Hydrocarbon Proton Exchange Membranes for Fuel Cells: Do We Need New Chemical Durability Testing Protocols?

The continued development of hydrocarbon proton exchange membranes (PEMs) for fuel cell applications has brought them close to commercial viability, increasing interest in their upscaled production and operational testing. The chemical accelerated stress test (AST) protocol developed by the US Department of Energy (DOE) for perfluorinated membranes has been widely adopted to evaluate membrane durability in fuel cells. However, due to different degradation mechanisms, its utility for hydrocarbon PEMs is questionable and may obscure critical degradation pathways. To design a more suitable protocol for hydrocarbon PEMs, the effects of operating relative humidity (RH) and O2 partial pressure (pO2) during the AST were investigated in this work. Sulfonated poly(ether ether ketone) (sPEEK) was selected as a model PEM for study. It was found that degradation accelerates significantly with increasing RH and pO2, with a 15 times higher loss rate of ion exchange capacity (IEC) at open circuit voltage (OCV) under 100% RH, H2/O2, than under 30% RH, H2/air (DOE conditions). Effluent water analysis via UV–Vis and ion chromatography (IC) complements our findings and provides insight into the RH/pO2 dependent degradation mechanisms in sPEEK. The new AST conditions can not only provide valuable insight into critical stability aspects of hydrocarbon PEMs but also speed up material screening: at 100% RH, H2/O2, the tested membranes failed reliably within 40 h, while lasting 775 h under the DOE conditions.