Pore-Scale Modeling of Platinum Degradation in PEM Fuel Cells

Abstract

Heavy-duty vehicles rely primarily on diesel engines and produce disproportionately higher emissions in the transportation sector. They also lack feasible zero-emission alternatives due to their demanding range and load requirements. Polymer electrolyte membrane (PEM) fuel cells can offer a promising solution due to their long range, low weight, and fast refueling times. However, their application for heavy-duty vehicles is limited by durability issues which limit the fuel cell lifetime. In particular, platinum catalyst degradation in the cathode catalyst layer (CCL) is a critical degradation mechanism. Understanding the mechanisms of platinum degradation through physics-based modeling is essential for developing more durable fuel cells. This thesis first presents an in-depth literature review of physics-based modeling approaches for platinum degradation, classifying the most widely used modeling frameworks and examining how recent studies have utilized and expanded upon them. The review then identified a key gap: the lack of pore-scale models capable of investigating the effects of the CCL microstructure and spatial properties on platinum degradation. To address this gap, a transient pore-scale model of platinum degradation is first developed using pore network modeling. The model couples a performance solver (which simulates the oxygen reduction reaction) with a degradation solver (which simulates the degradation mechanisms of platinum dissolution, oxidation, and mass loss to the membrane). The model is able to reproduce key degradation trends including spatial variations in surface area loss and transient changes in platinum oxide coverage and ion concentration. Importantly, the pore-scale resolution reveals new microstructural insights, such as how constrictions between agglomerates lead to localized degradation and how the length of transport path correlates with platinum loss. Building on the model developed, microstructural and spatial design strategies were systematically investigated. Strategies through modifications in the porosity, agglomerate size, and platinum placement were evaluated for their effects on performance and degradation. It was found that: (1) increasing porosity uniformly or near the membrane reduces platinum loss but worsens performance, although mid-CCL constrictions can minimize this trade-off, (2) reducing agglomerate size enhances performance and has negligible impact on degradation, and (3) increasing platinum loading near the membrane through larger particle sizes promotes more uniform degradation. A combined case study integrating these strategies demonstrates simultaneous improvements in initial performance, total degradation, and degradation uniformity. These results indicate that while individual strategies have distinct trade-offs, combining complementary strategies can retain their advantages while minimizing the drawbacks. Overall, the works presented in this thesis demonstrate the utility of pore-scale modeling for uncovering microstructural insights and identifying mitigation strategies for platinum degradation through the CCL design, which can ultimately contribute to the advancement of PEM fuel cells as a viable clean energy technology for heavy-duty vehicles.