Solvation effect of [BMIM]Cl/AlCl3 ionic liquid electrolyte

Four kinds of solvents, methylene chloride (CH2Cl2), toluene (TOL), ethyl acetate (EtoAc), and dimethyl carbonate (DMC), were added into the 1-butyl-3-methylimidazolium chloroaluminate ionic liquid ([BMIM]Cl/AlCl3 IL) to improve the property of the IL-based electrolyte for aluminum (Al) electrolysis. Solvation effects of these solvents on the physical properties, electrochemical properties, and electroactive ion of the [BMIM]Cl/AlCl3 IL-based electrolyte were investigated. It was found that the viscosity of the IL electrolyte decreased after adding CH2Cl2 or TOL, and the conductivity increased while the change of the conductivity and the viscosity moved in the opposite direction with the addition of EtoAc or DMC. The UV-Vis spectra showed that the electroactive Al ion continued existence after adding CH2Cl2 or TOL, but there was solvation effect between TOL and BMIM+. On the other hand, both EtoAc and DMC had solvation effects with electroactive Al ion and BMIM+. The results illustrated that solvation effect was the key effect on the reduction of Al on the cathode, and further influenced the morphology of the Al deposits. In addition, the concentration polarization reduced and the efficiency of electrolysis improved after adding CH2Cl2 or TOL. Furthermore, bright Al coatings were obtained finally by adding CH2Cl2 into [BMIM]Cl/AlCl3 IL electrolyte (when the mole ratio of CH2Cl2 is around 66.7~75%).