantly result in poor battery performance. It has been demonstrated that a sulfur host with high adsorption energy and excellent catalytic activity/selectivity can effectively enhance the cycle stability and rate capability of LSBs. As a result, a variety of hosts, such as metal compounds, heterojunctions, defect matrices, and single metal atom catalysts, have been widely developed. Interestingly, single metal atom catalysts with a unique electronic structure, low metal content, theoretical 100% atom utilization efficiency, and high catalytic performance can effectively promote the conversion of different lithium polysulfides intermediates and provide abundant absorption sites for sulfur-contained species, thereby optimizing the redox reaction kinetics of the sulfur cathode and shuttle behavior of the soluble lithium polysulfides. Various single metal atom catalysts, mainly including iron, cobalt, nickel, zinc, tungsten, vanadium, molybdenum, and manganese, have been developed via atomic bonding, spatial confinement, and defect engineering strategies to solve the key challenges of sulfur cathode since single metal atom catalysts were for the first time to be utilized as catalytic agents for LSBs. In this review, the interaction among support materials in single metal atom catalysts, atomically dispersed metal catalytic sites, and the sulfur cathode were addressed in detail, providing a basis for the development of high-performance single metal atom catalysts. Furthermore, advanced characterization techniques such as in situ Raman spectroscopy, X-ray absorption spectroscopy, cyclic voltammograms, and electrochemical impedance spectroscopy, were employed to investigate the catalytic effect of single metal atom catalysts. Notably, the effects of the coordination environment on the catalytic activity and selectivity of single metal atom catalysts were systematically discussed. Simultaneously, the catalytic mechanism of single metal atom catalysts with different metal/nonmetallic atoms and coordination configurations was elucidated using theoretical calculations. In addition, some significant challenges of single metal atom catalyst in LSBs were proposed. It is believed that this review will provide a novel insight into the optimization of atomic catalysts with high activity and catalytic selectivity toward long-lifespan LSBs.
2023, 39(5): 221101
doi: 10.3866/PKU.WHXB202211019
Abstract:
Advanced oxidation processes (AOPs), especially AOPs that use transition metals as catalyst activated oxidants, are extremely effective in removing organic pollutants; they can completely degrade pollutants into CO2 and H2O. Thus, they have been widely studied in the field of water treatment. However, owing to the low catalytic efficiency and metal leakage, their applicability is currently limited. In this paper, the composite catalyst CoFe2O4/CuO containing spinel cobalt ferrite and copper oxide was successfully prepared by the chemical precipitation and sol-gel methods with two steps. The prepared CoFe2O4/CuO was characterized using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD), and its ability to remove sulfamethoxazole (SMX) with different AOPs was evaluated. Characterization results show that CoFe2O4 and CuO are well-complexed together, and the catalyst has good crystallinity. The effects of peracetic acid (PAA) concentration, catalyst dosage, common interfering substances (Cl−, HCO3−, SO42−, and HA) in water, and different radical scavengers on SMX removal were also investigated. The results show that CoFe2O4/CuO has the characteristics of both CoFe2O4 and CuO. Compared with CoFe2O4 or CuO alone, CoFe2O4/CuO exhibits an excellent activation performance for PAA. Under the optimal reaction conditions (catalyst dosage = 20 mg·L−1, c(PAA) = 200 μmol·L−1, c(SMX) = 10 μmol·L−1), the degradation rate of SMX reaches 92% within 90 s. The existence of Cu+/Cu2+ electron pairs can convert Co from the high valence to low valence state and accelerate the conversion of Co2+/Co3+, thereby improving the catalytic performance. An increase in the PAA concentration increases the removal efficiency of SMX; however, too high a concentration lowers removal efficiency. Compared to acidic or alkaline conditions, the CoFe2O4/CuO reaction system exhibits a better removal rate of SMX under neutral conditions. The common interfering substances in the environment have different effects on the CoFe2O4/CuO reaction system. Cl− promotes the degradation of SMX by producing Cl•, HCO3− and HA inhibit the removal of SMX because of their quenching effect on free radicals, and SO42− has no significant effect on the progress of the reaction. The XPS characterization results before and after the reaction show that the valence state of Co changes, indicating that Co is the main element involved in the activation of PAA. Radical quenching experiments demonstrate that the organic radical (R―O•) plays a dominant role in the removal of SMX. Further, the removal rate of SMX decreases after the catalyst is subjected to 3 recycle; nevertheless, it achieves a relatively rapid degradation of SMX (85% within 10 min).![]()
Advanced oxidation processes (AOPs), especially AOPs that use transition metals as catalyst activated oxidants, are extremely effective in removing organic pollutants; they can completely degrade pollutants into CO2 and H2O. Thus, they have been widely studied in the field of water treatment. However, owing to the low catalytic efficiency and metal leakage, their applicability is currently limited. In this paper, the composite catalyst CoFe2O4/CuO containing spinel cobalt ferrite and copper oxide was successfully prepared by the chemical precipitation and sol-gel methods with two steps. The prepared CoFe2O4/CuO was characterized using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD), and its ability to remove sulfamethoxazole (SMX) with different AOPs was evaluated. Characterization results show that CoFe2O4 and CuO are well-complexed together, and the catalyst has good crystallinity. The effects of peracetic acid (PAA) concentration, catalyst dosage, common interfering substances (Cl−, HCO3−, SO42−, and HA) in water, and different radical scavengers on SMX removal were also investigated. The results show that CoFe2O4/CuO has the characteristics of both CoFe2O4 and CuO. Compared with CoFe2O4 or CuO alone, CoFe2O4/CuO exhibits an excellent activation performance for PAA. Under the optimal reaction conditions (catalyst dosage = 20 mg·L−1, c(PAA) = 200 μmol·L−1, c(SMX) = 10 μmol·L−1), the degradation rate of SMX reaches 92% within 90 s. The existence of Cu+/Cu2+ electron pairs can convert Co from the high valence to low valence state and accelerate the conversion of Co2+/Co3+, thereby improving the catalytic performance. An increase in the PAA concentration increases the removal efficiency of SMX; however, too high a concentration lowers removal efficiency. Compared to acidic or alkaline conditions, the CoFe2O4/CuO reaction system exhibits a better removal rate of SMX under neutral conditions. The common interfering substances in the environment have different effects on the CoFe2O4/CuO reaction system. Cl− promotes the degradation of SMX by producing Cl•, HCO3− and HA inhibit the removal of SMX because of their quenching effect on free radicals, and SO42− has no significant effect on the progress of the reaction. The XPS characterization results before and after the reaction show that the valence state of Co changes, indicating that Co is the main element involved in the activation of PAA. Radical quenching experiments demonstrate that the organic radical (R―O•) plays a dominant role in the removal of SMX. Further, the removal rate of SMX decreases after the catalyst is subjected to 3 recycle; nevertheless, it achieves a relatively rapid degradation of SMX (85% within 10 min).
2023, 39(5): 221202
doi: 10.3866/PKU.WHXB202212027
Abstract:
As an emerging technology for achieving carbon neutrality, the electrocatalytic CO2 reduction reaction (CO2RR) converts CO2 and water to valuable fuels and chemicals with power supply from renewable energy. Currently, the practical application of the CO2RR suffers from insufficient electrocatalytic performance in terms of selectivity, reaction rate, energy efficiency, and long-term stability. Electrolytes are considered as equally critical as catalysts for enhancing the CO2RR performance. From a catalysis perspective, electrolytes significantly affect the reaction microenvironments around catalytically active sites. From an electrochemical perspective, electrolytes determine the electric double layer structure. The electrocatalytic electrode/electrolyte interface where the CO2RR takes place is strongly influenced by electrolyte composition and identity. Thus, beyond catalyst design, rational electrolyte design is an alternative strategy for advancing the CO2RR towards industrial applications. This review presents important electrolyte effects in the aqueous CO2RR using the most recent studies, with an emphasis on those conducted under industrially-relevant reaction conditions. The effects of (local) pH, cations, and anions in aqueous inorganic electrolytes and their coupled effects with solid polymer electrolytes on tuning the activity and selectivity of the CO2RR are summarized. Although their influences on CO2RR performance are interconnected, pH effects, cation effects, and anion effects as well as electrolyte effects in membrane electrode assembly (MEA) electrolyzers are discussed separately considering the leading role of each factor. The experimentally observed performance dependence on the electrolyte composition and identity as well as the underlying reaction mechanism are discussed. The unique role of vibrational spectroscopies, such as surface-enhanced infrared absorption and Raman, in the characterization of electrode/electrolyte interfaces and the elucidation of electrolyte effects is highlighted. An innovative experimental strategy that involves the validation of specific adsorption of alkali-metal cations on the surface of an electrode by precisely monitoring the vibrational spectroscopic characteristics of probe molecules is highly recommended. The review focuses on the pH-dependent electrocatalytic performance and reaction pathways, the uncertain mechanisms of cation effects, and the roles of typical anions, such as bicarbonate and halides. Particularly, emphasis is given to the transport of key species, such as anions, cations, water, and products in ion exchange membranes, as well as their dynamic behaviors at the electrode/electrolyte interface in MEA CO2 electrolyzers. Although it has some drawbacks, anion exchange membranes are currently the most promising polymer electrolytes for practical application of the CO2RR. However, some emerging strategies based on cation exchange and bipolar membranes as well as tandem electrolysis processes are in progress. In all cases, a detailed understanding of electrolyte effects in the complex environments of MEA electrolyzers is indispensable for achieving performance enhancement. In conclusion, the remaining challenges and research opportunities in terms of the experimental and theoretical investigation of the electrolyte effects in the CO2RR process are proposed. This review provides novel insights into rational electrolyte design and useful guidelines for researchers in the field.![]()
As an emerging technology for achieving carbon neutrality, the electrocatalytic CO2 reduction reaction (CO2RR) converts CO2 and water to valuable fuels and chemicals with power supply from renewable energy. Currently, the practical application of the CO2RR suffers from insufficient electrocatalytic performance in terms of selectivity, reaction rate, energy efficiency, and long-term stability. Electrolytes are considered as equally critical as catalysts for enhancing the CO2RR performance. From a catalysis perspective, electrolytes significantly affect the reaction microenvironments around catalytically active sites. From an electrochemical perspective, electrolytes determine the electric double layer structure. The electrocatalytic electrode/electrolyte interface where the CO2RR takes place is strongly influenced by electrolyte composition and identity. Thus, beyond catalyst design, rational electrolyte design is an alternative strategy for advancing the CO2RR towards industrial applications. This review presents important electrolyte effects in the aqueous CO2RR using the most recent studies, with an emphasis on those conducted under industrially-relevant reaction conditions. The effects of (local) pH, cations, and anions in aqueous inorganic electrolytes and their coupled effects with solid polymer electrolytes on tuning the activity and selectivity of the CO2RR are summarized. Although their influences on CO2RR performance are interconnected, pH effects, cation effects, and anion effects as well as electrolyte effects in membrane electrode assembly (MEA) electrolyzers are discussed separately considering the leading role of each factor. The experimentally observed performance dependence on the electrolyte composition and identity as well as the underlying reaction mechanism are discussed. The unique role of vibrational spectroscopies, such as surface-enhanced infrared absorption and Raman, in the characterization of electrode/electrolyte interfaces and the elucidation of electrolyte effects is highlighted. An innovative experimental strategy that involves the validation of specific adsorption of alkali-metal cations on the surface of an electrode by precisely monitoring the vibrational spectroscopic characteristics of probe molecules is highly recommended. The review focuses on the pH-dependent electrocatalytic performance and reaction pathways, the uncertain mechanisms of cation effects, and the roles of typical anions, such as bicarbonate and halides. Particularly, emphasis is given to the transport of key species, such as anions, cations, water, and products in ion exchange membranes, as well as their dynamic behaviors at the electrode/electrolyte interface in MEA CO2 electrolyzers. Although it has some drawbacks, anion exchange membranes are currently the most promising polymer electrolytes for practical application of the CO2RR. However, some emerging strategies based on cation exchange and bipolar membranes as well as tandem electrolysis processes are in progress. In all cases, a detailed understanding of electrolyte effects in the complex environments of MEA electrolyzers is indispensable for achieving performance enhancement. In conclusion, the remaining challenges and research opportunities in terms of the experimental and theoretical investigation of the electrolyte effects in the CO2RR process are proposed. This review provides novel insights into rational electrolyte design and useful guidelines for researchers in the field.
