The past few decades have shown a rapid and continuous exhaustion of the available energy resources which may lead to serious energy global crises. Researchers have been focusing on developing new and renewable energy resources to meet the increasing fuel demand and reduce greenhouse gas emissions. A surge of research effort is also being directed towards replacing fossil fuel based vehicles with hybrid and electric alternatives. Energy storage is now seen as a critical element in future "smart grid and electric vehicle" applications. Electrochemical energy storage systems offer the best combination of efficiency, cost and flexibility, with redox flow battery systems currently leading the way in this aspect. In this work, a panoramic overview is presented for the various redox flow battery systems and their hybrid alternatives. Relevant published work is reported and critically discussed. A comprehensive study of the available technologies is conducted in terms of technical aspects as well as economic and environmental consequences. Some of the flow battery limitations and technical challenges are also discussed and a range of further research opportunities are presented. Of the flow battery technologies that have been investigated, the all-vanadium redox flow battery has received the most attention and has shown most promise in various pre-commercial to commercial stationary applications to date, while new developments in hybrid redox fuel cells are promising to lead the way for future applications in mechanically and electrically "refuelable" electric vehicles.

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The vanadium redox flow battery, which was first suggested by Skyllas -Kazacos and co-workers in 1985, is an electrochemical storage system which allows energy to be stored in two solutions containing different redox couples. Unlike commercially available batteries, all vanadium redox flow batteries have unique configurations, determined by the size of the electrolyte tanks. This technology has been proven to be an economically attractive and low-maintenance solution, with significant benefits over the other types of batteries. Moreover, the soaring demand for large-scale energy storage has, in turn, increased demands for unlimited capacity, design flexibility, and good safety systems. This work reviews and discusses the progress on electrodes their reaction mechanisms as key components of the vanadium redox flow battery over the past 30 years. In terms of future outlook, we also provide practical guidelines for the further development of self-sustaining electrodes for vanadium redox flow batteries as an attractive energy storage system.

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