(87m) Ion and Water Transport in Polynorbornene-Based Thin Film Membranes

Insight into ion transport within Anion Exchange Membranes (AEMs) is critical for crafting efficient polymer electrolytes, integral to fuel cells and numerous electrochemical technologies. One significant hurdle in characterizing ion transport in AEMs is the lack of an inclusive methodology that bridges macroscopic experimental data with simulation results and disentangles ion, water, and polymer dynamics at varying hydration states on a molecular level. In this study, we propose a methodology to probe unique ion transport mechanisms. This strategy encompasses the use of anion exchange thin films, interdigitated electrodes, two-dimensional Infrared spectroscopy (2D IR), and atomistic Molecular Dynamics (MD) simulations. We quantify bromide ion conductivities in polynorbornene-based thin films as a function of temperature and relative humidity, harnessing electrochemical impedance spectroscopy. Intriguingly, bromide ion conductivities exhibit Arrhenius-type behavior, and, for the first time, we employ activation energy (E_a) as a diagnostic tool to discern the transition between two transport mechanisms. Our study further enriches percolation theory by integrating empirical results with MD simulations. We merged ultrafast IR and 2D IR spectroscopy with simulations to examine water dynamics and their effects on ion behavior. Quantitatively, we discovered that the transition between two mechanisms is spurred by enriched solvation environments for anions and escalated percolation of water pathways at 55% RH. This new transport mechanism is underpinned by the robust network of water molecules and a significant population of water molecules in the second solvation shell, which reduces the bridging effect and eases constraints on ions. Our findings hold considerable implications for the engineering of AEMs as proficient ion-conducting polymers.