Effects of Cations on Electrochemical Behavior of Ni(II)/Ni in a Hydrophobic Ionic Liquid
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 , 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.
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 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.