When polymer electrolyte membrane (PEM) fuel cells are exposed to sub-freezing temperatures, the water present in the various components canfreeze resulting in ice formation. The effects of this freezing on theelectrical and mechanical properties of the components are not very wellunderstood, especially after repeated cycling between freezing andoperating temperatures. In this study we examine the effect of freeze/thawcycling on fully humidified fuel cells frozen to various sub-freezingtemperatures. The cells showed negligible performance degradation whencycled up to 40 times to as low as -40°C. However, the cells started to failwith an associated increase in the high frequency cell resistance within 10freeze/thaw cycles down to -80°C. In this paper we also report theconductivity and state of water of the Nafion membrane at sub-freezingtemperatures at various relative humidity conditions. A change inactivation energy associated with the freezing of water in the ionomer wasobserved at high relative humidities. This change in activation energy andthe associated phase transition can be avoided at low relative humidities orwhen the water is replaced with a non-freezing solvent.
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