Electrodeposition of Al from chloroaluminate ionic liquids with different cations

Four typical chloroaluminate ionic liquids (ILs) with different cations, namely 1-butyl-3-methylimidazoliumchloride [Bmim]Cl/AlCl3 (33.3/66.7 mol%), 1-butyl-3-methylpyridinium chloride [BMPyri]Cl/AlCl3 (33.3/66.7 mol%), 1-butyl-1-methylpyrrolidiniumchloride [Py-1,Py-4]Cl/AlCl3 (33.3/66.7 mol%), and trimethylphenylammonium chloride [TMPA]Cl/AlCl3 (33.3/66.7 mol%), were employed to deposit Al. The viscosity and ionic conductivity of these ILs were measured. The results showed that [Bmim]Cl/AlCl3 (33.3/66.7 mol%) had the lowest viscosity and the highest conductivity. Raman spectra showed that Al2Cl7 (-) was the main anion in the four systems. Cyclic voltammetry indicated that [Bmim]Cl/AlCl3 (33.3/66.7 mol%) had the highest reduction current for Al deposition in the four ILs. By comparing the quality of the Al coatings prepared at the same current density and temperature, it was found that compact and smooth Al deposits could be obtained from [Bmim]Cl/AlCl3 (33.3/66.7 mol%) at 303 K, while the temperature needed was higher than 333 K for the other three ILs to obtain Al deposits with the same quality. Based on the density functional theory (DFT) calculations, the differences in these properties were attributed to the different molecular structure and cation-anion interaction induced by the cations of these ILs.