Exploring the hydrated microstructure and molecular mobility in blend polyelectrolyte membranes by quantum mechanics and molecular dynamics simulations

Abstract

Quantum mechanics and molecular dynamics simulations were employed to examine several structural and dynamical characteristics in blend SPEEK–SPPO based membranes at varied water content and temperature values. QM results showed that water molecules were localized around the SPEEK and SPPO sulfonate groups due to the hydrogen bonding interactions, which caused proton dissociation at the increased hydrations. By increasing the hydration level, more water molecules occupied the sulfonate fragments because of the improved sulfonate–water interactions, whereas the hydrogen bond interaction of sulfonate–hydronium ion was weakened, enabling the hydronium ions to be away from the sulfonate groups. Based on water cluster size distribution and structure factor evaluations, it seemed that by improving the water content, isolated smaller aqueous clusters appeared under lower hydration levels, which merged together to form larger clusters comprising almost all molecules. Diffusivities for water and hydronium ion were observed to be enhanced by an increase in water uptake, which were attributed to the fact that larger hydrophilic clusters across the swollen blend SPEEK–SPPO PEMs promoted molecular mobility. Similarly, enhancing operational temperature gave rise to an enhancement in the membrane transport dynamics. Finally, predicted water and hydronium ion diffusion coefficients were noted to be smaller in hydrated SPEEK–SPPO membrane as compared to Nafion under identical conditions, which was in agreement with the experimental results.