Electron-Transfer Kinetics of V(IV)/V(V) and V(II)/V(III) Couples for Different State of Charge

The electrochemical behavior of the V(IV)/V(V) and V(II)/V(III) redox couples has been studied at a graphite electrode by cyclic voltammetry in sulfuric acid solutions. The diffusion coefficient (D) and the rate constant (k) of V(IV)/V(V) and V(II)/V(III) redox couples for varying state of charge (SOC) can be calculated by cyclic voltammetry. The values of D-a(diffusion coefficient of anode) and k(a)(the rate constant of anode) are increased for V(IV)/V(V) couple while decreased for V(II)/V(III) couple with increasing of SOC. Electrochemical impedance spectroscopy (EIS) is employed to study mechanism of the two couples in sulfuric acid medium. Characteristic EIS plots with the capacitive semicircle in high frequency and the Warburg tail in low frequency are observed for the two couples, implying that both the electrochemical and diffusion process play important roles in V(IV)/V(V) and V(II)/V(III) couples, the two electrode processes are controlled by both electron-transfer and mass-transfer.