Correlating solvation shell dynamics and ion transport in highly ordered nanoporous polymers

A molecular-level understanding of the role of hydration in ion transport is essential for developing next generation polymeric anion exchange membranes (AEMs). However, progress towards this goal is hindered by the structural heterogeneity of conventional membranes, complex ion-water and ion-membrane interactions, and dynamics that occur over femtosecond to millisecond timescales. We use RH-dependent 2DIR, EIS, and MD simulation to investigate the role of hydration in ion transport in a highly ordered synthetic AEM with structural uniformity. 2DIR reveals sub-diffusive ion hopping (τ₂), while EIS measurements yield ion conductivity (σ) and enthalpic and entropic barriers for ion transport. A nonlinear relationship between τ₂ and σ indicates strong ion-ion correlations, and a deviation from Nernst-Einstein behavior. We find that hydration primarily regulates ion hopping rates through tuning ion-pair interactions, thus influencing the activation barrier for ion transport in extremely nanoconfined AEMs. MD simulations reveal that a single-contact exchange structural transport mechanism dictates ion diffusion across all hydration levels. The associative ion hopping model based on multiple cation-anion contacts provides new design principles for efficient ion transport with improved permeability.