Cross-linked sulfonated polyimide membranes with excellent proton selectivity and ultra-long cycle life for vanadium flow batteries

The membrane always plays as a crucial component of vanadium flow battery (VFB), and its proton selectivity and stability determine battery efficiencies and life. Herein, a series of permselective cross-linked sulfonated polyimide (PFSPI-DNBC) membranes are constructed by using crown ether with proper cavity size as cross-linker to break the trade-off effect between proton conductance and vanadium ions blocking. In contrast to commerical Nafion 212 membrane, the as-optimized PFSPI-DNBC-15 membrane reveals outstanding comprehensive properties: same area resistance (0.21 Ω cm 2 ), excellent vanadium ions blocking (0.44×10 −7  cm 2  min −1 ) and 15.91 times higher proton selectivity (5.41×10 5  S min cm −3 ). Furthermore, the PFSPI-DNBC-15 membrane shows higher Coulomb efficiencies ( CE s: 97.45%–98.35%) and energy efficiencies ( EE s: 68.58%–87.76%) compared with Nafion 212 membrane ( CE s: 92.74%–95.26%; EE s: 67.39%–81.28%) at 100–280 mA cm −2 . Exhilaratingly, the PFSPI-DNBC-15 membrane shows remarkable chemical structural and morphological stabilities over 2280 VFB cycles at 180 mA cm −2 . The internal reasons of improved stability of PFSPI-DNBC-15 membrane are described by simulation calculations. To sum up, the PFSPI-DNBC-15 membrane possesses a bright application prospect in VFB.