On mechanical behavior and in-plane modeling of constrained PEM fuel cell membranes subjected to hydration and temperature cycles

Currently, ionomer membranes are used in a variety of specialized applications. Such applications include, but are not limited to, dialysis, electrolysis, membrane separators, reaction catalysts and the most promising application: polymer electrolyte fuel cells. Although their use is widespread, significant gaps in understanding the mechanical behavior of these materials still remain. Many ionomer membranes change their structure, and in turn, their mechanical properties in response to applied thermal and moisture conditions that are functions of position. It has been observed that constrained materials subjected to changing environmental conditions can exhibit unusual behavior, e.g., in some cases, mechanical failure is seen in the absence of external applied mechanical loads. This condition is especially important in polymer membranes (specifically Nafion®) used in polymer electrolyte membrane (PEM) fuel cells and is the major motivation of the present work. Laboratory characterization has been conducted to determine the mechanical properties of a proton exchange membrane with respect to temperature and relative humidity. Data recovered in these tests along with properties from literature have been used in finite element models to predict the behavior of membranes used in certain applications and geometries. The overall goal of this investigation was to characterize the mechanical response of ionomer membranes in in-plane constraint configurations subjected to variable hygro-thermal environments. Expansion/contraction mechanical response of the constrained membrane as a result of change in hydration and temperature is studied in uniform and non-uniform geometries and environments. With this information, mechanical failure modes can be analyzed which is necessary for durability modeling and life prediction. The present work concentrates on defining and understanding the basic mechanical behavior of ionomeric membranes clamped in a rigid frame, and subjected to changes in temperature and humidification.