Features of aluminum electrodeposition from 1,3-dialkylimidazolium chloride chloroaluminate ionic liquids

Abstract

The mechanism of aluminum electrodeposition from acidic 1-butyl-3-methylimidazolium chloride chloroaluminate ionic liquids ([BMIm]Cl ILs) is studied over a wide range of the molar ratio of AlCl3 to [BMIm]Cl (N) at 30 °C. The findings for AlCl3–[BMIm]Cl and AlCl3–1-ethyl-3-methylimidazolium chloride ([EMIm]Cl) are analysed and compared in order to identify dependences common for 1,3-dialkylimidazolium chloride ILs. Limiting currents were detected on the cathodic polarization curves at the aluminum | ionic liquid interface. Limiting currents are caused by the Al2Cl7– anions diffusion to the electrode surface. The limiting current density values for AlCl3–[BMIm]Cl are lower (ilim, 1.1 ≤ N ≤ 2.0 = 1.17 – 12.36 mA∙cm−2) than for AlCl3–[EMIm]Cl (ilim, 1.1 ≤ N ≤ 2.0 = 1.81 – 25.37 mA·cm−2). It is shown that anion AlCl4– can be reduced to metallic aluminum at cathodic overpotentials above 1.5 V. The diffusion coefficient of Al2Cl7– in AlCl3–[BMIm]Cl (7.4∙10−7 cm2∙s−1) is lower than in AlCl3–[EMIm]Cl (9.3∙10−7 cm2∙s−1) and is unaffected by aluminum chloride concentration in either system.