Measurement of Dissolved Oxygen Concentrations of Chloroaluminate Ionic Liquids in Dry Air

The electrodeposition of Al has been studied in various ionic liquids represented by the 1-ethyl-3-methylimidazolium chloride-AlCl₃ system. However, AlCl₃-based (chloroaluminate) ionic liquids are generally handled in an inert atmosphere of nitrogen or argon because they are highly hygroscopic and susceptible to hydrolysis through the reaction with moisture in the air. To maintain an inert atmosphere, a closed system that typically uses a glove box is required, but operation in a closed system leads to low productivity and high cost. Therefore, electrodeposition in an inert atmosphere is unsuitable for mass production on an industrial scale. To alleviate this problem, we have previously studied the feasibility of Al electrodeposition in dry air using chloroaluminate ionic liquids. Because the electrodeposition process in dry air does not require a glove box, it is considered to be more productive than the electrodeposition process under an inert atmosphere.

In our previous study, four different chloroaluminate ionic liquids consisting of different organic cations ([cation]Cl-AlCl₃) with a molar ratio of [cation]Cl : AlCl₃ = 1 : 2 were used as Al electrodeposition baths to study Al electrodeposition behavior in dry air. The cations used were 1-ethyl-3-methylimidazolium (EMI⁺), 1-butylpyridinium (BP⁺), 1-butyl-1-methylpyrrolidinium (BMP⁺), and trimethylphenylammonium (TMPA⁺). We found that electrodeposition in dry air using the bath composed of EMI⁺ or BP⁺ could not produce an Al film covering the entire surface of the substrate, whereas electrodeposition using the bath composed of BMP⁺ or TMPA⁺ could produce an Al film covering the entire surface of the substrate in dry air as well as in an inert atmosphere.

The reason for the difference in the results of Al electrodeposition in dry air according to cationic species has not yet been ascertained, but it is speculated that dissolved oxygen has some influence on the Al electrodeposition. In the present study, we measured the concentration of oxygen dissolved in four types of ionic liquids in dry air.

For measuring the dissolved oxygen concentration, ionic liquids with a composition of [cation] Cl: AlCl₃ = 1: 0.9 were used. This composition was chosen because only AlCl₄⁻, which is an ionic species that is difficult to be reduced to Al metal, is present in the bath when the AlCl₃ mole fraction is less than 50 mol% (Lewis base)in chloroaluminate ionic liquids. Consequently, Al electrodeposition does not occur, and low extent of electrochemical reaction other than Al electrodeposition at a potential of 0 V vs. Al / Al (III) or less can be investigated. The concentration of dissolved oxygen was measured by chronoamperometry using two types of electrodes, a macroelectrode and a microelectrode, both of which have a circular active area with a diameter in the order of millimeter and micrometer, respectively. In the case of chronoamperometric measurement with a macroelectrode, the reduction current of oxygen follows the equation in the upper row in Table 1, assuming that the reaction rate is controlled by the planar diffusion of oxygen from the bulk electrolyte to the surface of the electrode. Upon measuring the current–time response, two unknowns remain, namely, the concentration and the diffusion coefficient of dissolved oxygen; and it is not possible to determine the dissolved oxygen concentration through the chronoamperometric measurement using the macroelectrode alone. In the case of chronoamperometric measurement with a microelectrode, the geometry of the diffusion of the reactant to the electrode surface changes to semi-spherical, and the reduction current of oxygen thus follows the equation in the lower row in Table 1. The concentration and diffusion coefficient of dissolved oxygen can thus be determined through the two chronoamperometric measurements.

The chronoamperometric measurements revealed that there was no significant difference in the oxygen concentrations of the ionic liquids with different cationic species. The results that we have obtained so far are as follows: 1) In an inert atmosphere, an Al film covering the entire surface of the substrate can be electrodeposited from a bath containing any cation species. 2) In dry air, an Al film covering the entire surface of the substrate can only be electrodeposited from some baths composed of specific cation species. 3) There is no significant difference in the dissolved oxygen concentrations of the baths in dry air, even though the cation species in the baths are different. In consideration of these results, the reason for the different results of Al electrodeposition in dry air according to the cationic species is neither dissolved oxygen alone nor cationic species alone but the combined action of both dissolved oxygen and the cation.

Figure 1