Electrodeposition of Nanocrystalline Aluminum: Breakdown of Imidazolium Cations Modifies the Crystal Size

In this paper we describe the aluminum electrodeposition in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide and 1-ethyl-3-methylimidazolium chloride , respectively. It is found that, besides microcrystalline aluminum, nanocrystalline aluminum can also be obtained by electrodeposition from both ionic liquids at room temperature without additives. The crystal refinement is due to a cathodic decomposition of the imidazolium ions to a certain extent giving rise to nanocrystalline aluminum. The electrochemical behavior was studied by cyclic voltammetry, galvanostatic, and potentiostatic polarization experiments complemented by scanning electron microscopy/energy-dispersive X-ray analysis. The upper phase of the biphasic ionic liquid and the Lewis acidic ionic liquid are both subject to some decomposition at low electrode potentials: the color of the ionic liquids instantly changes from colorless to purple, starting near the working electrode. The subsequent electrodeposition performed on various substrates no longer leads to microcrystalline but rather to nanocrystalline aluminum, presumably because cation breakdown products act as a crystal refiner during the electrodeposition.