Resolving charge-transfer and mass-transfer processes of VO2+/VO2+ redox species across the electrode/electrolyte interface using electrochemical impedance spectroscopy for vanadium redox flow battery

Abstract

Electrochemical impedance spectroscopy is used to investigate the charge-transfer and mass-transfer processes of VO²⁺/VO₂⁺ (V⁴⁺/V⁵⁺) redox species across the carbon-modified glassy carbon disk electrode/electrolyte interface. The features of the EIS patterns depend on the potential, concentrations of the redox species and mass-transport conditions at the electrode/electrolyte interface. With the starting electrolyte containing either only V⁴⁺ or V⁵⁺ redox species, EIS shows a straight line capacitor feature, as no oxidation or reduction reaction take place at the measured open circuit potential (OCP). With the electrolyte containing equimolar concentration of V⁴⁺ and V⁵⁺, EIS pattern has both charge-transfer and mass-transfer features at the equilibrium potential. The features of the charge-transfer process are observed to be influenced by the mass-transfer process. Optimum concentrations of the V⁴⁺/V⁵⁺ redox species and supporting H₂SO₄ electrolyte are required to resolve the EIS features corresponding to the underlying physical processes. The semi-infinite linear diffusion characteristics of the V⁴⁺/V⁵⁺ redox species observed with a static condition of the electrode converges to that of a finite diffusion under hydrodynamic condition.