Temperature-Dependent Structural Transformations of Electrodeposited Al from Acidic Imidazolium-Based Tetrachloroaluminate Ionic Liquid
Authors: Jiang Junhua , Wang Congjian 
Published: December 2020

The electrodeposition of aluminum (Al) has been extensively investigated for a range of emerging applications in electrical, electronic, and automotive industries due to its excellent corrosion and wear tolerance. One highly prospective process is the Al electrodeposition from an electrolytic bath comprised of an air and moisture-stable ionic liquid, such as 1-ethyl-3-methylimidazolium chloride ([EMIm]Cl), and an aluminum precursor, which is normally aluminum chloride (AlCl3). Recent spectroscopic studies have shown that Al deposition results from electrochemical reduction of chloroaluminate complexes. In general, the chloroacidity determines reactivity and electrochemistry in the ionic liquid electrolyte. However, the Al deposition chemistry has been complicated by the chemical nature of different chloroaluminate complexes species, their chemical equilibria, and interconversion [20, 21]. The [AlCl4]-, [Al2Cl7]-, and Cl- are the main anions in the solution of alkylimidazolium chloride and AlCl3. More complicated anions such as [Al3Cl10]-, [Al4Cl13]-, and [Al2OCl5]-, have also been detected when the AlCl3 concentration is high.

The distribution of the chloroaluminate complexes in the AlCl3/[EMIm]Cl system is heavily dependent on AlCl3 concentration and reaction temperatures. When the molar ratio is less than 50%, [AlCl4]- is the only chloroaluminate complex present. In contrast, when is greater than 50%, the [AlCl4]- concentration decreases and [Al2Cl7]- is formed. When is further increased to 75%, the [Al3Cl10]- concentration reaches 48%, accompanied by [Al2Cl7]-, [Al4Cl13]- and Al2Cl6. Our thermodynamic calculations indicate that increasing temperature mainly affects the distribution of [AlCl4]-, [Al2Cl7]- and [Al4Cl13]- when is higher than 50%.

As of now, standardized and reproducible procedures have not yet been established due to challenges with the sensitivity of the process chemistries to environments and operation conditions. The properties of Al deposits were affected by many factors such as: the composition of the mixture (AlCl3-to-IL ratio), substrate-pretreatment, stirring, additives, and reaction temperatures. In order to elucidate how the temperature affects the quality of Al deposits, we have focusedly investigated microstructure changes of Al thin films deposited from an acidic imidazolium-based tetrachloroaluminate bath as a function reaction temperature using several materials characterization techniques. Our presentation will introduce the interesting structural changes of Al films with increasing temperatures, and their correlation with the process chemistry and the kinetics of the electrodeposition.

References:

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[2] G. Franzen, B.P. Gilbert, G. Pelzer, E. DePauw, The anionic structure of room-temperature organic chloroaluminate melts from secondary ion mass spectrometry, Org. Mass Spectrom. 21 (1986) 443–444.