Molten Salt Electrolyte Research Articles

A high-performance graphite-graphite dual ion battery based on AlCl3/NaCl molten salts

A novel double graphite dual ion battery (DIB) based on low-temperature inorganic molten salt (AlCl3/NaCl) electrolytes is first investigated. As a result, it is found that chloroaluminate ions are intercalated/de-intercalated at the cathode and metallic aluminum are dissolved and deposited at the anode. In addition, the self-discharge phenomenon of the DIB is systematically studied and found that it has obvious self-charging phenomenon due to the adsorption of the charged ions when standing at low voltage. Due to low cohesion and the relatively high working temperature for the inorganic molten salt, the DIB exhibits unexpected electrochemical performance. The initial capacity is 183.8 mAh g−1 at 1 A g−1, and the capacity is 132 mAh g−1 even at 4 A g−1. After 700 cycles, the capacity retention rate is 79.5% and the coulombic efficiency increase from 85.1% to 93.9%. Given the advantages of cost-effectiveness and environmental friendliness, the DIB will play a decisive role in the future of aluminum-based batteries.

An ultra stable and efficient bifunctional cathode catalyst for iron molten air batteries

Iron molten air battery possesses cost-efficiency and high theoretical specific energy density merits, and is one of the promising battery systems for future application in energy storage. Although substantial efforts and significant progress have been made in recent years, it is still one of the major challenges to explore a stable and efficient bifunctional catalyst in high-temperature molten salt environments. Herein, the NiMnO3–La2O3 composite is synthesized on porous nickel foam substrates by a hydrothermal method and used as a cathode catalyst for the iron molten air battery. The results show that the NiMnO3–La2O3 composite has excellent stability and durability. After 250 cycles in a 500 °C molten salt electrolyte, the microscopic morphology of the catalyst is almost the same as that before using. Moreover, the NiMnO3–La2O3 composite exhibits excellent oxygen evolution reaction (OER)/oxygen reduction reaction (ORR) activity, less discharge polarization, and excellent rate performance. The iron molten air battery with NiMnO3–La2O3 composite cycles stably over 250 cycles with the highest Coulombic efficiency of 99.8% and the average discharge potential of 1.14 V at 500 °C in the air. This work opens a new avenue toward developing a stable and efficient bifunctional cathode catalyst for the iron molten air batteries.

Effects of additive NaI on electrodeposition of Al coatings in AlCl3-NaCl-KCl molten salts

Effects of NaI as an additive on electrodeposition of Al coatings in AlCl3-NaCl-KCl (80-10-10 wt-%) molten salts electrolyte at 150 °C were investigated by means of cyclic voltammetry, chronopotentiometry, scanning electron microscopy and X-ray diffraction (XRD). Results reveal that addition of NaI in the electrolyte intensifies cathodic polarization, inhibits growth of Al deposits and increases number density of charged particles. The electrodeposition of Al coatings in the AlCl3-NaCl-KCl molten salts electrolyte proceeds via three-dimensional instantaneous nucleation which however exhibits irrelevance with NaI. Galvanostatic deposition results indicate that NaI could facilitate the formation of uniform Al deposits. A compact coating consisting of Al deposits with an average particle size of 3 µm was obtained at a current density of 50 mA·cm−2 in AlCl3-NaCl-KCl molten salts electrolyte with 10 wt-% NaI. XRD analysis confirmed that NaI could contribute to the formation of Al coating with a preferred crystallographic orientation along (220) plane.

Investigation on the Electrochemical Behaviour and Deposition Mechanism of Neodymium in NdF3–LiF–Nd2O3 Melt on Mo Electrode

Neodymium was electrochemically deposited from NdF3–LiF–Nd2O3 molten salt electrolyte onto the Mo electrode at temperatures close to 1273 K. Cyclic voltammetry and chronoamperometry measurements were the applied electrochemical methods. Metallic neodymium is obtained by potentiostatic deposition. The optical microscopy and XRD were used to analyze the electrolyte, the working electrode surface, and the deposit on the electrode. It was established that Nd(III) ions were reduced to Nd metals in two steps: Nd(III) + e− → Nd(II) at potential ≈−0.55 V vs. W and Nd(II) + 2e− → Nd(0) at ≈−0.83 V vs. W. Both of these processes are reversible and under mass transfer control. Upon deposition under the regime of relatively small deposition overpotential of −0.10 V to −0.20 V, and after the electrolyte was cooled off, Nd metal was observed at the surface of the Mo electrode. CO and CF4 were gases registered as being evolved at the anode. CO and CF4 evolution were observed in quantities below 600 ppm and 10 ppm, respectively.

Preparation of Magnesium Salts Composed of Perfluoro Anions and These Electrochemical Behaviors in Molten Salts

Magnesium secondary batteries are expected to be one of the popular post-lithium ion batteries due to their high capacity and the bountiful resources of this element. Reversible plating by the reduction of Mg2+ and stripping by the oxidation of Mg metal are critical problems for this battery. In this study, we attempted to investigate the dependence of the Mg plating/stripping behavior on the anion species. Various Mg salts containing perfluoro anions were synthesized and their thermal stabilities were measured by thermogravimetry. For the Mg salts possessing a high thermal stability, Mg molten salt electrolytes were developed with potassium or cesium salts having common anions and their plating/stripping behaviors were investigated.

Synergistic effect of quinary molten salts and ruthenium catalyst for high-power-density lithium-carbon dioxide cell

With a recent increase in interest in metal-gas batteries, the lithium-carbon dioxide cell has attracted considerable attention because of its extraordinary carbon dioxide-capture ability during the discharge process and its potential application as a power source for Mars exploration. However, owing to the stable lithium carbonate discharge product, the cell enables operation only at low current densities, which significantly limits the application of lithium-carbon dioxide batteries and effective carbon dioxide-capture cells. Here, we investigate a high-performance lithium-carbon dioxide cell using a quinary molten salt electrolyte and ruthenium nanoparticles on the carbon cathode. The nitrate-based molten salt electrolyte allows us to observe the enhanced carbon dioxide-capture rate and the reduced discharge-charge over-potential gap with that of conventional lithium-carbon dioxide cells. Furthermore, owing to the ruthernium catalyst, the cell sustains its performance over more than 300 cycles at a current density of 10.0 A g−1 and exhibits a peak power density of 33.4 mW cm−2.

Recycling of Gold Using Anodic Electrochemical Deposition from Molten Salt Electrolyte

Recycling of precious metals is becoming increasingly important as our living standards improve and electronic technologies advance. Conventional recycling processes involve multi-step hydrometallurgical treatments, in which precious metals are dissolved in aqueous solutions using a combination of strong complexing agents and oxidizing agents. However, these processes are time consuming and produce a large amount of hazardous liquid waste. Here, we present a new recycling process for Au, which is based on the direct anodic electrochemical deposition of anionic Au ions dissolved in a molten salt electrolyte. Moreover, this process is simple and does not generate harmful waste. In this process, Au is alloyed with Na to facilitate its dissolution in a molten mixture of NaCl–NaI. The existence of Au as an anionic species (Aun−) in the molten salt was proven and its anodic deposition (oxidation reaction) by application of an anodic current was demonstrated. During the anodic deposition of Au, Na is deposited on the cathode and can be recovered and reused in the process. It is expected that this electrodeposition technique to recycle Au from scrap metal could result in the development of a new environmentally friendly Au recycling process that does not generate hazardous waste.

Transition Metal Phosphorous Trisulfides as Cathode Materials in High Temperatures Batteries

The challenging environment of high temperature and high pressure on the Venus surface limit the battery options for Venus landers and surface probes. High temperature batteries employing Li alloy anodes, molten salt electrolytes and FeS cathodes were demonstrated to be resilient and operational for several days. For further improvements in performance, i.e., both specific energy and operational life, new high-capacity cathode materials are needed. Transition metal phosphorus trisulfides (TMPS3) are promising with considerably higher (2X) specific capacity, specific energy and energy density, by virtue of their ability to react with more than two lithium ions. This papers describes the assessment of these cathodes for high temperature batteries to power future Venus landers and probes. Manganese, iron, cobalt and nickel phosphorus trisulfides were synthesized and characterized by Scanning Electron Microscopy (SEM)/Energy Dispersive X-ray Spectroscopy (EDAX) and X-ray Diffraction (XRD) and tested in our high-temperature laboratory cells at 475 °C using cyclic voltammetry (CV) and galvanostatic discharges at different rates. Mn, Fe and Ni phosphorus trisulfides showed reversible behavior in cyclic voltammetric measurements. In the discharge tests, NiPS3 displayed the highest capacity out of the three metal phosphorous trisulfides tested at both C/20 and C/720 rates, with higher voltages and slightly higher capacity than FeS, followed by FePS3, while MnPS3 displayed relatively poor performance at C/20. Cathodes extracted from the discharged cells contain the transition metal (Fe, Ni or Mn) and Li2S by XRD, as expected from the reaction scheme.

High temperature primary battery for Venus surface missions

The hostile environment of high temperature and CO2 pressure has limited previous Venus surface mission durations to less than 2 h giving only a glimpse of the Venus surface, despite the use of considerable insulation, phase-change materials, and similar heat sinks to isolate the payload and avionics from high surface temperatures. NASA has initiated studies to develop new power technologies that can tolerate the high Venus surface temperature of 465 °C and pressure of 92 bar, and operate for future long-duration Venus landers and probes. Such in-situ investigations are crucial to gather basic information on the crust, mantle, core, and bulk composition of Venus for understanding the evolutionary paths of Venus in relation to Earth. In this paper, we describe the development of high temperature primary batteries based on lithium alloy anodes, transition metal sulfide cathodes, and molten salt electrolytes. Among the various cathode materials examined for thermal stability and for electrochemical activity, FeS has emerged as the most suitable cathode. With optimized cell components, cell design, and operational parameters, laboratory test cells were fabricated, which demonstrated continuous operation of ~26 days at 475 °C. These batteries will enable new long-duration surface missions for detailed surface exploration of Venus.

A potential working electrode based on graphite and montmorillonite for electrochemical applications in both aqueous and molten salt electrolytes

The feasibility of a novel graphite–MMT composite electrode for electrochemical processes in aqueous and molten salt electrolytes has been investigated. The graphite–MMT composite electrodes (G-MMTCEs) were fabricated by preparing composites in deionized water, pressing the dry composite under 1.03 × 104 N ram force to obtain cylindrical electrodes (5.00 cm long and 1.00 cm in diameter) and firing the electrodes at around 550 °C for 1 h. The results indicate that the G-MMTCE containing 80% graphite showed the lowest resistivity (8.17 × 10−4 Ωm) and highest flexural strength (5.81 × 106 Nm2). The exponential decrease in resistivity from low to high graphite percentage is clearly observed for a series of G-MMTCEs. The lamella-like graphite structure held together by tiny clay particles accounts for the enhanced electrical and mechanical stability of the G-MMTCEs. It is also found that the fabricated G-MMTCE is very stable in molten salts as well as in aqueous electrolytes with different pH values. The G-MMTCE has advantages as a working electrode over a glassy carbon electrode (GCE) in analyte detection as well as in electropolymerization. A narrow working potential range and enhanced sensitivity are the major advantages of the G-MMTCE over the GCE under identical cell and measurement conditions.

Electrochemical synthesis of ammonia in molten salts

Abstract Ammonia is important feedstock for both fertilizer production and carbon-free liquid fuel. Many techniques for ammonia formation have been developed, hoping to replace the industrial energy-intensive Haber–Bosch route. Electrochemical synthesis of ammonia in molten salts is one promising alternative method due to the strong solubility of N3− ions, a wide potential window of molten salt electrolytes and tunable electrode reactions. Generally, electrochemical synthesis of ammonia in molten salts begins with the electro-cleavage of N2/hydrogen sources on electrode surfaces, followed by diffusion of N3−/H+-containing ions towards each other for NH3 formation. Therefore, the hydrogen sources and molten salt composition will greatly affect the reactions on electrodes and ions diffusion in electrolytes, being critical factors determining the faradaic efficiency and formation rate for ammonia synthesis. This report summarizes the selection criteria for hydrogen sources, the reaction characteristics in various molten salt systems, and the preliminary explorations on the scaling-up synthesis of ammonia in molten salt. The formation rate and faradaic efficiency for ammonia synthesis are discussed in detail based on different hydrogen sources, various molten salt systems, changed electrolysis conditions as well as diverse catalysts. Electrochemical synthesis of ammonia might be further enhanced by optimizing the molten salt composition, using electrocatalysts with well-defined composition and microstructure, and innovation of novel reaction mechanism.

High Temperature Primary Batteries for Venus Surface Missions

Because of the hostile environments of high temperature and pressure, previous Venus probes survived for just two hours. Future Venus surface missions need to survive for much longer durations of several days, in order to return useful science. NASA’s LISSE lander concept has been proposed to operate on the surface of Venus for 60 days. Operating at a temperature of 465 °C and an ambient (CO2) pressure of 92 bar for up to 60 days is a significant challenge and requires new battery chemistries and cell designs. With this objective, we have been investigating high temperature lithium primary batteries with lithium alloy anodes, transition metal sulfide cathodes and molten salt electrolytes. Various cathode materials were screened for thermal stability at high temperature, in contact with the molten salt. Among these cathodes, FeS exhibited the highest stability and laboratory cells were fabricated and tested at 475 OC at various discharge rates to demonstrate long-term operational capability. Initial single cell designs have been shown to be operational for up to 15 days, with the cathode utilization decreasing at the low rates of discharge. Factors determining selection of cathode material, electrolyte composition, separator, etc. will be discussed, along with plans to achieve a full 60 day discharge.

Corrosion of Directed Energy Deposited Cantor Alloy in Molten KNO3-NaNO3 for Solar Thermal Applications

High entropy alloys (HEA) with 4-5 component elements are being considered for various applications including nuclear and solar thermal energy generation due to desirable mechanical properties and corrosion resistance. Advanced heat transfer systems in both nuclear and solar thermal energy generation are being designed to utilize molten salts as the working fluid, allowing for a significant increase in hot-leg temperatures leading to increased efficiency. The use of additively manufactured HEAs may be beneficial to these systems but corrosion properties of HEAs have not yet been established for many of the common molten salt systems. Additionally, the unique microstructure of HEA components produced via additive manufacturing can potentially alter the corrosion properties of the component relative to a cast or forged microstructure. This study will focus on CoCrFeMnNi HEA Cantor alloy prepared via an additive manufacturing pathway using directed energy deposition (DED); this method will allow for the development of complex geometries not available through traditional manufacturing methods. To understand the corrosion of additively-manufactured HEAs, a sample of DED-manufactured Cantor alloy was exposed to a 40/60 weight percent mixture of molten KNO3-NaNO­3 at 500 °C. This environment is relevant to solar thermal power plants. Accelerated corrosion testing using potentiodynamic polarization was carried out. Also, the sample subsequently underwent 50 hours of exposure to the molten salt. The sample surface was analyzed with x-ray diffraction, x-ray photoelectron spectroscopy, and focused ion beam milling coupled with scanning electron microscopy/energy dispersive spectroscopy. The molten salt electrolyte was analyzed post-exposure via inductively-coupled plasma – optical emission spectroscopy. Cantor alloy exposed to this nitrate salt mixture developed an oxide film with small, densely packed, vertically oriented fin-shaped grains and an inhomogeneous composition profile through its depth. The results of these studies will be discussed. Acknowledgments: The authors thank Zachary Karmiol, research scientist at Materials Characterization Nevada, for technical input. One of the authors, JM, acknowledges the Graduate Research Fellowship from the US National Science Foundation. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1447692.

Corrosion of Candidate Alloys in Molten LiCl-Li2O-Li for Pyroprocessing of Used Nuclear Fuel

Pyrochemical reprocessing of used nuclear fuel (UNF) requires large structural process vessels and functional components fabricated from stainless steel 316L (SS316L) or another high-temperature molten salt-compatible material like Inconel 625 (I625). The unique chemistry of the molten salt electrolyte during processing of UNF leads to complex interactions between container materials and the electrolyte that have been well documented by our group. In the head-end process to incorporate oxide UNF into the pyrochemical reprocessing scheme, LiCl with 1-2 wt. % Li2O is utilized as an electrolyte to facilitate the electrolytic reduction of UO2 via the Fray-Farthing-Chem (FFC) process. The Li+ reduction potential is quite close to the UO2 reduction potential and it is generally recognized that lithium metal will be produced at the cathode during the reduction process. While much of the metallic lithium further facilitates the reduction of UO2 via a metallothermic pathway, forming Li2O, a fraction of the metallic lithium dissolves or otherwise disperses into the molten salt. The metallic lithium then diffuses throughout the salt and reacts with process vessels and other components. To further understand corrosion in these conditions, SS316L and I625 coupons were exposed to LiCl-1 wt. % Li2O-Li and LiCl-2 wt. % Li­2O-Li with lithium metal concentrations of 0, 0.3, 0.6 or 1.0 wt. %. These samples were then studied using focused ion beam (FIB) milling, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and energy dispersive spectroscopy (EDS). Results from these studies will be discussed. Acknowledgements: The authors thank Zachary Karmiol, research scientist at Materials Characterization Nevada, for technical input. One of the authors, JM, acknowledges the Graduate Research Fellowship from the US National Science Foundation. This work was supported by the United States Department of Energy (DOE) under contracts, DE-NE0008262, DE-NE0008236, and DE-NE0008572, and the United States Nuclear Regulatory Commission (NRC) under Contract NRC-HQ-13-G-38-0027. Dr. Kenny Osborne and Ms. Nancy Hebron-Israel serve as the DOE and NRC award program managers, respectively. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1447692.

Electrochemical Synthesis of Ammonia: Recent Efforts and Future Outlook.

Ammonia is a key chemical produced in huge quantities worldwide. Its primary industrial production is via the Haber-Bosch method; a process requiring high temperatures and pressures, and consuming large amounts of energy. In the past two decades, several alternatives to the existing process have been proposed, including the electrochemical synthesis. The present paper reviews literature concerning this approach and the experimental research carried out in aqueous, molten salt, or solid electrolyte cells, over the past three years. The electrochemical systems are grouped, described, and discussed according to the operating temperature, which is determined by the electrolyte used, and their performance is valuated. The problems which need to be addressed further in order to scale-up the electrochemical synthesis of ammonia to the industrial level are examined.

Ternary Low-Temperature Molten Salt Electrolyte for Sodium Ion Batteries

To meet the increasing demand of electricity (which is forecasted to double by 2050)1 and to mitigate the climate crisis associated with using fossil fuels, renewable and clean energy sources are becoming the ultimate choices to generate electricity. Electrochemical energy storage devices such as batteries play an important role in the efficient use of renewable energy, providing an efficient and sustainable way to store energy.1,2 Beyond-lithium rechargeable batteries based on earth-abundant elements, namely sodium ion batteries (SIBs), are believed to be the alternatives for large-scale storage. Nevertheless, the commercial applications of SIBs are restricted by absence of reliable and low-cost electrolyte favouring low-temperature, high-efficiency, and reversible metal deposition/stripping. Interestingly, the newly reported molten salts containing several metal halides act as a low-cost electrolyte, which exhibits high electrical conductivity and quick electrode kinetics, and produce even better performances than ionic liquid electrolytes.3,4 Nonetheless, the conventional molten salt electrolytes normally operate at a relatively high temperature (over 260 ), which is significantly inconvenient for outdoor operation. Actually, reducing the operation temperature is not only beneficial to improving cycle life by restraining temperature-related degradation (i.e. undesired side reactions), but also profitable to using lower cost cell materials and reduce the cost of batteries fabrication. This paper reports our development of a low-temperature inorganic molten salt electrolytes with good electrochemical performances.5 The metal chloride eutectic mixture is used as electrolyte, whose melting points are dependent on its precursors and composition, so the relationship of mole ratio of metal chlorides and electrolyte’s melting points are investigated in our work. The influences of additives in chloride-based eutectic mixture are also studied, to decrease the melting point of the electrolyte. Herein, the electrochemical properties of this electrolyte are investigated in battery cells using Na metal as anode and metal fluoride cathode respectively. The battery charge /discharge behaviour and electrochemical impedance spectra (EIS) is tested using GAMRY potentiostat. Materials characterization techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM),X-ray spectroscopy (XPS) and Raman spectroscopy are used to characterise the electrode materials and to understand the effect of electrolyte on the electrode materials.

Electrolytic behavior of SrCl2 and BaCl2 in LiCl molten salt during oxide reduction in pyroprocessing

Ba and Sr elements in spent nuclear fuels are supposed to accumulate in LiCl molten salt electrolyte by forming chlorides (i.e., BaCl2, SrCl2) during oxide reduction in pyroprocessing. An electrochemical examination was performed in a LiCl–BaCl2–SrCl2 system to identify the effect of the BaCl2 and SrCl2 on the C anode-based oxide reduction process. BaCl2 and SrCl2 dissolved in LiCl were electrochemically stable up to 5 wt% with the C anode system, suggesting that no concern for their accumulation is needed in this range in the electrochemical aspect for the process design of the oxide reduction.

A Study on the Decontamination Possibility of Simulated Radioactive Inconel 600 – Co Alloy Using Electrorefining in LiCl-KCl Electrolyte

As the operating period of a nuclear power plant increases, the demand for dismantling of nuclear facilities has been rising, and the dismantling market is expected to continue to expand steadily. The Inconel 600, which was used as a material for the steam generator, has problem with crack and corrosion, and it is currently replaced with a steam generator of Inconel 690. It has been reported that if this steam generator is corroded or cracked, the cooling water contaminated with radioactive elements such as cobalt, strontium, may penetrate into the cracks. Therefore, development of decontamination technology for radioactive metal waste is essential to minimize waste during dismantling process.This study investigates the decontamination possibility of Inconel 600 in LiCl-KCl-CoCl2-NiCl2 molten salt electrolyte. Inconel 600 – cobalt alloy was used for anode as a simulated radioactive metal waste. After electrorefining process, the cobalt remains in the molten salt. Therefore, CoCl2 was added to the electrolyte to imitate the case when the radioactive metal waste was dissolved and concentrated in the electrolyte. Electrochemical analysis was performed by cyclic voltammetry (CV) to confirm the redox behavior of Ni ions in molten salt electrolytes. Decontamination experiments were performed by chrono potentiometry (CP) to analyze the oxidation behavior of Inconel 600 – cobalt alloy (anode) and the reduction behavior of Ni on Ni plate electrodes (cathode). Decontamination factor(DF) of the deposits produced by electrorefining was analyzed by ICP.

Electrodeposition of Silicon with a Liquid Gallium Cathode in Molten Salts

Further development of solar power depends largely on the availability of inexpensive solar grade silicon. Electrodeposition is a candidate method to produce high purity silicon at a reasonable cost. A new approach using a liquid gallium cathode was used to study deposition of silicon from molten salt electrolytes containing K2SiF6. Electrochemical studies and electrolysis to deposit silicon from molten eutectic KCl–KF with additions of K2SiF6 were carried out at 650 oC. Silver wire and glassy carbon rod were used as working electrodes, while glassy carbon rod or silver wire were counter electrodes. A silicon plate reference electrode was used. Liquid gallium in a small alumina tube with tungsten lead was the cathode during electrolysis while glassy carbon or silicon was the anode. Silicon was found to deposit inside a liquid gallium cathode during electrolysis. The current efficiency was found to be quite high, and consistently above 80 %.

Reduction of Carbon Dioxide to Carbon Nanostructures in Molten Salt: The Effect of Electrolyte Composition

The effect of the composition of the molten salt electrolyte on the morphology of carbon produced via reduction of carbon dioxide has been studied. Three different molten salt compositions (CaCl2-CaO-Na2Co3, CaCl2-Na2Co3, and CaCl2-CaO) were applied at constant potential difference of 3.0 V and carbon dioxide flow of ~ 1 mL/s for 120 min, and the current was recorded at 10-s intervals. The powder products obtained from the solidified electrolyte and cathode surface were then characterized by scanning electron microscopy and energy-dispersive spectroscopy. Rod-like structures with diameter of 120 nm formed on the surface of the steel cathode after electrolysis using CaCl2-0.5% M CaO-2% M Na2CO3 or CaCl2-2% M Na2CO3.