Highly ion-selective sulfonated polyimide membranes with covalent self-crosslinking and branching structures for vanadium redox flow battery

With an innovatively synthesized X-shaped tetramine monomer 4,4',4'',4'''-(1H,3'H-[5,5'-bibenzo[d]imidazole]) tetraaniline with imidazole groups, a series of novel sulfonated polyimide membranes with covalent self-crosslinking and branching structures (sc-bSPI-x) are designed and prepared for application in vanadium redox flow battery (VRFB). The synergistic effects of protonated imidazole and sulfonic acid are supposed to create proton transport channels and Donnan exclusion, leading to break the trade-off between proton conductance and vanadium ion resistance. Among all the prepared sc-bSPI-x membranes, the sc-bSPI-14 membrane with 14% theoretical cross-linking degree shows the highest ion selectivity (2.78 × 105 S min cm−3), which is about 6.5 times of commercial Nafion 212 membrane (0.43 × 105 S min cm−3). The VRFB performances including self-discharge time, coulombic and energy efficiencies at 80–200 mA cm−2 of sc-bSPI-14 membrane are superior to those of Nafion 212 membrane (sc-bSPI-14: self-discharge time 46 h, CE = 97.6–99.2% and EE = 82.9–63.2%; Nafion 212: self-discharge time 11 h, CE = 86.5–94.5% and EE = 78.4–61.4%). Meanwhile, the 1000-time cycling charge–discharge test of sc-bSPI-14 membrane at 140 mA cm−2 is performed, demonstrating its outstanding operational stability. The results indicate that the proposed sc-bSPI-14 membrane is a promising candidate for VRFB application.