Electrochemical Behavior of Vanadium Redox Couples on Carbon Electrode

Enormous work has been devoted to the vanadium redox flow battery (VRFB) in recent years; however, the electrochemical behavior of the vanadium redox couples in solutions has not been well understood. In the present work, the transition between the adjacent redox couples (V2+ -> V3+ -> V4+ -> V5+) was extensively investigated on a typical electrode material (Vulcan XC72 carbon black) by the cyclic voltammetry (CV) in various potential windows. It was found that the two redox reactions, V3+/2+ and V5+/4+, could be seen as quasi-reversible electrochemical processes. With the simulation of the CV results, the standard rate constant was found to be (1.5 +/- 0.4)* 10(-3) cm s(-1) (V5+/4+) and (2.0 +/- 0.4)* 10(-3) cm s(-1) (V3+/2+), and the diffusion coefficient was determined to be (3.2 +/- 0.5)* 10(-6) cm(2) s(-1) (V5+), (3.3 +/- 0.5)* 10(-6) cm(2) s(-1) (V4+), (9.0 +/- 1.3)* 10(-7) cm(2) s(-1) (V3+) and (9.0 +/- 1.3)* 10(-7) cm(2) s(-1) (V2+), respectively. In comparison, V4+/3+ was rarely observed in the thermodynamic potential region, of which the anodic and cathodic waves overlapped with the other two redox couples. Also, the kinetic parameters were extracted by the Nicholson analysis for comparison. In terms of the electrochemical kinetics, Vulcan XC72 meets the requirement as the electrode material in VRFB, which yields promising electrochemical properties and is worth further investigating. The above findings shed lights on both the mechanism of redox reactions and the charging/discharging processes in the VRFB. (C) 2016 The Electrochemical Society. All rights reserved.