The Influence of Free Acid in Vanadium Redox-Flow Battery Electrolyte on “Power Drop” Effect and Thermally Induced Degradation

Abstract

A series of vanadium redox-flow battery (VRFB) electrolytes at 1.55 m vanadium and 4.5 m total sulfate concentration are 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 in total acidity to 3 m, expressed as concentration of sulfuric 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 is evaluated based on decrease in vanadium concentration in the electrolyte series after exposure to the temperature of 45 °C and based on characterization of catholytes series using 51V, 17O, and 1H nuclear magnetic resonance spectroscopy.