A novel fluorinated acid-base sulfonated polyimide membrane with sulfoalkyl side-chain for vanadium redox flow battery

A novel fluorinated acid-base sulfonated polyimide membrane (s-f-SPI) with trifluoromethyl (-CF3), side-chain-type-SO3H and benzimidazole groups was synthesized by a 2-step method for vanadium redox flow battery applications. A series of physico-chemical properties of s-f-SPI membrane were researched systematacially. The acid-base pairs between the side-chain-type -SO3H and protonated imidazole groups provided more channels for proton-transport, enhancing the proton conductivity (0.51×10−1 S cm−1 at ∼25 °C), and Donnan effect of the imidazole groups impeded the penetration of vanadium-ion (3.95×10−7 cm2 min−1). In VRFB tests, s-f-SPI membrane exhibited a coulombic efficiency (CE) of 98.1% and an energy efficiency (EE) of 81.2% at a current density of 100 mA cm−2, higher than those of Nafion 212 (CE: 94.1%, EE: 76.5%). Furthermore, the VRFB with s-f-SPI membrane showed a stable performance in the 500 cycles charging-discharging test at 100 mA cm−2, and the morphology and structure remained unchanged after the cycle test. These results demonstrate that the s-f-SPI membrane has great potential for VRFB applications.