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.

The need for large scale energy storage has become a priority to integrate renewable energy sources into the electricity grid. Redox flow batteries are considered the best option to store electricity from medium to large scale applications. However, the current high cost of redox flow batteries impedes the wide spread adoption of this technology. The membrane is a critical component of redox flow batteries as it determines the performance as well as the economic viability of the batteries. The membrane acts as a separator to prevent cross-mixing of the positive and negative electrolytes, while still allowing the transport of ions to complete the circuit during the passage of current. An ideal membrane should have high ionic conductivity, low water intake and excellent chemical and thermal stability as well as good ionic exchange capacity. Developing a low cost, chemically stable membrane for redox flow cell batteries has been a major focus for many groups around the world in recent years. This paper reviews the research work on membranes for redox flow batteries, in particular for the all-vanadium redox flow battery which has received the most attention.

The vanadium redox flow battery pioneered by Skyllas‐Kazacos et al. at the University of New South Wales (UNSW) is currently considered as one of the few electrochemical energy storage systems suitable for use in the large‐scale utility applications that are emerging in response to the increasing global implementation of renewable energy technologies for the mitigation of greenhouse gas emissions. While the original all‐vanadium redox flow battery (G1 VRF) has already been successfully implemented in a wide range of stationary field trials in Japan, U.S.A., Austria, Italy and Australia, further cost reduction has been needed for its widespread market up‐take. In this paper, up to 80% efficiency is reported for the latest 5–10 kW G1 VRF battery stack employing low‐cost stack materials that are expected to achieve the necessary cost structure for the majority of stationary energy storage applications. While the G1 VRF battery has shown high efficiencies and excellent cycle life, however, its low energy density has restricted its use in mobile applications. The new Generation 2 vanadium bromide redox battery (G2 V/Br) patented by UNSW in 2001 has been shown to potentially double the energy density of the G1 VRF battery, allowing mobile applications to be considered. In this paper, the performance of the G2 V/Br is presented at a range of temperatures and the use of complexing agents is shown to successfully bind any bromine produced during charging to prevent the formation of bromine vapours. Copyright © 2009 John Wiley & Sons, Ltd.

As part of a project aimed at increasing the energy density of the vanadium redox battery for mobile applications, a number of additives was evaluated as precipitation inhibitors to enhance the stability of supersaturated vanadyl electrolytes in . While the saturation solubility of vanadyl sulfate in is less than at , supersaturated vanadyl sulfate solutions could readily be prepared. A blank solution of vanadyl sulfate in began to precipitate after 22 days at , reaching an equilibrium concentration of after 80 days. In the presence of or sodium hexametaphosphate, however, there was no sign of precipitation after 80 days at . ©1999 The Electrochemical Society