Anodic Dissolution of Copper in the Aluminum Chloride-1-Ethyl-3-Methylimidazolium Chloride Ionic Liquid

Abstract

The ever popular AlCl3-1-ethyl-3-ethylimidazolium chloride (EtMeImCl) ionic liquid (IL) system offers a unique solvation environment for many metal ions. In fact, the use of these chloroaluminate ionic liquids for the electrochemical surface finishing of metals was recently reviewed.1 These ionic liquids display adjustable Lewis acidity that depends directly on the mole ratio of the AlCl3 to the organic salt. The basic AlCl3-EtMeImCl mixture, which contains coordinately unsaturated Cl-, provides a solvation environment similar to aqueous HCl and NaCl, except of course without the complications due to the H2O solvent or H3O+. The opposite situation prevails in the Lewis acidic ionic liquid because the main chemically active ionic component of the ionic liquid is the coordinately unsaturated species, [Al2Cl7]-. Thus, there are no “hard” ligands available to form stable, isolable anionic complexes with the anodization product. Recently, both composition regions of the AlCl3-EtMeImCl IL system have been explored for the electrodissolution/electropolishing of Al.2 The advantages afforded by anodic dissolution of metals in this ionic liquid relative the aqueous acids and salt solutions commonly used for this purpose are quite obvious, i.e., the avoidance of the oxide surface films that invariably complicate the metal electropolishing process. In this paper, we report the thermodynamics and kinetics of the anodic dissolution of copper metal in the acidic and basic compositions of the AlCl3-EtMeImCl ionic liquid. In the former, the dissolution of copper at a Cu-RDE exhibits mixed control. No limiting current or passivation due to the formation of an insoluble surface film was apparent. In the basic composition region, mixed control was observed at small overpotentials, but as the overpotential was increased, a limiting current was obtained that scaled linearly with the Cl- concentration in the ionic liquid. The connection between these results and the mechanism of the dissolution process will be discussed.____________________________________________ 1. T. Tsuda, G. R. Stafford and C. L. Hussey, J. Electrochem. Soc., 164, H5007 (2017).2. C. Wang and C. L. Hussey, J. Electrochem. Soc., 163, H1186 (2015).