Author: Wong, Ka Hung
Chemical membrane degradation is a major limiting factor for polymer electrolyte fuel cell (PEFC) durability and lifetime. While the effects of chemical membrane degradation are characterized in the literature, the underlying mechanism is not fully understood. This motivates the development of a comprehensive in situ chemical membrane degradation model addressed in this work to determine the linkages between the membrane electrolyte assembly (MEA) macroscopic phenomena, in situ operating conditions, and the temporal membrane degradation process. Chemical membrane degradation through OH radical attack on the membrane, where the radical is produced by decomposition of hydrogen peroxide in the presence of contaminants such as Fe2+, is comprehensively investigated. A redox cycle of iron ions is discovered within the MEA which sustains the Fe2+ concentration in the membrane and results in the most severe chemical degradation at open circuit voltage (OCV). The cycle is suppressed at lower cell voltages leading an exponential decrease in Fe2+ concentration in the membrane and associated membrane degradation rate, which suggests that intermediate cell voltage operation would efficiently mitigate chemical membrane degradation and extend the fuel cell lifetime. Effectiveness of membrane additives (e.g., ceria) in mitigating the membrane degradation is explored. At high cell voltages, abundant Ce3+ ions are available in the membrane to quench hydroxyl radicals which is the primary mitigation mechanism observed at OCV conditions. However, the mitigation is suppressed at low cell voltages, where electromigration drives Ce3+ ions into the cathode catalyst layer (CL). Without an adequate amount of Ce3+ in the membrane, the hydroxyl radical scavenging is significantly reduced. Moreover, the modeling results reveal that proton starvation may occur in the cathode CL due to local Ce3+ accumulation and associated reductions in proton conductivity and oxygen reduction kinetics. Significant performance tradeoffs in the form of combined ohmic and kinetic voltage losses are therefore evident. A lower initial Ce3+ concentration is demonstrated to mitigate voltage losses without compromising membrane durability at high cell voltages. However, the harmful Fe2+ concentration in the membrane increases with the Ce3+ concentration, which suggests that ceria-supported MEAs can experience higher rates of degradation than baseline MEAs at low cell voltages. Strategic MEA design is recommended in order to ensure membrane durability at low cell voltages.
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Thesis advisor: Kjeang, Erik
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