Coatings by refractory metal carbides: Deposition from molten salts,properties, application

High-temperature electrochemical synthesis and currentless transfer in molten salts were used to obtain coatings constituted by carbides of refractory metals (Mo₂C, Cr₇C₃, NbC, and TaC). It was found that the Mo₂C/Mo composite synthesized from a chloride-carbonate-molybdate melt has the highest catalytic activity. It was shown that the Mo₂C catalytic coating preserves its properties for at least 5000 h of tests. The protective properties of refractory metal carbides of composition Cr₇C₃, NbC, and TaC significantly improve the corrosion resistance of steel articles in concentrated solutions and raise their wear resistance by an order of magnitude.     

Electrochemical synthesis of hafnium silicides

The method of cyclic voltammetry was used to study cathodic processes in a molten salt of NaCl-KCl-NaF (10 wt %)-K₂HfF₆-K₂SiF₆. The parameters of electrochemical synthesis allowing synthesizing hafnium silicides are determined and hafnium silicide HfSi₂ is obtained using galvanostatic electrolysis.     

Quantum-Chemical Analysis of the Electron Transfer Mechanism in Model System MgNbF<Subscript>7</Subscript> + 12MgCl<Subscript>2</Subscript> by the Method of Frontier Molecular Orbitals

By the example of quantum-chemical analysis of the model system MgNbF₇ + 12MgCl₂, the possibilities of the method of frontier molecular orbitals in studying the mechanism of electron transfer in molten salts are demonstrated. The rich information provided by this method allows recommending it as a tool for testing the hypotheses on the mechanism of charge transfer in electrochemical systems.     

Molten salt synthesis (MSS) of Cu2Mo6S8—New way for large-scale production of Chevrel phases

The Chevrel phase (CP), Mo₆S₈, was found to be an excellent cathode material for rechargeable magnesium batteries. Mo₆S₈ is obtained by a leaching process of Cu₂Mo₆S₈, which removes the copper. A new method of Cu₂Mo₆S₈ production was developed. In contrast to the well-known solid-state synthesis of CP, the method is based on the reaction in a molten salt media (KCl). A fast kinetics of this reaction allows using less active, but more convenient precursors (sulfides instead of sulfur), decreasing temperature and synthesis duration, as well as operation in the inert atmosphere instead of dynamic evacuated systems. It was shown that the composition and the electrochemical behavior of the products obtained by MSS and by the solid-state synthesis are identical. Thus, the molten salt method is extremely attractive for the large-scale production of the active materials for Mg batteries.     

Inorganic Molten Salts as Solvents for Cellulose

Inorganic molten salts can be used as efficient solvents for cellulose in a wide range of degrees of polymerization. Furthermore, molten salts can be applied as reaction medium for the derivatization of cellulose. For both dissolution and derivatization of cellulose, knowledge of the solution state as well as information about chemical interactions with the solvent system is essential. Using the melts of LiClO₄·3H₂O, NaSCN/KSCN/LiSCN·2H₂O and LiCl/ZnCl₂/H₂O as cellulose solvents, factors which determine the dissolving ability will be discussed. Besides the specific structure of the molten salt hydrate, the cation and the water content of the melt are the most important factors for the dissolving capability of a molten salt hydrate system. FT-Raman spectroscopy, ⁷Li and ¹³C NMR spectroscopy were applied to describe solvent–cellulose interactions and the state of cellulose dissolved in the molten salts. Using Raman and solid state NMR spectroscopy it was proved that cellulose is amorphous in the frozen solvent system. The application of inorganic molten salts as a medium for cellulose functionalization is demonstrated for cellulose carboxymethylation and acetylation.     

Low‐Temperature Molten‐Salt Production of Silicon Nanowires by the Electrochemical Reduction of CaSiO3

Silicon is an extremely important technological material, but its current industrial production by the carbothermic reduction of SiO₂ is energy intensive and generates CO₂ emissions. Herein, we developed a more sustainable method to produce silicon nanowires (Si NWs) in bulk quantities through the direct electrochemical reduction of CaSiO₃, an abundant and inexpensive Si source soluble in molten salts, at a low temperature of 650 °C by using low‐melting‐point ternary molten salts CaCl₂–MgCl₂–NaCl, which still retains high CaSiO₃ solubility, and a supporting electrolyte of CaO, which facilitates the transport of O²⁻ anions, drastically improves the reaction kinetics, and enables the electrolysis at low temperatures. The Si nanowire product can be used as high‐capacity Li‐ion battery anode materials with excellent cycling performance. This environmentally friendly strategy for the practical production of Si at lower temperatures can be applied to other molten salt systems and is also promising for waste glass and coal ash recycling.     

Study on a separation method of radionuclides (Ba, Sr) from LiCl salt wastes generated from the electroreduction process of spent nuclear fuel

In this paper, a separation method of radionuclides (Ba, Sr) from LiCl salt wastes generated from the electroreduction process of spent nuclear fuel was studied to recover pure LiCl salts and reduce radioactive wastes. The method consisted of chemical conversion process of BaCl₂ and SrCl₂ in LiCl molten salts by using lithium compounds and vacuum distillation process of LiCl salts. In the chemical conversion, BaCl₂ and SrCl₂ in LiCl molten salts were mainly converted into (Ba,Sr)CO₃ or (Ba,Sr)SO₄. Contents of Ba and Sr in LiCl salts recovered from the vacuum distillation process were equal to about 0.01 of initial concentrations of Ba and Sr in LiCl molten salts. These results will be utilized to recycle the LiCl salt wastes.     

Pure and Metal-confining Carbon Nanotubes through Electrochemical Reduction of Carbon Dioxide in Ca-based Molten Salts

To be successfully implemented, an efficient conversion, affordable operation and high values of CO₂-derived products by electrochemical conversion of CO₂ are yet to be addressed. Inspired by the natural CaO-CaCO₃ cycle, we herein introduce CaO into electrolysis of SnO₂ in affordable molten CaCl₂-NaCl to establish an in situ capture and conversion of CO₂. In situ capture of anodic CO₂ from graphite anode by the added CaO generates CaCO₃. The consequent co-electrolysis of SnO₂ and CaCO₃ confines Sn in carbon nanotube (Sn@CNT) in cathode and increases current efficiency of O₂ evolution in graphite anode to 71.9 %. The intermediated CaC₂ is verified as the nuclei to direct a self-template generation of CNT, ensuring a CO₂-CNT current efficiency and energy efficiency of 85.1 % and 44.8 %, respectively. The Sn@CNT integrates confined responses of Sn cores to external electrochemical or thermal stimuli with robust CNT sheaths, resulting in excellent Li storage performance and intriguing application as nanothermometer. The versatility of the molten salt electrolysis of CO₂ in Ca-based molten salts for template-free generation of advanced carbon materials is evidenced by the successful generation of pure CNT, Zn@CNT and Fe@CNT.     

Mg二次电池正极材料Cu2Mo6S8的合成与表征

采用CuS.H2O、MoS2、Mo为原料,用熔盐法(KCl为熔盐)合成了谢弗雷尔相的Cu2Mo6S8正极材料,并用XRD,SEM,循环伏安测试,充放电测试对材料的结构和电化学性能进行研究。XRD结果表明本Cu2Mo6S8正极材料属于R3空间群,具有良好的晶型。电化学测试表明,当材料在电压0.2～2 V范围内进行充放电时,其放电比容量在90 mAh.g-1左右,循环稳定性和可逆性均良好。该材料的充放电曲线中在1.2 V和1.9 V分别有还原峰,0.7 V和1.0 V分别有氧化峰,与伏安曲线相对应。     

Overall Carbon-neutral Electrochemical Reduction of CO2 in Molten Salts using a Liquid Metal Sn Cathode

An overall carbon-neutral CO₂ electroreduction requires enhanced conversion efficiency and intensified functionality of CO₂-derived products to balance the carbon footprint from CO₂ electroreduction against fixed CO₂. A liquid Sn cathode is herein introduced into electrochemical reduction of CO₂ in molten salts to fabricate core–shell Sn−C spheres (Sn@C). An in situ generated Li₂SnO₃/C directs a self-template formation of Sn@C. Benefitting from the accelerated reaction kinetics from the liquid Sn cathode and the core–shell structure of Sn@C, a CO₂-fixation current efficiency higher than 84 % and a high reversible lithium-storage capacity of Sn@C are achieved. The versatility of this strategy is demonstrated by other low melting point metals, such as Zn and Bi. This process integrates energy-efficient CO₂ conversion and template-free fabrication of value-added metal-carbon, achieving an overall carbon-neutral electrochemical reduction of CO₂.     

Competitive Coordination of Chloride and Fluoride Anions Towards Trivalent Lanthanide Cations (La3+ and Nd3+) in Molten Salts

Molten salt electrolysis is a vital technique to produce high-purity lanthanide metals and alloys. However, the coordination environments of lanthanides in molten salts, which heavily affect the related redox potential and electrochemical properties, have not been well elucidated. Here, the competitive coordination of chloride and fluoride anions towards lanthanide cations (La³⁺ and Nd³⁺) is explored in molten LiCl-KCl-LiF-LnCl₃ salts using electrochemical, spectroscopic, and computational approaches. Electrochemical analyses show that significant negative shifts in the reduction potential of Ln³⁺ occur when F⁻ concentration increases, indicating that the F⁻ anions interact with Ln³⁺ via substituting the coordinated Cl⁻ anions, and confirm [LnCl_xF_y]^3−x−y (y_max=3) complexes are prevailing in molten salts. Spectroscopic and computational results on solution structures further reveal the competition between Cl⁻ and F⁻ anions, which leads to the formation of four distinct Ln(III) species: [LnCl₆]³⁻, [LnCl₅F]³⁻, [LnCl₄F₂]³⁻ and [LnCl₄F₃]⁴⁻. Among them, the seven-coordinated [LnCl₄F₃]⁴⁻ complex possesses a low-symmetry structure evidenced by the pattern change of Raman spectra. After comparing the polarizing power (Z/r) among different metal cations, it was concluded that Ln−F interaction is weaker than that between transition metal and F⁻ ions.     

Electrochemical formation of LiAl alloy in molten dimethylsulfone at 150°C

The electrochemical behaviour of the Li⁺/Li couple is examined briefly in molten dimethylsulfone (DMSO₂) at 150°C. It has been found that the Li⁺/Li system is reversible. However, electrodeposited lithium exhibits some instability which severely limits the use of the lithium anode in molten DMSO₂ based secondary cells. Therefore we have undertaken the study of the electroformation of the LiAl alloy in 1 mol kg⁻¹ LiClO₄/DMSO₂ at 150°C by investigating the electrochemical incorporation of lithium into an aluminium electrode by potentiostatic and galvanostatic techniques. The results obtained from electrochemical measurements and X-ray diffraction experiments have proved that incorporation of Li in Al in molten DMSO₂ leads to the formation of the β LiAl alloy. Analysis of the chronocoulometric curves has allowed the different processes limiting the rate of incorporation of Li and Al to be specified. Moreover, the diffusion coefficient of lithium into aluminium has been determined from chronoamperometric measurements: 0.7 × 10⁻¹⁰ ⪕ D_Li(α) ⪕ 1.4 × 10⁻¹⁰ cm² s⁻¹. Finally, the galvanostatic study has shown that the β LiAl alloy can be considered a promising anodic material for molten dimethylsulfone-based secondary batteries.     

Na2SO4 + NaCl molten salts corrosion mechanism of thermal barrier coatings used in ships

6–8 mass% Y₂O₃ stabilized ZrO₂ (6–8YSZ) thermal barrier coatings (TBCs) are widely applied to protect the hot ends of gas turbines in large navy ships. In this work, the 8YSZ TBCs were prepared by air plasma spraying technique, and their microstructure and phase composition were investigated. The hot corrosion mechanism of YSZ TBCs in molten salts consisting of 80% Na₂SO₄?+?20% NaCl at 900 °C was comprehensively analyzed. The results showed that the corrosion product Y₂(SO₄)₃ was formed due to the reaction between Na₂SO₄ media and the stabilizer Y₂O₃. As the result of the depletion of Y₂O₃ phase, the transformation from the tetragonal phase to monoclinic phase of ZrO₂ could not been totally inhibited, which consequently induced the 4–6 vol.% expansion and more cracks of YSZ TBCs. Meanwhile, the cracks could work as transfer paths for oxygen and molten salts. The kinetic analysis on hot corrosion also showed that more reaction products (from 2 to 8.1 mg cm^?1) were generated from 20 to 60 h due to more cracks generated by molten salts and oxygen infiltrating. More thermal grown oxides generated between ceramic layer, bonding layer and substrate, and the volume expansion caused by phase transition, increased the stresses in the coatings. Consequently, the peeling-off failure of 8YSZ TBCs could happen.

     

Direct electrochemical preparation of solid Fe(VI) ferrate,and super-iron battery compounds

The electrochemical generation of Fe(VI) salts eliminates requirements for chemical oxidants and decreases the synthesis complexity. An alternate electrochemical synthesis from an iron anode in alkaline electrolyte is presented for the in situ direct synthesis of the solid Fe(VI) salts, such as BaFeO₄. A variety of electrolysis conditions including cell configuration, applied current density, temperature, synthesis duration, electrolyte composition and concentration, and the molar ratio between the reactants have been explored. In situ electrochemically synthesized BaFeO₄ exhibits similar valence, IR absorption spectrum and X-ray powder diffraction as the chemically synthesized material.     

Foreword

Electrochemical behaviors of U⁴⁺ in LiCl–KCl–UF₄ eutectic and deposition of U metal were investigated. It was found that the presence of F^? has influence on the diffusion of U³⁺ and U⁴⁺ as comparing to data obtained in pure chloride molten salts. Electrochemical deposition of U was carried out by using pulse current electrolysis. Characterization results indicate that U metal was obtained at the cathode, implying U metal can be directly deposited from LiCl–KCl–UF₄ eutectic in this case and the extractive ratio is calculated to be 98%. Our results demonstrate feasible separation of U from LiCl–KCl–UF₄ molten salt by electrochemical method.     

Producing metallic titanium through electro-refining of titanium nitride anode

Titanium nitride (TiN) was used as consumable anode to produce metallic titanium in molten salts. The electrochemical dissolution of TiN was investigated. It was found that nitrogen (N₂) was monitored at the anode during electrolysis. The titanium ion species was changed between Ti^2 + and Ti^3 + depending on the electrochemically dissolving potentials of TiN. Furthermore, the product on the cathode was analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results show that pure titanium powders can be prepared by the TiN electro-refining in a molten salt bath.     

Advancements and potentials of molten salt CO2 capture and electrochemical transformation (MSCC-ET) process

Electroreduction of CO₂ in molten salts, also called molten salt carbon capture and electrochemical transformation (MSCC-ET), can convert CO₂ into value-added carbonaceous materials or CO in a comparably high rate with high energy efficiency. It shows a promising potential to contribute to the earth's atmospheric carbon balance, the intermittent renewable energy storage, carbon materials production, and air quality control of the local environment. This short review briefly introduces the principle of the MSCC-ET process, the state-of-the-art of the process, its potential of commercialization in terms of process efficiency, product marketing and economy, and finally, the opportunities and challenges in future research and development.     

Electrochemical Study of Potassium Fluoride in a Cryolite-Aluminum Oxide Molten Salt

Selective and efficient electrochemical methods to characterize aluminum are necessary. Current methods are based on potentiodynamic polarization, recurrent potential double pulses, chronopotentiometry, open-circuit chronopotentiometry, and potentiostatic electrolysis, but have not been used to characterize the deposition of aluminum in Na₃AlF₆-Al₂O₃-KF molten salts. The control processes of the formation of aluminum-tungsten intermetallic compounds, and the deposition of aluminum have been investigated by using steady-state potentiodynamic cathodic polarization curves. The dissolution loss rate of aluminum was determined with an increase in KF concentration by the analysis of recurrent potential double pulses. Using chronopotentiometry, it was confirmed that the deposition potential of aluminum shifted more negative as the KF concentration increased, and a higher KF concentrations induced a higher cathodic overpotential. From open-circuit potential measurements and scanning electron micrographs, it was concluded that aluminum(III) ions react with tungsten substrates to form an aluminum-tungsten compound, and the reaction mechanism of aluminum was determined. These electrochemical methods applied with aluminum electrolysis were accurate, efficient, and reliable.     

Chemische Reaktionen in Salzschmelzen. XVIII. Die Synthese des Methylzinntrichlorids,CH3SnCl3

Chemical Reactions in Molten Salts. XVIII. Synthesis of Methyltin Trichloride, Ch₃SnCl₃ A process for the synthesis of CH₃SnCl₃ by reaction of molten SnCl₂ and CH₂Cl infused salts is described. Out of the systems investigated with NaCl, KCl, AlCl₃, ZnCl₂, KZnCl₃, LiAlCl₄, NaAlCl₄, KAlCl₄, and CuAlCl₄, the systems with NaAlCl₄ and NaAlCl₄/KAlCl₄ proved best. We measured the space-time-yield (RZA) as a function of the salts admixed to SnCl₂ as well as a function of the reaction temperatur, gasification, and inlet rate of CH₃Cl respectively. The optimum reaction conditions are investigated and the system SnCl_2//NaAlCl₄ is described.     

The electrochemically assisted synthesis of ZnSiP2

Efforts directed toward the electrochemical synthesis of the ternary chalcopyrite ZnSiP₂ are discussed. In all cases reported here, the zinc source was a molten zinc cathode. Electrolysis current densities and temperatures are found to be important variables in determining whether the predominant products are ternary or binary phosphides. The binary phosphides are formed at temperatures below the boiling point of the zinc cathode (906°C). Above 1000°C, transport of zinc and silicon sources occurs readily. In the atmosphere of nearly pure elemental phosphorus maintained by the electrochemical reduction of the phosphorous source onto the cathode, the ternary phosphide is formed by the direct combination of the elements on the walls of the cell. The chemical nature of the silicon source is also found to be a critical factor influencing the formation, purity, and silicon content of the ternary phosphides. Silicon sources studied included: Mg₂Si, Na₂SiF₆, and SiO₂. The characteristics of ZnSiP₂ samples prepared in this fashion using a variety of silicon sources are described.