Electrochemistry of vanadium redox couples on nitrogen-doped carbon

The redox reaction of vanadium ions (V2+<-> 3+<-> 4+<-> 5+) on well-developed nitrogen-doped ordered mesoporous carbon (NOMC) was extensively investigated in different electrolyte solutions by electrochemical methods. It is found that both the electronic structure modulated by nitrogen doping and the enriched electrochemically active functional groups on NOMC favor the three electrochemical transitions between the adjacent couples, viz. V2+<-> 3+<-> 4+<-> 5+, as compared with Vulcan XC72 carbon black. Salient findings are as follows. First, the transition of V3+<-> 2+ is the same on the two distinctly different carbons, which indicates that this reaction is an outer-sphere charge transfer reaction. The concomitant hydrogen evolution reaction makes NOMC unsuitable to be used as a negative electrode material in flow batteries. Second, the transition of V5+<-> 4+ shows a quasi-reversible behavior, indicating that NOMC can be used as a positive electrode material. Simulation of cyclic voltammogram (CV) reveals that the standard rate constant and the adsorption equilibrium constant are (7.0 +/- 0.9)* 10(-3) cm s(-1) and 0.70 +/- 0.09 (both V4+ and V5+), respectively. Third, the transition of V4+<-> 3+ is recognized in the CV curve, which proceeds in a quasi-reversible reaction. The preceding adsorption of the symmetrical ions (V3+) is found to play a key role in determining the kinetics. Finally, for the two latter transitions, the content of dopant nitrogen yields a negligible effect on the electrochemical activity, excluding the possibility of its direct involvement in electrocatalysis. The above findings not only reveal the applicability of the nitrogen-doped carbon to be used as an electrode material in flow batteries, but also offer an in-depth understanding of the reaction mechanism of vanadium redox couples. (c) 2017 Elsevier Ltd. All rights reserved.