Freeze/thaw effects in PEM fuel cells

When polymer electrolyte membrane (PEM) fuel cells are exposed to sub-freezing temperatures, the water present in the various components can freeze resulting in ice formation. The effects of this freezing on the electrical and mechanical properties of the components are not very well understood, especially after repeated cycling between freezing and operating temperatures. In this study we examine the effect of freeze/thaw cycling on fully humidified fuel cells frozen to various sub-freezing temperatures. The cells showed negligible performance degradation when cycled up to 40 times to as low as-40oC. However, the cells started to fail with an associated increase in the high frequency cell resistance within 10 freeze/thaw cycles down to-80oC. In this paper we also report the conductivity and state of water of the Nafion ® membrane at sub-freezing temperatures at various relative humidity conditions. A change in activation energy associated with the freezing of water in the ionomer was observed at high relative humidities. This change in activation energy and the associated phase transition can be avoided at low relative humidities or when the water is replaced with a non-freezing solvent