Analysis of Ionomer Distribution and Pt/C Agglomerate Size in Catalyst Layers by Two-Stage Ion-Beam Processing

Ionomer distribution in catalyst layers (CLs) of polymer electrolyte fuel cells has garnered much attention because it affects proton and gas transfer. In this study, a novel visualization method of the overall through-plane ionomer and platinum-supported carbon (Pt/C) distributions in the CLs using two-stage ion-beam processing is proposed. The first stage is the formation of a flat and smooth cross-section using a broad ion beam. The second stage is the selective removal of the materials in the CL by a focused-ion beam. Scanning ion microscopic images were obtained after the first and second stages. The ionomer and Pt/C distributions were then obtained by image processing. CLs were prepared with the ionomer-to-carbon (I/C) ratio varied from 0.5 to 3.0. The effect of the dispersion process on the structure of the CL was also studied. With increasing I/C ratio, a thin ionomer layer was formed at the interface with the polymer electrolyte membrane. This behavior is attributed to deposition of ionomer during solvent evaporation. Ionomer thickness, agglomerate size of Pt/C, and pore size were evaluated. The agglomerate size of Pt/C was found to be affected by both I/C ratio and the dispersion process.