Effects of Cations on Electrochemical Behavior of Ni(II)/Ni in a Hydrophobic Ionic Liquid
Authors: Zhu Yan-Li , Katayama Yasushi , Miura Takashi 
Published: February 2020

Nickel is one of the important constituents for such functional materials as decoration, corrosion- and heat-resistance and magnatic alloys [1,2]. Electrodeposition method has been shown to be a good way to prepare nickel and nickel-based alloys since it has low cost, low requirement and rapid deposition rate [3]. Much interest in the electrodeposition from ionic liquids is due to the fact that there is no hydrogen evolution in these electrolytes in addition to the advantages of their wide electrochemical potential windows, high thermal stability and negligible low vapor pressure [4-6]. The electrodeposition of Ni has been mainly studied in acidic chloroaluminate [7,8] and pyrrolidinium ionic liquids [9]. The crystal morphology of the deposit is different from each other. Especially the difference in the crystal size is significant, probably due to the different adsorption ability of cations on the substrate and/or Ni deposit [9]. In this paper, the effects of the cations of 1-ethyl-3-methylimidazolium (EMI+), 1-methyl-propylpiperidinium (PP13
+), and 1-butyl-1-methylpyrrolidinium (BMP+) on the electrochemical process of Ni(II)/Ni were investigated in room-temperature ionic liquids containing bis(trifluoromethylsulfonyl)amide (TFSA-).
The UV-vis spectra showed that the changes in cations have little effect on the solvation structure of Ni(II) in TFSA--based ionic liquid although the molar absorption coefficients in each electrolyte are different. In these ionic liquids, Ni(II) is considered to be octahedrally coordinated by three TFSA- anions.
The typical cyclic voltammograms of a Pt electrode in EMITFSA, BMPTFSA and PP13TFSA containing 0.05 mol dm-3 Ni(TFSA)2 are shown in Figure 1. The cathodic peak potential in each ionic liquid was close to one another probably because the coordination environment of Ni(II) is the same in these three ionic liquids. The difference in the cathodic peak current densities reflects the difference in the viscosity of the ionic liquids [10-12]. The order of their viscosities is: EMITFSA < BMPTFSA < PP13TFSA. Thus, the order of the cathodic peak current densities becomes: EMITFSA > BMPTFSA > PP13TFSA, as seen in Figure 1.
Electrodeposition of Ni on copper substrates at 50 oC in the ionic liquids containing 0.05 mol dm−3 Ni(TFSA)2 was performed by galvanostatic electrolysis with the current density of -0.05 mA cm-2. From the images of the deposits in Figure 2, the morphology of the deposits varied with the changes in the cations of the ionic liquids. The potential for the reduction of Ni takes place where the adsorption of cations on the surface of the cathodic electrode becomes significant [13], as shown in Figure 1. Thus, the surface of the cathodic electrode is mainly covered by the cations. The changes in cation will inevitably lead to the changes in the state of the cathodically polarized electrode surface and then the morphology of the deposits.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the financially support of the Japanese Government (Monbukagakusho) Scholarship and the National Natural Science Foundation of China (Grant No. 51304024).
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Figure captions:

Figure 1. Cyclic voltammograms of a Pt electrode in BMPTFSA, EMITFSA and PP13TFSA containing 0.05 mol dm-3 Ni(TFSA)2 at 25 oC. Scan rate: 100 mV s-1.

Figure 2. SEM images of the deposits obtained on Cu substrates in PP13TFSA, BMPTFSA and EMITFSA containing 0.05 mol dm-3 Ni(TFSA)2 at 50 oC. Current density: -0.05 mA cm-2. Charge density: 3.6 C cm-2.

Figure 1