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

The VO2+/VO2+ redox reaction takes place in the catholyte solution of the all-vanadium redox flow battery (VRFB), one of the few options to electrochemically store energy from intermittent renewable sources on a large scale. However, the sluggish redox kinetics of the VO2+/VO2+ couple limit the power density of the VRFB, which increases the footprint of the power converters and increases capital costs. Therefore, catalysis of the redox reaction and a deeper understanding of its intricate reaction pathways is desirable. 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 1(0 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.