Role of Water Molecules in Enabling Site Hopping and Vehicular Transport Mechanisms in Polynorbornene-Based Anion Exchange Membrane

Nealey, Paul

Role of Water Molecules in Enabling Site Hopping and Vehicular Transport Mechanisms in Polynorbornene-based Anion Exchange Membrane

Zhongyang Wang,^⸹ Ge Sun^，^⸹ Mrinmay Mandal,^‡, Paul A. Kohl, ^‡, Juan de Pablo,^⸹ Shrayesh N. Patel,^⸹and Paul F. Nealey ^⸹

^‡ School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia, 30332-0100, United States

^⸹Pritzker School of Molecular Engineering, University of Chicago, Chicago, Illinois 60637, USA

Ion exchange membranes are at the heart of electrochemical conversion and storage devices such as fuel cells ¹, water electrolyzers ², CO₂ electrolyzers ³. redox flow batteries ⁴, and reverse electrodialysis ⁵. Anion exchange membrane fuel cells (AEMFCs) have attracted enormous attention as alternatives to replace perfluorinated, sulfonic acid-based proton exchange membrane fuel cells (PEMFCs) ⁶ because alkaline membrane electrode assemblies (MEAs) composed of anion exchange ionomers (AEIs) and AEMs that allow the use of Ni ^{7, 8}, Fe ⁹, and Ag ¹⁰ based precious-group-metal (PGM) free catalysts in alkaline environments for hydrogen oxidation reactions (HORs) and oxygen reduction reactions (ORRs). However, the lack of understanding of ion transport mechanisms at different hydration levels of an anion exchange membrane hinders the rational design of the MEAs in an AEMFC. Here we investigate site hopping and vehicular transport mechanisms using anion exchange thin films, interdigitated electrodes, and atomistic molecular dynamics simulations. Halide ion (Br^-, Cl^- and I^-) conductivities in polynorbornene-based thin films are measured as a function of temperature and relative humidity using electrochemical impedance spectroscopy. Halide ions show Arrhenius behaviors, and activation energy (Ea) is for the first time used as an indicator for detecting the transition of site hopping and vehicular transport mechanisms. Using atomistic molecular dynamics simulation, we quantitatively demonstrate that the transition of site hopping and vehicular mechanisms is aided by better solvation environments of anions and more percolated water pathways.

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