The custom designed X-ray transparent fixture was made with a 9 mm (length) x 4 mm (width) active area and consisted of two co-flow parallel straight channels each having 1 mm width and separated by a 250 μm wide central land region with additional land regions at the two peripheral sides. After assembling a single fuel cell within it, pure mechanical degradation in form of in situ hygrothermal fatigue was generated within the membrane by subjecting the assembled fuel cell held at 80°C to successive cycles of 2 min wet and 2 min dry states with nitrogen gas on both anode and cathode sides to eliminate chemical degradation. A laboratory-based XCT system, ZEISS Xradia 520 Versa®, was used to obtain 3D tomographic images at two different length scales: (i) low resolution (2.1 μm voxel size) large field of view (FOV) scans for inspecting the overall membrane damage; and (ii) high resolution (1.1 μm voxel size) zoomed scans of selected regions of interest for a detailed structural investigation. Tomographic data of identical locations were acquired periodically at every 500 wet/dry cycles to track membrane damage development over time.
No cracks had appeared within the membrane up to 1500 wet/dry cycles, whereas a significant number of through-thickness membrane cracks had developed at 2000 cycles. This result suggests that fatigue-driven mechanical degradation progresses non-linearly over time via distinct crack initiation/propagation events. The majority of the membrane crack development occurred under the channel regions which is consistent with higher tensile stresses predicted in these regions by simulation studies . A strong correlation was observed between the presence of beginning-of-life (BOL) MEA defects, mainly cathode catalyst layer cracks and membrane—catalyst layer delamination, and eventual formation of membrane cracks at those locations (cf. Figure 1). In many cases, the shape of newly developed membrane cracks resembled that of the BOL catalyst layer cracks suggesting that localized stress concentration effects may influence both crack initiation and propagation within the membrane. Overall, the novel approach for 4D same-location tracking of membrane degradation reported in this work shows significant potential for improved fundamental understanding of the membrane crack development process during mechanical degradation in fuel cells.
Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Ballard Power Systems through an Automotive Partnership Canada grant.
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