Anion effects on the redox kinetics of positive electrolyte of the all-vanadium redox flow battery

The kinetics of the VO2 +/VO2+ redox reaction have been investigated in 1 M sulfuric and 1 M phosphoric acid by cyclic voltammetry, chronoamperometry, electrochemical impedance spectroscopy and flow battery tests. It was found that in 1 M phosphoric acid the electron transfer constant k0 is up to 67 times higher than in 1 M sulfuric acid. At higher over-potentials the determined currents match for the two electrolytes. This over-potential dependent difference in electron transfer constant is explained by variable contributions from three reaction mechanisms for the oxidation of VO2 + to VO2+, and by the presence of adsorbed intermediates for the reduction of VO2+. This study shows that the redox kinetics of the VO2 +/VO2+ can be considerably accelerated by altering the chemical environment of the vanadium ions, and that this effect can also be transferred into a flow battery.