Proton Transport Mechanisms in PFSA and SDAPP Ionomer Dispersions: Effects of Water Content in Water/2-Propanol Mixtures

Abstract

In this study, we measured ionic conductivities of two types of ionomers in dispersions, containing water and 2-propanol solvent mixtures. Using ionic conductivity theory, we decoupled vehicular and structural proton diffusion coefficients of these ionomer dispersions and acid solutions. Our results reveal that solvent composition and the identity of ions influence both the solution’s ionic conductivity and proton transport dynamics. In the case of ionomer dispersions, Pemion, a member of the sulfonated Diels–Alder poly(phenylene) (SDAPP) family, exhibited a higher ionic conductivity per mole of SO3H compared to its perfluorosulfonic acid (PFSA) counterpart, Nafion, in similar solvent mixtures, due to a higher structural diffusion coefficient. We attribute this finding to Pemion’s chemical architecture, mainly its lower equivalent weight (EW), which we posit results in more closely spaced sulfonate groups requiring fewer water molecules to bridge them. This closer spacing frees additional water molecules, providing pathways for proton hopping, facilitating the participation of dissociated protons in the transient hydrogen bonded network responsible for structural diffusion.