Aluminum deposition and dissolution in [EMIm]Cl-based ionic liquids-kinetics of charge-transfer and the rate-determining step

The kinetics of the dissolution and deposition of aluminum from a first generation ionic liquid consisting of AlCl₃/1-ethyl-3-methylimidazolium chloride (molar ratio 2:1) was studied. Electrochemical impedance spectroscopy shows that the double layer capacitance and the charge–transfer resistance depend on the state of the electrode surface. The impedance spectra are strongly influenced by mass transport. The rate–determining step of the aluminum deposition, as determined from the cathodic Tafel slope evaluated from current step experiments, was found to be either a chemical step, releasing the complexing agent chloride, while aluminum is in the divalent oxidation state (AlCl₃⁻ → AlCl₂ + Cl⁻) or an electron transfer from the divalent to the monovalent aluminum occurring twice for the overall reaction to occur once (Al²⁺ + e⁻ → Al⁺). The rate–determining step for aluminum dissolution was found to be the transfer of an electron from elemental aluminum to the monovalent oxidation state (Al⁰ → Al⁺ + e⁻). A linear slope in the low cathodic overpotential region of the Tafel plot suggests a change in the cathodic rate–determining step. The Tafel slope indicates a chemical step, releasing the complexing agent chloride, after the last electron transfer (AlCl⁻ → Al⁰ + Cl⁻) to be the rate–determining step for overpotentials below 50 mV. Density functional theory calculations support the proposed reduction and oxidation mechanisms.