Ionic liquid electrolytes composed of AlCl3 and primary/secondary alkylamine hydrochlorides with varying alkyl chain lengths for Al electrodeposition

Abstract

The electrodeposition of Al is essential for applications such as electroplating, electrorefining, and rechargeable Al batteries. However, the standard electrolyte currently used for Al electrodeposition, viz., 1-ethyl-3-methylimidazolium chloride–AlCl₃ ionic liquid, is prohibitively expensive for widespread industrial application. This study explored alternative ionic liquid electrolytes composed of AlCl₃ and primary or secondary alkylamine hydrochlorides (RNH₃Cl and R₂NH₂Cl) with varying alkyl chain lengths (R = methyl (Me), ethyl (Et), n-propyl (n-Pr), n-butyl (n-Bu)) for Al electrodeposition. The phase states of a series of mixtures of alkylamine hydrochlorides and AlCl₃ at different molar ratios were examined to identify the conditions required for forming a single liquid phase. Notably, a single liquid phase was obtained at room temperature (30°C) for n-PrNH₃Cl–AlCl₃, n-BuNH₃Cl–AlCl₃, and n-Bu₂NH₂Cl–AlCl₃ ionic liquids. These ionic liquids exhibit conductivities of approximately 10 mS cm⁻¹ at 30 °C. Galvanostatic electrodeposition studies using these electrolytes confirmed that Al electrodeposition occurs with current efficiencies exceeding 90 % at current densities of 5–12 mA cm⁻². This study demonstrates that alkylamine hydrochloride–AlCl₃ ionic liquids with an appropriate alkyl chain length of the amine can serve as promising low-cost electrolytes for Al electrodeposition.