Microstructure Characterization of Catalysis Layers during Ink Drying Process for Polymer Electrolyte Membrane Fuel Cells

Abstract

A catalyst ink for proton exchange membrane fuel cells (PEMFCs) is generally prepared by various mixing methods by dispersing platinum or platinum alloy nanoparticles supported on carbon blacks with ionomers in a specific solvent or solvent combination. The catalyst ink is deposited on the surface of the membrane or diffusion media and is typically subjected to elevated temperatures to facilitate rapid removal of solvent. Large carbon agglomerates resulting from sub-optimal ink dispersion and drying conditions can limit catalyst utilization, inhibit mass transport in the catalyst layer, and damage the membrane and possibly also the gas diffusion media [1, 2]. Micro-structural evolution of the catalyst ink can be controlled by interactions of the ionomer in the catalyst-ionomer ink and by the effect of ink solvent composition on those interactions during the ink drying process. This presentation will describe the results of in-situ/ex-situ X-ray scattering studies of the evolution of the cathode catalyst layer during the ink drying process to determine the impact of solvent removal rates and solvent identity on the structural evolution of the cathode catalyst layer. These studies also included operando ink drying at room temperature, under an atmosphere containing the solvent, and a high temperature. Ultra-small angle X-ray scattering (USAXS) was used to measure the agglomerate size distribution during the ink drying. A small environmental chamber was used to examine drying phenomena of high solid-content inks and dispersions to determine structural evolution during ink drying. The effects of ionomer concentration, catalyst concentration, and solvent composition on the microstructure of the catalyst inks and electrode are correlated with the MEA performance and operando diagnostic data. The goal of these studies is to guide the MEA fabrication process to optimize MEA performance and durability.