3D Analysis of PEM Fuel Cell Membrane Cracks Using X-Ray Computed Tomography
An end-of-life (EOL) MEA sample, subjected to cyclic open circuit voltage (COCV) accelerated stress test (AST) protocol , is analyzed upon failure using the XCT technique and compared with a similar beginning-of-life (BOL) sample. Based on their reach in the through-plane direction, cracks penetrating the entire membrane thickness are categorized into: (i) Exclusive cracks that remain confined within the membrane; and (ii) Non-exclusive cracks that extend into and penetrate through the anode and/or cathode catalyst layers. The very presence of Exclusive cracks (cf.Fig. 1), without any adjoining cracks in the catalyst layers, indicates the likelihood of crack initiation within the membrane. An examination of 9 membrane cracks spread over a 0.88 mm2area reveals that more than 50% of the cracks fall into the Exclusive category. This percentage is found to be significantly higher than the 13% overall interaction of catalyst layer cracks with the membrane cracks.
In the in-plane direction, some membrane cracks are found to propagate as a single entity forming a curved I-shape while the others branch out once forming a Y-shape. Distribution of the two shapes is approx. equal among the analyzed cracks. The smaller Y-cracks (<20 μm average branch length), which are likely to be in their initial phase of development, tend to propagate at equal rate in all three directions.
The total in-plane crack length in the membrane is found to have a linear relationship with the maximum crack width. The crack width remains almost uniform along its length and tapers sharply at the ends indicating that the crack propagation in the membrane is mechanical in nature caused by the cyclic in-plane stresses. The effect of these stresses seems to have been exacerbated by the observed non-uniform reduction in membrane thickness resulting in stress concentration locations within the membrane. During this analysis, a strong probability for crack development is observed at such locations with membrane thickness found to be approx. 30% lower at the crack sites than its average EOL bulk value.
The work reported here is unprecedented from the perspective that, for the first time, a truly 3D view of a membrane crack is achieved; thus, making this comprehensive analysis possible. These results, establish the XCT as a breakthrough approach for reliable failure analysis of fuel cell membranes.
This research is financially supported by Ballard Power Systems and Automotive Partnership Canada (APC). The authors thank Chan Lim, Lida Ghassemzadeh and Erin Rogers for providing samples and technical support.
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