Enhanced proton selectivity and stability of branched sulfonated polyimide membrane by hydrogen bonds construction strategy for vanadium flow battery

Vanadium redox flow battery (VFB) is promising for use as an energy storage/conversion device, however membrane imposes a limitation in enhancing the performance of VFB. Herein, a novel diamine monomer bis(2-trifluoromethyl-4-aminobenzene) amine containing imino groups is synthesized by nucleophilic substitution and reduction reactions. Then, we develop four branched sulfonated polyimide (I-bSPI) membranes with 40%-70% theoretical sulfonation degrees for application in VFB. The trade-off between proton conduction and vanadium ion blocking of I-bSPI membrane is broken, and the stability of I-bSPI membrane is also effectively enhanced owing to hydrogen bonds network and Donnan repulsion effect. Surprisingly, the proton selectivity of I-bSPI-50 membrane is 4.6 times that of commercial Nafion 212 membrane. In addition, I-bSPI-50 membrane exhibits superior coulomb and energy efficiencies to Nafion 212 membrane at 100-300 mA cm(-2). And I-bSPI-50 membrane has steady efficiencies and high capacity holding abilities during 600-time cycles. This work illustrates a promising pathway to obtain cost-effective membrane by hydrogen bonds construction strategy for VFB application.