Molecular Dynamics Simulations of Proton Diffusion in the Short-Side-Chain Perfluorosulfonic Acid Ionomer

Abstract

This computational study seeks to understand why low equivalent weight short-side-chain (SSC) perfluorosulfonic acid membranes exhibit higher proton conductivities than Nafion when hydrated at similar water contents. The diffusion of protons in the SSC ionomer at water contents of 3, 6, and 13 H2O/SO3H were investigated through classical molecular dynamics simulations. We developed a unique force field set based on torsion profiles derived from extensive electronic structure calculations of a two side chain fragment of the SSC ionomer and simulated a system consisting of a single 40 repeat unit of the polymer (F3C-{[CF2-CF(OCF2CF2SO3H)]-(CF2CF2)3}40-CF3) corresponding to an equivalent weight of 578 at each of the three water contents. After an equilibration time of approximately 450 ps at 300 K where the density was fixed at 1.67 g/cm3, structural information was collected from 2 ns production runs at 315C. We calculated proton (as a hydronium ion) diffusion coefficients of 2.83×10-7, 1.37×10-6, and 3.87×10-6 cm2/s at the three water contents which show excellent agreement to experimentally measured diffusion coefficients at the lower water content only.