Investigation of Pt-Based Nanoparticle Degradation in Catalyst Layers through Automated Electron Tomography

Proton exchange membrane fuel cells (PEMFC) are promising devices for the deployment of hydrogen-powered heavy-duty vehicles, which provide higher efficiencies than internal combustion engine vehicles and similar driving ranges and fueling times. However, PEMFCs still encounter durability challenges, mainly due to catalyst degradation in the cathode. Mitigating these performance losses requires a better understanding of the degradation mechanisms under heavy-duty accelerated stress tests (ASTs) ¹. Electron tomography is a key tool for high-resolution three-dimensional analysis of Pt and PtCo nanoparticle spatial distribution to determine the rate and type of degradation as a function of position on the carbon support ².<br/><br/>In this work, we developed an automated electron tomography workflow performed in scanning transmission electron microscopy (STEM) mode and utilizing SerialEM ³ for data acquisition. We applied a super-voxel model-based iterative reconstruction with adaptive regularization (MBIR-ARAR) for artifact reduction ⁴ and performed segmentation of the nanoparticles and carbon using the commercial software Avizo. This workflow allowed us to differentiate Pt-based nanoparticles located on the exterior of the carbon support from those within the pore structure, which enabled assessment of the relative stability of interior versus exterior nanoparticles.<br/><br/>In this study, the catalyst powder was scraped from electrodes and drop-cast on tomography TEM grids. We compared Pt and PtCo catalyst particle size distributions at the beginning of test (BOT) and end of test (EOT). The findings indicated a noticeable increase in particle size, regardless of location, and a significant portion of particles remaining within the carbon pores. Simultaneously, there is evidence of an expanding size distribution of closed pores after the AST, possibly stemming from carbon corrosion. These observed degradation mechanisms challenge the assumption that Pt dissolving primarily on the carbon support interior and re-depositing on the surface is a major contributor to ECSA decline, suggesting that factors such as carbon corrosion, dissolution of exterior nanoparticles, and/or particle coalescence also contribute to ECSA loss. This shows the importance of automating the electron tomography workflow, i.e., acquisition, reconstruction, and visualization, for increasing sampling and determining statistical parameters for the measurements. The outlook for utilizing low-dose cryo-tomography for limiting damage to the catalyst, support, and especially proton-conducting ionomer will be discussed ⁵.<br/><br/><u>References:</u><br/><br/>1. Cullen, D. A. <i>et al.</i> New roads and challenges for fuel cells in heavy-duty transportation. <i>Nat. Energy</i> <b>6</b>, 462–474 (2021).<br/>2. Padgett, E. <i>et al.</i> Editors’ Choice—Connecting Fuel Cell Catalyst Nanostructure and Accessibility Using Quantitative Cryo-STEM Tomography. <i>J. Electrochem. Soc.</i> <b>165</b>, F173 (2018).<br/>3. Mastronarde, D. N. Automated electron microscope tomography using robust prediction of specimen movements. <i>J. Struct. Biol.</i> <b>152</b>, 36–51 (2005).<br/>4. Ziabari, A. <i>et al.</i> Reducing Artifacts in BF and HAADF-STEM Images of Pt/C Fuel Cells using MBIR-ARAR. <i>Microsc. Microanal.</i> <b>29</b>, 1385–1387 (2023).<br/>5. Girod, R., Lazaridis, T., Gasteiger, H. A. & Tileli, V. Three-dimensional nanoimaging of fuel cell catalyst layers. <i>Nat. Catal.</i> <b>6</b>, 383–391 (2023).<br/><br/>This material is based on work performed by the Million Mile Fuel Cell Truck (M2FCT) Consortium, technology manager Greg Kleen, which is supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office. Electron microscopy research was supported by the Center for Nanophase Materials Sciences (CNMS), which is a US Department of Energy, Office of Science User Facility at Oak Ridge National Laboratory.