Molten NaCl-KCl Research Articles

Electrodeposition of tantalum and its electrochemical behaviors in molten NaCl-KCl

This paper systematically studied the electrochemical behaviors of tantalum ions in NaCl-KCl molten salt, and pure metallic tantalum was collected on the cathode. Through a series of electrochemical methods such as cyclic voltammetry, square wave voltammetry, and chronopotentiometry, the reduction and diffusion processes of tantalum ions in NaCl-KCl molten salt were determined. The diffusion coefficient and nucleation mode of tantalum ions were investigated, and the electrodeposition was carried out at a certain potential. The results showed that the electrode reduction process of tantalum ion in NaCl-KCl molten salt at 1023 K is a two-step reaction Ta(V)→Ta(II)→Ta, and the reaction Ta(V)→Ta(II) was reversible and controlled by diffusion. The nucleation mode of tantalum ion was determined to be instantaneous nucleation by chronoamperometry. The cathode deposition product was analyzed by SEM, EDS and XRD, and it was confirmed that the deposition product was granular and the main component was tantalum.

Mechanism for electrochemical reduction of Zr(IV) in molten NaCl–KCl

The electrochemical behavior of Zr (IV) at molybdenum and nickel electrodes was investigated by cyclic voltammetry, square-wave voltammetry, chronoamperometry, and chronopotentiometry at 1023 K to investigate the mechanism through which zirconium was electrochemically reduced. The number of electrons transferred in the two-step reduction of Zr(IV) at a molybdenum electrode in molten NaCl–KCl was calculated from the square-wave voltammetry results. Zr(IV) reduction to Zr (0) was found to be a two-step reaction described by the equations Zr(IV)+2e−→Zr(II) and Zr(II)+2e−→Zr (0). The diffusion coefficient for the transfer of Zr(IV) to the molybdenum electrode in molten NaCl–KCl was calculated from the chronoamperometry results. The phase and structure of the product of Zr(IV) reduction at a nickel electrode were determined by X-ray diffractometry and by scanning electron microscopy with energy dispersive X-ray spectroscopy, and a NiZr intermetallic compound was found.

Preparation of MoB2 Nanoparticles by Electrolysis of MoS2/B Mixture in Molten NaCl-KCl at 700 °C

MoB2 is synthesized by the electrochemical reduction of solid MoS2/B mixture in molten NaCl-KCl at 700 °C. Unlike the traditional methods, the electrolysis method employs the low-cost MoS2 feedstock and the boronization reaction happens at a low temperature of 700 °C. The electrochemically induced boronization involves two steps: the electrochemical desulfurization to generate Mo and the reaction of Mo with B to form MoB2. The S2− released from the reduction of MoS2 transfers to the carbon anode and is oxidized to sulfur gas, realizing a green synthetic process. In addition, the influences of molar ratio of MoS2 and amorphous boron and electrolysis cell voltage on the phase composition and morphology of electrolytic products were studied. The obtained MoB2 particles possess a uniform nodular morphology. Overall, this paper provides a straightforward and green process to prepare MoB2 nanoparticles using economically affordable raw materials at low temperature, and this method can be extended to prepare other borides.

Influence of nature of the fluoride ion on the electrochemical behaviors of Nb(V) in molten NaCl-KCl

Influence of nature of the fluoride ion on the electrochemical behaviors of Nb(V) in molten NaCl-KCl

Preparation and molten salts corrosion behavior of high-entropy (Ca,Sr,Ba)TMO3 (TM= Zr, Hf and Ta): Promising protecting materials of reference electrode for metal smelting through fused salts electrolysis

The protecting materials of reference electrode with good corrosion resistance to molten salts play vital roles in realizing smelting of refractory and rare earth metals through fused salts electrolysis. BaZrO3 has been considered as a candidate protecting material of reference electrode for its good chemical stability and high-temperature corrosion resistance. However, the unsatisfactory corrosion resistance to molten salts hinders its practical application. In order to enhance the corrosion resistance to molten salts, three perovskite structured high-entropy oxides HE (Ca,Sr,Ba)TMO3 including (Ca,Sr,Ba)(Zr,Hf)O3, (Ca,Sr,Ba)(Zr,Ta)O3 and (Ca,Sr,Ba)(Zr,Hf,Ta)O3, were designed and prepared in this work. The as-prepared HE (Ca,Sr,Ba)(Zr,Hf)O3 and (Ca,Sr,Ba)(Zr,Hf,Ta)O3 ceramics are single-phase, while the as-prepared HE (Ca,Sr,Ba)(Zr,Ta)O3 ceramic contains 11.6 vol.% of (Ca,Sr,Ba)2Ta2O7. These HE (Ca,Sr,Ba)TMO3 ceramics possess better corrosion resistance to molten NaCl-KCl salts than BaZrO3, among which (Ca,Sr,Ba)(Zr,Hf,Ta)O3 shows the best corrosion resistance to molten NaCl-KCl salts. The mass loss per unite area of HE (Ca,Sr,Ba)(Zr,Hf,Ta)O3 after corrosion by molten NaCl-KCl salts at 800 °C for 8 h is mere 2.6×10−6 g/mm2 which is ca. 1/60 of that of BaZrO3 (∼1.7×10−4 g/mm2), and its surface after corrosion is plain and smooth without any new phase detectable after corrosion. It also exhibits better corrosion resistance to molten NaF-KF salts than BaZrO3. The mass loss per unite area after corrosion by molten NaF-KF salts at 800 °C for 8 h is only 2.9×10−5 g/mm2 which is smaller than that of BaZrO3 (3.6×10−5 g/mm2), the corrosion caused by molten NaF-KF salts only occurs on the surface and the thickness of the corrosion layer is mere ∼8 µm, being less than 1/10 of that of BaZrO3 (∼100 µm). These results indicate that the three HE (Ca,Sr,Ba)TMO3, especially (Ca,Sr,Ba)(Zr,Hf,Ta)O3, are promising as protecting materials of reference electrode for the smelting of refractory and rare earth metals through fused salts electrolysis.

Ionic Conductivities of Molten Alkali Metal Chloride Binary Systems by Equilibrium Molecular Dynamics Simulation: Composition and Temperature Dependence

Conductivity of molten alkali metal chlorides is the foundation of molten-salt electrolysis process. In the study, equilibrium molecular dynamics was performed to investigate the ionic conductivities of the molten alkali metal chloride binary systems. The calculated values were in good agreement with the experimental results. Simple databases for conductivity have been built based on the verified algorithm. Mathematic expressions of conductivity vs. temperature & composition have also been obtained according to calculated data. Ionic conductivities of these melts exhibited positive dependences on temperature. Adding a smaller alkali metal cation into molten alkali chlorides could improve the conductive performance. With the gradual addition of LiCl, ionic conductivities of molten LiCl-NaCl, LiCl-KCl, LiCl-RbCl, and LiCl-CsCl all increased monotonously. The conductivities of molten NaCl-KCl, NaCl-RbCl, and NaCl-CsCl also increased with the continuous addition of NaCl. The improvement effects of LiCl on ionic conductivities of molten KCl, RbCl, and CsCl are more significant than those of NaCl.

Extraction of Cu and Ni from Low-Ni Matte by a Combined Chemical Precipitation and Molten Salt Electrolysis Approach

The extraction of Cu and Ni from low-Ni matte in an efficient and green manner is essential to harvest value-added metals with minimum environmental footprint. Herein, a combined approach of chemical precipitation and molten salt electrolysis was used to extract valuable metals (Cu and Ni) from the low-Ni matte. First, the low-Ni matte was converted and separated into CuS and NiS by ammonium chloride roasting, and then, it was water leached and precipitated. Fe was removed from the leachate solution by adjusting the pH value, and the separation of Cu and Ni was carried out by adjusting the temperature, S-to-Ni molar ratio, and precipitation time. The obtained CuS and NiS were then separately converted to sulfur gas at the anode and Cu and Ni at the cathode in molten NaCl-KCl. The reduction behaviors of CuS and NiS were studied using cyclic voltammetry (CV). This study proposes an eco-friendly way to extract valuable metals from low-Ni matte.

Molten salt in-situ exfoliation of graphite to graphene nanoplatelets applied for energy storage

The in-situ exfoliation of graphite in molten NaCl–KCl salt at 750 °C yielded a homogeneous composite, from which graphene nanoplatelets (GnP) could be easily and completely separated. The GnP product displayed relatively large-size particles (12 μm, TEM, SEM, laser diffraction) and low defect density (Raman, XPS), indicating the superiority of the molten salt exfoliation approach over the conventional liquid-phase approach for producing GnP. The mechanism of GnP production involves the formation of cavities in the molten salt, such that impregnation of the graphite by the molten material facilitated gentle exfoliation of the graphite to GnP. The methodology for the in-situ production of GnP in molten salt is highly applicable in the field of thermal energy storage, where molten inorganic salts are one of the most widely used heat-transfer fluids, such as phase-change materials. In graphite-GnP-salt composite, the thermal conductivity is enhanced by more than one order of magnitude compared to the neat salt matrix, which is essential for efficient thermal energy storage applications.

Corrosion performance of slurry aluminide coatings in molten NaCl–KCl

The corrosion performance of water-based slurry aluminide coatings elaborated on iron- and nickel-based materials is investigated in molten chlorides as candidate heat transfer fluids (HTF) for thermal energy storage (TES) in third generation concentrated solar power (CSP) plants. This work presents a screening of four different materials (ferritic-martensitic P91 steel, austenitic stainless steel 316L, Inconel 600 and high-purity nickel) to investigate the influence of the alloying elements (e.g. Fe, Cr, Ni contents) on the microstructure and corrosion performance of slurry aluminide coatings. Individual metallic samples were diffusion-coated with slurries containing Al microparticles and subsequently exposed to molten NaCl–KCl at 700 °C for 100 h under argon. The experimental observations indicate that the performance of the aluminide coatings is governed by the precipitation of secondary phases within the B2 aluminide matrix rather than its intrinsic Al concentration (where B2 refers to either FeAl, (Fe,Ni)Al or NiAl intermetallic compounds). Of those, Fe-rich aluminide coatings were found to be more resistant to molten chlorides than Ni-rich ones. This is attributed to the greater solubility of Cr in iron aluminides than in nickel aluminides, preventing the precipitation of Cr-rich intermetallic compounds and/or Cr-rich carbides within the B2 matrix. Cr-rich phases were selectively dissolved upon exposure leaving the coating matrix with void channels. According to residual coating thickness measurements, the following ranking with increasing corrosion resistance can be given for Al-slurry coated materials: Inconel 600 < high-purity nickel < austenitic stainless steel 316L < ferritic-martensitic P91 steel.

Mechanism for Electrochemical Reduction of Zr (IV) in Molten NaCl–KCl

The electrochemical behavior of Zr(IV) at molybdenum and nickel electrodes was investigated by cyclic voltammetry, square-wave voltammetry, chronoamperometry, and chronopotentiometry at 1023 K to investigate the mechanism through which zirconium was electrochemically reduced. The number of electrons transferred in the two-step reduction of Zr(IV) at a molybdenum electrode in molten NaCl–KCl was calculated from the square-wave voltammetry results. Zr(IV) reduction to Zr(0) was found to be a two-step reaction described by the equations Zr(IV)+2e−→Zr(II) and Zr(II)+2e−→Zr(0). The diffusion coefficient for the transfer of Zr(IV) to the molybdenum electrode in molten NaCl–KCl was calculated from the chronoamperometry results. The phase and structure of the product of Zr(IV) reduction at a nickel electrode were determined by X-ray diffractometry and by scanning electron microscopy with energy dispersive X-ray spectroscopy, and a NiZr intermetallic compound was found.

Mechanism for Electrochemical Reduction of Zr(IV) in Molten NaCl–KCl

Mechanism for Electrochemical Reduction of Zr(IV) in Molten NaCl–KCl

Molten Electrolyte-Modulated Electrosynthesis of Multi-Anion Mo-Based Lamellar Nanohybrids Derived from Natural Minerals for Boosting Hydrogen Evolution.

The multi-anion molybdenum-based nanohybrids, N-doped β-Mo2C/MoP/MoOx (denoted as MoCPO), serving as a highly efficient catalyst for hydrogen evolution reaction (HER), are fabricated via a simple and scalable electrosynthesis in molten NaCl-KCl, which integrates pyrolysis/electroreduction/compounding into a one-pot strategy using polyphosphazenes (PPAs) and earth-abundant molybdenite (mainly MoS2) as precursors. The deliberately selected PPA and molten electrolyte ensure the unique lamellar nanostructures and the blending of multiple anions of C, N, P, and O in the obtained catalyst, specifically, triggering the in situ formation of the structural oxygen vacancies (VO) in MoCPO. The nature of the hybrids can be regulated by adjusting the synthesis condition. The optimized hybrid displays a low overpotential of 99.2 mV at 10 mA cm-2 for HER in 0.5 M H2SO4 and stays active over a broad pH range. The theoretical calculations reveal that VO in the hybrids serves as favorable active sites, thus contributing to the superior HER activity. Moreover, MoCPO is also effective for overall water splitting as a bifunctional catalyst.

Corrosion behaviors of 316 stainless steel and Inconel 625 alloy in chloride molten salts for solar energy storage

AbstractCorrosion behaviors of 316 stainless steel (316 ss) and Inconel 625 alloy in molten NaCl–KCl–ZnCl2 at 700°C and 900°C were investigated by immersion tests and electrochemical methods, including potentiodynamic polarization and electrochemical impedance spectroscopy. X-ray diffraction and scanning electron microscopy/energy dispersive spectroscopy were used to analyze the phases and microstructures of the corrosion products. Inconel 625 alloy and 316 ss exhibited high corrosion rates in molten chlorides, and the corrosion rates of these two alloys accelerated when the temperature increased from 700°C to 900°C. The results of the electrochemical tests showed that both alloys exhibited active corrosion in chloride molten salt, and the current density of 316 ss in chloride molten salt at 700°C was 2.756 mA/cm−2, which is about three times the value for Inconel 625 alloy; and the values of the charge transfer resistance (Rt) for Inconel 625 were larger than those for 316 ss. The corrosion of these two alloys is owing to the preferred oxidation of Cr in chloride molten salt, and the corrosion layer was mainly ZnCr2O4 which was loose and porous and showed poor adherence to metal.

Corrosion mechanisms of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy in molten chlorides. Part I: NaCl–KCl binary system

With suitable thermophysical properties and commercial viability, molten chlorides are promising candidates to replace nitrate salts as heat transfer fluids (HTF) and thermal energy storage (TES) materials for next-generation concentrated solar power (CSP) plants. Nevertheless, structural materials including steels and nickel-based alloys experience severe corrosion in molten chlorides. The explicit understanding of the corrosion mechanisms is therefore decisive in order to propose viable technical solutions to increase the durability of these materials. In this work, the corrosion behavior of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy is investigated in molten NaCl–KCl at 700 °C under Ar. The salt mixtures after exposure as well as the corroded samples were simultaneously characterized. The systematic metallographic study after the water-free preparation indicates that electrochemical processes play a major role in the overall corrosion mechanisms. In both P91 steel and Inconel 600, the formation of micro-galvanic pairs between Cr-rich phases (anodic sites) and the alloy matrix (cathode) leads to the selective dissolution of Cr from the alloys resulting in the formation of subsurface voids. The influence of O2 and of various alloying elements on the corrosion kinetics is discussed. Based on the experimental observations, several corrosion mechanisms combining electrochemical reactions, solid-state diffusion and chloridation-oxidation processes have to be taken into account.

Electrochemical behavior of niobium ions in molten KCl-NaCl

The reduction mechanism of Nb(V) in molten KCl-NaCl at 1023K was investigated by a series of electrochemical techniques such as cyclic voltammetry, square wave voltammetry and chronopotentiometry. It was found that there were three steps in the reduction of Nb(V) as Nb(V)→Nb(III)→Nb(I)→Nb, and electrochemical reaction, Nb(V)→Nb(III), is a quasi-reversible diffusion controlled process. The diffusion coefficient of Nb(V) was also obtained by cyclic voltammetry, square wave voltammetry and chronoamperometry. The relationship between current density and the square root of time shows that the nucleation of niobium belongs to the instantaneous nucleation on molybdenum electrode in molten KCl-NaCl-NbCl5 (0.43×10−3mol/cm3) at 1023K at −3.10V vs. Cl2/Cl−. Moreover, the metallic niobium was deposited on molybdenum wire by chronoamperometry, and the products were analyzed by scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX). The results showed that the metallic niobium has a high purity of 99.98%.

Bionic Structural Design and Electrochemical Manufacture of WC/N-Doped Carbon Hybrids as Efficient ORR Catalyst

A biomass-derived WC/N-doped carbon nanocomposite (denoted as WC/CC) with unique bionic structures, as an efficient electrocatalyst toward oxygen reduction reaction (ORR), is simply prepared via an eco-friendly and scalable method in molten NaCl-KCl using bulk-WS2 as a precursor. Corn crumb (CC) from by-products of food industry provides a fibriform N-containing biochar backbone by in situ pyrolysis, while W from the in situ electroreduction of WS2 would infiltrate into the carbon matrix under the effect of an applied electric field, contributing to the crystallization of WC along the fibers so that gnarled root-like composites are fabricated. The electrocatalytic behavior of the prepared materials are investigated in detail, the optimized sample (WC/CC-1/2) exhibits a decent onset potential of 0.865 V vs RHE for ORR in 1 M KOH solution and a four-electron pathway is dominant. This work provides a simple and feasible strategy for synthesizing high-performance ORR catalysts using biochar templates.

Interphase Distribution of Elements during Two-Stage High Temperature Electrochemical Treatment of Lead-Bismuth Alloys

Experimental simulation of the separation of the components of lead-bismuth alloy in a chloride melt is conducted. Using X-ray fluorescence, emission spectral and X-ray powder diffraction analyses, the interphase distribution of elements is estimated during high-temperature (550 С) electrochemical treatment of lead-bismuth alloys in molten NaCl–KCl–PbCl2 –ZnCl2 . Regimes of the two-stage process are substantiated when in the first stage (U = 7.4–15.6 V, ia = 0.5 A/cm2 ) the bismuth content in anode alloy is brought to 48.4 wt.% and then (U = 4.9-13.5 V, ia = 0.3-0.4 A/cm2 ) crude bismuth is separated containing, wt.%: 93.6 Bi, 4.1 Pb, 0.086 Ag, 0.0066 As, 0.006 Sb, 0.0013 Cu, 0.001 Sn, and 0.0014 Zn. The final anode product contains 93% bismuth and 0.4% lead from the original alloy. Crude lead obtained at the cathode contains, wt.%: 95.8–96.3 Pb, 0.007–0.06 Bi, 0.002–2.9 Na, 1.2–3.6 Zn, and ≤ 0.1 Cu. Sublimation of chlorides saturated with lead and action of oxygen from the air are the reasons for heterogenization of the electrolyte that gives rise to an increase in working voltage in the electrolytic cell. The relatively low anodic and cathodic current efficiency (50 and 38% respectively) are associated with electrical recharging of polyvalent ions on electrodes. Use of technology for treating intermediate products and waste refining will make it possible to organize production of crude bismuth from mineral and secondary lead-containing raw materials. Prospects for improving the efficiency of the process are associated with the sealing of electrolyzers and preliminary pyrometallurgical refining of lead-bismuth alloys from impurities (Cu, Zn, Sb, As, and Sn).

Electric Field-Driven Interfacial Alloying for in Situ Fabrication of Nano-Mo2C on Carbon Fabric as Cathode toward Efficient Hydrogen Generation

A binderless composite cathode for efficient electrocatalytic hydrogen evolution reaction (HER), Mo2C-decorated carbon cloth (denoted as CC/MC), is simply fabricated via a novel and unique strategy which involves a solid-solid phase interfacial electrochemical reaction between carbon fiber and bulk-MoS2 in molten NaCl-KCl (700 °C). MoS2, evenly coated on carbon cloth (CC), is electrochemically reduced in situ and readily reacts with the carbon fibers of CC current collector to form a Mo2C nanoparticle layer. The experiment and calculation results show that the applied electric field results in a declining migration barrier of Mo vacancies in Mo2C lattice, which promotes the diffusion of Mo atoms into carbon across the interfacial Mo2C layer, thereby impelling the combination of Mo with C in depth. The electrochemical tests indicate that the optimized cathode (CC/MC-2) exhibits a small overpotential of 134.4 mV at 10 mA cm-2 and stays stable for HER in acidic media. The catalytic capacity for N2 reduction of CC/MC-2 is analyzed. In addition, a Ni-doped Mo2C-modified carbon fabric electrode with enhanced HER activity (η10 = 96.6 mV) can be prepared through a similar process.

Facile synthesis of porous Mo2C/C composites by using luffa sponge-derived carbon template in molten salt media.

Herein, we report the synthesis of a new type of porous Mo2C/C composite by using luffa sponge-derived carbon template and ammonium molybdate ((NH4)6Mo7O24•4H2O) in molten NaCl–KCl salt media. The product exhibits a higher specific surface area and three-dimensional porous structure, including macrochannels, micropores and mesopores. The desirable porous structure results from the carbon template structure and Mo2C coating formed.

Production of Fine Titanium Powder from Titanium Sponge by the Shuttle of the Disproportionation Reaction in Molten NaCl–KCl

Production of Fine Titanium Powder from Titanium Sponge by the Shuttle of the Disproportionation Reaction in Molten NaCl–KCl