Molecular dynamics study of structure and transport of water and hydronium ions at the membrane/vapor interface of Nafion

Through the use of molecular dynamics simulation, we examine the structural and transport properties of water and hydronium ions at the interface of a Nafion polymer electrolyte membrane and a vapor phase. The effect of humidity was studied by examining water contents of 5%, 10%, 15%, and 20% by weight. We observe a region of water depletion in the membrane near the vapor interface. We report the vehicular diffusion of hydronium ions and water as components parallel and perpendicular to the interface. In the interfacial region, for hydronium ions, we find that the component of the vehicular diffusivity parallel to the interface is largely unchanged from that in the bulk hydrated membrane, but the component perpendicular to the interface has increased, due to local decrease in density. We find similar behavior with water in the interfacial region. On the basis of these diffusivities, we conclude that there is no observable additional resistance to mass transport of the vehicular component of water and hydronium ions due to the interface. In terms of structure at the interface, we find that there is a decrease in the fraction of fully hydrated hydronium ions. This translates into a lower probability of forming Eigen ions, which are necessary for structural diffusion. Finally, we observe that the hydronium ions display a preferential orientation at the interface with their oxygen atoms exposed to the vapor phase.