This review article describes some selected novel molten salt electrochemical processes which have been created/developed by the author and his coworkers, with emphasis on the applications towards future energy systems. After showing a perspective of the applications of molten salt electrochemistry from the viewpoints of energy and environment, several selected topics are described in detail, which include nitride fuel cycle in a nuclear field, hydrogen energy system coupled with ammonia economy, thermally regenerative fuel cell systems, novel Si production process for solar cell and novel molten salt electrochemical processes for various energy and environment related functional materials including nitrides, rare earth-transition metal alloys, fine particles obtained by plasma-induced electrolysis, and carbon film. And finally, the author stresses again, the importance and potential of molten salt electrochemistry, and encourages young students, scientists and researchers to march in a procession hand in hand towards a bright future of molten salts.

Among the next 30 years, we have to implement a practical measure in the global energy/environmental problems, solving the followings: (1) replacing the fossil fuels without CO₂ emission, (2) no severe accidents, (3) no concern on military, (4) minimizing wastes, (5) economical, (6) few R&D investment and (7) rapid/huge global application supplying about half of the total primary energy till 50 years later. For this purpose the following system was proposed: THORIMS-NES [Thorium Molten-Salt Nuclear Energy Synergetic System], which is composed of (A) simple fission Molten-Salt power stations (FUJI), and (B) fissile-producing Accelerator Molten-Salt Breeder (AMSB). It has been internationally prepared a practical Developmental Program for its huge-size industrialization of Th breeding fuel cycle to produce a new rational primary energy. Here it is explained the social meaning, the conceptual system design and technological bases, especially, including the molten fluoride salt technology, which was developed as the triple-functional medium for nuclear-engineering, heat-transfer and chemical engineering. The complex function of this system is fully achieved by the simplified facility using a single phase molten-salt only.

The electrochemical behaviors of divalent iron species were investigated in a room-temperature molten salt (RTMS) consisting of 1-n-butyl-1-methylpyrrolidinium cation (BMP⁺) and bis(trifluoromethylsulfonyl)imide (TFSI⁻) anion. The TFSI⁻-salt of Fe(II) was soluble in BMPTFSI at room temperature. The cyclic voltammetric data for the Fe(III/Fe(II) processes at 1.1 V vs. Ag/Ag(I) showed quasi-reversible kinetics on a platinum electrode. The diffusion coefficient of Fe(II) determined by chronoamperometric techniques was 9.7 × 10⁻⁸ cm² s⁻¹ at 25°C. After cathodic reduction of Fe(II) at −2.0 V, furthermore, metallic Fe was electrodeposited on a copper substrate.

The electrodeposition of cobalt was investigated in a hydrophobic room-temperature molten salt, 1-n-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPTFSI), containing Co(TFSI)₂ as a Co source. The cathodic reduction wave of Co(II) species was confirmed by cyclic voltammetry. The electrodeposit obtained by the potentiostatic reduction on a nickel substrate was found to be metallic Co by both XPS and EDX analyses. The diffusion coefficient of Co(II) was estimated to be 1.0 × 10⁻⁷ cm² s⁻¹ at 27.8°C. The electrodeposition of Co on the Pt substrate seemed to involve the three-dimensional instantaneous nucleation under diffusion control.

The electrochemical behaviours of lanthanum and neodymium were investigated in a molten LiCl-KCl eutectic by chronopotentiometry. A chronopotentiometric method was devised to minimize the error in defining the surface area of the working electrode. The immersed depth of the working electrode was altered in stages, and the relation between the change in the square root of the transition time and that of the surface area of the working electrode was substituted for the Sand equation using their absolute values. The diffusion coefficients of La(III) and Nd(III) were determined in the temperature range between 673 and 823 K and the validity of the Arrhenius law was also verified by plotting the variation in the logarithm of the diffusion coefficient versus the inverse of the temperature.

Molecular dynamics simulations have been performed on molten (Nd_1/3, M)Cl (M = Li, Na, K) in order to investigate the ionic correlation among the dynamical properties such as the self-diffusion coefficient (D), the self-exchange velocity (SEV, v), compared with the corresponding experimental results and the structure of molten NdCl₃-alkalichloride mixtures. The simulated results revealed that the calculated v and D of alkali metal decrease as the composition of rare earth increases. The decrease ratio of the calculated v and D was influenced on the ionic radius of the alkali metal. Finally, the sequence of the calculated SEV’s for the various compositions were consistent with those of the referred experimental results.

The electrochemical reduction of tetravalent titanium species was investigated in hydrophobic 1-n-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPTFSI) room-temperature molten salt (ionic liquid). Titanium tetrabromide was soluble in BMPTFSI regardless of the presence of BMPBr as a Lewis base. The stepwise reduction from Ti(IV) to Ti(III) and probably Ti(II) was observed in BMPTFSI containing TiBr₄ without BMPBr. The potentiostatic cathodic reduction gave some deposits at 180°C. The reduction of Ti(IV) species at −2.3 V led to the deposition of some Ti compounds containing TFSI⁻ anions. The electrodeposit containing Ti without TFSI⁻ anions was obtained by the potentiostatic cathodic reduction below −3.0 V.

The ionic conductance behavior of a polymeric gel electrolyte system based on an ionic liquid dissolving magnesium salt has been investigated. The ion gel was composed of poly(ethylene oxide)-modified polymethacrylate (PEO-PMA) swollen with 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI) dissolving Mg[(CF₃SO₂)₂N]₂ as the second salt. The ionic conductivity varied with the gel composition. The highest conductivity, 8.8 × 10⁻⁴ S cm⁻¹ at 20°C, was obtained for the system containing 80 wt% of the liquid component consisting of 20 mol% of the magnesium salt and 80 mol% of the ionic liquid. Raman spectra of the polymeric ion-gel revealed that the ethylene oxide units in the polymer matrix assist to dissociate the magnesium salt in the polymeric gel, which would enable the magnesium species to conduct even in the solid state at room temperature.

The electrochemical behavior of Fe electrode has been studied in NaCl- or NaBr-saturated AlX₃-MX [X = Cl, Br; M = Li, Na, K] melts at a temperature of 185°C. The passivation of Fe electrode was suggested from cyclic voltammetry and chronopotentiometry. The performance of the Fe/FeX₂ positive electrode in AlCl₃-AlBr₃-KCl-NaCl-LiCl-NaBr melt was investigated at 120-250°C using a rechargeable cell Al/electrolyte/FeX₂. The charge-discharge capacity of the Fe/FeX₂ electrode depended upon the temperature.

The effect of the addition of LiTFSI in both an imidazolium (EMITFSI) and a quaternary ammonium (HTMATFSI) ionic liquid on their conductivities have been estimated. The conductivities of both ionic liquid solutions were markedly decreased when 10 mol% of LiTFSI was dissolved. Ethylene carbonate (EC) and dimethyl carbonate (DMC) were used as additives to improve the conductivity of LiTFSI/ionic liquid solutions. DMC, expected to decrease viscosity, is more effective than EC, expected to dissociate LiTFSI, for HTMA-based system. On the other hand, the contribution of both additives was similar for EMI-based system.

Equilibrium structures and harmonic vibrational frequencies of an ion pair model composed of EMI⁺ and BF₄⁻ (EMI⁺-BF₄⁻) were calculated by Density Functional Theory with SCRF calculation in order to examine local molecular structures of ions in the EMIBF₄ ionic liquid. With increasing the relative dielectric constant (ε) of the medium from 1 to 25, intermolecular distance between the EMI⁺ and the BF₄⁻ moieties at equilibrium configuration was increased, concomitant with a change in relative molecular orientation. Accordingly, the interaction energy between EMI⁺ and BF₄⁻ decreased with the increase of dielectric constants. The splitting of the B-F stretching frequencies (F₂ mode of a free BF₄⁻ ion) of the EMI⁺-BF₄⁻ ion pair became smaller with the increase of dielectric constants, which gave theoretical IR spectra more representative of the observed one than those calculated under vacuum.

The double layer capacitance properties of activated carbon fibers (ACFs) in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF₄) were discussed in comparison with propylene carbonate solution of 0.5 M (C₂H₅)₄NBF₄ (TEABF₄/PC) as typical organic electrolyte for electric double layer capacitor (EDLC). In the case of using TEABF₄/PC, the ACF electrodes showed very stable cycle-ability of the capacitance, but narrow micropores caused small capacitances by strong ion-sieving to TEA⁺ cation. The capacitance of the ACFs in EMImBF₄ were higher than those in TEABF₄/PC. However, their capacitances of the ACFs with narrow micropores dramatically decreased with the increasing cycle number, indicating that EMIm⁺ is adsorbed in the micropore irreversibly. The capacitance behavior of the ACFs in EMImBF₄ can be explained by the stereo-chemical matching of the plane-like structure of EMIm⁺ cation with slit-like micropore of ACF and a strong adsorption of EMIm⁺ and carbon surface. Ketjen black (mesoporous carbon black) had high capacitance and good cycle-ability in EMImBF₄ regardless of comparably low surface area to the ACF with narrow micropores. It suggests that wide-pore such as mesopores is effective in ion adsorption/desorption in EMImBF₄.

Raman spectra have been obtained for peroxide ion in Na₂O₂ up to 950 K. Drastic changes were observed in the spectra around 780 K, which could be attributed to the phase transition between the β and α forms. The spectra of peroxide ions in molten NaOH were also obtained at 610 K, just above the eutectic temperature, and the peak could be observed up to 900 K for the melt containing 20 mol% Na₂O₂. This evidently shows that peroxide ions are able to be present in molten NaOH. An analysis of the peak position suggested the analogous local structure around peroxide ions in the melt with that in the crystal.

The performance of electric double layer capacitor (EDLC) with an ionic liquid, 1-ethy-3-methylimidazolium tetrafluoroborate (EMIBF₄) electrolyte containing LiBF₄, has been investigated and compared with that of EDLC containing EMIBF₄ without LiBF₄ as well as containing a conventional organic electrolyte, triethylmethylammonium tetrafluoroborate/propylene carbonate (TEMABF₄/PC). EDLC with EMIBF₄ + LiBF₄ showed an excellent electrochemical stability; the electrolyte was not decomposed even at a cell voltage of ca. 3.6 V. Calculated energy density in EDLC with EMIBF₄ + LiBF₄ was highest among EDLCs with the cited electrolytes. The addition of LiBF₄ to EMIBF₄ also improved double layer capacitance especially at a positive electrode.

The differential scanning calorimetry was performed in order to discuss the thermochemical properties of room temperature molten salts (RTMSs) coexisting with porous inorganic powders such as α-Al₂O₃, ZrO₂, SiO₂, and SiC. Whereas the melting point of bulk sample was observed at 25.8°C, it decreased with an increase of the specific surface area of the solid phase. The molar enthalpy of fusion also decreased. It is suggested that the phase transition behavior is influenced by the coexistence of the porous solid phase.

The electrical conductivity was measured for the coexisting system containing the α-Al₂O₃ powder and RTMS, i.e. trimethyl-n-propylammonium bis(trifluoromethylsulfonyl)imide (TMPATFSI) and binary RTMS containing TMPATFSI with addition of 1,1,1-trifluoro-N-[(trifluoromethyl) sulfonyl] methanesulfonamide (HTFSI) as a proton source; H_xTMPA_1−xTFSI. For the system containing the inorganic powder and TMPATFSI, the electrical conductivity increased exponentially as the liquid content increased, and depended on the specific surface area of the powder. The conductivity showed the temperature dependence following the VTF (Vogel-Tammann-Fulcher) equation. Since HTFSI does not dissociate in the TMPATFSI and does not contribute to the conduction in the bulk system, the electrical conductivity decreased with the increase of the HTFSI content in the bulk system. However, in the coexisting system, the conductivity increased, and the activation energy decreased with the HTFSI content up to 0.4 of [HTFSI]/[HTFSI] + [TMPATFSI]. It is expected that the conductivity is enhanced near the solid surface by the contribution of proton of HTFSI.

The elucidation of the structures of electrical double layers formed in interphases between a room-temperature ionic liquid (RTIL) and a solid electrode at the molecular level is essential for the development of high-performance electrochemical devices. We have investigated the molecular structure in an 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF₄) | Au electrode interphase by means of in situ surface-enhanced infrared absorption spectroscopy (SEIRAS). The feature in the SEIRA spectra measured in a potential range of −1 to +0.2 V vs. Ag | AgCl suggests that the EMI⁺ reorients from a flat (parallel) to a vertical configuration as the electrode potential becomes less negative.

The structure of intermediate-range ordering (IRO) in sodium acetate (CH₃COONa) and sodium propionate (C₂H₅COONa) melts was investigated by X-ray diffraction (XRD) measurements. A peak of IRO was observed at 0.59 Å⁻¹ and 0.49 Å⁻¹ of Q, (d = 10.7 Å and 12.8 Å) for anhydrous sodium acetate and sodium propionate melt, respectively. The hexahydrate melts also showed a peak at the same position. The XRD measurements were also performed for mixed melts of anhydrous sodium propionate and sodium acetate in order to investigate the composition dependence of the IRO structure, from which the structure and stability of the IRO was discussed.

Structural change of SrF₂ from 300 K to above its melting point was firstly obtained by X-ray Absorption Fine Structure (XAFS) technique, and a distinct phase shift in XAFS oscillation was observed in the temperature range of superionic conductance phase transition of SrF₂. The decrease in coordination number of first neighboring fluoride ions around a strontium ion is considered highly correlated with the increase tendency of electric conductivity. In molten phase, the nearest Sr²⁺-F⁻ correlation structural parameters are evaluated to be N_SrF = 4.08 and R_SrF = 2.58 Å.

Ion-transport properties of room-temperature ionic liquids (RTILs), N, N-diethyl-N-methoxyethyl-N-methyl ammonium (DEME) with different anions, BF₄ and (CF₃SO₂)₂N, as well as their mixed systems with two different organic solvents, propylene carbonate (PC) and 1,2-dichloroethane (DCE), have been explored by means of conductometry and pulsed-field-gradient spin-echo (PGSE) NMR diffusivity measurements. In a highly concentrated range from the neat ionic liquids to 1 mol dm⁻³ solution, an increase in the organic solvent content contributes to an increase in the ionic diffusion coefficients, which is reflected in enhancement of the ionic conductivity. Such increase in the solvent content, on the other hand, promotes the ionic association, even if highly polar PC is introduced as the solvent.

2-Heptadecylimidazole (a) neutralized with a series of acids showed good thermal stability. The decomposition temperature of most salts exceeded 200 °C. Among these, a neutralized with HBF₄ showed enantiotropic liquid crystalline phase. Higher ionic conductivity and lower activation energy were found for the salt in the liquid crystalline phase. The a neutralized with HBF₄ was mixed with 11 mol% a to introduce the proton acceptor sites in the assembled a. The mixture showed a monotropic liquid crystalline phase. High ionic conductivity and low activation energy were also seen in the monotropic liquid crystalline phase.

Co-electrodeposition of boron and neodymium has been studied by means of different electrochemical methods in the NaCl-KCl-NaF-KBF₄-NdF₃ molten system at 1023 K. Conditions of the neodymium tetra- and hexaboride electrochemical synthesis has been defined on the base of voltammetric data.

It was shown that contact of oxide materials with LiCl-KCl eutectic melt containing UCl₄ and UCl₃ changes drastically the mechanism of uranium electroreduction. Transformation of voltammograms on addition of oxide ions (Li₂O) to the melt LiCl-KCl-UCl₄ melt was studied. Similar changes of voltammetric curves were observed during a long contact of oxide materials with LiCl-KCl-UCl₄ molten system due to appearance of oxide ions in the melt resulting from oxide materials corrosion.

The electrochemical behavior of room temperature ionic liquids (RTILs) based on perfluoroethyltrifluoroborate ([C₂F₅BF₃]⁻) was investigated with and without Li[BF₄]. The plating-stripping corresponding to the peak of lithium could be observed on a nickel electrode in the RTIL based on a quaternary ammonium cation. However, the addition of ca. 0.4 mol dm⁻³ of Li[BF₄] had almost no effect on the cathodic limit potential of EMI[C₂F₅BF₃]. The results of electrochemical quartz crystal microbalance (EQCM) measurements in propylene carbonate solution containing ca. 2.0 mol dm⁻³ of RTILs and 20 mmol dm⁻³ of LiBF₄ indicate that a solid-electrolyte interface formation, which inhibits the reductive decomposition of the EMI cation, was not significant in EMI[C₂F₅BF₃] with the addition of Li[BF₄].

The electrode reaction of Ce(III/Ce couple in the eutectic LiCl–KCl, on a Bi liquid electrode, was investigated in the temperature range of 673–823 K. Cyclic voltammetry, using a Bi pool electrode, suggest a quasi–reversible behaviour of the system, and the values of the kinetic parameters, k⁰ and α, as well as the reversible half wave potential, E^r_1/2, have been obtained. The differences between the equilibrium potential adopted by a Ce electrode and the E^r_1/2 observed with the same Ce(III) solution at the liquid electrode, were consistent with the activity coefficients of Ce in the liquid Bi phase taken from the literature. Because of the ease of application of cyclic voltammetry, it was suggested this to be used as a general method to obtain approximate values of the excess Gibbs energy change of Ce in liquid metal, and hence the partial molar enthalpy of mixing and the partial molar excess entropy. Electromotive force, emf, measurements for various intermetallic compounds in two–phase coexisting states were carried out using a Bi coated tungsten electrode. The activities and relative partial molar Gibbs energies of Ce were obtained for CeBi₂ and CeBi. The relative partial molar entropies and enthalpies of Ce as well as the standard Gibbs energies of formation for the Ce–Bi intermetallic compounds were calculated.

The anodic hydrogen electrode reaction was examined in the Lewis acidic 66.7-33.3 mole percent (m/o) aluminum chloride-1-ethyl-3-methylimidazolium chloride (AlCl₃-EtMeImCl) ionic liquid and the LiCl-buffered neutral AlCl₃-EtMeImCl ionic liquid containing LiAlH₄. Raman spectroscopy experiments revealed that the Al₂Cl₇⁻ anion in the acidic ionic liquid is directly involved in the dissolution of the LiAlH₄. Electrochemical hydrogen generation was verified by gas chromatography and XRD for Pd and Ti electrodes polarized in the ionic liquid. The hydrogen electrode reaction was irreversible, i.e., it followed an E_irC_ir mechanism in the 66.7 m/o ionic liquid at 298 K. However, it was quasireversible in the LiCl-buffered ionic liquid at 353 K and appeared to follow an E_qrC_ir mechanism.

The effect of lanthanum addition to the Li/Na eutectic carbonate on the oxygen reduction reaction has been investigated using a meniscus electrode made of a gold ring. This electrode could reduce the effect of the gas diffusion to the reaction. Therefore, this is the good electrode for a model of a porous gas diffusion electrode. The apparent standard exchange current density due to the lanthanum addition that is determined by a quasi-steady state voltammogram around the open circuit potential increased five times that without the additive. The reduction peak potential of the cyclic voltammogram decreased, and the reduction current density increased due to the lanthanum addition. The diffusion process of the oxygen species in the molten carbonate would affect these behaviors. Therefore, the charge transfer and the diffusion process should be enhanced by the lanthanum.

Molar conductivities of molten PbBr₂-NaBr and PbBr₂-KBr systems were evaluated using the data of molar volume and electrical conductivity which were measured by dilatometry and a conventional a.c. technique, respectively. The molar conductivities of both systems increase with increasing temperature. The molar conductivity isotherm of molten PbBr₂-NaBr system shows a gradual increase with increasing NaBr concentration, but that of molten PbBr₂-KBr system gives little change in spite of an increase of KBr concentration especially in a PbBr₂-rich region. These results suggest some complex formation in the molten salts and predominant complex ion such as PbBr₄²⁻ is expected from the results of Raman spectroscopy and neutron diffraction.

The new carbon electrodes containing alkali and/or alkali earth metal fluorides such as LiF-CaF₂ and CaF₂ were developed to prevent the occurrence of anode effect in electrochemical fluorination and were used as the anode for electrolytic synthesis of perfluorotrimethylamine, (CF₃)₃N, from mixed melt of (CH₃)₄NF·5HF and KF·2HF at 100°C. The ratio of (CF₃)₃N to the overall anode gas increased with increasing the concentration of (CH₃)₄NF and decreasing the current density, and the best value of 61.7% was obtained in electrolysis of the mixed melt of (CH₃)₄NF·5HF (40 mol%) and KF·2HF (60 mol%) at 20 mA cm⁻² with the 4 wt% CaF₂ added carbon anode. Also, the best value of its ratio in electrolysis at the same current density with the ordinary carbon anode (FE-5) was obtained in the mixed melt of (CH₃)₄NF·5HF (30 mol%) and KF·2HF (70 mol%) and was 81.4%. XPS analysis revealed that the surface of anode after electrolysis was mainly covered with the layer of the C-F intercalation compounds having the semi-covalent and ionic bonds. However, the new carbon anode was broken down in the mixed melt of (CH₃)₄NF·5HF (50 mol%) and KF·2HF (50 mol%) during electrolysis at 40 mA cm⁻². From these results, it is concluded that die alkali and/or alkali earth metal fluoride added carbons as well as the carbon (FE-5) is available as the anode for electrolytic production of (CF₃)₃N from the mixed melt having the molar fraction of KF higher than 0.143.

The thermal stabilities, ionic conductivities and oxygen reduction reaction (ORR) on Pt in 2-alkylimidazolium triflate (2RHimTf, R: methyl (Me), ethyl (Et), or propyl (Pr)), 2-alkylimidazolium tetrafluoroborate (2RHImBF₄) and acid-added systems as proton conductive room temperature molten salts (proton conductive RTMSs) have been investigated for application to intermediate temperature (from 100 to 200°C) fuel cells. The melting points of the 2EtHIm and 2 MeHIm salts were below room temperature. The mixture of 11 mol% trifluoromethanesulfonic acid (HTf) and l-ethyl-3-methylimidazolium triflate (11 mol% HTf + lEt3MeImTf), 2EtHImTf and 2EtHImBF₄ were stable up to 280, 390, and 250°C, respectively. The ionic conductivities of the 2RHImTf and 2RHImBF₄ were in the range from 10⁻² to 10⁻¹ S cm⁻¹ at 160°C, especially those of the 2EtHImTf and 2MeHImTf were greater than 0.10 S cm⁻¹. The order of the ORR rate on a Pt was HTf + lEt3MeImTf > HTf + 2EtHlmTf > 2EtHImTf (= 85 wt. % phosphoric acid) > 2 EtHImBF₄. The ORR rate in the HTf + lEt3MeImTf system was higher than that in HTf + 2EtHImTf, therefore the 1Et3MeIm cation is more adequate to the ORR on Pt than the 2EtHIm cation. The HTf added RTMSs showed higher ORR rates than the phosphoric acid and the neutral RTMSs. Therefore, the acidic room temperature molten salts would be better than the neutral one, regarding the ORR rate.

Raman spectra have been obtained for α- and β-NaFeO₂, Na₃FeO₃, Na₈Fe₂O₇, Na₅FeO₄, and Na₄FeO₃ from room temperature up to 833 K in the powder state under an inert atmosphere. The comparison of the spectra showed good applicability of Raman spectroscopy to the identification of these sodium iron double oxides. We have therefore investigated the corrosion reaction of a steel surface with Na₂O₂ powder using in situ Raman spectroscopy. The obtained spectra showed that a corrosion reaction occurs under 833 K, and the corrosion product was identified as Na₅FeO₄. A reaction mechanism for the corrosion based on the above results is proposed as follows: the system of Fe + Na₂O₂ produces corrosive melt under 833 K; it spreads over the surface of iron; the melts with a Na-rich composition produce Na₅FeO₄ in the initial state; the corrosion products distribute homogeneously on the surface. The corrosion reaction in a homogeneous powder mixture of Fe and Na₂O₂ has also been investigated using Raman spectroscopy with the help of XRD and DTA methods. The corrosion products were identified as double oxides whose compositions correspond to the stoichiometric molar ratio of Na to Fe in the starting materials, while the products from the surface reaction of steel with Na₂O₂ tended to have a spcified composition of Na₅FeO₄. The difference was reasonably explained using the mechanism presented above. It was found from the DTA measurements that Na₂O₂, in contrast to Na₂O and NaOH, has particularly strong corrosivity for iron.

The Raman spectra of molten lithium-potassium carbonate (LiKCO₃) coexisting with porous inorganic powder were measured. The effect of the surface properties of the solid phase and relationship between the electrical conductivity and phase transition behavior of molten carbonate was discussed. The stretching band of CO₃²⁻ ion (v₁) shifted toward lower wavenumbers with a decrease of the liquid content and an increase of the specific surface area of the solid phase caused by the interaction between ionic species and solid surface. The shift width of wavenumber also depends on the kinds of solid phase. The change of peak position for the system containing α-Al₂O₃ powder is larger than that for the system containing ZrO₂ powder. These tendencies correspond to the increase of the activation energy of the conductivity and the decrease of the melting point of the carbonate. For the results of conductivity and thermal analysis, the degree of the anomalous behavior of the molten carbonates by the solid phase was observed in the order of α-Al₂O₃ > γ-LiAlO₂ > ZrO₂. It is suggested that the influence of the solid phase on the anomalous properties related to the surface acidity.

The ionic liquid, trimethyl-n-hexylammonium bis((trifluoromehyl)sulfonyl)amide (TMHA-Tf₂N), with a wide electrochemical window of more than 5 V, is considered to be hydrophobic because of two –CF₃ groups in its Tf₂N⁻ anion. However, a small amount of water remains in the ionic liquid even after dehydration in vacuo, which causes some problems in metal electrodeposition when using the ionic liquid as a solvent. Therefore, we measured the water content of the ionic liquids, TMHA-Tf₂N and TMHA-Tf₂N containing M(Tf₂N)_n. (M = Li, Mg, Ni, Zn, Al, La, and Dy; n = 2, 3), as well as their kinematic viscosities and electrical conductivities in the temperature range of 25 to 130°C. The water content of TMHA-Tf₂N decreased with an increase in temperature from about 2300 ppm at 25°C to about 200 ppm at 130°C when water vapor pressure of the atmosphere was kept constant at 1600 Pa. At the same time, the kinematic viscosity decreased and the electrical conductivity increased. On the other hand, the water content of TMHA-Tf₂N containing M(Tf₂N)_n also decreased with an increase in temperature. The addition of M(Tf₂N)_n to TMHA-Tf₂N, increased the water content at a constant temperature, which resulted in slight increases in the kinematic viscosity and electrical conductivity. Walden’s plot of the conductivities against the viscosities, measured over various temperatures and water contents, gave a single straight line.

With the aim of achieving a continuous SiH₄ synthesis by a new molten salt electrochemical process, anodic reaction mechanism of metal grade Si electrode has been investigated in molten LiCl-KCl systems at 673 K. It has been found out that continuous anodic dissolution of Si is possible in an LiCl-KCl-LiF (5.0 mol%) melt, leading to the evolution of SiF₄ and SiCl₄ gases accompanied with the formation of a small amount of SiF₆²⁻ ion. While, continuous anodic Si dissolution and SiH₄ gas evolution were difficult due to the passivation of the Si electrode in an LiCl-KCl-LiF (5.0 mol%)-LiH (0.8 mol%) melt, indicating that the passivation was caused by H⁻ ion. However, it has been clarified that starting electrolysis in an LiCl-KCl-LiF (5.0 mol%) melt using an Si anode and an H₂ gas cathode opens up the possibility of avoiding the passivation and achieving a continuous SiH₄ synthesis.

Advantages of Si-Cu alloy anode for the SiH₄ synthesis by molten salt electrochemical process have been investigated by comparing the electrochemical behavior of Si-Cu alloy with that of bare Si in molten LiCl-KCl systems at 673 K. The results of cyclic voltammetry and potentiostatic electrolysis at 2.8 V (vs. Li⁺/Li) revealed that use of Si-Cu alloy allows increasing both the current efficiency and the reaction rate for SiH₄ evolution. It was also indicated that reducing Cu content is possible while maintaining sufficient current efficiency and reaction rate for SiH₄ synthesis. On the other hand, the effect of preventing the passivation of anode was not sufficient for the alloy prepared in this study, although the electrolysis was successfully sustained longer than the bare Si case. Metallographic requirements for the appropriate Si alloy were discussed aiming for the achievement of a continuous electrolysis in the future.

Direct electrodeposition of Mg metal containing La has been investigated in eutectic MgCl₂-NaCl-KCl melt including LaCl₃. Electrolysis in the salt was performed stably, and Mg metal containing La was electrodeposited. The La content in the deposited Mg strongly depended on the concentration of LaCl₃ in the molten salt and the cathode potential. Conversely, Mg metal containing the demanded amount of La should be produced easily by controlling the LaCl₃ concentration and the cathode potential. The corrosion resistivity of the Mg metal containing La in salt water was also measured. The corrosion resistivity changed with the La content in Mg, and was as good as that of AZ31 when the La content was about 0.5~3 mass%. It is concluded that the Mg metal containing La which shows good corrosion resistivity can be produced directly by the electrolysis in the molten salt.

The local structures of molten LaCl₃ and solid LaOCl at elevated temperatures were deduced from radial distribution analyses through La-L_III absorption-edge XAFS technique. The nearest neighbor La-Cl distance and the coordination number of Cl around La in molten LaCl₃ have so far been reported to be about 0.28 nm and nearly equal to 6 from the X-ray diffraction analysis. But the coordination number of Cl around La was evaluated at about 7 according to XAFS technique. By comparing the above results, the local structure of molten LaCl₃ was considered whether it would be described as an octahedral-like coordination scheme where La was surrounded by about six to seven Cl’s or the distinct schemes. A similar discussion was made on LaOCl with less solubility against molten salts.

The structure of the ionic species in an SnCl₂-1-ethyl-3-methylimidazolium chloride (EMIC) melt was examined using density functional theory (DFT) calculation and Raman spectroscopy. The Raman spectra of three different of SnCl₂-EMIC melts, 33/67 (33 mol% SnCl₂-67 mol% EMIC), 50/50, and 67/33 melts, were measured. The most stable structure of each SnCl₂, SnCl₃⁻, Sn₂Cl₄, Sn₂Cl₅⁻, EMI⁺, SnCl₃⁻ - EMI⁺ complex, or Sn₂Cl₅⁻ - EMI⁺ complex was obtained using the B3LYP method with the LanL2DZ basis set. SnCl₃⁻ was found to be contained in all of the examined SnCl₂-EMIC melts based on the assignment of the Raman spectra. Sn₂Cl₅⁻ was contained in the acidic melts (SnCl₂ > 50 mol%). The SnCl₃⁻ - EMI⁺ and Sn₂Cl₅⁻ - EMI⁺ complexes as a microscopic structure in the SnCl₂-EMIC melt were evaluated by a comparison between the harmonic frequencies and Raman spectra. In the SnCl₃⁻ - EMI⁺ complex, the SnCl₃⁻ was located on one side of EMI⁺ with a strong Coulombic force. Two possible structures of the Sn₂Cl₅⁻ - EMI⁺ complex were obtained by a comparison between the calculated and experimental Raman spectra. Sn₂Cl₅⁻ was also located on the same side of EMI⁺ as the SnCl₃⁻ - EMI⁺ complex.

A new process is studied to synthesize an intermetallic compound TiCr₂ directly from the oxide mixture of Cr₂O₃ and TiO₂. This method combines the calciothermic co-reduction and the in situ dissolution of the by-product CaO into the molten salt. The molten CaCl₂ was needed to dissolve CaO for complete reaction, and the reduction and alloying occurred simultaneously and quickly in this media. Because of the narrow compositional range of TiCr₂, however, a small amount of Ti and Cr coexisted with α-TiCr₂. The high temperature form, β-TiCr₂, was not found probably because it decomposed into α-TiCr₂ and Cr during cooling. The obtained alloy powder showed capability of hydrogen storage after activation.

A new room temperature molten salt (room temperature ionic liquid), 1-ethyl-2,3-dimethylimidazolium fluorohydrogenate (EDMIm(HF)_2.3F), has been prepared by the reaction of EDMImCl and large excess of anhydrous hydrogen fluoride (aHF). EDMIm(HF)_2.3F is composed of EDMIm cation and (HF)_nF anions (fluorohydrogenate anions) and exhibits the conductivity of 56.9 mS cm⁻¹ and viscosity of 6.8 cP at 298 K. Electrochemical window of EDMIm(HF)_2.3F determined by cyclic voltammetry using a glassy carbon electrode as a working electrode is about 2.9 V that is close to the window of 1-ethyl-3-methylimidazolium fluorohydrogenate (EMIm(HF)_2.3F) reported previously.

The relation between hydrogen pressure and the Warburg coefficient derived from AC impedance measurement at an Mo electrode indicates that hydrogen gas dissolves chemically into molten LiCl-KCl. The solubility coefficient of hydrogen in molten LiCl-KClat 673 K was estimated to be 6.1 × 10⁻⁶ mol H cm⁻³ atm^−1/2 from the amount of the dissolved hydrogen obtained by tensimetry. The diffusion coefficient of the dissolved hydrogen in this melt was calculated to be 2.6 × 10⁻⁵ cm² s⁻¹ from the Warburg coefficient and the solubility coefficient.

Limiting current densities of silver deposition in AlCl₃-NaCl-AgCl or LiCl-KCl-AgCl melts, and change in the equilibrium potential of silver with AlCl₃ concentration were measured to consider a large potential difference in the two melts. Diffusion rate of silver species in AlCl₃-NaCl-AgCl melt was concluded to be much slower than that in LiCl-KCl-AgCl melt. The formation of a silver complex in AlCl₃-NaCl-AgCl melt was suggested. The equation, Ag + AlCl₄⁻ = AgAlCl₄ + e or Ag + 2 A1Cl₄⁻ = Ag(AlCl₄)₂⁻ + e., might be a possible electrochemical reaction of silver in the melt.

To improve the oxidation resistance of TiAl intermetallic compounds under high temperature conditions, electrodeposition of an Al, Cr, and Ni layer was carried out by constant potential control or potential pulse control in AlCl₃-EMIC (1-ethyl-3-methyl-imidazolium chloride) ambient-temperature molten salt containing NiCl₂ and CrCl₂ at 338 K. The electrodeposition of Al, Cr, and Ni occurred at potentials lower than 0.0 V vs. A1/A1³⁺. Ni concentration in the deposit was about 9 at% at 0 V. However, it decreased with a decrease in potential to 1 at% at −0.2 V. The potential pulse technique enhanced the Ni concentration in the deposit to 20-27 at%. Cr concentration was also enhanced by pulse potential control: the concentration of Cr was raised to 15 at%. In all electrodeposits, sharp peaks corresponding to Al metal were detected with a relatively high background by XRD analysis. It is conceivable that deposit layers consisting of Al, Cr, and Ni may be amorphous.

An ionic liquid was chosen as a new type of fluid used to demonstrate the magnetohydrodynamic pumping principle using an ionic current derived from an electrochemical reaction. A simple pump was constructed and was successful in achieving moderate flow rates, as well as reversibility in flow direction and in electrochemical reaction. The Lorentz Law is applicable to this situation thus allowing the Lorentz force to be calculated.

The structural change of molten PbCl₂ by mixing with alkali chlorides was investigated by Pb L₃-edge XAFS measurements. By the mixing, the nearest neighbor Pb²⁺-Cl⁻ distance and its coordination number increased, while the Debye-Waller factor and the 3rd cumulant decreased. It is concluded that stability of the coordination structure (PbCl₆)⁴⁻ is constructed and its stability is enhanced by mixing with alkali chlorides.

We performed trial MD simulations for molten (Na-2Cs)Cl containing 10 wt% of UO₂²⁺ at 1100 K. In order to incorporate different function forms between alkali halides and UO₂, the Tosi-Fumi potentials used for the former were converted to the function form of the latter. From the viewpoints of both the bulk structure and local structure around UO₂²⁺, dependence of these structures was examined on the charge distribution in UO₂²⁺ using pair distribution functions, the coordination number, and snapshots. It was found that the bulk structure of the solvent was not influenced by the charge distribution in UO₂²⁺, although the local structure of UO₂²⁺ depended strongly on that in UO₂²⁺. Homogeneous dispersion of the UO₂²⁺ ions was observed in the case of the charge distribution in UO₂²⁺ with (Z_U, Z_O) = (4, −1), and the coordination number of CI⁻ around the U atoms was four.

The simulated channel boxes were electrolyzed in molten salts in order to fix the electrorefining conditions. The decontamination effects were investigated by using the samples of the actual spent channel boxes, which were obtained, at Hamaoka nuclear power station by Chubu Electric Power Inc. This paper outlined zircaloy wastes treatment and zirconium recovery process by electrorefining in molten salts.

The electrodeposition of indium-tin alloys on a platinum electrode from EMI·BF₄·Cl melts was investigated by cyclic voltammetry and potentiostatic electrolysis. The cyclic voltammogram of the melt containing 0.1 mol kg⁻¹ InCl₃ and 0.1 mol kg⁻¹ SnCl₂ presented two broad reduction waves on the cathodic scan from a rest potential and three oxidation waves on the reverse scan. The electrodeposits obtained by potentiostatic electrolysis were found to be the mixture of tin and InSn₄, of which the average mole ratio of indium contained in the electrodeposits increased with decreasing electrolysis potential in the range from −1.1 V to −1.4 V vs. Fe(III)/Fe(II).