S. A. Kuznetsov

Voltammetric study of electroreduction of silicon complexes in a chloride-fluoride melt

The method of cyclic voltammetry was used to study electroreduction of Si(IV) complexes in a molten salt of NaCl-KCl-NaF%)-K2SiF6. A two-stage discharge mechanism with a transfer of two electrons in each step is established. Herewith, the first electrochemical stage is complicated with the disproportionation reaction. Diffusion coefficients are determined for the Si(IV) and Si(II) complexes at the temperature of 1023 K.

Standard rate constants of charge transfer for Nb(V)/Nb(IV) redox couple in chloride-fluoride melts: Experimental and calculation methods

Standard rate constants ks of charge transfer for Nb(V)/Nb(IV) redox couple in NaCl-KCl (the equimolar mixture) -K2NbF7, KCl-K2NbF7, and CsCl-K2NbF7 melts are determined by using cyclic voltammetry. An unconventional series of the standard rate constants is found: ks (KCl) < ks (CsCl) < ks (NaCl-KCl). Ab initio calculations carried out by using a PC Gamess/Firefly quantum-chemical program showed that the charge transfer activation energy can change not monotonically in the Na-K-Cs series, in compliance with the reorganization energy relationship. This leads, in its turn, to nonmonotonic change in the charge transfer rate constants.

Catalytic Mo2C coatings for the water gas shift reaction: Electrosynthesis in molten salts

Electrosynthesis methods using molten salts are suggested for obtaining a new catalytic system based on the Mo2C/Mo composition for the water gas shift reaction. The coatings obtained by the discharge of the carbonate ion on a molybdenum substrate and by the simultaneous reduction of the electroactive species MoO42− and CO32− are catalytically more active than bulk Mo2C or the commercial catalyst Cu-ZnO-Al2O3 by one and three orders of magnitude, respectively.

Electrochemical and Thermodynamic Properties of EuCl3 and EuCl2 in an Equimolar NaCl-KCl Melt

Abstract The electroreduction of EuCl3 in an equimolar NaCl-KCl mixture was studied at 973 -1123 K by different electrochemical methods. This reduction of Eu(III) in NaCl-KCl melt occurs via two successive reversible steps involving transfer of one and two electrons. The diffusion coefficients of Eu(III) and Eu(II) were measured by linear sweep voltammetry and chronopotentiometry. The values found by these methods are in a good agreement. The standard rate constants for the redox reaction Eu(III) + e~ Eu(II) were calculated from cyclic voltammetry data. The sluggish kinetics of this reaction is discussed in terms of substantial rearrangement of the europium coordination sphere. Special attention was paid to the choice of working electrodes for the study of electrode reactions. The formal standard potentials E*E u (^ /Ba, E* eU (hr/eu and the formal redox potentials ^*Eu(ni)/Eu(ii) were determined from open-circuit potentiometry and linear sweep voltammetry data. The free Gibbs energy changes for the reaction EuCl3(so1) EuCl2(SOi.) + 1/2 Cl2(g.) and the equilibrium constants of the metal-salt reaction 2Eu(III) + Eu 3 Eu(II) were calculated. The thermodynamics of the formation of dilute solutions of europium di-and trichloride in an equimolar NaCl-KCl melt were determined. It was shown that electrochemical transient techniques give the possibility of the determination of the relative partial molar mixing enthalpy of europium trichloride and dichloride in NaCl-KCl melt.

Standard Rate Constants of Charge Transfer of Nb(V)/Nb(IV) Redox Pair in Equimolar NaCl-KCl Melt

Standard rate constants of charge transfer (ks) on platinum and glassy carbon electrodes for Nb(V)/Nb(IV) redox pair in the NaCl-KCl-K2NbF7 melt are determined using the method of cyclic voltammetry in the temperature range of 973 to 1123 K. It is found that ks increases with increasing temperature and when we pass from glassy carbon to platinum electrode. The “apparent” activation energies of charge transfer are determined; it is shown that the charge transfer between the Nb(V) and Nb(IV) complexes is quasi-reversible and is controlled predominantly by the diffusion.

Electrochemical synthesis of Mo2C catalytical coatings for the water-gas shift reaction

The electroreduction of CO32− ions on a molybdenum cathode in a NaCl-KCl-Li2CO3 melt was studied by cyclic voltammetry. The electrochemical synthesis of Mo2C on molybdenum substrates has been performed at 1123 K for 7 h with a cathodic current density of 5 mA cm−2. If molybdenum carbide is present as a thin (ca. 500 nm) film on a molybdenum substrate (Mo2C/Mo), its catalytic activity in the water gas-shift reaction is enhanced by at least an order of magnitude compared to that of the bulk Mo2C phase.

Electrochemical synthesis of rare-earth metal (Eu, Nd) borides in molten salts

Rare-earth metal borides are widely used in different fields of modern techniques. Electrochemical synthesis at moderate temperatures (973–1023 K) is a cost-effective alternative to direct reaction techniques. The present work reports the mechanism and kinetics of boron and europium, boron and neodymium joint electrodischarge in chloride-fluoride molten systems. The optimum regimes of europium and neodymium borides electrodeposition are worked out on the base of voltammetric experiments. Europium compound is synthesized as a single-phase EuB6 product, while neodymium compounds is co-deposited as NdB4 and NdB6.

Electrodeposition of hafnium and hafnium-based coatings in molten salts

At present, coatings are mostly produced by CVD, PVD methods, plasma, and detonation spraying. Molten salts for coatings deposition are also becoming highly attractive. They provide wide possibilities for coating production of hafnium by electrodeposition, electrochemical synthesis, and precise surface alloying. Consideration is given to all these methods. The production of heat-resistant coatings from hafnium and niobium-hafnium alloys, and the electrochemical syntheses of copperhafnium solder and HfB2 coatings are also discussed.

Experimental and calculation methods of studying the effect of second coordination sphere on standard rate constants of charge transfer for Cr(III)/Cr(II) redox couple in chloride melts

The cyclic voltammetry method was used to determine the standard rate constants of charge transfer (ks) on a glassy carbon electrode for the Cr(III)/Cr(II) redox couple in the systems of NaCl-KCl-CrCl3, KCl-CrCl3, and CsCl-CrCl3 in the temperature range of 973–1173 K. It was shown that ks grows at an increase in the temperature and decreases as sodium cations are replaced by potassium and cesium cations in the second coordination sphere of chromium complexes. The calculations carried out using the PC GAMESS/Firefly quantum-chemical software by means of the DFT technique showed that the values of the charge transfer activation energy change monotonously in the series of Na-K-Cs in accordance with the ratio of reorganization energies. In its turn, this leads to monotonous variation of the charge transfer rate constants.

Electrochemical Synthesis of Niobium-Hafnium Coatings in Molten Salts

Graphite is widely used in technology because of its unique properties. A drawback of graphite is its low heat resistance in oxidizing atmospheres. To increase its heat resistance, Nb-Hf protective coatings were synthesized. Electrodeposition of niobium coatings on graphite with subsequent precise surface alloying of niobium with hafnium was studied. Electrochemical synthesis of Nb-Hf coatings from molten salt systems containing compounds of niobium and hafnium was used too. It was shown that Nb-Hf coatings with a planar growing front can be obtained if the concentration and therefore the limiting current density of the more electropositive component Nb is kept low. Nb-Hf coatings with a thickness of 20 - 30 μm have been obtained in this way from an NaCl-KCl-K2NbF7 (1 wt%)-K2HfF6 (10 wt%)-NaF (5 wt%) melt, above the limiting current density of niobium deposition.