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Assessment of CO tolerance for PEMFC Pt-alloy anode catalysts
FCBSeptember
8/28/02
9:34 AM
Page 12
RESEARCH TRENDS
characterized to determine their prospects as effective anode materials for methanol fuel cells in both acidic and alkaline media. The coatings are partially amorphous, with good corrosion resistance to H2SO4 and KOH. Heat treatment increases the alloy catalytic activity. The alloys exhibit low overvoltage even on prolonged electrolysis, and have good corrosion resistance in acidic and alkaline media. Electrochemical parameters for methanol oxidation depend on the heat treatment, deposition potential and deposit microstructure. T. Shobba, S.M. Mayanna and C.A.C. Sequeira: J. of Power Sources 108(1/2) 261–264 (1 June 2002).
Assessment of CO tolerance for PEMFC Pt-alloy anode catalysts Assessment of CO tolerance for different anode catalysts (Pt/C, PtRu/C, PtSn/C) at 50°C and PtRu/C at different operating temperatures in PEMFCs was evaluated. AC impedance measurements for oxidation of an H2+2% CO mixture reported that at a critical (onset) potential, ‘pseudo-inductive’ behavior appears, different for different anode catalysts and strongly dependent on working temperatures. The researchers propose that this onset potential for the pseudo-inductive behavior is used as a new criterion for evaluating CO tolerance for different anode catalysts at different operating temperatures. Y.-J. Leng, X. Wang and I.-M. Hsing: J. of Electroanalytical Chemistry 528(1/2) 145–152 (14 June 2002).
Plasma-treated PP composite electrolyte membranes Methanol-impermeable thin polymer electrolyte membranes were prepared by impregnating Nafion® solution in porous polypropylene (PP) membranes for DMFCs. To enhance the impregnation amount and stability of Nafion molecules, not only the solvent in commercial Nafion solution was exchanged with ethanol, but the PP membrane surface and pores were treated with Freon-116 plasma gas. Proton conductivities of the prepared Nafion/PP composite membranes were about 10 times lower, but resistance to methanol permeability was about 10 times higher than commercial Nafion membranes. The impregnation features of Nafion molecules in the plasma-treated and non-treated membrane pores were compared by examining cross-sectional membrane morphologies. B. Bae, B.-H. Chun, H.-Y. Ha, I.-H. Oh and D. Kim: J. of Membrane Science 202(1/2) 245–252 (15 June 2002).
12
Fuel Cells Bulletin
Improved PEMFC water management using a microporous sublayer A microporous PTFE/carbon sublayer between the carbon paper and catalyst layer plays a crucial role in PEMFCs, with the layer reducing any differences among carbon papers. When the sublayer is present, the carbon paper functions merely as a support that provides the sublayer’s mechanical strength. A sublayer is extremely useful if the carbon paper is prone to flooding. Sublayers of different thickness containing 24, 35 and 45% PTFE were tested. A four-cell stack could operate stably at a current density of 145 mA/cm2 using unhumidified air and hydrogen when sublayered carbon paper was employed. Z. Qi and A. Kaufman: J. of Power Sources 109(1) 38–46 (15 June 2002).
Design considerations for miniaturized PEMFCs This work considers the design of a miniaturized PEM fuel cell for 0.5–20 We portable devices. The design is implemented on silicon substrate to take advantage of advanced Si processing technology, to minimize production costs. The reduced length scales afforded by Si processing allow designs that would be prohibited by excessive ohmic losses in larger systems. The effects of the secondary current distribution on two competing cell designs are modeled. Key integration issues and engineering trade-offs relevant to all miniaturized fuel cell systems – air movement, fuel delivery and water balance, thermal management and load handling – are discussed. J.P. Meyers and H.L. Maynard: J. of Power Sources 109(1) 76–88 (15 June 2002).
Enhanced PEMFC performance by steaming or boiling the electrode Treating electrodes or catalyst-coated membranes with steam or boiling water for as short as 10 min dramatically increases their performance when subsequently tested in PEM fuel cells. The treatment was effective for numerous electrodes consisting of various types of carbon-supported Pt catalysts with different Pt loadings. It is proposed that the treatment increases the number of active sites or regions in the catalyst layer, leading to enhanced catalyst utilization. Z. Qi and A. Kaufman: J. of Power Sources 109(1) 227–229 (15 June 2002).
Suppression of methanol crossover in Pt-dispersed PEMs A new polymer electrolyte membrane has been developed to suppress methanol crossover in
DMFCs. Platinum nanocrystals were highly dispersed in a Nafion® 117 film (Pt-PEM) to catalyze oxidation of crossover methanol with O2. An appreciable increase in the cathode potential resulted from the reduced amount of methanol reaching the cathode. Distribution profiles of specific resistance in the thickness direction were measured using Pt probes inserted into the Pt-PEM under various operational conditions. Humidification of oxygen gas was effective to achieve both uniform resistance distribution and high cathode performance. H. Uchida, Y. Mizuno and M. Watanabe: J. Electrochem. Soc. 149(6) A682–687 (June 2002).
CO poisoning of hydrogen oxidation reaction in PEMFCs CO tolerance of hydrogen oxidation was investigated on nanocrystalline Pt and binary Pt alloys. Current/potential behavior was derived from half-cells under actual PEMFC operating conditions. Kinetic analyses have shown that CO poisoning on Pt/C, PtRu/C and PtSn/C catalysts occurs through a free Pt site attack mechanism, involving bridge- and linear-bonded adsorbed CO. Hydrogen oxidation currents are generated on the vacancies of a CO-adsorbed layer created when some of the bridge-bonded CO molecules are oxidized. Linearly adsorbed CO is oxidized at higher overpotentials, leading to more holes on the CO layer and thus faster hydrogen oxidation. G.A. Camara, E.A. Ticianelli, S. Mukerjee, S.J. Lee and J. McBreen: J. Electrochem. Soc. 149(6) A748–753 (June 2002).
Influence of anode flow rate/cathode oxygen pressure on CO poisoning The anode flow rate of a PEM fuel cell involving Pt anode electrocatalyst strongly influences the single cell performance when the H2 feed contains trace CO. The performance drops dramatically due to CO poisoning as the anode flow rate increases until a large overpotential is reached when it levels off. This effect is reversible, and depends on the actual CO concentration in the anode chamber which in turn depends on the feed content, flow rate and CO oxidation kinetics on Pt. Oxygen permeating across the PEM from the cathode side also appreciably affects the anode overpotential by providing another route for CO oxidation. A CO inventory model explains the observed phenomena in a PEMFC operating with H2/CO as anode feed and a cathode feed with different oxygen pressures. J. Zhang, T. Thampan and R. Datta: J. Electrochem. Soc. 149(6) A765–772 (June 2002).
September 2002
8/28/02
9:34 AM
Page 12
RESEARCH TRENDS
characterized to determine their prospects as effective anode materials for methanol fuel cells in both acidic and alkaline media. The coatings are partially amorphous, with good corrosion resistance to H2SO4 and KOH. Heat treatment increases the alloy catalytic activity. The alloys exhibit low overvoltage even on prolonged electrolysis, and have good corrosion resistance in acidic and alkaline media. Electrochemical parameters for methanol oxidation depend on the heat treatment, deposition potential and deposit microstructure. T. Shobba, S.M. Mayanna and C.A.C. Sequeira: J. of Power Sources 108(1/2) 261–264 (1 June 2002).
Assessment of CO tolerance for PEMFC Pt-alloy anode catalysts Assessment of CO tolerance for different anode catalysts (Pt/C, PtRu/C, PtSn/C) at 50°C and PtRu/C at different operating temperatures in PEMFCs was evaluated. AC impedance measurements for oxidation of an H2+2% CO mixture reported that at a critical (onset) potential, ‘pseudo-inductive’ behavior appears, different for different anode catalysts and strongly dependent on working temperatures. The researchers propose that this onset potential for the pseudo-inductive behavior is used as a new criterion for evaluating CO tolerance for different anode catalysts at different operating temperatures. Y.-J. Leng, X. Wang and I.-M. Hsing: J. of Electroanalytical Chemistry 528(1/2) 145–152 (14 June 2002).
Plasma-treated PP composite electrolyte membranes Methanol-impermeable thin polymer electrolyte membranes were prepared by impregnating Nafion® solution in porous polypropylene (PP) membranes for DMFCs. To enhance the impregnation amount and stability of Nafion molecules, not only the solvent in commercial Nafion solution was exchanged with ethanol, but the PP membrane surface and pores were treated with Freon-116 plasma gas. Proton conductivities of the prepared Nafion/PP composite membranes were about 10 times lower, but resistance to methanol permeability was about 10 times higher than commercial Nafion membranes. The impregnation features of Nafion molecules in the plasma-treated and non-treated membrane pores were compared by examining cross-sectional membrane morphologies. B. Bae, B.-H. Chun, H.-Y. Ha, I.-H. Oh and D. Kim: J. of Membrane Science 202(1/2) 245–252 (15 June 2002).
12
Fuel Cells Bulletin
Improved PEMFC water management using a microporous sublayer A microporous PTFE/carbon sublayer between the carbon paper and catalyst layer plays a crucial role in PEMFCs, with the layer reducing any differences among carbon papers. When the sublayer is present, the carbon paper functions merely as a support that provides the sublayer’s mechanical strength. A sublayer is extremely useful if the carbon paper is prone to flooding. Sublayers of different thickness containing 24, 35 and 45% PTFE were tested. A four-cell stack could operate stably at a current density of 145 mA/cm2 using unhumidified air and hydrogen when sublayered carbon paper was employed. Z. Qi and A. Kaufman: J. of Power Sources 109(1) 38–46 (15 June 2002).
Design considerations for miniaturized PEMFCs This work considers the design of a miniaturized PEM fuel cell for 0.5–20 We portable devices. The design is implemented on silicon substrate to take advantage of advanced Si processing technology, to minimize production costs. The reduced length scales afforded by Si processing allow designs that would be prohibited by excessive ohmic losses in larger systems. The effects of the secondary current distribution on two competing cell designs are modeled. Key integration issues and engineering trade-offs relevant to all miniaturized fuel cell systems – air movement, fuel delivery and water balance, thermal management and load handling – are discussed. J.P. Meyers and H.L. Maynard: J. of Power Sources 109(1) 76–88 (15 June 2002).
Enhanced PEMFC performance by steaming or boiling the electrode Treating electrodes or catalyst-coated membranes with steam or boiling water for as short as 10 min dramatically increases their performance when subsequently tested in PEM fuel cells. The treatment was effective for numerous electrodes consisting of various types of carbon-supported Pt catalysts with different Pt loadings. It is proposed that the treatment increases the number of active sites or regions in the catalyst layer, leading to enhanced catalyst utilization. Z. Qi and A. Kaufman: J. of Power Sources 109(1) 227–229 (15 June 2002).
Suppression of methanol crossover in Pt-dispersed PEMs A new polymer electrolyte membrane has been developed to suppress methanol crossover in
DMFCs. Platinum nanocrystals were highly dispersed in a Nafion® 117 film (Pt-PEM) to catalyze oxidation of crossover methanol with O2. An appreciable increase in the cathode potential resulted from the reduced amount of methanol reaching the cathode. Distribution profiles of specific resistance in the thickness direction were measured using Pt probes inserted into the Pt-PEM under various operational conditions. Humidification of oxygen gas was effective to achieve both uniform resistance distribution and high cathode performance. H. Uchida, Y. Mizuno and M. Watanabe: J. Electrochem. Soc. 149(6) A682–687 (June 2002).
CO poisoning of hydrogen oxidation reaction in PEMFCs CO tolerance of hydrogen oxidation was investigated on nanocrystalline Pt and binary Pt alloys. Current/potential behavior was derived from half-cells under actual PEMFC operating conditions. Kinetic analyses have shown that CO poisoning on Pt/C, PtRu/C and PtSn/C catalysts occurs through a free Pt site attack mechanism, involving bridge- and linear-bonded adsorbed CO. Hydrogen oxidation currents are generated on the vacancies of a CO-adsorbed layer created when some of the bridge-bonded CO molecules are oxidized. Linearly adsorbed CO is oxidized at higher overpotentials, leading to more holes on the CO layer and thus faster hydrogen oxidation. G.A. Camara, E.A. Ticianelli, S. Mukerjee, S.J. Lee and J. McBreen: J. Electrochem. Soc. 149(6) A748–753 (June 2002).
Influence of anode flow rate/cathode oxygen pressure on CO poisoning The anode flow rate of a PEM fuel cell involving Pt anode electrocatalyst strongly influences the single cell performance when the H2 feed contains trace CO. The performance drops dramatically due to CO poisoning as the anode flow rate increases until a large overpotential is reached when it levels off. This effect is reversible, and depends on the actual CO concentration in the anode chamber which in turn depends on the feed content, flow rate and CO oxidation kinetics on Pt. Oxygen permeating across the PEM from the cathode side also appreciably affects the anode overpotential by providing another route for CO oxidation. A CO inventory model explains the observed phenomena in a PEMFC operating with H2/CO as anode feed and a cathode feed with different oxygen pressures. J. Zhang, T. Thampan and R. Datta: J. Electrochem. Soc. 149(6) A765–772 (June 2002).
September 2002