Molecular structure and transport dynamics in perfluoro sulfonyl imide membranes

We report a detailed and comprehensive analysis from classical molecular dynamics simulations of the nanostructure of a model of hydrated perfluoro sulfonyl imide (PFSI) membrane, a polymeric system of interest as a proton conductor in polymer electrolyte membrane fuel cells. We also report on the transport dynamics of water and hydronium ions, and water network percolation in this system. We find that the water network percolation threshold for PFSI, i. e. the threshold at which a consistent spanning water network starts to develop in the membrane, is found to occur between hydration levels (lambda) 6 and 7. The higher acidity of the sulfonyl imide acid group of PFSI compared to the sulfonic acid group in Nafion, as computationally characterized in our earlier ab initio study (Idupulapati et al 2010 J. Phys. Chem. A 114 6904-12), results in a larger fraction of ` free' hydronium ions at low hydration levels in PFSI compared to Nafion. However, the calculated diffusion coefficients of the H3O+ ions and H2O molecules as a function the hydration level are observed to be almost the same as that of Nafion, indicating similar conductivity and consistent with experimental data.