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Electrochemistry Communications 10 (2008) 213–216 www.elsevier.com/locate/elecom
Dicyanamide anion based ionic liquids for electrodeposition of metals Ming-Jay Deng a, Po-Yu Chen b,1, Tin-Iao Leong a, I-Wen Sun a,*,1, Jeng-Kuei Chang c, Wen-Ta Tsai c a Department of Chemistry, National Cheng Kung University, Tainan, Taiwan Faculty of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung, Taiwan c Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan b
Received 30 October 2007; received in revised form 21 November 2007; accepted 23 November 2007 Available online 3 December 2007
Abstract Electrodeposition of Al, Mn, Ni, Zn, Sn, and Cu are successfully demonstrnated in the ionic liquids (ILs) composed of 1-methyl-3alkylimidazolium or N-methyl-N-alkylpyrrolidinium cations with dicyanamide (DCA) anions. The DCA-based room-temperature ILs exhibit lower viscosities than those ILs based on BF4 , PF6 , and bis(triﬂuoromethylsulfonyl)imide (TFSI) anions. While most of the metal chlorides are insoluble in the BF4 , PF6 , and TFSI-based ILs, they exhibit good solubility in DCA-based ILs due to the strong complexing ability of DCA toward the transition metal ions. It is possible to alter the regular reduction sequence for particular metal ions in the DCA-based ILs. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Ionic liquid; Electrodeposition; Dicyanamide; Low viscosity
1. Introduction Low temperature ionic liquids (ILs) that are composed of organic cations such as tetraalkylammonium, alkylsubstituted imidazolium, alkyl-substituted pyrrolidinium, and various anions [1–9] have been examined as the electrolytic media for various electrochemical applications such as lithium ion batteries [10,11], solar cells [12,13], and fuel cells [14,15] and electrodeposition [16–20]. Because of the wide electrochemical window of ILs, electrodeposition of active elements which are diﬃcult to be reduced in aqueous solutions [21–25] can be achieved in ILs. From practical applications point of view, it is of great interest to search the ILs with good solubility, low cost, low viscosity, and wide potential window. The most popular ILs studied so far are those contain the weak Lewis bases PF6 , BF4 , TFSI, CF3 SO3 anions. *
Corresponding author. E-mail address: [email protected]
(I-Wen Sun). ISE member.
1388-2481/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.11.026
However, the literature indicated that transition metals are typically not very soluble in these neutral ILs. Recently, ILs based on dicyanamide (DCA) anion has been synthesized and characterized . The DCA is a Lewis base having good ligand properties [6–9] and it is expected that transition metal salts would dissolve well in the DCAbased ILs by forming complexed anions. This feature makes it easier to prepare transition metal solutions for electrodeposition. Furthermore, the DCA-based ILs exhibit lower viscosity than their TFSI counterparts . Lower viscosity implies more eﬃcient mass transport and higher conductivity for electrochemical applications. Undoubtedly, DCA-based ILs are interesting solvents for electrodeposition. However, the use of the DCA-based ILs for electrodeposition of metals has not been investigated in the literature. The reduction potential of metal complexes with DCA may be more negative than those of the metal chloride complexes. To explore their potential utility for electrodeposition, several DCA-based ILs were synthesized in this report. Their viscosity and density were measured and the electrodeposition of representative
M.-J. Deng et al. / Electrochemistry Communications 10 (2008) 213–216
metals including Cu(I), Zn(II), Sn(II), Ni(II), Mn(II), and Al(III) species in these ILs was conﬁrmed. 2. Experimental The 1-ethyl-3-methylimidazolium dicyanamide (EMIDCA), 1-butyl-3- methylimidazolium dicyanamide (BMI– DCA), and N-butyl-N-methyl pyrrolidinium dicyanamide (BMP-DCA) ILs were prepared following the previous literature . The ILs were mixed with acetone and dichloromethane sequentially and ﬁltrated to remove NaCl precipitates and vacuum dried at 393 K. All electrochemical experiments were performed under a puriﬁed nitrogen atmosphere in a glove box (Vacuum Atmospheres Co.). Anhydrous metal chlorides, including ZnCl2 (98%, Aldrich), AlCl3 (99.9%, Fluka), NiCl2 (99.99%, Aldrich), CuCl (99.999%, Strem), SnCl2 (99.99%, Aldrich), and Mn pieces (99.9%, Alfa Aesar) were used as purchased. Except Mn(II), solutions were prepared by dissolving these anhydrous metal chlorides in the DCA-based ILs. Mn(II) was introduced into the ILs by anodic dissolution of the Mn piece, even though MnCl2 is soluble in the ILs. All electrochemical experiments were carried out with an EG&G PARC Model 263A potentiostat/galvanostat controlled by EG&G Model 270 software. Experiments were performed in a three-electrode cell. A platinum wire (Alfa Aesar, 99.95%) immersed in ferrocene/ferrocenium (Fc/ Fc+ = 50/50 mol%) BuMePy-TFSI solution contained in a glass tube with porous Vycor tip (Bioanalytical Systems, MF-2042) was used as a reference electrode . Density and viscosity measurements were carried out at 301 K with the procedures described in the literatures [26,27]. 3. Results and discussion 3.1. The density and visocity of DCA-based ILs The EMI-DCA, BMI-DCA, and BMP-DCA ILs were dried under vacuum at 393 K for 24 h before being measured with the airtight dilatometer and viscometer. No obvious reaction such as smoking was observed when AlCl3, which is highly sensitive to moisture, was added to the ILs, indicating that moisture level in the ILs must be insigniﬁcant. Shown in Table 1 are the density and viscosity values measured for the DCA-based ILs prepared in this study together with those of several other ILs reported in the literature [3,24,28,29]. It should be noted that chloride impurities may be diﬃcult to be completely removed from the DCA-based ILs, and thus, may aﬀect the measured values of the viscosity or other physical properties of the ILs. However, the viscosity obtained in this study is in consistent with that have been reported previously . As can be seen, the viscosities of ILs composed of dicyanamide anions are relatively lower than other pairs. Although the charge in the DCA anion is mainly centered on the amide nitrogen, delocalization across the whole molecule is likely,
Table 1 Physical properties of the selected room temperature ionic liquids Composition
EMI-DCA EMI-BF4 EMI-TFSI BMI-DCA BMI-TFSI BMI-BF4 BMI-PF6 BMP-DCA BMP-TFSI Bu3MeN-TFSI HeMe3N-TFSI
1.057 1.240 1.520 1.061 1.429 1.170 1.370 0.952 1.379 1.253 1.330
19.8 (301 K) 37.7 (295 K) 34.0 (295 K) 32.6 (301 K) 52.0 (293 K) 233 (303 K) 312 (303 K) 45.3 (301 K) 67.2 (301 K) 386 (303 K) 153 (298 K)
This study   This study    This study   
(301 K) (295 K) (295 K) (301 K) (292 K) (303 K) (303 K) (301 K) (301 K) (303 K) (293 K)
and it is expected to produce weak ion–ion interaction that is one of the most important facts for producing ILs with low melting point and low viscosity. The much lower viscosity of DCA-based ILs implies more eﬃcient mass transport and higher conductivity. 3.2. Electrochemical windows of BMP-DCA and EMI-DCA ILs Fig. 1a and b shows the cyclic voltammograms of the neat EMI-DCA and BMP-DCA ILs, respectively, recorded at a glassy carbon electrode. Similar results were obtained at a platinum electrode. The EMI -DCA IL exhibits a cathodic potential limit near 2.4 V, and anodic potential limit near +1.2 V (vs Fc/Fc+), whereas the BMP-DCA IL exhibit a cathodic potential limit near 3.5 V, and anodic potential limit near +1.2 V (vs Fc/Fc+). The cathodic potential limit, which is determined by the reduction of the organic cations (EMI and BMP), is comparable to the EMI-TFSI , and BMP-TFSI  ILs, respectively. The cathodic limit is important for electrodeposition of metals. Since BMP-DCA IL shows a wider cathodic potential window than EMI-DCA IL, it was chosen for electrodeposition of the more reactive Al and Mn metals. 3.3. The electrodeposition in the BMP-DCA IL AlCl3 dissolves easily in the BMP-DCA IL; the color of the resulting solution is pale yellow. Fig. 1c shows the cyclic voltammogram of the BMP-DCA IL containing 1.6 M AlCl3 recorded on a glassy carbon electrode at room temperature. The electrode is scanned initially from the open circuit potential (OCP) to the negative direction at a sweep rate of 50 mV/s. The voltammogram exhibits a broad reduction wave followed by a stripping peak on the reverse scan. Moreover, a current loop which is indicative of a nucleation process is apparent. The area of the oxidation wave is, however, fairly smaller than that of the reduction part indicating that not all the electrodeposited Al can be completely oxidized during the anodic scan. After the electrodeposition, the electrodeposited sample was analyzed by an energy dispersive spectroscopy (EDS). The EDS result indicated that an Al layer was deposited on the Cu sub-
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Fig. 1. Staircase cyclic voltammograms of (a) neat EMI-DCA, (b) neat BMP-DCA at GC electrode, (c) 1.6 M Al(III)/ BMP-DCA solution at GC electrode, (d) 0.1 M Mn(II)/ BMP-DCA solution at Pt electrode, respectively. (e) 0.05 M Sn(II) solution at Pt electrode, (f) 0.05 M Zn(II) solution at Pt electrode, (g) 0.05 M Ni(II) solution at Pt electrode, and (h) 0.05 M Cu(I) solution at GC electrode, respectively. Scan rate: 50 mV/s. Temperature: 301 K.
strate surface. For illustration, the SEM micrograph of an Al-deposited sample is shown in Fig. 2. The ability of electrodepositing Al from the BMP-DCA IL is of importance because in previous literature aluminum can be electrodeposited almost only from the highly water-sensitive Lewis acidic chloroaluminate ILs [2,16,19] and the water-stable TFSA-based ILs reported by Endres’ group [31–33]. Mn(II) species can be introduced into the IL by anodic dissolution of a metallic Mn electrode. This implies that Mn(II) is solvated or complexed by the DCA anions. The typical staircase cyclic voltammogram of the Mn(II) species in the IL at a platinum electrode is presented in Fig. 1d where the potential was initially scanned from OCP toward the negative direction at a rate of 50 mV/s. The voltammogram exhibits a reduction wave coupled with a small but sharp and symmetric stripping peak on the reverse scan. The area of the oxidation wave is much smaller than that of the reduction wave indicating that the Mn electrodeposits produced during the cathodic scan could not be reoxidized completely. Similar behavior was also observed for the electrodeposition of Mn in BMP-TFSI, and Zn–Mn alloys in Bu3MeN-TFSI [23,24].
Fig. 2. SEM micrographs of the as-deposited surface of Al, and Ni, respectively, that were obtained from the BMP-DCA and EMI-DCA, respectively.
3.4. The electrodeposition in the EMI-DCA IL In this work, EMI-DCA was examined for electrodeposition of metals including Sn, Zn, Ni and Cu. It is found that SnCl2, ZnCl2, NiCl2, and CuCl dissolve readily in the EMIDCA IL. The successful dissolution of the transition metals in EMI-DCA is in consistent with the previous report . As shown in Fig. 1e–h, respectively, the staircase cyclic voltammograms of these metal ions recorded on a platinum electrode at room temperature are typical for metal electrodeposition; in each voltammogram, a current loop which is indicative of a nucleation process is apparent. EDS data taken for the electrodeposits obtained from these solutions showed strong peak for Sn, Zn, Ni, and Cu respectively. For illustration, a SEM micrograph of the Ni-deposited sample is shown in Fig. 2. It is interesting to note from Fig. 1 that the redox potential of these metal in the IL has the relationship as: Sn(II) > Cu(I) > Ni(II) > Zn(II), while such relationship in aqueous solution is Cu(I) > Sn(II) > Ni(II) > Zn(II). This may be due to the strong complexing ability of DCA toward transition metal ions. It is noteworthy that although the electrodeposition of Ni and Zn in aqueous is common, the electrodeposition of Ni or Zn from ILs was only achieved in Lewis acidic ILs such
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as chloroaluminates and chlorozincates except the choline chloride-urea eutectic [17,34] and the TFSA-based ILs . In Lewis basic ILs containing excess chloride ions, Zn(II) and Ni(II) exist as the fully coordinated ZnCl24 and NiCl24 complex anions which can not be reduced to the metal form within the cathodic limit of the ILs. Thus, the ability to electrodepositing these two metals in the DCA-based ILs is of signiﬁcance. Another fact observed in Fig. 1 is that the thermodynamic deposition potentials of Sn and Cu are very close to each other. This is especially favorable for the electrodeposition of Cu–Sn alloys. 4. Conclusion The results observed in this study reveal that DCAbased ILs would be useful electrolytes for electrodeposition. The dicyanamide anion based ILs exhibit much lower viscosity than their TFSI pairs, and are good solvents for metal ions resulting from the high complexing ability of DCA. These features would highly beneﬁt the electrochemical studies. While Ni and Zn could not be electrodeposited from most Lewis basic or neutral ILs reported in literature, they can be obtained from the DCA-based ILs. Moreover, the ability of electrodepositing reactive metals such as Al and Mn is of signiﬁcance. More detailed investigations for Cu, Sn, Mn, Ni, Al, and their alloys are in progress and will be reported in the near future. Acknowledgment The work was supported by the National Science Council of Republic of China, Taiwan (NSC 96-2113-M-006015). References  J.S. Wilkes, J.A. Levisky, R.A. Wilson, C.L. Hussey, Inorg. Chem. 21 (1982) 1263.  G.R. Staﬀord, C.L. Hussey, in: R. Alkire, D. Kolb (Eds.), Advances in Electrochemical Science and Engineering, 275, Wiley-VCH, Weinheim, Germany, 2002.  J. Sun, M. Forsyth, D.R. MacFarlane, J. Phys. Chem. B 102 (1998) 8858.  M. Yamagata, Y. Katayama, T. Miura, J. Electrochem. Soc. 153 (2006) E5.  Yasushi Katayama, Ryuta Fukui, Takashi Miura, J. Electrochem. Soc. 154 (2007) D534.
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