Electrodeposition of Aluminum from AlCl3 / Glyme Solutions at Room Temperature
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.