Molecular Dynamics Simulations of Proton Transport in 3M and Nafion Perfluorosulfonic Acid Membranes

Proton transfer and local structures in 3M (EW 825) and Nafion (EW 890) membranes are investigated in this study by both standard nonreactive molecular dynamics and the self-consistent iterative multistate empirical valence bond method, which is capable of simulating multiple reactive protons and accounting for the Grotthuss mechanism of proton transport. The Nafion and 3M systems have the same backbone, so we can isolate and compare the effect of the different side chains by calculating the radial distribution functions (RDFs), self-diffusion constants, and other properties for three hydration levels at 5, 9, and 14 at 300 and 353 K. The conformations of the 3M and Nafion side chains are also compared. We found that even though many results are similar for both F3C and SPC/Fw water models, certain trends such as the sulfonate clustering can depend on the water model selected. The relationship between the different RDFs for the sulfonate, water, and hydronium is discussed. The self-diffusion constants of water for both membranes are found to be close with respect to each water model selected, even though the experimental values for 3M at 300 K are higher. The calculated self-diffusion constants of the excess protons are found to be higher for 3M than Nafion for hydration levels 9 and 14 at 300 K but statistically the same at 353 K.