The Influence of Free Acid in Vanadium Redox‐Flow Battery Electrolyte on "Power Drop" Effect and Thermally Induced Degradation

A series of vanadium redox‐flow battery (VRFB) electrolytes at 1.55 M vanadium and 4.5 M total sulfate concentration is prepared from vanadyl sulfate solution and tested under conditions of appearance of ""power drop"" effect (discharge at high current density from high state‐of‐charge). A correlation between the initial electrolyte composition, the thermal stability of catholyte and the susceptibility of VRFB to exhibit a ""power drop"" effect is derived. The increase of total acidity to 3 M, expressed as concentration of sulphuric acid in precursor vanadyl sulfate solution, enables ""power drop"" free operation of VRFB at least at 75 mA cm‐2. Thermally‐induced degradation of electrolyte was evaluated based on decrease in vanadium concentration in the electrolyte series after exposure to the temperature of 45°C and based on chacterization of catholytes series using 51V, 17O, 1H nuclear magnetic resonance spectroscopy.