Viscoelastic Stress Model and Mechanical Characterization of Perfluorosulfonic Acid (PFSA) Polymer Electrolyte Membranes

Many of the premature failures in the PEM fuel cells are attributed to crossover of the reactant gas from pinholes or through-the-thickness flaws in the membranes. The formation of these pinholes is not fully understood, although mechanical stress is often considered one of the major factors in their initiation and/or propagation. This paper reports evidence of pinhole failure from mechanical stress by cycling between wet and dry conditions in a normally built single 50cm² fuel cell. In an effort to understand the source of the mechanical stress, to quantify the magnitude, and to correlate its role in membrane failure, a membrane stress model based on linear viscoelastic theory was developed. The effects of temperature, water content, and time are accounted for in the membrane stress model. To satisfy the inputs for the membrane model and to characterize the mechanical behavior of the polymer electrolyte membrane, a series of experiments was performed. Using commercially available Nafion® NR111 membrane as a model material, swelling of 15% and shrinkage of 4% were found from a hydration and de-hydration cycle. Data on elastic moduli versus relative humidity showed discontinuity at the vapor and liquid water transition. We also found that creep compliance master curves can be obtained by double-shifting the compliance curves according to the time-temperature-moisture superposition principle, which significantly simplifies the modeling effort. Combining data on hygro-expansion, elastic moduli, and creep compliance data through the membrane stress model, it was found that the de-hydration process induces significant stress in the membrane. Due to fluctuations in fuel cell operating conditions, the membrane and the associated components are subject to mechanical fatigue which may mechanically degrade the membrane of PEM fuel cells and eventually lead to pinhole formation.