Electrodeposition of Aluminum Using Chloroaluminate Ionic Liquids Under Dry Air Conditions

Abstract

Electrodeposition of Al at near room temperature using chloroaluminate ionic liquids, represented by the 1-ethyl-3-methylimidazolium chloride−AlCl3 system, has been extensively investigated as a technique for the fabrication of Al coatings. Such coatings have many potential applications, e.g., as corrosion-protective and decorative coatings. The electrodeposition of Al is usually performed under an inert gas atmosphere such as nitrogen and argon in a closed system using a glove box to protect the bath from absorbing moisture. Exposure of chloroaluminate ionic liquids to ambient air readily causes hydrolysis, which makes Al electrodeposition impossible. However, operation in a closed system reduces the productivity and increases the cost of the electrodeposition process, making it unsuitable for a large-scale production in industry. In this study, the feasibility of the electrodeposition of Al using chloroaluminate ionic liquids under dry air conditions was examined. Such dry air conditions can be maintained in a system that is half-open to ambient air using a dehumidifier. As the electrodeposition process in dry air does not require the use of a glove box, it should be much more productive than in inert atmosphere. Four kinds of chloroaluminate ionic liquids composed of different organic cations ([cation]Cl−AlCl3) with a molar ratio of [cation]Cl:AlCl3 = 1:2 were employed as the bath for Al electrodeposition. The cations employed were 1-ethyl-3-methylimidazolium, 1-butylpyridinium, 1-butyl-1-methylpyrrolidinium, and trimethylphenylammonium. Electrochemical behavior of Al in these baths in dry air was examined by cyclic voltammetry. Galvanostatic electrodeposition was performed and deposits from these baths in dry air were inspected by X-ray diffraction, scanning electron microscopy, and elemental analysis. The current efficiency was also assessed. These experiments clarified the baths that can produce Al films in dry air with as good quality as the ones obtained in an inert atmosphere. The reactivity between the organic cation and superoxide ion generated by the cathodic reduction of dissolved oxygen seemed to be responsible for the difference in the quality of the Al films electrodeposited from these baths in dry air. The reactions of dissolved oxygen in these baths are discussed.