No 5 (2023)

The processes of cathodic reduction of U(III) ions to metal in a low–melting LiCl–KCl–CsCl eutectic at the temperature range 650–850 K on tungsten, gallium and cadmium electrodes in an inert gas atmosphere have been studied by non-stationary and stationary electrochemical methods. Reagents without contain impurities of moisture, oxygen and their compounds were used in the experiments. All major operations were performed in a dry glove box. The following methods were used to analyze the electrochemical processes: cyclic voltammetry, square-wave voltammetry and potentiometry at zero current. On cyclic voltammogram of the molten LiCl–KCl–CsCl–UCl₃ solution on an inert tungsten electrode, one cathode current peak corresponding to the deposition of metallic uranium and one anode current peak associated with its dissolution were recorded. It was found that the potential of the cathode peak was shifted to a region of more negative values with an increase of the scan rate. The value of the cathode peak current was directly proportional vs. the square root of the polarization rate, but this dependence does not pass through the origin. Consequently, the system of U(III)/U(0) couple was irreversible and proceeds in one stage. It was found that on square-wave voltammograms in the studied “electrochemical window” the deposition of uranium on liquid reactive gallium and cadmium electrodes was carried out at more positive values than on inert tungsten electrode. It was established that this potential shift was associated with the formation of intermetallic compounds of uranium with the material of reactive electrodes. The values of the alloy formation potentials were determined. For identification of the composition of cathode deposits, potentiostatic electrolysis was performed. By X–ray diffraction analysis, it was found that the formation of the intermetallic compounds Ga₃U and Ga₂U occurs on the gallium reactive electrode, and Cd₁₁U occurs on the cadmium one. The conditions of their formation during the electrolysis of molten LiCl–KCl–CsCl–UCl₃ solutions were established. The reaction of the electrochemical extraction of uranium from molten LiCl–KCl–CsCl–UCl₃ electrolyte was investigated on liquid reactive electrodes at different duration of electrolysis. It was found that the electrochemical extraction of uranium exceeds 97% on both Ga and Cd electrodes.

A mathematical model is presented for the electrolytic synthesis of a crystalline cathode deposit UO₂–ZrO₂ with simultaneous and continuous electrochemical and chemical reactions occurring on the electrode. Uranium dioxide is formed by an electrochemical reaction during the reduction of uranyl ions \({\text{UO}}_{2}^{{2 + }}{\text{,}}\) zirconium is emerges by a chemical exchange reaction. Using the Faraday and Fick’s equations, an expression was obtained for calculating the content of zirconium dioxide in the UO₂–ZrO₂ system, which adequately describes the process of its synthesis in the NaCl–KCl–UO₂Cl₂–ZrCl₄ melt. Qualitative coincidence of the geometric shape of the dependences, and, in some cases, quantitative correspondence of the calculated and experimental values of the zirconium dioxide concentration on the process conditions (ZrCl₄ concentration, current density and duration of electrolysis, temperature) was established. The discrepancy between the values is explained by the volatilization of a part of ZrCl₄ from the electrolyte during electrolysis, which was not taken into account when deriving the analytical equation.

Due to the possibility of controlling composition and morphology, one of the promising methods for obtaining silicon and its materials is the electrolysis of molten salts. However, this requires data on the influence of various factors on the kinetics of silicon electrodeposition. In this work, an effect of the cathode substrate material on the kinetics of electroreduction of silicon ions in a low-fluoride melt (wt %) 57KCl–43CsCl with the addition of 2.8 wt % K₂SiF₆ at a temperature of 730°C was studied by cyclic voltammetry and chronoamperometry. Interacting and indifferent materials for silicon were chosen as substrates: glassy carbon, silver, and nickel. On the glassy carbon electrode, the electroreduction of silicon ions proceeds in the potential region more negative than –0.05 V, on the silver electrode, more negative than 0.05 V, and on the nickel electrode, more negative than 0.40 V relative to the potential of the silicon quasi-reference electrode. For all the studied substrates, a cathode process is observed, which is not electrochemically reversible. In this case, according to chronoamperometry measurements, the stage of nucleation of a new phase at the cathode does not affect the kinetics of the process under study. Presumably, in the case of glassy carbon and silver, irreversibility can be caused by a delayed discharge, while silicon electrodeposition on a nickel electrode is accompanied by the formation of nickel silicides. From the voltammetric and chronoamperometric dependences, the diffusion coefficient of silicon ions to the glassy carbon electrode was estimated, the values of which were 1.5 · 10^–5 and 1.2 · 10^–5 cm²/s, respectively.

The structure of the crystallization surface of the system (Li, Na), Pb // SO₄, WO₄ has been studied in a wide concentration and temperature range in order to identify compositions with optimal physicochemical parameters that can be used as the basis for the synthesis of highly dispersed lead tungstate with high yield and purity. The system (Li₂WO₄–Na₂WO₄)_evt–PbSO₄, which is a diagonal section of the system (Li, Na), Pb // SO₄, WO₄, was chosen as a working system for solving the task set in the work. For the first time, the concept of a “complex component” is used in the work, which is a mixture of lithium and sodium tungstates, as well as lithium and sodium sulfates at the vertices of the square of compositions. The complex components are eutectic compositions of the corresponding lithium tungstates, sodium and their sulfates. This approach to the study of the “resulting” triple mutual system (Li, Na), Pb // SO₄, WO₄ on the vertices of which complex components are located allowed us to take advantage of the noticeable differences between the studied system and the original triple mutual systems Li, Pb // SO₄, WO₄ and Na, Pb // SO₄, WO₄. It is shown that the studied system (Li, Na), Pb // SO₄, WO₄ has a number of advantages both in terms of melting temperatures of the eutectic mixture on the side of Li₂,Na₂(WO₄)₂–Li₂,Na₂(SO₄)₂, and in terms of the shift of the line of joint crystallization of phases, which leads to a noticeable increase in the crystallization surface lead tungstate. In this regard, before proceeding to the production of lead tungstate, we estimated the thermodynamic probability of the reaction underlying the synthesis of lead tungstate on the basis of the Temkin-Schwarzman method and the Van’t-Hoff isotherm equation of chemical reactions. Calculations have shown that all metabolic processes proceed with high negative Gibbs energies. The obtained samples of lead tungstate were analyzed by the X-ray phase analysis method on the X-ray diffractometer Dron-6, and their dispersion was determined on the Fritsch Analysette 22 Nanotek Plus laser particle analyzer. The presented results of a theoretical analysis of the possibility of implementing a method for synthesizing lead tungstate in melts of the (Li₂WO₄–Na₂WO₄)_evt–PbSO₄ system and experimental material for its implementation can become the basis for the development of technology for obtaining highly dispersed lead tungstate powders.

The discharge characteristics of the elements of a thermally activated chemical current source (HAB) containing NiCl₂–CoF₂–MoO₃ mixtures as a positive electrode are investigated. It is established that molybdenum oxide stabilizes the discharge plateau and increases the discharge voltage at temperatures above 530°C. The discharge curve has a stepwise character. The number of steps of the discharge curve is determined by the operating conditions of HAB. The low-voltage stage (less than 0.4 V) corresponds to the reduction of lithium molybdates, which are formed by the interaction of molybdenum oxide with the reduction products of transition metal halides. A study of the cathode reduction products by the methods of XRD, STA and SEM was carried out. It is established that during the discharge of the HAB element, the initial components of the cathode mixture are restored to metals that form a dendritic matrix. The DSC curves of the salt fraction formed during electrochemical reactions have a number of thermal effects corresponding to the temperatures of joint melting of a triple mixture of lithium halides LiF–LiCl–LiBr and eutectic dual systems LiF–LiCl, LiCl–Li₂O, in which transition metal halides and lithium molybdates are dissolved.