Resolving charge-transfer and mass-transfer processes of VO(2+)/VO(2)(+) redox species across the electrode/electrolyte interface using electrochemical impedance spectroscopy for vanadium redox flow battery

Electrochemical impedance spectroscopy is used to investigate the charge-transfer and mass-transfer processes of VO(2+)/VO(2)(+) (V(4+)/V(5+)) 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 V(4+) or V(5+) 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 V(4+) and V(5+), 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 V(4+)/V(5+) redox species and supporting H(2)SO(4) electrolyte are required to resolve the EIS features corresponding to the underlying physical processes. The semi-infinite linear diffusion characteristics of the V(4+)/V(5+) redox species observed with a static condition of the electrode converges to that of a finite diffusion under hydrodynamic condition.