Influence of Cr3+ concentration on the electrochemical behavior of the anolyte for vanadium redox flow batteries

The composition of electrolyte affects to a great extent the electrochemical performance of vanadium redox flow batteries (VRB). The effects of Cr3+ concentration in the anolyte on the electrode process of V(V)/V(IV) couple have been investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). It was found that Cr3+ causes no side reactions, but affects the electrochemical performance of V(V)/V(IV) redox reaction, including the reaction activity, the reversibility of electrode reaction, the diffusivity of vanadium ions, the interface film impedance, and the electrode reaction impedance. The experimental results show that Cr3+ within a certain concentration range can improve the reversibility of electrode reaction and the diffusion of vanadium ions. With the Cr3+ concentration increasing from 0 to 0.30 g L-1, the reversibility of V(V)/V(IV) reaction increases, while the diffusion resistance decreases. Correspondingly, the diffusion coefficient of vanadium ions increases from (5.48-6.77) x 10(-7) to (6.82-8.44) x 10(-7) cm(2) s(-1), an increase of similar to 24%. However, the diffusion resistance increases and the diffusion coefficient decreases when Cr3+ concentration is over 0.30 g L-1, while the impedances of the interface, the film as well as the charge transfer increase continuously. As a result, Cr3+ with a certain concentration improves the diffusion and mass transfer process, but the resistances of the film, the interface, and the charge transfer rise. Furthermore, Cr3+ concentration of no more than 0.10 g L-1 has few effect on the electrode reaction process, and that of no more than 0.30 g L-1 is favorable to the diffusion of vanadium ions.