Towards an all-vanadium redox-flow battery electrolyte: electrooxidation of V(III) in V(IV)/V(III) redox couple

The electrochemistry of the V(IV)/V(III) redox couple influences the initial pre-charging of the electrolyte in vanadium redox-flow batteries to produce a catholyte and anolyte containing V(V)/V(IV) and V(III)/V(II) couples respectively. The electrochemical conversion of V(III) to V(IV) at a glassy carbon electrode has been studied in detail by cyclic voltammetry in sulfuric acid solutions containing 1.6 M and 0.25 M of V(III) species. A sigmoid-shaped voltammetric response with a half-wave potential at 0.1 V vs. Hg/Hg2SO4 was observed at low scan rates and was attributed to the oxidation of V(III) at an oxidatively pretreated glassy carbon electrode. The attribution of voltammetric responses was consistent with results from a spectroelectrochemical study in 0.25 M vanadium electrolyte solution at a reticulated glassy carbon electrode. Initial pre-charging of V3.5+ electrolyte in vanadium redox-flow batteries is assumed to be accompanied by the generation of V(V) in the catholyte and its chemical reaction with V(III) to form V(IV).