The composition of electrolyte affects to a great extent the electrochemical performance of vanadium redox flow batteries (VRB). The effects of Cr 3+ 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 Cr 3+ 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 Cr 3+ within a certain concentration range can improve the reversibility of electrode reaction and the diffusion of vanadium ions. With the Cr 3+ 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) × 10 −7 to (6.82-8.44) × 10 −7 cm 2 s −1 , an increase of ~24%. However, the diffusion resistance increases and the diffusion coefficient decreases when Cr 3+ 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, Cr 3+ 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, Cr 3+ 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. Vanadium redox flow batteries (VRB), a high efficient and clean rechargeable device for energy conversion and storage, have promising prospects in the fields of generating electricity using renewable power sources, peak regulation of electric networks, emergency power, remote supply and electric vehicles. They have unique advantages such as high power density, excellent cycling stability, high reliability, simple operation and convenient maintenance [1]. The process of energy storage and release of VRB is generally achieved through an interconversion of soluble redox couples like V(V)/V(IV) and V(III)/V(II) in a electrolyte. Thus, the electrolyte is both the electroactive species and the reservoirs for energy storage, and has a determining influence on the electrochemical performance of VRB. The anolyte can be prepared directly by dissolving VOSO 4 in a dilute sulfuric acid solution [2]. However, it is usually prepared by dissolving V 2 O 5 in a dilute sulfuric acid with a reduction treatment because of the high cost of VOSO 4 [3]. V 2 O 5 is an intermediate product in the smelting process for producing steel in Pangang Group, but it usually contains small amounts of impurities, such as Cr 3+ , Mn 2+ ions, which exist also in the VOSO 4 prepared by using such V 2 O 5 . We have investigated the influence of Mn 2+ concentration on the electroc...
