Anion exchange membranes with high power density and energy efficiency for aqueous organic redox flow batteries

Membrane development in organic redox flow batteries (ORFBs) is of significant importance. Herein, we designed a series of anion exchange membranes made from poly( p -phenylene oxide) (PPO) with different degrees of functionalization, cationic moieties and crosslinking degrees. We tested them in a N,N,N -2,2,6,6-heptamethylpiperidinyl oxy-4-ammonium chloride/1,1′-dimethyl-4,4′-bipyridinium dichloride (TMA-TEMPO/MV) based ORFB single cell to investigate the structure-property-performance relationship. The common problem of active species crossover is solved by controlling the membrane water uptake below 70 wt.%, resulting in stable systems with coulombic efficiency of over 99.3%. The cell employing a membrane prepared from PPO and trimethylamine moiety attached via a C6-spacer achieved the best overall cell performance. It delivered high capacity retention (94% after over 100 consecutive cycles) and higher peak power density (293 mW•cm −2 at 16 mL•min −1 ) and energy efficiency (80%) than the commercial reference memane FAA-3-50. By controlling the water uptake below 70 wt.% and involving a C6 spacer, the obtained membranes maintain excellent ion selectivity and conductivity, resulting in enhanced battery performance that breaks through the commercial membrane.