Fracture Properties of Catalyst Coated Membranes

Recent studies have shown that a suitable approach for analyzing the mechanical durability of fuel cell stacks is to study the mechanical behavior of catalyst coated membranes (CCMs) under hygrothermal loading conditions [1–4]. Previous studies have also indicated that under cyclic mechanical and hygrothermal loadings, cracks are initiated and propagated inside the membrane [5,6] eventually leading to the cell/stack failure. The crack propagation rate of pure membranes is found to be a strong function of applied stress, temperature, and humidity [7]. Given the significant difference in mechanical properties of pure membrane and CCM [1], their crack propagation characteristics can also be expected to differ. In an operating fuel cell, this implies that a crack could potentially propagate differently when it has penetrated through the entire CCM compared to when it is confined within the membrane. To investigate this, a series of experiments were conducted to characterize the rate of crack propagation in the CCM. Rectangular specimens with a width of 10 mm and artificially created double edge cracks were used. The initial crack length on each side was approximately 0.7 mm. Under pre specified levels of environmental conditions (temperature and relative humidity), the specimens were subjected to cyclic mechanical loading [3–6] which led to crack propagation as shown in Fig. 1. The rate of crack propagation as a function of loading and environmental conditions was measured and analyzed. It is found that at elevated levels of relative humidity and temperature, the sensitivity of propagation rate to the applied mechanical loading increases.

In a parallel study, a fracture mechanics model based on Paris Law theory and capable of simulating the ex situ crack propagation in the CCM during typical fuel cell operating conditions is developed. The model incorporates the characteristic time, temperature, and humidity dependent elastic-viscoplastic mechanical behaviour of CCMs [1] through a sub model developed using the finite element method (FEM) in COMSOL Multiphysics^®. The stress-strain relationship of CCM simulated by the FEM sub model is validated at all combinations of  23 ºC and 70ºC temperature, 50% and 90% relative humidity; and 0.0001 s^-1 and 0.001 s^-1 strain rates. Fundamental fracture mechanics parameters, viz. J-integral, stress intensity factor (K), and configuration correction factor (ccf) are obtained iteratively for incremental changes in the crack length. These parameters together with the experimental crack propagation data enable the construction of Paris Curves at various temperature and humidity conditions. Information from the Paris Curves is used to predict the time taken by a CCM crack to increase from initial crack length a_i to final crack length a_f under typical fuel cell conditions.

The CCM crack propagation data collected and simulation capability developed during this work are considered to be important contributions towards developing a holistic understanding of mechanical fatigue and fracture phenomenon which are active during fuel cell operation and which ultimately lead to its failure. 

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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