Electrochemical Preparation of Ni-Fe in a Hydrophobic Ionic Liquid

The Nickel-iron (Ni-Fe) binary alloy is widely applied in electronics field for memory, recording and storage devices, and corrosion-resistant industries due to its beneficial magnetic property and dimensional stability. It has been fabricated using different techniques, like sputtering and ball milling [1]. Compared with these methods, the electrodeposition method has been shown to have low cost, low energy requirement, rapid deposition rate, and possibility of preparing Ni-Fe alloy with different compositions. However the by-product of hydrogen from the conventional Ni–Fe plating bath can affect and even change the properties of the Ni-Fe alloy [2]. Room-temperature ionic liquids containing bis(trifluoromethylsulfonyl)amide (TFSA^-) have attracted much attention in the electrodeposition of metals and alloys because of wide electrochemical potential windows and no hydrogen evolution. In addition, hydrophobicity of the ionic liquids is expected to be favorable for practical use. In the present study, electrochemical preparation of Ni-Fe was attempted in a hydrophobic room-temperature ionic liquid, BMPTFSA (BMP⁺: 1-butyl-1-methylpyrrolidinium) by electrochemical reduction of [Ni(TFSA)₃]^- and [Fe(TFSA)₃]^-.

         The typical cyclic voltammograms of a Pt electrode from 0.05 mol dm^-3 Ni(II) and 0.05 mol dm^-3 Fe(II) in BMPTFSA are shown in Figure 1. The cathodic peaks of Ni(II) and Fe(II) emerged into one wave, which is close to the cathodic waves of individual metals [3,4]. Electrodeposition of Ni-Fe on a copper substrate in 0.05 mol dm^-3 Ni(TFSA)₂ + 0.05 mol dm^-3 Fe(TFSA)₂/BMPTFSA was performed by the potentiostatic electrolysis with a potential of -1.5 V at 70 ^oC. The black deposit was confirmed to consist of nickel oxide and iron oxide by XPS and EDX. Oxygen probably comes from the oxidation of Ni and Fe in the air. Thus, the cathodic peak can be ascribed to the co-deposition of Ni-Fe. The one broad anodic peak in the anodic scan in Figure 1 is assignable to the anodic dissolution of Ni or Fe from the alloy deposited during the preceding cathodic scan. The cathodic and anodic peak current densities were larger with respect to those of individual metals since the concentration of electroactive species in the ionic liquid are higher compared to the individuals.

         Electrodeposition on a copper substrate from 0.05 mol dm^-3 Ni(TFSA)₂ + 0.05 mol dm^-3 Fe(TFSA)₂/BMPTFSA at 25 ^oC was performed by potentiostatic cathodic reduction of -1.9 V. The energy-dispersive X-ray analysis showed that the molar ratio of Ni to Fe in the deposit was about 1.8. The molar ratio of Ni to Fe in the deposit was larger than that in the electrolyte. This was opposite to the phenomenon in the aqueous solution, where Fe (E^o = -0.44 V) is more preferably deposited than Ni (E^o = -0.23 V) [5]. Electrodeposition of Ni-Fe alloy was classified as anomalous co-deposition in aqueous solution because of the preferential deposition of the less noble metal, Fe. It is proposed that Ni⁺_ads and Fe⁺_ads species compete for adsorption, and [FeOH]⁺ plays an inhibitor role for the nickel reduction on the surface of the electrode [5]. However, there is no OH^- in BMPTFSA. Thus, the inhibitor for the electrodepositon of Ni is not existed. Thus, the inhibitor for the electrodepositon of Ni is not existed. Since the formal potential of Ni(II)/Ni (E^o = -0.55 V vs. Ag/Ag⁺) is more positive than that of Fe(II)/Fe (E^o = -0.77 V) [6] in BMPTFSA, the reduction of Ni is preferred to be deposited on the electrode compared to that of Fe.

 ACKNOWLEDGMENTS

The authors gratefully acknowledge the financially support of the National Natural Science Foundation of China (Grant No. 51304024).

References:

1. H. Ataee-Esfahani, M. R. Vaezi, L. Nikzad, B. Yazdani and S. K. Sadrnezhaad, J. Alloys Compd., 484, 540 (2009).

2. C. W. Su, F. J. He, H. Ju, Y. B. Zhang and E. L. Wang, Electrochim. Acta, 54, 6257 (2009).

3. Y-L. Zhu, Y. Kozuma, Y. Katayama and T. Miura, Electrochim. Acta, 54, 7502 (2009).

4. Y-L. Zhu, Y. Katayama and T. Miura, J. Electrochem. Soc., 159, D699 (2012).

5. J. P. Chopart, O. Aaoubi and K. Msellak, J. Solid State Electrochem., 11, 703 (2007).

6. Y. Katayama, N. Tachikawa and T. Miura, Electrochemical Behavior of Some Divalent Metal Species in 1-Butyl-1-methylpyrrolidinium Bis(trifluoromethylsulfonyl)amide Ionic Liquid, poster COIL-3, Cairns, Australia, (2009).