Adjustment of Electrolyte Composition for All-Vanadium Flow Batteries and its Effect on the Thermal Stability of Electrolyte for Positive and Negative Half-Cells

Commercial electrolyte for vanadium flow batteries is modified by dilution with sulfuric and phosphoric acid so that series of electrolytes with total vanadium, total sulfate, and phosphate concentrations in the range from 1.4 to 1.7 m, 3.8 to 4.7 m, and 0.05 to 0.1 m, respectively, are prepared. The electrolyte samples of the series for positive and negative half-cells at various state-of-charges are produced by electrolysis and are investigated for stability in the range of temperatures from -20 to +65 °C. It is attempted to reveal a correlation between initial electrolyte formulation in terms of total vanadium and total sulfate concentrations, which are measurable parameters in practice, and electrolyte thermal stability properties. The study of negative electrolyte samples by headspace online mass spectrometry enables to detect hydrogen gas, which evolves by chemical reaction of vanadium(II) species with protons during thermally induced aging. The battery with vanadium electrolyte at 1.4 m total vanadium, 4.7 m total sulfate, and 0.1 m phosphate concentrations displays more stable operation in terms of capacity decay during galvanostatic charge–discharge cycles than the battery with electrolyte at 1.7 m vanadium, 3.8 m sulfate, and 0.05 m phosphate concentrations under the same conditions.