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2 Vanadium Redox Flow Battery Review Electrolyte Considerations Whilst the cost of the vanadium electrolyte is essentially fixed by the market price for vanadium, the performance of a VRFB is heavily influenced by the chosen electrolyte [43]. Typically, vanadium ions are dissolved in sulphuric acid, with the maximum concentration of vanadium limited by the lowest solubility of the four different redox states, within the chosen operating temperature range. The solubility of the V2+, V3+ and V4+ ions increase with increasing temperature, but decrease with increased sulphuric acid concentration. The decrease in solubility with sulphuric acid is due to the common-ion effect, whereby each of these vanadium redox states precipitate as vanadium sulphate salts. However, the solubility of the V5+ ion decreases with temperature and increases with sulphuric acid concentration, as the V5+ species precipitates as V2O5. The electrolyte concentration is typically 1.6 – 2 M of vanadium and 2 – 5 M sulphate, with higher sulphate recommended for operation in warmer climates. Using a mixed electrolyte with hydrochloric acid and sulphuric acid enables higher vanadium concentrations, by reducing the common ion effect. However, the presence of Cl- ions in the electrolyte introduces a significant safety issue. During charging the cell potential increases and as the SOC approaches 100%, the charging becomes mass transfer limited and the overpotential increases. If the potential goes too high, Cl- ions will be oxidised to form Cl2, a toxic gas. The performance advantages of the mixed electrolyte however are significant, with a stable 2.5 M vanadium concentration. This increased concentration reduces concentration polarization losses, increasing the efficiency, power density and energy density of the battery. Addition of phosphoric acid has been shown to increase the kinetics for the positive redox couple. The apparent rate constant for the VO2+ / VO2+ redox couple in a 1 M solution of phosphoric acid was 6.1 × 10−4 cm s-1, 70 times larger than the 9.2 × 10−6 cm s-1 recorded with a 1 M sulphuric acid solution. When a mixed acid electrolyte of 0.5 M phosphoric and 0.5 M sulphuric was used, the rate constant was still significantly higher at 1.2 × 10−4 cm s-1. As the phosphoric acid is not consumed this is a form of homogenous catalysis, which may be the result of the phosphate ion forming a complex that increases the inherent rate constant [62]. 19PDF Image | Electron Transfer Kinetics in Redox Flow Batteries
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