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

The VO^{2 +}/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 VO^{2 +}/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 VO^{2 +}/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 VO^{2 +} to VO₂ ⁺, and by the presence of adsorbed intermediates for the reduction of VO₂ ⁺. This study shows that the redox kinetics of the VO^{2 +}/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.