Investigation On The Electrode Process Of V(Ⅳ)/V(Ⅴ) Species And Modification Of Graphite Felt As Electrode In A Vanadium Redox Flow Battery

Vanadium redox flow battery (VRB) is a new energy storage system developed quickly in recent years owed to its specific properties such as environmental friendship, deep-discharge capability, no cross contamination and high reliability. However, many problems of electrode materials restrict the development of VRB. Nowadays, metal electrode, composed electrode and carbon fiber electrode are used in VRB. Graphite felt (GF) is used as the typical electrode of VRB for its good electrical conductivity, high porosity, specific surface area, corrosion resistance and low cost. In order to improve the electrochemical activity of GF materials, many modification methods have been investigated in recent years. Although the properties of VRB were affected by many factors such as membranes, electrodes, the study of electrode reaction kinetics for the optimization of battery performance is equally important.In this paper, the electrochemical behavior ofⅤ(Ⅲ)/Ⅴ(Ⅳ) andⅤ(Ⅳ)/Ⅴ(Ⅴ) couples on graphite and GF electrodes has been respectively studied by cyclic voltammetry and AC impedance. The results showed that the electrode process ofⅤ(Ⅳ)/Ⅴ(Ⅴ) on graphite was under mixed control of electrochemical reaction and diffusion, and the temperature had a great effect on reaction rate.While theⅤ(Ⅲ)/Ⅴ(Ⅳ) couple was a quasi-reversible process controlled by diffusion, and the temperature had little influence on reaction rate. The electrode process ofⅤ(Ⅳ)/Ⅴ(Ⅴ) on GF was controlled by electrochemical reaction.In this work, the GF was pretreated with H2O2, and then modified by SnO2 with the method of electrodeposition. The morphologies of pretreated and modified GF were characterized by scanning electron microscope (SEM), and the electrochemical properties of GF modified by SnO2 were investigated by cyclic voltammogram. The results showed that the surface of GF became smoother because the sediments on it were eliminated after pretreatment, and the SnO2 coating uniformly growthed on the surface of GF. Meanwhile, the results showed that the peak current and duration time of oxidation ofⅤ(Ⅱ)/Ⅴ(Ⅲ) couple,Ⅴ(Ⅲ)/Ⅴ(Ⅳ) couple, andⅤ(Ⅳ)/Ⅴ(Ⅴ) couple enhanced after SnO2 modified, which indicated that SnO2 performed a catalytic action upon the reaction of vanadium ions. The oxygen evolution was delayed, which widened the electrochemical window of GF electrode in VOSO4 solution.