Diffusion coefficient and nucleation density studies on electrochemical deposition of aluminum from chloroaluminate ionic liquid electrolytes

In this study, aluminum was electrodeposited from ionic liquids comprising a melt of alkyl imidazolium chlorides and aluminum chloride (AlCl3). The ionic liquids utilized in this study were 1-ethyl-3-methyl-imidazolium chloride (EMIC)-AlCl3, 1-butyl-3-methylimidazolium chloride (BMIC)-AlCl3, and 1-hexyl-3-methyl-imidazolium chloride (HMIC)-AlCl3 at the AlCl3 mole fraction of 0.667 (molar ratio of 1:2). The electrochemical behavior of chloroaluminate species in the three ionic liquids was investigated by cyclic voltammetry (CV) and chronoamperometry (CA) techniques at different temperatures. The cyclic voltammograms indicated that the reduction of Al2Cl7− species to metallic aluminum followed a diffusion-controlled phenomenon. Moreover, even at a less negative applied potential, the higher current density was obtained for EMIC-AlCl3 ionic liquid. That indicates that EMIC-AlCl3 favors less energy consumption during the electrodeposition. The chronoamperometric analysis revealed that the onset of aluminum deposition from such ionic liquids proceeds via a three-dimensional instantaneous nucleation process. The concentrations of Al2Cl7− ions in the ionic liquids were calculated based on the thermodynamic data, which were 2520, 2271, and 2035 mol m−3 for EMIC-AlCl3, BMIC-AlCl3, and HMIC-AlCl3, respectively. The diffusion coefficient (D) values of Al2Cl7− species in such ionic liquids were also calculated at various temperatures. The D values determined from the CA technique at 363 K are 2.16 × 10−11, 1.03 × 10−11, and 0.87 × 10−11 m2 s−1 for EMIC-AlCl3, BMIC-AlCl3, and HMIC-AlCl3, respectively. In addition, the D value increased as temperature increased and decreased as the hydrocarbon group in the ionic liquid increased. The calculated grain sizes of nucleation range from 2 to 4 μm for these ionic liquids and are in good agreement with experimental data obtained from SEM micrographs.