Estimation of thermodynamic properties of the ternary molten salt system, LiF-NaF-BeF2, by the modified Peng-Robinson equation

The molten salt reactor (MSR), which is one of the generation IV reactors, can meet the demand of transmutation and breeding. The thermodynamic properties of the molten salt system like LiF-NaF-BeF₂ influence the design and construction of the fuel salt and coolant in the MSR for the new generation. In this paper, the equation of state of the ternary system 15%LiF-58%NaF-27%BeF₂, over the temperature range from 873.15 to 1 073.15 K at one atmosphere pressure, is described using a modified Peng-Robinson (PR) equation. The densities of the ternary system and its components are estimated by this equation directly, and compared with the experimental data. Based on the equation of state, the other thermodynamic properties such as the enthalpy, entropy and heat capacity at constant pressure are estimated by the residual function method and the fugacity coefficient method respectively. The densities calculated by PR equation are highly in agreement with the experimental data, and the enthalpy, entropy and heat capacity evaluated by the two different methods are consistent with each other. It can be concluded that the modified PR equation can be applied to evaluate the density of the molten salt system, and it is recommended that it be used as the basis to estimate the enthalpy, entropy and heat capacity of the molten salt system.     

First‐principles based computational study on nucleation and growth mechanisms of U on Mo(110) surface solvated in an eutectic LiCl–KCl molten salt

We utilize first principles density functional theory (DFT) calculations and ab‐initio molecular dynamic (AIMD) simulations to identify underlying mechanisms elucidating the initial stage of electrocrystallization process of U on Mo(110) surface in a eutectic LiCl–KCl molten salt at T = 773 K. Our results clearly unveil surprisingly different principles on the nucleation of U in the media from that under vacuum conditions. U nanoclusters exposed to vacuum completely collapse into flat atomic layers on Mo(110) surface similar to an electrodeposition process. On the other hand, Cl ions in eutectic molten salt thermodynamically drive crystallite formation consisting of UCl_n (n = 3–6) through agglomeration of U atoms. Those crystallite gradually grows into bigger nuclei by adsorbing on Mo(110) surface. We propose that those behaviors are understandable only with revised conventional theories and that atomic level interactions among U, LiCl–KCl molten salt and Mo(110) surface play a key role to describe the atomic‐scale dendrite formation of U in the electrorefining process. Our study can be one of the basic steps to design efficient electrorefining systems by identifying the fundamental cause of the experimentally observed uranium nucleation phenomena. Copyright © 2016 John Wiley & Sons, Ltd.     

Molten salt strategy to synthesize alkali metal-doped Co9S8 nanoparticles embedded,N, S co-doped mesoporous carbon as hydrogen evolution electrocatalyst

A molten salt strategy was proposed to prepare a series of electrocatalysts for hydrogen evolution reaction (HER) with salts as templates, which consisted of Co₉S₈ nanoparticles (NPs) and N, S co-doped mesoporous carbons. The porosities, heteroatoms contents, and crystalline structures of the final electrocatalysts were determined by the types of salts and the calcining temperatures. When KCl/NaCl, KCl/LiCl, and NaCl/CaCl₂ were adopted as the molten salts, Co₉S₈ NPs embedded, N, S co-doped carbons were obtained. However, when CaCl₂ and ZnCl₂ were used as the molten salts, Co₉S₈ could not be synthesized. The characterization results exhibited that alkali metal atoms could be introduced in the lattices of Co₉S₈. The combination of experimental and theoretical results revealed that Na and K atoms doping improved the electrocatalytic performance for HER. GTCo900-KCl/NaCl possessed the best HER activity, delivering a current density of 10 mA cm⁻² at 54 mV in acidic media, 142 mV in neutral media, and 103 mV in alkaline media.     

Molten salt electrochemical production and in situ utilization of hydrogen for iron production

The low voltage electrochemical extraction of iron from iron oxide powder is reported. Hydrogen gas can be generated in molten LiCl at 660 °C by the electrochemical decomposition of steam at a voltage as low as only 0.97 V. Interestingly, individual submicrometer-sized iron oxide particles simply immersed in the melt can directly be reduced to semi-spherical metallic iron particles with a particle size of about 5 μm under the influence of hydrogen gas generated in-situ in the molten salt. The direct reduction of Fe₂O₃ powder, instead of sintered Fe₂O₃ pellets, which are commonly used as the cathode in the conventional molten salt electro-deoxidation processes, enhances the simplicity, energy consumption and the reaction kinetics of the molten salt hydrogen reduction process. In this method, the bottom surface of the graphite crucible acts as the cathode, resulting in a high rate of hydrogen cations discharge, hence a high current. The high temperature and low voltage characteristics of this method offer a scalable strategy for direct reduction of metal oxide particles in a green and low-cost way, where there is no requirement for the compaction of metal oxide powders into pellets. The mechanism involved in this hydrogen production and utilization methodology is discussed.     

Molten salt synthesis of high-entropy alloy AlCoCrFeNiV nanoparticles for the catalytic hydrogenation of p-nitrophenol by NaBH4

High-entropy alloy (HEA) AlCoCrFeNiV nanoparticles were prepared from oxide precursors using a molten salt synthesis method without an electrical supply. The oxide precursor was directly reduced by CaH₂ reducing agent in molten LiCl at 600°C-700°C or molten LiCl–CaCl₂ at 500°C-550°C. When the reduction was conducted at 700°C, a face-centered cubic (FCC) structure produced, as identified by X-ray diffraction analysis. With lower reduction temperatures, the FCC structure was absent, replaced by a body-centered cubic (BCC) structure. With a reduction temperature of 550°C, the resulting sample was composed of highly pure HEA AlCoCrFeNiV nanoparticles with a BCC structure of 15 nm. Analyses by scanning electron microscopy/transmission electron microscopy with energy-dispersive X-ray spectroscopy confirmed the formation of homogeneous HEA AlCoCrFeNiV with a nanoscale morphology. In the hydrogenation reaction of p-nitrophenol by NaBH₄, the AlCoCrFeNiV nanoparticles (produced at 550°C) exhibited a catalytic activity with ～90% conversion and 16 kJ/mol activation energy.     

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.     

Visualization of electrolyte volatile phenomenon in DIR-MCFC

Volatilization of molten salt is one of the factors that control the performance of molten carbonate fuel cells (MCFC). Volatilization of molten salt promotes cross-leakage and the corrosion of metallic components. Moreover, piping blockage is caused by the solidification of volatile matter. Because reforming catalysts filling the anode channel are polluted by molten salt volatile matter in direct internal reforming molten carbonate fuel cells (DIR-MCFC), the volatilization of molten salt is an especially serious subject. However, neither the behaviour nor the volatilization volume of molten salt volatile matter has heretofore been elucidated on. Because molten salt volatile matter that has strong alkalinity cannot be supplied directly to an analyzer, its volatilization volume is small, and analytical accuracy is poor. Therefore, an attempt has been made to elucidate about the electrolyte volatile phenomenon in an MCFC by using a non-contact image measurement technique. A 16 cm² MCFC single cell frame has an observation window and an irradiation window. The image of the volatile phenomenon is shown by irradiating a YAG laser light sheet 2 mm thick from an irradiation window into the anode channel, and taking measurements from an observation window with a high spatial resolution video camera (12 bit). As a result, though the volatile matter is not observed in an anode channel at OCV, the volatile matter flows in a belt-like manner from the inlet side near the electrode toward the outlet at a current density of 150 mA cm⁻². In addition, volatile matter is difficult to observe with the conventional thickness of an anode electrode. Because the composition of these volatile matters is 15Li₂CO₃/85K₂CO₃ (the result of conversion into molten salt) by ion chromatography analysis, it is not an electrolyte (62Li₂CO₃/38K₂CO₃) but rather the volatile matter of potassium, such as KOH. Therefore, it is understood that the volatile matter K₂CO₃ is generated as KOH, comprising the water generated by the cellular reaction with an electrolyte, in a reaction with CO₂. Conversely, the volatile matter flows to the surface of the cathode electrode without regard to changes in current density. In addition, the volatile matter exists on the electrode surface, although it decreases more or less with the conventional thickness of the cathode electrode. Therefore, it is understood that the volatile matter, in order for it not to be related to the cellular reaction, does not comprise the dispersion of molten salt.     

Synthesis of porous nitrogen and sulfur co-doped carbon beehive in a high-melting-point molten salt medium for improved catalytic activity toward oxygen reduction reaction

The development of unique, reliable and scalable synthesis strategies for producing dual-heteroatom-doped nanostructured carbon materials with improved activity toward electrochemical oxygen reduction reaction (ORR) presents an intriguing technological challenge in the field of catalysis. Herein, we report a method to synthesize a three-dimensional (3D) N and S Co-doped carbon beehive (NS-CB) with open structure by direct pyrolysis of egg white in a high-melting-point molten salt medium, e.g. NaCl/KCl, under inert atmosphere. Physicochemical characterization shows that NS-CB possess hierarchical pores (including micro- and mesopore) with a high specific surface area of 1478 m² g^?1, which is obviously larger than as-prepared carbons synthesized in only NaCl (CNaCl) or KCl (CKCl) as molten salt medium. Importantly, 50% of the pore volume is contributed by micropores with average pore size of 1.4 nm, which is the ideal pore size for ORR. The remaining 50% of the pore volume is made of mesopores and open macropores, assembled in the form of interconnected carbon sheets. Due to its hierarchical structure and high specific surface area, NS-CB shows high ORR activity comparable to commercial Pt/C catalyst in KOH electrolyte in terms of the half wave potential and the onset potential of ORR. NS-CB also exhibits markedly high stability as an ORR catalyst.     

Redox reaction in 1-ethyl-3-methylimidazolium–iron chlorides molten salt system for battery application

The redox reaction between divalent and trivalent iron species in binary and ternary molten salt systems consisting of 1-ethyl-3-methylimidazolium chloride (EMICl) with iron chlorides, FeCl₂ and FeCl₃, was investigated as a candidate of the half-cell reactions of novel rechargeable redox batteries based on low temperature molten salt systems. A reversible one-electron redox reaction between divalent and trivalent iron species was observed on a platinum electrode in EMICl–FeCl₂–FeCl₃ system at 130 °C. The voltammetric data indicated that divalent and trivalent iron species were FeCl₃⁻ and FeCl₄⁻ complex anions, respectively. Combined with another proper redox couple, this molten salt system can be utilized in a rechargeable redox battery on account of its low melting temperature and reversible redox reaction.     

Molten salt-promoted Ni–Fe/Al2O3 catalyst for methane decomposition

Bimetallic Ni–Fe/Al₂O₃ catalysts were prepared by the molten salt method, and the catalytic performance of the Ni–Fe/Al₂O₃ catalysts with the KCl–NiCl₂ melt for methane decomposition was evaluated at 800 °C. The catalysts and carbon products were characterized by XRD, SEM/EDS, XRF and Raman spectroscopy techniques. The results show that molten salt-promoted Ni–Fe/Al₂O₃ catalysts exhibit high activity and long-term stability up to 1000 min time on stream without any deactivation. The carbon products over the molten salt-promoted Ni–Fe/Al₂O₃ catalysts are in the form of small granular particles instead of filamentous carbon for the catalyst without molten salt. The promotional effect of the molten salt may attribute to the higher wettability of the Fe–Ni alloy by molten salt, which can prevent the catalysts from deactivation due to carbon encapsulation.     

The roles of anion and solvent transport during the redox switching process at a poly(butyl viologen) film studied by an EQCM

In this study, three electrolytes (KCl, LiCl, and KNO₃, each at 0.5 M in aqueous solution) were chosen to study the ion and solvent effect on the redox performance of poly(butyl viologen) (PBV) thin-films between its di-cation and radical-cation state, which is referred as its first redox couple. Before considering the role of ionic transport on the redox process, the exchange between ferrocyanide and anion should be completed. Since the deposition solution of PBV contains potassium ferrocyanide, the residual ferrocyanides inside the films would be exchanged by smaller anions from the bulk solution during the redox reaction of PBV. From cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM) results, the exchange was almost complete around 50 cycles when scanning the potential within its first redox range. After completion of the exchange process, the transfer would reach a steady state. At 50 cycles, the EQCM results suggested that the transport involves anions and water only for both being extracted upon reduction and being inserted upon oxidation. Therefore, we could obtain the molar fluxes of Cl⁻, NO₃⁻, and water. Besides, the average numbers of accompanying water were calculated to be about 24.8 per Cl⁻ and 14.2 per NO₃⁻ upon redox switching process. The instantaneous water to anion molar ratios at any potential were also obtained for Cl⁻ and NO₃⁻.     

Enhancing photoelectrical performance of dye-sensitized solar cell by doping with europium-doped yttria rare-earth oxide

The doped rare-earth oxide europium-doped yttria (Y₂O₃:Eu³⁺) is introduced into the TiO₂ film electrode in a dye-sensitized solar cell. As a luminescence medium, Y₂O₃:Eu³⁺ improves the light harvesting via a conversion luminescence process and increases the photocurrent, as a p-type dopant, Y₂O₃:Eu³⁺ elevates the energy level of the oxide film and increases the photovoltage. When the TiO₂ electrode is doped by 3 wt.% Y₂O₃:Eu³⁺, the cell light-to-electric conversion efficiency is improved by a factor of 1.14 compared to that of a cell without Y₂O₃:Eu³⁺ doping.     

Multi‐scale computational study of the molten salt based recycling of spent nuclear fuels

By applying a rigorous computational procedure combining first principles density functional theory (DFT) calculations and statistical mechanics, we acquire thermochemical properties of materials for a pyroprocessing system recycling spent nuclear fuels. Cluster expansions to DFT obtained energies parameterize atomic interaction potentials of Cl‐Cl and Cl‐U adsorbed on W(110) surface from a molten salt (KCL‐LiCl). Using these databases of the long‐range and multibody interactions, Monte Carlo simulations identify thermodynamically stable configurations of the adsorbates on the W(110) surface in grand canonical open system at T = 773 K. Our results indicate that Cl atoms adsorbed at the interface of the molten salt and W(110) surface substantially drive electrochemical deposition of U ions at low chemical potential of Cl. This behavior, however, stops after approximately 1/3 ML coverage of U because the atomic sites on W(110) surface are mostly blocked by adsorbed Cl, which implies that the attractive interactions of Cl‐W are stronger than Cl‐U as well as the repulsive interactions between U atoms are effective at these coverage ranges. We also predict the solubility limit of U ion in the molten LiCl‐KCl phases at T = 773 K should be about 5 atomic percent, which well agrees with previous reports by experimental measurements. This study indicates that accurate characterization of the stable Cl structure at the interface is vital for understanding the fundamental mechanisms of recycling spent nuclear fuels and for screening high functional electrode materials in the pyroprocessing system. Copyright © 2014 John Wiley & Sons, Ltd.     

Coal gasification by CO2 gas bubbling in molten salt for solar/fossil energy hybridization

Coal gasification with CO₂ (the Boudouard reaction: C+CO₂=2CO, Δ_rH°=169.2 kJ/mol at 1150 K), which can be applied to a solar thermochemical process to convert concentrated solar heat into chemical energy, was conducted in the molten salt medium (eutectic mixture of Na₂CO₃ and K₂CO₃, weight ratio=1/1) to provide thermal storage. When CO₂ gas was bubbled through the molten salt, higher reaction rates were observed compared to the case without CO₂ gas bubbling (CO₂ gas was streamed over the surface of the molten salt). Thus the coke formed by coal pyrolysis was well suspended in the molten salt by CO₂ gas bubbling. When the CO₂ flow rate was increased from 15 to 60 μmol/s, the CO evolution rate was increased (15 to 26 μmol/s). However, CO₂ conversion efficiency was decreased (50 to 22%). Based on the maximum CO evolution rate (26 μmol/s), solar thermal energy from a solar farm (300×300 m²) could be converted to chemical energy at a rate of 50,000 kJ/s by the coal (23 ton as C) gasification process studied here. This assumes 50% solar heat to chemical energy conversion efficiency which can be generally obtained by the actual solar experiments.     

Thermodynamic calculation and reference electrode calibration for high temperature molten salts

Abstract

Molten salts are important reaction media for chemical and electrochemical processing and have recently attracted attention for their potential in reprocessing and partitioning spent nuclear fuels. Electrochemical measurements are a convenient tool for exploring thermodynamic and kinetic properties of molten salts, but inconsistency in acquired data may arise from the use of inaccurate reference electrodes and differences in thermodynamic calculations. A thermodynamic approach to the calculation of half cell potentials for reactions in molten salts is proposed. As examples, chlorine/chloride and lithium ion/lithium half cell potentials in LiCl–KCl eutectic are thermodynamically analysed. The Ag/AgCl reference electrode is discussed as an example of a high temperature reference electrode. A technique involving in situ transient reduction of constitutive metal ions for the calibration of high temperature reference electrodes is developed which may enable the consistency of acquired data using different reference electrodes in a variety of molten salts. The thermodynamic approach and calibration technique may be extended to ionic liquid and other media at high and low temperatures.     

Synthesis,characterization and electrochemical properties of La2-xEuxNiO4+δ Ruddlesden-Popper-type layered nickelates as cathode materials for SOFC applications

Eu doped La₂NiO₄ powders, with the general formula La_2-xEu_xNiO_4+δ denoted as LENO_x (for x = 0, 0.2, 0.4, 0.6 and 0.8), were synthesized via the mechanical milling reaction method. The Eu³⁺ doping content has a remarkable influence on structural and electrochemical properties. The phase identification and morphology were studied by X-ray diffraction (XRD), Raman spectroscopy, Infrared spectroscopy (IR), A laser size analyzer and scanning electron microscopy (SEM). Lattice parameters were calculated using the Rietveld method. It was observed that the lattice parameter values in LENO_x systems varied with the amount of Eu³⁺. The latter was symmetrically deposited by spin coating on both surfaces of an Ce_0.8Sm_0.2O_1.9 (SDC) electrolyte and studied using AC impedance spectroscopy. The electrochemical properties were studied using two-probe impedance spectroscopy and results showed that the ASR of LENO_x was enhanced by the Eu³⁺ dopant content x. Results also showed that LNEO_0.2 had the lowest Area specific resistance (ASR) at 700 °C and it was therefore concluded that doping with the appropriate amount of Eu³⁺ can further improve the properties of a nickelate cathode.     

Achieving high performance for aluminum stabilized Li7La3Zr2O12 solid electrolytes for all solid‐state Li‐ion batteries: A thermodynamic point of view

For the solid‐state reaction synthesis of Al containing Li₇La₃Zr₂O₁₂, various precursors have been used. Since there is a lack of general agreement for choosing precursors, a quantitative approach to build a consensus is required. In this study, a thermodynamic point of view for selecting the precursors in the field of Li₇La₃Zr₂O₁₂ synthesis was covered according to the Gibbs free energy and enthalpy change of precursors' decomposition reactions. In terms of Gibbs free energy change calculations, LiOH, La(OH)₃, and Al(OH)₃ were favorable whereas, LiOH, La₂O₃, and Al(OH)₃ were the preferred precursors for the enthalpy change calculations. Pellets prepared by using the favored precursors calculated from enthalpy change showed improved densification, higher ionic conductivity (2.11 × 10^?4 S/cm), and lower activation energy (0.23 eV) compared with Gibbs free energy change. As a thermodynamically favored aluminum precursor, Al(OH)₃ was discussed in the present study and hinders the ionic conductivity in comparison to Al₂O₃.     

Particle size influence of starting batches on phosphorescence behaviour of Sr4Al14O25 based bluish green phosphors

Abstract

Photoluminescent material with long afterglow is a kind of energy storage material that can absorb both ultraviolet (UV) and visible lights from sunlight, and gradually releases the energy in the dark at a certain wavelength. These sorts of materials have great potential for various device applications and have been widely studied by many researchers. In recent years, it has also been reported that 2SrO.3Al₂O₃/Eu²⁺ and 4SrO.7Al₂O₃/Eu²⁺ phosphors as green and blue emitters have even higher quantum efficiencies. To determine the initial particle size effect on the phosphorescence behaviour, Eu²⁺/Dy³⁺ doped Sr₄Al₁₄O₂₅ phosphors were synthesised by mixing 4SrO and 7Al₂O₃ with a flux (H₃BO₃) through high temperature solid state reaction method under weak reducing atmosphere. Such an influence on the crystalline structure and emission colour of phosphorescent pigments was studied by means of X-ray diffraction, scanning electron microscope (SEM), particle size analysis, excitation and emission spectroscopy. The results showed that the emission wavelength of the phosphorescence pigments shifted from green to blue region due to the decrease in average particle sizes of the phosphor batches, forming different types of strontium aluminate crystals.     

Thermophysical properties of copper compounds in copper–chlorine thermochemical water splitting cycles

This paper examines the relevant thermophysical properties of compounds of copper that are used in thermochemical water splitting cycles. There are four variants of such Cu–Cl cycles that use heat and electricity to split the water molecule and produce H₂ and O₂. Since the energy input is mainly in the form of thermal energy, the Cu–Cl water splitting cycle is more efficient than water electrolysis, if the electricity generation efficiency for electrolysis is taken into account. Various chemicals are recycled within the plant, while the overall effect is splitting of the water molecule. The system includes several reactors, heat exchangers, a spray dryer, and an electrochemical cell. This paper identifies the available experimental data for properties of copper compounds relevant to the Cu–Cl cycle analysis and design (Cu₂OCl₂, CuO, CuCl₂, CuCl). It also develops new regression formulae to correlate the properties, which include: specific heat, enthalpy, entropy, Gibbs free energy, density, formation enthalpy and free energy. No past literature data are available for the viscosity and thermal conductivity of molten CuCl, so estimates are provided. The properties are evaluated at 1 bar and a range of temperatures from ambient to 675–1000 K, which are consistent with the operating conditions of the cycle. Updated calculations of chemical exergies are provided as follows: 21.08, 6.268, 82.474, and 75.0 kJ/mol for Cu₂OCl₂, CuO, CuCl₂ and CuCl, respectively. For molten CuCl, the estimated viscosity varies from 1.7 to 2.6 mPa s for the envisaged range of temperatures. A Riedel-like equation is proposed to correlate the vapor pressures with the temperature for molten CuCl.     

Highly selective and efficient ammonia synthesis from N2 and H2O via an iron-based electrolytic-chemical cycle

Ammonia production via electroreduction of N₂ and water under mild conditions is emerging as a promising alternative to the fossil fuels-reliance and CO₂ emitting Haber-Bosch process. However, the achievement of high Faradaic efficiency and high ammonia formation rate is still challenging. Here, we demonstrate how ammonia can be selectively produced from N₂ and H₂O via a two-step iron-based cyclic process using a molten hydroxide electrolyte. The first step is the production of Fe by electrochemical reduction of Fe₂O₃. The second step is the steam-hydrolysis of Fe with bubbling N₂ to produce NH₃ and reform Fe₂O₃. Both reaction steps proceed isothermally at 250 °C in a molten salt electrolytic cell without switching of temperature and needing separation of the mediator, resulting in more easily putting into industrial practice. The cycle achieves an ultrahigh Faradaic efficiency of 79.8% at 1.15 V and a high ammonia formation rate of 1.34 × 10⁻⁸ mol s⁻¹ cm⁻² at 1.75 V. This is a critical advance in breaking the domination of hydrogen evolution reaction (HER) competition to achieve highly selective and efficient NH₃ synthesis from N₂ and H₂O beyond reliance of fossil fuels.