Electrochemical Aluminum Deposition from Fluorinated Ionic Liquids
Authors: Liberale Francesco , Ispas Adriana , Bund Andreas , Magagnin Luca 
Published: February 2020

Aluminum is one of the metals which cannot be deposited by traditional water-based electrolytes due to its reduction potential being below that of hydrogen evolution: Al coatings are nowadays mainly obtained by vapor phase techniques, which guarantee conformal and uniform deposits but which implies costs and technology complexity which are obviously higher than those of traditional electrodeposition methods. A possible solution could be that of using non-aqueous solvents, characterized by wider potential windows and improved chemical inertness. Among them, Ionic Liquids(ILs), generically defined as ionic systems having a melting point below 100°C, are being investigated for this purpose and many scientific reports still exist on this subject [1-8]. The handling and control of the deposition process with ILs’ based electrolytes still presents delicate aspects: almost all the procedure must be indeed conducted in lots of cases in a controlled atmosphere, usually in a glovebox. This obviously adds complexity to the process, higher costs and time consuming. Searching for innovative solution to overcome these obstacles could attract the real concrete interest from different industrial sectors. Lots of researchers are nowadays trying to synthesize ILs which can be handled and used in ambient atmosphere [9-13]: these could bring enormous advantages both in the salt dissolution step and in electrodeposition one, provided that the salt used as a metal source is not hygroscopic and therefore can be treated in ambient atmosphere.

In this study a series of fluorinated ionic liquids with a sulphonate anion is investigated, namely different perfluoro-3-oxa4,5 dichloro-pentan sulphonate ionic liquids. The strong hydrophobic nature of the liquids is meant to allow aluminum deposition in ambient atmosphere. These ionic liquids have the same anionic structure and different cations. They are electrochemically characterized with cyclic voltammetry tests to determine their electrochemical stability window with and without the Al salt, namely AlCl3. The influence of the deposition potential and of the temperature on the quality of the deposited Al layer was studied. The electrochemical evidences show that Al deposition is feasible but, when deposition tests are performed in air, a uniform Al deposit is not obtained: the presence of Al is almost always associated with contaminating agents like Cl and, when an intense Al peak is revealed by EDS analysis, it is unfortunately confined to isolated limited regions of the sample. Some of these selected ionic liquids, those which give some traces of Al deposition in air, have been purified at high temperature and under vacuum to reduce the water content and subsequently tested in glove box. Their water content was then analysed by Karl-Fisher titration. Some 1-Ethyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide (EmImTFSI) was added to the IL/AlCl3 mixture to reduce its viscosity and make the Al deposition feasible. Cyclovoltammetries and chronoamperometric tests performed with the Quartz Crystal Microbalance (QCM) gave positive indications as concerns Al deposition, as confirmed by SEM analysis. Cyclovolammogramms and chronoamperometric deposition experiments on a copper electrode were afterwards conducted. The electrochemical evidences are partially confirmed by SEM image of the deposit: a uniform Al is not obatained, but Al is detected in some regions of the substrate, with the level of contaminants being reduced with respect to the not purified dissolutions used in ambient condition.

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