Study on Uranium Redox-flow Battery with High Energy Efficiency

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More than 120 million tons of depleted uranium has been stored all over the world even at the end of 1999, and an development of its peaceful utilization is an important issue. One of the most outstanding property of actinide chemistry lies in reversible electrode reaction between III/IV valences, and V/VI valences. Provided this character is used, a redox-flow battery possessing high energy efficiency even if comparing with the existing vanadium battery is expected.Energy efficiency was theoretically studied in 2003. In vanadium battery, positive electrode reaction VO_2^++4H^++e^-→VO^<2+>+2H_2O is inner-sphere reaction accompanied with dissociation of oxygen. Due to activation overpotential 16% of energy are lost in discharge and charge cycle at current density 70mA/cm2. In contrast, since reactions of both electrolytes are rapid in actinides, loss of energy by activation overpotential remains in 2% in the case of Np. We constructed the Np battery and energy efficiency was studied through charge and discharge experiments.In 2004, uranium battery which equips with U(V) and U(III) as positive and negative electrolytes, respectively, was constructed and demonstrated. As a result, this uranium battery discharged electricity enough for rotating rotor of motor was confirmed. On the other hand, concentration of U(V) and U(III) in discharged and charged state, respectively, decays in half decay period for around several hours. Since this means that stability of uranium complex is not enough, in 2005, investigation of U(V) and U(III) complex, which were merely studied up to present, were performed. Consequently, preparation of U(V) complex solution with the half life exceeding a half year was successful. Also a preparation of the U(III) diamide complex was successful and during its half life of 11 hours in solution, electrode reaction of U(III)/U(IV) was studied.