Redox Flow Battery for Energy Storage

The redox flow battery has undergone widespread research since the early 1970s. Several different redox couples have been investigated and reported in the literature. Only three systems as such have seen some commercial development, namely the all-vanadium (by VRB-ESS), the bromine-polysulfide (RGN-ESS) and the zinc-bromine (Powercell) systems. The vanadium-bromine system may be an attractive replacement for the all-vanadium system due to its higher energy density with possible applications as energy storage systems for electric vehicles. Other redox flow battery systems have faced problems due to slow electrochemical kinetics of redox couples, membrane fouling, cross-contamination, high costs (mainly due to the membrane as well as inefficient cell stack design), poor sealing, shunt current losses and low energy capacity (due to the use of aqueous electrolytes). One of the main factors limiting further development of the redox flow battery so far is the high costs associated with the ion-exchange membrane. Focussed research in this as well as areas such as reactor characterization and electrode design is necessary to ensure the widespread commercialization of the technology. In this paper, various redox flow systems are discussed historically and technically and the latest developments are compared.