Molten Salt Systems Research Articles

Electrochemical behavior and dissolution processes of lithium metal in the LiCl–KCl–CsCl molten salt system

Electrochemical behavior and dissolution processes of lithium metal in the LiCl–KCl–CsCl molten salt system

Electrochemical behavior of tantalum ion in LiF-NaF-K2TaF7 molten salt system

It is of great significance to clarify the electrochemical behavior of Ta5+ in LiF-NaF-K2TaF7 molten salt systems for the preparation of tantalum metal by molten salt electrodeposition. The aim of this work was to explore the reaction steps and behavior of the reduction of metal Ta from Ta5+ in fluorine molten salt by electrochemical method. The cyclic voltammetry, square wave voltammetry, chronopotentiometry were used to determine the reduction steps of Ta5+. The results showed that there were two reduction steps which the first reduction step R1 was TaF72−+2e−⇌TaF3+4F− and the second reduction step R2 was TaF3+3e−→Ta+3F−. SEM, XRD and EDS results showed that the reduction product of Ta5+ in molten salt is Ta and the tantalum atom was selectively deposited and grew on the (200) crystal plane in priority.

The Form of Electrodeposited Iridium Ions in a Molten Chloride Salt and the Effects of Different Iridium Concentrations

A molten salt system was prepared using an optimized method. We studied the complex structure of Ir3+ ions in the molten salt system and the influence their concentration had on the quality of the coatings prepared via electrodeposition. Using TG-DSC and in situ XRD experiments, we studied the high-temperature characteristics and properties of IrCl3 alongside its thermal stability. Using in situ XRD and Raman spectroscopy, we analyzed the Ir ions’ complex structure and the variation in the molten salt system at high temperatures. Finally, the changes in the Ir ion concentration in the molten salt system and the influence of the microstructure of the coatings’ surfaces were investigated under different anode conditions. IrCl3 easily decomposes above 400 °C, and temperature increases accelerate the rate of this decomposition. When the NaCl-KCl-CsCl system is in a high-temperature, molten state, IrCl3 forms stable complex structures (IrCl6)3− and (IrCl6)2−, and the valence state of Ir will be transformed with the increase in temperature. Generating these complex structures is conducive to improving the Ir coating quality. During the electrodeposition process, too few Ir ions in the molten salt can lead to concentration polarization, affecting the quality of the coating. Application of the molten NaCl-KCl-CsCl system is conducive to the electrodeposition of Ir coatings in a suitable temperature range. At the same time, using Ir as the anode can enhance the quality of the coatings.

Natural convection effects in molten LiF-NaF-KF

Understanding mass transfer is of great importance to molten salt electrochemistry. However, natural convection has long been neglected in electrochemical measurements in stagnant molten salt systems due to the complex mathematical treatments. Here we reported the notable natural convection effects in molten LiF-NaF-KF during cyclic voltammetry and chronoamperometry tests. The established model showed that the convection-diffusion layer (CDL) convection dominated mass transfer under long time-scale tests in molten LiF-NaF-KF-EuF3. However, in the case of molten LiF-NaF-KF-K2TiF6, CDL convection predominated at dilute redox concentrations while density-driven convection came into dominance at high redox concentrations, because of the severe coordinate structure change of the redox couple. The derived thickness of the convection-diffusion layer, δconv., ranged from 135 to 183 μm, much higher than that in aqueous solutions (220–250 μm). Moreover, the increase in system temperature greatly enhanced natural convection effects in molten fluorides. Lastly, the use of microelectrode (re<39.7 μm for δconv.=160 μm) was demonstrated to be an effective way to suppress the intense natural convection effects.

Investigation of the electrochemical behaviour of praseodymium ions in a LiF-PrF3-Pr6O11 molten salt system

Understanding the electrochemical of paseodymium ions in molten salt is essential to improve electrolytic efficiency. To elucidate the electroreduction mechanism of praseodymium ions and overcome the limitations associated with regulating the yield of praseodymium metal in the industrial praseodymium electrolysis process, the electrochemical behaviour of praseodymium on the W working electrode surface in the LiF-PrF3-Pr6O11 molten salt system was determined by square–wave voltammetry, chronoamperometry, cyclic voltammetry and potentiometry analyses. The results indicated that the reduction of Pr3+ on the W cathode is a one-step quasireversible Pr3+/Pr reduction process controlled by diffusion of the LiF-PrF3 and (LiF-PrF3)eut-Pr6O11 electrolytes at 1223 K Pr3+ in the LiF-PrF3-Pr6O11 molten salt has a diffusion coefficient of DPr3+/Pr = 0.20 × 10−8–4.11 × 10−8 cm2·s−1. The incorporation of Pr6O11 increased the electrochemical activity of Pr3+ in the LiF-PrF3 system. Pr crystallization on the W electrode was achieved by three-dimensional progressive nucleation.

Quantum Chemical Calculation on ScCl3-NaCl-KCl Molten Salt System

Background: The study of the ionic structure of the ScCl3-NaCl-KCl molten salt system is of guiding significance for the production of metal Sc and Sc alloy by molten salt electrolysis using ScCl3-NaCl-KCl molten salt system as electrolyte. However, limited research has been conducted on the structural analysis of molten salt within this system using Raman spectroscopy. Methods: The Gaussian and GaussView programs are used to simulate Sc-Cl ionic groups that may exist in the ScCl3-NaCl-KCl molten salt system based on density functional theory (DFT) and calculate their theoretical Raman spectra. Then, the wave function analysis of these Sc-Cl ionic groups is carried out using the Multiwfn program. Results: When the ScCl3-NaCl-KCl system is used as the electrolyte, it is considered that the 8 Sc atom in ScCl74− ionic groups is most easily reduced by electrochemistry at the cathode when the concentrations of eight Sc-Cl ionic groups are the same. In Sc2Cl7− Sc2Cl82− and Sc2Cl93− ionic groups, the sites that are most likely to attract electrophiles to attack and react are 1Cl and 7Cl, respectively, and the two Sc atoms located in the "chlorine bridge" structure contribute the most to LUMO orbitals, and nucleophiles will preferentially attack the Sc atoms located in the "chlorine bridge" structure. For Sc2Cl7−、Sc2Cl82− and Sc2Cl93− ionic groups, which have two Sc atoms, the bonds formed between Sc and Cl in the "chlorine bridge" structure have the smallest bond order, so the bonds formed by Sc and Cl in the "chlorine bridge" structure break first during the reaction, and then the bond formed by Sc and Cl located at both ends of the whole structure breaks Conclusion: The key information, such as bond length, point group structure, and theoretical Raman spectral characteristic peak after optimization, are obtained by the Gaussian and GaussView programs. The net atomic charge in each ionic group, the direction of electron migration during the formation of the ionic groups, the sites where electrophilic and nucleophilic reactions are most likely to occur in each ionic group, and the order of bond breaking during chemical reactions are obtained by the Multiwfn program.

Scalable and Sustainable Zinc (II) Ions‐Glue‐Assisted Conversion of Biomass Waste Bits into Carbon Aerogels for Efficient Uranium Extraction

AbstractCarbon aerogels (CAs) have garnered significant attention due to their multifunctional applications. Biomass waste, abundantly generated by agriculture and industry, serves as a primary carbon source. However, developing a facile, sustainable, and efficient method to produce CAs from biomass waste remains challenging. In this study, a one‐step Zn2+ ion‐glue‐assisted carbonization technique was developed to produce large‐scale, high‐performance CAs. Various biomass materials (wood bits, peanut shells, bamboo bits, and straw waste) were treated in a molten salt system (ZnCl2/KCl) at 300 °C for 2 h to obtain large‐block CAs derived from biomass bits. Zn2+ ions cleave cellulose hydrogen bonds in natural biomass, facilitating the dehydration crosslinking reaction among cellulose, hemicellulose, and lignin, thus reconstructing the entire block structure. The resulting CAs exhibited high porosity (95 %) and low density (0.078 g/cm3). Numerous hydroxyl and carbonyl groups were preserved during the low‐temperature treatment, facilitating chemical modification for diverse applications. For instance, amidoxime functionalized CAs were utilized as filters for selective and highly efficient extraction of U(VI) from wastewater. The adsorption capacity and extraction efficiency reached 801.2 mg/g and 95 % with a flux rate of 6.1×103 L/m2 ⋅ h.

Scalable and Sustainable Zinc (II) Ions-Glue-Assisted Conversion of Biomass Waste Bits into Carbon Aerogels for Efficient Uranium Extraction.

Carbon aerogels (CAs) have garnered significant attention due to their multifunctional applications. Biomass waste, abundantly generated by agriculture and industry, serves as a primary carbon source. However, developing a facile, sustainable, and efficient method to produce CAs from biomass waste remains challenging. In this study, a one-step Zn2+ ion-glue-assisted carbonization technique was developed to produce large-scale, high-performance CAs. Various biomass materials (wood bits, peanut shells, bamboo bits, and straw waste) were treated in a molten salt system (ZnCl2/KCl) at 300 °C for 2 h to obtain large-block CAs derived from biomass bits. Zn2+ ions cleave cellulose hydrogen bonds in natural biomass, facilitating the dehydration crosslinking reaction among cellulose, hemicellulose, and lignin, thus reconstructing the entire block structure. The resulting CAs exhibited high porosity (95 %) and low density (0.078 g/cm3). Numerous hydroxyl and carbonyl groups were preserved during the low-temperature treatment, facilitating chemical modification for diverse applications. For instance, amidoxime functionalized CAs were utilized as filters for selective and highly efficient extraction of U(VI) from wastewater. The adsorption capacity and extraction efficiency reached 801.2 mg/g and 95 % with a flux rate of 6.1×103 L/m2 ⋅ h.

Novel assessment of molten salt oxidation for cation exchange resin treatment: Effective neutralization of sulfurous gas with Li2CO3-Na2CO3-K2CO3 system

The conventional thermal treatment of cation exchange resin substantially releases sulfurous gases, causing significant equipment corrosion and air pollution. In contrast, the Li2CO3-Na2CO3-K2CO3 as an alkaline molten system effectively neutralizes sulfur gas and mitigates waste gas production inherent in thermal oxidation methods. In the molten salt oxidation process, the volume concentration of SO2 was reduced by 81.7 % compared to that in the traditional thermal oxidation process, and this method reduces the generation of hazardous gases such as CO, CH4, and C2H6. The integration of online gas mass spectrometry and phase stability diagrams for carbonate and sulfur interception products demonstrate excellent thermodynamic stability during the molten salt oxidation (MSO) process. Moreover, a more accurate assessment of the acid gas neutralization capacity of the molten salt system is provided, and the acid gas neutralization capacity of the Li2CO3-Na2CO3-K2CO3 carbonate system can reach 82.58 % at 800 °C. The predominant contributors to acid gas neutralization are Na2CO3 and K2CO3, as evidenced by waste salt composition and ternary phase diagrams. The stable presence of Li2CO3 throughout the MSO process contributes to the lowering of the melting point of the carbonate system to 393 °C.

Technical Overview of Pyro-processing and Policy Considerations

Pyro-processing is an emerging technology crucial to the implementation of a closed nuclear fuel cycle. It is distinguished by its method of extracting and recycling actinides from fission products in spent nuclear fuels, utilizing high-temperature molten salt media. Despite its promise, the technology poses proliferation concerns that underscores the imperative for robust safeguards. While the safeguarding framework for aqueous reprocessing facilities that has been established over decades offers a foundational reference, the unique challenges posed by Pyro-processing necessitate a tailored approach. Amid these concerns, Pyro-processing is heralded as a viable strategy for nuclear waste management, with research increasingly focusing on its selectivity and efficacy within molten salt systems. This paper scrutinizes the proliferation risks inherent in Pyro-processing, delineates the requisite safeguards, and evaluates the feasibility of Pyro-processing, especially within the purview of the 123 Agreements between the United States and South Korea.

Molten Salt-Assisted Synthesis of Catalysts for Energy Conversion.

A breakthrough in manufacturing procedures often enables people to obtain the desired functional materials. For the field of energy conversion, designing and constructing catalysts with high cost-effectiveness is urgently needed for commercial requirements. Herein, the molten salt-assisted synthesis (MSAS) strategy is emphasized, which combines the advantages of traditional solid and liquid phase synthesis of catalysts. It not only provides sufficient kinetic accessibility, but effectively controls the size, morphology, and crystal plane features of the product, thus possessing promising application prospects. Specifically, the selection and role of the molten salt system, as well as the mechanism of molten salt assistance are analyzed in depth. Then, the creation of the catalyst by the MSAS and the electrochemical energy conversion related application are introduced in detail. Finally, the key problems and countermeasures faced in breakthroughs are discussed and look forward to the future. Undoubtedly, this systematical review and insights here will promote the comprehensive understanding of the MSAS and further stimulate the generation of new and high efficiency catalysts.

Li2O concentration influenced local structure and properties of molten LiCl salt by machine learning driven molecular dynamics simulation

Pyroprocessing is one of the promising technologies for the reprocessing of spent nuclear fuel. In the electrolytic reduction process of pyroprocessing, LiCl serves as the electrolyte with a minor addition of Li2O as the initial oxygen ion input. Investigating the impact of Li2O concentration on the molten salt system is essential for enhancing the efficiency of electrolytic reduction. In the present study, the LiCl-Li2O system is simulated using deep potential molecular dynamics simulation. The properties including self-diffusion coefficient, shear viscosity, ionic conductivity, heat capacity, radial distribution functions, coordination numbers, short-range order, medium-range order, Voronoi structure, and angle distribution functions are explored at different Li2O concentrations. This study reveals that the variations of physical properties at different Li2O concentrations in LiCl molten salt can be attributed to the changes in local structure. For example, unnormal high proportions of two Voronoi structures are found in the molten salt when the Li2O concentration increases, which may explain the changes of properties. These findings offer valuable insights for refining the parameters of electrolytic reduction and elucidating the correlation between microscopic and macroscopic phenomena.

Cathodic Decomposition Electrodes as Standard Reference Electrodes for Molten Salts: Example of the Lithium Eutectic Electrode for the LiCl-KCl Eutectic

Alternatives to the widely-used standard anodic decomposition reference electrodes in molten salts are necessary to enable more easily reproduced thermochemical and electrochemical data in molten salt electrolytes. The class of standard reference electrodes called cathodic decomposition electrodes (CDEs) are easily constructed and can be used to make thermochemical measurements in molten salts more directly compared to anodic decomposition electrodes. The lithium eutectic electrode (LEE) was chosen as a sample test case for validation and was applied to thermochemical measurements of electroactive species in molten LiCl-KCl eutectic. Transient measurements were made to measure the Li+/Li reduction potential at zero current in pure LiCl-KCl eutectic relative to a Li-alloy reference electrode to validate the reference potential of the LEE. Literature-reported electromotive force measurements against Li-alloy reference electrodes were used to generate a relationship between the LEE and the standard chlorine electrode and this relationship was used to evaluate measured and reported formal potential measurements for the LiCl-KCl-GdCl3 system. This work demonstrates the general framework for defining CDEs for any molten salt system and a method for calibrating external reference electrodes against a CDE standard reference electrode, improving the ease of obtaining thermochemical and electrochemical measurements in any molten salt system.

Diffusion Behaviors of CaCl2-NaCl Molten Salt under an Electric Field: A Deep Potential Molecular Dynamics Simulation.

CaCl2-NaCl molten salt is one of the widely used electrolytes, and the effect of the electric field on it needs to be considered in the environment of manufacturing metals and alloys as well as in battery applications. To be closer to the state of the CaCl2-NaCl molten salt system in practical applications, this study uses the training-based deep potential, combined with the ionic structure, to analyze the electric field effects on the drift velocity, ionic mobility, ion diffusion, and viscosity of the CaCl2-NaCl system by molecular dynamics simulations. It is shown that the electric field has a stronger effect on the ion diffusion behavior parallel to the direction of the electric field in the CaCl2-NaCl molten salt system. Due to the looser coordinate structure of Na+, the interaction force between Na-Cl is small compared with that between Ca-Cl. Na+ is easily driven and more sensitive to the electric field, whose drift velocity, ionic mobility, and diffusion coefficient are larger than those of the other two ions, with an order of Na+ > Cl- > Ca2+. All ionic drift velocities increase linearly as electric field intensity increases and the ionic mobilities tend to be constant. The diffusion coefficient parallel to the direction of the electric field exponentially increases and the viscosity tends to exponentially decrease with the increase in the electric field intensity. This work is important for accurately predicting the properties and performance of molten salts and their improvement for applications such as solar thermal energy conversion.

Computational insights into the structural, thermodynamic and transport properties of CaF2-MgF2 binary fluoride system at high temperatures

The structural, thermodynamic and transport properties of the CaF2-MgF2 molten salt system were investigated with ab initio molecular dynamics (AIMD), system-specific neural network interatomic potentials (NNIPs) and universal PreFerred Potentials (PFP). We trained an NNIP model using AIMD data as input and used this potential to efficiently simulate the interactions within a large supercell in a temperature range of 1273–1773 K. The Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) code was employed to validate our trained NNIP model. The Matlantis software with universal PFP is also presented to prove its feasibility for MD calculations and can be considered as a useful alternative simulation tool for higher-order systems where existing potentials are not readily available. We calculated structural and thermodynamic properties including radial distribution function (RDF), angular distribution function (ADF), specific heat capacity, ionic self-diffusivity, and viscosity. Our results indicate that the system exhibited a high degree of structural disorder, with the Ca, Mg, and F ions forming a liquid solution. Using PFP, the positions of the first peak in RDFs for Ca-F and Mg-F pairs are only slightly left-shifted (<0.05 Å), and the estimated viscosity of the melt decreases from 4.613 mPa·s to 1.846 mPa·s with an increase in temperature from 1273 K to 1773 K, in agreement with the NNIP trained specifically for CaF2-MgF2. Our results provide valuable insights into the properties of the CaF2-MgF2 system at high temperatures and serve as predictive models for the development of new electrolytes that could be used for silicon epitaxy by adding silica.

Investigation of the Relationship between Molten Fluoride Salt Structure and Fluoroacidity Using Chromium As a Probe Redox Ion

Molten fluoride salts have been studied for decades due to their applications in the production of aluminum as well as in the operation of advanced nuclear reactors. Advanced fission and fusion nuclear reactor designs may include molten fluoride salts as coolants, fuel carriers (fission), or fuel breeding blankets (fusion). Relevant to their nuclear applications are the thermophysical and thermochemical properties of the salt. These include viscosity and thermal conductivity, as well as density, vapor pressure, and heat capacity. Underlying these properties are the interactions of the ions constituting the liquid, characterized by their coordination number, correlation times, and the possible formation of polymeric networks in the melt. The composition and structure of a melt, that is, what ions are present and how they interact, dictates the macroscopic properties relevant to reactor operation. Fluoroacidity, or the activity of dissociated fluoride anions, is one metric that may be used to describe the relationship between salt chemistry and structure. Overall, understanding the relationship between molten fluoride salt chemistry and structure enables deeper understanding and, thus, prediction of property changes that may take place throughout a reactor’s lifetime.In this poster, we present cyclic and square wave voltammetry studies of chromium speciation and diffusivity in various molten fluoride salts, connecting results with current understanding of the structure of the melts. Chromium is a thermodynamically favored corrosion product in molten salt systems, and in this study, it serves as a probe ion in different fluoride melts. Chromium trifluoride is added to various fluoride solvents and the diffusion coefficients of its various oxidation states are calculated and interpreted in light of salt structure. Experimental apparatus design and know-how for high-temperature molten salt electrochemistry are also highlighted.

Closed-Loop Direct Upcycling of Spent Ni-Rich Layered Cathodes into High-Voltage Cathode Materials.

Facing the resource and environmental pressures brought by the retiring wave of lithium-ion batteries (LIBs), direct recycling methods are considered to be the next generation's solution. However, the contradiction between limited battery life and the demand for rapidly iterating technology forces the direct recovery paradigm to shift toward "direct upcycling." Herein, a closed-loop direct upcycling strategy that converts waste current collector debris into dopants is proposed, and a highly inclusive eutectic molten salt system is utilized to repair structural defects in degraded polycrystalline LiNi0.83Co0.12Mn0.05O2 cathodes while achieving single-crystallization transformation and introducing Al/Cu dual-doping. Upcycled materials can effectively overcome the two key challenges at high voltages: strain accumulation and lattice oxygen evolution. It exhibits comprehensive electrochemical performance far superior to commercial materials at 4.6V, especially its fast charging capability at 15C, and an impressive 91.1% capacity retention after 200 cycles in a 1.2Ah pouch cell. Importantly, this approach demonstrates broad applicability to various spent layered cathodes, particularly showcasing its value in the recycling of mixed spent cathodes. This work effectively bridges the gap between waste management and material performance enhancement, offering a sustainable path for the recycling of spent LIBs and the production of next-generation high-voltage cathodes.

Determining phase-transition temperatures of molten salt mixtures through microsecond-scale staircase voltammetry

Determining the phase-transition temperature is crucial in the design of molten salt reactors, as operational temperatures need to be established considering the safety margins related to the thermodynamic stability of the molten salt system. This study introduces a novel approach to determine the phase-transition temperature based on rapid electrical conductivity measurements using a microsecond-scale staircase voltammetry technique. The phase-transition temperature is determined near distinct points where the electrical conductivity abruptly changes, indicating phase transitions. The phase-transition temperatures of three LiCl–KCl molten salt systems, determined using the proposed approach based on electrical conductivity measurements, are with relative errors less than 3% compared with reported phase transition data based on conventional thermal analysis techniques.

In situ desulfurization mechanism of molten salt thermal treatment for waste tires

Pyrolysis is regarded as a practical way for waste tires to achieve waste-to-resource objectives. However, the environmental risk of pollution emissions in product utilization caused by the migration of sulfur is an important issue in the waste tire disposal industry. This study introduced a solution to using the binary molten salt system (NaOH-Na2CO3) as a heat transfer and desulfurization medium for the thermal utilization of waste tires. The results demonstrated that molten salts had in-situ desulfurization capability at 425 °C −575 °C and inhibited the migration of sulfur to gas–liquid-solid products with the total sulfur ratio reduced by 50 % − 67 %. The sulfur-containing radical intermediates could be extensively consumed by molten salt, leading to a low concentration level of H2S and COS in gaseous products. Simultaneously, molten salt blocked the traditional pathway of the sulfhydryl radical with inorganic additives, weakening the sulfur enrichment to char. Additionally, molten salt enhanced heat transfer during pyrolysis, facilitating the decomposition of thiazole and the polymerization of sulfur-containing benzene ring (2,3-dimethyl-thiophene to benzothiophene). The online monitoring further corroborated that COS release was preceded by H2S at high temperatures (>500 °C) along with the full degradation of the waste tires. Higher desulfurization efficiency of molten salt was achieved with feedstocks of small particle size, as it allows for adequate exposure of the reaction contact sites. This approach provides a new opportunity for in-situ desulfurization of waste tire pyrolysis via molten salt thermal treatment.

The role of the local structure on the thermal transport within dissociated and complexed Molten salt mixtures

Contrary to the solid state, the understanding of thermal transport within molten salts remains limited. This study aims at establishing a robust theoretical framework for describing and predicting thermal transport in complex and dissociated multicomponent molten salt mixtures, considering both the composition and temperature variables. The proposed approach, based on kinetic theory, considers the local perturbation of kinetic energy through mass fluctuation, weighted by a function dependent on the thermal conductivity of the uncorrelated system and the local structure. The local structure is described in terms of the concentration of free and complex ions, determined either experimentally (using NMR) or through equilibrium atomistic simulations. The thermal transport within several molten salt systems, including KCl, Al2O3, LiF-KF, LiF-NaF-KF (FLiNaK), KCl-MgCl2, NaCl-KCl-MgCl2, and the NaF-AlF3 system, are presented as case studies using the proposed approach. Furthermore, a molecular-level perspective of thermal conduction within molten salts is discussed.