Microscopic Insights into the Inhomogeneous Degradation of Pt Alloy Catalysts in PEM Fuel Cells

Abstract

Fuel cell electric vehicles (FCEVs) powered by proton exchange membrane fuel cells (PEMFCs) offer a sustainable, low-carbon alternative to the world’s current energy structure based on fossil fuels. Recently, application of PEMFC to heavy-duty vehicles (HDVs) has attracted significant interest for reducing greenhouse gas emissions in this market. Relative to passenger vehicles, HDVs require extended lifetimes and high operating costs, leading to an increased emphasis on the durability and efficiency of membrane electrode assembly components [1].Degradation of the oxygen reduction reaction (ORR) cathode catalyst is one of the main challenges for heavy duty applications. Several degradation mechanisms have been identified, such as Ostwald ripening, particle coalescence/migration, and particle detachment, the observation of which relies on transmission electron microscopy (TEM) [2,3]. Through TEM, inhomogeneous degradation at the micro- and nanoscale during operation have been observed: (1) the Pt loading decreases toward the cathode/membrane interface and creates a depletion zone adjacent to the interface [2,4] and (2) different rates of electrochemical surface area loss of PtCo supported on various porous carbons,[5] leading to the speculation of different degradation rates for particles at the exterior vs. interior carbon surfaces.To elucidate these inhomogeneous degradation mechanisms, we employ high-throughput image analysis and electron tomography methods to analyze fuel cell cathodes before and after accelerated stress tests (ASTs) performed in both nitrogen and air environments. An automated, high-throughput method was developed that provides statistically relevant analysis of PtCo particle size, shape, and loading distribution. This method combines automated image or energy-dispersive spectroscopy (EDS) map acquisition using a commercial software (MAPS, Thermo Fisher Scientific) and custom Python codes to analyze high-angle annular dark field (HAADF) and EDS spectrum images. With the high-throughput method, we reveal the inhomogeneous distribution of PtCo particle size, Pt loading, and Co content across the entire electrode. In addition, an electron tomography workflow is under development to measure PtCo particle size and composition on the exterior and interior of the porous carbon support. The effect of cathode gaseous environment on the distribution of particle size, loading, and composition is discussed at the microscale across the electrode and at the nanoscale for exterior and interior surfaces of primary carbon particles. Findings from this study will provide new mitigation strategies and design principles to enable durable fuel cell catalysts.[6]References[1] DA Cullen et al., Nat. Energy. 6 (2021), p. 462.[2] PJ Ferreira et al., J. Electrochem. Soc. 152 (2005), p. A2256.[3] R Borup et al., Chem. Rev. 107 (2007), p. 3904.[4] H Yu et al., Electrochim. Acta 247 (2017), p. 1169.[5] M Ko et al., J. Electrochem. Soc. 168 (2021), p. 24512.[6] This material is based on work performed by the Million Mile Fuel Cell Truck (M2FCT) Consortium, technology managers Greg Kleen and Dimitrios Papageorgopoulus, which is supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office, and by U.S. Department of Energy, Office of Nuclear Energy, Fuel Cycle R&D Program, and the Nuclear Science User Facilities. 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.This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).