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In this paper, a high energy density vanadium redox battery employing a 3 M vanadium electrolyte is reported. To stabilise the highly supersaturated vanadium solutions, several additives were evaluated as possible stabilizing agents for the thermal precipitation of supersaturated V(V) solutions at elevated temperatures. The Blank 3 M V(V) solution in a sulfuric acid supporting electrolyte containing 5 M total sulfates, showed thermal precipitation after 3 days, while the solution containing 1 wt% H 3 PO 4 additive increased the induction time for precipitation to over 47 days at 30 • C. After 32 days, 3 M V(V) solutions containing 1 wt% sodium pentapoIyphosphate, 1 wt% K 3 PO 4 and 2 wt% (NH 4 ) 2 SO 4 + 1 wt% H 3 PO 4 showed final V(V) concentrations of 2.7, 2.7 and 2.6 M respectively, compared to 2.4 M V(V) in the Blank solution. From the screening tests, selected additives were used in vanadium redox flow cell cycling studies employing a 3 M vanadium electrolyte. The cell was subjected to 90 charge-discharge cycles and no precipitation or capacity loss was observed in the presence of 1wt% H 3 PO 4 + 2 wt% ammonium sulfate. These results demonstrate that a significant enhancement in the energy density of the VRB can be achieved in the presence of additives that act as precipitation inhibitors for the vanadium ions. The UNSW All-vanadium redox flow battery (VRB) has been attracting considerable commercial interest in recent years, with several companies currently manufacturing VRB systems in Japan, USA, Austria and China. Although its energy density is relatively low compared to Li-ion batteries, the VRB offers excellent cycle life, energy efficiency and reasonable costs for storage capacities greater than 4 hours, making it ideal for large-scale energy storage in a range of stationary applications. [1][2][3][4][5] Efforts to increase the energy density of the vanadium redox flow battery have included the use of a mixed halide electrolyte in the case of the vanadium bromide redox battery developed by Skyllas-Kazacos and co-workers in Australia 6 and a mixed HCl-H 2 SO 4 supporting electrolyte as proposed by researchers at the Pacific Northwest National Laboratories in the USA. 7 In both cases however, the use of HCl and/or HBr in the electrolytes introduces the risk of acid, chlorine or bromine vapors at elevated temperatures or during overcharge, and therefore restricts the operating state-ofcharge range of the batteries, while also presenting a potential safety hazard.In contrast to both the VBr and mixed acid vanadium chemistries, the original UNSW All-Vanadium Redox Flow Battery employs sulfuric acid as the supporting electrolyte, so any overcharge reactions produce oxygen gas at the positive electrode, making it a much safe system. Current VRB systems employ solutions of between 1.6 and 2 M vanadium ions in sulfuric acid with total sulfate concentrations ranging between 4 and 5 M and utilise the V(II)/V(III) and V(IV)/V(V) couples in the negative and positive half-cell electrolytes respectively. Increasing...

The vanadium redox battery currently employs solutions of up to 2 M V(II)/V(III) and 2 M V(IV)/V(V) as the negative and positive half‐cell electrolytes. This concentration is limited by the solubility of the different vanadium ions in the temperature range of 10 to 40°C. Generally, the solubility of V(II), V(III), and V(IV) increases with an increase in temperature; however, the V(V) electrolyte suffers from the effect of thermal precipitation at temperatures of 40°C and above. While thermal precipitation is a serious problem in solutions of V(V) concentrations between 1.5 and 2.0 M, a surprising result was observed at concentrations above 3.0 M. As the results presented here show, at higher vanadium concentrations the V(V) solution demonstrated increased stability and there was no evidence of thermal precipitation over a 30 day period at temperatures above 40°C.

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A promising approach to improving the energy density of the all-vanadium redox flow battery while also saving on raw materials costs, is to eliminate the positive half-cell electrolyte and replace it with an air electrode to produce a hybrid vanadium-oxygen redox fuel cell (VOFC). This concept was initially proposed by Kaneko et al. in 1992 and first evaluated at the University of New South Wales by Menictas and Skyllas-Kazacos in 1997. In this project the performance of the VOFC over a range of temperatures and using different types of membranes and air electrode assemblies was evaluated. Despite early problems with the membrane electrode assemblies that saw separation of the membrane due to swelling and expansion during hydration, with improved fabrication techniques, this problem was minimized and it was possible to operate a 5-cell VOFC system for a total of over 100 h without any deterioration in its performance.

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