A sulfonated polyimide containing imidazole ring with a low vanadium ion permeable for vanadium redox flow battery

A new type of sulfonated polyimide membrane (6f-abSPI) containing benzimidazole group and trifluoromethyl group (-CF3) is synthesized by one-step copolymerization. The 6f-abSPI-60 membrane with the highest ion selectivity is selected and applied to all vanadium redox flow battery (VRFB). Compared with Nafion 212 (N212) membrane and 6f-SPI-60 membrane, the vanadium ion permeability of 6f-abSPI-60 membrane (1.78×10-7cm2 min-1) is 1/13 lower than that of N212 membrane (2.33×10-6 cm2 min-1) and 1/2 lower than 6f-SPI-60 membrane (3.60×10-7 cm2 min-1). At the same time, the 6f-abSPI-60 membrane (2.69 × 105 S min cm−3) has significant selectivity, which is 5.6 times higher than N212 membrane (0.48 × 105 S min cm−3). In VRFB tests, the coulombic efficiency (CE) of 6f-abSPI-60 membrane (98.88%) is significantly higher than that of N212 membrane (92.99%) and 6f-SPI-60 membrane (97.65%) at 100 mA cm-2, and the energy efficiency (EE) also reaches a higher value (77.29%). Additionally, the VRFB equipped with the 6f-abSPI-60 membrane exhibits consistent and reliable performance throughout the 100 cycles of charging and discharging at the current densities of 100 mA cm−2. The good stability of 6f-abSPI membranes is explained by calculating C atoms' natural bond orbital (NBO) charges on different polyimide (PI) rings.