Stable covalent cross-linked polyfluoro sulfonated polyimide membranes with high proton conductance and vanadium resistance for application in vanadium redox flow batteries

The performance of vanadium redox flow batteries (VRFBs) is largely determined by the membrane as a separator. To address the trade-off issue between the proton conductance and vanadium resistance of sulfonated polyimide (SPI) membranes, novel covalent cross-linked polyfluoro sulfonated polyimide (PFSPI-PAA-X) membranes are developed by designing and synthesizing polyfluoro non-sulfonated diamine and adopting hydrophilic polyacrylic acid (PAA) as the cross-linking agent. The structures, morphology and physico-chemical properties of PFSPI-PAA-X membranes are systematically investigated. PFSPI-PAA-X membranes possess improved chemical stabilities in contrast to the pure PFSPI membrane. Among all PFSPI-PAA-X membranes, the PFSPI-PAA-25 membrane (0.15 Ω cm2) shows the lowest area resistance, which is even a little lower than that of the Nafion 212 membrane (0.16 Ω cm2). And, the PFSPI-PAA-25 membrane also has an extremely low vanadium permeability of 5.90 × 10−9 cm2 min−1, which is two orders of magnitude lower than that of the Nafion 212 membrane (7.53 × 10−7 cm2 min−1). The VRFB performances including the self-discharge time (PFSPI-PAA-25: 52.3 h, Nafion 212: 17.6 h), efficiencies at 60–300 mA cm−2 (PFSPI-PAA-25: CE = 97.3–99.9% and EE = 90.7–73.6%, Nafion 212: CE = 90.9–96.5% and EE = 84.7–72.0%) and peak power density (PFSPI-PAA-25: 451.4 mW cm−2, Nafion 212: 427.9 mW cm−2) of the PFSPI-PAA-25 membrane are all superior to those of the Nafion 212 membrane. Meanwhile, the PFSPI-PAA-25 membrane shows outstanding durability during the 500 times charge–discharge cycle test. Therefore, the as-prepared PFSPI-PAA-25 membrane with excellent cost-performance ratio has a bright future in replacing the Nafion 212 membrane for application in VRFBs.