A novel permselective branched sulfonated polyimide membrane containing crown ether with remarkable proton conductance and selectivity for application in vanadium redox flow battery

As a key component of vanadium redox flow battery (VRFB), the ideal membrane with high proton conductance together with high vanadium ions resistance is urgently required. In this work, a permselective monomer di (aminobenzo)-18-crown-6 with proper aperture of 0.26-0.32 nm is synthesized to prepare branched sulfo-nated polyimide containing crown ether (ce-bSPI-x) membranes. In virtue of the excellent balance between vanadium ions resistance and proton conductance, the ce-bSPI-60 membrane possesses the optimum physico-chemical properties among all ce-bSPI-x membranes. Subsequently, in comparison with commercial Nafion 212 membrane, the ce-bSPI-60 membrane demonstrates higher coulomb efficiencies (CEs) (96.50%-99.39%) and energy efficiencies (EEs) (69.36%-85.45%). Furthermore, the ce-bSPI-60 membrane can achieve 80% of EE at 140 mA cm-2, which is in a superior position compared with other SPI-based membranes reported in recent years. 1000-time cycles at 140 mA cm-2 without obvious structural and morphological changes proves the excellent durability of ce-bSPI-60 membrane. To sum up, the as-prepared ce-bSPI-60 membrane exhibits a great potential to match the demand of application in VRFB.