Membranes for vanadium-air redox flow batteries

In recent decades, redox flow battery systems with independently tunable power and energy storage capacity have been regarded as promising energy storage systems. Among the various types of redox flow batteries, the vanadium/air redox flow battery is considered to have great potential, owing to its high energy density and wide operating temperature range. However, until now, the performance of vanadium/air redox flow batteries has been largely constrained by several challenges associated with its components, including the ion exchange membrane. As one indispensable component influencing both the performance and the cost of this system, the ion exchange membrane is considered to have a major effect and therefore has been studied as a key research goal in the past decades. To provide a comprehensive understanding of the membranes for vanadium/air redox flow batteries, this chapter summarizes the research outcomes in this field of study in recent years. Various aspects, including the working principles of vanadium-air redox flow battery systems, functional requirements of the membranes, membrane classifications, and mechanisms of species transport across the membrane, are discussed. Some potential strategies for enhancing membrane performance are also identified.