Abstract
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.
Highlights
Results and DiscussionE. sanding or electrochemical coating with aluminum), the impedance spectra intercept the x—axis at (345 ± 25) Ω on average
Electrochemical impedance spectroscopy suggests that the fresh electrodeposited aluminum film interacts with the electrolyte
This might be either the formation of a passive film on the surface or corrosion of the deposit in the electrolyte leading to a reduced micro roughness
Summary
E. sanding or electrochemical coating with aluminum), the impedance spectra intercept the x—axis at (345 ± 25) Ω on average This represents the Ohmic resistance of the electrolyte, RE. The charge—transfer resistance decreased after aluminum was deposited when compared to the sanded electrode surface. The chloroaluminate species or chloride anions (Eq 13) might form an adsorbed layer or react with the freshly deposited aluminum This layer might cause the described increase of the charge—transfer resistance. The time constant of the electrochemical double layer, tDL, was calculated from the double layer capacitance and the electrolyte resistance evaluated via impedance spectroscopy A concentration gradient evolves due to the electrochemical reaction in front of the electrode causing a time dependent concentration overpotential, hC(t) (Eq 9).[54,55] The time tC which is needed to cause a decrease of the surface concentration by 10 % can be calculated by Eq 11.39
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