Influence of solvent on ion conductivity of Polybenzimidazole proton exchange membranes for vanadium redox flow batteries

Abstract Polybenzimidazole (PBI) is a kind of proton transport membrane material, and its ion conductivity is a key factor affecting its application in vanadium redox flow batteries (VRFBs). The casting solvent of PBI has a significant influence on the acid doping level of PBI membranes which is closely related to ionic conductivity. In this paper, 3,3′-diaminobenzidine (DABz) and 4,4′-Dicarboxydiphenylether (DCDPE) were used as raw materials by solution condensation to prepare the PBI with ether bond groups. The chemical structure of PBI was determined by 1H NMR and FT-IR, and the prepared PBI had good solubility which can be dissolved in a variety of solvents. The PBI proton exchange membranes were prepared by solution coating with 5 different solvents of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), 1-methyl-2-pyrrolidone (NMP), methane sulfonic acid (MSA). The effects of different solvents on the ion conductivity and physicochemical properties were discussed in detail. The results showed that the PBI membrane prepared by using MSA as solvent (the PBI + MSA membrane) exhibits high water uptake, acid doping level and low vanadium ion permeability. The VRFB assembled with the PBI + MSA membrane exhibited higher coulombic efficiency (CE) 99.87% and voltage efficiency (VE) 84.50% than that of the commercial Nafion115 membrane at 100 mA·cm−2, and after 480 cycles, the EE value can still be maintained at 83.73%. The self-discharge time of a single battery was recoded to be as long as 1000 h. All experimental data indicated that MSA is the best solvent for casting PBI membrane.