Transient Covalency in Molten Uranium(III) Chloride.

Uranium is arguably the most essential element in the actinide series, serving as a crucial component of nuclear fuels. While U is recognized for engaging the 5 orbitals in chemical bonds under normal conditions, little is known about its coordination chemistry and the nature of bonding interactions at extreme conditions of high temperature. Here we report experimental and computational evidence for the shrinkage of the average U-ligand distance in UCl upon the solid-to-molten phase transition, leading to the formation of a significant fraction of short, transient U-Cl bonds with the enhanced involvement of U 5 valence orbitals.

Exploring Cr and molten salt interfacial interactions for molten salt applications.

Molten salts play an important role in various energy-related applications such as high-temperature heat transfer fluids and reaction media. However, the extreme molten salt environment causes the degradation of materials, raising safety and sustainability challenges. A fundamental understanding of material-molten salt interfacial evolution is needed.

Heterogeneous Structure, Mechanisms of Counterion Exchange, and the Spacer Salt Effect in Complex Molten Salt Mixtures Including LaCl.

Complex molten chloride salt mixtures of uranium, magnesium, and sodium are top candidates for promising nuclear energy technologies to produce electricity based on molten salt reactors. From a local structural perspective, LaCl is similar to UCl and hence a good proxy to study these complex salt mixtures. As fission products, lanthanide salts and their mixtures are also very important in their own right.

Temperature-Dependent Morphological Evolution during Corrosion of the Ni-20Cr Alloy in Molten Salt Revealed by Multiscale Imaging.

Understanding the mechanisms leading to the degradation of alloys in molten salts at elevated temperatures is significant for developing several key energy generation and storage technologies, including concentrated solar and next-generation nuclear power plants. Specifically, the fundamental mechanisms of different types of corrosion leading to various morphological evolution characteristics for changing reaction conditions between the molten salt and alloy remain unclear. In this work, the three-dimensional (3D) morphological evolution of Ni-20Cr in KCl-MgCl is studied at 600 °C by combining in situ synchrotron X-ray and electron microscopy techniques.

Evolution of micro-pores in Ni-Cr alloys via molten salt dealloying.

Porous materials with high specific surface area, high porosity, and high electrical conductivity are promising materials for functional applications, including catalysis, sensing, and energy storage. Molten salt dealloying was recently demonstrated in microwires as an alternative method to fabricate porous structures. The method takes advantage of the selective dissolution process introduced by impurities often observed in molten salt corrosion.

Radiation-induced reaction kinetics of Zn with e and Cl˙ in Molten LiCl-KCl eutectic at 400-600 °C.

Molten chloride salts are currently under consideration as combined coolant and liquid fuel for next-generation molten salt nuclear reactors. Unlike complementary light-water reactor technologies, the radiation science underpinning molten salts is in its infancy, and thus requires a fundamental mechanistic investigation to elucidate the radiation-driven chemistry within molten salt reactors. Here we present an electron pulse radiolysis kinetics study into the behaviour of the primary radiolytic species generated in molten chloride systems, , the solvated electron (e) and di-chlorine radical anion (Cl˙).

A high temperature cell for investigating interfacial structure on the molecular scale in molten salt/alloy systems.

In this work, we describe the design and development of an in situ neutron reflectometry cell for high temperature investigations of structural changes occurring at the interface between inorganic salts, in their molten state up to 800 °C, and corrosion resistant alloys or other surfaces. In the cell, a molten salt is confined by an annular ring of single crystal sapphire constrained between the sample substrate and a sapphire plate using two gold O-rings, enclosing a liquid salt volume of 20 ml, along with a dynamic cell volume to accommodate expansion of the liquid with heating. As a test case for the cell, we report on an in situ neutron reflectometry measurement of the interface between a eutectic salt mixture of MgCl-KCl (32:68 molar ratio) and a single crystal sapphire substrate at 450 °C, resulting in the formation of a 60 Å layer having a scattering length density of 1.

A Holistic Approach for Elucidating Local Structure, Dynamics, and Speciation in Molten Salts with High Structural Disorder.

To examine ion solvation, exchange, and speciation for minority components in molten salts (MS) typically found as corrosion products, we propose a multimodal approach combining extended X-ray absorption fine structure (EXAFS) spectroscopy, optical spectroscopy, molecular dynamics (AIMD) simulations, and rate theory of ion exchange. Going beyond conventional EXAFS analysis, our method can accurately quantify populations of different coordination states of ions with highly disordered coordination environments via linear combination fitting of the EXAFS spectra of these coordination states computed from AIMD to the experimental EXAFS spectrum. In a case study of dilute Ni(II) dissolved in the ZnCl+KCl melts, our method reveals heterogeneous distributions of coordination states of Ni(II) that are sensitive to variations in temperature and melt composition.

X-ray scattering reveals ion clustering of dilute chromium species in molten chloride medium.

Enhancing the solar energy storage and power delivery afforded by emerging molten salt-based technologies requires a fundamental understanding of the complex interplay between structure and dynamics of the ions in the high-temperature media. Here we report results from a comprehensive study integrating synchrotron X-ray scattering experiments, molecular dynamics simulations and rate theory concepts to investigate the behavior of dilute Cr metal ions in a molten KCl-MgCl salt. Our analysis of experimental results assisted by a hybrid transition state-Marcus theory model reveals unexpected clustering of chromium species leading to the formation of persistent octahedral Cr-Cr dimers in the high-temperature low Cr concentration melt.

Formation of three-dimensional bicontinuous structures via molten salt dealloying studied in real-time by in situ synchrotron X-ray nano-tomography.

Three-dimensional bicontinuous porous materials formed by dealloying contribute significantly to various applications including catalysis, sensor development and energy storage. This work studies a method of molten salt dealloying via real-time in situ synchrotron three-dimensional X-ray nano-tomography. Quantification of morphological parameters determined that long-range diffusion is the rate-determining step for the dealloying process.

Photoinduced Strong Metal-Support Interaction for Enhanced Catalysis.

Strong metal-support interaction (SMSI) construction is a pivotal strategy to afford thermally robust nanocatalysts in industrial catalysis, but thermally induced reactions (>300 °C) in specific gaseous atmospheres are generally required in traditional procedures. In this work, a photochemistry-driven methodology was demonstrated for SMSI construction under ambient conditions. Encapsulation of Pd nanoparticles with a TiO overlayer, the presence of Ti species, and suppression of CO adsorption were achieved upon UV irradiation.

Unraveling Local Structure of Molten Salts via X-ray Scattering, Raman Spectroscopy, and Molecular Dynamics.

In this work, we resolve a long-standing issue concerning the local structure of molten MgCl by employing a multimodal approach, including X-ray scattering and Raman spectroscopy, along with the theoretical modeling of the experimental spectra based on molecular dynamics (AIMD) simulations utilizing several density functional theory (DFT) methods. We demonstrate the reliability of AIMD simulations in achieving excellent agreement between the experimental and simulated spectra for MgCl and 50 mol % MgCl + 50 mol % KCl, and ZnCl, thus allowing structural insights not directly available from experiment alone. A thorough computational analysis using five DFT methods provides a convergent view that octahedrally coordinated magnesium in pure MgCl upon melting preferentially coordinates with five chloride anions to form distorted square pyramidal polyhedra that are connected via corners and to a lesser degree via edges.

Low-Cost Transformation of Biomass-Derived Carbon to High-Performing Nano-graphite via Low-Temperature Electrochemical Graphitization.

Graphite, an essential component of energy storage devices, is traditionally synthesized via an energy-intensive thermal process (Acheson process) at ∼3300 K. However, the battery performance of such graphite is abysmal under fast-charging conditions, which is deemed essential for the propulsion of electric vehicles to the next level. Herein, a low-temperature electrochemical transformation approach has been demonstrated to afford a highly crystalline nano-graphite with the capability of tuning interlayer spacing to enhance the lithium diffusion kinetics in molten salts at 850 °C.

Radiation-Assisted Formation of Metal Nanoparticles in Molten Salts.

Knowledge of structural and thermal properties of molten salts is crucial for understanding and predicting their stability in many applications such as thermal energy storage and nuclear energy systems. Probing the behavior of metal contaminants in molten salts is presently limited to either foreign ionic species or metal nanocrystals added to the melt. To bridge the gap between these two end states and follow the nucleation and growth of metal species in molten salt environment , we use synchrotron X-rays as both a source of solvated electrons for reducing Ni ions added to ZnCl melt and as an atomic-level probe for detecting formation of zerovalent Ni nanoparticles.

Structure and dynamics of the molten alkali-chloride salts from an X-ray, simulation, and rate theory perspective.

Molten salts are of great interest as alternative solvents, electrolytes, and heat transfer fluids in many emerging technologies. The macroscopic properties of molten salts are ultimately controlled by their structure and ion dynamics at the microscopic level and it is therefore vital to develop an understanding of these at the atomistic scale. Herein, we present high-energy X-ray scattering experiments combined with classical and ab initio molecular dynamics simulations to elucidate structural and dynamical correlations across the family of alkali-chlorides.

Revealing 3D Morphological and Chemical Evolution Mechanisms of Metals in Molten Salt by Multimodal Microscopy.

Growing interest in molten salts as effective high-temperature heat-transfer fluids for sustainable energy systems drives a critical need to fundamentally understand the interactions between metals and molten salts. This work utilizes the multimodal microscopy methods of synchrotron X-ray nanotomography and electron microscopy to investigate the 3D morphological and chemical evolution of two-model systems, pure nickel metal and Ni-20Cr binary alloy, in a representative molten salt (KCl-MgCl 50-50 mol %, 800 °C). In both systems, unexpected shell-like structures formed because of the presence of more noble tungsten, suggesting a potential route of using Ni-W alloys for enhanced molten-salt corrosion resistance.

Temperature Dependence of Short and Intermediate Range Order in Molten MgCl and Its Mixture with KCl.

This article addresses the non-Debye-Waller temperature behavior in the intermediate range order for molten MgCl and its mixtures with KCl from a theory, Molecular Dynamics, and experimental X-ray scattering perspective and puts these findings in the context of discussions and controversies extending at least four decades. We find that these liquids are defined by two structural motifs. The first motif is associated with chains of positive-negative charge alternation; the second motif, which results in a prepeak in the structure function (), is associated with the interaction of Mg and Cl ions that do not belong to the same charge alternation chain or aggregate.

Connections between the Speciation and Solubility of Ni(II) and Co(II) in Molten ZnCl.

Understanding the factors that control solubility and speciation of metal ions in molten salts is key for their successful use in molten salt reactors and electrorefining. Here, we employ X-ray and optical absorption spectroscopies and molecular dynamics simulations to investigate the coordination environment of Ni(II) in molten ZnCl, where it is poorly soluble, and contrast it with highly soluble Co(II) over a wide temperature range. In solid NiCl, the Ni ion is octahedrally coordinated, whereas the ZnCl host matrix favors tetrahedral coordination.

Elucidating Ionic Correlations Beyond Simple Charge Alternation in Molten MgCl-KCl Mixtures.

The development of technologies for nuclear reactors based on molten salts has seen a big resurgence. The success of thermodynamic models for these hinges in part on our ability to predict at the atomistic level the behavior of pure salts and their mixtures under a range of conditions. In this letter, we present high-energy X-ray scattering experiments and molecular dynamics simulations that describe the molten structure of mixtures of MgCl and KCl.

Enabling chloride salts for thermal energy storage: implications of salt purity.

Molten salts for use as heat transfer fluids in concentrated solar or nuclear power plants have experienced a resurgence over the past decade with a special focus on chloride-based salt mixtures, particularly for use in concentrating solar power and fast-spectrum nuclear reactors. Salt purification, specifically oxide removal, is required even for high purity commercial salts and can be achieved using many different methods. Carbochlorination, however, proves most effective according to thermodynamics and produces a gaseous byproduct easily removed from the salt.