Heterogenous Degradation of Cathode Catalyst Layers: Influence of O2 Partial Pressure and Various Stressors Under Load Cycling Conditions

Abstract

Proton exchange membrane fuel cells (PEMFCs) are an important technology to reduce greenhouse gas emissions in the heavy-duty vehicle automotive fleet. Of particular importance is achieving high performance and durability of the cathode catalyst layer (CCL) which uses platinum group metals (PGMs) to catalyze the kinetically sluggish oxygen reduction reaction (ORR).1 Understanding limits of performance under specific stressors and correlating stressors to changes in the microstructure of the membrane electrode assembly (MEA) and properties of the catalyst will enable better understanding of potential pathways to enhance durability of the PEMFCs.Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) studies have demonstrated through-plane differences in the local Pt content and nanoparticle size with regard to the gas diffusion layer (GDL)-catalyst layer interface, middle of the catalyst layer, and membrane (PEM)-catalyst layer interface.2,3 We have previously shown that there are spatial through-plane differences (e.g., F/Pt profile as a benchmark and nanoparticle size assessment) throughout the CCL depth of MEAs when ran under potential cycling accelerated stress test (AST) conditions under N2 with varied stressors (i.e., temperature, relative humidity, dwell time at upper and lower potential limits, etc.).3 It has also been demonstrated that adjusting these stressors can either mitigate or enhance the rate of degradation under potential cycling conditions.4 Historically, N2-based ASTs have been used, including in our past work, to characterize aging behavior in the CCL. Here, we present an analysis on the effect of oxygen presence in the cathode, more similar to real load/unload PEMFC operation. The elucidation of the through-plane spatial degradation with focus on influence of varied O2 partial pressure and comparison to lower temperature, relative humidity, and upper potential limit at 21% O2 will be discussed. More specifically, the differences in the F/Pt profile, nanoparticle size, depletion layer thickness, Pt band location within the membrane, etc. and its implications on real-world PEMFC testing will be presented.References(1) Cullen, D. A.; Neyerlin, K. C.; Ahluwalia, R. K.; Mukundan, R.; More, K. L.; Borup, R. L.; Weber, A. Z.; Myers, D. J.; Kusoglu, A. New Roads and Challenges for Fuel Cells in Heavy-Duty Transportation. Nat. Energy 2021, 6 (5), 462–474. https://doi.org/10.1038/s41560-021-00775-z.(2) Schneider, P.; Batool, M.; Godoy, A. O.; Singh, R.; Gerteisen, D.; Jankovic, J.; Zamel, N. Impact of Platinum Loading and Layer Thickness on Cathode Catalyst Degradation in PEM Fuel Cells. J. Electrochem. Soc. 2023, 170 (2), 024506. https://doi.org/10.1149/1945-7111/acb8df.(3) Coats, M.; Medina, S.; Braaten, J.; Cheng, L.; Craig, N.; Johnston, C.; Pylypenko, S. Quantitative Analysis of Fuel Cell Cathode Catalyst Layer Degradation Using Scanning Transmission Electron Microscopy and Energy Dispersive Spectroscopy. ECS Meet Abstr 2022, MA2022-02 1434. https://doi.org/10.1149/MA2022-02391434mtgabs.(4) Kneer, A.; Wagner, N. A Semi-Empirical Catalyst Degradation Model Based on Voltage Cycling under Automotive Operating Conditions in PEM Fuel Cells. J. Electrochem. Soc. 2019, 166 (2), F120–F127. https://doi.org/10.1149/2.0641902jes.