Chloride Melts Research Articles

Corrosion of Austenitic Steels and Their Components in Molten Nacl–Kcl–UCl3

The fused halides can be used for nuclear fuel reprocessing and as working media for molten salt nuclear reactors. However practical implementation of such technologies is limited by the problem of finding suitable corrosion resistant materials capable of long-term working in contact with molten salts. From the economical point of view the application of stainless steels as construction materials for the molten salt media is one of the most prospective ways. In the present study the corrosion behaviour of metallic iron, nickel, chromium, molybdenum, AISI 316L and 12Kh18N10T (analogue of AISI 321) austenitic steels was studied in NaCl–KCl–UCl3(1 wt.% U) melts at 750 ºC. Gravimetric measurements served as a basis for estimating corrosion resistance of studied materials. Quenched melts were analyzed using ICP-AES method. The surface of the alloy samples after corrosion tests was characterised using XRD, SEM and Х-ray microanalysis. The foils of corroded materials were examined by TEM. The corrosion rates of studied metals in NaCl–KCl–UCl3melt decrease in the following order: Cr>Fe>Ni>Mo, which correlates well with the formal standard potentials of metals in chloride melts indicating electrochemical nature of the corrosion processes. Molybdenum is the only corrosion resistant metal in the uranium-containing chloride melts and it can be used as a construction material for molten salt reactors. The major products of stainless steel corrosion in chloride melts are iron, chromium and manganese species. The surface of the corroded samples of austenitic stainless steels was depleted in manganese and chromium and enriched in nickel and molybdenum. Intergranular corrosion was observed for all types of austenitic steels. In terms of intensity and depth of the corrosion layer in NaCl–KCl–UCl3 the studied steels can be ranged in the order AISI 321>AISI 316L that correlates with the amount of excessive phases formed (figure). Figure 1

Corrosion of Zirconium in Halogenide Melts

We propose a method for the evaluation of stability of the corrosion products of zirconium in melts of different compositions. The method is based on the analysis of the dependence of the equilibrium potential of zirconium electrode on the initial concentration of its compounds in the oxidation state 4+. It is discovered that zirconium compounds in the oxidation state 2+ are in equilibrium with the metal in all investigated systems and their stability depends on the composition of the melt and decreases in the following order: (NaBr–KBr) > KBr ≥ KCl > (LiCl–KCl). It is also established that, parallel with zirconium compounds in the oxidation state 2+, the investigated systems contain its compounds in the other oxidation states. The stability of these compounds in bromide melts decreases in the following order: Zr(II) (NaBr–KBr) > Zr(IV) (NaBr–KBr) > Zr(II) (KBr) > Zr(IV) (KBr). In chloride melts, the observed sequence has a somewhat different form: Zr(II) (KCl) > Zr(IV) (KCl) > Zr(II) (LiCl–KCl). Lithium is present among the corrosion products in LiCl–KCl melts. We also determined the thermodynamic characteristics (ΔH and ΔS) of equilibria of the corrosion products of zirconium with the investigated melts.

Raman spectra and conductivity of PbO–PbCl2–CsCl melts

The structure and the conductivity of homogeneous CsCl–PbCl2–PbO melts with different compositions are studied. The Raman spectra (RS) of the chloride and oxide–chloride melts containing lead oxide have been measured. It is found that the CsCl (71.3 mol %)–PbCl2 (28.7 mol %) system with the addition of 12 mol % PbO and the CsCl (18.3 mol %)–PbCl2 (81.7 mol %) system with the addition of 18 mol % PbO contain Pb3O2Cl+ oxide–chloride groups. The temperature dependences of the electrical conductivity are measured in the PbCl2–CsCl chloride melts containing 28.7 and 81.7 mol % lead chloride and in homogeneous molten mixtures CsCl (71.3 mol %)–PbCl2 (28.7 mol %) with the addition of 12 mol % PbO and the CsCl (18.3 mol %)–PbCl2 (81.7 mol %) with the addition of 18 mol % PbO. Significant degradation of the ion conductive properties of the PbO–PbCl2–CsCl oxide–chloride melts as compared to those of the PbCl2–CsCl chloride electrolytes has been revealed. It is shown that the decrease in the electrical conductivity of the PbCl2–CsCl oxide–chloride melts as lead oxide is added is due to the presence of Pb3O2Cl+ oxide–chloride groups.

Liquidus temperatures and phase equilibria in the BaCl2–MCl–BaO systems

The liquidus temperatures of the BaO–BaCl2–MCl systems (with M = alkali metal) are determined by thermal analysis. The caloric effects observed during melting of the barium-containing chloride eutectic with barium oxide additions are studied. A chemical mechanism of barium oxide dissolution in the melts is confirmed. X-ray diffraction patterns taken for the melt solidified after experiment indicate the presence of barium oxychloride Ba4OCl6 in the solid phase. It is shown that the significant increase in the liquidus temperature in adding the barium oxide to barium-containing chloride molten mixtures is related to substantial changes in their composition and structure.

ELECTROCHEMICAL SYNTHESIS OF ULTRAFINE INTERMETALLIC POWDERS BASED ON LANTHANUM AND COBALT FROM CHLORIDE MELTS

The paper studies the combined electroreduction of lanthanum LaCl 6 3– and cobalt CoCl 4 2– chloride complexes in an equimolar KCl–NaCl melt at a tungsten electrode. As it follows from the results of voltampere measurements, potentials of La and Co reduction at a tungsten electrode coated with preliminary reduced cobalt differ by more than 1,0 V. Therefore, the electrochemical synthesis of La and Co intermetallics is possible only in the kinetic regime. The paper develops the method for the high-temperature electrochemical synthesis of lanthanum and cobalt intermetallic powders in halide melts. The paper determines conditions for realization of the combined electroreduction of La 3+ and Co 2+ ions, on the basis of which the paper demonstrates the principal possibility of the direct electrochemical synthesis of lanthanum intermetallics. The paper establishes that the electrosynthesis mechanism is determined by electrolysis conditions. Under potentiostatic conditions (when potentials are more negative than potentials for reduction of the more electronegative element – lanthanum) the combined reduction of La and Co takes place, and the synthesis process is determined by the activation energy of the interaction reaction between lanthanum and cobalt at the cathode. In general, the electrosynthesis of La- and Co-based intermetallics is determined by the following interrelating parameters – the composition of the electrolytic bath and the voltage across it. The paper describes the high-temperature electrochemical synthesis of lanthanum- and cobalt-based ultrafine powders of intermetallics кобальта (Сo 3 La 2 , Сo 7 La 2 , Co 13 La) from chloride melts.

Production of Nd-Fe Foundry Alloy by Electrolysis in Molten Salts

Nd-Fe-B-based permanent magnets have the highest magnetic characteristics. To improve their quality, one generally applies the technique of solid-phase alloying by materials with high content of neodymium, dysprosium, terbium, i.e. rare-earth metal (REM)-Fe(Co) foundry alloys. The paper presents the results of the study of the manufacturing process of Nd-Fe foundry alloys, with composition being close to eutectic, by electrolysis of neodymium oxides and fluorides from chloride and fluoride melts. It is shown that in the oxide electrolysis a sufficiently high current efficiency (over 98 %) is obtained. This is a promising method of obtaining of rare earth-containing materials.

Intervalence Charge Transfer of the Nb(V)/Nb(IV) Redox Couple in Alkali Chloride Melts: Experiment and Quantum-Chemical Calculations

The standard rate constants of charge transfer (ks) for the Nb(V)/Nb(IV) redox couple in KCl-NbCl5 and CsCl-NbCl5 chloride melts have been determined. The redox process in the NaCl-KCl-NbCl5 melt was shown being reversible; hence, the determination of ks appeared impossible. Values of ks in the KCl-NbCl5 melt are larger than in the CsCl-NbCl5 melt. Previously, a quantum-chemical calculations of nM+·[NbCl6] model systems (M = Na, K, Cs), containing a complex anion with cations outer-sphere (OS) shell (nM+) have been done. It was found that compositions with an intermediate number of OS cations n < nlim, where nlim is the limiting number of OS cations bound to a given complex, are thermodynamically most stable. In this study a quantum-chemical calculations were performed for estimation of the temperature effect on the composition of the stable particles in the extended systems M+·[NbCl6]+18MCl (M = Na, K, Cs).

Dissolution of Calcium Titanate in Calcium Chloride Melt and Its Application to Titanium Electrolysis

Dissolution of Calcium Titanate in Calcium Chloride Melt and Its Application to Titanium Electrolysis

Electrode and Redox Potentials of Molybdenum and Stability of Molybdenum Chloro-Species in Alkali Chloride Melts

Molybdenum electrode and Mo(III)/(IV) redox potentials were measured in fused alkali chlorides. Experiments were performed in individual salts (LiCl, NaCl, KCl, RbCl and CsCl) and in several binary and ternary eutectic or low-melting mixtures between 633 and 1173 K (depending on the melting point of the solvent salt). Formal standard electrode potentials E*Mo/Mo(III) and redox potentials E*Mo(III)/Mo(IV) in respect to Cl−/Cl2 couple and Gibbs free energy change of formation of molybdenum(III) chloride in alkali chloride melts were calculated. Electronic absorption spectra of Mo(III) ions were recorded, and spectroscopic parameters of MoCl63− complex ions determined. High temperature spectroscopy measurements were used to study the stability of Mo(III) chloro-species in fused chlorides and the reaction rates of Mo(III) ions disproportionation were also determined. Diffusion coefficients of molybdenum ions in LiCl-KCl-CsCl and NaCl-CsCl eutectic based melts were determined from the results of electrochemical measurements.

Electrochemical Behavior of Titanium Ions in Various Molten Alkali Chlorides

Cyclic voltammetry and square wave voltammetry were performed for TiCl3 at 1023 K, in molten LiCl, LiCl-KCl, NaCl-CsCl and CsCl, respectively. The anodic process from Ti(III) to Ti(IV) is a simple reversible process in all the alkali chloride melts. Ti(III) can be reduced directly to Ti in one step, in CsCl, while Ti(II) will appear before Ti in LiCl, LiCl-KCl and NaCl-CsCl. The electrode potentials of redox couples of titanium species decrease with the decrease of the polarization power of solvent alkali ion.

Difference in the Mechanism of Electrochemical Deoxidation of Conducting and Non-conducting Solid Oxide Preforms: An Experimental Demonstration with TiO2 and SiO2 Pellet Electrodes in CaCl2 Melt

Powder compacted and sintered solid oxide preforms, generally shaped as thin pellets, have been used as the negative electrode of a molten salt electro-deoxidation cell and deoxidation of the electrode is greatly influenced by its electrical conductivity. The deoxidation pattern of electron conducting and non-conducting oxide preforms should, therefore, be different. In order to demonstrate this experimentally, novel electrochemical experiments were carried out with electron conducting TiO2 and non-conducting SiO2 pellet electrodes in calcium chloride melt at 1173 K. The experimental results have shown, as already reported by many previously, that three physically distinct phases, viz. the solid electron conductor, the oxide and the electrolyte melt should coexist for electro-deoxidation of non-conducting SiO2 electrode but only two physically distinct phases, viz. the oxide and the electrolyte melt need only to coexist in the case of conducting TiO2 electrode. It is demonstrated in this study, for the first time, that the 3 Phase Interline (3PI) mechanism, proposed to explain electro-deoxidation of solid oxides in general, stands reduced to 2 Phase Interface (2 PI) mechanism in the case of conducting oxides.

Factors Influencing the Direct Electrochemical Reduction of Nb2O5 Pellets to Nb Metal in Molten Chloride Salts

Powder compacted and sintered Nb2O5 pellets were cathodically polarised against graphite anode in calcium chloride melt at 1173 K to study the influence of various factors on the electrochemical reduction of the oxide. The parameters were; duration and temperature of electrolysis, open porosity of pellets, nature of anode, mode of electrolysis and configuration of the oxide cathode. The experiments were also conducted in KCl, KCl-25 mol% CaCl2 and NaCl melts to understand the effect of melt composition on the electroreduction. Different Ca–Nb–O and Nb–O intermediates were found in the pellets electrolysed for different durations of time in CaCl2 melt which eventually reduced to Nb. The current efficiency of the process decreased with increasing duration of electrolysis. Decrease in electrolysis temperature from 1173 to 1073 K led to the decrease in the rate of reduction of the oxide pellets. Pellets with high open porosity reduced faster. Carbon contamination of the melt was relatively less when pyrolytic graphite was used as anode. Of all the melts studied, the reduction was found to be better in calcium chloride melt, that too when alumina crucible was used as container of the melt.

Layered Dion-Jacobson type niobium oxides for photocatalytic hydrogen production prepared via molten salt synthesis

A class of Dion-Jacobson type layered perovskite niobium oxides (AB2Nb3O10 with A=K, Rb, Cs and B=Ca, Sr, Ba) was prepared via molten salt method for the first time. By dissolving oxide and carbonate precursors in A-cation chloride melts, this type of synthesis needs only two hours of preparation time resulting in highly crystalline layered niobates at a much shorter synthesis time compared to conventional syntheses, like the solid state reaction. Beside detailed materials characterization, we investigated the influence of A-cation and B-cation variation on the band gap and photocatalytic activity for hydrogen production. It was found that the band gap decreases with the increase in size of the B-cation. A strong dependence on interlayer spacing (influenced by the size of the A-cation) and lattice relaxation can be derived from the steady-state hydrogen evolution rates.

Taguchi optimisation of process parameters for electrowinning of praseodymium metal in molten salt electrolysis

Electrowinning of praseodymium (Pr) metal from its chloride melt has been investigated above its melting point employing molybdenum as cathode and graphite as anode. Experiments for the electrolysis were carried with wide range of conditions based on Taguchi method. This approach has been adopted to arrive at optimum conditions for the electrowinning of Pr metal with maximum current efficiency. The percentage effect of process parameters such as temperature, electrolyte composition, current density on current efficiency was also studied. The electro-deposited product was characterised by X-ray diffraction, Wavelength dispersive X-ray fluorescence and ICP-AES techniques.

Different types of liquid immiscibility in carbonatite magmas: A case study of the Oldoinyo Lengai 1993 lava and melt inclusions

Oldoinyo Lengai is situated within the Gregory Rift Valley (northern Tanzania) and is the only active volcano erupting natrocarbonatite lava. This study investigates the texture and mineralogy of the June 1993 lava at Oldoinyo Lengai, and presents petrographic evidence of liquid immiscibility between silicate, carbonate, chloride, and fluoride melt phases. The 1993 lava is a porphyritic natrocarbonatite consisting of abundant phenocrysts of alkali carbonates, nyerereite and gregoryite, set in a quenched groundmass, composed of sodium carbonate, khanneshite, Na-sylvite and K-halite, and a calcium fluoride phase. Dispersed in the lava are silicate spheroids (<2mm) with a cryptocrystalline silicate mineral assemblage wrapped around a core mineral. We have identified several textural features preserved in the silicate spheroids, melt inclusions, and carbonatite groundmass that exhibit evidence of silicate-carbonate, carbonate-carbonate and carbonate-halide immiscibility. Rapid quenching of the lava facilitated the preservation of the end products of these liquid immiscibility processes within the groundmass. Textural evidence (at both macro- and micro-scales) indicates that the silicate, carbonate, chloride and fluoride phases of the lava unmixed at different stages of evolution in the magmatic system.

A Spectroscopic and Electrochemical Study of Molybdenum(IV) and Tungsten(IV) Species in Alkali Chloride Melts

Alkali chloride based melts can be employed as the working media for electrowinning and electrorefining refractory metals, including molybdenum and tungsten. In addition, chloride melts are suitable for manufacturing various items of these metals and recycling molybdenum and tungsten containing scrap. Knowing electrochemical properties and speciation of the metals of interest in the molten electrolyte is essential for designing a feasible process. Molybdenum and tungsten are the polyvalent metals and follow the general trend of increasing the lowest stable oxidation state in the high temperature chloride ionic media as one moves down the group from 3d to 5d transition metal. In the group 6 of the periodic table the lowest oxidation state of chromium in fused chlorides is +2, and this increases to +3 for molybdenum and +4 for tungsten. The oxidation state +4 is unusual for molybdenum in molten chlorides and the aim of the present study was finding the conditions of Mo(IV) species formation, determining their stability and comparing molybdenum(IV) and tungsten(IV) species behavior and electrochemical properties. The experiments were performed between 500–900 oC in the melts based on LiCl–KCl–CsCl, NaCl–KCl–CsCl and NaCl–CsCl eutectic mixtures. Anodic dissolution of metallic tungsten results in the formation of predominantly W(IV) ions. Increasing temperature causes slight increase of the average oxidation state of tungsten in the melt. The electrode potentials of tungsten were determined by potentiometry and the speciation characterized by electronic absorption spectroscopy measurements. Prolonged holding of tungsten-containing chloride melts in contact with silica results in partial oxidation of W(IV) ions. Red-ox processes involving Mo(III) and Mo(IV) ions were studied by cyclic voltammetry and potentiometry combined with in situ electronic absorption spectroscopy measurements. Anodic oxidation of Mo(III) chloro-ions on a glassy carbon electrode results in formation of Mo(IV) species. Mo(III)-Mo(IV) oxidation-reduction is reversible. Molybdenum(IV) chloro-species, MoCl6 2– are stable at 550-600 oC. At higher temperatures a disproportionation reaction occurs: 2 MoCl6 2– → MoCl6 3– + MoCl6 –. Molybdenum(V) ions decompose and molybdenum pentachloride sublimes from the melt: 2 MoCl6 – → MoCl5↑ + Cl–. Reactions of molybdenum and tungsten dioxides with hydrogen chloride in fused alkali chlorides were also investigated by in situ spectroscopy measurements. Depending on temperature and cationic composition of the melt the following reactions take place in the case of molybdenum: MoO2 + 4 HCl + 2 Cl– → MoCl6 2– + H2O MoO2 + 2 HCl → MoOCl2↑ + H2O Decreasing temperature or increasing mean alkali cation radius favors MoCl6 2–formation. Reaction of MoO2with HCl is a convenient method of preparing Mo(IV) containing chloride melts. Molybdenum and tungsten dioxides can be prepared by hydrogen reduction of the respective trioxides and the reduction processes were also studied in the course of the present work. The authors are grateful to Dr. A.V. Chukin for the help with XRD measurements.

Separation of Lanthanides and Actinides in a Chloride Melt - Liquid Metal System: The Effect of Phase Composition

Pyrochemical processes employing high temperature molten salts can be used for treating spent nuclear fuels (SNF), especially with a high burn-up and after low cooling time. Before returning the fissile materials to the fuel cycle, fission products must be separated and the required separation factor depends on the nuclear reactor type. Alkali metal chlorides are normally considered as the working media for the pyrochemical reprocessing. Metallic or nitride SNFs can be anodically dissolved in the melt, whereas oxide fuels can be reduced to the metallic state and then subjected to the anodic dissolution. SNF arising from molten salt reactors can be directly used for the reprocessing. Rare earth elements (yttrium, lanthanum and lanthanides) represent an important group of fission products with the electrode potentials more negative than those of uranium and plutonium. During SNF dissolution these elements will also dissolve. Liquid metals can be effectively employed for the subsequent separation of the fissile materials from the fission products. The separation factor Θ depends on the electrode potentials (E*) of separating elements in the salt phase and their activity coefficients (γ) in the metallic alloy. If the separating metals form in the salt melt the ions in the same oxidation state (like actinides and lanthanides), the separation factor can be expressed as lgΘMe1/Me2 = [n·F / (2.303·R·T)]·(E*Me2 – E*Me1) + (lgγMe1 – lg γMe2). In the present study the effect of temperature, cationic composition of the alkali chloride based melts, and composition of the liquid metal were considered, and separation of uranium from neodymium was studied as an example. Gallium, indium, tin and their mixtures of various compositions were chosen as low melting metals. Thermodynamic and electrochemical properties of uranium and neodymium were determined between 573–1073 K thus allowing to model separation processes. Cationic composition of the salt phase has little effect on the separation factor, whilst the effect of the metallic phase nature is more pronounced. Efficiency of separation (Θ value) increases with lowering temperature. An example of thermodynamically possible Nd/U separation factors for various liquid metals and LiCl–KCl–CsCl eutectic melt is presented in the Fig. Here experimentally determined values of E* and γ were used for the calculations. To confirm the results of calculations, experiments on separating Nd and U were conducted. The experiments were performed under various conditions (e.g., static or with stirring the phases) and the experimentally achieved separation factors compared with the theoretical expectations. Figure 1

The Effect of the Second Coordination Sphere on Vanadium Speciation in Molten Chlorides

Molten salts have a wide range of unique properties that make them particularly useful in many areas of modern technology. From the fundamental point of view, analysis of speciation of vanadium in chloride melts leads to deeper understanding the nature of complex ion formation in ionic media. In the present work formation of vanadium complex ions in 3LiCl-2KCl, NaCl-KCl and NaCl-2CsCl mixtures at 450-750 oC was studied employing high temperature electronic absorption spectroscopy. The electronic absorption spectra (EAS) were recorded in the course of dissolution of vanadium(II) and vanadium(III) chlorides, anodic dissolution of metallic vanadium, and chlorination of various vanadium oxides (V2O3, V2O4, V2O5). Dissolution of VCl3 in fused alkali chlorides leads to the formation of V(III) species. Average oxidation state of vanadium in the quenched melt samples was 2.9-3.1. Spectral curves were resolved into two individual bands attributed to the 3T1g→3T1g(P) and 3T1g→3T2g electronic transitions in the octahedral VCl6 3-complex. The positions of the band maxima shift towards longer wavelengths with increasing average radius of alkali metal cation, in agreement with the expected decrease of the polarizing effect of the second coordination sphere. Vanadium dichloride added to the alkali chloride melts partly disproportionated forming V(III) species and vanadium metal. The resulting spectrum represented a superposition of the absorption bands of V2+ and V3+ions. The average oxidation state of vanadium in the quenched melts was higher than two and the concentration of vanadium was noticeably lower than expected. The best method for preparing vanadium(II) containing melts is the anodic dissolution of the metal. Position of the maximum in the EAS of V(II) in 3LiCl-2KCl at 450 oC (18770 cm-1) is very close to that of V(III), being shifted to higher energies only by about 900 cm-1 (Figure). This band corresponds to 4A2g→4T1g (P) electronic transition in the octahedral VCl6 4- complex (d3-configuration). Second band, with the maximum at 11890 cm-1 is attributed to 4A2g→4T1g electron transition in the same ion. Increasing temperature and the mean radius of the alkali metal cation on the solvent melt resulted in certain changes of EAS of vanadium(II)-containing melts, i.e., the intensity of the second peak decreased and a new, third, band appeared around 13300 cm-1 (Figure). The position of this third peak did not change with temperature and was the same in different melts. This band is attributed to 4T1→4T1 (P) electronic transition in the tetrahedral VCl4 2- ion. Therefore, increasing temperature and the mean radius of the salt-solvent cation shifts the VCl6 4-–VCl4 2-equilibrium towards the tetrahedral species. The results of in situ spectroscopy measurements conducted during the reactions of various vanadium oxides with hydrogen chloride in alkali chloride melts showed that the nature of the products formed does not depend on the type oxide and the flow rate of HCl. Oxidation state of vanadium in all the resulting melts was close to four. The EAS corresponded to the absorption of vanadyl-ions. Oxygen-containing vanadium(IV) species exhibited somewhat anomalous behavior in NaCl-2CsCl melt as the temperature was varied. Increasing temperature resulted in decreasing intensity of the band at 13000-14000 сm-1. This is explained by the coordination environment of vanadyl species in NaCl-2CsCl melts, different from the melts containing smaller cations. This conclusion was confirmed by XRD and Raman spectroscopy analysis of the quenched melts. Figure. Resolution into Gaussian bands of EAS recorded after anodic dissolution of vanadium metal in 3LiCl-2KCl (a) and NaCl-2CsCl (b) eutectic melts at 450 and 750 oC, respectively. Figure 1

Availability Evaluation of SiC Anode in Chloride Molten Salt Electrolysis

INTRODUCTION Carbon or graphite is commonly used to anode material in molten salt electrolysis because of their stability and reasonable price. However, they are consumed with CO or CO2generation by electrochemical reaction with oxide ion, and the consumption is undesirable in electrolysis in chloride melt. In this study, the anodic behavior of carbide was investigated to develop the inert anode material. EXPERIMENTAL A eutectic mixture of LiCl-KCl (41:59 in mole) with Li2O was used as an electrolytic bath. Anode was a silicon carbide (SiC) rod, and a graphite rod was also used for comparison. Cathode was an Al plate, and reference electrode was a Ag/Ag+ couple, and its potential was calibrated with a Li/Li+couple. The electrodes were inserted into the molten salt at 723 K in an Ar atmosphere. Reactions were studied by cyclic voltammetry and potentiostatic electrolysis. The generated gas by potentiostatic electrolysis was analyzed by gas chromatography. After the electrolysis, the specimen was analyzed by SEM-EDX and XRD. RESULTS and DISCUSSION Fig.1 shows typical cyclic voltammograms at the SiC and graphite anode. At the graphite electrode, oxidation current increased around 2.65V (vs. Li/Li+, the same hereinafter) and 3.5V, and arose suddenly above 3.8V. At the SiC electrode, oxidation current was increased around 2.65V, 3.2V, 3.5V and arose suddenly above 4.0V. Bubble formation was seen around 3.5V and above 3.8V (4.0V) at both electrodes. The generated gas at 3.5V was identified as CO2 by gas chromatograph, and it was considered that the generated gas above 3.8V (4.0V) was Cl2. Gas was not generated around 3.2V at the SiC electrode, and SiO2was detected on by XRD after potentiostatic electrolysis at 3.2V. The mass of the SiC electrode increased by potentiostatic electrolysis at 3.2V for 4h and 6h, while it decreased by the electrolysis for 8h, as shown in Table 1. The mass didn’t change by potentiostatic electrolysis at 3.5V for 4h, but it decreased by the electrolysis for 6h and 8h. The mass of the SiC electrode tended to decreased with the electrolysis time at both potentials. The mass decreased by the electrolysis at 4.5V even for 4h. The O content after the potentiostatic electrolysis at higer potential was smaller than that at lower potential as shown in Table 2. The surface of the SiC electrode after the electrolysis was uneven and a dense SiO2layer was not found at the cross-section. From the result mentioned above, SiO2 formation occurred below 3.2V at SiC, and CO2 was generated around 3.5V. This result indicate that SiO2 formation precedes CO2 generation at SiC, but that the formed SiO2 cannot prevent the consumption of SiC by CO2 generation. The formed SiO2 layer was still uneven and partially peered out during electrolysis. The formation of a dense and coherent SiO2 layer should be necessary to prevent the consumption of SiC by electrolysis in molten chloride. Figure 1

Intervalence Charge Transfer of the Nb(V)/Nb(IV) Redox Couple in Alkali Chloride Melts: Experiment and Quantum-Chemical Calculations

The standard rate constants of charge-transfer (k s) for the Nb(V)/Nb(IV) redox couple were determined by cyclic voltammetry in KCl – NbCl5 and CsCl – NbCl5 chloride melts at glassy carbon and platinum electrodes. The Nb(V) electrochemical reduction process in the NaCl–KCl (equimolar mixture) – NbCl5 melt was shown being reversible even at high polarization scan rate (v = 2.0 Vs-1); hence, the determination of k s appeared impossible. Values of k s are larger in KCl–NbCl5 melt than in CsCl–NbCl5 one over the entire studied temperature range; the values of k s obtained at platinum electrode are larger than those for glassy carbon electrode1. Quantum-chemical calculations were carried out at the DFT/B3LYP theory level with use of the quasi-relativistic basis set Stuttgart. For Na-system under consideration the ECP basis Crenbl was used too. Previously,2-7 in quantum-chemical calculations of nM+·[NbF7], and nM+·[NbCl6], model systems (M = Na, K, Cs; n = 1÷n lim), containing a complex anion with a cation outer-sphere (OS) shell, it was found that compositions with an intermediate number of OS cations m <n lim, where n lim is the limiting number of OS cations bound to a given complex, are thermodynamically most stable. Typically, the change in the values calculated activation energies Eact of charge transfer on the Nb complex consistent with the change of the experimental standard rate constants of charge transfer in a series of molten salts type (Na–K)Cleq–KCl–CsCl. However, in chloride melts containing [NbCl6] complex, there is an abnormally large computed Eact value in the Na-system. This indicates the difference in the mechanisms of charge transfer in the Na-system, on the one hand, and the K-, Cs-systems on the other. In this work the calculated data for extended model systems such as 2M+∙[NbCl6] + 18MCl (M – Na, K, Cs) are analyzed in spirit of works.8-9 In this study, temperature dependences of parameters analyzed are follow-up calculated. It was found that at temperatures (T ≥873K) compositions of the most stable particles in these systems differ. Namely, in the Na-system it corresponds to electroneutral particle Na+∙[NbCl6]-,and in the K-, Cs-systems the anion [NbCl6]- is the most stable. Contribution in Eact of the work of particle approach to the cathode surface for these cases is essentially different. This may explain the anomalous value of the Na-system Eact compared with K-, Cs-systems. Model systems of the present approximation level do not allow to consider such effects, their analysis is possible only in model systems of the higher level of approach. Acknowledgments The work was financially supported by Russian Foundation for Basic Research (15-03-02290_a). References A.V. Popova, V.G. Kremenetsky, S.A. Kuznetsov, Russ. J. Electrochem., 2014, 50 (9), 807.S.A. Kuznetsov, V.G. Kremenetsky, A.V. Popova, O.V. Kremenetskaya, V.T. Kalinnikov, Dokl. Phys. Chem., 2009, 428 (2), 209.V.G. Kremenetsky, O.V. Kremenetskaya, S.A. Kuznetsov, V.T. Kalinnikov, Dokl. Phys. Chem. 2011, 437 (2) 75.A.V. Popova, V.G. Kremenetsky, V.V. Solov’ev, L.A. Chernenko, O.V. Kremenetskaya, A.D. Fofanov, S.A. Kuznetsov, Russ. J. Electrochem., 2010, 46 (6), 671.Yu.V. Stulov, V.G. Kremenetsky, S.A. Kuznetsov, Int. J. Electrochem. Sci., 2013, 8(5), 7327.A.V. Popova, V.G. Kremenetsky, S.A. Kuznetsov, ECS Trans., 2013. 50 ( 11 ), 367.A.V. Popova, V.G. Kremenetsky, S.A. Kuznetsov, ECS Trans., 2014. 64 (4), 171.V.G. Kremenetsky, O.V. Kremenetskaya, S.A. Kuznetsov, V.T. Kalinnikov, Dokl. Phys. Chem., 2013, 452 (2), 213.V.G. Kremenetsky, O.V. Kremenetskaya, Russ. J. Inorg. Chem., 2013, 58 (12), 1523.