(Invited) Understanding Fuel Cell Materials Degradation Via Advanced Electron Microscopy Techniques

Abstract

Polymer electrolyte membrane (PEM) fuel cell performance degradation, particularly associated with the cathode catalyst layer, can be directly attributed to the stability and durability of individual material constituents comprising the membrane electrode assemblies (MEAs), including the electrocatalyst, catalyst support, recast ionomer, polymer membrane, and gas diffusion-microporous layers. The structural and chemical changes of these MEA constituents are being quantified via advanced electron microscopy methods to elucidate the specific degradation mechanisms contributing to performance loss, with the ultimate goal being microstructural and compositional improvements that will enhance MEA durability. Research efforts at Oak Ridge National Laboratory have been focused on the high-resolution microstructural and microchemical characterization of as-fabricated fuel cell materials (e.g., the individual materials constituents and the same materials incorporated in fresh MEAs), these materials/MEAs subjected to accelerated stress tests (ASTs) designed to degrade specific MEA components, and field-aged MEAs. These studies are used to establish critical processing-microstructure-performance correlations and to elucidate the individual materials changes contributing to measured MEA degradation, performance loss, and failure. Analytical microscopy methods have been used to mechanistically explain the degradation of various carbon support structures, to quantify Pt loss due to migration, catalyst coalescence, and to understand the structure, distribution, and composition of thin ionomer films within catalyst layers. Understanding and quantifying the structural and compositional changes of the materials comprising the MEA during electrochemical-aging will allow for the implementation of processing changes and critical materials development that are required for optimizing PEM fuel cell durability and performance. ________________________________________________________________________ Research sponsored by (1) the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy and (2) Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences (CNMS), which is an Office of Science User Facility.