Self-healing anion exchange membrane for pH 7 redox flow batteries

Abstract

Anion exchange membranes (AEMs) separate the redox active electrolytes and transfer the charge carriers in redox flow batteries. During the long-term operation of a redox flow battery, the formation and propagation of cracks in AEMs are detrimental, resulting in capacity loss and self-discharge, while replacing the cracked membranes would increase the cost. Here we incorporate self-healing to an AEM through reversible and temperature-controlled Diels-Alder (DA) reaction. We first synthesized a block copolymer from vinylbenzyl chloride and 2-((4-vinylbenzyloxy) methyl) furan by RAFT polymerization. The benzylic chloride moiety was quaternized with trimethylamine to conduct anions, while bismaleimide was added to dynamically connect the furan moieties via DA reaction. By solution casting and reacting at 80 °C, an AEM with dynamic network was prepared. When evaluated for practical application, we found the AEM has a Cl− conductivity of 32.7 mS cm−1 at 80 °C and man-made cracks on the membrane can be self-healed. We then tested its performance in a pH7 aqueous organic redox flow battery, where it delivered stable cycling performances over 100 consecutive charge/discharge cycles, with a coulombic efficiency of >97% and an energy efficiency of >79%. This membrane is promising for practical applications and what we report here is a useful strategy in rendering conventional AEMs with self-healing property.