Investigation on Operating Conditions Influencing the Aluminum Electrolysis Using Chloroaluminate Ionic Liquids
Authors: Ui Koichi , Kobayashi Satoshi , Takeguchi Tatsuya , Tsuda Tetsuya , Ueda Mikito , Nunomura Jyunji , Honkawa Yukio , Kojima Yoichi 
Published: December 2020

Our group has focused on the Al electrolysis using room-temperature ionic liquids (RTILs). Al can be deposited from the chloroaluminate ILs, but a practical technology for depositing Al from even the ILs has not been established. There are several problems such as the low limiting current density and the deposition in a dendritic form. In particular, the surface smoothness of electrolytic Al foil is required for practical application. Many articles on the Al electrodeposition using the ILs have ever been reported. However, few articles systematically investigate the correlation between the Al electrodeposits and the operating conditions (parameters). Ueda and co-workers reported that the surface smoothness was improved by adding 1,10-phenanthroline anhydrate (OP) to a Lewis acidic AlCl3-EMIC (1-ethyl-3-methylimidazolium chloride) melt [1]. In order to effectively scale up from the laboratory level to the practical level, it is desirable to make the above correlation clear sufficiently. In this study, we investigated on the parameters influencing to obtain a smooth electrolytic Al foil.

The chloroaluminate ionic liquids consisting of anhydrous AlCl3 and 1-ethyl-3-methylimidazolium chloride (EMIC) for 2:1 molar ratio were prepared as an electrolyte in an Ar-filled glove box. 20 mmol dm−3 OP (1,10-phenanthroline anhydrate) was added to the electrolyte as an additive. Galvanostatic electrolysis method was carried out in a conventional three-electrode cell with stirring at room temperature (RT) and 50 ℃. A Ti plate was employed as a cathode. The electricity was controlled to 30 C cm−2. Surface morphology was observed using a field-emission scanning electron microscope (FE-SEM). Arithmetic mean roughness (Sa) was observed using an atomic force microscope (AFM).

The current efficiency of the resulting Al foil at the operating temperature of RT and 50 oC was 99.6% in the latter as compared to 84.8% in the former at a current density of 52.6 mA cm−2. The current efficiencies at the operating temperature of 50 oC were higher than those of RT also at other current densities. The Al foil was obtained without the reductive decomposition of EMI+ cation (-2.2 V vs. Al / Al (Ⅲ)) even at the high current density (63.2 mA cm−2) by increasing the operating temperature to 50 oC. Regardless of the addition of OP, the current efficiencies were more than 90% even at the current density of 50 mA cm-2.

Fig. 1 shows the FE-SEM images of the Al foil obtained at the various operating conditions. The crystal grain shape was the same, like a texture, regardless of the operating conditions. In contrast, the crystal grain size became larger with increasing the operating temperature. Moreover, the addition of OP to the AlCl3-EMIC melt at 50 oC also affected the crystal grain size to be small while the addition of OP did not affect the overpotential for the electrolysis. The AEM images showed that the Sa value became larger with increasing the operating temperature. By adding OP to the melt, the Sa value became extremely smaller, indicating that the addition of OP strongly suppresses the roughness.

These results indicate that the smooth electrolytic Al foil would be obtained even with high operating temperature and high current density by adding OP to the AlCl3-EMIC melt.

Acknowledgements

This work is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

References

[1] H. Takahashi, C. Namekata, T. Kikuchi, H. Matsushima, and M. Ueda, J. Surf. Finish. Soc. Jpn., 68, 208 (2017).

Figure 1