Enhancing proton conductivity of polybenzimidazole membranes by introducing sulfonate for vanadium redox flow batteries applications

Acid doped polybenzimidazole membrane can transport protons and is a potential proton exchange membrane for vanadium redox flow battery applications. But the proton conductivity of polybenzimidazole membranes is just moderate; the vanadium redox flow battery assembled with polybenzimidazole membranes usual cannot achieve satisfactory voltage efficiency. Therefore, the main aim of this study is to prepare high proton conductivity polybenzimidazole membranes. A series of sulfonated polybenzimidazoles with different sulfonation degree are synthesized. The 5-sulfoisophthalic acid monosodium salt is introduced not only to control the sulfonation degree of the polymer but also to enhance the hydrophilicity of polybenzimidazole and further increase the proton conductivity of polybenzimidazole membrane. The physicochemical properties and single cell performance of sulfonated polybenzimidazoles membranes are thoroughly assessed. The results indicate that the water uptake of membranes are enhanced as the sulfonation degree increases and is superior to increase the proton conductivity. But sulfonate can also form hydrogen bonds with the basic groups - N=in imidazole rings and block the protons transport. Ultimately, sulfonated polybenzimidazole membrane with the sulfonation degree of 30% exhibites the highest proton conductivity as well as energy efficiency.