An important problem concerning rare earth metals and molten salts is the reprocessing of nuclear fuel, since lanthanides are always present in spent nuclear fuel. According to the technology being developed for pyroelectrochemical processing of nuclear waste spent fuel is converted into molten salts by anodic dissolution, followed by electrochemical extraction of actinides from lanthanides.

Thus a knowledge of the electrochemistry and thermodynamics of lanthanide compounds in molten salt systems is very useful for the understanding the recycling of spent nuclear fuel.

The electroreduction of YbCl₃ in alkali chloride melts (NaCl-KCl, KCl, CsCl) was studied in the temperature range 973-1173 K by different electrochemical methods. The diffusion coefficients (D) for Yb(III) and Yb(II) were determined by linear sweep voltammetry, chronopotentiometry and chronoamperometry methods. Decreasing values of D were obtained when the cation in the second coordination sphere changed from Na to Cs. It was shown that such changes are due to the decreasing of counter polarizing effect of cations with increasing cationic radius.

The standard rate constants of charge transfer (k_s) for the Yb(III)/Yb(II) redox couple were calculated on the basis of cyclic voltammetry data using Nicholson’s approach. The following row of the standard rate constants of charge transfer has been experimentally determined: k_s (KCl) < k_s (CsCl) < k_s (NaCl-KCl).

The formal redox potentials E^*_{Yb(III)/Yb(II} were obtained in alkali chlorides melts from the linear sweep voltammetry data. From the values of the formal redox potentials were calculated the Gibbs energies and equilibrium constants for the reaction:

YbCl_2(sol.) +1/2 Cl_2(g.) « YbCl_3(sol.) (1)

It was determined the increasing of the formal entropy for the reaction (1) in transition from NaCl-KCl to CsCl melt that associated with a greater degree of ordering of the reaction products due to the complex formation.

Influence of fluoride ions (NaF) on the electrochemical behavior of the Yb(III)/Yb(II) redox couple in an equimolar NaCl-KCl melt was studied. It was determined а decrease of diffusion coefficients and standard rate constants, as well as a shift of the formal redox potentials to the negative region.