Influence of Electrolytic Condition on Surface Smoothness of Electrolytic Aluminum Foil from AlCl₃-EMIC Melt

1. Introduction The Hall-Héroult process is known as an Al smelting process, which consumes a large amount of electricity in melting and electrolysis. Therefore, our group has focused on electrolysis using room-temperature ionic liquids (RTILs). Al can be deposited from RTILs, but a practical technology for depositing Al from even RTILs has not been established. There are several problems such as the low limiting current density and the deposition in a dendritic form at least. In particular, surface smoothness of electrolytic Al foil is required for practical application. 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 this study, using the AlCl3-EMIC melt, we investigated the effect of the addition of OP to the above melt on the properties such as surface smoothness of electrolytic Al foil. 2. Experimental An AlCl3-EMIC (molar ratio 2:1) melt was used as an electrolyte. 20 mmol dm-3 OP was added to it as an additive. A three-electrode cell was used for the electrolysis. A Ti plate was used as a working electrode. Galvanostatic electrolysis method was used. 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) of was observed using an atomic force microscope (AFM). 3. Results and Discussion The current efficiency of the electrolytic Al foil at the electrolytic temperature of room temperature 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 electrolytic temperature of 50 oC were higher than those of room temperature also at other current densities. The electrolytic 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 electrolytic 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 electrolytic Al foil obtained at the various electrolytic conditions. The crystal grain shape was the same, like a texture, regardless of the electrolytic conditions. In contrast, the crystal grain size became larger with increasing the electrolytic 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 electrolytic. The AEM images showed that the Sa value became larger with increasing the electrolytic temperature. By adding OP to the melt, the Sa value became extremely smaller. Based on these results, by increasing the electrolytic temperature and adding OP to the AlCl3-EMIC melt at 50 oC, a smooth electrolytic Al foil was found to be obtained with high current density. This study is supported by NEDO. References [1] H. Takahashi, C. Namekata, T. Kikuchi, H. Matsushima, and M. Ueda, J. Surf. Finish. Soc. Jpn., 68, 208 (2017). Figure 1