Fracture Properties of Fuel Cell Membranes

Understanding the crack propagation behavior of common perfluorosulfonic acid (PFSA) ionomer membranes under hygrothermal and mechanical cyclic loading is of vital significance in designing more durable fuel cell stacks (1). Hence, we have developed a test procedure to shed light on the crack propagation characteristics of membranes as a function of loading and environmental conditions. Rectangular specimens with a width of 10 mm and cracks with an average length of 0.7 mm on both sides are used. The specimens are placed under cyclic mechanical loading in a controlled level of environmental conditions (temperature and humidity). The crack propagation rate in the membrane is measured as a function of temperature, humidity and the amplitude of mechanical loading (Figure 1). It is found that a small linear decrease in the magnitude of the force results in an exponential decrement in the crack propagation rate. Furthermore, for a given state of mechanical load, it is found that increasing the relative humidity and/or temperature can significantly increase the crack propagation rate.

In a parallel study, a fracture mechanics-based model capable of simulating the in-situ crack initiation and propagation in the membrane during fuel cell operation is also developed. The elastic-viscoplastic nature of PFSA membranes is well-established in (2). A finite element method (FEM) based constitutive model that can simulate this behavior is used to relate the stress and strain at different environmental conditions (3). Numerical simulations reveal the presence of cyclic mechanical stresses within the membrane due to dynamic hygrothermal conditions present in a running fuel cell that can cause the initiation and propagation of cracks through mechanical membrane degradation. The dynamic stress-field thus obtained is coupled with the fracture mechanics model. In-situ test cases for various temperature, humidity and strain rate conditions are run to fully understand their effect on crack propagation rates and ultimate failure of the membranes.

Acknowledgements:

This research is supported by Ballard Power Systems and the Natural Sciences and Engineering Research Council of Canada through an Automotive Partnership Canada (APC) grant.

References

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2. M.A. Goulet, R.M.H. Khorasany, C. De Torres, M. Lauritzen, E. Kjeang, G.G. Wang, N. Rajapakse, J. Power Sources 234 38 (2013).

3. R.M.H. Khorasany, M.A. Goulet, A.S. Alavijeh, E. Kjeang, G.G. Wang, R.K.N.D. Rajapakse, J. Power Sources 252 176 (2014).