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 mvanadium and 4.5 mtotal 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 degrees C and based on characterization of catholytes series using(51)V,O-17, and(1)H nuclear magnetic resonance spectroscopy.