The Local Changes in the Physicochemical Properties during Cu Electrodeposition in an Ionic Liquid Composed of [CuCl2]–
Authors: Serizawa Nobuyuki , Kuwahara Suguru , Li Xiaoyi , Katayama Yasushi 
Published: December 2024

The binary equimolar mixtures of cuprous chloride, CuCl, and some organic chlorides have been reported to behave as an ionic liquid containing [CuCl2]– [1]. The high concentration of the Cu(I) species is advantageous for the application to the electrodeposition. In addition, electrical charge transferred during electrodeposition is half that of the conventional aqueous system containing Cu(II) species, because the dissolved Cu species in these ionic liquids are monovalent. On the other hand, the viscosity of the mixture has been reported to depend on the composition [2], suggesting that the local viscosity near the electrode is expected to change during the electrodeposition of Cu. We have reported the changes in the local viscosity and the transient electrolyte structure during the deposition and dissolution of metals in some ionic liquids using an electrochemical quartz crystal microbalance (EQCM) [3, 4], since the resonance resistance (R
res) of a quartz crystal electrode reflects the physicochemical properties, the product of density and viscosity, of the electrolyte contacting on the electrode. In the present study, the local changes in the physicochemical properties during the deposition and dissolution of Cu were investigated in BMP[CuCl2] (BMP+: 1-butyl-1-methylpyrrolidinium) using the EQCM.

BMPCl was synthesized by the reaction of 1-methylpyrrolidine with butyl chloride in acetonitrile. The electrolyte was prepared by mixing the equimolar amount of BMPCl and CuCl in an Ar filled glovebox. The electrochemical measurement was conducted with a three-electrode cell using an Ag wire immersed in BMPTFSA (TFSA–: bis(trifluoromethylsulfonyl)amide) containing 0.1 M AgCF3SO3 as a reference electrode (Ag|Ag(I)). The EQCM analysis of the reactions was performed using an impedance type EQCM system with a Pt-coated quartz crystal electrode. The dissolved species in the electrolyte was identified by Raman spectroscopy. The deposits were characterized using a scanning electron microscope and an X-ray diffractometer (XRD).

The equimolar mixture of BMPCl and CuCl was found to be a homogeneous liquid at 298 K. The peak assignable to [CuCl2]– was observed at 229 nm in the Raman spectrum of the equimolar mixture, indicating that the dissolved Cu(I) species in the electrolyte was [CuCl2]–. The electrodeposits obtained by the potentiostatic electrolysis at –2.0 V vs. Ag|Ag(I) was identified as metallic Cu by XRD. The resonance frequency of the quartz crystal electrode decreased and increased during the deposition and dissolution of Cu, respectively, due to the change in the mass of the electrode. In addition, R
res changed during the reactions, suggesting that the local physicochemical properties near the electrode changed during the deposition and dissolution of Cu.

The present work was partially supported by the Tokyo Ohka Foundation for The Promotion of Science and Technology and JSPS KAKENHI Grant Number JP23H03826.

[1] J. R. Silkey and J. T. Yoke, J. Electrochem. Soc., 127, 1091 (1980).

[2] S. A. Bolkan and J. T. Yoko, J. Chem. Eng. Data, 31, 194 (1986).

[3] N. Serizawa, Y. Katayama, and T. Miura, J. Electrochem. Soc., 156, D503 (2009).

[4] N. Serizawa, S. Seki, K. Takei, H. Miyashiro, K. Yoshida, K. Ueno, N. Tachikawa, K. Dokko, Y. Katayama, M. Watanabe, and T. Miura, J. Electrochem. Soc., 160, A1529 (2013).