Novel highly efficient branched polyfluoro sulfonated polyimide membranes for application in vanadium redox flow battery

Branched polyfluoro sulfonated polyimide (BPFSPI) membranes with different degree of branching and degree of sulfonation are designed and prepared for application in vanadium redox flow battery (VRFB). The optimized BPFSPI-10-50 membrane exhibits a similar proton conductivity (2.94 × 10 −2  S cm −1 ) as that of the commercial Nafion 212 membrane (3.20 × 10 −2  S cm −1 ) and a much higher membrane selectivity (3.16 × 10 5  S min cm −3 ) than Nafion 212 membrane (0.42 × 10 5  S min cm −3 ). Meanwhile, BPFSPI-10-50 membrane is durable in V(V) + H 2 SO 4 solutions for more than 30 d. Both coulomb efficiency ( CE ) and energy efficiency ( EE ) of BPFSPI-10-50 membrane are higher than those of Nafion 212 membrane ( CE : 94.2–98.9% vs 73.9–94.9%, EE : 68.0–86.9% vs 67.7–77.9%) in VRFB (40–200 mA cm −2 ) test. Besides, BPFSPI-10-50 membrane displays slower self-discharge rate and excellent discharge capacity retention compared to Nafion 212 membrane. Furthermore, the VRFB with BPFSPI-10-50 membrane shows very stable CE and EE (>98% and 76%) over 200 charge-discharge cycles. Accordingly, the BPFSPI-10-50 membrane with superior cost-performance ratio has an outstanding prospect for application in VRFB.