Electron-Transfer Kinetics of Np[sup 3+]∕Np[sup 4+], NpO[sub 2][sup +]∕NpO[sub 2][sup 2+], V[sup 2+]∕V[sup 3+], and VO[sup 2+]∕VO[sub 2][sup +] at Carbon Electrodes

Like vanadium redox couples ( and ), neptunium is a good candidate for the active materials of a redox-flow battery because it possesses two isostructural and reversible redox couples ( and ). The standard rate constants of the neptunium electrode reactions were first determined at four types of carbon electrodes. The values for and are and , respectively, at the plane of pyrolytic graphite [PG( plane)] and also and , respectively, at plastic formed carbon (PFC). The standard rate constants for the and electrode reactions were also first determined at PG( plane) and PFC electrodes. The determined values are larger at the PFC electrode, and for and , respectively, than at the PG( plane) electrode. There is about a order of difference between the values of neptunium and vanadium ions, and this difference was discussed in reference to the homogeneous electron-exchange rate constants of these redox couples.