Impact of Irregular Microporous Layer on Chemo-Mechanical Membrane Degradation Investigated by 4D in-Situ Fuel Cell Visualization

Membrane durability is critical to the performance and safe operation of proton exchange membrane fuel cells (PEMFC). To address chemical and mechanical membrane degradation mechanisms, substantial mitigation strategies have been developed to enhance the strength of membranes. However, as a system, it is also important to make sure that the membrane electrode assembly (MEA) subcomponents and their integration do not produce unexpected non-uniformities in the manufacturing process, which may lead to premature membrane failure. The interaction between the membrane and gas diffusion media, such as gas diffusion layers (GDLs) and microporous layers (MPLs), was deemed as one of the major root causes for many membrane failure modes [1]. Previously reported mechanisms include elevated membrane buckling induced by micro sags or voids on MPL surface, and local impingement due to GDL fiber protrusion [1]. These non-uniformity features within the MPL and GDL can be natural irregularities from manufacturing, or unexpected damage during handling, and are difficult to control and characterize due to the random nature of the porous fibrous structure of the GDL. In a previous talk [2], the impact of though-plane GDL holes was discussed as they may relate or contribute to the combined chemo-mechanical membrane degradation mechanism. Both accelerated stress testing (AST) and X-ray computed tomography (XCT) imaging results suggest that GDL holes can be harmful to chemical and mechanical membrane degradation and the level of severity depends on their size and location. As a result, further in-depth research is warranted to capture the salient interactions between membrane degradation mechanisms and GDL non-uniformities. The objective of the present work is to determine the influences of missing MPL spots on fuel cell membrane chemo-mechanical durability. Compared to through-plane GDL holes, the fiber structure of the GDL was left intact, while only the MPL layer was completely removed at select locations. The study was carried out using a previously disclosed four dimensional (three spatial dimensions plus one temporal dimension) in-situ XCT visualization technique [3], where membrane degradation was traced at different life stages over time. The MEAs used in this work were composed of GORE-SELECT® membrane, crack free Pt based catalyst layers (CLs), and AvCarb® GDLs with customized smooth and crack free MPL. Circular missing MPL spots were artificially created by laser micromachining, with precisely controlled laser beam energy to only remove the MPL without damaging the GDL substrate. Missing MPLs of multiple controlled sizes were placed at strategic locations, both under flow channels and lands, on anode or cathode GDLs. The MEAs were custom designed small scale MEAs [3] for in-situ XCT visualization, and were subjected to a custom AST protocol imparting combined chemo-mechanical stresses [3] in an alternating pattern. The results confirmed that missing MPL spots are indeed harmful to the membrane durability, mainly through GDL fiber impingement. The impact level of the missing MPL non-uniformity highly depends on its location. Under flow channels, membrane buckling and associated CL crack formation were the major failure modes induced by missing MPL. Regardless of which electrode had the missing MPL, CL cracks tended to initiate either from GDL impinging locations or within voids of the GDL substrate. However, both membrane buckling and CL cracks were moderate, and their influence on MEA performance and durability was minor. Under the supporting lands, GDL impinging and membrane penetration became the major failure modes instead of membrane buckling and CL cracks, which was assessed to be due to the elevated through-plane compression. GDL impinging can be detrimental to MEA performance and membrane durability by penetrating through the membrane and raise electrode shorting, which was validated through a control experiment with MPL-free GDL. Keywords: fuel cell; membrane durability; gas diffusion layer; mechanical degradation; chemical degradation; X-ray computed tomography Acknowledgements: This research was supported by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, Western Economic Diversification Canada, Ballard Power Systems, and W.L. Gore & Associates. This research was undertaken, in part, thanks to funding from the Canada Research Chairs program. References: [1] D. Ramani, N.S. Khattra, Y. Singh, F.P. Orfino, M. Dutta, E. Kjeang, Journal of Power Sources 512 (2021) 230431. [2] Y. Chen, A. Bahrami, N. Kumar, F.P. Orfino, M. Dutta, E.N. Alvar, M. Lauritzen, E. Setzler, A. Agapov, E. Kjeang, Meet. Abstr. MA2023-02 (2023) 1780. [3] Y. Chen, M. Bahrami, N. Kumar, F.P. Orfino, M. Dutta, M. Lauritzen, E. Setzler, A.L. Agapov, E. Kjeang, J. Electrochem. Soc. 170 (2023) 114526. Figure 1