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2016 Volume 37 Issue 7
2016, 37(7):
Abstract:
2016, 37(7): 987-987
doi: 10.1016/S1872-2067(16)62461-0
Abstract:
2016, 37(7): 988-993
doi: 10.1016/S1872-2067(16)62481-6
Abstract:
Cathode catalyst layers (CLs) with varying ionomer (Nafion) contents were prepared and the direct methanol fuel cell structure and catalytic behavior were investigated as a function of ionomer content. CL roughness and thickness increased with increasing Nafion content. Contact angle measurements determined that CL hydrophilicity also increased as a function of Nafion content. Poor bonding between the CL, microporous layer, and the proton exchange membrane was obtained when the ionomer content was too low. The electrochemical surface areas (ESAs) were found to increase with increasing Nafion content before reaching an asymptote at elevated loading levels. However, upon increasing the ionomer content above 30 wt.%, the water and oxygen mass transfer properties were difficult to control. Considering the above conditions, N30 (30 wt.% Nafion) was found to be the optimal level to effectively extend the three-phase boundaries and enhance cell performance.
Cathode catalyst layers (CLs) with varying ionomer (Nafion) contents were prepared and the direct methanol fuel cell structure and catalytic behavior were investigated as a function of ionomer content. CL roughness and thickness increased with increasing Nafion content. Contact angle measurements determined that CL hydrophilicity also increased as a function of Nafion content. Poor bonding between the CL, microporous layer, and the proton exchange membrane was obtained when the ionomer content was too low. The electrochemical surface areas (ESAs) were found to increase with increasing Nafion content before reaching an asymptote at elevated loading levels. However, upon increasing the ionomer content above 30 wt.%, the water and oxygen mass transfer properties were difficult to control. Considering the above conditions, N30 (30 wt.% Nafion) was found to be the optimal level to effectively extend the three-phase boundaries and enhance cell performance.
2016, 37(7): 994-998
doi: 10.1016/S1872-2067(15)61075-0
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Silver nanoparticles prepared by the direct reduction of AgNO3 in aqueous solution were compacted into coins and used as the cathode for the electrocatalytic carboxylation of 1-phenethyl bromide with CO2. The influences of the working electrode, charge, current density and temperature were investigated. Under optimized conditions, 98% yield of 2-phenylpropionic acid was obtained. The reaction was performed under very mild conditions and no added catalyst was required in the electrolyte. Yields that varied from moderate to excellent were also achieved with other benzyl bromides. This electrode has good stability and reusability, and the yield and selectivity of 2-phenylpropionic acid could be maintained during reuse for 10 times.
Silver nanoparticles prepared by the direct reduction of AgNO3 in aqueous solution were compacted into coins and used as the cathode for the electrocatalytic carboxylation of 1-phenethyl bromide with CO2. The influences of the working electrode, charge, current density and temperature were investigated. Under optimized conditions, 98% yield of 2-phenylpropionic acid was obtained. The reaction was performed under very mild conditions and no added catalyst was required in the electrolyte. Yields that varied from moderate to excellent were also achieved with other benzyl bromides. This electrode has good stability and reusability, and the yield and selectivity of 2-phenylpropionic acid could be maintained during reuse for 10 times.
2016, 37(7): 999-1015
doi: 10.1016/S1872-2067(16)62455-5
Abstract:
The electrochemical reduction of CO2 into liquid fuels especially coupling with the intermittent renewable electricity offers a promising means of storing electricity in chemical form, which reduces the dependence on fossil fuels and mitigates the negative impact of anthropogenic CO2 emissions on the planet. Although converting CO2 to fuels is not in itself a new concept, the field has not substantially advanced in the last 30 years primarily because of the challenge of discovery of structural electrocatalysts and the development of membrane architectures for efficient collection of reactants and separation of products. This overview summarizes recent advances in catalytic conversion of CO2 and presents the challenges and future directions in producing value-added fuels.
The electrochemical reduction of CO2 into liquid fuels especially coupling with the intermittent renewable electricity offers a promising means of storing electricity in chemical form, which reduces the dependence on fossil fuels and mitigates the negative impact of anthropogenic CO2 emissions on the planet. Although converting CO2 to fuels is not in itself a new concept, the field has not substantially advanced in the last 30 years primarily because of the challenge of discovery of structural electrocatalysts and the development of membrane architectures for efficient collection of reactants and separation of products. This overview summarizes recent advances in catalytic conversion of CO2 and presents the challenges and future directions in producing value-added fuels.
2016, 37(7): 1016-1024
doi: 10.1016/S1872-2067(15)61125-1
Abstract:
Rechargeable Li-CO2 batteries provide a promising new approach for carbon capture and energy storage technology. However, their practical application is limited by many challenges despite much progress in this technology. Recent development in Li-CO2 batteries is presented. The reaction mechanism with an air cathode, operating temperatures used, electrochemical performance under different CO2 concentrations, stability of the battery in different electrolytes, and utilization of different cathode materials were emphasized. At last, challenges and perspectives were also presented. This review provides a deep understanding of Li-CO2 batteries and offers important guidelines for developing reversible and high efficiency Li-CO2 batteries.
Rechargeable Li-CO2 batteries provide a promising new approach for carbon capture and energy storage technology. However, their practical application is limited by many challenges despite much progress in this technology. Recent development in Li-CO2 batteries is presented. The reaction mechanism with an air cathode, operating temperatures used, electrochemical performance under different CO2 concentrations, stability of the battery in different electrolytes, and utilization of different cathode materials were emphasized. At last, challenges and perspectives were also presented. This review provides a deep understanding of Li-CO2 batteries and offers important guidelines for developing reversible and high efficiency Li-CO2 batteries.
2016, 37(7): 1025-1036
doi: 10.1016/S1872-2067(16)62480-4
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Polymer membrane fuel cells represent important sustainable energy devices because their operation involves zero emissions and low temperatures and their components exhibit low toxicity. Among the various components of such cells, the electrocatalyst plays the vital role of enhancing the output power density and/or working lifetime. Over the past several decades, numerous strategies have been proposed to address the challenges of electrocatalyst activity and/or durability. Herein, we review the applications of polyelectrolytes in electrocatalysts, including the enhancement of both catalytic nanoparticles and support materials. The effects of polyelectrolytes with regard to controlling the size, composition and morphology of catalytic nanoparticles, as well as the modification of support materials were summarized. In addition, the future possibilities for the research and development of polyelectrolytes in the field of catalyst design and synthesis are discussed.
Polymer membrane fuel cells represent important sustainable energy devices because their operation involves zero emissions and low temperatures and their components exhibit low toxicity. Among the various components of such cells, the electrocatalyst plays the vital role of enhancing the output power density and/or working lifetime. Over the past several decades, numerous strategies have been proposed to address the challenges of electrocatalyst activity and/or durability. Herein, we review the applications of polyelectrolytes in electrocatalysts, including the enhancement of both catalytic nanoparticles and support materials. The effects of polyelectrolytes with regard to controlling the size, composition and morphology of catalytic nanoparticles, as well as the modification of support materials were summarized. In addition, the future possibilities for the research and development of polyelectrolytes in the field of catalyst design and synthesis are discussed.
2016, 37(7): 1037-1048
doi: 10.1016/S1872-2067(16)62477-4
Abstract:
Direct methanol fuel cells (DMFCs) are promising for use in portable devices because of advantages such as high fuel energy density, low working temperature and low emission of pollutants. Nanotechnology has been used to improve the performance of DMFCs. Catalytic materials composed of small, metallic particles with unique nanostructure supported on carbons or metal oxides have been widely investigated for use in DMFCs. Despite our increased understanding of this type of fuel cell, many challenges still remain. This paper reviews the current developments of nanostructured electrocatalytic materials and porous electrodes for use in DMFCs. In particular, this review focuses on the synthesis and characterization of nanostructured catalysts and supporting materials. Both computational and experimental approaches to optimize mass transportation in porous electrodes of DMFCs, such as theoretical modeling of internal transfer processes and preparation of functional structures in membrane electrode assemblies, are introduced.
Direct methanol fuel cells (DMFCs) are promising for use in portable devices because of advantages such as high fuel energy density, low working temperature and low emission of pollutants. Nanotechnology has been used to improve the performance of DMFCs. Catalytic materials composed of small, metallic particles with unique nanostructure supported on carbons or metal oxides have been widely investigated for use in DMFCs. Despite our increased understanding of this type of fuel cell, many challenges still remain. This paper reviews the current developments of nanostructured electrocatalytic materials and porous electrodes for use in DMFCs. In particular, this review focuses on the synthesis and characterization of nanostructured catalysts and supporting materials. Both computational and experimental approaches to optimize mass transportation in porous electrodes of DMFCs, such as theoretical modeling of internal transfer processes and preparation of functional structures in membrane electrode assemblies, are introduced.
2016, 37(7): 1049-1061
doi: 10.1016/S1872-2067(15)61059-2
Abstract:
The high cost of Pt-based catalysts and the sluggish dynamics of the oxygen reduction reaction (ORR) severely hinder the rapid development of fuel cells. Therefore, the search for inexpensive, non-noble metal catalysts to substitute Pt-based catalysts has become a critical issue in the ORR research field. As an earth-abundant element, the use of Cu to catalyze the ORR has been explored with the ultimate target of finding a replacement for Pt-based catalysts in fuel cells. This review mainly focuses on recent research progress with Cu-based ORR catalysts and aims to aid readers' understanding of the status of development in this field. The review begins with a general update on the state of knowledge pertaining to ORR. This is followed by an overview of recent research based on Cu nanomaterial catalysts, which comprises Cu complexes, compounds, and other structures. Charting the development of Cu-based ORR catalysts shows that designing Cu-based materials to mimic active enzymes is an effective approach for ORR catalysis. By collecting recent developments in the field, we hope that this review will promote further development of Cu-based ORR catalysts and their application in fuel cells.
The high cost of Pt-based catalysts and the sluggish dynamics of the oxygen reduction reaction (ORR) severely hinder the rapid development of fuel cells. Therefore, the search for inexpensive, non-noble metal catalysts to substitute Pt-based catalysts has become a critical issue in the ORR research field. As an earth-abundant element, the use of Cu to catalyze the ORR has been explored with the ultimate target of finding a replacement for Pt-based catalysts in fuel cells. This review mainly focuses on recent research progress with Cu-based ORR catalysts and aims to aid readers' understanding of the status of development in this field. The review begins with a general update on the state of knowledge pertaining to ORR. This is followed by an overview of recent research based on Cu nanomaterial catalysts, which comprises Cu complexes, compounds, and other structures. Charting the development of Cu-based ORR catalysts shows that designing Cu-based materials to mimic active enzymes is an effective approach for ORR catalysis. By collecting recent developments in the field, we hope that this review will promote further development of Cu-based ORR catalysts and their application in fuel cells.
Photoelectrochemical degradation of acetaminophen and valacyclovir using nanoporous titanium dioxide
2016, 37(7): 1062-1069
doi: 10.1016/S1872-2067(15)61101-9
Abstract:
Electrochemically treated nanoporous TiO2 was employed as a novel electrode to assist in the photoelectrochemical degradation of acetaminophen and valacyclovir. The prepared electrode was characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Cyclic voltammetry (CV), Mott-Schottky plots, ultraviolet-visible light (UV-vis) absorbance spectroscopy, and a total organic carbon (TOC) analyzer were employed to investigate the photoelectrochemical degradation of acetaminophen and valacyclovir. The results indicated no obvious removal of acetaminophen and valacyclovir over 3 h when separate photochemical degradation and electrochemical oxidation were employed. In contrast, acetaminophen and valacyclovir were rapidly eliminated via photoelectrochemical degradation. In addition, electrochemically treated nanoporous TiO2 electrodes significantly enhanced the efficacy of the photoelectrochemical degradation of acetaminophen and valacyclovir, by 86.96% and 53.12%, respectively, when compared with untreated nanoporous TiO2 electrodes. This enhanced performance may have been attributed to the formation of Ti3+, Ti2+, and oxygen vacancies, as well as an improvement in conductivity during the electrochemical reduction process. The effect of temperature was further investigated, where the activation energy of the photoelectrochemical degradation of acetaminophen and valacyclovir was determined to be 9.62 and 18.42 kJ/mol, respectively.
Electrochemically treated nanoporous TiO2 was employed as a novel electrode to assist in the photoelectrochemical degradation of acetaminophen and valacyclovir. The prepared electrode was characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Cyclic voltammetry (CV), Mott-Schottky plots, ultraviolet-visible light (UV-vis) absorbance spectroscopy, and a total organic carbon (TOC) analyzer were employed to investigate the photoelectrochemical degradation of acetaminophen and valacyclovir. The results indicated no obvious removal of acetaminophen and valacyclovir over 3 h when separate photochemical degradation and electrochemical oxidation were employed. In contrast, acetaminophen and valacyclovir were rapidly eliminated via photoelectrochemical degradation. In addition, electrochemically treated nanoporous TiO2 electrodes significantly enhanced the efficacy of the photoelectrochemical degradation of acetaminophen and valacyclovir, by 86.96% and 53.12%, respectively, when compared with untreated nanoporous TiO2 electrodes. This enhanced performance may have been attributed to the formation of Ti3+, Ti2+, and oxygen vacancies, as well as an improvement in conductivity during the electrochemical reduction process. The effect of temperature was further investigated, where the activation energy of the photoelectrochemical degradation of acetaminophen and valacyclovir was determined to be 9.62 and 18.42 kJ/mol, respectively.
Influence of transition metal modification of oxide-derived Cu electrodes in electroreduction of CO2
2016, 37(7): 1070-1075
doi: 10.1016/S1872-2067(16)62465-8
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The modification of oxide-derived Cu electrode with Ni, Zn, and Au was examined to improve the catalytic activity of CO2 electroreduction. The experimental results showed that Ni modification increased the Faraday efficiency of the formation of formic acid and n-propanol. The Faraday efficiency relating to the formation of the liquid products was as high as 34.3% at -1.5 V versus the saturated calomel electrode reference potential. In contrast, modification with Zn reduced the formic acid formation efficiency but enhanced the alcohol formation efficiency. Finally, modification with Au suppressed the selectivity toward the formation of both formic acid and alcohols.
The modification of oxide-derived Cu electrode with Ni, Zn, and Au was examined to improve the catalytic activity of CO2 electroreduction. The experimental results showed that Ni modification increased the Faraday efficiency of the formation of formic acid and n-propanol. The Faraday efficiency relating to the formation of the liquid products was as high as 34.3% at -1.5 V versus the saturated calomel electrode reference potential. In contrast, modification with Zn reduced the formic acid formation efficiency but enhanced the alcohol formation efficiency. Finally, modification with Au suppressed the selectivity toward the formation of both formic acid and alcohols.
2016, 37(7): 1076-1080
doi: 10.1016/S1872-2067(15)61070-1
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Using 1-butyl-3-methyl- imidazolium bromide (BMIMBr) as the supporting electrolyte and magnesium as the sacrificial anode, a new and highly efficient electrochemically catalytic route was developed for the synthesis of cyclic carbonates from epoxides and CO2. Based on the cooperative action of BMIMBr and an electrogenerated magnesium salt obtained under a N2 atmosphere, CO2 reacted with a wide range of epoxides to readily generate cyclic carbonates in moderate to excellent yields under mild conditions.
Using 1-butyl-3-methyl- imidazolium bromide (BMIMBr) as the supporting electrolyte and magnesium as the sacrificial anode, a new and highly efficient electrochemically catalytic route was developed for the synthesis of cyclic carbonates from epoxides and CO2. Based on the cooperative action of BMIMBr and an electrogenerated magnesium salt obtained under a N2 atmosphere, CO2 reacted with a wide range of epoxides to readily generate cyclic carbonates in moderate to excellent yields under mild conditions.
2016, 37(7): 1081-1088
doi: 10.1016/S1872-2067(15)61048-8
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A novel catalyst for CO2 electroreduction based on nanostructured SnO2 was synthesized using a facile hydrothermal self-assembly method. The electrochemical activity showed that the catalyst gave outstanding catalytic activity and selectivity in CO2 electroreduction. The catalytic activity and formate selectivity depended strongly on the electrolyte conditions. A high faradaic efficiency, i.e., 56%, was achieved for formate formation in KHCO3 (0.5 mol/L). This is attributed to control of formate production by mass and charge transfer processes. Electrolysis experiments using SnO2-50/GDE (an SnO2-based gas-diffusion electrode, where 50 indicates the 50% ethanol content of the electrolyte) as the catalyst, showed that the electrolyte pH also affected CO2 reduction. The optimum electrolyte pH for obtaining a high faradaic efficiency for formate production was 8.3. This is mainly because a neutral or mildly alkaline environment maintains the oxide stability. The faradaic efficiency for formate production declined with time. X-ray photoelectron spectroscopy showed that this is the result of deposition of trace amounts of fluoride ions on the SnO2-50/GDE surface, which hinders reduction of CO2 to formate.
A novel catalyst for CO2 electroreduction based on nanostructured SnO2 was synthesized using a facile hydrothermal self-assembly method. The electrochemical activity showed that the catalyst gave outstanding catalytic activity and selectivity in CO2 electroreduction. The catalytic activity and formate selectivity depended strongly on the electrolyte conditions. A high faradaic efficiency, i.e., 56%, was achieved for formate formation in KHCO3 (0.5 mol/L). This is attributed to control of formate production by mass and charge transfer processes. Electrolysis experiments using SnO2-50/GDE (an SnO2-based gas-diffusion electrode, where 50 indicates the 50% ethanol content of the electrolyte) as the catalyst, showed that the electrolyte pH also affected CO2 reduction. The optimum electrolyte pH for obtaining a high faradaic efficiency for formate production was 8.3. This is mainly because a neutral or mildly alkaline environment maintains the oxide stability. The faradaic efficiency for formate production declined with time. X-ray photoelectron spectroscopy showed that this is the result of deposition of trace amounts of fluoride ions on the SnO2-50/GDE surface, which hinders reduction of CO2 to formate.
2016, 37(7): 1089-1095
doi: 10.1016/S1872-2067(15)61077-4
Abstract:
The nanostructure of the catalytic electrode has a great effect on the performance of direct methanol fuel cells (DMFCs), including catalyst utilization, precious metal loading, water balance, and oxygen mass transfer. In this work, ordered arrays of platinum nanorods with different diameters were directly grown onto microporous layers by electrodeposition via a sacrificial template, and were used as the catalytic cathode for passive DMFCs. The use of these ordered electrodes led to a dramatic decrease in cathode polarization behavior. The maximum power density of passive DMFCs fabricated with catalytic electrodes of 200 and 100 nm Pt nanorod arrays were 17.3 and 12.0 mW/cm2, respectively. The obtained improvement in performance was ascribed to the fact that the ordered nanostructured electrode not only increased the electrochemically active surface area and the catalyst utilization, but also enhanced oxygen mass transfer and water balance in the system.
The nanostructure of the catalytic electrode has a great effect on the performance of direct methanol fuel cells (DMFCs), including catalyst utilization, precious metal loading, water balance, and oxygen mass transfer. In this work, ordered arrays of platinum nanorods with different diameters were directly grown onto microporous layers by electrodeposition via a sacrificial template, and were used as the catalytic cathode for passive DMFCs. The use of these ordered electrodes led to a dramatic decrease in cathode polarization behavior. The maximum power density of passive DMFCs fabricated with catalytic electrodes of 200 and 100 nm Pt nanorod arrays were 17.3 and 12.0 mW/cm2, respectively. The obtained improvement in performance was ascribed to the fact that the ordered nanostructured electrode not only increased the electrochemically active surface area and the catalyst utilization, but also enhanced oxygen mass transfer and water balance in the system.
2016, 37(7): 1096-1102
doi: 10.1016/S1872-2067(15)61063-4
Abstract:
Nitrogen-doped carbon materials exhibiting high oxygen reduction reaction activity were prepared via the pyrolysis of a poly-p-phenylenediamine/carbon black composite. The as-synthesized catalyst showed excellent catalytic activity in alkaline solution, and outperformed commercial Pt/C in KOH solution (0.1 mol/L), as demonstrated by the higher current density and the more positive half-wave potential. Scanning electron microscopy and N2 adsorption-desorption analyses indicated that a composite structure, in which the N-rich surface of the poly-p-phenylenediamine had an increased active center concentration and the high external surface area of the carbon black was conducive to the mass transport, is highly beneficial in terms of promoting the oxygen reduction reaction. However, the activity of this catalyst underwent an obvious decrease following exposure to air for 30 d. X-ray photoelectron spectroscopy showed that the oxygen content in the catalyst was increased by prolonged air exposure. O 1s spectrum showed increases in the C=O and C-O components, suggesting that atmospheric oxygen reacted with the catalyst. This oxidation leaded to the deactivation of active center, thus the catalytic activity decreased. Based on these results, the stability in air of nitrogen-doped carbon materials must be taken into consideration when assessing applications as alternatives to platinum-based materials.
Nitrogen-doped carbon materials exhibiting high oxygen reduction reaction activity were prepared via the pyrolysis of a poly-p-phenylenediamine/carbon black composite. The as-synthesized catalyst showed excellent catalytic activity in alkaline solution, and outperformed commercial Pt/C in KOH solution (0.1 mol/L), as demonstrated by the higher current density and the more positive half-wave potential. Scanning electron microscopy and N2 adsorption-desorption analyses indicated that a composite structure, in which the N-rich surface of the poly-p-phenylenediamine had an increased active center concentration and the high external surface area of the carbon black was conducive to the mass transport, is highly beneficial in terms of promoting the oxygen reduction reaction. However, the activity of this catalyst underwent an obvious decrease following exposure to air for 30 d. X-ray photoelectron spectroscopy showed that the oxygen content in the catalyst was increased by prolonged air exposure. O 1s spectrum showed increases in the C=O and C-O components, suggesting that atmospheric oxygen reacted with the catalyst. This oxidation leaded to the deactivation of active center, thus the catalytic activity decreased. Based on these results, the stability in air of nitrogen-doped carbon materials must be taken into consideration when assessing applications as alternatives to platinum-based materials.
2016, 37(7): 1103-1108
doi: 10.1016/S1872-2067(16)62471-3
Abstract:
Fe/N/C is a promising non-platinum catalyst for the oxygen reduction reaction (ORR). Even so, mass transfer remains a challenge in the application of Fe/N/C to proton exchange membrane fuel cells, due to the high catalyst loadings required. In the present work, mesoporous Fe/N/C was synthesized through heat treatment of KJ600 carbon black coated with poly-2-aminobenzimidazole and FeCl3. The as-prepared Fe/N/C possesses a unique hollow-shell structure that contains a buffer zone allowing both water formation and vaporization, and also facilitates the mass transfer of gaseous oxygen. This catalyst generated an oxygen reduction reaction activity of 9.21 A/g in conjunction with a peak power density of 0.71 W/cm2.
Fe/N/C is a promising non-platinum catalyst for the oxygen reduction reaction (ORR). Even so, mass transfer remains a challenge in the application of Fe/N/C to proton exchange membrane fuel cells, due to the high catalyst loadings required. In the present work, mesoporous Fe/N/C was synthesized through heat treatment of KJ600 carbon black coated with poly-2-aminobenzimidazole and FeCl3. The as-prepared Fe/N/C possesses a unique hollow-shell structure that contains a buffer zone allowing both water formation and vaporization, and also facilitates the mass transfer of gaseous oxygen. This catalyst generated an oxygen reduction reaction activity of 9.21 A/g in conjunction with a peak power density of 0.71 W/cm2.
2016, 37(7): 1109-1118
doi: 10.1016/S1872-2067(16)62454-3
Abstract:
Significant progress has been made in the development of non-precious metal electrocatalysts (NPMEs) during the past decade. Correspondingly, there is an urgent demand for an appropriate measurement method to be established for the reliable evaluation of NPMEs. In this study, platinum and graphite counter electrodes were used to investigate the impact of counter electrode material on the accelerated durability testing (ADT) of NPMEs in acidic medium. Platinum used as the counter electrode in a traditional three-electrode electrochemical cell was found to dissolve in acidic medium and re-deposit on NPME coated on the working electrode during ADT. Such re-deposition causes the oxygen reduction reaction (ORR) performance of NPMEs to remarkably improve, and thus will seriously mislead our judgment of NPMEs if we are unaware of it. The phenomenon can be avoided using a graphite counter electrode.
Significant progress has been made in the development of non-precious metal electrocatalysts (NPMEs) during the past decade. Correspondingly, there is an urgent demand for an appropriate measurement method to be established for the reliable evaluation of NPMEs. In this study, platinum and graphite counter electrodes were used to investigate the impact of counter electrode material on the accelerated durability testing (ADT) of NPMEs in acidic medium. Platinum used as the counter electrode in a traditional three-electrode electrochemical cell was found to dissolve in acidic medium and re-deposit on NPME coated on the working electrode during ADT. Such re-deposition causes the oxygen reduction reaction (ORR) performance of NPMEs to remarkably improve, and thus will seriously mislead our judgment of NPMEs if we are unaware of it. The phenomenon can be avoided using a graphite counter electrode.
2016, 37(7): 1119-1126
doi: 10.1016/S1872-2067(16)62456-7
Abstract:
Pure graphitic nitrogen (G-N) was doped on carbon black by Hummers method and a following heat treatment was used to transform the G-N to pyridinic (P)-N. An oxygen reduction reaction (ORR) study showed that the G-N site doped on carbon gave a two-electron ORR with H2O2 as the main product, while the P-N site gave the four-electron process of ORR and decreased the production of H2O2. The results help the understanding and design of doped N-based ORR electrocatalysts.
Pure graphitic nitrogen (G-N) was doped on carbon black by Hummers method and a following heat treatment was used to transform the G-N to pyridinic (P)-N. An oxygen reduction reaction (ORR) study showed that the G-N site doped on carbon gave a two-electron ORR with H2O2 as the main product, while the P-N site gave the four-electron process of ORR and decreased the production of H2O2. The results help the understanding and design of doped N-based ORR electrocatalysts.
2016, 37(7): 1127-1133
doi: 10.1016/S1872-2067(15)61104-4
Abstract:
The development of a non-precious metal electrocatalyst (NPME) with a performance superior to commercial Pt/C for the oxygen reduction reaction (ORR) is important for the commercialization of fuel cells. We report the synthesis of a NPME by heat-treating Co-based metal organic frameworks (ZIF-67) with a small average size of 44 nm. The electrocatalyst pyrolyzed at 600 ℃ showed the best performance and the performance was enhanced when it was supported on BP 2000. The resulting electrocatalyst was composed of 10 nm Co nanoparticles coated by 3-12 layers of N doped graphite layers which as a whole was embedded in a carbon matrix. The ORR performance of the electrocatalyst was tested by rotating disk electrode tests in O2-saturated 0.1 mol/L KOH under ambient conditions. The electrocatalyst (1.0 mg/cm2) showed an onset potential of 1.017 V (vs. RHE) and a half-wave potential of 0.857 V (vs. RHE), which showed it was as good as the commercial Pt/C (20 μgPt/cm2). Furthermore, the electrocatalyst possessed much better stability and resistance to methanol crossover than Pt/C.
The development of a non-precious metal electrocatalyst (NPME) with a performance superior to commercial Pt/C for the oxygen reduction reaction (ORR) is important for the commercialization of fuel cells. We report the synthesis of a NPME by heat-treating Co-based metal organic frameworks (ZIF-67) with a small average size of 44 nm. The electrocatalyst pyrolyzed at 600 ℃ showed the best performance and the performance was enhanced when it was supported on BP 2000. The resulting electrocatalyst was composed of 10 nm Co nanoparticles coated by 3-12 layers of N doped graphite layers which as a whole was embedded in a carbon matrix. The ORR performance of the electrocatalyst was tested by rotating disk electrode tests in O2-saturated 0.1 mol/L KOH under ambient conditions. The electrocatalyst (1.0 mg/cm2) showed an onset potential of 1.017 V (vs. RHE) and a half-wave potential of 0.857 V (vs. RHE), which showed it was as good as the commercial Pt/C (20 μgPt/cm2). Furthermore, the electrocatalyst possessed much better stability and resistance to methanol crossover than Pt/C.
2016, 37(7): 1134-1141
doi: 10.1016/S1872-2067(16)62472-5
Abstract:
Nafion-membrane-based proton exchange fuel cells (PEMFCs) typically operate at below 100 ℃. However, H3PO4-doped polybenzimidazole (PBI)-based PEMFCs can operate at 100-200 ℃. This is advantageous because of accelerated reaction rates and enhanced tolerance to poisons such as CO and SO2, which can arise from reformed gas or the atmosphere. However, the strong adsorption of phosphoric anions on the Pt surface dramatically decreases the electrocatalytic activity. This study exploits the "third-body effect", in which a small amount of organic molecules are pre-adsorbed on the Pt surface to inhibit the adsorption of phosphoric anions. Pre-adsorbate species inhibit the adsorption of phosphoric anions, but can also partially occlude active sites. Thus, the optimum pre-adsorbate coverage is studied by correlating the oxygen reduction reaction (ORR) activity of Pt with pre-adsorbate coverage on the Pt surface. The influence of the pre-adsorbate molecule length is investigated using the organic amines, butylamine, octylamine, and dodecylamine, in both 0.1 mol/L HClO4 and 0.1 mol/L H3PO4. Such amines readily bond to the Pt surface. In aqueous HClO4 electrolyte, the ORR activity of Pt decreases monotonically with increasing pre-adsorbate coverage. In aqueous H3PO4 electrolyte, the ORR activity of Pt initially increases and then decreases with increasing pre-adsorbate coverage. The maximum ORR activity in H3PO4 occurs at a pre-adsorbate coverage of around 20%. The effect of molecular length of the pre-adsorbate is negligible, but its coverage strongly affects the degree to which phosphoric anion adsorption is inhibited. Butylamine adsorbs to Pt at partial active sites, which decreases the electrochemically active surface area. Adsorbed butylamine may also modify the electronic structure of the Pt surface. The ORR activity in the phosphoric acid electrolyte remains relatively low, even when using the pre-adsorbate modified Pt/C catalysts. Further development of the catalyst and electrolyte is required before the commercialization of H3PO4-PBI-based PEMFCs can be realized.
Nafion-membrane-based proton exchange fuel cells (PEMFCs) typically operate at below 100 ℃. However, H3PO4-doped polybenzimidazole (PBI)-based PEMFCs can operate at 100-200 ℃. This is advantageous because of accelerated reaction rates and enhanced tolerance to poisons such as CO and SO2, which can arise from reformed gas or the atmosphere. However, the strong adsorption of phosphoric anions on the Pt surface dramatically decreases the electrocatalytic activity. This study exploits the "third-body effect", in which a small amount of organic molecules are pre-adsorbed on the Pt surface to inhibit the adsorption of phosphoric anions. Pre-adsorbate species inhibit the adsorption of phosphoric anions, but can also partially occlude active sites. Thus, the optimum pre-adsorbate coverage is studied by correlating the oxygen reduction reaction (ORR) activity of Pt with pre-adsorbate coverage on the Pt surface. The influence of the pre-adsorbate molecule length is investigated using the organic amines, butylamine, octylamine, and dodecylamine, in both 0.1 mol/L HClO4 and 0.1 mol/L H3PO4. Such amines readily bond to the Pt surface. In aqueous HClO4 electrolyte, the ORR activity of Pt decreases monotonically with increasing pre-adsorbate coverage. In aqueous H3PO4 electrolyte, the ORR activity of Pt initially increases and then decreases with increasing pre-adsorbate coverage. The maximum ORR activity in H3PO4 occurs at a pre-adsorbate coverage of around 20%. The effect of molecular length of the pre-adsorbate is negligible, but its coverage strongly affects the degree to which phosphoric anion adsorption is inhibited. Butylamine adsorbs to Pt at partial active sites, which decreases the electrochemically active surface area. Adsorbed butylamine may also modify the electronic structure of the Pt surface. The ORR activity in the phosphoric acid electrolyte remains relatively low, even when using the pre-adsorbate modified Pt/C catalysts. Further development of the catalyst and electrolyte is required before the commercialization of H3PO4-PBI-based PEMFCs can be realized.
2016, 37(7): 1142-1148
doi: 10.1016/S1872-2067(15)61064-6
Abstract:
We studied the alloying effect in Ir-based alloys on the catalysis of the hydrogen oxidation reaction (HOR) in both acidic and alkaline medium. IrFe, IrNi and IrCo alloy catalysts with nanoparticle size of <5 nm were obtained by our solvent-vaporization plus hydrogen reduction method. The second metal played an important role in tuning the crystal structure and surface electronic structure of the Ir-based alloy catalyst. Among the IrFe, IrCo and IrNi alloy catalysts, Ni induced a mid-sized contraction of the Ir lattice, and gave the best HOR activity in both acidic and alkaline medium. In acidic medium, the weakening of the Ir-Had interaction caused by the electronic effect of M (M = Fe, Ni, Co) alloying is responsible for the enhancement of HOR activity. The oxophilic effect of the catalytic metal surface, which affects OHad adsorption and desorption and surface Had coverage, has a large impact on the HOR activity in the case of alkaline medium.
We studied the alloying effect in Ir-based alloys on the catalysis of the hydrogen oxidation reaction (HOR) in both acidic and alkaline medium. IrFe, IrNi and IrCo alloy catalysts with nanoparticle size of <5 nm were obtained by our solvent-vaporization plus hydrogen reduction method. The second metal played an important role in tuning the crystal structure and surface electronic structure of the Ir-based alloy catalyst. Among the IrFe, IrCo and IrNi alloy catalysts, Ni induced a mid-sized contraction of the Ir lattice, and gave the best HOR activity in both acidic and alkaline medium. In acidic medium, the weakening of the Ir-Had interaction caused by the electronic effect of M (M = Fe, Ni, Co) alloying is responsible for the enhancement of HOR activity. The oxophilic effect of the catalytic metal surface, which affects OHad adsorption and desorption and surface Had coverage, has a large impact on the HOR activity in the case of alkaline medium.
2016, 37(7): 1149-1155
doi: 10.1016/S1872-2067(15)61117-2
Abstract:
Hierarchical nitrogen-doped carbon nanocages (hNCNC) with large specific surface areas were used as a catalyst support to immobilize Pt nanoparticles by a microwave-assisted polyol method. The Pt/hNCNC catalyst with 20 wt% loading has a homogeneous dispersion of Pt nanoparticles with the average size of 3.3 nm, which is smaller than 4.3 and 4.9 nm for the control catalysts with the same loading supported on hierarchical carbon nanocages (hCNC) and commercial Vulcan XC-72, respectively. Accordingly, Pt/hNCNC has a larger electrochemical surface area than Pt/hCNC and Pt/XC-72. The Pt/hNCNC catalyst exhibited excellent electrocatalytic activity and stability for methanol oxidation, which was better than the control catalysts. This was attributed to the enhanced interaction between Pt and hNCNC due to nitrogen participation in the anchoring function. By making use of the unique advantages of the hNCNC support, a heavy Pt loading up to 60 wt% was prepared without serious agglomeration, which gave a high peak-current density per unit mass of catalyst of 95.6 mA/mg for achieving a high power density. These results showed the potential of the Pt/hNCNC catalyst for methanol oxidation and of the new hNCNC support for wide applications.
Hierarchical nitrogen-doped carbon nanocages (hNCNC) with large specific surface areas were used as a catalyst support to immobilize Pt nanoparticles by a microwave-assisted polyol method. The Pt/hNCNC catalyst with 20 wt% loading has a homogeneous dispersion of Pt nanoparticles with the average size of 3.3 nm, which is smaller than 4.3 and 4.9 nm for the control catalysts with the same loading supported on hierarchical carbon nanocages (hCNC) and commercial Vulcan XC-72, respectively. Accordingly, Pt/hNCNC has a larger electrochemical surface area than Pt/hCNC and Pt/XC-72. The Pt/hNCNC catalyst exhibited excellent electrocatalytic activity and stability for methanol oxidation, which was better than the control catalysts. This was attributed to the enhanced interaction between Pt and hNCNC due to nitrogen participation in the anchoring function. By making use of the unique advantages of the hNCNC support, a heavy Pt loading up to 60 wt% was prepared without serious agglomeration, which gave a high peak-current density per unit mass of catalyst of 95.6 mA/mg for achieving a high power density. These results showed the potential of the Pt/hNCNC catalyst for methanol oxidation and of the new hNCNC support for wide applications.
2016, 37(7): 1156-1165
doi: 10.1016/S1872-2067(15)61106-8
Abstract:
The potential (E)-dependent vibrational behavior of a saturated CO adlayer on Au-core Pd-shell nanoparticle film electrodes was investigated over a wide potential range, in acidic, neutral, and basic solutions, using in situ surface-enhanced Raman spectroscopy (SERS). Over the whole of the examined potential region (-1.5 to 0.55 V vs. NHE), the peak frequencies of both the C-OM and the Pd-COM band (here, M denotes the multiply-bonded configuration) displayed three distinct linear regions: dνC-OM/dE decreased from ~185-207 (from -1.5 to -1.2 V) to ~83-84 cm-1/V (-1.2 to -0.15 V), and then to 43 cm-1/V (-0.2 to 0.55 V); on the other hand, dνPd-COM/dE changed from ~-10 to -8 cm-1/V (from -1.5 to -1.2 V) to ~-31 to -30 cm-1/V (-1.2 to -0.15 V), and then to -15 cm-1/V (-0.2 to 0.55 V). The simultaneously recorded cyclic voltammograms revealed that at E < -1.2 V, a hydrogen evolution reaction (HER) occurred. With the help of periodic density functional theory calculations using two different (2 × 2)-3CO slab models with Pd(111), the unusually high dνC-OM/dE and the small dνPd-COM/dE in the HER region were explained as being due to the conversion of COad from bridge to hollow sites, which was induced by the co-adsorbed hydrogen atoms formed from dissociated water at negative potentials.
The potential (E)-dependent vibrational behavior of a saturated CO adlayer on Au-core Pd-shell nanoparticle film electrodes was investigated over a wide potential range, in acidic, neutral, and basic solutions, using in situ surface-enhanced Raman spectroscopy (SERS). Over the whole of the examined potential region (-1.5 to 0.55 V vs. NHE), the peak frequencies of both the C-OM and the Pd-COM band (here, M denotes the multiply-bonded configuration) displayed three distinct linear regions: dνC-OM/dE decreased from ~185-207 (from -1.5 to -1.2 V) to ~83-84 cm-1/V (-1.2 to -0.15 V), and then to 43 cm-1/V (-0.2 to 0.55 V); on the other hand, dνPd-COM/dE changed from ~-10 to -8 cm-1/V (from -1.5 to -1.2 V) to ~-31 to -30 cm-1/V (-1.2 to -0.15 V), and then to -15 cm-1/V (-0.2 to 0.55 V). The simultaneously recorded cyclic voltammograms revealed that at E < -1.2 V, a hydrogen evolution reaction (HER) occurred. With the help of periodic density functional theory calculations using two different (2 × 2)-3CO slab models with Pd(111), the unusually high dνC-OM/dE and the small dνPd-COM/dE in the HER region were explained as being due to the conversion of COad from bridge to hollow sites, which was induced by the co-adsorbed hydrogen atoms formed from dissociated water at negative potentials.
2016, 37(7): 1166-1171
doi: 10.1016/S1872-2067(15)61082-8
Abstract:
Fe and Co porphyrins and phthalocyanines are excellent catalysts for the oxygen reduction reaction (ORR) and are promising alternatives to Pt in fuel cells. However, the stability of these molecular catalysts in acidic media is poor. This study explores whether demetalation through proton exchange causes these metal macrocyclic catalysts to be unstable in acidic media. We first present a theoretical scheme for investigating exchange reactions of metal ions in metal macrocyclic compounds with protons in acidic media. The equilibrium concentrations of metal ions in solution when various metalloporphyrins (MPs) and metallophthalocyanines (MPcs) are brought into contact with a strongly acidic solution (pH = 1) were then estimated using density functional theory calculations; these values were used to evaluate the stability of these metal macrocyclic compounds against demetalation in acidic media. The results show that Fe, Co, Ni, and Cu phthalocyanines and porphyrins have considerable resistance to exchange with protons, whereas Cr, Mn, and Zn phthalocyanines and porphyrins easily undergo demetalation through ion exchange with protons. This suggests that the degradation in the ORR activity of Fe and Co macrocyclic molecular catalysts and of carbon materials doped with Fe(Co) and nitrogen, which are believed to have metal-nitrogen coordination structures similar to those of macrocyclic molecules as ORR catalytic centers, is not the result of replacement of metal ions by protons. The calculation results show that electron-donating substituents could enhance the stability of Fe and Co phthalocyanines.
Fe and Co porphyrins and phthalocyanines are excellent catalysts for the oxygen reduction reaction (ORR) and are promising alternatives to Pt in fuel cells. However, the stability of these molecular catalysts in acidic media is poor. This study explores whether demetalation through proton exchange causes these metal macrocyclic catalysts to be unstable in acidic media. We first present a theoretical scheme for investigating exchange reactions of metal ions in metal macrocyclic compounds with protons in acidic media. The equilibrium concentrations of metal ions in solution when various metalloporphyrins (MPs) and metallophthalocyanines (MPcs) are brought into contact with a strongly acidic solution (pH = 1) were then estimated using density functional theory calculations; these values were used to evaluate the stability of these metal macrocyclic compounds against demetalation in acidic media. The results show that Fe, Co, Ni, and Cu phthalocyanines and porphyrins have considerable resistance to exchange with protons, whereas Cr, Mn, and Zn phthalocyanines and porphyrins easily undergo demetalation through ion exchange with protons. This suggests that the degradation in the ORR activity of Fe and Co macrocyclic molecular catalysts and of carbon materials doped with Fe(Co) and nitrogen, which are believed to have metal-nitrogen coordination structures similar to those of macrocyclic molecules as ORR catalytic centers, is not the result of replacement of metal ions by protons. The calculation results show that electron-donating substituents could enhance the stability of Fe and Co phthalocyanines.
2016, 37(7): 1172-1179
doi: 10.1016/S1872-2067(15)61089-0
Abstract:
Rechargeable Na-O2 batteries have attracted significant attention as energy storage devices owing to their theoretically high energy storage capacity and the natural abundance of sodium. However, practical applications of this type of battery still suffer from low specific capability, poor cycle stability, instable electrolytes, and unstable polymer binders. Herein, we report a facile method of synthesizing binder free and flexible cathodes with Co3O4 nanowire arrays vertically grown onto carbon textiles. When employed as a cathode for Na-O2 batteries, this cathode exhibits superior performance, including a reduction of charge overpotential, high specific capacity (4687 mAh/g), and cycle stability up to 62 cycles. These enhanced performance can be attributed to the synergistic effect of the porosity and catalytic activity of the Co3O4 nanowire catalyst.
Rechargeable Na-O2 batteries have attracted significant attention as energy storage devices owing to their theoretically high energy storage capacity and the natural abundance of sodium. However, practical applications of this type of battery still suffer from low specific capability, poor cycle stability, instable electrolytes, and unstable polymer binders. Herein, we report a facile method of synthesizing binder free and flexible cathodes with Co3O4 nanowire arrays vertically grown onto carbon textiles. When employed as a cathode for Na-O2 batteries, this cathode exhibits superior performance, including a reduction of charge overpotential, high specific capacity (4687 mAh/g), and cycle stability up to 62 cycles. These enhanced performance can be attributed to the synergistic effect of the porosity and catalytic activity of the Co3O4 nanowire catalyst.
