Hydrocarbon\nProton Exchange Membranes for Fuel Cells:\nDo We Need New Chemical Durability Testing Protocols?

The continued development of hydrocarbon\nproton exchange\nmembranes\n(PEMs) for fuel cell applications has brought them close to commercial\nviability, increasing interest in their upscaled production and operational\ntesting. The chemical accelerated stress test (AST) protocol developed\nby the US Department of Energy (DOE) for perfluorinated membranes\nhas been widely adopted to evaluate membrane durability in fuel cells.\nHowever, due to different degradation mechanisms, its utility for\nhydrocarbon PEMs is questionable and may obscure critical degradation\npathways. To design a more suitable protocol for hydrocarbon PEMs,\nthe effects of operating relative humidity (RH) and O<sub>2</sub> partial\npressure (<i>p</i><sub>O2</sub>) during the AST were investigated\nin this work. Sulfonated poly(ether ether ketone) (sPEEK) was selected\nas a model PEM for study. It was found that degradation accelerates\nsignificantly with increasing RH and <i>p</i><sub>O2</sub>, with a 15 times higher loss rate of ion exchange capacity (IEC)\nat open circuit voltage (OCV) under 100% RH, H<sub>2</sub>/O<sub>2</sub>, than under 30% RH, H<sub>2</sub>/air (DOE conditions). Effluent\nwater analysis via UV–Vis and ion chromatography (IC) complements\nour findings and provides insight into the RH/<i>p</i><sub>O2</sub> dependent degradation mechanisms in sPEEK. The new AST conditions\ncan not only provide valuable insight into critical stability aspects\nof hydrocarbon PEMs but also speed up material screening: at 100%\nRH, H<sub>2</sub>/O<sub>2</sub>, the tested membranes failed reliably\nwithin 40 h, while lasting 775 h under the DOE conditions.