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.