An Assessment of Aluminum Electrodeposition from Aluminum Chloride/4-ethylpyridine Ionic Liquid at Ambient Temperature

The electrodeposition of aluminum from the ionic liquids (ILs) obtained from complexation of aluminum chloride with a neutral ligand, 4-ethylpyridine (AlCl₃/4-EP), was assessed using voltammetry, chronoamperometry, and constant potential electrolysis. It is found that reduction current of the AlCl₃/4-EP with a molar ratio of 1.3/1 is higher than that with a molar ratio of 1.1/1, suggesting that the reduction of [AlCl₂(4-EP)]+ is thermodynamically easier than [AlCl₂(4-EP) ₂]⁺. While the Al(III)/Al redox is chemically reversible, the reaction is not diffusion limited but kinetically limited resulted from the slow bond-breaking of the bulky 4-EP ligands in the [AlCl₂(4-EP)]⁺complex cations as well as the adsorption of un-complexed 4-EP molecules on the electrode surface. The kinetic limitation was also observed in the chronoamperometric study of nucleation process. Electrodeposition of Al was conducted on copper foils at 310 K and characterized with scanning electron microscope (SEM) and X-ray diffraction (XRD). The SEM images reveal that the morphologies of the aluminum deposits are dependent on the composition of the IL and the deposition potential whereas the XRD analysis confirms that all the deposits have same crystal texture.