Vanadium Redox Flow Batteries

Abstract A redox flow battery (RFB) is a special type of electrochemical storage device. Electric energy is stored in electrolytes which are in the form of bulk fluids stored in two vessels. Power conversion is realized in a stack, made of electrodes, membranes, and bipolar plates. In contrast to conventional lead–acid or lithium-ion batteries, the energy conversion unit and energy storage unit are separate devices. From this point of view RFBs are more like reversible fuel cells in that the stack is the power conversion unit and is built in a filter press design; or, compared with other storage technologies, like pumped hydro systems, where power conversion is realized by pumps and turbines, but capacity by higher and lower elevation reservoirs. This advantage leads to applications where higher or various ratios of capacity to power (kilowatt-hours per kilowatt) are needed or advantageous—usual are ratios from 5:1 to 10:1. The most common and mature RFB is the vanadium redox flow battery (VRFB) with vanadium as both catholyte (V 2+ , V 3+ ) and anolyte (V 4+ , V 5+ ). No cross-contamination from anolyte to catholyte is possible and hence this is one of the most simple electrolyte systems known. Other electrolyte systems could be cheaper (Fe/Cr) or more efficient, but currently these are not available. Other important advantages of the VRFB include long-lasting operation time, long lifecycle; good stability; ease of regeneration or recycling of the electrolyte; and very low flammability. Disadvantages include low energy density both gravimetric and volumetric so that mobile applications are excluded, and currently they are relatively expensive when compared to lead–acid or lithium-ion batteries. Their higher cost is largely due to lower maturity and thus they do not have the advantage that mass production would bring about.