(Invited) Aluminum Electrodeposition in AlCl3–1-Ethyl-3-Methylimidazolium Chloride–Urea Melts
Authors: Tsuda Tetsuya , Miyakawa Ryutaro , Kuwabata Susumu 
Published: July 2022

Several non-aqueous solvents for aluminum (Al) electrodeposition have been reported so far. Currently most researches on Al electrodeposition are conducted in Lewis acidic chloroaluminate ionic liquids (ILs) and deep eutectic solvents (DESs), which contain AlCl3 over 50 mol% and have a liquid phase at room temperature. Recently these solvents have also drawn attention as electrolytes for Al metal anode anion-rechargeable batteries.1 Only a few years ago, we have reported that Al electrodeposits with unique 2D structure are obtained in a 60.0-40.0 mol% AlCl3–urea DES without any difficulty, but not in the same composition chloroaluminate IL.2 Herein, in order to understand the unique 2D structure fabrication in the AlCl3–urea, we examine Al electrodeposition in the Lewis acidic chloroaluminate ternary melts consisting of AlCl3, 1-ethyl-3-methylimidazolium chloride ([C2mim]Cl), and urea.
The preparation and purification processes for AlCl3, [C2mim]Cl, and urea, were identical with those described in previous articles.3 The molar fraction of AlCl3 used in the melts was 60 mol%. The sum of molar fraction for the [C2mim]Cl and urea was 40 mol% and these molar fraction was changed in the range of 0 ~ 40 mol%. The electrochemical 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 using the specially designed two-electrode airtight electrochemical cell.
As described in previous article, Al deposition initiates at ca. 0 V (vs. Al(III)/Al) in both 60.0-40.0 mol% AlCl3–[C2mim]Cl IL and 60.0-40.0 mol% AlCl3–urea DES by the following commonly known electrode reaction.2,3

4 [Al2Cl7]− + 3 e− ⇌ Al + 7 [AlCl4]−

Typical SEM images of the Al electrodeposits obtained in both electrolytes under the same conditions are exhibited in Fig. 1. In the 60.0-40.0 mol% AlCl3–[C2mim]Cl, dense Al crystals were uniformly electrodeposited on the Cu electrode. Interestingly, in the 60.0-40.0 mol% AlCl3–urea, 2D structured Al deposits, i.e., Al nanoplatelets, were obtained. Such unique Al deposits were yielded at the bath temperature over 313 K, if the applied current density was -5 ~ -20 mA cm-2. EDX analyses of all the electrodeposits revealed that the resultant deposits are pure Al without chloride contamination. XRD measurements strongly suggested that the Al deposits shown in Fig. 1a and 1b are polycrystalline Al and (111)-preferentially oriented Al, respectively. To the best of our knowledge, this anomalous electrodeposition behavior recognized in the 60.0-40.0 mol% AlCl3–urea has not been known in the chloroaluminate IL. Further investigation was conducted by the electrodeposition in the ternary AlCl3–[C2mim]Cl–urea melts under the same condition that can produce Al nanoplatelets in the AlCl3–urea. In the present study, we used three types of melts, 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 melts provoked the morphology variation in the Al deposits. The 2D structure appeared in only the 60.0-10.0-30.0 mol% AlCl3–[C2mim]Cl–urea, suggesting that the urea is highly involved in the nanoplatelet formation. Because direct observation of the electrodeposition process enables to propose the plausible formation mechanism, the airtight electrochemical cell for operando digital microscope was newly designed. Further insights obtained through the operando observation will be reported in our presentation.

References

T. Tsuda, in Next Generation Batteries, K. Kanamura, Ed., Springer, Singapore, pp. 565-580 (2021).

T. Tsuda, R. Miyakawa, A. Konda, Y. Ikeda, and S. Kuwabata, Program & Book of Abstracts (BoA) of 69th Annual ISE Meeting, p. 1512 (2018).

T. Tsuda, G. R. Stafford, and C. L. Hussey, J. Electrochem. Soc., 164, H5007 (2017) and references therein.

Figure 1