Revealing the Effects of Impurities among Phosphoric Acid Electrolytes for the Stability of All-Vanadium Redox Flow Batteries

Impurities in the electrolyte have a large impact on the efficiency and stability of all-vanadium redox flow batteries. Herein, this work tries to introduce the electronegativity to explain the effects of the impurities on the stability and electrochemical performance of vanadium electrolyte. The stability of vanadium sulfate acid redox flow batteries is evaluated in an orthogonal experiment with six control factors at three levels in the presence of low-concentration impurities (0–50 ppm) in phosphoric acid electrolytes. The factors affecting the stability of the positive and negative electrolytes in polypropylene tubes are ranked as follows: temperature (Temp.) > state of charge (SOC) > Al3+ > NH4+ > Fe2+ > Si/F and Al3+ > SOC > Temp. > NH4+ > Si/F > Fe2+, respectively. Especially, Al3+ impurity concentrations between 0 and 200 ppm increased the electrochemical activity and reversibility in 0.1 M H3PO4 and 3 M H2SO4 containing 0.1 M VOSO4 for VO2+/VO2+ couple, but decreased above 50 ppm in 0.1 M H3PO4 and 3 M H2SO4 containing 0.05 M V2(SO4)3 for V2+/V3+ couple. The presence of 50 ppm of Al3+ in the vanadium electrolyte slows down the mass transfer of the V(IV)/V(V) and V(II)/V(III) redox reactions.