Recreating Fuel Cell Catalyst Degradation in Aqueous Environments for Identical-Location Scanning Transmission Electron Microscopy Studies

Abstract

Proton exchange membrane fuel cells (PEMFCs) have long been studied for applications in sustainable transportation. The recent shift in focus from passenger vehicle to heavy duty applications places increased demand on the efficiency and durability of the components used in membrane electrode assemblies (MEAs).[1] Of particular interest are the oxygen reduction reaction electrocatalysts used in the fuel cell cathode. While the relaxed capital cost requirements for heavy duty applications allows for increased precious metal loadings, the four-fold increase in operating lifetime requires improved catalysts which can express their high activity not only at beginning-of-life, but also after 25k-hour equivalent accelerated stress tests (ASTs).This paradigm shift prioritizes efforts aimed at understanding and subsequently controlling catalyst degradation. In situ characterization methods can be instrumental in this effort, but it is critical that the operating conditions in these studies reliably replicate the type and degree of degradation observed in the operating fuel cell.[2,3] In this study, identical-location scanning transmission electron microscopy (IL-STEM) is used to compare catalyst degradation in an aqueous environment to degradation observed ex situ in MEAs following AST cycling.[4] For traditional catalysts, the aqueous environment appears to replicate the dissolution and coalescence mechanisms observed for Pt catalysts with small starting particle sizes (<3nm) supported primarily on the surfaces of graphitized carbon supports. In evaluating more stable alloy catalysts (4-5nm) supported on high surface area carbon supports, the large discrepancy between degradation in the aqueous and MEA environment was observed. In particular, the Ostwald ripening mechanism, which is more dominate in these high surface area carbons, seemed absent in the aqueous environment.[5]To better replicate the type and degree of degradation in the MEA environment, various experimental parameters such as temperature, acid type/concentration, and upper and lower potential limits were explored. By extending the cycling potential window from 0.6-0.95 Vvs . to 0.4-1.0 Vvs . RHE in an sulfuric acid electrolyte containing Pt ions, the Ostwald ripening mechanism was enhanced, resulting in an end-of-test particle size distribution and composition which better match MEA tests. These refined conditions can now be used to perform in situ and identical-location experiments that will play an important role in the development of robust catalysts for heavy-duty vehicle applications.[6]References[1] D. A. Cullen, et al. Nat. Energy 2021, 6 (5), 462–474.[2] J. A. Gilbert, et al. Electrochim. Acta 2015, 173, 223–234.[3] I. Martens, et al. ACS Energy Lett. 2021, 6 (8), 2742–2749.[4] S. Rasouli, et al. Nano Lett. 2019, 19 (1), 46–53.[5] E. Padgett, et al. J. Electrochem. Soc. 2019, 166 (4), F198–F207[6] This material is primarily based on work performed by the Million Mile Fuel Cell Truck (M2FCT) Consortium (https://millionmilefuelcelltruck.org), 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 was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.