Like vanadium redox couples (V2+/V3+ and VO 2+/VO2)+, neptunium is a good candidate for the active materials of a redox-flow battery because it possesses two isostructural and reversible redox couples (Np3+Np4+ and NpO 2+/NpO22+). The standard rate constants k0 of the neptunium electrode reactions were first determined at four types of carbon electrodes. The k0 values for Np3+/Np4+ and NpO2+/NpO 22+ are 1.9 × 10-2 and 1.5 × 10-2 cm 2-1, respectively, at the c plane of pyrolytic graphite [PG(c plane)] and also 1.4 × 10-2 and 2.1 × 10-2 cm s-1, respectively, at plastic formed carbon (PFC). The standard rate constants for the V2+/V3+ and VO 2+/VO2+ electrode reactions were also first determined at PG(c plane) and PFC electrodes. The determined k0 values are larger at the PFC electrode, k0 = 5.3 × 10 -4 and 8.5 × 10-4 cm s-1 for V 2+/V3+ and VO2+/VO2+, respectively, than at the PG(c plane) electrode. There is about a 102 order of difference between the k0 values of neptunium and vanadium ions, and this difference was discussed in reference to the homogeneous electron-exchange rate constants of these redox couples.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Renewable Energy, Sustainability and the Environment
- Surfaces, Coatings and Films
- Materials Chemistry