Liquid Composed of Only Ions
Mikito Ueda , Professor
Faculty of Engineering, Graduate School of Engineering(Materials Science and Engineering, Department of Applied Science and Engineering, School of Engineering)
High school : Tomakomai National College of Technology (Hokkaido)
Academic background : background: Graduate School of Engineering, Hokkaido University
- Research areas
- Electrochemistry, Surface Treatment
- Research keywords
- Molten Salts, Ionic Liquids, Recycle, Fuel Cell System
What Kind of Research are You Conducting?
Figure: Ionic Liquid Composed of Chloroaluminate and Imidazolium Ions.
A student learns about oxidation and the reduction of ions in high school. For example, when we try to reduce the Na+ or Al3+ ions in aqueous solutions, however, the Na+ or Al3+ ions are not reduced in the solutions because the electrolysis reaction of water in solutions occurs before the reduction reaction of the Na+ or Al3+ ions. Therefore, liquid without water is required to produce metallic Al through the electrolysis of Al ions. My research area involves molten salts and ionic liquids that contain no water. One can produce active metals by reducing the active metals ions such as Al ions in molten salts or ionic liquids. Molten salt is an inorganic salt which is melted at a temperature more than 100°C in order to keep the molten state. On the other hand, most ionic liquids, which are composed of organic salt ions, have a melting point lower than room temperature. Ionic liquid is also a solvent for reducing active metal ions at relatively low temperatures. The photograph shows an ionic liquid for the electrodeposition of Al. This liquid is a mixture of imidazolium ions and complex ions of chloroaluminate.
When we use molten salts and ionic liquids as a media for the reduction reactions of ions, my research also involves metal production or electroplating. When used as media for oxidation reactions from metals to ions, my research involves batteries. Using these liquids, I am studying the electroplating of Al or the electrorefining of metallic Na contained in used, large-size batteries.
Electroplating of Aluminum
Figure Mirror –like Al Electroplating
Because Al forms a thin and dense oxide film on the surface, it features excellent corrosion resistance. As a resource, there is a large supply of Al found on earth, and therefore, it is a very attractive metal. Al electroplating using an ionic liquid can be applied to many fields. For example, when Al is electroplated to a steel surface, the corrosion-prone surface of steel is improved by Al electroplating and the strength of the steel is maintained. However, it’s not so easy to form the flat Al film by electroplating, because Al tends to grow into random directions during electrodeposition. We found that by adding organic compounds to the electrolyte, it was absorbed onto the growing tips enabling us to control the growing directions to some extent, resulting in a flatter surface formation. The photograph shows a prototype of a mirror-like surface of Al electroplating. While there is still progress to be made for this surface, further investigation in the additives and in the electroplating conditions will lead to the formation of truly mirror-like surfaces using electroplating.
Electrorefining of Sodium
Figure: Granular-Shape Liquid Na Deposited on Electrode
There are large-size sodium-sulfur (Na-S) secondary batteries at electric power companies or factories in Japan. Of course, the battery contains metallic Na and has a long lifetime of about fifteen years. A lot of metallic Na emitted after used the battery must dispose safely. Furthermore, the Na that is used for this battery is all supplied by imports. To secure the resource of sodium in Japan and dispose batteries safely, I am developing an electrorefineing process of the Na which is collected from used batteries, using the ionic liquid electrolyte. The photograph shows granular-shape liquid Na deposited on the electrode. The refined Na is found to be the same high purity that is used in raw materials for batteries, and I am developing a large scale electrorefinening cell to produce a lot of metallic Na continuously.
What Will be Your Next Goal?
As previously described, Al electroplating is formed by reducing Al ions in ionic liquid, but the opposite reaction process is also possible. We can make Al batteries when combining an Al oxidation reaction and another reduction reaction in the electrolyte. When Al is oxidized to Al3+ ions, three electrons are emitted in the electrode reaction. Therefore, it can provide an electric current three times more compared to the electrode reaction of a monovalent ion. Using Al as a battery material may produce a high-performance battery. I am developing fuel cells in which Al is embedded in the ionic liquid electrolyte. There are many problems to be solved in the development of the fuel cell. I think that a large amount of scrap Al disposed throughout the world can be used for electrode materials. By establishing this type of fuel cell, I can create a sustainable process which is compatible with the disposal of scrap Al while collecting energy from the cell. And, I would like to contribute to the solutions for both environmental and energy issues. That is why I must find an ionic liquid with excellent characteristics in the future