UC Irvine

In response to growing demands for decarbonization, high temperature proton exchange membrane fuel cells (HT-PEMFC) have been explored as alternatives to low temperature PEMFCs due to improved fuel tolerance and efficiency and simplified thermal management systems. Phosphoric acid doped quaternary ammonium-biphosphate ion-pair coordinated polymer membranes have recently emerged as a more durable alternative to traditional phosphoric acid doped polybenzimidazole membranes. The integration of these novel membranes with protonated phosphonated poly(penta)fluorostyrene catalyst layer ionomers has not been extensively explored. This dissertation investigates the effect of humification on these ion-pair membrane HT-PEMFCs by examining the role of water in catalyst layer degradation and by developing methods to examine and quantify this degradation at 160˚C. First, we conducted a systemic study on the effect of feed gas humidification and water generation on HT-PEMFC stability. Next, catalyst accelerated stress tests were performed to compare catalyst stability under dry and humidified conditions at high temperatures. Humidification and water management were found to play a critical role in electrode durability, while temperature and phosphoric acid played less dominant roles. To quantify cell degradation at high temperatures, we developed a novel method of quantifying the electrochemical surface area and identifying surface coverage using CO displacement and CO stripping techniques. Relative humidity was found to strongly influence onset potentials of the hydrogen desorption and oxidation reactions. High coverage of phosphoric acid species was found to occur at low potentials, underscoring the extent of phosphoric acid poisoning in HT-PEMFCs. Lastly, a platinum (Pt) catalyst protected by graphene nanopockets was subjected to stability testing and catalyst accelerated stress testing. This modified catalyst demonstrated improved stability and durability relative to commercial Pt catalysts, attributed to the reduction of phosphoric acid species coverage on Pt sites. These key findings provide a guideline for future catalyst testing methodology and inform the design of catalyst materials for operation at 160˚C in phosphoric acid rich environments.