Controlling Ionomer Film Morphology through Altering Pt Catalyst Surface Properties for Polymer Electrolyte Membrane Fuel Cells

Abstract

In this study, molecular dynamics simulations were used to understand the effects of the surface properties of the catalyst on ionomer film morphology by switching the nature of the catalyst surface from hydrophilic to hydrophobic. Equilibrium structures and radial distribution functions reveal that the surface properties affect the ionomer film morphology and the dispersion solvent on the catalyst surface. For a Pt surface, water molecules and the sulfonate groups of the ionomer are mainly situated in close proximity to the Pt surface, which is due to the hydrophilicity of water and the sulfonate groups that interact strongly with the hydrophilic Pt surface. In the case of the alkylthiol-modified Pt surface, dipropylene-glycol molecules and the backbones of the Nafion ionomers are distributed in the immediate proximity of the alkylthiol-modified Pt surface owing to favorable hydrophobic-hydrophobic interactions. In addition, the accompanying transport properties of water molecules also depend on the nature of the catalyst surface. The calculated diffusion coefficients show that water molecules diffuse faster on the alkylthiol-modified Pt surface than on the pure Pt surface and that the diffusion coefficient gradually decreases as the amount of the dipropylene-glycol/water solvent used to disperse the Nafion solution is reduced.