Energy efficiency of neptunium redox battery in comparison with vanadium battery

A neptunium ion possesses two isostructural and reversible redox couples (Np3+/Np4+ and NPO2+/NpO22+) and is therefore suitable as an active material for a redox-flow battery. Since the plastic formed carbon (PFC) is known to show the largest k(0) values for Np(IV)/Np(III) and Np(V)/Np(VI) reactions among various carbon electrodes, a cell was constructed by using the PFC, with the circulation induced by bubbling gas through the electrolyte. In discharge experiments with a neptunium and a vanadium battery using the cell, the former showed a lower voltage loss which suggests a smaller reaction overvoltage. Because of the high radioactivity of the neptunium, it was difficult to obtain sufficient circulation required for the redox-flow battery, therefore a model for evaluating the energy efficiency of the redox-flow battery was developed. By using the known k(0) values for neptunium and vanadium electrode reactions at PFC electrodes, the energy efficiency of the neptunium battery was calculated to be 99.1% at 70 mA cm(-2), which exceeds that of the vanadium battery by ca. 16%. (c) 2005 Elsevier B.V. All rights reserved.