Quantitative Analysis of Fuel Cell Cathode Catalyst Layer Degradation Using Scanning Transmission Electron Microscopy and Energy Dispersive Spectroscopy

Abstract

Proton exchange membrane fuel cells (PEMFCs) are a central technology for the electrification of the heavy-duty automotive fleet, ultimately enabling reduced carbon emissions. However, improvements in lifetime and cost are necessary before mass adoption of PEMFC vehicles is achieved. Significant research into analyzing and mitigating degradation of catalyst layers (CLs) is ongoing and this work highlights a transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) approach aimed at quantifying electrochemically active surface area (ECSA) loss through the thickness of CLs resulting from electrochemical-applied, catalyst-specific accelerated stress tests or ASTs.Several mechanisms contribute to degradation of the catalysts in PEMFCs, including Ostwald ripening, coalescence, and formation of a Pt band within the membrane, which result in decreased active surface area throughout the CL. TEM has often been used in conjunction with electrochemical testing to infer changes in the catalyst nanoparticle size, distribution, and changes to the support materials within the catalyst layer (CL) to investigate catalyst. Typically, high-magnification TEM with high-resolution are used to infer localized changes within the CL by examining shape and size to provide information concerning the degradation mechanism. Low-magnification TEM is typically used to observe changes in the support, ionomer, and catalyst distribution. Here we use a quantitative approach to show elemental differences within the CL by using scanning transmission electron microscopy and energy dispersive spectroscopy maps (STEM/EDS) for a given MEA that underwent catalyst-specific ASTs. This approach uses the fluorine to platinum ratio (F/Pt) from a fresh sample as a benchmark to track loss of Pt through the thickness of the CL of aged samples. This information can then be used to compare differences in various locations on the MEA in relationship to gas inlets and outlets as well as changes induced by different stressors during the catalyst ASTs. This analysis complements studies using bulk techniques, such as electrochemical ECSA via cyclic voltammetry and micro-X-ray diffraction (µ-XRD), to confirm trends in CL degradation.