This review article describes some selected novel molten salt electrochemical processes which have been challenged by the author and his co-workers. Metallic Na is produced with high current efficiency by the electrolysis of molten NaCl-ZnCl2 or NaOH using β-alumina diaphragm. Two kinds of chlorine recovery from HCl gas are possible by electrochemical processes in molten LiCl-KCl. A novel SiH4 production method has been proposed by the use of Si electrode in a molten LiCl-KCl-LiH system. N2 gas is cathodically reduced to form N3− ion in molten LiCl-KCl, which encourages the author to develop new applications, e.g., Li-N2 thermally regenerative fuel cell and thermogalvanic cell. Electrochemical implantation of nitrogen to form various metal nitrides is possible by the use of anodic reaction of N3− ion in a molten LiCl-KCl-Li3N system. Electrochemical implantation and displantation can be applied to form transition metal-rare earth alloys in molten LiCl-KCl containing rare earth chloride. Finally, as non-conventional electrochemical reactions, discharge electrolysis to form fine metal or carbon particles and electrochemical plantation of Zr on ceramics are introduced.
Sodium manganese composite oxides were prepared by the so1-gel method using manganese acetate, sodium acetate and tartaric acid. Manganese dioxides were obtained by extraction of sodium ion from the composite oxides in nitric acid solution. It was confirmed from XRD patterns that γ-MnO2 phase was formed in low molar ratio (Na/Mn ≦ 0.3) of sodium acetate to manganese acetate in starting materials, and the formation of α-MnO2 phase was observed in a higher ratio. The specific surface areas were 230-240 m2 g−1. The discharge capacities of the products heated at 375°C as a cathode material in lithium/manganese oxide battery were 250-260 mAh g−1 regardless of Na/Mn ratio. The capacities were higher than that of the electrolytic manganese dioxide (207 mAh·g−1). It was found that the composite oxide prepared using the sol-gel method is hopeful as a cathode material for lithium/manganese dioxide battery.
LaNi5−xMx(M = Al, Co, x = 0, 0.3, 0.5) electrodes were treated by immersing them in boiling alkaline solution to remove the oxide films from the alloy surface. The electrodes treated by immersing them in a boiling 6 mol dm−3 KOH solution for 2 h were activated rapidly, and the capacity was increased 10∼67%. The hydrogen absorbing reaction of the treated electrodes has been investigated using the AC impedance method and XPS, and has been compared with the non-treated electrode. From these results, it was found that the oxide films on the alloy surface were removed and the number of Ni clusters on the alloy surface was increased by the alkaline treatment. Therefore, the hydrogen absorbing reaction in which the diffusion of hydrogen atoms into the alloys follows the formation of hydrogen atoms was improved.
In order to develop a new method of forming porous titanium dioxide, anodic oxidation and chemical etching of titanium were investigated in aqueous sodium hydroxide solution. From an observation by a scanning electron microscope, the formation of porous structure was ascertained at the surface of titanium plate after anodic oxidation. The pore size and film thickness of the formed TiO2 were 50 nm∼1 µm and 100 nm∼1 µm, respectively, which were influenced strongly by reaction time, sodium hydroxide concentration, and bath temperature. In comparative experiments of chemical etching, porous TiO2 was also formed at the surface of titanium plate. The pore size and film thickness of the TiO2 formed by this method was similar to the films which obtained by anodic oxidation method. The results of X-ray photoelectron spectroscopy, X-ray diffraction, and X-ray fluorescence indicated that the films obtained from both methods were composed of amorphous TiO2. An advantage of the anodic oxidation method is that the growth rate of oxide film can be controlled by applied current.
Oxygen reduction properties of gas diffusion-type oxygen electrodes loaded by 50 wt% with perovskite-type oxides, La1−xAxMnO3(A = Na, K, Rb, 0.0 ≦ x ≦ 0.2), were investigated in 8 mol dm−3 KOH aqueous solution at 60°C under air flow. Among these oxides, La0.8Rb0.2MnO3 gave the highest electrode performance, i.e., current density of 341 mA cm−2 at −150 mV vs. Hg/HgO. This oxide was found to be highly active for the direct 4-electron reduction of oxygen as revealed by a rotating ring-disk electrode (RRDE) analysis. Electrode performances changed with a change in A or x over the oxides, and tended to be higher with the oxide which exhibited a smaller amount of oxygen desorption in temperature-programmed desorption (TPD) experiments. On the basis of the iodometry and electron spin resonance (ESR) analysis, the 4-electron reduction was suggested to take place most favorably at the sites composed of a pair of Mn3+ and Mn4+ on the oxide surface. The electrode loaded with La0.8Rb0.2MnO3 was confirmed to be fairly stable over a continuous operation for 100 h under a galvanostatic condition of 300 mA cm−2. The same electrode allowed to construct a zinc-air battery with a maximum power density as large as 293 mW cm−2 at a cell voltage of 0.7 V.
Zinc oxides modified with Al3+, Mg2+, and Cu2+ were used as photocatalysts for hydroxyl radical formation and degradation of methylviolet. The oxides with larger ESR signal of defect species around g=1.96 were effective for enhancement of photocatalytic activity.