Electrodeposition of Aluminum Nanoplatelet from AlCl3–1-Ethyl-3-Methylimidazolium Chloride–Urea Mixtures

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Aluminum electrodeposition is generally conducted in nonaqueous solvents because of its negative electrode potential (-1.66 V (vs. NHE)). Among the solvents, Lewis acidic chloroaluminate ionic liquids (ILs) and deep eutectic solvents (DESs), which contain AlCl3 over 50 mol% and are handled at room temperature, are commonly employed for the electrodeposition process. Recently, these solvents are also expected as the electrolyte for Al-anion secondary battery.1 A great number of articles on Al technologies using AlCl3–1-ethyl-3-methylimidazolium chloride ([C2mim]Cl) IL2 and AlCl3–urea DES3 have been reported so far. However, there is no report on Al electrodeposition from the Lewis acidic chloroaluminate ternary mixtures consisting of AlCl3, [C2mim]Cl, and urea. In this study, we examine the electrochemical behavior of Al deposition process and the association between the morphology variations in the Al electrodeposits and electrodeposition conditions. The procedures used for the preparation and purification of the AlCl3, [C2mim]Cl, and urea, were identical with those described in previous articles.2,3 The molar fraction of AlCl3 in each mixture was 60 mol%. The sum of molar fraction for the [C2mim]Cl and urea was 40 mol%. Their molar fraction was changed in the range of 0 ~ 40 mol%. The electroanalytical measurements were conducted using a three-electrode cell. The working electrode was a Pt disk electrode. An Al plate was used for the counter electrode. The reference electrode was an Al wire immersed directly in the electrolyte. Galvanostatic electrodeposition was conducted using Cu plate electrodes. All the experiments were carried out in an Ar gas-filled glove box with O2 and H2O < 1 ppm. The resulting Al samples were characterized by SEM, EDX, and XRD. Operando digital microscope observation for the Al electrodeposition process was performed by using the specially designed two-electrode airtight electrochemical cell. Figure 1 shows SEM images of the pure Al electrodeposits prepared in 60.0-40.0 mol% AlCl3–[C2mim]Cl IL and 60.0-40.0 mol% AlCl3–urea DES under the same electrodeposition conditions. EDX analysis of the electrodeposits revealed that they are pure Al without chloride contamination. When the IL electrolyte was used, the Cu substrate surface was covered with dense crystals. Meanwhile, in the case of the DES electrolyte, surprisingly Al nanoplatelets having very smooth surface were yielded at 333 K but not 293 K, when the applied current density was -5 ~ -20 mA cm-2. The Al nanoplatelets were readily obtained at the bath temperature over 313 K. The surface morphology of the electrodeposit obtained at 293 K was similar to that shown in Fig. 1a. XRD patterns of the Al deposits revealed that specimens shown in Fig. 1a and 1b were Al polycrystalline Al and (111)-preferentially oriented Al, respectively (Fig. 2a and 2e). The unexpected electrodeposition behavior recognized in the DES solvent has not been observed in the chloroaluminate IL. In order to get further insights concerning the interesting behavior, the electrodeposition was conducted in the ternary AlCl3–[C2mim]Cl–urea mixtures under the same condition that can produce Al nanoplatelet in the AlCl3–urea DES. Here we used three types of mixtures, 60.0-30.0-10.0 mol%, 60.0-20.0-20.0 mol%, and 60.0-10.0-30.0 mol% AlCl3–[C2mim]Cl–urea. As we expected, the increase in the urea molar faction in the ternary mixture caused the variation in the surface morphology. The nanoplatelet structure appeared in only the 60.0-10.0-30.0 mol% AlCl3–[C2mim]Cl–urea. It suggests that the urea contributes the formation of unique platelet structure. Because direct observation of the electrodeposition process enables to propose the plausible formation mechanism, airtight electrochemical cell for the operando digital microscope was newly designed. From the video clip during the electrodeposition process, two-dimensional crystal growth of the Al deposits was visually recognized in the 60.0-40.0 mol% AlCl3–urea DES. The detailed information obtained from the operando observation including the video clip will be given in our presentation. References T. Tsuda, G. R. Stafford, and C. L. Hussey, J. Electrochem. Soc., 164, H5007 (2017) and references therein.J. S.Wilkes, J. A. Levisky, R. A.Wilson, and C. L. Hussey, Inorg. Chem., 21, 1263 (1982).H. M. A. Abood, A. P. Abbott, A. D. Ballantyne, and K. S. Ryder, Chem. Commun., 47, 3523 (2011). Figure 1