Influence of Pulse Electrolytic Conditions on Properties of Electrolytic Aluminum Foil Using Chloroaluminate Ionic Liquids

Abstract

An Aluminum (Al) foil is industrially manufactured by the rolling method, whereas the production of Al foil by the electrolytic method (electrolytic Al foil) has not yet been established. Therefore, our group has focused on the Al electrolysis using room-temperature ionic liquids (ILs). Al can easily be deposited from the chloroaluminate ILs, whereas a practical technology for depositing Al from the ILs has still not been established. In the above ILs system, there are several problems such as a low limiting current density and deposition in a dendritic form. Thereby, the high deposition rate and the surface roughness (smoothness) of the electrolytic Al foil are required for practical application. Many articles on the Al electrodeposition using the ILs have ever been reported. However, there are few articles that systematically investigate the correlation between the Al deposits 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]. Using a constant-current electrolysis method, our group investigated on the parameters influencing to obtain a smooth electrolytic Al foil at higher current density [2]. Consequently, the smooth Al foil was successfully obtained even with high operating temperature (50 ℃) and high current density (52.6 mA cm−2) by adding OP to the AlCl3-EMIC melt. In general, a pulse electrolysis method has a thinner diffusion layer than a constant-current electrolysis method, so that electrolysis at a higher current density is possible. Therefore, the crystal grains are refined, and a smooth Al foil can be expected to be obtained. The Al deposits obtained by a pulse electrolysis method exhibited brighter and flatter surfaces compared to those obtained under the same average current density by a constant-current electrolysis method [3]. 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 the influence of pulse electrolytic conditions on the properties of the electrolytic Al foil. The chloroaluminate ILs consisting of anhydrous AlCl3 and EMIC for 2:1 molar ratio were prepared as an electrolyte in an Ar-filled glove box. 20 mmol dm−3 OP was added to the electrolyte as an additive. A pulse 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 total charge was controlled to 30 C cm−2. The pulse-current parameters employed in this study were as follows: pulse current density (CD) of 52.6 to 126 mA cm−2, pulse frequency (f) of 100 Hz, on-current period (ton) of 5 ms, and off-current period (toff) of 5 ms. The arithmetic mean roughness (Sa) of the foil surface was observed using an atomic force microscope (AFM). In pulse electrolysis at RT, the current efficiency was more than 90% at a CD of 52.6 to 73.7 mA cm−2, but less than 70% at 94.7 mA cm−2. When the operating temperature was increased from RT to 50 °C, pulse electrolysis at a high current density became possible, and the current efficiency was 88.3% at 126 mA cm−2. Figure 1 shows the AFM images and the Sa values of the resulting Al foil obtained on the Ti plate substrate under the various operating conditions. Sa value became higher with increasing the operating temperature from RT to 50 °C. At the operating temperature of 50 °C, Sa value became lower with increasing the CD from 52.6 to 84.2 mA cm−2, indicating that increased current density improves smoothness. In summary, the current efficiency at a high CD was enhanced by increasing the operating temperature, but the surface roughness made coarse. In this case, smoothness can be improved by increasing the CD. Acknowledgements This work is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).