Aluminum deposition mainly adopts the preparation method of molten salt with high temperature, which has the disadvantages of serious corrosion of equipment, high energy consumption and others. In virtue of the ionic liquids, high purity aluminum by deposition method can be obtained at room temperature and this method does not have strong corrosion of high temperature molten salt, and the energy consumption is greatly reduced.
Most of the ionic liquids used for aluminum deposition contain the composition of AlCl3, and by changing the molar ratio of organic salts to AlCl3, the Lewis acidity can be designed. Furthermore, equilibrium reactions of AlCl3 happened in the ionic liquids system is shown in Figure 1. In addition, the most representative ionic liquids system used in aluminum deposition can be classified into pyridine ionic liquids, quaternary ammonium ionic liquids and imidazole ionic liquids.
Figure 1. Equilibrium reactions of AlCl3 happened in the ionic liquids system.
- Pyridine ionic liquids for aluminum deposition: AlCl3-ethyl pyridine bromide ionic liquid is the first ionic liquid system that can maintain stable melting state at room temperature, and it can be used for aluminum deposition. A study group obtained 0.6-1 mm dense aluminum coating at room temperature by using an electrolyte composed of 67% toluene, 32% AlCl3-ethyl pyridine bromide ionic liquid and 1% methyl-t-butyl ether. Furthermore, when the molar ratio of AlCl3 and ethyl pyridine bromide is in the range of 2:1-0.75:1, it can form transparent molten salt with low viscosity and near room temperature melting point. Furthermore, 99.99% purity aluminum coating can be obtained by using the pyridine ionic liquids.
- Quaternary ammonium ionic liquids for aluminum deposition: With the progress of technology, some research groups reported the ionic liquid system of AlCl3-alkane aryl ammonium chloride ionic liquid. Compared with AlCl3-ethyl pyridine bromide ionic liquid system, AlCl3-alkane aryl ammonium chloride ionic liquid has the advantages of easy availability, low cost and high purity, which is very suitable for electrochemical research electrolyte. Based on this, a group of researchers used AlCl3-alkane aryl ammonium chloride ionic liquid system to electrodeposit aluminum on tungsten electrode. It is found that the average electrodeposition efficiency of aluminum is 88%.
- Imidazole ionic liquids for aluminum deposition: The use of imidazole ionic liquids in aluminum deposition is one of the most studied systems in recent years. Aluminum with 98.15% purity can be obtained by electrodeposition using imidazole ionic liquid.
Figure 2. SEM micrographs of aluminum electrodeposits obtained on Al substrates by using imidazole ionic liquid. a) 20 mA/cm2 ; b) 30 mA/cm2 ; c) 50 mA/cm2 ; and d) 70 mA/cm2.
- T, Jiang, et al. Electrodeposition of aluminum from ionic liquids: Part I—electrodeposition and surface morphology of aluminum from aluminum chloride (AlCl3)–1-ethyl-3-methylimidazolium chloride ([EMIm]Cl) ionic liquids[J]. Surface and Coatings Technology, 2006.