Three-phase Interline Research Articles

Modelling diffusion controlled electro-deoxidation of metal oxide to metal in molten salt

Diffusion is a fundamental irreversible process intervening in the evolution of many out-of-equilibrium systems and is successfully described by Fick’s law obtained from non-equilibrium thermodynamics. Despite this, numerical simulations of solid state electro-deoxidation in the diffusion-controlled regime in molten salt remain elusive. Here, a new model for diffusion controlled three-phase interline (3PI) penetration in a porous cathode during electro-deoxidation is validated against experimental observation. This penetrating 3PI model is applied at high overpotential and benchmarked using the oxygen ionisation TiO2(s) + 4e− → Ti(s) + 2O2− at the 3PI. The model couples slow diffusive transport and fast oxygen ionisation while assuming a negligible ohmic potential drop in bulk molten CaCl2 electrolyte. The 14 nm s−1 penetration rate of the 3PI and the order of magnitude of 3PI currents (derived from an exchange current density and cathodic transfer coefficient of 0.32 A cm−2 and 0.01, respectively) in the chronoamperometric data for porous cathodes are in good agreement with experimental observation.

Electrolytic reduction of Gd2O3-ZnO in NaCl-CaCl2 Molten Salt

The electrochemical behaviors of Gd2O3, ZnO and Gd2O3-ZnO in NaCl-CaCl2 molten salt was studied by cyclic voltammetry and square wave voltammetry, and the corresponding constant potential electrolysis and XRD analysis indicated that the reduction processes were in agreement with the process that 3PI (metal∣oxide∣electrolyte three-phase interline) expanded with time from surface to interior of the cathode. Moreover, the addition of ZnO could promote the electrochemical reduction of Gd2O3. According to the electrochemical behavior (CV) and the characterization (XRD and SEM-EDS) of electrolytic product, the intermetallic compounds, GdZn12, Gd2Zn17 and Gd13Zn58, were generated during the electrolysis of Gd2O3-ZnO cathode without dendritic deposits.

Research on synthesis and nucleation mechanism of silicon nanowire by silver catalysis in molten salt

Molten salt electrolytic synthesis of silicon is identified as a potential industry method due to its thorough deoxidization of oxide and short-flow synthetic progress. However, figuring out a more efficient and sustainable strategy for the increase of the reduction process as well as the control of product morphology still needs endless research. In this study, benefiting from the high electrical conductivity of metallic silver and the easy separation of silver from silver-silicon alloys during the electrolysis of SiO2, metallic silver was introduced to electric-catalyze the production of silicon. Firstly, metallic silver was introduced into the silica matrix by a simple silver mirror reaction. Secondly, the mixed Ag\\SiO2 precursor was located at the cathode for molten salt electrolysis. Finally, silicon nanowires were herein achieved in molten NaCl–CaCl2. Mechanisms of metal Ag played during the electric-polarization process of silica and the electrochemical nucleation of silicon were further analyzed by X-ray diffraction (XRD), Scanning Electron Microscope (SEM), Field-Emission Scanning Electron Microscope (FESEM), Transmission Electron Microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Raman spectra. The results demonstrate that Ag helps construct Si nanowires via a classic Ag-Si binary alloy-droplet process and provided quantitative guidance for the large-yield production of silicon nanowires via adding 0.08–0.17 g Ag versus 1 g SiO2, controlling the temperature at 850–900 °C and electrolysis voltage at 2.6–2.8 V (2.6V at first 1 h and 2.8V at last 5 h). The synthesized silicon nanowires present two types of linear structures based on two three-phase interlines (3PIs) models and a streamed long-straight nanowire is observed.

Liquid zinc assisted electro-extraction of molybdenum

• Co-deposition of Mo and Zn were investigated which lead Mo ions to deposit under a less cathodic potential. • Electrolytic cell was designed utilizing liquid zinc as cathode arranged according to the density. • Pure molybdenum was obtained by removing zinc in Zn-Mo alloy, where Zn-Mo alloy was produced through liquid zinc assisted electro-extraction. Improvements of the direct electro-desulfurization of molybdenum disulfide in NaCl-KCl eutectic molten salt by employing a liquid zinc electrode was studied in this work. According to the previous studies, one of the rate controlling step of the MoS 2 desulfurization process is the sulfur ions’ diffusion through solid MoS 2 pellet, and a decreasing reaction rate is considered to result from the poor conductivity between the loosely connected partial reduced molybdenum particles. Based on these facts, liquid zinc electrode, along with a graphite crucible as the container, was employed to collect the reduced molybdenum particles as long as keep providing a fresh three-phase interline (3PI) as the current collector. Transient electrochemical technologies were introduced to study the electrochemical behavior of molybdenum ions with and without the appearance of zinc ions. Zn-Mo alloy was obtained through the potentiostatic electrolysis process, and molybdenum powders mainly sank into the bottom of liquid zinc due to the gravity. Metallic molybdenum shall be obtained by distilling the electrolysis product.

Quasi-solid-state electrolyte for rechargeable high-temperature molten salt iron-air battery

Molten salts are a unique type of electrolyte enabling high-temperature electrochemical energy storage (EES) with unmatched reversible electrode kinetics and high ion-conductivities, and hence impressive storage capacity and power capability. However, their high tendency to evaporate and flow at high temperatures challenges the design and fabrication of the respective EES devices in terms of manufacturing cost and cycling durability. On the other hand, most of these EES devices require lithium-containing molten salts as the electrolyte to enhance performances, which not only increases the cost but also demands a share of the already limited lithium resources. Here we report a novel quasi-solid-state (QSS) electrolyte, consisting of the molten eutectic mixture of Na2CO3-K2CO3 and nanoparticles of yttrium stabilized zirconia (YSZ) in a mass ratio of 1:1. The QSS electrolyte has relatively lower volatility in comparison with the pristine molten Na2CO3-K2CO3 eutectic, and therefore significantly suppresses the evaporation of molten salts, thanks to a strong interaction at the interface between molten salt and YSZ nanoparticles at high temperatures. The QSS electrolyte was used to construct an iron-air battery that performed excellently in charge-discharge cycling with high columbic and energy efficiencies. We also propose and confirm a redox mechanism at the three-phase interlines in the negative electrode. These findings can help establish a simpler and more efficient approach to designing low-cost and high-performance molten salt metal-air batteries with high stability and safety.

Electro-reduction processes of U3O8 to metallic U bulk in LiCl molten salt

U3O8, the most stable state of uranium oxides, is a key intermediate compound for pyroprocessing of used nuclear fuel. However, compared to UO2, the electro-deoxidation mechanism of U3O8 is still cursory due to the complicated U-O intermediate phases. In this paper, the electro-deoxidation processes of U3O8 were investigated via diverse methods. A molybdenum metallic cavity electrode loaded with U3O8 powder was adopted to investigate the electrochemical behavior via cyclic voltammetry and square wave voltammetry. Sintered U3O8 pellets were used as cathodes to obtain electrolytic products with varying applied voltages or duration time. The composition and microstructure of electrolytic products were characterized by Raman spectroscopy, X-ray diffraction, scanning electron microscope and energy dispersive spectrometer. Based on above analysis results, electro-reduction processes of porous U3O8 pellets were proposed. First, U3O8 undergoes a rapid transition to U4O9 (U4O9-y). Then, the intermediate compound was gradually reduced to UO2 from the outer layer to the center of the bulk due to the limited migration rate of O2−. Before the transformation completed, metallic U generated on the surface of the bulk through direct electro-deoxidation and Li metal thermal reduction. With the increase of porosity and the propagation of metal-oxide-electrolyte three-phase interline, the U3O8 pellet was finally reduced to metallic U bulk.

Tuning the preferentially electrochemical growth of carbon at the “gaseous CO2-liquid molten salt-solid electrode” three-phase interline

It has been widely accepted that the electrochemical reduction of CO2 in molten carbonate is implemented by the reduction of CO32− at the cathode that releases O2− to capture CO2 and replenish CO32−. This paper aims to reveal the direct electrochemical reduction of gaseous CO2 in molten salts and the growth behaviors of carbon at the CO2/molten salt/electrode interline. The electrochemical reduction of gaseous CO2 takes place at the three-phase interline of gaseous CO2/liquid molten salt/solid electrode (GLS-3PI) when the temperature is over 700 °C. The reduction of CO2 at the GLS-3PI results in the preferential growth of carbon, posing a potential risk to short-circuiting the electrolysis cell. An insulative sheath can prevent the CO2 contacting the GLS-3PI, thereby avoiding the preferential growth of carbon and getting rid of the risk of the short circuit. This paper offers new insights into understanding the direct electrochemical reduction of CO2 at the GLS-3PI and provides an isolation strategy to engineer the molten salt electrolysis cells.

Transformation of the Three-Phase Interlines During the Electrochemical Deoxidation of TiO2

Transformation of the Three-Phase Interlines During the Electrochemical Deoxidation of TiO2

Electro-deoxidation modelling of titanium dioxide to titanium

The ability to control the degree of supersaturation using electric potential in electrochemical nucleation and growth is generally limited to studies in metal electrodeposition and electrochemical oxide formation. Here we report in-situ observations of 2D crystal nucleation and thin layer growth during the solid state electro-deoxidation of TiO2 in CaCl2 melt. The nucleation mechanism is found to be progressive-like according to the non-dimensional Scharifker-Hill model where relatively slow Ti nucleation occurs on a large number of active sites. After the formation of a thin layer, changing the design of the porous cathode or inert anode has a profound effect on the rate and distribution of electro-deoxidation on the Ti/TiO2/melt three-phase interline.

Direct Electrolytic Reduction of Solid Ta2O5 to Ta with SOM Process

A process that uses the solid-oxide–oxygen-ion conducting membrane has been investigated to produce tantalum directly from solid Ta2O5 in molten CaCl2 or a molten mixture of 55.5MgF2-44.5CaF2 (in wt pct). The sintered porous Ta2O5 pellet was employed as the cathode, while the liquid copper alloy, saturated with graphite powder and encased in a one-end-closed yttria-stabilized-zirconia (YSZ) tube, acted as the anode. The electrolysis potential in this method is higher than that of the Fray–Farthing–Chen Cambridge process because the YSZ membrane tube blocks the melts to electrolyze, and only Ta2O5 is will be electrolyzed. The microstructures of reduced pellets and a cyclic voltammogram of solid Ta2O5 in molten CaCl2 were analyzed. In addition, the influence of particle size and porosity of the cathode pellets on metal-oxide-electrolyte, three-phase interlines was also discussed. The results demonstrate that the sintering temperature of cathode pellets and electrolytic temperature play important roles in the electrochemical process. Furthermore, this process can be used to produce Ta metal efficiently without the expensive cost of pre-electrolysis and generation of harmful by-products.

Formation of Si nanowires by the electrochemical reduction of SiO2 with Ni or NiO additives.

Various morphologies of silicon nanowires (SiNWs) were successfully prepared by the electrochemical reduction of silica mixed with different additives (Au, Ag, Fe, Co, Ni, and NiO, respectively). Straight SiNWs were extensively obtained by the electro-reduction of porous Ni/SiO2 blocks in molten CaCl2 at 900 °C. The SiNWs had a wide diameter distribution of 80 to 350 nm, and the Ni-Si droplets were found on the tips of the nanowires. The growth mechanism of SiNWs was investigated, which could reveal that the nano-sized Ni-Si droplets formed at the Ni/SiO2/CaCl2 three-phase interlines. Based on the mechanism proposed, NiO particles with sub-micrometer size were selected as the additive, and straight SiNWs with diameters of 60 to 150 nm were also prepared via the electrochemical process.

Formation of Si nanowires by the electrochemical reduction of porous Ni/SiO2 blocks in molten CaCl2

Silicon nanowires (SiNWs) were prepared by the electrochemical reduction of solid Ni/SiO2 blocks in molten CaCl2 at 1173K. The SiNWs have diameter distributions ranging from 80 to 350nm, and the nickel–silicon droplets are found on the tips of the nanowires. The growth mechanism of SiNWs was investigated, which confirmed that the nano-sized nickel–silicon droplets formed at the Ni/SiO2/CaCl2 three-phase interline. The droplets lead to the oriented growth of SiNWs. Formation of nano-sized nickel–silicon droplets suggests that this method could be a potential way to produce nano-sized metal silicides.

Titanium production in rotationally symmetric electrochemical reactors

Solid state electroreduction of porous TiO2 to Ti immersed in CaCl2 melt occurs in electrochemical reactors using predominantly graphite rod anodes. Here we describe the solid state electroreduction of TiO2 in electrochemical reactors using graphite annulus anodes by both experiment and numerical simulation. This new experimental system shows electronic background currents that are ∼3 times higher than that previously reported, experimental peak currents in excellent agreement with the current predicted in our thin layer three-phase interline (3PI) model, and large pores in Ti cathodes due to a Kirkendall-like effect. The latter increases the surface area of Ti cathodes indicating potential applications for enhancing bone cell ingrowth in medical implants as well as lowering the bone-metal stiffness mismatch.

EXPERIMENTS ON HEAT TRANSFER CHARACTERISTICS OF SHEAR-DRIVEN LIQUID FILM IN CO-CURRENT GAS FLOW

Experiments are performed to study the liquid film behavior and corresponding local heat transfer to shear-driven liquid film flow of water in the cocurrent nitrogen gas flow. The heated channel has a cross section of 30mm in width and 5mm in height, where the bottom is operated as a heating surface of 30mm in width and 100mm in length. The heated section is divided into segments to evaluate the local heat transfer coefficients. Under most gas Reynolds numbers, the local heat transfer coefficients are increased with increasing heat flux, where three mechanisms are important; (i) increase of areas along the three-phase interline around dried areas, (ii) rewetting of dried areas by the transverse liquid flow pushed by the generation of bubbles at the side edges of duct, (iii) microlayer evaporation during nucleate boiling in the film flow. The existence of duct corners makes the phenomena more unsteady and non-uniform in the transverse direction.

The FFC Cambridge process and its relevance to valorisation of ilmenite and titanium-rich slag

The Fray–Farthing–Chen (FFC) Cambridge process was patented in 1998 for low cost and clean electrochemical extraction of metals and synthesis of alloys directly from the mineral precursors, particularly oxides (including slags) and sulfides, with the aid of molten salts. In this paper, the technical success of this unique method is explained from the basic electrode reaction thermodynamics and mechanisms at the compound/metal/electrolyte three-phase interlines (boundaries). Selected recent innovations are then introduced towards more efficient cathode design and practices. Finally, the prospects of the FFC Cambridge process are discussed for processing the ilmenite ore and the so-called titanium-rich slag to produce Fe–Ti alloys and its commercial potential in the future titanium industry.

瞬时离子释放扩散模型及熔盐电解固态WS<sub>2</sub>制备纳米W的液相扩散研究

Over the past few years, preparation of metals by molten salt electrolysis of solid metal oxide (or sulfide) cathodes has attracted numerous interests from both academic and industrial communities. According to the three-phase interlines theories, diffusion of O2- (or S2-) ions should play an important role in the solid cathode process, however, it has seldom been studied due to the absence of both convenient theoretical and experimental methods. In this work, we introduced an instantaneous ion release diffusion model (IIRD Model) for porous electrodes as theoretical base, and fabricated mini porous electrodes by filling WS2/W mixture into the cylindrical cavities in a Mo foil substrate and performed double potential operations to study the liquid diffusion of S2- ions in the solid sulfide cathode in the equi-molar NaCl-KCl melts. The experimental results were in good accord with the theoretical equation. By curve fitting, the diffusion coefficient of S2- ions in the electrolysis generated porous W layer was evaluated as 9.2×10-6 cm2/s at 973 K, with the diffusion activation energy of about 53.4 kJ/mol. The study indicates that electrochemical reduction of WS2 to nanometer W can be achieved very quickly, and the electrolysis of solid WS2 cathode should be rate determined by the liquid diffusion of S2- ions in the generated porous W layer.

Reaction Mechanism on Preperation of Ta with SOM Process

Tantalum metal was prepared by Solid Oxygen-ion Conducting Membrane (SOM) technology from Ta2O5 directly in molten CaCl2. Through reaction mechanism of the metal-oxide-electrolyte three-phase interline (3PIs), the influence of pellets microstructure on the electrolysis process and products morphology were discussed. The results demonstrate that forming pressure, particle size and porosity of cathode pellets are significant factors for three-phase interline during electrolysis process. It was found that the big porosity and small particle can enhance the deoxidization rate, Contrarily, the compact out-layer of tantalum metal was formed. While the Ta2O5 pellet was pressed by 4MPa and sintered at 1150°C for 2 h has rational porosity and particle, the electrochemical reduction can be achieved via two steps: Ta2O5Ta2O5-xTa.

Electropolymerization of polypyrrole at the three-phase interline: Influence of polymerization conditions

Free-standing polypyrrole film was electrosynthesized at the interface between an HClO4 aqueous solution and a pyrrole chloroform solution through a dynamic three-phase interline electropolymerization (D3PIE) process. The cyclic voltammetary measurements showed that the onset potential for the oxidation of pyrrole monomers was ∼0.2V vs. Hg/Hg2SO4, and the radial expansion of conductive polymer film gradually increased the reaction currents. Under potentiostatic conditions, higher potentials, higher counterion (perchlorate) concentrations and higher monomer concentrations all caused a faster growth of polymer, and the aqueous perchlorate ions appeared the most sensitive factor for the reaction currents. In the initial stage of electropolymerization, the reaction currents and the diameters of polymer films increased linearly with the operating times, and a simple model can be used to describe the growth behavior of the circular polymer film. However, several factors can cause the deviation from an ideal linear relation in the later stage of electropolymerization. Higher aqueous perchlorate concentration increased the penetration amount of perchlorate ions to organic solvent phase, resulting in the longitudinal growth of PPy polymers. In addition, the prepared PPy films exhibited significantly different microstructures between the side toward water and the side toward organic solvent, and the aqueous perchlorate concentration and monomer concentration both affect the morphologies of films greatly. In order to observe a well-defined expansion of three-phase interline, lower concentrations of aqueous perchlorate ions and monomers are preferable.

Fabrication of free-standing conductive polymer films through dynamic three-phase interline electropolymerization

Poly(3,4-ethylenedioxythiophene) (PEDOT) and polypyrrole (PPy) free-standing conductive polymer films are prepared through a dynamic solid/liquid (oil)/liquid (water) three phase interline electropolymerization (D3PIE) process. The oil phase (dichloromethane) contains only hydrophobic monomer (3,4-ethylenedioxythiophene or pyrrole), while the aqueous phase contains only dopant electrolytes. A platinum wire is immersed vertically across the O/W interface and is acted as working electrode. The electropolymerization starts from the three phase interline junction of water, dichloromethane and Pt wire and expands along the O/W interface so that a free-standing film forms. Furthermore, both the PEDOT film and PPy film exhibit heterogeneous microstructure because of this unique electropolymerization process. The novel D3PIE method proves to be applicable to the preparation of a variety of free-standing conductive polymer films with controllable size and microstructure.

Electrochemical extraction of Fe–Ti–Si alloys direct from Ti bearing compound ores

This study demonstrated the successful direct electrochemical reduction of Ti bearing compound ores to Fe–Ti–Si alloys by using the solid oxide membrane (SOM) process. This electrochemical process was carried out in molten CaCl2 at 1100°C and 3·5–4·0 V, the pressed cylindrical pellet of Ti bearing compound ores sintered in air at 1150°C for 2 h acted as a cathode, and the carbon saturated liquid metal contained in an yttria stabilised zirconia tube served as an anode. The electrolytic characteristics were investigated and the products obtained from different conditions were characterised by means of scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. It is found that Ti bearing compound ores can be directly electrochemically reduced to Fe–Ti–Si alloys, the titanomagnetite mixed in the compound ores is first deoxidised to the metal Fe, and the pure Fe extends into the centre of the pellet through the metal/oxide/electrolyte three-phase interlines, which may promote the subsequent electrochemical reactions effectively, and lead to the electrodeoxidation process proceed fast. Importantly, it is observed that some metallic impurities in the Ti bearing compound ores, such as the magnesium, calcium and aluminium, are partly or completely removed during the electrolysis.