Abstract
Today, existing methods of recycling spent nuclear fuel do not solve the problem of separation of transuranium elements and lanthanides. Electrolysis of molten salts is promising for separation of lanthanides and actinides. To solve this problem, it is necessary to have experimental data on the electrochemical behavior of various lanthanides in molten salts.
The aim of this study is the electrochemical investigation by cyclic voltammetry of the Sm(III)/Sm(II) redox couple in NaCl-KCl, KCl and CsCl melts containing SmF3.
Electrochemical studies were carried out in a temperature range of 973-1173 K by cyclic voltammetry using an AUTOLAB PGSTAT 20 potentiostat with a package of application programs GPES (version 4.4.). The sweep rate (v) varied between 0.1 up to 3.0 V s-1. The melt container was a glassy carbon crucible of SU-2000 brand, which also served as an auxiliary electrode. Tungsten wire was used as a working electrode and platinum wire was utilized as a quasi-reference electrode.
It was found in NaCl-KCl-SmF3, KCl-SmF3 and CsCl-SmF3 melts that the recharge process was reversible up to the scan rate 1.0 V s-1. In this range of sweep rate the diffusion coefficients (D) of Sm(III) were calculated using the Randles–Shevchik equation. The diffusion coefficients decrease with a change of the composition of the second coordination sphere from sodium to cesium. Similar dependencies for D values are well known. It is associated with a decrease of the counter-polarizing effect during the transition from Na to Cs, which in turn causes a decrease of the metal – ligand bond length and increase the strength of samarium complexes.
A transition from reversible to quasi-reversible process was found at v > 1.0 V s-1 in NaCl-KCl-SmF3, KCl-SmF3 and CsCl-SmF3 melts.
The standard rate constants of charge transfer (ks) of the redox couple Sm(III)/Sm(II) were determined by cyclic voltammetry in all studied melts by using the Nicholson's equation, which is valid for quasi-reversible processes.
The following series of the standard charge transfer constants was found ks(CsCl)<ks(KCl)<ks(NaCl–KCl). According to the theory of elementary charge transfer, the smaller and stronger bond complexes require a higher rearrangement energy, and in consequence, the charge transfer proceeds at a slower rate. Therefore, a decrease of the standard rate constants for the redox reaction would have been expected, which is in an agreement with obtained experimental results.
