Recent developments in sol-gel based polymer electrolyte membranes for vanadium redox flow batteries - A review

The current importance of energy production from renewable sources stimulated interest in energy storage systems. Recent research has focused on developing vanadium redox flow batteries (VRFB) for large scale energy storage owing to their cost-effectiveness, flexible design, energy efficiency and long cycle life. Nafion the preferred membrane in VRFB systems has an overall cost of more than 11% in these systems. However, due to its drawbacks of high permeability and high cost limit its suitability for commercial use. Many researchers are therefore keen to develop new chemical designs for synthetic membranes, such as perfluorinated, partially fluorinated, hydrocarbon based and organic-inorganic composite membrane. The sol-gel process, which is mainly based on hydrolysis and condensation reaction offers the possibility of homogeneous preparation of membranes, leading to high proton conductivity and reduced vanadium ion permeability, thereby greatly increasing the cell efficiency. This review summarizes recent development in the synthesis and applications of sol-gel based proton exchange membranes for VRFB system, on which many researchers have been working in recent years. We also discuss critical research areas and the future development of cost-effective sol-gel based membranes for attractive energy storage systems.