The effect of humidity on the chemical stability of two types of membranes ͓i.e., perfluorosulfonic acid type ͑PFSA, Nafion 112͒ and biphenyl sulfone hydrocarbon type, ͑BPSH-35͔͒ was studied by subjecting the membrane electrode assemblies ͑MEAs͒ to open-circuit voltage ͑OCV͒ decay and potential cycling tests at elevated temperatures and low inlet-gas relative humidities. The BPSH-35 membranes showed poor chemical stability in ex situ Fenton tests compared to that of Nafion membranes. However,

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... der fuel cell conditions, BPSH-35 MEAs outperformed Nafion 112 MEAs in both the OCV decay and potential cycling tests. For both membranes, ͑i͒ at a given temperature, membrane degradation was more pronounced at lower humidities and ͑ii͒ at a given relative humidity operation, increasing the cell temperature accelerated membrane degradation. Mechanical stability of these two types of membranes was also studied using relative humidity ͑RH͒ cycling. Due to decreased swelling and contraction during wet-up and dry-out cycles, Nafion 112 lasted longer than BPSH-35 membranes in the RH cycling test.