Insights into structure and properties of catalyst-ionomer interfaces in a PEM fuel cell cathode from atomistic molecular dynamics simulations

• A Pt/C-ionomer thin film model is developed for molecular dynamics simulations. • The ionomer thin film structure is sensitive to the oxidation state of the carbon support. • Ionomer equivalent weight affects thin film density and oxygen transport resistance. • Ionomer equivalent weight impacts proton conductivity within the thin film. • Ionomer sidechain length affects the water clusters and proton conductivity. The structure and properties of the catalyst-ionomer interface at the cathode exert a major impact on the performance of polymer electrolyte membrane fuel cells. The interface is affected by both the chemical structure of the ionomer and the potential-dependent changes to the catalyst/support surface during operation. This work presents molecular dynamics simulations of the catalyst-ionomer interface for an expanded Pt/C-ionomer thin film model. Simulations reveal that the structure of the ionomer film is sensitive to the oxidation state of the carbon support, with the preferential ionomer orientation shifting from backbone-towards-carbon to sidechain-towards-carbon oxide. The equivalent weight of the ionomer is shown to determine ionomer packing at the catalyst surface, which could impact the local oxygen transport resistance. The equivalent weight also influences the local proton concentration (or pH) and the proton conductivity at the catalyst-ionomer interface. Shorter sidechain length also increases conductivity by forming larger water clusters that act as channels for hydronium mobility. Overall, the presented simulations demonstrate how the ionomer composition could be tuned to enhance performance via its impact on kinetic, ohmic, and transport losses in fuel cell voltage.