Effect of Deposition Conditions on Properties of Aluminum Deposit Obtained from Aluminum Chloride-Acetamide Melts
Authors: Ito Yuga , Takeguchi Tatsuya , Ui Koichi 
Published: December 2024

Introduction

Since the standard electrode potential of aluminum (Al) (-1.67 V vs. SHE) is lower than that of hydrogen, Al electrodeposition from aqueous solution systems is difficult. Therefore, Al electrodeposition using nonaqueous solution systems, such as molten salts, ionic liquids, organic solvents, and deep eutectic solvents, have been widely studied. However, the synthesis cost of nonaqueous solution systems is not small. Therefore, Al electrodeposition using deep eutectic solvents has been studied, including from the viewpoint of low cost. For example, in the AlCl3-urea melt, the effect of mixing with EMIC and current density on the surface morphology of Al deposits has been reported.1 The AlCl3-acetamide (AcA) melt has been reported to produce better Al deposits than the ionic liquid AlCl3-EMIC melts in dry air. 2 However, in the Al deposition using the AlCl3-AcA melts, the effect of deposition conditions such as deposition potential on the properties of the deposit has not been clarified.3 Therefore, in this study, we obtained basic knowledge of Al electrodeposition using the AlCl3-AcA melt and then examined the effect of the deposition conditions such as deposition potential and operating temperature on the properties of the Al deposit.

Experimental

The electrolyte was prepared by mixing anhydrous AlCl3 and AcA in a molar ratio of 1.5 : 1. A three-electrode cell was fabricated using a glassy carbon (GC) electrode or a Cu plate as the working electrodes, an Al plate as the counter electrode, and an Al wire dipped in AlCl3-EMIC (molar ratio 2 : 1) melts as the reference electrode. Cyclic voltammetry (CV) was carried out for the electrochemical measurements. A potentiostatic electrodeposition method was performed with stirring at 50-90℃. The observation and crystal structure of the deposit were evaluated by ULV-SEM and XRD, respectively.

Results and Discussion

In the CVs, the Redox waves were observed in the AlCl3-AcA melt at the operating temperature of 50 °C and a scanning rate of 10 mV s−1. When the potentiostatic electrodeposition method was performed at the operating temperature of 50-90 °C and the set potential of −0.5 V (vs. Al / Al (Ⅲ)), the current efficiencies were about 90%.
The effect of the electrodeposition conditions on the surface morphology of the Al deposit was investigated. Figure 1 shows the ULV-SEM images of the deposits obtained the Cu plate substrate from the AlCl3-AcA melt at the operating temperature of 50 °C and 90 °C. At the operating temperate of 50 °C, the grain shape was fine-scale, similar to the surface morphology of the deposit in the AlCl3-urea melt.4 In contrast, at the operating temperature of 90 °C, the grain shape changed to fine grains.
Next, the effect of the operating temperature on the crystal structure of the Al deposits was investigated. The XRD patterns showed that all the peaks were attributed to Al, yielding a single phase of Al(fcc). In order to now discuss the crystal orientation of the resulting Al deposit, the orientation coefficient (N) was calculated using Wilson’s equation. A value of N greater than 1 indicates the preferential orientation to that plane. The preferential orientation to the {110} plane was observed at the operating temperature of 50 °C, but the preferential orientation to the {110} plane decreased as the bath temperature increased to 90 °C. Although the Al deposits from the AlCl3-based ionic liquids were mostly non-oriented, the surface morphology of the Al deposits obtained from the AcA bath was scale-like and the crystal structure showed preferential orientation.
Based on these results, it was revealed that the grain shape and the preferential orientation of the resulting Al deposit obtained from the AlCl3-AcA melt would be correlated and temperature dependent.

References

T. Tsuda, et al., J. Electrochem. Soc, 169, 092520 (2022).
M. Yamagami, et al., J. Electrochem. Soc, 169, 062502 (2022)
V. S. Cvetković, et al., Trans. Nonferrous Met. Soc. China, 30, 823 (2020).
L. Min, et al., Electrochim. Acta, 180, 811 (2015).

Figure 1