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Lithium-ion batteries are crushed in lime water in an inert N2 atmosphere to inactivate them. N2 flow reduces the risk of hydrogen explosion, and water absorbs the reaction heat.

The effects of temperature, Ti(III) ion concentration, and current density on the electrodeposition of Ti films were investigated in the eutectic LiF–LiCl melt at 823–973 K. The Ti(III) ions were prepared by adding Li2TiF6 and Ti metal to the melt. The diffusion coefficients of Ti(III) were 1.4, 1.8, 2.3, and 3.2 × 10−5 m2 s−1, at 823, 873, 923, and 973 K, respectively. Galvanostatic electrolysis was conducted at 823–973 K. The surface roughness (S a) of the Ti films decreases with decreasing temperature. Thus, the electrodeposition of Ti films was conducted at the lowest temperature of 823 K with various Li3TiF6 concentrations (0.55–7.1 mol%) and cathodic current densities (50–1200 mA cm−2). The S a was lower at higher Ti(III) ion concentrations and lower current densities. The smoothest Ti films with a S a of 1.23 μm and a thickness of 10 μm were obtained at a cathodic current density of 50 mA cm−2 and Li3TiF6 concentration of 7.1 mol%.

We propose a novel diamond synthesis method based on molten salt electrolysis. In our method, carbon deposition and hydrogen generation occur simultaneously, and hydrogen reacts selectively with carbon atoms that possess sp 2 hybrid orbitals to form CH4 gas. Therefore, only carbon with sp 3 hybrid orbitals grows to form a diamond. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Raman spectroscopy analysis confirmed that diamond was synthesized by potentiostatic electrolysis at 1.1 V vs Li+/Li with a 10 C cm−2 charge density in molten LiCl–KCl–K2CO3–KOH at 973 K.

This study evaluated the refining ability of a proposed production process for solar-grade silicon utilizing the electrolytic reduction of SiO₂ on a liquid zinc electrode in molten salt. The distribution behaviors of impurity elements during the precipitation of solid silicon from a liquid Si–Zn alloy were studied by thermodynamic calculations at 923 K. In the precipitation experiment, silicon granules were recovered from a liquid Si–Zn alloy, which was prepared from metallurgical-grade silicon. The impurity removal ratios exceeded 99% for C, Al, and Ca, and 90% for Fe. High removal ratios were attained for B and O as well. As the post-processing, a silicon ingot was produced from the precipitated silicon granules by the floating zone method. The Zn residue in the precipitated silicon was completely evaporated during the floating zone refining. The total content of metallic elements (Al, Ca, Fe, Ti, and Zn) was lower than 0.2 ppmw, even though metallurgical-grade silicon was used as the starting material.

1. Introduction Titanium is known as a metal which has excellent properties such as high specific strength and high corrosion resistance. However, there are two problems that prevent widespread use of titanium. One is the high smelting cost and the other is the poor workability. As a method to solve these problems, titanium plating is an attractive technique because superior surface properties of titanium can be utilized. For some applications, physical vapor deposition (PVD) is practically used as a titanium plating method. However, the PVD method has high plating cost and can be used for only simple-shaped substrates. On the other hand, electrodeposition of titanium in high-temperature molten salt is a promising plating method which is expected to have lower cost and can be used for complex shaped substrates. Accordingly, there have been many reports on the titanium electrodeposition in high-temperature molten salts [1–3]. We have already reported the electrochemical behaviors of Ti(III) ions and the electrodeposition of titanium in KF–KCl and LiF–LiCl molten salts [4–7]. Furthermore, we found that LiF–LiCl has an advantage for electrodepositing smooth titanium films due to its lower melting point (774 K at the eutectic composition [8]); smoother titanium films were obtained at lower temperature by suppressing grain growth of titanium [9]. Thus, in the present study, we investigated the effect of electrolysis conditions such as current density and concentration of Ti(III) ions on smoothness of titanium films in LiF–LiCl eutectic molten salt. 2. Experimental The experiments were conducted in LiF–LiCl eutectic molten salt in an Ar glove box at 823 K. Li2TiF6(0.50–5.0 mol%) and Ti sponge (0.33–3.3 mol%) were added to the bath and Ti(IV) ions were converted to Ti(III) ions by comproportionation reaction. Ni plate, Mo flag, and Au flag electrodes were used as the working electrode. The counter and reference electrodes were Ti rods. The potential of the reference electrode was calibrated by Cl2/Cl− potential measured at a glass-like carbon rod electrode. Samples were prepared by galvanostatic electrolysis of Ni plate substrates. The samples were analyzed by SEM/EDX after washing with distilled water and 1M Al(NO3)3 aqueous solution to remove adhered salts. 3. Result and Discussion The electrodeposition was carried out by changing the added amounts of Li2TiF6from 0.50 to 5.0 mol% and Ti sponge from 0.33 to 3.3 mol%. The cathodic current density was also varied from 25 to 1000 mA cm−2. The surface roughness of the obtained samples was measured by SEM. As a result, smooth Ti films were obtained at high Ti(III) concentrations and low current densities. As a typical example of the smooth film, Fig. 1 shows an appearance of the sample prepared at 50 mA cm−2 and 60 C cm−2 after addition of 3.0 mol% Li2TiF6and 2.0 mol% Ti sponge. The electrodeposited titanium film had metallic luster. Fig. 2 shows a cross-sectional SEM image of the film, indicating the smooth and compact deposit. In the poster session, the solubility of Ti(III) ions will be presented. Also, the effects of Ti(III) ion concentration and cathodic current density on the morphology will be discussed. Acknowledgement The present address of Kouji Yasuda is Graduate School of Engineering, Kyoto University. References [1] H. Takamura, I. Ohno, and H. Numata, J. Jpn. Inst. Metals, 60, 388 (1996). [in Japanese] [2] A. Robin and R.B. Ribeiro, J. Appl. Electrochem., 30, 239 (2000). [3] V. V. Malyshev and D. B. Shakhnin, Mater. Sci., 50, 80 (2014). [4] Y. Norikawa, K. Yasuda, and T. Nohira, Mater. Trans., 58, 390 (2017). [5] Y. Norikawa, K. Yasuda, and T. Nohira, Electrochemistry, 86, 99 (2018). [6] Y. Norikawa, K. Yasuda, and T. Nohira, J. Electrochem. Soc., 166, D755 (2019). [7] Y. Norikawa, K. Yasuda, and T. Nohira, J. Electrochem. Soc., 167, 082502 (2020). [8] J. Sangster and A. D. Pelton, J. Phys. Chem. Ref. Data, 16, 509 (1987). [9] Y. Norikawa, K. Yasuda, K. Numata, M. Ogawa, M. Majima, and T. Nohira, Abstract of The 50th Symposium on Molten Salt Chemistry, p. 76, Tokyo, Japan (2018). Figure 1

The authors worked as working group members in the Molten Salt Committee of the Electrochemical Society of Japan in FY 2015–2016 to collect and summarize information on books and literatures devoted to molten salts. They covered a wide range of aspects of molten salts such as handling/measurement techniques, thermophysical properties, phase diagrams, thermodynamic data, electrochemical data, and structures. A total of 89 books and database literature sources were organized by content. The group also carried out an activity to archive the technical data of the electrolytic smelting of Al metal in the Hall-Héroult process that has operated in Japan. We hope this activity report will allow molten salt researchers to access trustworthy information conveniently.

The electrodeposition of W was studied in detail using CsF–CsCl–WO3. Prior to electrodeposition, the WO3 solubility was confirmed to be 1.0 mol% at 773 K and increase with temperature. To investigate the effect of temperature on the crystal structure, electrodeposition was conducted at 6–25 mA cm−2 between 773 and 923 K with a unified charge density of 90 C cm−2. X-ray diffraction analysis confirmed that the crystal structures of the electrodeposited W films were β-W at 773 and 823 K, a mixed phase (α-W and β-W) at 873 K, and α-W at 923 K. The shape of the crystal grains varied with temperature: grains of β-W obtained at 773 and 823 K were spherical, while those of α-W obtained at 923 K were angular. Scanning electron microscopy observations showed that W films with smoother surfaces were obtained at lower current densities at all temperatures. In particular, a dense and smooth W film (surface roughness: 0.66 μm, thickness: 10 μm) was obtained at 6 mA cm−2 and 773 K. When the charge density was increased to 210 C cm−2 at 6 mA cm−2 and 773 K, a W film with a smooth surface and thickness of 30 μm was obtained.

1. Introduction Ti has superior properties, such as high specific strength, corrosion resistance, biocompatibility, and is used for many fields such as airplane, chemical plant, consumer products. However, its high manufacturing cost and poor processability have hindered its further spread. Thus, a number of smelting processes and processing methods have been investigated to reduce the cost and spread the use of Ti. Among them, plating Ti metal on inexpensive substrates is one of the methods that can utilize the surface properties of Ti. Electrodeposition is a promising plating method from the viewpoints of the cost and the flexibility in shape of substrate. Therefore, electrodeposition of Ti metal in high-temperature molten salts has been investigated for a long time [1–3]. Our group has reported the electrodeposition of Ti films with compact and smooth surfaces in molten KF–KCl and LiF–LiCl containing Ti(III) ions at 923 K [4–7]. We also reported that the Ti films obtained in LiF–LiCl at 823 K had smoother surfaces [8]. In the present study, we investigated the effect of temperature on the grain growth and smoothness of Ti films electrodeposited in LiF–LiCl eutectic melt at 823–973 K. 2. Experimental The experiments were carried out in eutectic LiF–LiCl melt (30:70 mol%, melting point 774 K) at 823–973 K. The melt was placed in a nickel crucible and the experiments were conducted in an airtight Kanthal container in an argon glove box. As Ti(III) ion sources, Li2TiF6 (2.00 mol%) and Ti sponge (1.33 mol%) were added to the melt. The added amounts of Ti sponge were twice the amounts necessary to generate Ti(III) ions by comproportionation reaction between Ti(IV) ions and Ti(0) according to Eq. 1. Ti(0) + 3Ti(IV) → 4Ti(III) (1) Ni plates were used as working electrodes, and Ti rods was used as counter and reference electrodes. The samples prepared by galvanostatic electrolysis were analyzed by XRD and SEM/EDX after washing with distilled water to remove adhered salts. 3. Results and Discussion Fig. 1 shows the optical and surface SEM images of the samples obtained by galvanostatic electrolysis at 823, 873, 923, and 973 K. The cathodic current density was 50 mA cm−2 and electrolysis time was 20 min. All the samples have a metallic luster and are confirmed to be Ti metal by XRD analysis. However, the brightness differs between samples, with the Ti film at 823 K showing the highest brightness. From the surface SEM images, the crystal grain size of Ti increases as the temperature rises. This tendency is reasonably explained by previous work on the temperature dependence on grain growth of Ti [9]. Also, the value of surface roughness (Sa) measured by SEM increases as the temperature increases. The smoothest Ti film with Sa = 2.05 ± 0.22 μm was obtained at 823 K. These results show that Ti film with smoother surface can be electrodeposited by suppressing the grain growth of Ti at lower temperatures. To obtain thicker films, electrolysis was conducted for longer time at 823 K, where grain growth is suppressed. Fig. 2 shows the optical and SEM images of the samples obtained by galvanostatic electrolysis at a cathodic current density of 50 mA cm−2 and charge densities of 60, 150, and 450 C cm−2. As shown in the SEM images, the thickness of the Ti film increases with the increase in charge density. Although some deposition onto the edge was observed at 450 C cm−2, the thickness of the Ti film reaches 100 μm. Acknowledgement A part of this work was supported by JSPS Fellows grant number 19J15015. A part of this study was conducted as a collaboration with Sumitomo Electric Industries, Ltd. The present address of Kouji Yasuda is Graduate School of Engineering, Kyoto Univ. References [1] D. Wei, M. Okido, and T. Oki, J. Appl. Electrochem., 24, 923 (1994). [2] A. Robin and R. B. Ribeiro, J. Appl. Electrochem., 30, 239 (2000). [3] V. V. Malyshev and D. B. Shakhnin, Mater. Sci., 50, 80 (2014) [4] Y. Norikawa, K. Yasuda, and T. Nohira, Mater. Trans., 58, 390 (2017). [5] Y. Norikawa, K. Yasuda, and T. Nohira, Electrochemistry, 86, 99 (2018). [6] Y. Norikawa, K. Yasuda, and T. Nohira, J. Electrochem. Soc., 166, D755 (2019). [7] Y. Norikawa, K. Yasuda, and T. Nohira, J. Electrochem. Soc., 167, 082502 (2020). [8] M. Unoki, Y. Norikawa, K. Yasuda, and T. Nohira, ECS Trans., 98, 393 (2020) [9] K. Okazaki and H. Conrad, Metall. Trans., 3, 2411 (1972). Figure 1

1. Introduction Tungsten is a metal with excellent properties such as high heat resistance, high strength, and low thermal expansion, and is used for carbide tools, heat sinks, and fusion reactor diverters. However, due to its hardness and brittleness, tungsten is difficult to process into complex shapes and thin films. If tungsten plating on materials with good processability becomes possible, the range of applications will be greatly expanded. Therefore, as one of the plating methods, electrodeposition of tungsten in high-temperature molten salts has been investigated [1–3]. Our group has previously reported the electrodeposition of α-W films in molten KF–KCl–WO3 at 923 K and mixed phase films of α-W and β-W in molten CsF–CsCl–WO3 at 873 K [4]. We also reported that β-W films with mirror-like surface were electrodeposited in molten CsF–CsCl–WO3 at 773 K [5]. In the present study, we investigated the effect of bath temperature on the smoothness and crystal structure of W films electrodeposited from molten CsF–CsCl–WO3. We varied the temperature from 773 to 923 K and the cathodic current densities from 6 to 25 mA cm−2, and analyzed the electrodeposited W films on Cu plate substrates by XRD and SEM/EDX. 2. Experimental The experiments were carried out in eutectic CsF–CsCl melt (50:50 mol%, melting point 713 K) with WO3 (1.0 mol%) additive at various temperatures in the range of 773–923 K. The melt was placed in a graphite crucible and the experiments were conducted in an argon atmosphere in an airtight SS container. Cu plates were used as working electrodes and a glass-like carbon rod was used as a counter electrode. A Pt wire was used as a quasi-reference electrode and the potential was calibrated by Cs+/Cs potential measured at a Ag electrode. Samples prepared at cathodic current density of 6–25 mA cm−2 were washed with distilled water to remove adhered salts, and then analyzed by XRD and SEM/EDX. 3. Results and Discussion Fig. 1 shows the appearance of the samples obtained at 6–25 mA cm−2 and 773–923 K. No electrodeposits were obtained at 25 mA cm−2 and 773 K, which is due to the co-deposition of Cs metal fog because the potential during the electrolysis was close to 0 V with respect to Cs+/Cs potential. In other conditions, gray or silvery colored deposits were obtained. In particular, a mirror-like surface was obtained at 6 mA cm−2 and 773 K, indicating that the electrodeposited W film had a highly smooth surface. Fig. 2 presents the XRD patterns of the electrodeposits; all the XRD patterns were attributed to metallic W. However, only β-W was detected below 823 K, both α-W and β-W were detected at 873 K, and only α-W was detected at 923 K. This result indicates that the crystal structure of W obtained in molten CsF–CsCl–WO3 varies with bath temperature. To investigate the smoothness of each obtained W film, we performed cross-sectional SEM observation and surface roughness (Sa) measurement. Fig. 3 shows the cross-sectional SEM images. At all bath temperatures, the lower the current density, the smoother the surface becomes, and the higher the current density, the more uneven the surface becomes. This is because the concentration gradient of W(VI) ions in the diffusion layer becomes larger, and the electrodeposition of W proceeds preferentially on the convex surface. It was found at the same low current density that the lower the bath temperature, the higher the smoothness. In particular, the mirror-like W film obtained at 6 mA cm−2 and 773 K showed almost no irregularity in the cross-sectional SEM image. The surface roughness Sa was as small as 0.32 μm, indicating that the W film was extremely dense and smooth. The reason for the smoother surface at lower bath temperatures is speculated to be due to the crystal structure of β-W and the suppression of crystal growth. Acknowledgement The present address of Kouji Yasuda is Graduate School of Engineering, Kyoto Univ. References [1] S. Senderoff and G. W. Mellors, Science, 153, 3743 (1966). [2] A. Katagiri, M. Suzuki, and Z. Takehara, J. Electrochem. Soc., 138, 767 (1991). [3] Y. Liu, Y. Zhang, F. Jiang, B. Fu, and N. Sun, J. Nucl. Mater., 442, S585 (2013). [4] T. Nohira, T. Ide, X. Meng, Y. Norikawa, and K. Yasuda, J. Electrochem. Soc., 168, 046505 (2021). [5] X. Meng, Y. Norikawa, and T. Nohira, Electrochem. Commun., 132, 107139 (2021). Figure 1

Optimized conditions for Ti film electrodeposition were investigated in water-soluble KF–KCl (45:55 mol%) molten salt at 923 K. K₃TiF₆ was prepared in the melt via a comproportionation reaction between K₂TiF₆ and Ti sponges. The K₃TiF₆ solubility was determined to be 1.48 mol%. Galvanostatic electrolysis was conducted on Ni plate substrates at various K₃TiF₆ concentrations (0.28, 0.70, and 1.43 mol%) and current densities (25–150 mA cm⁻²). Surface and cross-sectional scanning electron microscopy observations showed that adherent, compact, and smooth Ti films were obtained at higher K₃TiF₆ concentrations and lower current densities. A corrosion test was conducted for the Ti-coated Ni plate using linear sweep voltammetry, which showed that the Ti films have no cracks or voids.

To establish a next-generation production process for SOG-Si, we proposed the electrochemical reduction process of SiO2 using a liquid Zn cathode in molten CaCl2. In this study, we focused on the dissolved SiO2 to improve the productivity of the process. The ionic species and electrochemical reduction of silicate ions at solid graphite and liquid Zn electrodes in molten CaCl2 with various O2−/SiO2 ratios (r O2−/SiO2 ) at 1123 K were investigated. Ionic species of silicates in the melts were determined by Raman spectroscopy. The dominant species were SiO3 2− (r O2−/SiO2 = 1.0) and SiO4 4− (r O2−/SiO2 = 1.5 and 2.0). Cyclic voltammetry, XRD, and SEM revealed that the electrochemical reduction of SiO3 2−, Si2O7 6−, and SiO4 4− occurs at the graphite electrode from 1.35 V, 1.12 V, and 0.75 V vs Ca2+/Ca, respectively. The electrochemical reduction potentials of silicate ions at the liquid Zn electrode shifted to more negative values in the order SiO3 2− > Si2O7 6− > SiO4 4−. The reaction observed at 0.60 V was attributed to the simultaneous electrochemical reduction of silicate ions and formation of Ca–Zn alloy. The indirect reduction of silicate ions by Ca–Zn alloy was also confirmed.

The effects of temperature and current density on the electrodeposition of Si films in molten KF–KCl–K2SiF6 were investigated at 923–1073 K. The peak current density of Si deposition in cyclic voltammetry increased as the temperature rose. The diffusion coefficient of Si (IV) ions measured by chronoamperometry increased through a rise in temperature. The activation energy for the diffusion of Si (IV) ions, 28.0 kJ mol−1, agreed with that for the viscosity of KF and KCl. The crystallinity of the Si films prepared by galvanostatic electrolysis on Ag substrates was measured based on electron backscatter diffraction. The largest crystallite size in the deposited Si increased with the deposition temperature, from a submicron size at 923 K, to several tens of microns at 1073 K. Moreover, even at the same temperature of 1073 K, larger crystallite sizes were observed for the Si deposit at 100 mA cm−2 as compared with that at 300 mA cm−2, which was explained based on the crystallization rate of Si.

In situ synchrotron X-ray diffraction was used to investigate the electrochemical reduction of SiO2 to Si in molten CaCl2 at 1123 K for the first time. The present technique enabled direct determination of intermediate products at high temperature, without cooling and washing treatments. Based on the diffraction data, the Ca2SiO4 phase was detected inside the electrode, but not at the electrode edge. These results were explained by the presence of different concentrations of O2− ions in molten CaCl2 permeating different regions of the electrode.

The electrochemical reduction of two types of simplified simulants of vitrified radioactive waste, simulant 1 (glass component only: SiO2, B2O3, Na2O, Al2O3, CaO, Li2O, and ZnO) and simulant 2 (also containing long-lived fission product oxides, ZrO2, Cs2O, PdO, and SeO2), was investigated in molten CaCl2 at 1103 K. The behavior of each element was predicted from the potential−pO2− diagram constructed from thermodynamic data. After the immersion of simulant 1 into molten CaCl2 without electrolysis, the dissolution of Na, Li, and Cs was confirmed by inductively coupled plasma atomic emission spectrometry and mass spectrometry analysis of the samples. The scanning electron microscopy/energy dispersive X-ray and X-ray diffraction analyses of simulants 1 and 2 electrolyzed at 0.9 V vs. Ca2+/Ca confirmed that most of SiO2 had been reduced to Si. After the electrolysis of simulants 1 and 2, Al, Zr, and Pd remained in the solid phase. In addition, SeO2 was found to remain partially in the solid phase and partially evaporate, although a small quantity dissolved into the molten salt.

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 cm²s⁻¹.

The effects of temperature and electrolysis potential on the alloying rate, structure, and mechanical strength for the Dy-alloyed Hastelloy C-276 samples, where Hastelloy C-276 is a Ni-based alloy containing Cr and Mo, were investigated in a molten LiF–CaF2–DyF3 (0.30 or 0.50 mol%) system at 1123–1323 K and a molten LiCl–KCl–DyCl3 (0.50 mol%) system at 873 K. The microstructure was studied by electron microscopy and energy-dispersive X-ray spectrometry analyses, and the mechanical strength of the formed Dy-alloys was evaluated using punch tests. The alloying rate was influenced by the electrolysis potential and significantly by the temperature. Phase separation into DyNi2 and Cr–Mo was observed, and a layered structure perpendicular to the depth direction was formed. The pitch of the layered structure was found to depend on the electrolysis potential, suggesting that the diffusion rate of Cr and Mo determined the structure. The Dy-alloyed samples electrolyzed at a more negative potential in the LiCl–KCl–DyCl3 melt exhibited a higher mechanical strength. The Dy-alloyed samples obtained in the LiF–CaF2–DyF3 melt at 1223 K and 1323 K exhibited a low mechanical strength owing to the large grain size of the agglomerated Cr–Mo alloy phase.

The electrodeposition of crystalline Si films on graphite substrates was investigated in KF–KCl molten salts at 1073 K. The optimum K₂SiF₆ concentration and current density to obtain adherent, compact, and smooth films were investigated using surface and cross-sectional scanning electron microscopy. The crystallinity of the deposited Si films was measured by X-ray diffraction and electron backscatter diffraction techniques. By photoelectrochemical measurements in CH₃CN–TBAPF₆–Fc at room temperature, the Si film electrodeposited on the graphite substrate at 100 mA cm−₂ for ₃0 min in molten KF–KCl–K₂SiF₆ (3.5 mol%) was found to be an n-type semiconductor. When SiCl₄ was used as the Si source, the melt with a higher molar ratio of KF deposited smoother Si films on the graphite substrates. The Si films electrodeposited in molten KF–KCl after the introduction of SiCl₄ gas (2.37 mol%) were confirmed to be p-type by photoelectrochemical measurements in CH₃CN–TBAClO₄–EVBr₂. The characteristics of the electrodeposited Si film (p-type or n-type) is determined by the contaminating impurities (B, P, and Al).

The electrochemical formation of Ni–Nd intermetallic compounds has been investigated in a molten CaCl2–NdCl3 system. For further understanding the thermodynamic properties of different Ni–Nd intermetallic compounds, an electrochemical investigating method was used. The temperature dependences of the Nd3+/Nd potential and two-phase coexisting potentials of the Ni–Nd intermetallic compounds were determined via open-circuit potentiometry using Mo and Ni flag electrodes, respectively. Expressing the two-phase coexisting potentials with respect to the Nd3+/Nd potential, the relative partial molar Gibbs energies of Nd were calculated. The Nernst and Gibbs-Duhem equations were used to determine the activities of Ni and Nd, respectively. The relative partial molar Gibbs energies of Ni were calculated using its activities. Thereafter, the relative partial molar enthalpies and entropies of Ni and Nd were obtained using the temperature dependences of the relative partial molar Gibbs energies. Finally, the standard Gibbs energies, enthalpies, and entropies of formation of the various Ni–Nd intermetallic compounds were calculated using the relative partial molar Gibbs energies of Nd and Ni. The obtained thermodynamic parameters were compared with previously reported values obtained by the CALPHAD method and modeling calculations.

The effect of adding Si powder/granules on the electrochemical reduction rate of SiO2 granules in molten CaCl2 was investigated. Various starting materials were prepared by mixing different amounts (up to 80 mass pct) of Si powder/granules (four different sizes

1.Introduction In recent years, the demand for solar cells as a clean energy source has been increasing. Among the various types of solar cells, crystalline Si solar cells dominate the solar cell market due to their high efficiency, excellent stability, and abundant natural resources. However, the major disadvantages of the conventional Si substrate manufacturing method are the low productivity of the Siemens process and the considerable kerf loss in the Si slicing process. As the demand for crystalline Si solar cells continues to grow, there is a strong need to develop alternative manufacturing methods for Si substrates. As a promising method, the direct formation of Si films on inexpensive substrates has been investigated [1]. We have proposed the electrodeposition of Si films from molten KF–KCl which has relatively high fluoride-ion concentration at eutectic composition (KF:KCl = 45:55 mol%, m.p.: 878 K) and high solubility in water [2]. We have already studied the optimum conditions for obtaining adherent, compact, and smooth Si films in molten KF–KCl–K2SiF6 at 923 K [3]. Furthermore, good Si films was obtained from KF–KCl after bubbling SiCl4 as a Si-ion source [4]. In this study, we focused on CsF–CsCl eutectic melt (CsF:CsCl = 50:50 mol%, m.p.: 713 K [5]) as a new electrolyte for Si electrodeposition. Molten CsF–CsCl has lower operable temperature and higher fluoride-ion concentration than molten KF–KCl at the eutectic composition. Moreover, CsF and CsCl have extremely high solubility in water (CsF; 573, CsCl; 191 g per 100 g H2O at 293 K [6]), indicating that the solidified salts can be easily removed. In this study, Si was electrodeposited on Ag plate substrates at 923 K at various Si-ion concentrations and cathodic current densities. The obtained Si films were analyzed by XRD and SEM/EDX to determine the optimum electrodeposition conditions. 2. Experimental The experiments were conducted in CsF–CsCl eutectic melt in an Ar glovebox. The experimental temperature was 923 K and Cs2SiF6was used as the Si ions source. Ag plate and Ag flag electrodes were used as the working electrodes. The counter and reference electrodes were Si rods. The potential of the reference electrode was calibrated by Cl2/Cl− potential measured at a glass-like carbon electrode. Samples prepared by galvanostatic electrolysis of Ag plates substrates were analyzed by SEM/EDX after washing with distilled water to remove adhered salts. 3. Results and Discussion The electrodeposition was carried out by changing the added amounts of Cs2SiF6 from 0.50 to 3.5 mol%. The cathodic current density was also varied in the range of 25–800 mA cm−2. As typical deposits, Fig. 1 shows SEM images of Si prepared at (a) 600, (b) 100, and (c) 50 mA cm−2 in CsF–CsCl–Cs2SiF6 (2.0 mol%). At a higher current density, Si was nodule-shaped because the concentration gradient of Si(IV) ions increased near the electrode, and Si deposition proceeded preferentially on the convex part close to the bulk molten salt. At medium current densities (e.g., 100 mA cm−2), the electrodeposited Si was film-like and the surface was smooth. Fig. 2 shows a cross-sectional SEM image of the film, indicating the smooth and compact deposit. The thickness is about 20 μm (the theoretical film thickness: 25 μm), indicating a relatively high current efficiency. Furthermore, it was found that the upper current limit to obtain Si with smooth surface is higher at the higher Cs2SiF6 concentration; Si films with smooth surface were obtained even at 600 mA cm−2 in CsF–CsCl–Cs2SiF6 (3.5 mol%). At low current density, Si was film-like, but many humps were observed due to insufficient overvoltage and difficulty in nucleation. Acknowledgement The present address of Kouji Yasuda is Graduate School of Engineering, Kyoto Univ. References [1] E. Juzeliunas and D. J. Fray, Chem. Rev., 120, 1690 (2020). [2] K. Maeda, K. Yasuda, T. Nohira, R. Hagiwara, and T. Homma, J. Electrochem. Soc., 162, D444 (2015). [3] K. Yasuda, K. Maeda, T. Nohira, R. Hagiwara, and T. Homma, J. Electrochem. Soc., 163, D95 (2016). [4] K. Yasuda, K. Maeda, R. Hagiwara, T. Homma, and T. Nohira, J. Electrochem. Soc., 164, D67 (2017). [5] J. M. Sangster and A. D. Pelton, J. Phys. Chem. Ref. Data, 16, 509 (1987). [6] D. R. Lide, CRC Handbook of Chemistry and Physics, 88th Edition, CRC Press, Boca Raton (2007). Figure 1

This study investigated the ionic species and electrochemical reduction of silicate ions at a solid graphite electrode in molten NaCl–CaCl2 eutectic melts with various concentrations of O2− ion at 1023 K. Silicate ion species in the melts with various O2−/SiO2 ratios ( r O 2 − / SiO 2 ) were determined by Raman spectroscopy. The dominant species was SiO3 2− for r O 2 − / SiO 2 = 1.0, and SiO4 4− for r O 2 − / SiO 2 = 1.5 and 2.0. From cyclic voltammetry, XRD, and SEM analyses, electrochemical reduction was indicated for SiO3 2− and SiO4 4− at more negative than 1.0 V and 0.80 V vs Na+/Na, respectively. Formation of CaSi2 was confirmed at 0.50 V in all molten salts with r O 2 − / SiO 2 = 1.0, 1.5, and 2.0. The potential ranges for pure Si deposition are almost the same in molten salts with r O 2 − / SiO 2 = 1.0 and 1.5.

The electrochemical formation of Neodymium–Nickel (Nd–Ni) alloys was investigated in a molten CaCl2–NdCl3 (1.0 mol%) system at 1123 K. Cyclic voltammograms for Molybdenum (Mo) and Ni electrodes showed the electrodeposition/dissolution of metallic Nd and the formation/dissolution of Nd–Ni alloys, respectively. The equilibrium potential of Nd3+/Nd was determined at 0.27 V (vs. Ca2+/Ca) by open-circuit potentiometry for a Mo electrode. The potentials of 0.48, 0.68, and 0.95 V, corresponding to the two-phase coexisting states of (NdNi2 + NdNi3), (NdNi3 + NdNi5), and (NdNi5 + Ni), respectively, were confirmed using energy-dispersive X-ray spectroscopy and X-ray diffraction of the Ni electrode electrolyzed samples. The optimum electrolysis conditions for the Nd and Dysprosium (Dy) separation were discussed, and the standard Gibbs energies of formation were calculated for Nd–Ni alloys.

The electrochemical reduction of vitrified nuclear waste simulant glass produced by melting oxides of 33 different elements, including Si and four types of long-lived fission product elements (Cs, Zr, Pd, and Se), was investigated in molten CaCl2 at 1103 K. Before the experiment, potential−pO2− diagrams were constructed from the thermodynamic data to predict the behaviors of the constituent elements during the electrochemical reduction. In the first experiment, small crucible electrodes containing approximately 100 mg of the simulant were electrolyzed at 0.6–1.1 V vs. Ca2+/Ca. X-ray diffraction analysis confirmed that SiO2 was reduced to Si. In addition, the formation of XB6 (X = Ca, Si, or rare earth elements (REEs)) was confirmed. In the second experiment, a large crucible electrode containing approximately 10 g of the simulant was galvanostatically reduced at −2 A for 5 h. The distribution ratios of the constituent elements to the solid product and to the molten salt were calculated from the result of composition analysis using inductively coupled plasma atomic emission spectrometry and mass spectrometry. In particular, four types of long-lived fission product elements (Cs, Zr, Pd, and Se) were investigated. Consequently, alkali and alkaline earth elements, including Cs, were dissolved into the molten salt. Si mostly remained in the solid product as elemental silicon. Most of the other elements, including Zr and Pd, were also found in the solid product. Se and Zn were indicated to partially evaporate depending on the duration of electrolysis.

1. Introduction Recently, the demand for solar cells has been increasing to realize low-carbon society. Among various types of solar cells, crystalline Si solar cells have dominated the photovoltaic market due to the high efficiency, excellent stability, and abundant natural resources. However, the low productivity of the Siemens process and the considerable kerf loss in the Si-slicing step are the main drawbacks in the conventional production method of the Si substrates. Since the demand for crystalline Si solar cells will continue to increase, the development of an alternative production method of Si substrate is strongly required. One of the expected methods is a direct formation of Si films on an inexpensive substrate [1–3]. We have proposed the electrodeposition of Si films from KF–KCl molten salt, in which SiCl4 gas is used as a Si source [4,5]. We have already investigated the optimum conditions for obtaining adherent, compact, and smooth Si films in molten KF–KCl–K2SiF6 at 923 K [6], and the effect of temperature and current density on the Si electrodeposition in molten KF–KCl–K2SiF6 at 923–1073 K [7]. However, the detailed analysis including the evaluation of semiconductor characteristic has not been performed for the Si films. In the present study, we prepared crystalline Si films on graphite substrates under several conditions in molten KF–KCl–K2SiF6 at 1023 K. The obtained Si films were analyzed by XRD and SEM/EDX. The semiconductor characteristic was also evaluated by a photoelectrochemical measurement which had been used in the report by Bard et al. [2,3]. 2. Experimental 2.1 Electrodeposition of Si The electrodeposition was conducted in molten KF–KCl–K2SiF6 in Ar atmosphere at 1023 K in a glove box. As a Si source, 3.5 mol% of K2SiF6 was added to the bath. A graphite plate was used as the working electrode. The counter and the reference electrodes were Si rods. Si films prepared by galvanostatic electrolysis were analyzed by XRD, SEM/EDX, and a photoelectrochemical measurement. 2.2 Photoelectrochemical measurement Photoelectrochemical measurement was conducted by linear sweep voltammetry. The electrolyte was prepared by mixing TBAClO4 (0.3 M) and EVBr2 (0.05 M) in CH3CN at room temperature. 2TBAClO4 + EVBr2 → EV(ClO4)2 + 2TBABr↓ After the reaction was completed, the supernatant liquid in which EV(ClO4)2 was dissolved was used as the electrolyte. A Pt plate and an Ag+/Ag electrode were used as the counter and reference electrode, respectively. A Xe lamp (100 mW cm−2) was used as a light source. The light was chopped at a frequency of 1 Hz during the linear sweep voltammetry. 3.Result and discussion 3.1 Electrodeposition of Si Galvanostatic electrolysis was conducted under the conditions of the added amount of K2SiF6 of 3.5 mol% and the cathodic current density of 100 mA cm−2 for 15 min. As shown in Fig. 1(a), the deposit on a graphite substrate has dark gray color. From XRD analysis, the deposit was confirmed to be crystalline Si. Fig. 1(b) shows a surface SEM image of the Si film, indicating compact and smooth surface with crystal grains of around 10 μm. The current efficiency was calculated to be 90.1 % from the amount of electricity and the weight increase. 3.2 Photoelectrochemical measurement The Si film on a graphite substrate was covered with an insulating coating to expose only the Si part and to prescribe the electrode area. Fig. 2 shows a photoresponse of the Si film during the linear sweep voltammetry, where the light is chopped at a frequency of 1 Hz. Cathodic currents change depending on the light chopping, indicating that the obtained Si film is p-type semiconductor. Acknowledgement The present address of Kouji Yasuda is Graduate School of Engineering, Kyoto University. R eferences [1] E. Juzeliunas and D. J. Fray, Chem. Rev., 120, 1690 (2020). [2] J. Zhao, H. Yin, T. Lim, H. Xie, H. Hsu, F. Forouzan, A. J. Bard, J. Electrochem. Soc., 163, D506 (2016). [3] X. Zou, L. Ji, J. Ge, D. R. Sadoway, E. T. Yu, and A. J. Bard, Nat. Commun., 10, 5772 (2019). [4] K. Maeda, K. Yasuda, T. Nohira, R. Hagiwara, and T. Homma, J. Electrochem. Soc., 162, D444 (2015). [5] K. Yasuda, K. Maeda, R. Hagiwara, T. Homma, and T. Nohira, J. Electrochem. Soc., 164, D67 (2017). [6] K. Yasuda, K. Maeda, T. Nohira, R. Hagiwara, and T. Homma, J. Electrochem. Soc., 163, D95 (2016). [7] K. Yasuda, K. Saeki, T. Kato, R. Hagiwara, and T. Nohira, J. Electrochem. Soc., 165, D825 (2018). Figure 1

1. Introduction Titanium metal has superior properties such as high specific strength, high corrosion resistance, biocompatibility, etc. In addition to these properties, crustal abundance of titanium is more than 40 times larger than those of commonly used copper and nickel. However, titanium is not used widely and commonly due to two problems: the high cost of present smelting method and the poor workability of itself. Therefore, both a new smelting method and a processing method are required. Plating titanium metal on an inexpensive substrate is one of the methods to resolve these problems. Electrodeposition of titanium is a promising plating method from the viewpoints of the cost and the flexibility in shape of substrate. Thus, electrodeposition of titanium metal has been investigated for a long time using high-temperature molten salts [1–4]. We have already reported that compact, smooth, and adherent titanium films are electrodeposited in molten KF–KCl and LiF–LiCl containing Ti(III) ions at 923 K [5–8]. However, crystal grains of titanium become large with the increase of a film thickness, which results in a rough surface. Concerning the crystal grain size, Wei et al. electrodeposited titanium in molten LiCl–NaCl–KCl–K2TiF6 (+ Ti metal) at 723–973 K and reported that the grain size became small as the temperature decreased [1]. This report suggests that titanium films are expected to have a smoother surface at a lower electrodeposition temperature. Therefore, in the present study, we investigated the effect of temperature on morphology and smoothness of electrodeposited titanium films in LiF–LiCl eutectic melt at 823–973 K. The effect of temperature on the electrochemical behavior of Ti(III) ions was also studied. 2. Experimental The experiments were conducted in LiF–LiCl eutectic melt (LiF:LiCl = 30:70 mol%, m.p.: 774 K [9]) in an Ar glovebox. The experimental temperature was varied from 823 K to 973 K. Li2TiF6(2.00 mol%) and Ti sponge (1.33 mol%) were added to the bath and Ti(III) ions were prepared by comproportionation reaction. Ni plate, Mo flag, and Au flag electrodes were used as the working electrodes. The counter and reference electrodes were Ti rods. The potential of the reference electrode was calibrated by Li+/Li potential measured at a Mo flag electrode. Samples prepared by galvanostatic electrolysis of Ni plate substrates were analyzed by SEM/EDX after washing with distilled water to remove adhered salts. 3. Results and Discussion Galvanostatic electrolysis was conducted at 823, 873, 923, and 973 K. Fig. 1 shows the optical and surface SEM images of the samples. The cathodic current density and electrolysis time were 100 mA cm−2 and 10 min, respectively. The surface roughness (Sa) was measured by SEM, which is also shown in Fig. 1. All the samples have a metallic luster. The sample at 823 K has the highest brightness with metallic luster. The surface SEM observations indicate that the crystal grain size increases as the temperature rises. This tendency is reasonably explained by a previous work on the temperature dependent grain growth of Ti [10]. The value of Sa also increases as the temperature increases. The smoothest Ti film with Sa = 2.05 ± 0.22 μm was obtained at 823 K. These results conclude that Ti film with smoother surface can be electrodeposited at a lower temperature by suppressing the grain growth of Ti. In the presentation, more detailed analysis results and the effect of temperature on the electrochemical behavior of Ti(III) ions will be discussed. Acknowledgement A part of this work was supported by JSPS Fellows grant number 19J15015. The present address of Kouji Yasuda is Graduate School of Engineering, Kyoto Univ. References [1] D. Wei, M. Okido, and T. Oki, J. Appl. Electrochem., 24, 923 (1994). [2] H.Takamura, I. Ohno, and H. Numata, J. Jpn. Inst. Metals, 60, 388 (1996). [in Japanese] [3] A. Robin and R. B. Ribeiro, J. Appl. Electrochem., 30, 239 (2000). [4] V. V. Malyshev and D. B. Shakhnin, Mater. Sci., 50, 80 (2014) [5] Y. Norikawa, K. Yasuda, and T. Nohira, Mater. Trans., 58, 390 (2017). [6] Y. Norikawa, K. Yasuda, and T. Nohira, Electrochemistry, 86, 99 (2018). [7] Y. Norikawa, K. Yasuda, and T. Nohira, J. Electrochem. Soc., 166, D755 (2019). [8] Y. Norikawa, K. Yasuda, and T. Nohira, J. Electrochem. Soc., 167, 082502 (2020). [9] J. Sangster and A. D. Pelton, J. Phys. Chem. Ref. Data, 16, 509 (1987). [10] K. Okazaki and H. Conrad, Metall. Trans., 3, 2411 (1972). Figure 1

Introduction Tungsten metal is known for its robustness, especially high melting and boiling points and hardness. However, its hardness and brittleness make it difficult to work. Thus, electrodeposition of flat and dense tungsten films is worth investigating. There are a number of reports on the electrodepositon of tungsten from high temperature molten salts such as fluorides [1], chlorides [2], oxides [3], etc. It is known that dense and coherent tungsten deposits are more easily obtained from molten fluorides compared to molten chlorides. One drawback of typical fluoride melts like LiF–NaF–KF is difficulty in removing the adhered molten salts by water washing because LiF and NaF have limited solubility to water [4]. Although good tungsten deposits can be electrodeposited from molten oxides [3], the operation temperature tends to be very high, typically over 1123 K. From this background, we proposed new electrolyte baths, molten KF–KCl [5,6] and CsF–CsCl. Since all the components salts of KF, KCl, CsF and CsCl have large solubility to water [4], solidified salts on deposited films can be easily removed by water washing. Results & Discussion First, electrodeposition of tungsten was investigated in eutectic KF–KCl melt to which WO3 was added at 923 K. Cyclic voltammograms at copper and gold electrodes indicated the deposition of tungsten metal at more negative potential than 1.55 V vs. K+/K. The effects of electrolysis conditions on the morphology of deposits were studied by changing the cathodic current density from 12.5 to100 mA cm−2. As shown in Fig.1(a), a dense tungsten film with a thickness of approximately 20 µm was obtained on a copper substrate electrode at 12.5 mA cm−2 for 120 min. From the Raman spectra of molten KF–KCl and molten KF–KCl–WO3 at 923 K, the coordination structure of W(VI) complex ion was indicated to be fac-[WO3F3]3−. Secondly, eutectic CsF–CsCl melt was used to lower the operation temperature. After adding 2.0 mol% of WO3, the electrodeposition was investigated using copper and gold electrodes at 823–923K. As shown in Fig.1(b), a flat and dense tungsten film was obtained by galvanostatic electrolysis at 12.5 mA cm−2 for 120 min at 873 K. Acknowledgement This study is supported by Sumitomo Electric Industries, Ltd. The present address of Kouji Yasuda is Graduate School of Engineering, Kyoto University. References [1] S. Senderoff, G. Mellors, Science, 153, 1475 (1966). [2] M. Masuda, H. Takenishi, A. Katagiri, J. Electrochem. Soc., 148, C59 (2001). [3] K. Koyama, M. Morishita, T. Umezu, Electrochemistry, 67, 667 (1999). [4] D. R. Lide (Ed.), CRC Handbook of Chemistry and Physics, 88th Edition, Chap.4, p.43-101, CRC Press, Boca Raton (2007). [5] K. Maeda, K. Yasuda, T. Nohira, R. Hagiwara, T. Homma, J. Electrochem. Soc., 162, D444 (2015). [6] T. Nohira, Molten Salts, 62, 3 (2019). Figure 1