On the development of metallic inert anode for molten CaCl2–CaO System

Some fundamental aspects related to inert anode development in molten CaCl₂–CaO were investigated based on thermodynamic analysis, electrochemistry of metals and solubility of oxide measurements. The Gibbs free energy change of several key anodic reactions including electro-stripping of metals, electro-formation of metallic oxides, electro-dissolution of metallic oxides as well as oxygen and chlorine evolution was calculated and documented, for the first time, as a reference to develop metallic inert anode in chloride based melts. The anodic behaviors of typical metals (Ni, Fe, Co, Mo, Cu, Ag, and Pt) in the melt were investigated. The results confirmed the thermodynamic stability order of metals in the melts and revealed that surface oxide formation can increase the stability of the electrodes in CaO containing melt. Furthermore, solubility of several oxides (NiO, Fe₂O₃, Cr₂O₃, Co₃O₄, NiFe₂O₄) in pure CaCl₂ or CaCl₂–CaO melts was measured to evaluate the stability of oxide coating or a cermet inert anode in the melt. It was found that the solubility of NiO decreased with increasing CaO concentration, while that of Fe₂O₃ increased. Ni coated with NiO film had much higher stability during anodic polarization.     

Compatibility Issues of Yttria‐Stabilized Zirconia Solid Oxide Membrane in the Direct Electro‐Deoxidation of Metal Oxides

Direct electro‐deoxidation of metal oxides has become quite popular in the production of metals and alloys. In this process, metal oxide cathode is directly reduced to a metal in a molten CaCl₂ salt bath. The anode material used is graphite. Over the years, graphite is reported to cause numerous process difficulties. Recently, based on the solid oxide membrane technology, yttria‐stabilized zirconia (YSZ) has been tested as oxygen ion conducting membrane for the anode. The success of using a membrane implies its long‐term stability in the bath. In this paper, it is seen that YSZ chemically degrades in a static melt of CaCl₂ or CaCl₂–CaO. The degradation occurs by leaching of yttria into solution leading to the formation of monoclinic zirconia which, being porous, reacts with the molten electrolyte to form calcium zirconate. However, on application of voltage, YSZ degrades via a different mechanism. The metallic calcium produced during electrolysis increases the electronic conductivity of the salt, apparently leading to the electrochemical reduction of zirconia to ZrO_2?x. As a result, localized pores are formed which allow the infiltration of salts. Addition of yttria to the salt is seen to prevent both the chemical and electrochemical degradation of the membrane.     

A direct electrochemical route from oxide precursors to the terbium-nickel intermetallic compound TbNi5

Successful direct electrochemical reduction of mixed powders of terbium oxide (Tb₄O₇) and nickel oxide (NiO) to the intermetallic compound, TbNi₅, is demonstrated in molten CaCl₂ at 850 °C by constant voltage (2.4-3.2 V) electrolysis. The reduction mechanism was investigated by cyclic voltammetry using a molybdenum cavity electrode in conjunction with characterisations of the products from both constant voltage and potentiostatic electrolysis under different conditions by XRD, SEM and EDX. It was found that the reduction started from NiO to Ni, followed by that of Tb₂O₃ (resulting from Tb₄O₇ decomposition) on the pre-formed Ni to form the intermetallic compound. The reduction speed increased with increasing the cell voltage, but the speed gain was counterbalanced by decreased current efficiency and increased electric energy consumption. At 2.4 V, the current efficiency reached 63.2%, and the energy consumption by electrolysis was as low as 3.2 kWh/kg TbNi₅ when the oxide phase was converted fully to the metal phase (XRD) in 4 h. The oxygen level in the produced TbNi₅ could readily reach 1800 ppm by electrolysis at 3.2 V for 12 h with the energy consumption being 18.9 kWh/kg TbNi₅.     

Electrochemical reduction of cerium oxide into metal

The Fray Farthing and Chen (FFC) and Ono and Suzuki (OS) processes were developed for the reduction of titanium oxide to titanium metal by electrolysis in high temperature molten alkali chloride salts. The possible transposition to CeO₂ reduction is considered in this study. Present work clarifies, by electro-analytical techniques, the reduction pathway leading to the metal. The reduction of CeO₂ into metal was feasible via an indirect mechanism. Electrolyses on 10 g of CeO₂ were carried out to evaluate the electrochemical process efficiency. Ca metal is electrodeposited at the cathode from CaCl₂–KCl solvent and reacts chemically with ceria to form not only metallic cerium, but also cerium oxychloride.     

Investigation on electrochemical reduction process of Nb2O5 powder in molten CaCl2 with metallic cavity electrode

The direct electrochemical reduction process of Nb₂O₅ powder was investigated by cyclic voltammetry and constant potential electrolysis with a novel metallic cavity electrode in molten calcium chloride at 850 °C. The products of both constant potential and constant voltage electrolysis were characterized by XRD, SEM and EDX. CaNb₂O₆ was formed upon addition of solid Nb₂O₅ into molten CaCl₂ when CaO was present. During the electrolysis solid Nb₂O₅ was reduced to various niobium oxides of lower oxidation states, including some composite oxides, and then was converted completely to metallic niobium near −0.35 V (vs. Ag/AgCl), which was more positive than the reduction potential of Ca²⁺. Constant potential electrolysis was applied at the potentials near the reduction current peaks derived from the cyclic voltammetry curves, and cell voltages were monitored. The voltage was near 2.4 V when the oxide was metallized at −0.35 V (vs. Ag/AgCl). Nb₂O₅ pellet could be used to prepared metallic niobium at cell voltage 2.4 V in a larger electrolysis bath filled with calcium chloride at 850 °C. The experiment results further demonstrated the direct electrochemical reduction mechanism of Nb₂O₅ powder in a molten system.     

Toward optimisation of electrolytic reduction of solid chromium oxide to chromium powder in molten chloride salts

Electrochemical reduction of solid Cr₂O₃, in the form of an assembled cathode of porous pellets attached to a current collector, to chromium powder was investigated in molten CaCl₂ and a molten equimolar mixture of CaCl₂ and NaCl. The study focused on the influence of pellets preparation conditions, cell voltages and temperatures on the reduction process. Analyses were reported of the characteristics of the current-time plots of the constant voltage electrolysis under different conditions, cyclic voltammograms of solid Cr₂O₃ in the molten equimolar mixture of CaCl₂ and NaCl, the microstructures and elemental compositions of the reduced pellets. Particularly, attention was given to the intermediate product of the electrolysis, calcium chromites of various stoichiometries, aiming to achieve a better understanding and optimisation of the reduction process.     

Direct electrolytic reduction of solid alumina using molten calcium chloride-alkali chloride electrolytes

Solid alumina was reduced by electro-deoxidation to aluminium metal containing 1.8 and 5.4 at% Ca in molten CaCl₂–NaCl and CaCl₂–LiCl electrolytes at 900 °C, respectively. The potential-pO²⁻ diagrams for the Al–O–M–Cl (M = Na or Li, or/and Ca) system were constructed to predict equilibrium phase relationships in the electrolytes at 700 and 900 °C. It was found that calcium aluminates were formed as the main intermediate reaction products and were subsequently reduced to form the Al-rich Al–Ca alloys during electro-deoxidation. Calcium and/or lithium, at reduced activities, were created at the cathode especially at 700 °C at the same time as the ionization of the oxygen from the cathode, which resulted in Al₂Ca formation. The experimental results were consistent with the thermodynamic predictions.     

Direct selective extraction of titanium silicide Ti5Si3 from multi-component Ti-bearing compounds in molten salt by an electrochemical process

Titanium silicide (Ti₅Si₃) has been extracted directly from complex Ti-bearing compounds by electro-deoxidation. A pressed cylindrical pellet of the multi-component compounds acted as a cathode, and carbon-saturated liquid metal contained in a solid-oxide oxygen-ion-conducting membrane (SOM) tube served as an anode. This electrochemical process was carried out in a molten CaCl₂ system at 950–1050 °C and 3.5–4.0 V. The parameters of electrolysis and the electrolytic characteristics were investigated, and the morphology and phase composition of the final products were examined. Ti₅Si₃ was directly extracted from Ti-bearing compounds. Electrolysis time, applied potential, and electrolysis temperature are the dominant factors that affect the characteristics of the final products. The mechanisms of the extraction of Ti₅Si₃ and the removal of metallic elements (Ca, Mg, and Al) are suggested based on our experimental results and theoretical analysis. The optimal conditions of the electrolysis are a temperature of ∼1050 °C and a potential of 3.5–4.0 V, which result in rapid reduction and good product morphology. The excellent oxidation resistance of the extracted Ti₅Si₃ is confirmed.     

熔盐电解NiO制备Ni的机理探讨

通过固态NiO在CaCl_2-NaCl熔盐中直接电化学还原制备Ni的电解产物随时间变化分析,石墨阳极破损研究以及对NiO低于理论分解电压的槽电压分解现象的解释,探讨了熔盐电解NiO制备Ni的电解反应机理。认为电解反应基本服从氧离子化机理,且NiO阴极片在低于理论分解电压的槽电压下,电化学还原反应和热还原反应同时发生。     

Electrochemical characterization of La<Subscript>0.8</Subscript>Sr<Subscript>0.2</Subscript>MnO<Subscript>3</Subscript>-coated NiO as cathodes for molten carbonate fuel cells

La_0.8Sr_0.2MnO₃ was coated on porous NiO cathode using a simple combustion process. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed in the cathode characterizations. The electrochemical behavior of La_0.8Sr_0.2MnO₃-coated NiO cathodes (LSM–NiO) were also evaluated in a molten 62 mol%Li₂CO₃+38 mol%K₂CO₃ eutectic at 650 °C under the standard cathode gas condition by electrochemical impedance spectroscopy (EIS). The impedance response of the NiO and LSM–NiO cathode at different immersion times is characterized by the presence of depressed semicircles in the high frequency range and an extension at low frequencies. Impedance analysis showed that the behavior of the developed cathode was similar to that of the conventional nickel oxide cathode. The LSM–NiO showed a lower dissolution and a better catalytic efficiency superior to the state-of-the-art NiO value. Thus the cathode prepared with coating method to coat La_0.8Sr_0.2MnO₃ on the surface of NiO cathode is able to reduce the solubility of NiO to lengthen the lifetime of MCFC while maintaining the advantages of NiO cathode. The LSM–NiO shows promise as an alternate cathode in molten carbonate fuel cells (MCFCs).     

Current efficiency studies for graphite and SnO2-based anodes for the electro-deoxidation of metal oxides

Studies were performed investigating the electrochemical reduction of chromium oxide (Cr₂O₃) by electro-deoxidation by utilising either a graphite anode or a tin oxide (SnO₂) based anode. Potentiostatic electrolysis was performed at 3.0 V for both a graphite and for a SnO₂-based anode, and also 2.0 V for a graphite anode. The cathode reduction purity, anode mass change, anode potential relative to a glassy carbon pseudo-reference and current efficiency were measured and compared. The key observations are that substituting a SnO₂-based anode for a graphite anode led to greater current efficiencies for electro-deoxidation. This was attributed to the lack of contamination of the melt by carbon and the lower cathode potential due to the higher anodic potential when using tin oxide based anodes for the same applied voltage. The current efficiency was also found to decrease with both anode materials when higher anode surface areas or lower current densities were used. Again this was attributed to a decrease in anodic potentials and a corresponding increase in the cathodic potential resulting in a greater number of parasitic reactions occurring at the cathode.     

Direct electrochemical reduction of titanium dioxide in Lewis basic AlCl3–1-butyl-3-methylimidizolium ionic liquid

The direct electrochemical reduction of titanium dioxide (TiO₂) to metallic titanium at room temperature is firstly studied in Lewis basic AlCl₃–1-butyl-3-methylimidizolium (AlCl₃–BMIC) ionic liquid. In this study, cyclic voltammetry, potentiodynamic polarization, sampled current voltammetry and X-ray photoelectron spectroscopy (XPS) techniques were utilized. Analysis of the cyclic voltammetry suggested that TiO₂ film can be reduced to metallic Ti. The sampled current voltammetry was applied to elaborate the reduction mechanism and the results showed that this reduction process may include two steps. When the output potential difference of 2.8 V was applied, a TiO₂ cylindrical pellet was partly reduced to metallic Ti. However, due to the very slow reaction rate, there was only about 12 wt% of TiO₂ was reduced during the electrolysis time of 48 h. It was predicted that the process for the direct reduction of solid TiO₂ would be explained as follows: given enough cathode potential, the reduction happened at the cathode/ionic liquids interface, where the oxygen was ionized, then dissolved in the ionic liquid and discharged at the anode, with the metallic Ti left at the cathode.     

Preparation and characterization of CaO nanoparticles from Ca(OH)2 by direct thermal decomposition method

CaO is an important material because of its application as catalyst and effective chemisorbents for toxic gases. In this research CaO nanoparticles were prepared via direct thermal decomposition method using Ca(OH)₂ as a wet chemically synthesized precursor. Nanocrystalline particles of Ca(OH)₂ have been obtained by adding 1 and 2 M NaOH aqueous solutions to 0.5 M CaCl₂·2H₂O aqueous solutions at 80 °C. The precursor [Ca(OH)₂] was calcined in N₂ atmosphere at 650 °C for 1 h. Samples were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), infrared spectrum (IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Brunaure–Emett–Teller (BET). SEM images showed that CaO nano-particles were nearly spherical in morphology. TEM images illustrated that produced CaO nano-particles had the mean particle size of 91 and 94 nm for 1 and 2 M NaOH concentration, respectively. As a result, this method could be used for production of CaO nano-particles on large-scale as a cheap and convenient way, without using any surfactant, organic medium or complicated equipment.     

Novel electrochemical measurements on direct electro-deoxidation of solid TiO<Subscript>2</Subscript> and ZrO<Subscript>2</Subscript> in molten calcium chloride medium

A solid metal oxide cathode undergoes significant chemical changes during the molten salt electro-deoxidation process. The changes in the chemical composition lead to changes in the electrical resistivity and potential of the electrode. Two novel electrochemical techniques, based on these two parameters, have been employed to study the electro-deoxidation of solid TiO₂ and ZrO₂ in molten calcium chloride at 900 °C. The in situ resistance measurements carried out by the IR drop method conclusively proved that TiO₂ electrode remains highly conducting throughout the electro-deoxidation process and hence is amenable for reduction. The ZrO₂ electrode, on the other hand, developed very high resistance midway in the electro-deoxidation, and could not be reduced completely. The resistance measurements give strong indication that the electron-transfer reactions taking place at the cathode determine the rate and efficiency of the electro-deoxidation process to a great extent. The low-current galvanostatic electro-deoxidation of TiO₂ electrodes, in conjunction with a graphite pseudo reference electrode to monitor the half cell potentials, showed that the metal oxide passes through two stages during the electrolysis; a high current, low resistant stage 1, where Ca²⁺ ions are inserted to the metal oxide cathode to produce different intermediate compounds and stage 2 where electro-deoxidation of the cathode take place continuously. Removal of oxygen, from the cathode, in stage 1 of the electro-deoxidation is considered to be insignificant. The anodic and cathodic voltages in this stage remained more or less stable at ~1.4 V and ~−1 V, respectively. When the oxygen ions in the melt were depleted at the end of this stage, both the anode and cathode potentials were increased in the anodic direction and this behaviour suggested that the graphite pseudo reference electrode was changed from a C/CO electrode in stage 1 to a Ca²⁺/Ca electrode in stage 2.     

Direct electrochemical preparation of CeNi5 and LaxCe1−xNi5 alloys from mixed oxides by SOM process

A novel SOM process was used to prepare CeNi₅ and La_xCe_1−xNi₅ hydrogen storage alloys directly from their mixed oxides. The electrolytic reduction was carried out in molten CaCl₂ system at 1000 °C. The reduction mechanism was investigated by analyzing the chemical compositions and the phase constitutions of the intermediate products of electrolysis. The results suggested that the reduction of NiO-CeO₂ may take place in two steps: first, NiO was reduced into Ni and CeO₂ reacted with CaCl₂ to form CeOCl, then Ni reacted with CeOCl leading to the formation of CeNi₅. It was found that the reduction rate increased while decreasing the pressure load of the mixed oxide pellets. Furthermore, CeNi₅ could not be produced if the pressure load was lower than 10 MPa. It was also found that the pellets of NiO-CeO₂ could be completely reduced to CeNi₅ alloy by the SOM process, which was greatly excelled than FFC process. The successful preparation of La_xCe_1−xNi₅ (x = 0-1) alloy reported here suggests that the SOM process may be promising for the industrial application of producing such alloys.     

Effect of optical basicity on the stability of yttria‐stabilized zirconia in contact with molten oxy‐fluoride flux

Solid oxide membrane (SOM) electrolysis process can produce high‐purity silicon from SiO₂ dissolved in molten oxy‐fluoride flux at elevated temperatures. Yttria‐stabilized zirconia (YSZ), the preferred material for the oxygen‐conducting membrane for this application, is found to degrade over time upon exposure to the silica‐containing molten oxy‐fluoride flux. This YSZ degradation is caused by the acidity of the dissolved silica, especially when the optical basicity of the molten flux is lower than that of the yttria present in the YSZ membrane. To counteract this mismatch, the addition of CaO, a basic oxide, to the flux can adjust the optical basicity of the flux and successfully mitigate the YSZ membrane degradation. The detailed correlation between the rate of YSZ membrane degradation and the optical basicity of the flux is investigated by systematically testing a series of flux compositions. It is found that as the oxide optical basicity in the flux approaches that of the yttria in the YSZ, the degradation of the YSZ membrane is mitigated and essentially vanishes when the flux acidity with respected to the yttria is neutralized. This approach provides a guideline for eliminating membrane degradation during the production of silicon using the SOM electrolysis process.     

Study of a Li–Ni oxide mixture as a novel cathode for molten carbonate fuel cells by electrochemical impedance spectroscopy

Li–Ni oxide mixtures with high lithium content are considered to be an alternative cathode material for molten carbonate fuel cells (MCFCs). The electrochemical behaviour of Li_0.4Ni_0.6O samples has been investigated in a Li–K carbonate melt at 650 °C by electrochemical impedance spectroscopy as a function of immersion time and O₂ and CO₂ partial pressure. The impedance spectra have been interpreted using a transmission line model that includes contact impedance between reactive particles. The Li_0.4Ni_0.6O powder particles show structural changes due to high lithium leakage and low nickel dissolution from the reactive surface to the electrolyte during the first 100 h of immersion. After this time, the structure seems to be stable. The partial pressures of O₂ and CO₂ affect the processes of oxygen reduction and Li–Ni oxide oxidation. X-ray diffraction and chemical analysis performed on samples before and after the electrochemical tests have confirmed that the lithium content decreases. SEM observations reveal a reduction in grain size after the electrochemical tests.     

Yttria-stabilized zirconia as membrane material for electrolytic deoxidation of CaO–CaCl<Subscript>2</Subscript> melts

This article is devoted to the study of the stability of an yttria-stabilized zirconia membrane used in the electrolysis of molten CaCl₂–CaO mixtures at 850 °C. Intensiostatic and potentiostatic electrolysis were carried for periods ranging from 10 to 20 h. Post-mortem composition profiles across the zirconia membrane were determined using Raman spectroscopy and microprobe analysis. The membrane degradation was analyzed in terms of synergetic parameters, i.e., chemical, electrochemical, and thermomechanical effects.     

Electrochemical reduction behavior of U3O8 powder in a LiCl molten salt

The reduction path of the U₃O₈ powder vol-oxidized at 1200 °C has been determined by a series of electrochemical experiments in a 1 wt.% Li₂O/LiCl molten salt. Various reaction intermediates are observed by during electrolysis of U₃O₈. The formation of the metallic uranium is caused from two different reduction paths, a direct reduction of uranium oxide and an electro-lithiothermic reduction. As the uranium oxide is converted to the metallic uranium, the lithium metal is more actively formed in the cathode basket. The reducibility of the rare earth oxides with the U₃O₈ powder has been tested by constant voltage electrolysis. The results suggest the advanced vol-oxidation could lead to the enhancement in the reducibility of the rare earth fission products.     

Performance of a novel Ni/Nb cathode material for molten carbonate fuel cells (MCFC)

In this work the performance of NiO and a novel cathode material preoxidized nickel–niobium alloy were investigated. It is found that under a cathode atmosphere of p(CO₂)/p(O₂) = 0.67 atm/0.33 atm, the equilibrium solubility of nickel ions in (Li_0.62, K_0.38)₂CO₃ melt at 650 °C is about 17 ppm for the nickel oxide electrode and 8 ppm for the preoxidized nickel–niobium alloy electrode. The improvement in the stability of material in the melt may be attributed to the formation of a more dense nodular structure for the nickel–niobium alloy electrode when compared with a Ni electrode during preoxidation. The formation of a dense nodular structure for the nickel–niobium alloy electrode depresses the dissolution of NiO from the electrode into the carbonate melt and, accordingly, enhances the stability of the electrode material in the melt. The polarization performance of the NiO cathode was improved by electrodeposition of niobium. As far as the thermal stability and the polarization performance are concerned, the preoxidized nickel–niobium alloy can be considered as a candidate for the cathode material of MCFCs.