Determining the Critical Plastically Dissipated Energy for Fatigue Crack Growth in PEM Fuel Cell Membrane and Its Environmental Sensitivity

Durability of membranes, remains a major obstacle to widespread commercialization of polymer electrolyte membrane fuel cells in the automotive industry. Chemical and mechanical stressors play a role, but the general consensus is that mechanical fatigue, caused by swelling and deswelling of the membrane, is a major contributing factor in its degradation and failure. In order to better understand and predict the fatigue behavior, this study outlines a method to determine the critical plastically dissipated energy (C-PDE) for a typical fuel cell membrane material, Nafion NRE 211. The method employs a numerical simulation of a fatigue test specimen, along with one data point for the Paris-regime fatigue crack growth for that specimen. The results show that the C-PDE, like other properties of Nafion, is a function of both temperature and humidity of the test conditions. The C-PDE decreases with increasing temperature and exhibits a nonmonotonic dependence on humidity. At 50% RH, increasing the temperature from 23 degrees C to 70 degrees C results in a drop of the C-PDE for NRE 211 from 3.72 mJ/mm to 1.69 mJ/mm while at 90% RH, the same temperature increase results in a drop of C-PDE from 7.69 mJ/mm to 0.873 mJ/mm. Under a given environmental condition, the C-PDE determined from a single data point, is used to predict the fatigue crack growth for other loads in the Paris-regime and the results are compared to additional experimental data for verification. The numerical and experimental results are in generally good agreement, showing that future characterization of the material using this model, under a wide range of environmental conditions, can be conducted with minimal laboratory testing.