Electron-transfer kinetics of Np3+/Np4+, NpO 2+/NpO22+, V2+/V 3+, and VO2+/VO2+ at carbon electrodes

Tomoo Yamamura, Nobutaka Watanabe, Takashi Yano, Yoshinobu Shiokawa

Research output: Contribution to journalArticlepeer-review

175 Citations (Scopus)


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.

Original languageEnglish
JournalJournal of the Electrochemical Society
Issue number4
Publication statusPublished - 2005 May 20

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Renewable Energy, Sustainability and the Environment
  • Surfaces, Coatings and Films
  • Electrochemistry
  • Materials Chemistry


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