Investigation of Non-Aqueous Electrolyte and Crossover of Perfluorsulfonate Ion Exchange Membrane in a Non-Aqueous Flow Battery
Authors: Lou Kun , Zawodzinski Thomas A 
Published: February 2020

From aqueous to non-aqueous redox flow batteries, the general membrane properties are unchanged: electronically isolating anolyte and catholyte and allowing conductance through the membrane to keep charge balance. However, on top of the high cost(1), existing membranes for non-aqueous flow battery inherit the natural disadvantage of non-hydrogen-bonding transport compared to all-vanadium redox system while leaving the door wide open for redox species to pass through. Crossover-induced capacity fade remains one of the most challenging problems for non-aqueous flow battery(2). Efforts have been made to address the issue. For example, composite membranes(3), crosslinking(4) and size exclusion approaches (5) have all been used. Herein, we use Perfluorsulfonate Ion Exchange Membrane (PFSA) as a benchmark to study the crossover behavior in non-aqueous system.
For membranes swollen with non-aqueous solvents, tetraalkylammonium cations in the membrane have outperform lithium ion in terms of ionic conductivity(6). However, a large volume of swelling is observed in our study and also in literature(3). In fact, our previous vibrational data suggested that only a small portion of the organic solvent in the membrane participates in solvation of cations, leaving a substantial portion of the solvent molecules as potential carriers to dissolve and transport redox species across the membrane(7). The solvent mediated process, crossover of redox species through PFSA will be presented from a fundamental, interaction-oriented perspective. In-depth understanding of interactions among redox species, solvent and membrane are obtained by combining various spectroscopic measurements with transport measurements. This understanding is crucially necessary to shed light on future membrane design exclusive for non-aqueous flow batteries.

Acknowledgement

We gratefully acknowledge the support of this work by the U.S. Department of Energy, Office of Electricity Delivery and Energy Reliability (Dr. Imre Gyuk). We also thank 3M for providing membranes.

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