Understanding the Impact of Microporous Layer Cracks on Pt Catalyst Degradation

As the need for more flexible utilization of renewable energy sources increases, the uses of clean hydrogen and fuel cells are pursued by governments worldwide due to high gravimetric energy density of hydrogen. Polymer electrolyte fuel cells (PEFCs) have demonstrated great potential in passenger light-duty vehicles (LDVs), and their applications in heavy-duty vehicles (HDVs) have recently attracted significant attention due to their unique scalability1. Currently, the micro porous layer (MPL), the side of the gas diffusion layer that faces the catalyst layer in the membrane electrode assembly (MEA), is known to improve the performance of PEFCs at high current densities due to better heat, water management, and electron transport. And there has been an ongoing debate whether changes in water management through having cracks in the MPL will improve the durability of the catalyst. Cracks in the MPL are advantageous as they can facilitate removal of liquid water from the catalyst layer. However, too many cracks can result in poor contact between the MPL and the MEA2. Cracks can also pool water and block oxygen transport. In this study, electrocatalyst accelerated stress tests (ASTs) were performed on the MEAs prepared using MPLs with different density and configuration of cracks. The ASTs had a total of 90 000 cycles, with electrochemical characterization performed at beginning of life, after 10 000, 30 000, 60 000 and 90 000 (end of life). Each cycle consisted of potential cycling between 0.6 V to 0.9 V, with 3 seconds hold time for each. The effect of cracks in MPLs on electrocatalyst degradation was evaluated in both nitrogen (non-reactive) and air (reactive) gas environments at 100% relative humidity, 80oC, and 150 kPa backpressure. In-situ electrochemical characterization and extensive physical characterization was performed to understand the subtle differences in electrocatalyst degradation correlated to the use of crack-free and cracked MPLs. In particular, micro-X-ray fluorescence (XRF) map of identical locations before and after the AST were employed to study in-plane movement of Pt loading over the course of AST. Micro x-ray diffraction (XRD) maps were collected on the MEAs to correlate Pt loadings to Pt particle size distribution. The polarization curves, Tafel plots, and electrochemical surface area (ECSA) for MEAs with and without cracks in the MPLs all demonstrated similar trends in the cells’ durability, revealing that cracks in the MPLs do not significantly affect the performance of the cell at the beginning and also at the end of life. However, the XRF identical location maps showed significant in-plane movement of Pt over the course of the AST: decrease of Pt loading in the mapped region closer to the gas inlet and increase of Pt loading closer to the gas outlet. Moreover, higher degree of cracking was observed at the cell outlet after the AST, indicating that the catalyst layer mechanical aging is not homogeneous. References: Cullen, D. A. et al. New roads and challenges for fuel cells in heavy-duty transportation. Nat. Energy 6, 462–474 (2021). Gostick, J. T. et al. On the role of the microporous layer in PEMFC operation. Electrochem. Commun. 11, 576–579 (2009).