Rapid fuel cell performance degradation was observed after just 30 cold start/hot stop cycles. Relative humidity (RH) was found to be the primary operating parameter associated with the rapid voltage decay during this thermal cycling. It was found that repeated thermal cycling could affect the RH inside a fuel cell and, consequently, alter the water activity of membrane/ionomer. We propose an extension of the generally accepted reverse-current carbon corrosion mechanism to account for the RH effects. The rapid voltage decay was linked to a RH-dependent cathode carbon corrosion process which was attributed to a lag in the ionomer/membrane hydration state during the thermal cycling. It has been demonstrated that one can reduce the carbon corrosion rate by avoiding the combination of cold starts and hot stops. Recommended strategies for mitigation are discussed in the context of the corrosion mechanism.
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