Understanding Degradation of PtCo/C Catalysts in Polymer Electrolyte Fuel Cell Subjected to Two Accelerated Stress Tests

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Hydrogen is a promising fuel and chemical that has the potential to decarbonize many of the energy sectors, especially those with the largest carbon emission footprint. Hydrogen is especially valuable in the sectors of heavy-duty transportation, aviation and shipping because of its high gravimetric energy density. Heavy-duty vehicles (HDVs), although the minor portion of transportation sector at 7 % of the total vehicles emit 23% of the total greenhouse gas in the US1. Polymer electrolyte fuel cells (PEFCs) can potentially impact HDV sector because of their high efficiency, low weight and small footprint. PEFCs are already commercially deployed technologies but cost and durability are the areas that can be improved to enable large-scale adoption of the PEFCs. PEFC stack is the largest cost components of the fuel cell system and within the stack Pt-based cathode catalyst layer is large cost contributor. Pt and Pt-based catalysts are used on the cathode side to promote oxygen reduction reaction (ORR), which is sluggish.The state-of-the-art Pt-based catalyst for ORR is Pt-Co alloy, which has higher mass activity than Pt, enabling lower loadings of Pt catalyst used in PEFCs. Its increased activity towards ORR at the beginning of life (BOL) testing can also translate to the higher activity at the end of life (EOL), granted that Co dissolution from the catalyst can be mitigated. Co leaching results in reduced mass activity towards the ORR and also ionomer and membrane poisoning by Co2+ ions2. Pt shell that is formed on top of Pt-Co alloy particles should prevent cobalt dissolution but Pt dissolution also occurs during the PEFC operation, which exposes Co underneath the Pt shell resulting in further dissolution of both. Therefore, it is difficult to stabilize PtCo catalysts especially during harsh protocol of cycling during accelerated stress tests (ASTs).In this work, a membrane electrode assemblies (MEAs) with PtCo supported on Vulcan and high-surface area (HSA) carbon were fabricated. These MEAs were subjected to two ASTs: one that is the standard Department of Energy (DOE) catalyst degradation AST and the other that is introduced by M2FCT DOE consortia. The work shows that PtCo/HSA has higher degree of polarization loss, especially during the M2FCT protocol because of higher degree of Co leaching. The Co leaching was detected through HFR increase and also through proton transport resistance increase in the catalyst layer. It was further confirmed through transmission electron microscopy studies. We also observed that M2FCT protocol was harsher on the catalyst durability compared to the DOE catalyst degradation AST.