Molecular Dynamics Study of the Nanoscale Proton Density Distribution at the Ionomer-Catalyst Interface

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The proton density in the cathode catalyst layer is a crucial variable to define the local reaction conditions in and thereby determine the performance and lifetime of polymer electrolyte fuel cells. We present a molecular modeling study to examine the proton density distribution in a water-filled, slab-like pore. The slab is confined by two distinct boundaries, the first consisting of a platinum single-crystalline surface and the second being a thin and dense skin layer of proton-conducting ionomer. Classical molecular dynamics simulations serve as a valuable tool to rationalize the impact of the molecular structure and properties of the ionomer film as well as the adsorption and charging state of the metal surface on structure and properties of interfacial water and the spatial distribution of protons. Boundary conditions at the metal surface are obtained from explicit quantum mechanical simulations at the DFT level. The oxide coverage at the metal surface and the water layer thickness are considered as crucial parameters. Results of detailed structural analyses elucidate the impact of a structured layer of near-surface water molecules at the platinum surface on accumulating protons at the interface.