Molten Carbonate Salt Research Articles

Oxidative Dissolution of Tungsten Metal in Na2CO3 under Ar–O2–CO2 Atmosphere

A novel process for the recycling of tungsten from hard tool scraps using carbonate molten salts is proposed. As a fundamental study, the oxidative dissolution of tungsten metal into molten Na2CO3 under Ar–O2 and Ar–O2–CO2 atmospheres was investigated at 1173 K. The results of the oxidative dissolution experiments and the detection of CO gas evolution by infrared spectroscopy revealed the existence of multiple oxidation mechanisms by peroxide/superoxide ions and carbonate ions. The feasibility of each oxidant was discussed by calculating potential– diagrams in molten Na2CO3 at 1173 K. The relationship between the potential and the reaction was investigated using cyclic voltammetry and potential– diagrams.

Electrochemical graphitization conversion of CO2 through soluble NaVO3 homogeneous catalyst in carbonate molten salt

Abstracts Electrochemical conversion of CO2 to carbon in molten salts is a promising route for the high-value utilization of greenhouse gases. Low graphitization degree of carbon materials is the limiting issue for potential applications. In the past, catalytic effect for the electrochemical conversion of CO2 was not achieved in molten salts due to carbon coverage on heterogeneous solid electrode. A novel and feasible homogeneous catalytic strategy by soluble NaVO3 in carbonate molten salt was proposed for the enhanced electrochemical conversion of CO2 to carbon with high graphitization degree. It was found that graphite consisting of carbon nanotubes and sheets were more easily produced by adding NaVO3 into molten salt. The graphitization degree of carbon products was obviously improved and ID/IG value was only 0.428. During CO2 electrolysis, vanadate in the form of V2O74− will be co-electroreduced to V2O3. Meanwhile, V2O74− ions can be regenerated by the spontaneous reaction between the newly-formed V2O3 and carbonate, which also accelerates the production of graphite. Therefore, a continuous homogeneous in-situ catalytic cycle was achieved for the enhanced graphitization conversion of CO2. It provides a novel and effective way to prepare carbon materials with high graphitization degree from CO2.

Carbonate molten salt solar thermal pilot facility: Plant design, commissioning and operation up to 700 °C

Concentrated solar power (CSP) is not currently cost competitive with conventional power generation or other solar energy technologies, but it is attractive because it integrates commercially viable large-scale thermal energy storage (TES). To improve performance and reduce costs, supercritical carbon dioxide (s-CO2) power cycles have been proposed to increase the thermal-to-electric conversion to values above 50%. However, to achieve this target, the solar energy collected by the receiver and stored in the TES must be delivered to the power turbine at a temperature at or above 700 °C, while current advanced plants only provide thermal energy at 565 °C. For the first time, a high temperature (700 °C) molten salt experimental pilot plant for CSP was designed, built, and tested by Abengoa using carbonate salts as the heat transfer fluid. We demonstrate the feasibility of large-scale, industrially relevant operation between 400 and 700 °C, thus providing a solution to integrate higher efficiency s-CO2 power cycles. We address the key points in material compatibility, component design, instrumentation and system integration. Operational aspects such as preheating and filling procedures, start-up, maximum temperature reached, and the main challenges faced are described in detail. The heat losses in the loop and components are analyzed. We show that with appropriate electrical heat tracing elements, operation with carbonates is feasible and that the main subsystems that must be redesigned are the high temperature tanks and pumps. Lessons learned from the plant operation are summarized here to guide the design of future commercial scale high temperature molten salt plants.

ExxonMobil invests in fuel-cell carbon capture

ExxonMobil and FuelCell Energy have formed a pact worth up to $60 million to optimize carbonate fuel cells to capture carbon dioxide. FuelCell Energy uses a molten carbonate salt as the electrolyte in fuel cells that convert fuel into electricity. The two firms have worked together since 2016 to test running power plant exhaust rather than air through the fuel cells. While the cells produce electricity, their membranes also concentrate CO2, doing so more efficiently than the amine solvents often used to capture CO2, ExxonMobil says. The oil company is also working with Mosaic Materials on metal-organic frameworks that capture CO2 (see page 38).

Structure and diffusion of molten alkali carbonate salts at the liquid-vacuum interface

The liquid-vacuum interface of molten alkali carbonate salts is studied with molecular dynamics simulations. Three salts comprised of LixNayKzCO3 near their respective eutectic concentrations are considered to understand the distribution of ions relative to a liquid-vacuum interface and their diffusivity. These simulations show that each of the cations accumulate at the interface preferentially compared to carbonate. The cation ordering is found to inversely correspond to cation radius, with K being the most likely occupant at the surface, followed by Na, Li, and then the anion. Similar to other studies, the carbonate is found to diffuse more slowly than the cations, but we do observe small differences in diffusion between compositions that present opportunities to optimize ion transport. These results hold consequences for our understanding of ion behavior in molten carbonate salts and the performance of devices employ these electrolytes.

On the effect of cold-rolling on the corrosion of SS316L alloy in a molten carbonate salt

Interactions between stainless steel 316L and eutectic Li2CO3 + K2CO3 + Na2CO3 at 450 °C were investigated for thermal energy storage. Scanning Electron Microscopy (SEM), Electron Back-Scatter Diffraction (EBSD), Scanning Kelvin Probe Force Microscopy (SKPFM), X-ray Photoelectron Spectroscopy (XPS), neutron diffraction pattern, material loss, micro-hardness, polarization and impedance measurements were used to compare the alloy's response in unrolled (0%) condition versus 20% and 30% cold-rolled conditions. Cold-rolling increased the number of grains, grain boundaries and density of dislocations. Initially, faster corrosion accompanied by more areas of localized attack was confirmed by Volta potential measurements. However, recovery and the formation of a surface film were found to decelerate corrosion for longer times. Cold-rolling-induced dislocations were found to facilitate carbon diffusion and subsequently carburize the material leading to increased corrosion resistance. Consequently, the overall long-time corrosion rate was not noticeably affected by cold-rolling.

Exploration of alkali cation variation on the synthesis of carbon nanotubes by electrolysis of CO2 in molten carbonates

The electrochemical splitting of CO2 in molten carbonate salt electrolytes can form a valuable product, carbon nanotubes, and with this product a value-incentivized pathway of climate change mitigation by transforming CO2 from a greenhouse gas pollutant into a useful resource. Instability of the anode can provide a challenge for a commercialization of this electrochemical technology. A nickel superalloy (Inconel 718) anode is shown to be highly stable for the scalable controllable, electrosynthesis of nano-carbons from CO2 in molten carbonates salts. Unlike pure nickel anodes that degrade (corrode) in the presence of potassium carbonate containing molten electrolytes, Inconel is stable as an anode even in these electrolytes. This opens up more cost-effective electrolytes (less expensive than lithium carbonate), and the systematic variation of alkali cation carbonates is explored. Coupled with a brass cathode, uniform carbon nanotubes (CNT) are efficiently synthesized by electrolysis of CO2 in either pure Li2CO3 or in ternary electrolytes, Li2CO3-Na2CO3-LiBO2 or Li2CO3-K2CO3-LiBO2. These results increase the foundation of understanding of the electrolytic splitting of carbon dioxide and provide the data for developing a high-efficiency CO2 electroreduction for the production of carbon nanotubes.

The effects of temperature and molten salt on solar pyrolysis of lignite

Molten salt pyrolysis driven by concentrated solar radiation is well positioned to utilize solar energy and lignite effectively. This study focused on the effects of temperature (500, 600, 700 and 800 °C) and molten carbonate salt (Li2CO3-Na2CO3-K2CO3) on properties of char obtained from lignite pyrolysis, as well as gas and tar products for revealing their formation mechanism and transformation process. Molten salt pyrolysis of HulunBuir lignite produced more gas products and less char compared to conventional pyrolysis owing to the enhanced heat transfer and catalytic effect of molten salt. The char yield decreased from 58.4% to 43.4%, and the gas yield (especially CO2, H2 and CO) increased from 28.3% to 46.1% at 800 °C. CO2, CO and H2 production increased about 60.43%, 103.42% and 65.2% at 800 °C, respectively. Additionally, the presence of molten salt improved the tar quality with more hydrocarbon content (maximum increase of 5.8%) and less oxygenated compounds. The structure and reactivity relationship of char was characterized by XRD, BET, SEM, FTIR, Raman spectroscopy and TGA. Molten salt generated char had a higher reactivity due to the increase of disorder, surface area, microporosity (maximum of 71.74%) and active sites.

Corrosion evaluation of alumina-forming alloys in carbonate molten salt for CSP plants

The use of carbonate molten salts for the new generation of concentrated solar power (CSP) plants have been considered and analysed during the last years to improve the efficiency of energy generation. However, the high temperature and corrosivity of the salts pose a risk on safety and profitability of the technology requiring more resistant materials. In this study, two alumina-forming austenitic (AFA) alloys corresponding to modified OC4 and HR224 grades, were exposed to the eutectic ternary Li2CO3K2CO3Na2CO3 (32.1–34.5–33.4 wt%) salt mixture at 650 °C for 1000 h. The evolution of weight change along the exposure time and analysis of the resulting scales by means of scanning electron microscopy (SEM), X-Ray diffraction (XRD), and glow discharge optical emission spectroscopy (GDOES) revealed a good performance of both steel grades associated with the formation of multi-layered corrosion products. Whereas both alloys undergo external oxidation with the formation of NiO, internal oxidation with the formation of two spinels is the case of HR224. The rate of corrosion is significantly lower than those reported in molten carbonated in non-AFA alloys, allowing to recommend OC4 and HR224 for use in the carbonate-based CSP.

Intermediate temperature electrochemical properties of lutetium-doped SrCeO3/SrZrO3-molten carbonate composite electrolyte

A SrCe0.6Zr0.3Lu0.1O3-α electrolyte was synthesized via a citric-nitrate auto-combustion (CNA) method using Sr(NO3)2, Zr(NO3)4·5H2O, (NH4)2Ce(NO3)6, Lu2O3, HNO3 and citric acid as raw materials. Subsequently, it reacted with the molten carbonate salt to obtain the SrCe0.6Zr0.3Lu0.1O3-α-Li2CO3-Na2CO3 composite electrolyte. The morphology, structure, conductivity and fuel cell performance of the obtained samples were investigated. SrCe0.6Zr0.3Lu0.1O3-α is fully dense after calcinated at 1540 °C for 5 h SrCe0.6Zr0.3Lu0.1O3-α and Li2CO3-Na2CO3 combined with each other to form a homogeneous interconnected structure. The highest power density and conductivity of SrCe0.6Zr0.3Lu0.1O3-α-Li2CO3-Na2CO3 are 255 mW cm−2 and 8.6 × 10−2 S cm−1 at 600 °C, respectively.

Study on the compressed ion layer and the specific heat of the phase change materials doping charged single-walled carbon nanotubes

In this paper, the thermal effect of charged single-walled carbon nanotubes (SWCNT) exerting with the molten carbonate salt phase change materials are investigated through molecular dynamics (MD) simulation. The results show that the surface charge of the SWCNT can affect the compressed ion layer surrounding the SWCNT, and in turn affect the specific heat (Cp) enhancement for the nanoparticle-enhanced phase change materials (NPCM). First, the ions inside the compressed ion layer are found to be redistributed and Li+, K+ and CO32- are all denser than the bulk. Moreover, the absorption of the ions inside the compressed ion layer can be significantly affected by changing the surface negative charge. The SWCNT surface charge can in turn stimulates enrichment of positive and negative charges, and in turn results in greater pairwise energy. As a consequence, the enrichment of positive and negative charges is supposed to be the main reason for the enhancement on Cp of the NPCM. The specific heat capacity of the NPCM are investigated and the results show that Cp is enhanced by adding the charged SWCNT. It is shown that SWCNT carrying negative charges possesses the best ability to enhance Cp of carbonate salts and the enhancement can be up to 19.16%. The conclusions provide a theoretical basis for the application of charged nanoparticles in enhancing the thermal property of PCM.

Physical characteristics of capacitive carbons derived from the electrolytic reduction of alkali metal carbonate molten salts.

Carbons have been synthesized through the reduction of molten carbonate systems under varied conditions. The mechanism and kinetics of carbon electrodeposition has been investigated. Carbon morphologies include amorphous, graphite-like, and spherical aggregate phases. Increased graphitic character is observed in carbons electrodeposited at more cathodic potentials, particularly at higher temperatures. Bonding has been investigated and oxygen functionalised sp2 and sp3 structures have been identified. The level of functionalization decreases in carbons with reduced amorphous and increased graphitic character. Thermal decomposition of electrodepositied carbons has been investigated and zero order kinetics have been identified. A relationship has been identified between elevated oxygen functionalization and increased pseudo-capacitance, with carbons deposited at 0.15 A cm−2 showing capacitances of 400 F g−1 in 0.5 M H2SO4 at sweep rates of 10 mV s−1.

Molecular dynamics simulations of the thermodynamic properties and local structures on molten alkali carbonate Na2CO3

Molten carbonate salts as phase change materials have received particular attention for high-temperature thermal energy storage and heat transfer applications due to desirable thermal characteristics such as wide operating temperature range, low causticity and excellent thermal stability. In this study, molecular dynamics (MD) simulations were performed on molten alkali carbonate Na2CO3 based on an effective pair potential model, a Born-Mayer type combined with a Coulomb term. The temperature dependences of thermodynamic properties including the density, thermal expansion coefficient, specific heat capacity, shear viscosity, thermal conductivity and ion self-diffusion coefficient were simulated in detail from 1150 to 1500 K, which was all difficult to achieve from experiments on account of high-temperature extreme conditions. The simulation results were in satisfactory agreement with experimental data available with high calculation accuracies. In particular, both the equilibrium molecular dynamics (EMD) and non-equilibrium molecular dynamics (NEMD) simulations were tried to calculate the shear viscosity and thermal conductivity, and the NEMD method turned out to be more suitable for molten alkali carbonate salts with higher calculation accuracies of 15.11% and 22.48% for shear viscosity and thermal conductivity separately. Moreover, the radial distribution functions (RDF) and coordination number curves of the molten salt were characterized to explore the temperature dependences of macroscopic properties from microscopic view, and the results suggested that the changes of thermodynamic properties with temperature were induced by the distance changes between Na2CO3 particles. Besides, it could be concluded that the structure of CO32− was inferred reasonably to be ortho-triangular pyramid from the comprehensive analysis of local structures including the angular distribution functions (ADF).

Microplasma Anode Meeting Molten Salt Electrochemistry: Charge Transfer and Atomic Emission Spectral Analysis.

Molten salt electrolysis is normally conducted with solid anode, such as noble metal or graphite, which has defects such as high cost or emission of carbon oxide. Herein, we report that a microplasma based on atmospheric-pressure glow discharge could act as a kind of gaseous anode for electrolysis in molten salt. When the Ag/Ag+ redox couple was chosen as the research object, the microplasma anode could initiate charge-transfer reactions in the molten salt and Ag could be electrodeposited with current efficiency of above 90%. The microplasma anode has also shown excellent anticorrosive performance in both chloride and carbonate molten salt. Furthermore, the microplasma anode could potentially serve as an excitation source of atomic emission spectrometry (AES), making it possible to determine the concentration of Ag ions in the molten salt in situ and in real-time. With properties such as being carbon-free and having corrosion resistance and extensive utilization for analysis, the microplasma anode has opened a new direction for molten salt electrochemistry.

Anomalous Increase in Specific Heat of Binary Molten Salt-Based Graphite Nanofluids for Thermal Energy Storage

An anomalous increase of the specific heat was experimentally observed in molten salt nanofluids using a differential scanning calorimeter. Binary carbonate molten salt mixtures were used as a base fluid, and the base salts were doped with graphite nanoparticles. Specific heat measurements of the nanofluids were performed to examine the effects of the composition of two salts consisting of the base fluid. In addition, the effect of the nanoparticle concentration was investigated as the concentration of the graphite nanoparticles was varied from 0.025 to 1.0 wt %. Moreover, the dispersion homogeneity of the nanoparticles was explored by increasing amount of surfactant in the synthesis process of the molten salt nanofluids. The results showed that the specific heat of the nanofluid was enhanced by more than 30% in the liquid phase and by more than 36% in the solid phase at a nanoparticle concentration of 1 wt %. It was also observed that the concentration and the dispersion homogeneity of nanoparticles favorably affected the specific heat enhancement of the molten salt nanofluids. The dispersion status of graphite nanoparticles into the salt mixtures was visualized via scanning electron microscopy. The experimental results were explained according to the nanoparticle-induced compressed liquid layer structure of the molten salts.

Carbon Spaghetti from the Reduction of Molten Carbonate Salts

The synthesis of new forms of carbon is of great use in the production of many new materials and comforts of modern life. The synthesis of carbon has previously been achieved through many paths, however a path that has been given some major interest in recent years as a medium temperature method of producing carbon products with interesting physical and electrochemical characteristics is the reduction of molten carbonate salts to form value-added carbon products [1]. Of these products, it has recently been noted that carbon nanofibers may form during the reduction of molten Li2CO3 in the presence of trace elements of transitional metals [2]. If carbon nanotubes can be formed through the reduction of molten carbonates it would be of great benefit for many fields, embodying a cheap, low energy cost method of producing these materials. This research investigates the formation of carbon tube structures from molten Li2CO3-K2CO3-Na2CO3 eutectic at 600°C-700°C using copper substrates (Figure 1). Findings suggest that optimal conditions for nanotube synthesis involve nucleation and growth steps, which occur concurrently with the formation of further carbon phases. The formed fibers are hollow tubes which may be capped unless broken, with two major size distributions, thick tubes and thin tubes. The thin tubes have diameters in the order of 300-400 nm. [1] Hughes, M.A., J.A. Allen, and S.W. Donne, Carbonate Reduction and the Properties and Applications of Carbon Formed Through Electrochemical Deposition in Molten Carbonates: A Review. Electrochimica Acta, 2015. 176: p. 1511-1521. [2] Ren, J., et al., One-Pot Synthesis of Carbon Nanofibers from CO2. Nano Letters, 2015. 15(9): p. 6142-6148. Figure 1

The Inductive and Capacitive Characteristics of Carbonate-Derived Carbons

Due to the increasing modern dependence on electronic devices, the production of carbon materials for supercapacitive and battery-based applications is an area of current research and innovation. It has been known for a considerable time that carbons can be formed through the electrolytic reduction of molten carbonate salts [1], however, little research has been put into in-depth understanding of the electrochemical characteristics of these materials. Electrochemical impedance spectroscopy can tell us a lot about the suitability of these materials for supercapacitive applications. This research investigates how the characteristics of carbons derived through molten carbonate reduction change based on synthesis conditions, with a particular focus being placed on the inductive and capacitive behaviour of the materials and on how inductances arise (Figure 1 (i)). This is done through the application of equivalent circuit modelling (Figure 1 (ii)) to cells constructed from synthesised and activated carbons, and the subsequent mathematical modelling of the system using complex non-linear least squared error curve fitting (CNLS) based on the interior-point algorithm in the MATLAB program. The frequency-dependant capacitance of the materials (Figure 1 (iii)) is reported and used as a point of comparison. Optimal carbon synthesis conditions of 500°C or low deposition current densities of 0.15 A/cm2 have been found to both reduce inductive behaviour, and to produce carbons with capacitances as high as 160 F/g, close to 90% of that of the examined activated carbon. [1] Hughes, M.A., J.A. Allen, and S.W. Donne, Carbonate Reduction and the Properties and Applications of Carbon Formed Through Electrochemical Deposition in Molten Carbonates: A Review. Electrochimica Acta, 2015. 176: p. 1511-1521. Figure 1

Corrosion behavior of aluminide diffusion coatings in low temperature molten carbonate electrolysis environments

Interest on low‐temperature molten carbonate electrolysis processes (T < 600 °C) for advanced energy conversion and storage applications appears to be progressively growing in literature. However, the corrosion of metallic materials in molten carbonate salts in equilibrium with the acidic CO2‐rich electrolysis gas atmospheres has been scarcely studied. In this work, the corrosion stability of a low‐activity FeAl aluminide diffusion coated 316 L stainless steel has been tested in a ternary carbonate eutectic mixture Li2CO3‐Na2CO3‐K2CO3 by 48 h‐immersion tests at 500 °C under CO2 atmosphere. Before the corrosion testing, the coated steel samples were pre‐oxidized in air at 750 °C in order to generate a protective thin alumina film on the aluminide coating surface. The salt immersion results indicate that FeAl aluminide coatings are not stable in acid molten carbonate environments due to rapid transformation of the pre‐existing alumina film into a non‐protective mixed Fe‐Al oxide crystalline layer. Corrosion degradation has been found to consist of both general and localized forms of attack. Factors affecting FeAl aluminide coating corrosion have been analyzed and discussed in detail.

Corrosion resistance of Cr/Ni alloy to a molten carbonate salt at various temperatures for the next generation high-temperature CSP plants

The corrosion behaviour of a commercial HR3C steel in a molten eutectic ternary Li2CO3-Na2CO3-K2CO3 mixture was studied by isothermal immersion tests at 700 °C and 800 °C for 2000 h. It was found that in both cases the corrosion products were arranged in a multilayer structure, with LiFeO2, LiCrO2, NiO and FeCr2O4 as the main compounds. Similar results to those obtained at 700 °C were reached in the tests performed at 800 °C but 1000 h earlier, which seems to indicate that the process was accelerated at higher temperature. Furthermore, regarding the superficial crystalline phase formed and according to the XRD and SEM examinations, it was found that on increasing the test temperature, a continuous transformation occurred from the rhombohedral to the cubic phase of the LiFeO2 oxide layer. This transformation took place at both temperatures when the tests proceed and it was also accelerated by increasing the temperature from 700 to 800 °C. Thus, the process seems to be slower at 700 °C, which complicates the use of HR3C at 800 °C in the molten eutectic ternary Li2CO3-Na2CO3-K2CO3 salt mixture. These results suggest that the use of protective coatings should be assessed and that this material should be tested at 700 °C under more aggressive conditions, such as flowing salt, in future studies.

Molecular dynamics simulations of the local structures and thermodynamic properties on molten alkali carbonate K2CO3

Molten carbonate salts have received particular attention for high-temperature thermal energy storage and heat transfer applications due to desirable thermal characteristics, such as wide operating temperature range, low causticity and excellent thermal stability. In this study, molecular dynamics (MD) simulations were performed on molten alkali carbonate K2CO3 based on an effective pair potential model, a Born-Mayer type combined with a Coulomb term. The radial distribution functions (RDF) and coordination number curves of the molten salt were characterized to explore the temperature dependences of macroscopic properties from microscopic view. The results suggest that the distance between K2CO3 particles is getting larger with temperature increasing, resulting in the increase of molar volume and the diminished ability of resistance to shear deformation and heat transfer by vibration between ions. Besides, it can be concluded that the structure of CO32− is inferred reasonably to be ortho-triangular pyramid from the comprehensive analysis of local structures including the angular distribution functions (ADF). Moreover, the thermodynamic properties were simulated in detail from 1200 to 1600 K including the density, thermal expansion coefficient, specific heat capacity, sheer viscosity, thermal conductivity and ion self-diffusion coefficient, which was hard to be measured from experiments under high-temperature extreme conditions. All the simulation results are in satisfactory agreement with available experimental data with high accuracy, and the minimum simulation error is as low as 1.42%.