Molten Salt Medium Research Articles

Compatibility of SiC--and MAX phase-based ceramics with a KNO3-NaNO3 molten solar salt

In this work, several ceramic materials were exposed together with two reference structural materials (i.e., 316 L stainless steel and Inconel 600) to a molten solar salt (40 wt% KNO3 and 60 wt% NaNO3) for 1000 h at 600 °C to investigate their compatibility with the molten salt medium, thus assessing their potential use in concentrated solar power (CSP) applications. The exposed ceramics included different SiC grades (solid state-sintered, liquid phase-sintered, and silicon-infiltrated) and MAX phase-based materials (Maxthal® 211 & 312 (nominally, Ti2AlC & Ti3SiC2, respectively), Cr2AlC, Nb4AlC3, (Nb,Zr)4AlC3, and a cermet comprising 40 vol% Fe and 60 vol% (Nb,Zr)4AlC3). All SiC grades were chemically stable in the molten salt, whereas all Nb-containing MAX phase ceramics were severely oxidized. Comparing the two Maxthal® grades showed that the 312 was chemically more stable than the 211, and both grades formed a Na-based oxide scale. Interestingly, Cr2AlC showed practically no interaction with the molten salt during the performed exposure, forming a stable, sub-micrometre-thick Cr7C3 scale. Hence, it may be considered as promising structural/coating material candidate for the targeted CSP application.

Selective morphological analysis of cerium metal in electrodeposit recovered from molten LiCl-KCl eutectic by radiography and computed tomography

This paper presents, for the first time, a study to analyze the surface morphology of metal extracted from a high temperature molten salt medium in the electrodeposit using x-ray radiography and computed tomography. Widely used methods such as scanning electron microscopy and inductively coupled plasma-optical emission spectrometry/mass spectrometry are destructive and the related processes are often subject to the air condition. The x-ray imaging can provide rich information of the target sample in a non-destructive way without invoking hydrolysis or oxidation of a hygroscopic sample. In this study, the x-ray imaging conditions were optimized as following: tube voltage at 100 kVp and the current exposure time product at 8.8 mAs in our in-house x-ray imaging system. LiCl-KCl and cerium metals used in this work produced substantially distinguishable contrasts in the radiography due to their distinctive attenuation characteristics, and this difference was well quantified in the histograms of brightness. Electrodeposits obtained by chronoamperometry and chronopotentiometry demonstrated a completely different behavior of electrodeposition even at the same applied charge. In particular, computed tomography and volumetric analysis clearly showed the structural and morphological dissimilarity. The structure of cerium metal in the electrodeposit was successfully separated from the chloride salt structure in the CT image by an image segmentation process.

Electrochemical and Raman Spectroscopic Investigations on the Speciation and Behavior of Chromium Ions in Fluoride Doped Molten LiCl-KCl

The speciation and behavior of chromium ions in LiCl-KCl-CrF3 and LiCl-KCl-LiF-CrF3 melts were investigated by using cyclic voltammetry, square wave voltammetry, chronopotentiometry and Raman spectroscopy. Electrochemical reduction of Cr(III) to Cr(0) in LiCl-KCl-CrF3 is a two-step process mediated by the formation of Cr(II). Most Cr(III) ions exist in the form of CrCl6 3− while CrCl6-xFx 3− (x ≤ 3) ions are minor in the molten salt medium. Based on the cyclic voltammetric results obtained at different potential scanning rates, the diffusion coefficients of Cr(III) and Cr(II) in LiCl-KCl-CrF3 melt were determined between 723 and 873 K, and their dependence on temperature can be described as lnDCr(III) = −4.90-4322.66/T and lnDCr(II) = −4.71-4007.09/T respectively. The diffusion coefficients of chromium ions in LiCl-KCl-CrF3 are higher than those in LiCl-KCl-CrCl3, which can be explained by the hopping mechanism. In addition, a series of apparent standard potentials of the Cr(III)/Cr(II) and Cr(II)/Cr(0) couples were obtained over the same temperature range using the open-circuit chronopotentiometry technique. With the addition of LiF into the LiCl-KCl-CrF3 mixture, the dominant chromium species change from CrCl6 3− to CrCl6-xFx 3− (x ≤ 6), which is supported by the appearance of the new peaks in both square wave voltammograms and Raman spectra.

Carbonization of transition metals in molten salts.

The carbonization of transition metals in molten salts was performed to study the effect of electrochemical polarization and molten salt medium on the carbonization process. The carbonization of niobium (Nb) has been systemically investigated in Ar atmosphere and molten salts. The effect of particle size and structure of the starting materials, molten salt medium, and electric field on the formation of NbC was studied to reveal the dynamic barriers in the carbonization process. A native oxide layer and/or niobate derivatives generated on the Nb particles are the main barriers for the mass transfer of carbon and Nb towards each other. The results showed that molten salts can accelerate the formation of NbC by enhancing the diffusion of carbon, and the kinetic barrier can be effectively eliminated by supplying negative polarization to the cathode in molten salts to remove the oxide barriers, thereby enhancing carbonization. Besides Nb, tungsten (W), molybdenum (Mo), titanium (Ti), and tantalum (Ta) can also be carbonized in molten salts with the assistance of an applied electric field.

Facile molten salt synthesis of atomically thin boron nitride nanosheets and their co-catalytic effect on the performance of carbon nitride photocatalyst

Two-dimensional atomically thin boron nitride (BN) nanosheets possess many distinctive properties but their applications are still being hindered by the technical difficulties in their synthesis methods, due to the strong lip-lip interactions between their basal planes. In this paper, we reported a facile and scalable bottom-up approach to synthsizing atomically thin BN nanosheets in a molten salt medium. As-prepared BN nanosheets can be used as an effective co-catalyst to improve the catalytic performance of carbon nitride photocatalyst, owing to the interfacial effect induced by their addition. In the case of photodegradation of rhodamine B, for example, the photodegradation rate of carbon nitride could be approximately doubled when 5 wt% as-prepared BN nanosheets were added.

Synthesis, Structure and Electronic Properties of Graphitic Carbon Nitride Films

Dark-colored shiny flakes of graphitic carbon nitride materials produced by reacting dicyandiamide C2N4H4 in a KBr/LiBr molten salt medium were determined to have a C/N ratio near 1.2:1. The compou...

Molten salt synthesis of hierarchical porous N-doped carbon submicrospheres for multifunctional applications: High performance supercapacitor, dye removal and CO2 capture

Hierarchical porous N-doped carbon submicrospheres (HPNCs) are synthesized at 1000 °C by a facile molten salt method using diaminomaleonitrile as a precursor, iron trichloride as a catalyst, and ZnCl2-KCl to form the molten salt medium. The results indicated that the pore structure of resultant HPNCs could be tuned by changing the amount of iron catalyst, and those prepared with adding 7.1 wt% FeCl3·6H2O showed the highest specific surface area (3155 m2/g) and largest hierarchical pore volume (2.2 cm3/g) with micropore and mesopore surface areas of 808 m2/g and 2347 m2/g, respectively, in addition to their high mesopore ratio up to 84.4%. The resultant HPNCs showed outstanding specific capacitance up to 455 F/g in 6 M KOH electrolyte solution at a current density of 0.5 A/g, a considerably high energy density of 93.6 Wh/kg and high capacitance retention of 95% after 5000 cycles. Moreover, they exhibited high adsorption capacities of 812.2 mg/g and 805.2 mg/g, for methylene blue and methyl orange adsorption, respectively, and high CO2 adsorption capacity of 3.4 mmol/g at 25 °C and at an equilibrium pressure of 1 bar.

Kinetics of the Reduction of Rare Earth Metals in LiCl–KCl–Cscl Eutectic Melt

Modeling and developing high temperature processes involving rare earth metals relies on comprehensive data concerning their behavior in molten salt media. The aim of the present study was determining kinetic parameters of reduction of lanthanum, praseodymium and neodymium ions in the melts based on the ternary eutectic mixture of lithium, potassium and cesium chlorides. The experiments were performed at 400–750 oC on a tungsten working electrode employing cyclic voltammetry and chronopotentiometry. Potentials were measured vs. chlorine reference electrode. Reduction of Ln(III) ions to Ln(0) for all the elements studied occurred as a single stage process. The process is reversible and controlled by diffusion. Transition time of the process was determined and diffusion coefficients of Ln3+ ions calculated.

Magnesiothermic Reduction of Silicon Dioxide to Obtain Fine Silicon Powder in Molten Salt Media: Analysis of Reduction Mechanism

Magnesiothermic Reduction of Silicon Dioxide to Obtain Fine Silicon Powder in Molten Salt Media: Analysis of Reduction Mechanism

Corrosion of Structural Alloys in High-Temperature Molten Fluoride Salts for Applications in Molten Salt Reactors

Hastelloy N®, a nickel-based alloy, and 316 stainless steel are among the candidate structural materials being considered for the construction of the molten salt reactor (MSR). Most of the proposed MSR concepts use molten fluoride salts as coolant which can be quite corrosive to structural alloys. The results of studies on the corrosion behavior of the two alloys in molten Li2BeF4 (FLiBe) salt at 700°C are discussed. This salt is being considered as the primary coolant for MSR designs featuring solid fuel particles, but the reported results also provide insights into the corrosion in MSR designs where the uranium fuel is dissolved in the molten fluoride salt. Corrosion was observed to occur predominantly by de-alloying of Cr from the alloy surface and into the molten salt, with more pronounced attack occurring along the grain boundaries than in the bulk grains. The magnitude and the mechanisms of corrosion were different for corrosion tests performed in graphite and metallic capsules, a result warranting recognition given the coexistence of structural alloys and graphite in the molten salt medium in the MSR.

A Review of Electrochemical and Non-Electrochemical Approaches to Determining Oxide Concentration in Molten Fluoride Salts

The design of reactor core and heat transfer systems for molten salt reactors (MSRs) and fluoride-salt cooled high temperatures reactors (FHR) necessitates a precise understanding of oxide contribution to thermophysical and chemical properties of the molten salt medium [1]. The dissolved oxide content in the melt can vary during reactor operation due to transmutation or migration to/from outside the core. Therefore, it is important to develop capabilities to quantify the oxide concentration at reactor conditions such as temperatures exceeding 500 oC. As part of the oxide sensor design process, a literature review of electrochemical and non-electrochemical approaches to determining oxide concentration in molten fluoride salts was conducted and an evaluation of various methodologies is presented here. This evaluation was the basis for selecting electrochemistry as the most promising technique with which to develop an oxide sensor.

Low onset potential on single crystal Ta3N5 polyhedron array photoanode with preferential exposure of {001} facets

The photoelectrochemical performance of photoanode depends significantly on the surface properties. Enormous efforts have been spent on fabricating photoanode with exposure of high-activity facets. However, precise control of the Ta3N5 facet exposure is still highly challenging, due to its harsh fabrication condition. In this study, it was found that Ta3N5 polyhedron can be grew through a bottom-up growth process in molten salt. Molten salt media in a semi-closed reactor provides a steady growth environment so that the Ta3N5 growth units are able to assemble in thermodynamically and kinetically favorable way. The preferential exposed facets are determined to be {001} facets. Due to the stronger built-in electric field on {001} facets and the small effective mass of hole along the [001] axis, an onset potential of 0.6 V versus reversible hydrogen electrode (RHE) and a photocurrent of 5.6 mA cm−2 at 1.23 versus RHE have been achieved. Our findings may open an avenue to grow the nitride or oxynitride semiconductor crystals with desired structures for achieving efficient solar energy conversion and storage.

Fabrication and characterization of ternary La1‐xCexB6 by a novel molten salt synthesis route

AbstractA novel molten salt synthesis route has been developed to prepare ternary La1‐xCexB6 (x = 0.2, 0.4, 0.6, 0.8), using LaCl3, CeCl3, and NaBH4 as reactants and eutectic LiCl/KCl salt as molten salt medium. All the synthesized ternary La1‐xCexB6 are single‐phase solid solution, and their lattice constant decreases linearly with increasing x. Morphology, microstructure, and elements composition of La0.6Ce0.4B6 which is a typical representative of the series of La1‐xCexB6 are analyzed in detail.

Preparation of in situ grown silicon carbide whiskers onto graphite for application in Al2O3-C refractories

C-SiC composite powders were prepared by salt-assisted synthesis from Si powders, graphite, and a molten salt medium (NaCl and NaF) with the molar ratio of Si/C = 1/2 at 1300 °C for 3 h. After the C-SiC composite powders part and complete replacement of the graphite, the mechanical properties, oxidation resistance and slag-corrosion resistance of the Al2O3-C materials were studied by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), as well as with dedicated equipment. The results indicated that SiC whiskers, with lengths of 10–50 nm, formed on the surface of the flake graphite, and the activation energy of oxidation of the C-SiC composite powder increased by 45.72 kJ mol−1 as compared to that of flake graphite. Furthermore, the decarburization area and slag erosion area of the Al2O3-C material decreased after 3 wt% of C-SiC composite powder was substituted for the flake graphite. Meanwhile, the cold modulus of rupture was maintained when 3 wt% of C-SiC composite powder was added. This improved both the oxidation and slag resistance of the Al2O3-C materials.

Tuning the extent of porosity and composition of N-doped carbon materials by NaNO3 and its effect on electrochemical activity

A facile one-pot method using NaNO3 as etching agent in a molten salt medium is developed to tune the porous structure and composition of N-doped carbon materials derived from biomass at high temperatures. SEM and TEM analyses reveal that by increasing the amount of NaNO3, the structures of the obtained carbons become more open and graphene-like layers are formed. N2 adsorption/desorption isotherms show that the hierarchical porous structure could be efficiently tuned by adjusting the amount of NaNO3 in the salt mixture. It is observed that the specific surface area of the as-prepared N-doped carbon using NaNO3 can reach 1430.1 m2 g−1, a much higher value than that of the sample prepared in the absence of NaNO3. The micropore ratio decreases with increased amount of NaNO3, from 49.42% to 9.98%. Whilst the surface composition of the obtained carbon materials is affected by NaNO3. The N content increases and O decreases respectively, by increasing the amount of NaNO3. Analysis on the N 1s high resolution reveals that the sum of pyridinic-N and graphitic-N increased, whereas the sum of pyridinic-N oxide and NOx decreases from NC-0 to NC-0.75. It is also found that the changes of the porous structure and the oxygen-containing groups from NC-0 to NC-0.75 led to changes in hydrophilicity. When the as-prepared carbon materials are used as catalysts for the oxygen reduction reaction, high catalytic activity values are obtained, sample NC-0.5 shows the best performance due to the porous structure and doping effect of N atoms. More importantly, it is concluded that producing N-doped carbon materials from soybean dregs in molten salt medium using NaNO3 as etching agent could provide a promising approach for the synthesis of opened N-doped carbons of controlled porous structures and large specific surface areas based on biomass precursors.

Syntheses of MgCO3 and Na2Mg(CO3)2 through solid-gas reactions mediated by alkali nitrates

In this work, we devised a set of molten salt-mediated reactions that can make use of gaseous reagents in forming solid products. In order for gaseous molecules to be reacted, its solubility in the molten salt medium needs to be high, which condition, in turn, requires the molten salt material to have a low melting point. We used alkali nitrate (ANO3, A = Li, Na, K, Rb, and Cs) as the reaction medium, for their low melting points, for the reactions of CO2(g) with various Mg-reagents. Reactions between 4MgCO3·Mg(OH)2·4H2O or Mg(OH)2 with CO2 in the presence of an ANO3 above a certain quantity (30 wt% with respect to the MgO content in the Mg-reagent) produced single phase MgCO3, after washing the ANO3 with water. On the contrary, the same reactions without ANO3 did not show any sign of incorporating gaseous CO2 in the final products. Similarly, the reaction of a mixture between Na2CO3 and 4MgCO3·Mg(OH)2·4H2O or Mg(OH)2 with CO2 in an ANO3 medium produced Na2Mg(CO3)2, a double carbonate of MgCO3 with Na2CO3. The X-ray diffraction patterns and the thermogravimetry data under a CO2 flow of MgCO3 and Na2Mg(CO3)2 indicate their high phase purity. The effects of various synthesis parameters such as the nature of ANO3 and the amounts on the products were systematically investigated.

Microporous carbons derived from melamine and isophthalaldehyde: One-pot condensation and activation in a molten salt medium for efficient gas adsorption

In the present work, mixture of melamine and isophthalaldehyde undergo simultaneous polymerization, carbonization, and in situ activation in the presence of molten salt media through a single all-in-one route to design microporous carbons with high specific surface areas (~3000 m2/g). The effect of the activation temperature and molten salts on the polymerization process and final texture of the carbon was explored. Carbon materials prepared at 700 °C, in the presence of KOH (referred as MIK-700), exhibited a narrower pore-size distribution ~1.05 nm than those prepared in the presence of the eutectic KOH-NaOH mixture (MIKN). Additionally, MIK-700 possesses an optimum micropore volume (1.33 cm3/g) along with a high nitrogen content (2.66 wt%), resulting in the excellent CO2 adsorption capacity of 9.7 mmol/g at 273 K and 1 bar. Similarly, the high specific area and highest total pore volume play an important role in H2 storage at 77 K, with 4.0 wt% uptake by MIKN-800 (specific surface area and pore volume of 2984 m2/g and 1.98 cm3/g, respectively.) Thus, the facile one-step solvent-free synthesis and activation strategy is an economically favorable avenue for designing microporous carbons as an efficient gas adsorbents.

Synthesis of Li-Ion Conducting Garnet Li7La3Zr2O12 Powders in Molten Salt Media: Implications for Future Li-Ion Batteries

Recently, solid-state electrolytes have been a highly active area of research for future Li-ion batteries due to the potential for drastically improved energy density and safety. Among these materials, garnet structured lithium lanthanum zirconate (Li7La3Zr2O12, LLZO) shows particular promise owing to the high ionic conductivity of its cubic polymorph, inertness, and electrochemical stability against metallic lithium. Herein we report the facile preparation of phase-pure cubic LLZO via molten salt synthesis at temperatures substantially lower than conventional solid-state reactions. Fine powders of cubic LLZO were obtained with primary particle sizes in the submicron range. Depending on the consolidation conditions, dense ceramics with Li+ conductivity values above 0.371 mS cm-1 could be obtained. It is also observed that the effect of hydration has a profoundly deleterious effect on sintering and densification, and that this effect can be mitigated by the simple addition of LiOH before sintering to reverse hydration and aid densification. Qualitative discussions on the mechanisms of LLZO formation in the molten salt medium are discussed, in addition to implications for future scalable processing of LLZO electrolytes.

A Review of Electrochemical and Non-Electrochemical Approaches to Determining Oxide Concentration in Molten Fluoride Salts

The design of reactor core and heat transfer systems for molten salt reactors (MSRs) and fluoride-salt cooled high temperatures reactors (FHR) necessitates a precise understanding of the parameters affecting thermophysical and chemical properties of the molten salt medium. Oxides constitute a significant component of the impurities in alkali metal halide melts commonly implemented in MSRs. They are present initially in the raw materials due to the manufacturing process. If present in high enough concentrations, oxides can affect molten salt chemistry and thermodynamics, potentially affecting its thermophysical properties and corrosivity to structural materials. The dissolved oxide content in the melt can vary during reactor operation due to transmutation or migration to/from the environment. Therefore, it is important to develop capabilities to quantify the oxide concentration at reactor conditions. The benefit is threefold: firstly, measuring oxide concentration precisely and accurately will allow us to correlate oxide concentration with variation in thermophysical and chemical properties; secondly, knowing these correlations can facilitate the formulation of more incisive experiments to more accurately determine the activity of oxide in molten fluorides ; thirdly, such an oxide measurement system can be deployed online in a reactor as a sensor to add to the suite of reactor diagnostic instrumentation. These goals in concert will help us gain a better understanding of the basic science of a molten salt solvent environment. In context of the ongoing effort to design an oxide sensor for molten salt systems, a review of the literature of electrochemical and non-electrochemical approaches to determining oxide concentration in molten fluoride salts was conducted and an evaluation of their merits is presented here. This evaluation served as the basis on which we selected square wave voltammetry as the most promising technique with which to develop an oxide sensor.

Engineering morphology and porosity of N,S‐doped carbons by ionothermal carbonisation for increased catalytic activity towards oxygen reduction reaction

A facile and one-pot method using NaCl/KCl as a molten salt medium was developed to tune the porous structure of N,S-doped carbon derived from biomass at high temperature. With increasing the NaCl/KCl amount, the structures of the obtained carbons revealed by scanning electron microscopy become more and more open. N2 adsorption/desorption isotherm shows that the hierarchical porous structure could be efficiently tuned by adjusting the amount of NaCl/KCl salt mixture. Specific surface area (SSA) of as-prepared N,S-doped carbon could reach 1478.6 m2 g−1. The relative ratio of micropore/mesopore drops with the increased NaCl/KCl. When the obtained carbons are used as the catalysts for oxygen reduction reaction (ORR), high catalytic activities are obtained, which result from the porous structure and doping effect of N atoms. Meanwhile, a volcano trend of the electrochemical performance for as-prepared carbons is also shown, which is a result of multiple factors possessing the opposite effect on ORR. Importantly, producing N,S-doped carbon from egg white in NaCl/KCl molten salt medium could provide a promising approach to synthesise the opened N,S-doped carbons with a controllably porous structure and large SSA based on biomass precursors.