PNNL

Abstract

We present a detailed analysis of the nanostructure of short side chain (SSC) perfluorosulfonic acid membrane and its effect on H2O network percolation, H3O+ and H2O diffusion, and mean residence times of H3O+ and H2O near SO3- groups based on molecular dynamics simulations. We studied a range of hydration levels (?) at temperatures of 300 and 360 K, and compare the results to our previous findings in the benchmark Nafion® membrane at 300 K. The water channel diameter is about 20% larger in Nafion, while the extent of SO3- clustering is more in SSC membrane. The calculated channel diameter is in excellent agreement with the recently proposed cylindrical water channel model of these membranes. The H2O network percolation occurs at comparable hydration levels (? ~ 5-7), and the diffusion coefficients of H2O and H3O+ are similar in SSC and Nafion membranes. Raising the temperature of the SSC membrane from 300 to 360 K provides a much bigger increase in proton vehicular diffusion coefficient (by a factor of about 4) than changing the side chain length. H3O+ ions are found to exchange more frequently with SO3- partners at the higher temperature. Our key findings are that a) the hydrophobic-hydrophilic separation in the two membranes is surprisingly similar; b) at all hydration levels studied, the longer side chain of Nafion is bent and is effectively equivalent to a short side chain in terms of extension into the water domain; and c) proton transport along the centre of the channel is improbable and vehicular proton transport occurs between SO3- groups. The simulations are validated by good agreement with corresponding experimental values for the simulated membrane density and diffusion coefficients of H2O.