Investigations on the Electrode Process of Concentrated V(IV)/V(V) Species in a Vanadium Redox Flow Battery

The electrode process of concentrated V(IV)/V(V) species has been studied at a graphite electrode by using cyclic voltammetry, low speed linear polarization, and impedance techniques. The results have revealed that in H2SO4 solution below 2 mol·L−1 containing 2.0 mol·L−1 V(IV), the electrode process of V(IV)/V(V) is controlled by the electrochemical polarization and diffusion with poor reversibility. When the concentration of H2SO4 is above 2 mol·L−1, the electrode process of V(IV)/V(V) turns to diffusion control with improvement of the reversibility. An increase in concentration of H2SO4 solution facilitates a decrease in impedance, whereas too high concentration of H2SO4 solution (>3 mol·L−1) results in a remarkable increase in the viscosity of solution leading to a large mass transportation polarization, and thus the impedance starts to increase a little. In 3 mol·L−1 H2SO4 solution, the reversibility and kinetics of V(IV)/V(V) are improved gradually and the characteristics of impedance also improves with an increase in concentration of V(IV) solution. But, when the concentration of V(IV) solution exceeds 2.0 mol·L−1, the viscosity of the solution is so high that the mass transportation polarization increases considerably, resulting in the deterioration of electrochemical performance of V(IV)/V(V) species and an increase in impedance. Therefore, by considering from the standpoint of increasing energy density and electrode kinetics comprehensively, the optimal concentration of H2SO4 is 3 mol·L−1 containing 1.5-2.0 mol·L−1 of V(IV).