Electrochemical behaviour of neptunium in the molten LiCl–KCl eutectic

The electrochemical properties of neptunium, dissolved in the LiCl–KCl eutectic, were investigated by transient electrochemical techniques, such as cyclic voltammetry (CV) and chronopotentiometry on inert tungsten electrodes. It was shown that Np4+ is reduced to Np0 by a two-step mechanism corresponding to the transitions of Np4+/Np3+ and Np3+/Np0. In the 400–550 °C (673–823 K) temperature range, the diffusion coefficients of Np3+ and Np4+ were found to be of similar magnitude and equal to D Np 3 + = 9.47 × 10 - 4 exp ( - 23.6 × 10 3 / RT ( K ) ) and D Np 4 + = 2.14 × 10 - 4 exp ( - 14.1 × 10 3 / RT ( K ) ) cm2 s−1, respectively. The apparent standard potentials of Np4+/Np3+ and Np3+/Np0 redox systems were determined to be E∘*(Np4+/Np3+) = −1.250 + 6.637 × 10−4 · T (K) V versus Cl2/Cl− and E∘*(Np3+/Np0) = −3.250 + 7.251 × 10−4 · T (K) V versus Cl2/Cl−, respectively. The Gibbs free energies of formation in diluted solutions of NpCl3 and NpCl4 in the LiCl–KCl eutectic were determined to be ΔG*∘ = −936.54 + 0.20654 · T (K) and −120.61 + 0.06405 · T (K) kJ mol−1, respectively. Between 400 °C and 550 °C, the activity coefficients γ NpCl 3 and γ NpCl 4 range between 9 − 125 × 10−6 and 2 − 10 × 10−3, respectively.