Many articles on the Al electrodeposition using the ILs have ever been reported. However, there are few articles that have systematically investigated 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 the influence of the operating conditions, such as operating temperature, current densities, and additives (OP), on the surface roughness (smoothness) and the deposition rate of electrolytic Al foil [2]. It was revealed that the operating temperature needed to be increased to enhance the deposition rate and achieve a current efficiency of 90% or more, and the OP addition was significantly necessary to enhance the smoothness.
In general, a pulse electrolysis method has a thinner diffusion layer than a constant-current electrolysis method, so that it allows electrolysis at a higher current density. Even in the ILs system, pulsed currents greatly improved surface morphology [3]. Therefore, the crystal grains are refined and a smooth Al foil can be expected to be obtained. Azumi and co-workers reported that duty ratio and pulse frequency were adjusted to obtain dense deposits composed of small Al particles in the current pulse polarization method [4].
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 deposition morphology of the electrolytic Al foil from the chloroaluminate ILs with / without OP.
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 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 mA cm−2, pulse frequency (f) of 5 to 100 Hz, and duty ratio of 0.5. The observations and crystal structure of the resulting Al foil were evaluated by FE-SEM, AFM, and XRD.
Current efficiencies of the resulting Al foil were more than 90% in the 5 to 100 Hz range with and without the OP addition to the above electrolyte. No brightness was observed on the resulting Al foil obtained from the OP-free bath, whereas a matte finish was observed on the resulting Al foil obtained from the OP additive bath, as in the constant-current electrolysis method [2]. Figure 1 shows the FE-SEM images of resulting Al foils obtained from (a) the OP-free bath and (b) the OP additive bath at 100 Hz. From these results, the addition of OP to the electrolyte reduced the crystal grain size. In addition, the AFM results showed that the addition of OP to the electrolyte decreased the arithmetic mean roughness (Sa) value of the resulting Al foil, indicating that the enhancement in smoothness of the resulting Al foil obtained from the OP additive bath.
Based on these results, it was revealed that the addition of OP to the electrolyte significantly enhanced the smoothness of the resulting Al foil even in a pulse electrolysis method.
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 et al., J. Surf. Finish. Soc. Jpn., 68, 208 (2017)., [2] K. Ui et al., J. Electrochem. Soc., 168, 056510, (2021)., [3] Q. Zhu and C. L. Hussey, J. Electrochem. Soc., 149, C268 (2002)., [4] K. Azumi et al., Electrochim. Acta, 56, 1130 (2011).