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An AlCl3 based ionic liquid with a neutral substituted pyridine ligand for electrochemical deposition of aluminum
Electrochimica Acta 160 (2015) 82–88
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
An AlCl3 based ionic liquid with a neutral substituted pyridine ligand for electrochemical deposition of aluminum Youxing Fang b , Kazuki Yoshii a , Xueguang Jiang b , Xiao-Guang Sun a , Tetsuya Tsuda c , Nada Mehio b , Sheng Dai a,b, * a b c
Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Department of Chemistry, University of Tennessee-Knoxville, Knoxville, TN 37916-1600, USA Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 August 2014 Received in revised form 3 February 2015 Accepted 3 February 2015 Available online 4 February 2015
A new ionic liquid (IL) based on a “neutral” ligand, 4-propylpyridine, is obtained via complexation with AlCl3. It is found that the asymmetric cleavage of AlCl3 generates AlCl2+ and AlCl4, and the former is coordinated by 4-propylpyridine to produce the Al-containing cations ([AlCl2(4-Pr-Py)2]+). The AlCl3/4-propylpyridine IL with a molar ratio of 1.3/1 is highly fluidic with a viscosity of 42.8 mPa s and an ionic conductivity of 5.0 104 S/cm at room temperature. In contrast to conventional ILs for electroplating aluminum in which the electrochemically active species are Al-containing anions (for example Al2Cl7), this new IL has an Al-containing cation as the electroactive species, which is beneficial to electrodeposition of aluminum. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Al-containing ionic liquid Al electroplating Al-containing cations neutral ligand
1. INTRODUCTION During the development of ionic liquids (ILs), metal-containing ILs have attracted wide interest for their catalysis, battery, and plating applications [1,2]. Most metal-containing ILs are composed of metal complex anions derived from anionic ligands such as Cl, CN, N(CN), SCN, NCS, which can decompose easily or are highly sensitive to water. As an alternative to negatively charged ligands, neutral ligands can also be used to form metal complexes. In the latter case, the corresponding metal complexes are cationic in nature. For example, our group reported silver- and zinccontaining coordination complexes with neutral organic amine ligands as the cations to produce hydrophobic ILs [3]. Later, Anderson et al. further prepared a series of metal-containing ILs based on complexation of transition metal cations (Cu, Fe, Mn, and Zn) with neutral alkanolamine ligands [4,5]. Notably, Ag-containing ILs derived from alkylamine [6] and pyridine-N-oxide [7] neutral ligands have been prepared for high-current-density electrodeposition of silver. As noted above, most metal-containing ILs are based on transition metals. For those main group metal-containing ILs, aluminum based ones are very common, which have been mainly used for electrodeposition of aluminum [8–20]. A common
* Corresponding author. Tel.: +18655767303; fax: +18655765235. E-mail address: [email protected] (S. Dai). http://dx.doi.org/10.1016/j.electacta.2015.02.020 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
feature of these aluminum based ILs is that the electroactive species are aluminum-containing anions such as Al2Cl7. The disadvantage of anionic species for electrodeposition is that under reductive conditions, they not only travel against the electric field but also compete with common cations such as N-alkyl pyridinium and imidazolium along the electric field, which will induce polarization and reduce the energy efficiency of the electrodeposition process. Therefore, development of ILs with aluminum-containing cations as the electroactive species is highly desirable. Unfortunately, ILs based on Al-containing cations are seldom reported. It is generally believed that the formation of the cationic aluminum complex species involves either the asymmetric cleavage of a halide-bridged dimeric compound or halide displacement, abstraction or elimination, in which a neutral ligand donor (or base) complexes with the cation to compensate the charge [21]. Following a similar approach, Abbott et al. recently reported that simple neutral amides, such as acetamide and urea, could be mixed with AlCl3 to form roomtemperature ILs, which were used both as catalysts for acetylation of ferrocene and as electrolytes for electrodeposition of aluminum [22]. Owing to the high concentration of functional groups (CONH2 or NH2CONH2) and short alkyl chains (CH3), these ILs cannot effectively shield the aluminum center and, thus, it is still moisture-sensitive. Alternatively, pyridine-based species could mitigate the moisture sensitivity issue because of the aromatic ring. Unfortunately, earlier reported AlCl3-pyridine complexes were solids with high melting points [23].
Y. Fang et al. / Electrochimica Acta 160 (2015) 82–88
In this study, we report the synthesis of new ILs based on AlCl3 and a derivative of pyridine, 4-propylpyridine (4-Pr-Py), and their use for electrodeposition of aluminum. These new ILs based on complexation with neutral ligands are fundamentally different from earlier ones based on the mixing of AlCl3 and N-alkyl pyridinium halide salts. First, the positive charge is on the aluminum in the former case, whereas it is localized on the pyridium cation in the latter case. Second, under reductive conditions, the neutral pyridine species are more stable than the cationic pyridinium species; therefore, they will not be reduced during the aluminum deposition process and affect the morphology. Finally, as mentioned earlier, the cationic aluminum species are favored over anionic aluminum species in aluminum electrodeposition. 2. MATERIALS AND METHODS Granular AlCl3 was purchased from Sigma–Aldrich and used as received. 4-propylpyridine was obtained from TCI (Tokyo Chemical Industry) chemical company. 1-Ethyl-3-methylimidazolium chloride (EMIC) purchased from Sigma–Aldrich was purified by recrystallization [24]. In an Ar-filled glovebox, a calculated amount of AlCl3 was added into 4-propylpyridine in a sealed vessel with a magnetic stirring bar. The reaction is exothermic so it is not necessary to heat the vessel to accelerate the dissolution of AlCl3. The as-prepared IL is shown in Fig. 1. Thermal gravimetric analysis was recorded on a Pyris 1 TGA. The differential scanning calorimetry (DSC) profile was acquired from NETZSCH (STA 409 PC) under an argon environment. The kinematic viscosity data was obtained via a Cannon–Fenske viscometer, and the dynamic viscosity data were calculated from the kinematic viscosity data and the density of the respective ILs. The mass spectrum analyses were performed using a JEOL AccuTOF-D time-of-flight (TOF) mass spectrometer with a DART (direct analysis in real time) ionization source from JEOL USA, Inc. The 27Al NMR spectra were recorded on a Bruker Avance 400 spectrometer with 1.0 M Al(NO3)3 aqueous solution as the external chemical shift standard. The electrochemical experiment was performed on CHI600D electrochemical analyzer. Pt wire (0.5 mm in diameter) sealed inside pyrex glass was used as the working electrode, and aluminum wires were used as both counter and reference electrode. SEM images are obtained by Hitachi S-4800 FEG scanning electron microscope with the EDS attachment.
Fig. 1. Digital photo of AlCl3/4-propylpyridine(1.3:1, molar ratio) IL.
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3. RESULTS AND DISCUSSION The new ILs were prepared by slowly adding calculated amounts of AlCl3 into 4-Pr-Py under magnetic stirring in an Ar-filled glove box. AlCl3 quickly reacts with 4-Pr-Py with heat being released. The cooled solution remains a liquid at room temperature as long as the mixing molar ratio of AlCl3 and 4-Pr-Py is in the range of 1.1:1 to 1.5:1. The liquids are optically transparent with a yellow color and are highly fluidic as shown in Fig. 1 for the IL with a molar ratio of 1.3:1. The formation of ILs (or complexes) between AlCl3 and 4-Pr-Py can be verified by thermal gravimetric analysis (TGA). As shown in Fig. 2, under an inert atmosphere of helium and a heating rate of 20 C/min, 4-Pr-Py evaporated slowly at the very beginning with a major onset temperature of 95 C. For AlCl3, the major evaporation onset temperature is 160 C. When the mixing ratio of AlCl3 and 4-Pr-Py is 1:1, the onset temperature increased to 270 C. When the ratio of AlCl3 and 4-Pr-Py was increased to 1.3:1, the onset temperature dropped to 250 C. Nonetheless, the onset decomposition temperatures of the complexes were significantly higher than those of each of the components, confirming that complexes are formed. In addition to TGA, mass spectra (MS) measurement was also used to observe the molecular and quasimolecular ion peaks. A typical example of the MS spectra for AlCl3/4-Pr-Py (1.1:1) is shown in Fig. 3. The major cationic species (Fig. 3, left) were [4-Pr-PyH]+ (m/z = 121) and [AlCl2(4-Pr-Py)2]+(m/z = 340). The anionic species [AlCl4] (m/z = 169) (Fig. 3, right) were also identified. However, Al2Cl7 (m/z = 275.5) was not observed in the spectrum. This result is similar to data that were previously reported for ILs based on the complexation of amide and AlCl3 [22]. It is assumed that the asymmetric cleavage of AlCl3 generates AlCl2+ and AlCl4, and 4-Pr-Py coordinates the former. The general formation can be summarized as follows: 2AlCl3 + 2 4-Pr-Py $ [AlCl2(4-Pr-Py)2]+ + AlCl4
(1)
Nuclear magnetic resonance (NMR) was also used to identify the coordinated aluminum species in the new ILs, with the wellstudied AlCl3/EMImCl IL as a reference. As seen in Fig. 4, the peak at 103 ppm for the AlCl3/EMImCl (mixture of 1:1 molar ratio) is attributed to aluminum containing specie AlCl4. As a comparison, two peaks were observed in the NMR spectra of AlCl3/4-Pr-Py IL (mixture of 1:1 molar ratio). The peak at 103 ppm was attributed to
Fig. 2. Thermal gravimetric analysis diagrams of AlCl3, 4-propylpyridine, and their complexes at different ratios under an inert atmosphere of helium and a heating rate of 20 C/min.
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Fig. 3. MS spectrum of AlCl3/4-propylpyridine (1.1:1) (4-Pr-Py = 4-propylpyridine, cationic species; right: anionic species).
the symmetrical tetrahedral anion AlCl4, and the other one at 108 ppm was assigned to the cationic aluminum species, corresponding to [AlCl2(4-Pr-Py)2]+, with similar tetrahedral structure. The downfield shift was caused by the coordination legend changes [25]. The two coordinated aluminum species observed by NMR spectra were consistent with those observed by mass spectroscopy. For both ILs, it is observed that the peak became broader with increased quantities of AlCl3. The increased number of coordination complexes in the system may be responsible for the observed peak broadening. Further investigation is underway. Note that the NMR tube generated the broad peak around 69 ppm observed in all IL samples. Though the analysis of infrared spectrum (IR) data is generally limited to identifying the stoichiometry, it is a valuable tool that can provide additional evidence that 4-Pr-Py is complexing with AlCl3. In the literature, the “ring” vibrations of pyridine and other bands in the region of 1700 cm1 and 1400 cm1 have been used to distinguish free base, coordinately bonded pyridine, and the pyridinium ion, which was used to characterize surface acidity [26,27]. As shown in Fig. 5, the two strong peaks of 4-Pr-Py observed at 1414 cm1 and 1602 cm1 were shifted to 1443 cm1 and 1629 cm1 after complexing with
AlCl3. This was consistent with the shifts observed for coordinately bonded pyridine complexes with BH3 [26] and AlCl3 [27], respectively. In addition, the two new peaks observed at 1072 cm1 and 1050 cm1 for the AlCl3/4-Pr-Py (1.3:1) IL mostly likely originated from the original 993 cm1 peak of neat 4-Pr-Py after complexation. Thus, the above IR spectra further confirms the
Fig. 5. FT-IR spectrum of 4-Pr-Py and AlCl3/4-Pr-Py.
Fig. 4. 27Al NMR spectra of AlCl3/EMIC ILs and AlCl3/4-PrPy ILs in different mixing molar ratio.
Fig. 6. Temperature (from 25 to 100 C) dependence of ionic conductivities of ionic liquids with different molar ratios of AlCl3:4-Pr-Py.
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Fig. 7. DSC thermograms of the ionic liquids with different ratios of AlCl3:4-Pr-Py at a heating rate of 10 C/min under nitrogen. (Note: the lines have been shifted perpendicularly for clarity).
Fig. 9. Cyclic voltammogram of AlCl3:4-Pr-Py at a molar ratio of 1.3:1 on a platinum working electrode with aluminum wire as both counter and reference electrode. Start potential: 1.0 V. Scan rate: 100 mV/s.
observation that the new IL is generated from the complexation of AlCl3 and the neutral ligand, 4-Pr-Py. The ionic conductivities of the new ILs were measured by electrochemical impedance spectroscopy with a home-made twoplatinum-electrode cell calibrated with a 0.1 M KCl aqueous solution [28,29]. Fig. 6 shows the temperature dependence of the ionic conductivities of the ILs, which follows the Vogel-FulcherTammann (VFT) equation s = s o exp [B/(T To)], where s o (S cm1), B (K), and To (K) are constants [30–32]. Moreover, it is well known that the ionic conductivity is directly coupled to the number of charge carriers and their mobility via the following equation:
the excess AlCl3 were directly coupled with AlCl4 to form Al2Cl7, as in the case of AlCl3/EMIC, the ionic conductivity would decrease because the number of charge carrier does not change while the ion mobility (Al2Cl7) would decrease (relative to AlCl4). Therefore, according to equation (3), the newly added AlCl3 must have undergone asymmetric cleavage to generate the AlCl2+ cation and the AlCl4 anion. According to equation (4), it is highly possible, even though we cannot confirm it directly, that the newly generated AlCl2+ might have abstracted one neutral ligand from [AlCl2(4-Pr-Py)2]+ to generate the [AlCl2(4-Pr-Py)]+ cation. The mobility of both AlCl2+ and [AlCl2(4-Pr-Py)]+ was higher than that of [AlCl2(4-Pr-Py)2]+, which was confirmed by the concurrent decrease in glass transition temperature (Tg) (Fig. 7). For example, the Tg is 66 C when the molar ratio was 1:1; however, the Tg decreased to 68 C and 72 C when the molar ratio of AlCl3 and
s ¼ Sn i q i mi
(2)
where ni, qi and mi and are the number, charge and mobility of carrier. The lowest ionic conductivity values were measured when the molar ratio of AlCl3 and 4-Pr-Py is 1:1. When the molar ratio was increased to 1.1:1 and 1.3:1, the ionic conductivity increased significantly as well, this observation must be a consequence of the increase in the number of both charge carrier number and the increased ion mobility. The huge jump in ionic conductivity discussed above can also serve as an indirect evidence for the nonexistence of anionic species of Al2Cl7. This is due to the fact that if
Fig. 8. Plot of fluidity vs. conductivity of AlCl3/4-Pr-Py with different molar ratios.
Fig. 10. Digital images (A and C) and scanning electron micrographs (B and D) of aluminum deposition on copper substrates. Electrodeposition was performed by the potentiostatic method (0.2 V vs. Al/Al3+) in the ionic liquid of AlCl3 and 4propylpyridine with a molar ratio 1.3:1 for 1 h (A and B) and 3 h (C and D) at room temperature.
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Fig. 13. Energy-dispersive X-ray spectroscopy (EDX) analysis of Al deposition on Cu foil.
Fig. 11. Cross-section SEM image of Al on Cu substrate. Electrodeposition was performed by the potentiostatic method (0.2 V vs. Al/Al3+) in the ionic liquid of AlCl3 and 4-propylpyridine with a molar ratio 1.3:1 for 3 h at room temperature.
4-Pr-Py was increased to 1.1:1 and 1.3:1, respectively. However, when the molar ratio was further increased from 1.3:1 to 1.5:1, the ionic conductivity hardly incresed, which might be attributed to the decreased ion mobility (Tg increases from 72 C to 71 C) and the possible formation of ionic aggregates or even ion pairs [33,34]. In addition to ionic conductivity, we also measured the viscosity of the ILs. As shown in Fig. 8, the conductivity steadily increased as the viscosity decreased, with the exception of one steep transition point at 50 C, which was clearly related to the melting point of the IL (49 C, Fig. 7). Special attention is directed to the comparison of the ionic conductivities at 50 C for the three ILs with molar ratio of AlCl3 and 4-Pr-Py being 1.1:1, 1.3:1 and 1.5:1, respectively. The viscosity increased when the molar ratio of AlCl3 was increased from 1.1 to 1.5, therefore, the corresponding increase of ionic conductivity in both Fig. 6 and Fig 8 can be clearly attributed to the increase in charger carrier number. As shown in Fig. 7, the IL was a solid with a melting point of 49 C when the molar ratios of AlCl3 and 4-Pr-Py were 1:1 and 1.1:1, respectively; it remained a liquid when the molar ratios were further increased to 1.3:1 and 1.5:1, respectively. For simplicity, the IL with a molar ratio of 1.3, with an ambient ionic conductivity of 5.0 104 S/cm, was used for the following cyclic voltammetry (CV) measurement and aluminum deposition. 2AlCl3 $ AlCl2+ + AlCl4
AlCl2+ + [AlCl2(4-Pr-Py)2]+ $ 2 [AlCl2(4-Pr-Py)]+
(3)
(4)
Fig. 9 shows the cyclic voltammogram of the IL on a platinum working electrode at a scan rate of 100 mV/s. The aluminum
Fig. 12. X-ray diffraction pattern of Al deposition on Cu foil.
Fig. 14. A: Cyclic voltammogram of AlCl3:4-Pr-Py at a molar ratio of 1.3:1 on a platinum working electrode with aluminum wire as both counter and reference electrode at 50 C. Start potential: 1.2 V. Scan rate: 100 mV/s. B: scanning electron micrographs of aluminum deposition on copper substrates. C: Cross-section SEM image of Al on Cu substrate. Electrodeposition (for B and C) was performed by the potentiostatic method (0.6 V vs. Al/Al3+) in the ionic liquid of AlCl3 and 4propylpyridine with a molar ratio 1.3:1 for 1 h at 50 C.
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Fig. 15. Cyclic voltammogram of AlCl3:EMIC (A and C) and AlCl3:4-Pr-Py (B and C) at a molar ratio of 1.3:1 on a platinum working electrode with aluminum wire as both counter and reference electrode for different time. Scan rate: 100 mV/s. Characterizations are continuously performed in the static ILs without protective gas.
deposition started at a potential of 0.16 V vs Al/Al3+, and there was a corresponding aluminum stripping peak at 0.35 V. It was noticed that the bulk deposition region showed a characteristic “nucleation loop” where the forward and backward current cross over [35]. The bulk electrolysis was carried out using a polished copper strip as the cathode, spiral aluminum wire as the counter electrode, and straight aluminum wire as the reference electrode. As conveyed in Fig. 9, the bulk electrolysis was carried out using a potentiostatic method; that is, a constant potential of 0.2 V vs Al/ Al3+ was used. The electrolysis was carried out for 1 and 3 h, respectively. Fig. 8 shows the digital images (A and C) and the scanning electron micrographs (B and D) of the electrodeposited aluminum on the copper substrates. A homogeneous, bright, adherent aluminum layer was obtained after plating for only 1 h (Fig. 10B). When the plating time was increased to 3 h, a thicker and uneven aluminum deposition was obtained (Fig. 10D) with a thickness of approximately 3.3 mm (Fig. 11). To confirm the purity of the deposition, the 1 h deposition film was further analyzed with x-ray diffraction and energy–dispersive x-ray spectroscopy. As shown in Figs. 12 and 13, the only signals correspond to the substrate copper and the deposited aluminum. However, for practical applications fast plating is preferable. Fig. 14A shows the cyclic voltammogram of the IL at 50 C with the typical reduction loop and stripping peak. In comparison to Fig. 9, the reduction current of Al at 0.6 V at 50 C was 50% higher than that at 0.4 V at room temperature. Another electrodeposition was conducted at 50 C with a potential held at 0.6 V for 1 h (Fig. 14B and C). It is found that a rough Al layer with a thickness of 3.9 mm was obtained, which was attributed to the preference of the dendritic growth at a higher deposition rate. These results show that ILs based on complexation of neutral pyridine ligand and AlCl3 are good candidates for the electrodeposition of aluminum, even though the detailed plating parameters need to be optimized. In addition, AlCl3/4-Pr-Py showed superior hydrolytic stability to that
of AlCl3/EMIC. For comparison, the ILs were taken out of the glove box and their hydrolytic stability were monitored in the air by CV conducted in a static beaker. As shown in Fig. 15, the current density of AlCl3/EMIC decreased quickly with time (Fig. 15a) while that of AlCl3/4-Pr-Py was relatively unaffected with time (Fig. 15b). Although the initial current density of AlCl3/EMIC was more than ten times greater than that of AlCl3/4-Pr-Py, it was reduce to only about 15% of AlCl3/4-Pr-Py after 70 min (Fig. 15c). These results demonstrate that the AlCl3/4-Pr-Py IL was more stable than AlCl3/EMIC IL upon exposure to air. 4. CONCLUSIONS We have shown that new ILs can be obtained from AlCl3 and a neutral ligand 4-propylpyriinde (4-Pr-Py). The physical properties of the ILs, such as thermal stability, glass transition temperature, melting point, and ionic conductivity, are closely related to the mixing ratio of the two components. This IL formed from aluminum complexation with neutral ligands generated aluminum-based cationic [AlCl2(4-Pr-Py)2]+ species and anionic AlCl4 species, that were confirmed by both NMR and MS measurements. We also show that these new ILs can be successfully used for electrodeposition of aluminum. Unlike traditional chloroaluminates, in which the electroactive species Al2Cl7 is very sensitive to moisture, the new electroactive cationic aluminum species is less moisture-sensitive because of the ligand complexation. In addition, the moisture stability of the cationic species can be further improved by tuning the ligand structure. Therefore, these new ligand-complexed ILs can be classified as the third generation of the chloroaluminate ILs. The discovery of a new air- and moisture-stable IL will not only inspire us to discover and design novel ILs but also can speed up their practical application in aluminum deposition for corrosion protection in various fields.
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ACKNOWLEDGEMENTS This work was funded by the Strategic Environmental research and Development Program (SERDP) (WP2316). References [1] F. Endres, Chemphyschem 3 (2002) 144–154. [2] S. Wilkes John, Ionic Liquids in Perspective: The Past with an Eye Toward the Industrial Future, Ionic Liquids, American Chemical Society (2002) 214–229. [3] J.F. Huang, H.M. Luo, S. Dai, J. Electrochem. Soc. 153 (2006) J9–J13. [4] T.M. Anderson, D. Ingersoll, A.J. Rose, C.L. Staiger, J.C. Leonard, Dalton Trans. 39 (2010) 8609–8612. [5] H.D. Pratt, A.J. Rose, C.L. Staiger, D. Ingersoll, T.M. Anderson, Dalton Trans. 40 (2011) 11396–11401. [6] D. Depuydt, N.R. Brooks, S. Schaltin, L. Van Meervelt, J. Fransaer, K. Binnemans, Chempluschem 78 (2013) 578–588. [7] J. Sniekers, N.R. Brooks, S. Schaltin, L. Van Meervelt, J. Fransaer, K. Binnemans, Dalton Trans. 43 (2014) 1589–1598. [8] A.P. Abbott, G. Frisch, K.S. Ryder, Annu. Rev. Mater. Res. 43 (2013) 335–358. [9] F.H. Hurley, Electrodeposition of aluminum, US Patent 2446331, 1948. [10] T.P. Wier Jr., F.H. Hurley, Electrodeposition of aluminum, US Patent, 2446349, 1948. [11] J.S. Wilkes, J.A. Levisky, R.A. Wilson, C.L. Hussey, Inorg. Chem. 21 (1982) 1263–1264. [12] J. Tang, K. Azumi, Electrochim. Acta 56 (2010) 1130–1137. [13] B. Li, C. Fan, Y. Chen, J. Lou, L. Yan, Electrochim. Acta 56 (2011) 5478–5482. [14] M. Johnston, J.J. Lee, G.S. Chottiner, B. Miller, T. Tsuda, C.L. Hussey, D.A. Scherson, J. Phys. Chem. B 109 (2005) 11296–11300.
[15] J.K. Chang, I.W. Sun, S.J. Pan, M.H. Chuang, M.J. Deng, W.T. Tsai, T I Met Finish 86 (2008) 227–233. [16] T. Tsuda, S. Arimoto, S. Kuwabata, C.L. Hussey, J. Electrochem. Soc. 155 (2008) D256–D262. [17] T. Tsuda, C.L. Hussey, G.R. Stafford, J.E. Bonevich, J. Electrochem. Soc. 150 (2003) C234–C243. [18] Q. Zhu, C.L. Hussey, Elec Soc S 99 (2000) 494–504. [19] W.R. Pitner, C.L. Hussey, G.R. Stafford, J. Electrochem. Soc. 143 (1996) 130–138. [20] Q. Liao, W.R. Pitner, G. Stewart, C.L. Hussey, G.R. Stafford, J. Electrochem. Soc. 144 (1997) 936–943. [21] D.A. Atwood, Coord. Chem. Rev. 176 (1998) 407–430. [22] H.M. Abood, A.P. Abbott, A.D. Ballantyne, K.S. Ryder, Chem. Commun. 47 (2011) 3523–3525. [23] D.D. Eley, H. Watts, J. Chem. Soc. (1952) 1914–1918. [24] J.S. Wilkes, J.A. Levisky, R.A. Wilson, C.L. Hussey, Inorg. Chem. 21 (1982) 1263–1264. [25] F. Coleman, G. Srinivasan, M. Swadzba-Kwasny, Angew. Chem. Int. Ed. 52 (2013) 12582–12586. [26] E.P. Parry, J. Catal. 2 (1963) 371–379. [27] Y.L. Yang, Y. Kou, Chem. Comm. (2004) 226–227. [28] C. Liao, N. Shao, K.S. Han, X.G. Sun, D.E. Jiang, E.W. Hagaman, S. Dai, Phys. Chem. Chem. Phys. 13 (2011) 21503–21510. [29] X.G. Sun, C. Liao, N. Shao, J.R. Bell, B.K. Guo, H.M. Luo, D.E. Jiang, S. Dai, J. Power Sources 237 (2013) 5–12. [30] H. Vogel, Phys. Z. 22 (1921) 645. [31] G. Tamman, W.Z. Hesse, Anorg. Allg. Chem. 156 (1926) 245–257. [32] G.S. Fulcher, J. Am. Ceram. Soc. 8 (1923) 339–355. [33] A.A. Fannin Jr., D.A. Floreani, L.A. King, J.S. Landers, B.J. Piersma, D.J. Stech, R.L. Vaughn, J.S. Wilkes, J.L. Williams, J. Phys. Chem. B 88 (1984) 2614–2621. [34] M. Lipsztajn, R.A. Osteryoung, J. Electrochem. Soc. 132 (1985) 1126–1130. [35] R. Wibowo, S.E.W. Jones, R.G. Compton, J. Phys. Chem. B 113 (2009) 12293–12298.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
An AlCl3 based ionic liquid with a neutral substituted pyridine ligand for electrochemical deposition of aluminum Youxing Fang b , Kazuki Yoshii a , Xueguang Jiang b , Xiao-Guang Sun a , Tetsuya Tsuda c , Nada Mehio b , Sheng Dai a,b, * a b c
Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Department of Chemistry, University of Tennessee-Knoxville, Knoxville, TN 37916-1600, USA Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 August 2014 Received in revised form 3 February 2015 Accepted 3 February 2015 Available online 4 February 2015
A new ionic liquid (IL) based on a “neutral” ligand, 4-propylpyridine, is obtained via complexation with AlCl3. It is found that the asymmetric cleavage of AlCl3 generates AlCl2+ and AlCl4, and the former is coordinated by 4-propylpyridine to produce the Al-containing cations ([AlCl2(4-Pr-Py)2]+). The AlCl3/4-propylpyridine IL with a molar ratio of 1.3/1 is highly fluidic with a viscosity of 42.8 mPa s and an ionic conductivity of 5.0 104 S/cm at room temperature. In contrast to conventional ILs for electroplating aluminum in which the electrochemically active species are Al-containing anions (for example Al2Cl7), this new IL has an Al-containing cation as the electroactive species, which is beneficial to electrodeposition of aluminum. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Al-containing ionic liquid Al electroplating Al-containing cations neutral ligand
1. INTRODUCTION During the development of ionic liquids (ILs), metal-containing ILs have attracted wide interest for their catalysis, battery, and plating applications [1,2]. Most metal-containing ILs are composed of metal complex anions derived from anionic ligands such as Cl, CN, N(CN), SCN, NCS, which can decompose easily or are highly sensitive to water. As an alternative to negatively charged ligands, neutral ligands can also be used to form metal complexes. In the latter case, the corresponding metal complexes are cationic in nature. For example, our group reported silver- and zinccontaining coordination complexes with neutral organic amine ligands as the cations to produce hydrophobic ILs [3]. Later, Anderson et al. further prepared a series of metal-containing ILs based on complexation of transition metal cations (Cu, Fe, Mn, and Zn) with neutral alkanolamine ligands [4,5]. Notably, Ag-containing ILs derived from alkylamine [6] and pyridine-N-oxide [7] neutral ligands have been prepared for high-current-density electrodeposition of silver. As noted above, most metal-containing ILs are based on transition metals. For those main group metal-containing ILs, aluminum based ones are very common, which have been mainly used for electrodeposition of aluminum [8–20]. A common
* Corresponding author. Tel.: +18655767303; fax: +18655765235. E-mail address: [email protected] (S. Dai). http://dx.doi.org/10.1016/j.electacta.2015.02.020 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
feature of these aluminum based ILs is that the electroactive species are aluminum-containing anions such as Al2Cl7. The disadvantage of anionic species for electrodeposition is that under reductive conditions, they not only travel against the electric field but also compete with common cations such as N-alkyl pyridinium and imidazolium along the electric field, which will induce polarization and reduce the energy efficiency of the electrodeposition process. Therefore, development of ILs with aluminum-containing cations as the electroactive species is highly desirable. Unfortunately, ILs based on Al-containing cations are seldom reported. It is generally believed that the formation of the cationic aluminum complex species involves either the asymmetric cleavage of a halide-bridged dimeric compound or halide displacement, abstraction or elimination, in which a neutral ligand donor (or base) complexes with the cation to compensate the charge [21]. Following a similar approach, Abbott et al. recently reported that simple neutral amides, such as acetamide and urea, could be mixed with AlCl3 to form roomtemperature ILs, which were used both as catalysts for acetylation of ferrocene and as electrolytes for electrodeposition of aluminum [22]. Owing to the high concentration of functional groups (CONH2 or NH2CONH2) and short alkyl chains (CH3), these ILs cannot effectively shield the aluminum center and, thus, it is still moisture-sensitive. Alternatively, pyridine-based species could mitigate the moisture sensitivity issue because of the aromatic ring. Unfortunately, earlier reported AlCl3-pyridine complexes were solids with high melting points [23].
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In this study, we report the synthesis of new ILs based on AlCl3 and a derivative of pyridine, 4-propylpyridine (4-Pr-Py), and their use for electrodeposition of aluminum. These new ILs based on complexation with neutral ligands are fundamentally different from earlier ones based on the mixing of AlCl3 and N-alkyl pyridinium halide salts. First, the positive charge is on the aluminum in the former case, whereas it is localized on the pyridium cation in the latter case. Second, under reductive conditions, the neutral pyridine species are more stable than the cationic pyridinium species; therefore, they will not be reduced during the aluminum deposition process and affect the morphology. Finally, as mentioned earlier, the cationic aluminum species are favored over anionic aluminum species in aluminum electrodeposition. 2. MATERIALS AND METHODS Granular AlCl3 was purchased from Sigma–Aldrich and used as received. 4-propylpyridine was obtained from TCI (Tokyo Chemical Industry) chemical company. 1-Ethyl-3-methylimidazolium chloride (EMIC) purchased from Sigma–Aldrich was purified by recrystallization [24]. In an Ar-filled glovebox, a calculated amount of AlCl3 was added into 4-propylpyridine in a sealed vessel with a magnetic stirring bar. The reaction is exothermic so it is not necessary to heat the vessel to accelerate the dissolution of AlCl3. The as-prepared IL is shown in Fig. 1. Thermal gravimetric analysis was recorded on a Pyris 1 TGA. The differential scanning calorimetry (DSC) profile was acquired from NETZSCH (STA 409 PC) under an argon environment. The kinematic viscosity data was obtained via a Cannon–Fenske viscometer, and the dynamic viscosity data were calculated from the kinematic viscosity data and the density of the respective ILs. The mass spectrum analyses were performed using a JEOL AccuTOF-D time-of-flight (TOF) mass spectrometer with a DART (direct analysis in real time) ionization source from JEOL USA, Inc. The 27Al NMR spectra were recorded on a Bruker Avance 400 spectrometer with 1.0 M Al(NO3)3 aqueous solution as the external chemical shift standard. The electrochemical experiment was performed on CHI600D electrochemical analyzer. Pt wire (0.5 mm in diameter) sealed inside pyrex glass was used as the working electrode, and aluminum wires were used as both counter and reference electrode. SEM images are obtained by Hitachi S-4800 FEG scanning electron microscope with the EDS attachment.
Fig. 1. Digital photo of AlCl3/4-propylpyridine(1.3:1, molar ratio) IL.
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3. RESULTS AND DISCUSSION The new ILs were prepared by slowly adding calculated amounts of AlCl3 into 4-Pr-Py under magnetic stirring in an Ar-filled glove box. AlCl3 quickly reacts with 4-Pr-Py with heat being released. The cooled solution remains a liquid at room temperature as long as the mixing molar ratio of AlCl3 and 4-Pr-Py is in the range of 1.1:1 to 1.5:1. The liquids are optically transparent with a yellow color and are highly fluidic as shown in Fig. 1 for the IL with a molar ratio of 1.3:1. The formation of ILs (or complexes) between AlCl3 and 4-Pr-Py can be verified by thermal gravimetric analysis (TGA). As shown in Fig. 2, under an inert atmosphere of helium and a heating rate of 20 C/min, 4-Pr-Py evaporated slowly at the very beginning with a major onset temperature of 95 C. For AlCl3, the major evaporation onset temperature is 160 C. When the mixing ratio of AlCl3 and 4-Pr-Py is 1:1, the onset temperature increased to 270 C. When the ratio of AlCl3 and 4-Pr-Py was increased to 1.3:1, the onset temperature dropped to 250 C. Nonetheless, the onset decomposition temperatures of the complexes were significantly higher than those of each of the components, confirming that complexes are formed. In addition to TGA, mass spectra (MS) measurement was also used to observe the molecular and quasimolecular ion peaks. A typical example of the MS spectra for AlCl3/4-Pr-Py (1.1:1) is shown in Fig. 3. The major cationic species (Fig. 3, left) were [4-Pr-PyH]+ (m/z = 121) and [AlCl2(4-Pr-Py)2]+(m/z = 340). The anionic species [AlCl4] (m/z = 169) (Fig. 3, right) were also identified. However, Al2Cl7 (m/z = 275.5) was not observed in the spectrum. This result is similar to data that were previously reported for ILs based on the complexation of amide and AlCl3 [22]. It is assumed that the asymmetric cleavage of AlCl3 generates AlCl2+ and AlCl4, and 4-Pr-Py coordinates the former. The general formation can be summarized as follows: 2AlCl3 + 2 4-Pr-Py $ [AlCl2(4-Pr-Py)2]+ + AlCl4
(1)
Nuclear magnetic resonance (NMR) was also used to identify the coordinated aluminum species in the new ILs, with the wellstudied AlCl3/EMImCl IL as a reference. As seen in Fig. 4, the peak at 103 ppm for the AlCl3/EMImCl (mixture of 1:1 molar ratio) is attributed to aluminum containing specie AlCl4. As a comparison, two peaks were observed in the NMR spectra of AlCl3/4-Pr-Py IL (mixture of 1:1 molar ratio). The peak at 103 ppm was attributed to
Fig. 2. Thermal gravimetric analysis diagrams of AlCl3, 4-propylpyridine, and their complexes at different ratios under an inert atmosphere of helium and a heating rate of 20 C/min.
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Fig. 3. MS spectrum of AlCl3/4-propylpyridine (1.1:1) (4-Pr-Py = 4-propylpyridine, cationic species; right: anionic species).
the symmetrical tetrahedral anion AlCl4, and the other one at 108 ppm was assigned to the cationic aluminum species, corresponding to [AlCl2(4-Pr-Py)2]+, with similar tetrahedral structure. The downfield shift was caused by the coordination legend changes [25]. The two coordinated aluminum species observed by NMR spectra were consistent with those observed by mass spectroscopy. For both ILs, it is observed that the peak became broader with increased quantities of AlCl3. The increased number of coordination complexes in the system may be responsible for the observed peak broadening. Further investigation is underway. Note that the NMR tube generated the broad peak around 69 ppm observed in all IL samples. Though the analysis of infrared spectrum (IR) data is generally limited to identifying the stoichiometry, it is a valuable tool that can provide additional evidence that 4-Pr-Py is complexing with AlCl3. In the literature, the “ring” vibrations of pyridine and other bands in the region of 1700 cm1 and 1400 cm1 have been used to distinguish free base, coordinately bonded pyridine, and the pyridinium ion, which was used to characterize surface acidity [26,27]. As shown in Fig. 5, the two strong peaks of 4-Pr-Py observed at 1414 cm1 and 1602 cm1 were shifted to 1443 cm1 and 1629 cm1 after complexing with
AlCl3. This was consistent with the shifts observed for coordinately bonded pyridine complexes with BH3 [26] and AlCl3 [27], respectively. In addition, the two new peaks observed at 1072 cm1 and 1050 cm1 for the AlCl3/4-Pr-Py (1.3:1) IL mostly likely originated from the original 993 cm1 peak of neat 4-Pr-Py after complexation. Thus, the above IR spectra further confirms the
Fig. 5. FT-IR spectrum of 4-Pr-Py and AlCl3/4-Pr-Py.
Fig. 4. 27Al NMR spectra of AlCl3/EMIC ILs and AlCl3/4-PrPy ILs in different mixing molar ratio.
Fig. 6. Temperature (from 25 to 100 C) dependence of ionic conductivities of ionic liquids with different molar ratios of AlCl3:4-Pr-Py.
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Fig. 7. DSC thermograms of the ionic liquids with different ratios of AlCl3:4-Pr-Py at a heating rate of 10 C/min under nitrogen. (Note: the lines have been shifted perpendicularly for clarity).
Fig. 9. Cyclic voltammogram of AlCl3:4-Pr-Py at a molar ratio of 1.3:1 on a platinum working electrode with aluminum wire as both counter and reference electrode. Start potential: 1.0 V. Scan rate: 100 mV/s.
observation that the new IL is generated from the complexation of AlCl3 and the neutral ligand, 4-Pr-Py. The ionic conductivities of the new ILs were measured by electrochemical impedance spectroscopy with a home-made twoplatinum-electrode cell calibrated with a 0.1 M KCl aqueous solution [28,29]. Fig. 6 shows the temperature dependence of the ionic conductivities of the ILs, which follows the Vogel-FulcherTammann (VFT) equation s = s o exp [B/(T To)], where s o (S cm1), B (K), and To (K) are constants [30–32]. Moreover, it is well known that the ionic conductivity is directly coupled to the number of charge carriers and their mobility via the following equation:
the excess AlCl3 were directly coupled with AlCl4 to form Al2Cl7, as in the case of AlCl3/EMIC, the ionic conductivity would decrease because the number of charge carrier does not change while the ion mobility (Al2Cl7) would decrease (relative to AlCl4). Therefore, according to equation (3), the newly added AlCl3 must have undergone asymmetric cleavage to generate the AlCl2+ cation and the AlCl4 anion. According to equation (4), it is highly possible, even though we cannot confirm it directly, that the newly generated AlCl2+ might have abstracted one neutral ligand from [AlCl2(4-Pr-Py)2]+ to generate the [AlCl2(4-Pr-Py)]+ cation. The mobility of both AlCl2+ and [AlCl2(4-Pr-Py)]+ was higher than that of [AlCl2(4-Pr-Py)2]+, which was confirmed by the concurrent decrease in glass transition temperature (Tg) (Fig. 7). For example, the Tg is 66 C when the molar ratio was 1:1; however, the Tg decreased to 68 C and 72 C when the molar ratio of AlCl3 and
s ¼ Sn i q i mi
(2)
where ni, qi and mi and are the number, charge and mobility of carrier. The lowest ionic conductivity values were measured when the molar ratio of AlCl3 and 4-Pr-Py is 1:1. When the molar ratio was increased to 1.1:1 and 1.3:1, the ionic conductivity increased significantly as well, this observation must be a consequence of the increase in the number of both charge carrier number and the increased ion mobility. The huge jump in ionic conductivity discussed above can also serve as an indirect evidence for the nonexistence of anionic species of Al2Cl7. This is due to the fact that if
Fig. 8. Plot of fluidity vs. conductivity of AlCl3/4-Pr-Py with different molar ratios.
Fig. 10. Digital images (A and C) and scanning electron micrographs (B and D) of aluminum deposition on copper substrates. Electrodeposition was performed by the potentiostatic method (0.2 V vs. Al/Al3+) in the ionic liquid of AlCl3 and 4propylpyridine with a molar ratio 1.3:1 for 1 h (A and B) and 3 h (C and D) at room temperature.
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Fig. 13. Energy-dispersive X-ray spectroscopy (EDX) analysis of Al deposition on Cu foil.
Fig. 11. Cross-section SEM image of Al on Cu substrate. Electrodeposition was performed by the potentiostatic method (0.2 V vs. Al/Al3+) in the ionic liquid of AlCl3 and 4-propylpyridine with a molar ratio 1.3:1 for 3 h at room temperature.
4-Pr-Py was increased to 1.1:1 and 1.3:1, respectively. However, when the molar ratio was further increased from 1.3:1 to 1.5:1, the ionic conductivity hardly incresed, which might be attributed to the decreased ion mobility (Tg increases from 72 C to 71 C) and the possible formation of ionic aggregates or even ion pairs [33,34]. In addition to ionic conductivity, we also measured the viscosity of the ILs. As shown in Fig. 8, the conductivity steadily increased as the viscosity decreased, with the exception of one steep transition point at 50 C, which was clearly related to the melting point of the IL (49 C, Fig. 7). Special attention is directed to the comparison of the ionic conductivities at 50 C for the three ILs with molar ratio of AlCl3 and 4-Pr-Py being 1.1:1, 1.3:1 and 1.5:1, respectively. The viscosity increased when the molar ratio of AlCl3 was increased from 1.1 to 1.5, therefore, the corresponding increase of ionic conductivity in both Fig. 6 and Fig 8 can be clearly attributed to the increase in charger carrier number. As shown in Fig. 7, the IL was a solid with a melting point of 49 C when the molar ratios of AlCl3 and 4-Pr-Py were 1:1 and 1.1:1, respectively; it remained a liquid when the molar ratios were further increased to 1.3:1 and 1.5:1, respectively. For simplicity, the IL with a molar ratio of 1.3, with an ambient ionic conductivity of 5.0 104 S/cm, was used for the following cyclic voltammetry (CV) measurement and aluminum deposition. 2AlCl3 $ AlCl2+ + AlCl4
AlCl2+ + [AlCl2(4-Pr-Py)2]+ $ 2 [AlCl2(4-Pr-Py)]+
(3)
(4)
Fig. 9 shows the cyclic voltammogram of the IL on a platinum working electrode at a scan rate of 100 mV/s. The aluminum
Fig. 12. X-ray diffraction pattern of Al deposition on Cu foil.
Fig. 14. A: Cyclic voltammogram of AlCl3:4-Pr-Py at a molar ratio of 1.3:1 on a platinum working electrode with aluminum wire as both counter and reference electrode at 50 C. Start potential: 1.2 V. Scan rate: 100 mV/s. B: scanning electron micrographs of aluminum deposition on copper substrates. C: Cross-section SEM image of Al on Cu substrate. Electrodeposition (for B and C) was performed by the potentiostatic method (0.6 V vs. Al/Al3+) in the ionic liquid of AlCl3 and 4propylpyridine with a molar ratio 1.3:1 for 1 h at 50 C.
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Fig. 15. Cyclic voltammogram of AlCl3:EMIC (A and C) and AlCl3:4-Pr-Py (B and C) at a molar ratio of 1.3:1 on a platinum working electrode with aluminum wire as both counter and reference electrode for different time. Scan rate: 100 mV/s. Characterizations are continuously performed in the static ILs without protective gas.
deposition started at a potential of 0.16 V vs Al/Al3+, and there was a corresponding aluminum stripping peak at 0.35 V. It was noticed that the bulk deposition region showed a characteristic “nucleation loop” where the forward and backward current cross over [35]. The bulk electrolysis was carried out using a polished copper strip as the cathode, spiral aluminum wire as the counter electrode, and straight aluminum wire as the reference electrode. As conveyed in Fig. 9, the bulk electrolysis was carried out using a potentiostatic method; that is, a constant potential of 0.2 V vs Al/ Al3+ was used. The electrolysis was carried out for 1 and 3 h, respectively. Fig. 8 shows the digital images (A and C) and the scanning electron micrographs (B and D) of the electrodeposited aluminum on the copper substrates. A homogeneous, bright, adherent aluminum layer was obtained after plating for only 1 h (Fig. 10B). When the plating time was increased to 3 h, a thicker and uneven aluminum deposition was obtained (Fig. 10D) with a thickness of approximately 3.3 mm (Fig. 11). To confirm the purity of the deposition, the 1 h deposition film was further analyzed with x-ray diffraction and energy–dispersive x-ray spectroscopy. As shown in Figs. 12 and 13, the only signals correspond to the substrate copper and the deposited aluminum. However, for practical applications fast plating is preferable. Fig. 14A shows the cyclic voltammogram of the IL at 50 C with the typical reduction loop and stripping peak. In comparison to Fig. 9, the reduction current of Al at 0.6 V at 50 C was 50% higher than that at 0.4 V at room temperature. Another electrodeposition was conducted at 50 C with a potential held at 0.6 V for 1 h (Fig. 14B and C). It is found that a rough Al layer with a thickness of 3.9 mm was obtained, which was attributed to the preference of the dendritic growth at a higher deposition rate. These results show that ILs based on complexation of neutral pyridine ligand and AlCl3 are good candidates for the electrodeposition of aluminum, even though the detailed plating parameters need to be optimized. In addition, AlCl3/4-Pr-Py showed superior hydrolytic stability to that
of AlCl3/EMIC. For comparison, the ILs were taken out of the glove box and their hydrolytic stability were monitored in the air by CV conducted in a static beaker. As shown in Fig. 15, the current density of AlCl3/EMIC decreased quickly with time (Fig. 15a) while that of AlCl3/4-Pr-Py was relatively unaffected with time (Fig. 15b). Although the initial current density of AlCl3/EMIC was more than ten times greater than that of AlCl3/4-Pr-Py, it was reduce to only about 15% of AlCl3/4-Pr-Py after 70 min (Fig. 15c). These results demonstrate that the AlCl3/4-Pr-Py IL was more stable than AlCl3/EMIC IL upon exposure to air. 4. CONCLUSIONS We have shown that new ILs can be obtained from AlCl3 and a neutral ligand 4-propylpyriinde (4-Pr-Py). The physical properties of the ILs, such as thermal stability, glass transition temperature, melting point, and ionic conductivity, are closely related to the mixing ratio of the two components. This IL formed from aluminum complexation with neutral ligands generated aluminum-based cationic [AlCl2(4-Pr-Py)2]+ species and anionic AlCl4 species, that were confirmed by both NMR and MS measurements. We also show that these new ILs can be successfully used for electrodeposition of aluminum. Unlike traditional chloroaluminates, in which the electroactive species Al2Cl7 is very sensitive to moisture, the new electroactive cationic aluminum species is less moisture-sensitive because of the ligand complexation. In addition, the moisture stability of the cationic species can be further improved by tuning the ligand structure. Therefore, these new ligand-complexed ILs can be classified as the third generation of the chloroaluminate ILs. The discovery of a new air- and moisture-stable IL will not only inspire us to discover and design novel ILs but also can speed up their practical application in aluminum deposition for corrosion protection in various fields.
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ACKNOWLEDGEMENTS This work was funded by the Strategic Environmental research and Development Program (SERDP) (WP2316). References [1] F. Endres, Chemphyschem 3 (2002) 144–154. [2] S. Wilkes John, Ionic Liquids in Perspective: The Past with an Eye Toward the Industrial Future, Ionic Liquids, American Chemical Society (2002) 214–229. [3] J.F. Huang, H.M. Luo, S. Dai, J. Electrochem. Soc. 153 (2006) J9–J13. [4] T.M. Anderson, D. Ingersoll, A.J. Rose, C.L. Staiger, J.C. Leonard, Dalton Trans. 39 (2010) 8609–8612. [5] H.D. Pratt, A.J. Rose, C.L. Staiger, D. Ingersoll, T.M. Anderson, Dalton Trans. 40 (2011) 11396–11401. [6] D. Depuydt, N.R. Brooks, S. Schaltin, L. Van Meervelt, J. Fransaer, K. Binnemans, Chempluschem 78 (2013) 578–588. [7] J. Sniekers, N.R. Brooks, S. Schaltin, L. Van Meervelt, J. Fransaer, K. Binnemans, Dalton Trans. 43 (2014) 1589–1598. [8] A.P. Abbott, G. Frisch, K.S. Ryder, Annu. Rev. Mater. Res. 43 (2013) 335–358. [9] F.H. Hurley, Electrodeposition of aluminum, US Patent 2446331, 1948. [10] T.P. Wier Jr., F.H. Hurley, Electrodeposition of aluminum, US Patent, 2446349, 1948. [11] J.S. Wilkes, J.A. Levisky, R.A. Wilson, C.L. Hussey, Inorg. Chem. 21 (1982) 1263–1264. [12] J. Tang, K. Azumi, Electrochim. Acta 56 (2010) 1130–1137. [13] B. Li, C. Fan, Y. Chen, J. Lou, L. Yan, Electrochim. Acta 56 (2011) 5478–5482. [14] M. Johnston, J.J. Lee, G.S. Chottiner, B. Miller, T. Tsuda, C.L. Hussey, D.A. Scherson, J. Phys. Chem. B 109 (2005) 11296–11300.
[15] J.K. Chang, I.W. Sun, S.J. Pan, M.H. Chuang, M.J. Deng, W.T. Tsai, T I Met Finish 86 (2008) 227–233. [16] T. Tsuda, S. Arimoto, S. Kuwabata, C.L. Hussey, J. Electrochem. Soc. 155 (2008) D256–D262. [17] T. Tsuda, C.L. Hussey, G.R. Stafford, J.E. Bonevich, J. Electrochem. Soc. 150 (2003) C234–C243. [18] Q. Zhu, C.L. Hussey, Elec Soc S 99 (2000) 494–504. [19] W.R. Pitner, C.L. Hussey, G.R. Stafford, J. Electrochem. Soc. 143 (1996) 130–138. [20] Q. Liao, W.R. Pitner, G. Stewart, C.L. Hussey, G.R. Stafford, J. Electrochem. Soc. 144 (1997) 936–943. [21] D.A. Atwood, Coord. Chem. Rev. 176 (1998) 407–430. [22] H.M. Abood, A.P. Abbott, A.D. Ballantyne, K.S. Ryder, Chem. Commun. 47 (2011) 3523–3525. [23] D.D. Eley, H. Watts, J. Chem. Soc. (1952) 1914–1918. [24] J.S. Wilkes, J.A. Levisky, R.A. Wilson, C.L. Hussey, Inorg. Chem. 21 (1982) 1263–1264. [25] F. Coleman, G. Srinivasan, M. Swadzba-Kwasny, Angew. Chem. Int. Ed. 52 (2013) 12582–12586. [26] E.P. Parry, J. Catal. 2 (1963) 371–379. [27] Y.L. Yang, Y. Kou, Chem. Comm. (2004) 226–227. [28] C. Liao, N. Shao, K.S. Han, X.G. Sun, D.E. Jiang, E.W. Hagaman, S. Dai, Phys. Chem. Chem. Phys. 13 (2011) 21503–21510. [29] X.G. Sun, C. Liao, N. Shao, J.R. Bell, B.K. Guo, H.M. Luo, D.E. Jiang, S. Dai, J. Power Sources 237 (2013) 5–12. [30] H. Vogel, Phys. Z. 22 (1921) 645. [31] G. Tamman, W.Z. Hesse, Anorg. Allg. Chem. 156 (1926) 245–257. [32] G.S. Fulcher, J. Am. Ceram. Soc. 8 (1923) 339–355. [33] A.A. Fannin Jr., D.A. Floreani, L.A. King, J.S. Landers, B.J. Piersma, D.J. Stech, R.L. Vaughn, J.S. Wilkes, J.L. Williams, J. Phys. Chem. B 88 (1984) 2614–2621. [34] M. Lipsztajn, R.A. Osteryoung, J. Electrochem. Soc. 132 (1985) 1126–1130. [35] R. Wibowo, S.E.W. Jones, R.G. Compton, J. Phys. Chem. B 113 (2009) 12293–12298.