Next‐Generation Vanadium Flow Batteries

Since the original all-vanadium flow battery (VFB) was proposed by UNSW in the mid-1980s, a number of new vanadium-based electrolyte chemistries have been investigated to increase the energy density beyond the 35 Wh l −1 of the original UNSW system. The different chemistries are often referred to as Generations 1 (G1) to 4 (G4) and they all involve vanadium as the main active material. The original Generation 1 (G1) utilises vanadium ions in a sulphuric acid supporting electrolyte with or without additives. Generation 2 (G2), the vanadium bromide flow cell (V/Br), also developed by researchers at UNSW Sydney, employs the same vanadium halide solution in both half-cells but utilises the V 2+ /V 3+ redox couple and the Br − /Br 3 − couple in the negative and positive half-cells, respectively. This system can provide a twofold increase in energy density with the possibility of bromine gas release being mitigated by the use of complexing agents. Researchers at the Pacific Northwest National Laboratory in the US proposed using a mixed-acid electrolyte consisting of H 2 SO 4 and HCl to support the vanadium ions. This system is often referred to as the Generation 3 VFB (G3) and the mixed-acid electrolyte enables higher concentrations of vanadium to be dissolved in the supporting electrolyte compared to G1, together with a wider operating temperature range. The vanadium oxygen fuel cell (VOFC) was initially proposed by researchers at the Electrotechnical Laboratories in Japan and demonstrated at UNSW Sydney. It is referred to as the Generation 4 (G4) VFB and utilises the V 2+ /V 3+ redox couple in the negative half-cell and the oxygen reduction reaction in the positive. As there is no positive electrolyte tank, the electrolyte volume is half that of the G1, G2, and G3 VFBs, providing a significantly higher theoretical energy density of approximately 150 Wh l −1 . While the G1 VFB is currently undergoing considerable commercialisation and the G3 mixed-acid electrolyte system has seen substantial field testing, the G2 and G4 technologies are still in the early stages of development, with further work still needed to achieve commercial uptake.