A study of VO2+/VO2+ and V3+/V2+ reactions on carbon-based electrodes – correlating reaction kinetics to electrode surface properties

Abstract

Vanadium redox flow battery (VRFB) shows great potential for large scale energy storage. The reaction kinetics of V^3+/2+ and VO²⁺/VO₂⁺ limit its efficiency. Carbon-based electrodes are typically used in VRFBs. Controversial results are reported in the literature on how the surface properties of carbon electrodes affect the reaction kinetics. In this work, 6 carbon based electrodes (Graphite rod presoaked in H₂SO₄ (Graphite-soaked), Graphite-untreated, Graphite-Pine, Edge plane pyrolytic graphite, Basal plane graphite, and glassy carbon (GC)) are studied with respect to the electrochemical surface property and reaction kinetics of VO²⁺/VO₂⁺ and V^3+/2+ redox couples. Cyclic voltammetry reveals that capacitance, carbonyl group density, and carboxylic group density of studied electrodes are dependent on the type of electrode and that soaking in H₂SO₄ leads to an increase in capacitance and functional group density. Diffusion coefficient, charge transfer coefficient, and reaction rate constant for both VO²⁺/VO₂⁺ and V^3+/2+ reactions are also dependent on the type of electrode. The diffusion coefficient of VO²⁺ increase linearly with the logarithm of carbonyl group density, and that of V³⁺ increase linearly with the logarithm of capacitance and carbonyl group density. Kinetic current is calculated from the charge transfer coefficient and reaction rate constant, and correlated to the surface properties. For VO²⁺ to VO₂⁺ reaction, slightly stronger relationship is observed for kinetic current vs logarithm of carbonyl group density than that vs the logarithm of capacitance and carboxylic group. For the V³⁺ to V²⁺ reaction, weak relationship between kinetic current and all the 3 properties are found.