Abstract
Water management is a serious concern for alkaline-exchange-membrane fuel cells (AEMFCs) because water is a reactant in the alkaline oxygen-reduction reaction and hydroxide conduction in alkaline-exchange membranes is highly hydration dependent. In this paper, we develop and use a multiphysics, multiphase model to explore water management in AEMFCs. We demonstrate that the low performance is mostly caused by extremely non-uniform distribution of water in the ionomer phase. A sensitivity analysis of design parameters including humidification strategies, membrane properties, and water transport resistance was undertaken to explore possible optimization strategies. Furthermore, the strategy and issues of reducing bicarbonate/carbonate buildup in the membrane-electrode assembly with CO2 from air is demonstrated based on the model prediction. Overall, mathematical modeling is used to explore trends and strategies to overcome performance bottlenecks and help enable AEMFC commercialization.
Highlights
Performance is mainly dominated by kinetics at both electrodes and dehydration of the cathode catalyst layer
H2 mass-transport limitations can become limiting due to anode flooding
Performance is governed by sluggish OH− conduction and oxygen-reduction-reaction kinetics in the cathode due to water deficiency at high current density
Summary
A dynamic electrochemical model of an alkaline fuel-cell stack was developed by Duerr to investigate the effects of the load changes on various fuel-cell parameters, such as electrolyte concentration and inlet gas pressure.[18] Water management in an alkaline fuel cell and AEMFC was investigated by Verhaert[19,20] and Jiao,[21] respectively, indicating that the performance is generally improved with anode and cathode humidification. Cathode drying is a critical water management issue for performance, and a possible reason for poor durability of cathode ionomers It is not clear yet what is the root cause for such a performance-limiting mechanism both experimentally and theoretically, which is the focus of this current study. Mustain and coworkers have recently demonstrated very high power densities after water management aspects were considered in terms of humidification and minimizing dryout and flooding of both the cathode and anode.[24]
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