In recent years, vanadium redox flow batteries (VRFB) have captured immense attraction in electrochemical energy storage systems due to their long cycle life, flexibility, high-energy efficiency, time, and reliability. In VRFB, polymer membranes play a significant role in transporting protons for current transmission and act as barriers between positive and negative electrodes/electrolytes. Commercial polymer membranes (such as Nafion) are the widely used IEM in VRFBs due to their outstanding chemical stability and proton conductivity. However, the membrane cost and increased vanadium ions permeability limit its commercial application. Therefore, various modified perfluorinated and non-perfluorinated membranes have been developed. This comprehensive review primarily focuses on recent developments of hybrid polymer composite membranes with inorganic TiO2 nanofillers for VRFB applications. Hence, various fabrications are performed in the membrane with TiO2 to alter their physicochemical properties for attaining perfect IEM. Additionally, embedding the -SO3H groups by sulfonation on the nanofiller surface enhances membrane proton conductivity and mechanical strength. Incorporating TiO2 and modified TiO2 (sTiO2, and organic silica modified TiO2) into Nafion and other non-perfluorinated membranes (sPEEK and sPI) has effectively influenced the polymer membrane properties for better VRFB performances. This review provides an overall spotlight on the impact of TiO2-based nanofillers in polymer matrix for VRFB applications.
Keywords: TiO2; cation exchange membrane; hybrid membrane; inorganic additive; ion selectivity; physicochemical properties; vanadium redox flow battery.