Abstract Graphite material was used as the electrode for an all-vanadium redox flow battery, and the electrode was modified by transition metallic ions to enhance its electrochemical behavior. An porous graphite composite electrode has high specific surface area and high current density. The electrode modified by transition metallic ions has improved catalysis behavior that can catalyze the V(II)-V(V) redox reaction showed by cyclic voltammograms. This article studied the impedance of the modified electrode by electrochemical impedance spectroscopy (EIS), and approved that the electrode modified by Co 2+ and Mn 2+ has a lower charge transfer resistance than the non-modified electrode. The effect of average particle size distribution is at lower frequencies that the slope of Warburg impedance is reduced by large particle size distribution. The voltage efficiency of the Co 2+ modified electrode test cell is 81.5%, which is higher than that of the non-modified electrode.