Various novel technological seeds of molten salt electrochemical processes (MSEPs) were created by the authors and their collaborators, which are currently under development for industrial applications. Among these, we describe four key processes: (1) electrochemical formation of carbon film, (2) plasma-induced discharge electrolysis for producing nanoparticles, (3) recycling crucial metals using a “bifunctional electrode”, and (4) electrolytic synthesis of ammonia from water and nitrogen under atmospheric pressure. Furthermore, we discuss their possible industrial applications in the fields of energy, the environment, resources, and materials with a view to the future low-carbon society.
We investigated effects of high lithium bis(fluorosulfonyl)imide (LiFSI) salt concentration or heat treatment before initial charge on charge-discharge performance of a graphite electrode in a 1-ethyl-3-methylimidazolium (EMImFSI)-based ionic liquid (IL) electrolyte. LiF was observed at the surface of graphite electrodes taken out from 2.00 mol kg−1 LiFSI/EMImFSI system and from 0.32 mol kg−1 LiFSI/EMImFSI system with pre-heat treatment before the initial charge. The surface LiF effectively suppresses the reductive decomposition of EMIm+. As a result, the irreversible capacity of initial cycle was significantly suppressed and the coulombic efficiency of subsequent cycles was greatly improved.
The redox reaction of tris(acetylacetonato)iron(III) (Fe(acac)3) on a glassy carbon electrode was investigated in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA). A one-electron transfer redox reaction of [Fe(acac)3]0/− showed the relatively negative potential of −1.4 V vs. Ag|Ag(I). The diffusion coefficient of Fe(acac)3 estimated by chronoamperometry was (1.4 ± 0.1) × 10−7 cm2 s−1. The redox properties examined in this study suggested that the redox reaction of [Fe(acac)3]0/− can be utilized in the negative half-cell reaction of redox flow batteries.
The cathodic reduction of nitrogen (N2) gas in molten chloride was investigated using a porous nickel (Ni) electrode. A LiCl-KCl-CsCl melt containing cathodically formed nitride ion (N3−) was sampled from an electrolytic bath and analyzed quantitatively by ion chromatography. The current efficiency of the cathodic reaction calculated from the ratio of quantity of the produced N3− was approximately 78% on average. The main reason why the measured efficiency did not reach 100% is considered as the amount of N3− increased more in the pores of the gas electrode than in the bulk of the electrolytic bath.
Static differential capacitance (Cdc) at the liquid-liquid interface between ionic liquids (ILs) and eutectic Ga-In alloy (EGaIn) has been measured using the pendant drop method for two ILs: 1-ethyl-3-methylimidazolium tetrafluoroborate ([C2mim+]BF4−) and 1-octyl-3-methylimidazolium bis(nonafluorobutanesulfonyl)amide ([C8mim+][C4C4N−]). The potentials of zero charge for the IL|EGaIn interfaces are shifted compared with the IL|Hg interfaces with an amount that can be considered by the difference in the work functions of EGaIn and Hg. The measured Cdc at the [C2mim+]BF4−|EGaIn interface has well reproduced the camel-shape potential dependence of Cdc at the Hg interface of the same IL at the negatively charged potential region. This suggests that there are little specific interaction between the IL ions with EGaIn and Hg. The [C8mim+][C4C4N−]|EGaIn has been compared with the [C8mim+]BF4−|Hg interface where IL-cation is the same but IL-anion is different. Also in that case, Cdc is similar to each other at the negatively charged potential region, which means that accumulated C8mim+ ions at the interface mainly govern the Cdc behavior.
Aluminum species forming in Lewis acidic chloroaluminate and bis(trifluoromethylsulfonyl)amide (TFSA−) mixed ionic liquids having 1-butyl-3-methylimidazolium (BMI+) was investigated by potentiometry and Raman spectroscopy. A decrease in the concentration of [Al2Cl7]− and an increase in that of [AlCl4]− with addition of BMITFSA into acidic BMICl-AlCl3 were observed by Raman spectroscopy. The redox potential of Al(III)/Al in BMICl-AlCl3 (1:2 in molar ratio) was found to be −0.68 V vs. an Ag|Ag(I) reference electrode, which was corresponding to −0.25 V vs. the ferrocene/ferrocenium couple at 25°C. The potentiometry of an Al electrode in the mixed ionic liquids with different compositions using the Ag|Ag(I) reference electrode suggested formation of a mixed ligand aluminum complex, Al(TFSA)Cl2, indicating the instability of [Al2Cl7]− against TFSA−.
An equimolar mixture of oligoether solvents, glymes, and Li salt yields a new subclass of ionic liquids (ILs), referred to as solvate ILs, when using an appropriate combination of glyme and Li salt. In this paper a triglyme-Li tetrabromoferrate complex [Li(G3)][FeBr4] is prepared by combining a crystalline glyme-Li salt complex, [Li(G3)]Br, and an iron(III) bromide Lewis acid, FeBr3, to demonstrate a solvate IL comprised entirely of complex ions. [Li(G3)][FeBr4] is characterized in terms of thermal properties, coordination structure, and transport properties. This study reveals that the melting point of [Li(G3)][FeBr4] is lower than 100°C and that highly dissociated complex ions, [Li(G3)]+ and [FeBr4]−, are present both in the molten state and in polar acetonitrile solutions. [Li(G3)][FeBr4] is thus deemed a good solvate IL. The reversible redox property of the [FeBr4]−/[FeBr4]2− couple is exploited for the catholytes of rechargeable lithium batteries. The catholyte based on this redox active solvate IL exhibits a highly reversible charge-discharge behavior.
Phase behavior of the [N2222][BF4]-[N3333][BF4] (N2222+ = tetraethylammonium, and N3333+ = tetrapropylammonium,) binary system has been investigated by differential scanning calorimetry and powder X-ray diffraction. Two solid-solid phase transitions are observed in differential scanning calorimetric curve in the range of x([N3333][BF4]) = 0.1–0.9 (x([N3333][BF4]) = the molar fraction of [N3333][BF4]) reflecting the solid-solid phase transition of each single salt, and only slight shift was observed for the transition temperatures. Powder X-ray diffraction patterns confirmed phase transitions. Although the amount of the minor constituent is low in the solid solution phase based on each single salt at 313, 373, and 423 K, Phase I of [N2222][BF4] at 483 K with the NaCl-type structure, which is regarded as an ionic plastic crystal (IPC) phase, can accommodate [N3333][BF4] up to the level where significant change in lattice parameter is observed (5.7% volume expansion). Drop of the liquidus line was observed for the salts with the mixing ratio x([N3333][BF4]) = 0.8 to 0.9 approaching to the eutectic temperature of 475 K. Ionic conductivity of Phase I increases by two orders of magnitude from x([N3333][BF4]) = 0 to 0.1 owing to the lattice expansion by inclusion of N3333+.
Electrochemical formation of selenium (Se) nanoparticles in an amide-type ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing selenium tetrachloride (SeCl4) was investigated in the presence of excess chloride anion on a glassy carbon electrode. Electrochemical reduction of [SeCl6]2− resulted in deposition of Se on the electrode surface. The deposited Se was further reduced electrochemically at the more negative potential region to form [Se2]2−, which was suggested to exist by ultraviolet-visible spectroscopy. Se nanoparticles dispersed in the ionic liquid were formed by the proportionation reaction between [SeCl6]2− and [Se2]2− in the ionic liquid. The Se nanoparticles were characterized by energy dispersive X-ray spectroscopy and transmission electron microscopy.
The solubility of sodium tungstate (Na2WO4) in a molten sodium hydroxide (NaOH) bath and its dependence on the partial pressure of water vapor were measured. This was done as part of an ongoing study of a new tungsten (W)-recycling process using molten NaOH. First, the nature of the chemical compound in the equilibrium solid phase in a molten NaOH bath was confirmed by adding excess amount of tungsten oxide as a tungstate ion source into the bath. X-ray diffraction analysis indicated that anhydrous Na2WO4 is the solid phase in equilibrium with the liquid phase of molten NaOH saturated with Na2WO4. Then, the solubility of the anhydrous Na2WO4 was measured under various partial pressures of water vapor. The results revealed that the solubility of anhydrous Na2WO4 drastically increased with the partial pressure of water vapor and the bath temperature from a molar fraction of 0.06–0.17. This value was much higher than those previously reported for molten nitrate and nitrites.
To clarify the relationship between adhesive strength of Al electroplating film and Mg alloy substrate, the amounts of γ-phase (Mg17Al12) and α-phase (Mg solid solution phase) in Mg alloy surface was investigated. Al electroplating film on Mg alloy was formed by current pulse electrolysis in AlCl3-1-ethyle-3-methyl-imidazolium chloride mixture ionic liquid at 283 K. It was found that the adhesive strength was affected by the ratio of γ-phase in the Mg alloys and it increased with increasing ratios of γ-phase on the Mg alloy surface. The results suggest that the adhesive strength of the electrodeposits on the γ-phase is stronger than that of the α-phase due to little oxide formation on the γ-phase.
To enhance three phase interfaces on graphite cathodes in an Al-Cl2 cell, graphite cathodes with different densities (1.27, 1.32, 1.40, 1.57, and 2.00 gcm−3) were evaluated in an 1-ethyl-3-methylimidazolium chloride -AlCl3 mixture ionic liquid electrolyte at 313 K. The relation between current and cell voltage or power and current of the 1.27 gcm−3 graphite cathode showed the best performance among the graphite materials with 5 graphite densities in the cathodes investigated here. Surface profiles of the graphite by mercury intrusion porosimetry suggested that there are effective pore diameters for penetration of the electrolyte, lower density graphite has more pores on the surface than graphite with higher densities.
Low-cost graphene nanoplatelet-polysulfone composite cathodes for a rechargeable aluminum battery with a Lewis acidic AlCl3–1-ethyl-3-methylimidazolioum chloride ([C2mim]Cl) ionic liquid are prepared by a standard slurry-coating method for the composite electrode, and conductive additives such as acetylene black (AB), ketjen black (KB), and vapor grown carbon fiber (VGCF) are explored in order to improve the cathode performance. All the cathodes show reversible electrode reactions related to the intercalation reaction of [AlCl4]− into the graphene nanoplatelets. The cathodes achieve a discharge capacity of ca. 70–80 mAh g−1 at 1000 mA g−1. However, the rate capability and the capacity retention rate strongly depend on the conductive additive species. The best cathode performance is obtained when the additive is an equal mixture of VGCF and KB. The specific capacity is ca. 55 mAh g−1 at 10000 mA g−1. The retention rate based on the capacity observed at 1000 mA g−1 exceeds 65%. A Ragone plot constructed from the data shown in this article suggests that an Al rechargeable battery with a VGCF-KB added graphene nanoplatelet-polysulfone composite cathode has great potential as a future high-power density rechargeable battery.
Si powder was produced by direct electrolytic reduction of SiO2 in molten CaCl2 at 1123 K. From the Si powder, Si ingots were obtained by a floating zone method. The concentrations of most metallic elements and of P in the Si ingots were lower than the acceptable levels for solar grade Si. The minority carrier lifetimes in the Si ingots were measured using a microwave photo conductivity decay method. The obtained values of ca. 1.0 µs were two orders of magnitude shorter than those observed in an Si ingot prepared from 10N purity Si.
The reduction of CaTiO3 to generate Ti powder with a low content of oxygen was examined by electrolysis in a CaCl2-CaO melt. Using various sizes of the raw oxide particles, the concentration profile of the residual oxygen in the cathode basket suggests a reduction mechanism involving calcium from the melt. The residual oxygen content was influenced by the dehydration method. Vacuum dehydration above the melting temperature and decomposition of CaCl2 hydrates below 550 K were required to eliminate water. Optimization of both water removal and the oxide particle size produced the lowest oxygen content (0.42 mass%O) in the powder.
The in-situ Fourier transform infrared spectroscopy of molten eutectic LiCl-KCl-CsCl was conducted in order to identify the dissolved ions in the molten salt electrolytes. The transflectance spectrum of the melts could be easily obtained at temperatures higher than 350°C using the commercially available diffuse reflectance optical system and an air-tight chamber with a built-in heater. The sharp absorption peak attributable to the stretching vibration of OH− was observed in the melt containing LiOH, indicating the availability of identification of dissolved species in the melts. The intensities of transflection possibly assignable to N–H bonds in imide (NH2−) and amide (NH2−) anions in the melt containing Li3N changed during supplying H2 gas because of the progress of the protonation reactions of these anions to form NH3. The in-situ analysis of the dissolved ions in the melts under the reaction conditions by infrared spectroscopy with the diffuse reflectance optical system can be a simple and powerful tool for understanding the reaction mechanism.
An ionic liquid system based on two amide anions; bis(fluorosulfonyl)amide anion (FSA−) and bis(trifluoromethanesulfonyl)amide anion (TFSA−), with a quaternary ammonium cation; N,N,N-trimethyl-N-propylammonium cation (TMPA+), was investigated over entire composition range. The system showed the binary phase diagram and the eutectic point was 5.5°C around the anion ratio of 1:2 (FSA−: TFSA−). Polymorphic behavior of solidified ionic liquid was observed by X-ray diffraction (XRD) and discussed comparing with ionic liquids composed of other anion; (fluorosulfonyl) (trifluoromethylslufonyl)amide anion (FTA−) or tetrafluoroborate anion (BF4−). In addition, the phase diagram of the ionic liquid system containing lithium cation with binary anion was confirmed. Raman spectroscopy suggested that lithium cation interacts more preferentially with TFSA− than FSA−.
The electrochemical behavior of Ti(III) ions in a eutectic KF–KCl molten salt was investigated using cyclic voltammetry, square wave voltammetry, and chronoamperometry at 923 K. Ti(III) ions were produced by the addition of 0.50 mol% of K2TiF6 and 0.33 mol% of Ti sponge to the melt. The reduction of Ti(III) ions to metallic Ti was observed as a single 3-electron wave around 0.33 V vs. K+/K in the square-wave voltammogram. The electrodeposition was conducted at a Mo electrode by galvanostatic electrolysis at −50 mA cm−2 for 20 min. The deposits were confirmed to be compact and adherent Ti metal films by scanning electron microscopy, energy dispersive X-ray analysis and X-ray diffraction analysis. The oxidation of Ti(III) to Ti(IV) was observed at 1.82 V vs. K+/K as a reversible electrochemical process. The diffusion coefficient of Ti(III) ions was determined to be 3.9 × 10−5 cm2 s−1.
The thermodynamic and transport properties of nitrite and nitrate salts, which are candidates for thermal energy storage, were investigated by differential scanning calorimetry (DSC), high-temperature pulse-field NMR, and molecular dynamics (MD) simulations. The potential parameters of NO2 for the MD simulations were newly developed by ab initio calculations and empirical approaches. The MD results for molten xNaNO2–(1 − x)NaNO3 salts (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) were comparable to the experimental density and Na self-diffusion coefficients. Moreover, the temperature-averaged heat capacities of the molten xNaNO2–(1 − x)NaNO3 systems were calculated from the changes in the energy and volume versus the temperature, which reproduced the results obtained from the DSC measurements well.