Study on the transport performance degradation of Nafion membrane due to the presence of Na+ and Ca2+ using molecular dynamics simulations

Abstract

Foreign cations have detrimental effects on the transport performance of polymer electrolyte membranes. In this study, the effects of two typical kinds of cations (Na+ and Ca2+) on the diffusion of hydronium ions (H3O+) and water molecules (H2O) in Nafion 117 membranes are investigated using molecular dynamics simulations, at the same time the effects of temperature are considered. The mechanisms of effects are analyzed using radial distribution functions, water clusters distribution and relative concentration distribution. Results show that the diffusion coefficients of H3O+ and H2O are significantly reduced by the presence of Na+ and Ca2+. Moreover, the impact of Ca2+ is more serious. Further investigations show that cations affect intramembrane transport properties by occupying sulfonic groups and disrupting the connectivity of water clusters. The difference in the effects of these two kinds of cations is discussed by hydrated radius, hydration degree of cations, and the interaction between cations and sulfonic groups. Increasing temperature can increase not only the diffusion coefficients, but also interaction between the cations and the sulfonic groups. However, the interaction between the sulfonic groups and H2O is decreased by the increasing temperature.