Characterization of Ionomer Thin Film Distributions in PEM Fuel Cell Catalyst Layers Using Advanced Analytical Microscopy

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, and ionomer films.  The structural and chemical changes of these MEA constituents are quantified via advanced electron microscopy methods to elucidate the materials-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 are 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), 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 materials factors contributing to MEA degradation, performance loss, and failure.  Recently, high-resolution analytical microscopy methods have been used to directly image/map the distribution and chemistry of the ionomer layers within catalyst electrodes, both on a mm-level for assessing “bulk” ionomer dispersions and at the nm-level to understand the continuity and uniformity of thin ionomer films.  This presentation will focus on understanding ionomer distributions as a function of initial ionomer chemistry (e.g., solvents used), electrocatalyst content and dispersion, and type of carbon support used. Additionally, the stability of the ionomer films in cathode catalyst layers after ASTs designed for catalyst degradation and carbon corrosion will be evaluated.

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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 a U.S. Department of Energy, Office of Science User Facility.