Molecular Dynamics Study of the Morphology of Hydrated Perfluorosulfonic Acid Polymer Membranes

Many morphological models have been proposed to describe the water swelling behavior and proton transport mechanism of perfluorosulfonic acid (PFSA) polymer membranes through experimental and modeling studies. However, the ongoing structural debate has not been completely resolved yet. We here conducted a series of all-atom molecular dynamics simulations of hydrated PFSA membranes to evaluate changes in the membrane morphology at different water contents. We found a similar dependence of the morphology on the water content between PFSA membranes with equivalent weight (EW) of 844 and 1144 g/equiv. That is, the morphology of the aqueous domain changes with increasing water content from a channel-network structure to a tortuous layered structure, and once attaining the tortuous layered structure, the water layer just thickened gradually by further increasing water content. Furthermore, we found more heterogeneous water domains in the higher-EW PFSA membrane, demonstrating the stronger aggregation behavior of the aqueous domains in the high-EW membranes. The variation of the PFSA membrane morphology observed here is useful to understand the proton transport mechanism and design new materials suitable for polymer electrolyte fuel cells in the near future.