Physical Modeling of the Proton Density in Nanopores of PEM Fuel Cell Catalyst Layers

In polymer electrolyte fuel cells, a foremost goal is to design catalyst layers with high performance at markedly reduced platinum loading. As a contribution towards this objective, we explore a simplified pore geometry to capture the impact of ionomer structure and metal charging properties on the proton density distribution and conductivity in relevant nanopores. The basic model is a cylindrical tubular pore confined by an ionomer shell and a solid platinum-coated core. The gap region between metal and ionomer is filled with water. We study how the surface charge density at the ionomer and the metal charging relation as well as geometric pore parameters affect the electrochemical performance. The density of charged side chains at the ionomer shell exerts a pronounced impact on the surface charge density at the Pt surface and thereby on the activity of the pore for the oxygen reduction reaction. The key parameter controlling the interplay of surface and bulk charging phenomena is the overlap of the Debye lengths of ionomer and metal surfaces in relation to the width of the gap. It allows distinguishing regions with weak and strong correlation between surface charge densities at ionomer shell and Pt core.