Superwetting Framework Polymer Membranes Enabling Fast TFSI^- Conduction for Nonaqueous Redox Flow Batteries.

Abstract

Nonaqueous redox flow batteries (NARFBs) can overcome the voltage limitation of aqueous batteries; however, their practical application is constrained by several membrane-related challenges, including the conductivity-selectivity tradeoff in porous separators, poor wettability of microporous membranes, and the incompatibility of ion exchange membranes with organic electrolytes. Here, we propose framework polymer membranes and introduce a diversity-oriented design strategy through which we have developed a microporous anion conductive membrane that is resistant to swelling and exhibits superb wettability with acetonitrile. We reveal that the enhanced affinity of robust micropores with acetonitrile allows for rapid and highly selective conduction of charge carrier ions, affording a TFSI^- diffusion coefficient of 8.12 × 10^-7 cm² s^-1 (23 times that for the weakly affinitive counterpart) and a high charge-carrier/electrolyte selectivity of 433. The membrane thereby enables NARFBs to operate at 80 mA cm^-2 under both room and sub-zero temperatures, while maintaining a constant Coulombic efficiency of greater than 99.5% over 900 cycles, thus improving the existing performance level. This study provides insights for designing membranes tailored for electrochemical devices that utilize organic electrolytes.