Molecular dynamics simulations of Nafion thin films at a platinum catalyst surface: Correlating structure with charging behaviour

Abstract

Electrocatalysis is greatly influenced by the local reaction environment, which is governed by the structure of the catalyst, the distribution of the electrolyte, and the local electric field. In catalytic systems comprised of complex molecular species like ionomers, the distribution of electrolyte can vary substantially, resulting in divers local reaction environments. In order to gain atom-scale insight into this micro-environment we construct a model system consisting of a platinum surface, varying levels of water, and a Nafion thin film and conduct molecular dynamics simulations. We employ a construction based on Voronoi tesselation to assemble a dense film of ionomer that fully covers the platinum substrate. An energy analysis reveals that water film configurations with thickness of less then 1.3 nm are stable. Simulations with charged platinum surfaces are analysed in view of electrostatic conditions and differential capacitance of the interface configuration. Trends observed in these properties can be interpreted in view of the crowding of hydronium ions or the Nafion film at the platinum surface. The presented workflow can be easily applied to investigate novel ionomers for use in PEMFCs.