A simple and effective method of enhancing the proton conductivity of polybenzimidazole proton exchange membranes through protonated polymer during solvation

Acid-doped polybenzimidazole (PBI) membranes are proton exchange membranes (PEM) which show promise for use in vanadium redox flow batteries (VRFBs). Fluorine-containing PBI membranes can withstand the strong acidity and oxidative electrolytes of VRFB due to their excellent oxidative resistance. However, the proton conductivity of dense fluorine-containing PBI membranes is moderate. In this work, the protic solvent methanesulfonic acid (MSA) was used to enhance the proton conductivity of hexafluoroisopropylidene group containing PBI (6FPBI) PEM and the performance of this membrane (6FPBI-MSA) was compared with that of a membrane (6FPBI-DMAc) that was manufactured using the aprotic solvent N,N-Dimethylacetamide (DMAc). MSA can protonate the nitrogen atoms in imidazole groups during polymer solvation, and the positively charged nitrogen atoms prevent the formation of intermolecular hydrogen bonds among polymers by electrostatic repulsion. Hence, the free volume and amorphous state of the 6FPBI-MSA were superior to those of 6FPBI-DMAc membrane. The proton conductivity of the 6FPBI-MSA membrane (8.371 mS cm⁻¹) is significantly higher than that of the 6FPBI-DMAc membrane (4.904 mS cm⁻¹). Correspondingly, the energy efficiency of the 6FPBI-MSA membrane is superior to that of the 6FPBI-DMAc membrane at various current densities. Protonation of 6FPBI using MSA during dissolution is an effective way to enhance its proton conductivity.