Electrodeposition of Aluminum from AlCl3 / Glyme Solutions at Room Temperature

Aluminum (Al) is used in many industrial fields because of its lightweight property, ubiquitous existence and corrosion resistance. Al is also attractive as a negative electrode material of Al ion rechargeable battery due to its low redox potential (–1.68 V vs. SHE) and high theoretical capacity (8042 Ah dm^–3). However, Al metal cannot be obtained from aqueous solutions since hydrogen evolution is dominant, while it can be electrodeposited from high temperature molten salts, organic solvents and ionic liquids [1]. Organic solvent such as tetrahydrofuran is highly volatile even at room temperature. Ionic liquids have some features such as flame resistance, low volatility and good electrical conductivity. However, ionic liquids are highly viscous and still expensive for practical use. Although AlCl₃ / dimethylsulfone electrolyte has some good features such as high conductivity and low toxicity, this electrolyte is usually solid at room temperature and electrodeposition was mainly performed above 110 °C. In this paper we used glymes (diglyme, triglyme, tetraglyme) as solvents. Glymes have boiling points and flash points over 100 °C which give low volatility at room temperature. We explored the Al redox behavior and the morphology of electrodeposited Al in AlCl₃-dissolved glyme solutions.

   All electrochemical experiments were performed at room temperature in an Ar-filled glove box. Al sheets were used as counter and reference electrodes. A Cu sheet was used as working electrode (WE) after being washed with acetone. AlCl₃ and glymes were mixed with molar ratio of AlCl₃: glyme = 1:5 and used as electrolytes. Only AlCl₃-dissolved diglyme solution was used for potentiostatic electrolysis at –1.0 V vs. Al. Molar conductivities and viscosities were measured at 28 °C in an Open Dry Chamber (Daikin) with H₂O < 40 ppm.

   Figure 1 shows the cyclic voltammograms (CVs) measured for the Cu electrode in the glyme solutions. Reduction and corresponding oxidation current density that originated from the deposition and dissolution of Al were observed only for AlCl₃-dissolved diglyme solution. Reduction and oxidation current density started at –0.7 V and –0.3 V vs. Al, respectively. On the other hand, sizable oxidation current density were not observed at around 0 V in the triglyme and tetraglyme solution but at +0.8 V vs. Al (see Fig1. b, c). However, potentiostatic electrolysis of Cu WE at +1.0 V vs. Al gave CuCl, confirmed by X-ray diffraction (XRD, not shown).

   TableⅠ lists molar conductivities and viscosities of the three kinds of glyme solutions. Although the diglyme solution has the highest molar conductivity and the lowest viscosity, the three kinds of glyme solutions have comparable conductivities and viscosities. Therefore the contrast redox ability among the three glyme solutions could not be explained by concentration of ionic species.

   Figure 2 shows photograph and XRD profiles of WE Cu sheet electrodeposited at –1.0 V vs. Al in the AlCl₃-dissolved diglyme solution. The Cu sheet was covered with black sponge deposits, which were identified as elemental Al (Fig. 2b). The coulombic efficiency for the deposition at –1.0 V vs. Al was calculated to be about 97% from the mass change using an Al sheet as WE.

[1] Y. Zhao and T. J. VanderNoot, Electrochim. Acta, 42(1), 3 (1997).