Electrochemical Investigation of Uranium Redox Flow Battery

by Y. Shiokawa et al.

　For the leveling of output power fluctuations of renewable energy sources, batteries for electricity power storage, including the redox flow battery as a strong candidate, are investigated. We have proposed the uranium redox flow battery on the basis of the quite unique feature that the single element possesses two redox couples with identical structures (UO2+/UO22+, U3+/U4+). Uranium satisfies the necessary condition for the excellent active material for the redox flow battery, i.e. the two battery reactions are reversible. Since the energy efficiency of charge-discharge cycles depends on the reversibility of the battery reactions, the uranium battery is expected to be of higher charge-discharge performance at a large current density even compared with the existing vanadium redox flow battery.

Uranium Battery
　Uranium battery requires an aprotic system because of the disproportionation reaction of U(V) observed in protic media. The battery is also effective to reuse the massive amount of depleted and recovered uranium.

　Our electrochemical investigation of uranium b-diketones, which show relatively high solubilities in aprotic solvents, reveals that (i) the large emf (>1V) is expected (Fig.2) and (ii) their electrode reactions involve ligand dissociation reactions. Since the latter feature prevent the high energy efficiency of the battery, the development of new active materials without the ligand dissociation are now underway.

Fig.1 Uranium redox flow battery for electricity power storage

Fig.2 Redox potentials for VI/V and IV/III of uranium complexes. Emf larger than 1V is expected by using appropriate ligands.

　A new uranium complex, prepared for preventing the ligand dissociation, shows a simple electrode reactions for both positive and negative electrodes in a single solvents (Fig.3). Since this complex seems to be an excellent material, further experiments including charge and discharge of the battery are planned by using the active materials.

Fig.3 Excellent reversibility of new uranium complexes for positive and negative electrolytes

Neptunium Battery
　Neptunium battery had been already tested in an aqueous system, where neptunium also possesses two reversible redox couples like uranium. In the small scale experiments (Fig.4), the reduction of emf is smaller than the vanadium cell (Fig.5) and thus the higher energy efficiency is expected.

Fig.4 Non-flow cell of neptunium battery after discharge

Activity of Research Staff of Radiochemistry of Metals Group

Preparation of actinide and rare earth metal by aqueous solutions　by Y. Shiokawa et al.

Electrochemical Investigation of Uranium Redox Flow Battery　by Y. Shiokawa et al.

Concentrated Spin Glass Effect in 5f Electronic Intermetallics　by Y. Shiokawa et al.