Anodization Behavior of Aluminum in Ionic Liquids with a Small Amount of Water

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We investigated the anodic oxidation of aluminum in ionic liquids (ILs) by the constant-voltage rising rate (C-V) method and constant-current density (C-C) method, with characterization by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The structure of the oxide layer and the reaction efficiency varied with the type of IL, the applied voltage, the forming method, and the water content. A homogeneous Al2O3 film with a superior dielectric property was formed by the C-V method (100 mV/s) in 1-butyl-3-methylimidazolium benzoate (BMIm-BEN, water content 0.81 wt%) at 40 V. Although a homogeneous barrier-type Al2O3 film was also formed in 1-butyl-3-methylimidazolium mandelate (BMIm-MAN, water content 0.34 wt%) at 40 V, some anion species existed in the film, and its jump-voltage characteristics was inferior to that of the film formed in BMIm-BEN. The relative permittivity of the film formed in BMIm-BEN was almost equal to that of the Al2O3 film formed in adipate (1.0 wt%) aqueous solution, but the dielectric constant of the film formed in BMIm-MAN was 1.5 times larger. In contrast, a heterogeneous porous Al2O3 film containing a large amount of anion molecules was formed in 1-ethyl-3-methylimidazolium acetate (EMIm-ACE, water content 0.79 wt%) at 40 V. Since the source of oxygen for anodic oxidation is the small amount of water in the ILs, the water content strongly affected the characteristics of the formed Al2O3 layer.