MD simulations of proton transport along a model Nafion surface decorated with sulfonate groups

We analyse the proton distributions in slab simulations of models of characteristic aqueous pores in polymer electrolyte membranes utilized in low temperature hydrogen and direct methanol fuel cells. In particular we calculate density profiles across the interfacial region and density distributions parallel to the interface for aqueous systems near nonpolar surfaces covered with static or tethered sulfonate groups. Three different model descriptions have been used, two based on a previously employed fluxional empirical valence bond model, and one using rigid ion and water models. The goal is to identify common characteristics, which are to first approximation independent of the particular choice of the details of the interaction and geometrical model. We observe that lateral (diffusive) proton motion shows only small barriers for reasonable choices of the arrangement of SO3− groups and that there is a significant coupling between lateral motion and motion perpendicular to the pore surface. We provide structural evidence that previously proposed surface and bulk transport mechanisms in a polymer electrolyte are not distinct mechanisms. Instead we conclude that the simulations are consistent with a common structural diffusion mechanism, modified by the topology of the landscape around the sulfonate groups.