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 the next generation of polymeric anion exchange membranes (AEMs). Structural heterogeneity of conventional membranes, complex ion-water and ion-membrane interactions, and overlapping timescales, significantly hamper progress toward a comprehensive understanding. In this work, we use RH-dependent 2DIR and, EIS along with 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 timescales, while EIS measurements yield ion conductivity and enthalpic and entropic barriers for ion transport. Comparing the sub-diffusive and diffusive ion motions suggests a structural transport mechanism dictates ion diffusion across all hydration levels. A nonlinear relationship between ion hopping time and ion conductivity indicates a deviation of ion motion from Nernst-Einstein (NE) behavior. Finally, we demonstrate that hydration primarily regulates ion hopping rates, through tuning ion-pair interactions, thus influencing the activation barrier for ion transport in extremely nanoconfined AEMs.