A cross-linked sulfonated polyimide membrane with regulated acid-base interaction and high-performance for vanadium redox flow battery

Researchers have attempted to introduce imidazole groups into sulfonated polyimide (SPI) to create acid–base interactions, which could effectively improve the physicochemical properties of membranes. However, the strong alkalinity of imidazole groups makes the deprotonation process difficult, thereby impeding the proton transport process. Herein, electron-withdrawing benzoyl groups was introduced by a cross-linking process to regulate the acid-base interactions, which successfully reduced the alkalinity of benzimidazole group, thereby reinforcing proton conduction. Meanwhile, the dense structure formed on the membrane effectively reduce the vanadium ion permeability. SPI membranes with a cross-linking time of 10 min exhibited low vanadium ion permeability (1.38 × 10 −7  cm 2  min −1 ) and the highest proton conductivity of 76.0 mS cm −1 , achieving the superb stability for 1500 cycles with the highest EE of 83.9 % at 120 mA cm −2 . This work offers a facile approach to solve the trade-off issue between proton conductivity and vanadium ions permeability for SPI-based proton exchange membranes.