Impact of GDL Hole on Chemo-Mechanical Membrane Degradation Investigated by 4D in-Situ Visualization

Abstract

Membrane electrode assembly (MEA) is the core unit of a fuel cell system, which is composed of proton exchange membrane (PEM), sandwiched on both sides by catalyst layers (CLs) and gas diffusion layers (GDLs). The membrane is subjected to various chemical and mechanical stresses during fuel cell operation, and these stresses will cause membrane degradation over time. Also, the interaction between membrane and other fuel cell components such as CLs and GDLs contributes uncertainty to the membrane degradation process. Previous morphological studies [1] on CLs and GDLs discovered common non-uniform features in these materials such as clusters, sags, cracks, and holes. In addition, such features could also appear during the MEAs assembly [1] process. Previous membrane degradation study discovered buckling driven membrane failure [2], which is strongly related to CL or GDL sags, cracks, and holes. Additionally, impinging driven membrane failure was also reported [2], which is caused by clusters and peaks on CL or GDL surfaces. Models were also developed to understand the mechanism of mechanical membrane degradation related to buckling. For instance, Ramani et al. [2] created a model to simulate membrane buckling into GDL surface pores, and identified stress concentration at the buckling center as the root cause of crack formation. Comparing MEAs made with high and low surface roughness GDLs, it was confirmed that GDL with low surface roughness has significant mitigatory impact on mechanical membrane durability. However, to the authors’ best knowledge, the level of impact of such non-uniform features under combined chemo-mechanical degradation, which is the most common form of membrane degradation during regular fuel cell operation, has not yet been reported.In this work, the objective is to understand the specific impacts of GDL holes as they relate or contribute to the combined chemo-mechanical membrane degradation mechanism and associated membrane durability in polymer electrolyte fuel cells. The study was carried out using a four dimensional (three spatial dimensions plus one temporal dimension) in-situ methodology achieved through X-ray computed tomography (XCT) visualization [3], and the MEAs were made of GORE-SELECT® membrane, Pt based CLs, and AvCarb® GDLs with smooth and crack free microporous layer (MPL). Through-thickness circular GDL holes were introduced with multiple controlled sizes at multiple strategic locations, on anode or cathode GDL. The MEAs were custom designed small scale MEAs with 0.39 cm2 active area for in-situ XCT visualization, and the cells were subjected to a custom accelerated stress test (AST) protocol imparting combined chemo-mechanical stresses in an alternating pattern [3]. The graphite endplates had narrower flow field compared to previously used ones [3], which can reduce membrane degradation rate and better deliver compressive stress to the MEA. A baseline MEA without GDL hole was first tested, with only minor membrane thinning observed. For the GDL hole MEAs, it was discovered that smaller holes were more impactful on membrane durability than larger holes. Among the three selected hole sizes (2.0, 0.5, and 0.2 mm2), through-plane membrane cracks were only observed under the smallest holes, both at the hole center and edge, which are the two stress concentration locations. On the CL surface, cracks were observed at the GDL hole center as a crack network, and along the hole edge, as indicated in the figure. Although the GDL holes are expected to mainly alter mechanical membrane degradation, it was discovered that MEAs with through-plane membrane cracks also had higher membrane thinning rate compared to the baseline. However, the non-uniformity MEAs without through-plane membrane cracks had similar membrane thinning rate as the baseline test. Therefore, the through-plane membrane cracks likely elevated chemical stressors as well; hypothetically, through increased gas crossover. The AST results suggest that GDL holes can be harmful to membrane chemo-mechanical degradation and the level of severity depends on their size and location. Keywords: fuel cell; membrane durability; gas diffusion layer defect; 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.