Electrochemical Behavior of Neptunium in NaCl–2CsCl Melt

Abstract

The electrochemical behavior of neptunium in NaCl-2CsCl melt at 823-923 K was investigated by cyclic voltammetry, differential pulse voltammetry, and open-circuit chronopotentiometry after polarization. The results show that Np 4+ ions are reduced to Np metal by a twostep mechanism via Np 3+ ions in the NaCl-2CsCl melt. The diffusion coefficients of Np 3+ and Np 4+ ions were determined from cyclic voltammograms. The apparent standard potentials of Np 3+ /Np 0 and Np 4+ /Np 3+ redox couples have been determined to be E*(Np 3+ /Np 0 ) = −3.353 + 7.67 × 10 −4 T and E*(Np 4+ /Np 3+ ) = −1.175 + 4.99 × 10 −4 T vs. Cl 2 /Cl − (V), respectively. The activity coefficients of Np 3+ and Np 4+ ions were also determined using the reported data on the Gibbs free energy of formation for NpCl 3 and NpCl 4 in a supercooled liquid state.