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.