A highly selective porous composite membrane with bromine capturing ability for a bromine-based flow battery

Bromine-based flow batteries (Br-FBs) are receiving more and more attention because of the high potential, high solubility, and low cost of the Br2/Br− redox couple. However, the self-discharge reaction caused by the bromine diffusion leads to serious and irreversible capacity decay of Br-FBs. Herein, a porous composite membrane is designed for Br-FBs, upon which a highly selective separation layer is constructed by introducing a bromine complexing agent. Apart from the intrinsically high selectivity on bromine, the complexing agent in the non-porous selective layer can complex with diffused bromine. Therefore, diffused bromine is captured by the separation layer, consequently suppressing the self-discharging of the battery. The zinc–bromine single flow battery (ZBSFB) assembled with the prepared porous composite membrane demonstrates a Coulombic efficiency of 97.42% and an energy efficiency of 85.31% at 40 mA/cm2. Moreover, this battery can operate stably at a high current density of 140 mA/cm2, much higher than that of the pristine membrane matrix (only 40 mA/cm2), enabling a higher-power density ZBSFB. Most importantly, this ZBSFB exhibits a much higher capacity retention rate after resting for 24 h than that based on a pristine membrane matrix (69.3% vs. 35.7%), suggesting a dramatically improved membrane selectivity. Therefore, the designed porous composite membranes can be a very promising prospect for Br-FB applications.