• Energy Research

Oxygen electrode reaction was investigated on a boron-doped diamond electrode in a LiCl―KCl eutectic melt. The standard formal potential of O 2 /O 2― decreases with the elevation of temperature. The potential at 773 K is 2.424 ± 0.003 V vs Li + /Li. The standard formal free energy change increases with the temperature elevation, calculated to be ―456.4 ± 0.5 kJ mol ―1 at 773 K. The standard formal entropy and enthalpy changes are determined to be ―151 ± 3 J K ―1 mol ―1 and ―573.5 ± 0.1 kJ mol ―1 , respectively, at 773 K.

Rare earth (RE) metals have several superior characters such as electric, magnetic, and fluorescence properties and are therefore indispensable as industrial materials throughout the world. Among the many applications of RE metals, the use of neodymium–iron–boron (Nd–Fe–B) magnets—the so-called neodymium permanent magnets—has been remarkably increasing recently. Because the properties of praseodymium (Pr) and Nd are similar, they are found in nature in the same ores and their separation is difficult. Thus, in some applications including permanent magnets, a Nd–Pr alloy called didymium (Di) is sometimes utilized. On the other hand, Nd–Fe–B magnets have the drawback of a relatively low Curie temperature of ~583 K. In order to maintain its superior coercive force even at high temperatures (above 473 K), where high-performance motors in EVs and HEVs operate, dysprosium (Dy) is necessary as an additive. One of the concerns pertaining to RE magnets is the uneven distribution of RE resources. While the present RE production capacity satisfies the demand and their price has lowered, there remains the potential for RE shortage in the future. On the basis of these supply circumstances, it can be concluded that the recycling or waste management of Nd–Fe–B magnet scraps is currently an urgent task. We have proposed a new separation and recovery process for RE metals from magnet scraps using molten salts and an alloy diaphragm as shown in the figure [1]. According to our previous studies, a specific RE element can be alloyed and de-alloyed rapidly with iron-group (IG) metals in molten salts by using electrochemical methods. In the proposed process, Nd, Dy, and Pr are separated because of differences in both the formation potentials and formation rates of the RE–IG alloys used as the diaphragm. In this study, selective formation of RE–Ni was investigated in molten NaCl–KCl–RECl3at 973 K. Based on our previous studies [2-4], the effective potential range for the separation of Dy from Pr and Nd is expected to be between 0.39 V and 0.48 V vs. Na+/Na. In this potential range, the thermodynamically stable phases are PrNi3, NdNi3, and DyNi2. Since the formation of DyNi2 is considerably faster than that of PrNi3 and NdNi3, it is expected that the formation of DyNi2 would proceed preferentially to those of NdNi3 and PrNi3. Then, the electrochemical formation of RE–Ni alloys were carried out at 0.42 V for 60 min in NaCl–KCl–0.50mol%RECl3. The thickness of the formed alloy layer was 4 µm for Nd-Ni, 2 µm for Pr-Ni, and 50 µm for Dy-Ni. These results suggest that Dy-Ni alloy is selectively formed in the melt containing both of NdCl3, DyCl3, and PrCl3. The sample was prepared by potentiostatic electrolysis at 0.42 V for 60 min in NaCl–KCl–0.50mol%NdCl3–0.50mol%DyCl3–0.50mol%PrCl3. The sample was analyzed by cross-sectional SEM and EDX. The formed alloy layer has a thickness around 60 µm with a composition of Nd:Dy:Pr = 1.3:29.8:0.7 at%. The high composition ratio, x Dy / (x Nd + x Pr), corresponding to 14.9 clearly indicates that Dy-Ni alloy can be selectively produced at this potential, and that RE-Ni alloy can be utilized as an alloy diaphragm with high separation ability for the proposed process. The results at different potentials are discussed in the presentation. References [1] T. Oishi, H. Konishi, T. Nohira, M. Tanaka, and T. Usui, Kagaku Kogaku Ronbunshu, 36, 299 (2010) (in Japanese). [2] K. Yasuda, S. Kobayashi, T. Nohira, and R. Hagiwara, Electrochim. Acta, 92, 349 (2013). [3] K. Yasuda, S. Kobayashi, T. Nohira, and R. Hagiwara, Electrochim. Acta, 106, 293 (2013). [4] K. Yasuda, K. Kondo, T. Nohira, and R. Hagiwara, J. Electroch em Soc., submitted.

AbstractThe discharge behavior of the Li/C2F battery in a 1 M LiClO4/propylene carbonate electrolyte is investigated.

Electrolytic production process for solar-grade Si utilizing liquid Si–Zn alloy cathode in molten CaCl2 has been proposed. Toward the establishment of the process, the behavior of liquid Zn metal was investigated in molten CaCl2 at 1123 K. The evaporation of Zn metal was largely suppressed by an immersion into molten salt, which enables the use of Zn electrode in spite of the high vapor pressure of Zn. The cyclic voltammetry suggested the reduction of SiO2 at 1.45 V vs. Ca2+/Ca on a Zn cathode. After the potentiostatic electrolysis at 0.9 V, Si particles with diameters of 2–30 µm were precipitated in the solidified Zn matrix by the slow cooling process of the produced liquid Si–Zn alloy. The alloying rate between solid Si and liquid Zn was measured as 4.56 μm s−1, and it linearly decreased with the Si content in liquid Zn phase.

A short review of ionic liquids (ILs) and their applications as electrolytes for electrochemical devices, such as electric double layer capacitors, fuel cells, lithium batteries, and solar cells, are presented here. The properties of ILs, such as non-volatility, non-flammability, wide liquid temperature ranges, and wide electrochemical windows, have the potential to be improved, including improvements in durability and safety, extending the operational temperature ranges and enabling improvements in power and energy densities of the devices.

Composite membranes consisting of N-ethyl-N-methylpyrrolidinium fluoro-hydrogenate (EMPyr(FH)1.7F) ionic liquid and poly(vinylidene fluoride hexafluoro-propylene) (PVdF-HFP) copolymer were successfully prepared in weight ratios of 5:5, 6:4, and 7:3 using a casting method. The prepared membranes possessed rough surfaces, which potentially enlarged the three-phase boundary area. The EMPyr(FH)1.7F/PVdF-HFP (7:3 weight ratio) composite membrane had an ionic conductivity of 41 mS·cm-1 at 120 °C. For a single cell using this membrane, a maximum power density of 103 mW·cm-2 was observed at 50 °C under non-humidified conditions; this is the highest power output that has ever been reported for fluorohydrogenate fuel cells. However, the cell performance decreased at 80 °C, which was explained by penetration of the softened composite membrane into gas diffusion electrodes to partially plug gas channels in the gas diffusion layers; this was verified by in situ a.c. impedance analysis and cross-sectional SEM images of the membrane electrode assembly.

AbstractDue to the monopolized supply of tungsten resource, it is important to efficiently recycle tungsten scrap for use as a secondary resource. The recycling of tungsten from cemented carbide tools by the molten carbonate method was investigated using simulated hard and soft scrap (carbide tool tips and WC powder, respectively). The oxidative dissolution of tungsten was examined in molten Na2CO3 under Ar–O2–CO2 atmospheres at 1173 K. Based on the immersion potentials of Cu, W, Co, C, and WC–Co, Cu2O was suggested to work as an oxidizing agent for tungsten dissolution. The oxidative dissolution rate for carbide tool tips with 12.8 mol% Cu2O addition reached 57 mg h−1 for the reaction time of 2.5 h, equivalent to 0.32 mm h−1. The decrease in the dissolution rate after 2.5 h was attributed to the decrease in the Cu(I) ion concentration in the melt and the inhibition of ion diffusion by the deposited metallic Cu. No violent reaction leading to explosion was observed, even for the oxidative dissolution of fine WC powder with a large surface area. Thus, this method provides significant safety improvements compared to the molten nitrate method. Graphical Abstract

The electrochemical reduction behavior of stratified SiO(2) granules in molten CaCl(2) at 1123 K (850 °C) was investigated to develop a new process for producing solar-grade silicon. The cross sections of the electrolyzed electrode prepared from a graphite crucible filled with SiO(2) granules were observed and analyzed. The visual and SEM observations indicate that the overall reduction proceeds via two different routes. In one route, the reduction proceeds along the granule surfaces from the SiO(2) near the conductor to the distant SiO(2). In the other route, the reduction proceeds from the granule surface to the core of each granule. The reduction along the granule surfaces is faster than that from the surface to the core. Fine SiO(2) granules are expected to be favorable for a high reduction rate.