Atomistic simulations of structure and dynamics of hydrated Aciplex polymer electrolyte membrane

Abstract

Aciplex is a perfluorosulfonic acid (PFSA) polymer electrolyte membrane, where its efficiency depends on hydration and temperature. In the present work, the nanostructure of the Aciplex membrane and transport of hydronium ions and water molecules are characterized using classical molecular dynamics simulations at varying hydrations and temperatures. An examination of radial distribution functions and scattering intensities shows that temperature has a negligible effect on membrane nanostructure at all hydration levels. The calculated structural factors and scattering intensities of water molecules closely resemble the experimental SAXS and SANS features of PFSA membranes. Further, for all hydration, the strong interactions between sulfonate groups of the pendant side chain arise only from inter-chain interactions. The stiffness of a pendant side chain limits the possibility of intra-chain interactions between sulfonate groups. The distance between adjacent sulfonate groups shows a variation of 3 Å from an average distance of 25 Å which shows a suitable orientation of the pendant side chain to maximize water–hydronium interactions with the sulfonate group. The radius of gyration shows an insignificant change with hydration and temperature which demonstrates that the membrane is a suitable electrolytic component for PEM fuel cells. The calculated diffusion coefficients of hydronium ions and water molecules are found to be in reasonable agreement with experimental data. The enlarged hydrophobic domains assisted by the rigid pendant side chain in a hydrated Aciplex membrane results in lower mobility of water molecules compared to Nafion.