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

Abstract

The anodic dissolution of aluminum metal was investigated in the Lewis acidic chloroaluminate ionic liquid, aluminum chloride-1-ethyl-3-methylimidazolium chloride. The investigation was conducted on aluminum rotating disk electrodes as a function of potential, ionic liquid composition, and temperature. Two different dissolution mechanisms were realized. At modest overpotentials, dissolution takes place under mixed kinetic-mass transport control. However, as the overpotential is increased to induce higher dissolution rates and/or the ionic liquid is made more acidic, the dissolution reaction transitions to a potential-independent passivation-like process ascribed to the formation of a porous solid layer of AlCl 3 (s). At a fixed temperature and composition, the limiting passivation current density displays Levich behavior and also scales linearly with the concentration of AlCl 4 − in the ionic liquid. The heterogeneous kinetics of the Al dissolution reaction were measured in the active dissolution potential regime. The exchange current densities were independent of the composition of the ionic liquid, and the anodic transfer coefficients were close to zero and seemed to be independent of the Al grain size. Room-temperature chloroaluminate ionic liquids are obtained by combining aluminum chloride with certain anhydrous quaternary ammonium chloride salts. The most popular examples of these wellknown salts are those based on the 1,3-dialkylimidazolium cations, notably 1-ethyl-3-methylimidazolium chloride (EtMeImCl).1 A unique and very versatile feature of these ionic liquids is their adjustable chloroacidity, which is based on the extant anions. This property is directly tied to the AlCl 3 content and is commonly expressed as the AlCl 3 /organic chloride salt ratio, mole fraction of AlCl 3 (x Al ), or percent mole fraction (m/o) of AlCl 3 . In this article, all compositions will be reported using the latter two conventions. Mixtures that contain less than 50 m/o AlCl 3 (x Al < 0.50) are Lewis basic due to excess unbound chloride ion, whereas those containing greater than 50 m/o AlCl 3 (x Al > 0.50) are Lewis acidic because they contain the coordinately unsaturated species, Al 2 Cl 7 − . Equimolar mixtures of the organic salt and AlCl 3 (x Al = 0.50) contain only AlCl 4 − and are designated as "neutral" ionic liquids.Acidic room-temperature chloroaluminates are of interest as solvents for the electroplating of aluminum and aluminum alloys due to the easily accessible redox reaction 2 4Al 2 Cl 7 − + 3eIt is also possible to electrochemically reduce the coordinately saturated species, AlCl 4 − , but this reaction is normally accessible only in alkali chloride-based systems such as AlCl 3 -NaCl where this anion can be reduced at more positive potentials than the alkali cation.3 However, this does not seem to be the case in chloroaluminates based on organic cations. Room-temperature chloroaluminates are safer, more stable alternatives to the traditional plating baths based on mixtures of aromatic hydrocarbons, alkali hal...

Effect of Aluminum Oxide on the Performance of Ionic Liquid-Based Aluminum–Air Battery

“…From the ex situ/in situ tests and thermodynamic-based analysis, it was found that a key parameter to control growth of the inert oxide film is the surface tension, which could be controlled by using proper amount of chloride ions in the electrolyte, and the cracks were formed on the oxide film by 21% when it was submerged into the electrolyte of molar ratio 1:1.5 of [EmIm]Cl and AlCl 3 , which is the electrolyte condition utilized in most research [7,19,33].…”

“…However, the OCV decreased significantly at the ratio of 1:2, which may be interpreted as the direct effect of Clion (i.e., corrosion) on the active area of the Al electrode, and the Clion affected not only the oxide film but also the Al active area. It is reported that at the ionic liquid ratio of 1:0.5, the dominant chloro-aluminate species in the electrolyte is AlCl4 - [7,8,19,33]. As the ratio of AlCl3 is increased from 1:0.5 to 1:1.5, the ionic liquid contains AlCl4 -and Al2Cl7 -species.…”

“…The oxide film can be modeled with the concept of double layer of dielectric constant ε' under the electric field E' as shown in Figure 4. It is reported that at the ionic liquid ratio of 1:0.5, the dominant chloro-aluminate species in the electrolyte is AlCl4 - [7,8,19,33]. As the ratio of AlCl3 is increased from 1:0.5 to 1:1.5, the ionic liquid contains AlCl4 -and Al2Cl7 -species.…”

Review—Electrochemical Surface Finishing and Energy Storage Technology with Room-Temperature Haloaluminate Ionic Liquids and Mixtures

“…In both ionic liquid systems, the Al anodization reaction is under mixed kinetic/mass-transport control at moderate overpotentials, but becomes a mass transport-limited process governed by the dissolution of a passive layer of solid AlCl 3 on the anode at more positive overpotentials. 55,56 Not surprisingly, in consideration of the reaction in Eq. 1, the anodic process is favored in slightly acidic melts, but highly disfavored under very acidic conditions.…”

Electrodeposition of indium from the ionic liquid trihexyl(tetradecyl)phosphonium chloride

“…Upon dissolution of iron, free chlorides are available making the passivation of iron by FeCl 2 very unlikely. The passivation of metals is reported in acidic chlorometallate ionic liquids (MCl 3 –ionic liquid mole ratio >0.50) 83–85. In Lewis-acidic ionic liquids the activity of free chlorides is insignificant.…”