Chemical and Mechanical Degradation of ePTFE-Reinforced Perfluorinated Sulfonic Acid Membranes in PEMFCs

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The durability of proton exchange membranes (PEMs) is a critical factor influencing the reliability and longevity of PEM fuel cells (PEMFCs). In the quest for durable fuel cell systems, expanded polytetrafluoroethylene (ePTFE) reinforced ionomer membranes stand as a promising avenue for designing membrane electrode assembly (MEA) compared to plain perfluorinated sulfonic acid (PFSA) membranes. The incorporation of ePTFE reinforcement introduces a complex microstructure that influences the membrane's ability to withstand chemical attack, mechanical stress, and oxidative damage during operation1,2. Despite several recent studies demonstrating enhanced durability in accelerated stress testing (AST)3–5, the intricate relationship between the membrane's microstructure, ePTFE reinforcement, and chemo-mechanical degradation in fuel cell operation has not been explored. In the present work, the durability of MEAs composed with ePTFE-reinforced ionomer membranes of 15 µm nominal thickness in PEMFCs was investigated using a combined chemical and mechanical accelerated stress test (CM-AST). The chemo-mechanical degradation of the ePTFE-reinforced membranes was characterized using X-ray computed tomography (XCT). The cell exhibited two degradation stages under CM-AST. The initial phase of 25 AST cycles was marked by a notable decline in cell performance. Subsequently, in the second phase, the cell maintained consistent operation until the conclusion of testing at the 65th cycle. The cell degradation rate for the first stage was 62 µV h-1 and the overall degradation rate was 58 µV h-1. The hydrogen crossover current density slowly increased from 3.3 mA cm−2 to 5.1 mA cm-2 at 80 °C under atmospheric pressure. A comprehensive analysis of cells using XCT showed significant decay of the catalyst layers and membrane thinning. 3D visualization by XCT scans of the entire active surface area confirmed the absence of pinholes or cracks in the membrane that explained the observed low hydrogen crossover. Chemical degradation was observed in the ionomer-rich layers, while no signs of ePTFE reinforcement degradation were found. Therefore, the reinforcement layer plays a key role in mitigating the chemical and mechanical degradation by limiting the gas crossover rather than altering the degradation process. The results concluded that by optimizing the microstructure of ePTFE, reinforced membranes can pave the way for more reliable and durable PEMFC systems.Keywords: Composite ionomer membranes, ePTFE reinforcement, PEMFC, chemo-mechanical degradation, accelerated stress testing, X-ray computed tomography References (1) Liu, W.; Suzuki, T.; Mao, H.; Schmiedel, T. Development of Thin, Reinforced PEMFC Membranes through Understanding of Structure-Property-Performance Relationships. ECS Trans. 2013, 50 (2), 51–64. https://doi.org/10.1149/05002.0051ecst.(2) Shi, S.; Weber, A. Z.; Kusoglu, A. Structure/Property Relationship of Nafion XL Composite Membranes. Journal of Membrane Science 2016, 516, 123–134. https://doi.org/10.1016/j.memsci.2016.06.004.(3) Ramani, D.; Singh, Y.; White, R. T.; Wegener, M.; Orfino, F. P.; Dutta, M.; Kjeang, E. 4D in Situ Visualization of Mechanical Degradation Evolution in Reinforced Fuel Cell Membranes. International Journal of Hydrogen Energy 2020, 45 (16), 10089–10103. https://doi.org/10.1016/j.ijhydene.2020.02.013.(4) Sadeghi Alavijeh, A.; Bhattacharya, S.; Thomas, O.; Chuy, C.; Kjeang, E. A Rapid Mechanical Durability Test for Reinforced Fuel Cell Membranes. Journal of Power Sources Advances 2020, 2, 100010. https://doi.org/10.1016/j.powera.2020.100010.(5) Tang, H.; Pan, M.; Wang, F.; Shen, P. K.; Jiang, S. P. Highly Durable Proton Exchange Membranes for Low Temperature Fuel Cells. J. Phys. Chem. B 2007, 111 (30), 8684–8690. https://doi.org/10.1021/jp073136t.