Electrochemical and Spectroscopic Studies of Zinc

DOI: 10.1002/celc.201500444

Articles

Electrochemical and Spectroscopic Studies of Zinc Acetate in 1-Ethyl-3-methylimidazolium Acetate for Zinc Electrodeposition Maryam Shapouri Ghazvini, Giridhar Pulletikurthi, Abhishek Lahiri, and Frank Endres*[a] with Zn2 + ions. A minimum concentration of 4 m Zn(OAc)2 was required to observe bulk zinc deposition, and in that case, the reducible species present in the solution were probably a mixture of Zn(OAc)2 and Zn(OAc) + . Furthermore, the scanning electron microscopy results revealed microcrystals of zinc composed of many nanostructures distributed on the substrate, and X-ray diffraction analysis indicated that a Zn–Au alloy along with Zn films was obtained.

In this paper, we report on the electrodeposition of zinc from the ionic liquid 1-ethyl-3-methylimidazolium acetate ([EMIm]OAc) with various concentrations of zinc acetate [Zn(OAc)2] at 100 8C. The electrochemical behavior of zinc complexes in [EMIm]OAc was determined by cyclic voltammetry. Raman and infrared spectroscopy were employed to study the coordination of Zn2 + ions with OAc¢ , and it was found that upon increasing the Zn(OAc)2 concentration the intensity of the band for the free acetate anion weakened, due to binding

1. Introduction Zinc is an interesting material for energy-storage devices; it is inexpensive, nontoxic, and a recyclable material.[1, 2] Electrodeposition of zinc from aqueous solutions has been studied for many years. The major impediments in aqueous electrolytes are the evaporation of the electrolyte from the cell, dendritic deposition, and passivation during cycling process.[3, 4] To overcome these issues, Zn deposition from ionic liquids is under exploration. These liquids are different from aqueous solutions for metal electrodeposition, which is due primarily to the differences in ionic structure and the way that the solvent acts as a ligand for the metal cation of interest. Ionic liquids have some advantages over aqueous electrolytes, including wider electrochemical window, thermal stability at elevated temperatures, nonflammability, and low volatility.[5–7] The electrodeposition of Zn from different ionic liquids has been reported in recent years. Electrodeposition of zinc from deep eutectic ZnCl2/choline chloride and from ZnCl2/ethylene glycol was reported by Abbott et al., who showed that the surface morphology of the zinc deposit could be a function of the relative nucleation and growth rates of the deposit.[8] Sun and co-workers reported Lewis acidic ionic liquids composed of 1-ethyl-3methylimidazolium chloride ([EMIm]Cl) and ZnCl2 to electrodeposit Zn.[9, 10] Apart from hygroscopic liquids, air- and waterstable ionic liquids have also been employed for the electrodeposition of zinc. 1-Ethyl-3-methylimidazolium dicyanamide ([EMIm]DCA) was tested as a potential electrolyte in secondary Zn-air batteries. Furthermore, the effect of different zinc salts

and substrates in this ionic liquid was investigated, and it was shown that the electrodeposited zinc had a uniform and dendrite-free morphology.[11] Moreover, high-quality Zn films were obtained from 1-butyl1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide by Doan et al.[12] In recent years, our group has investigated the electrodeposition of zinc from ionic liquids composed of the trifluoromethylsulfonate anion and different cations such as 1butyl-1-methylpyrrolidinium trifluoromethylsulfonate ([Py1,4]TfO), 1-ethyl-3-methylimidazolium trifluoromethylsulfonate ([EMIm]TfO), 1-methylimidazolium trifluoromethylsulfonate ([MIm]TfO), and 1-ethyl-2,3-dimethylimidazolium trifluoromethylsulfonate ([EMMIm]TfO) to show the influence of cations on the morphology and electrochemical behavior of the deposited zinc. Furthermore, the formation of zinc complexes according to spectroscopic investigations was described.[13–16] An interesting environmentally friendly ionic liquid is 1ethyl-3-methylimidazolium acetate ([EMIm]OAc), used herein as an electrolyte for Zn electrodeposition. It has attracted some attention due to its promising physical properties such as its low viscosity, low toxicity, and high biodegradability.[17–20] It also has high potential for applications in biocatalysis; it can dissolve strongly hydrogen-bonded materials such as cellulose,[21–23] and in addition, it has considerable ability to capture CO2,[24] which thus enables routes to transform it into valuable compounds. In this study, we show for the first time the electrodeposition of zinc from [EMIm]OAc. It comprises an investigation of the electrochemical behavior of various concentrations of zinc acetate in ionic liquids, Raman and IR spectroscopy analysis to understand the solvation of the Zn ions in the ionic liquids.

[a] M. S. Ghazvini, Dr. G. Pulletikurthi, Dr. A. Lahiri, Prof. Dr. F. Endres Institute of Electrochemistry Clausthal University of Technology Arnold-Sommerfeld-Strasse 6 Clausthal-Zellerfeld, 38678 (Germany) E-mail: [email protected]

ChemElectroChem 2016, 3, 598 – 604

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Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Articles 2. Results and Discussion The electrochemical window of neat [EMIm]OAc was measured at room temperature and at 100 8C. From Figure 1 it is clear that the onset of cation reduction at c1 and anion oxidation at a3 occur at ¢0.6 and + 2.6 V at 22 8C (i.e. room temperature) and ¢0.4 and + 2.4 V at 100 8C, respectively. At elevated temperatures, the electrochemical window is thus slightly reduced. Furthermore, two oxidation peaks at a1 and a2 are observed in both cases, which are attributed to the oxidation of decomposition products formed at c1.

Figure 2. Comparison of cyclic voltammograms at various concentrations of Zn(OAc)2 in [EMIm]OAc on gold at RT. Scan rate: 10 mV s¢1.

Table 1. Concentrations of Zn(OAc)2 in [EMIm].[a]

[EMIm]OAc mole [%]

0.2 0.5 1 2 3 4 5 6

3 7 14 25 33 40 45 50

97 93 86 75 67 60 55 50

mol L¢1 and the corresponding mole fractions of salt and ionic liquid. As shown in Figure 2, an oxidation process (a1) appears at a concentration of 4 m, which is indicative of the oxidation of electrodeposited zinc. The decreasing currents at c1 with increasing Zn(OAc)2 concentration are due to an increase in viscosity. Acetate is a bidentate ligand and its coordination with Zn2 + is shown in Figure 3 a.[29, 30] In the IR spectrum (Figure 3 b), the bands at n˜ = 1442 and 1537 cm¢1 are individually assigned to the symmetric and antisymmetric stretching modes of the carbonyl group of zinc acetate. These two bands are related to the carbonyl bonds, due to dissimilar masses and the bonding behavior, which are attributed to C¢O¢ and C=O bond stretching modes.[29–31] In Zn(OAc)2/[EMIm]OAc, these two bands shift to n˜ = 1323 and 1566 cm¢1, respectively. As can be seen in direct comparison with the [EMIm]OAc spectrum, the difference between these two bands increases from 95 to 244 cm¢1,[29, 30] which demonstrates that the nature of acetate bonding changes from a bidentate to a monodentate coordination structure.[29] Coordination of the carbonyl group with the metal cation (Zn2 + ) weakens the C=O stretching band, and consequently, the band location shifts to n˜ = 1566 cm¢1.

The lower electrochemical window at high temperatures shows that there are some kinetic phenomena influencing the electrochemical window. The cyclic voltammograms of 0.2 to 4 m Zn(OAc)2 in [EMIm]OAc on gold at room temperature are shown in Figure 2. In the cathodic regime, at low concentrations of Zn(OAc)2 in [EMIm]OAc (0.2–2 m), the cyclic voltammograms show an increasing current at about ¢0.6 V, but there is no visible Zn deposition in this concentration regime. Also, no equivalent oxidation process is observed in the backward scan. To verify the assumption of failed Zn deposition, a constant potential electrolysis was performed at ¢1 V, which revealed that no deposition was obtained. A similar electrochemical behavior was observed up to 4 m of Zn(OAc)2. This might be due to the absence of reducible Zn2 + species in the investigated liquids within its electrochemical window. This phenomenon was also observed in the case of Al deposition from other ionic liquids. It was stated that minimum concentrations of 1.6, 5.5, and 2.75 m AlCl3 are required for [Py1,4]TFSA, [EMIm]TFSA, and [Py1,4]TfO, respectively, to deposit Al [TFSA = bis(trifluoromethylsulfonyl)amide].[25–28] By increasing the concentration of Zn(OAc)2 above 4 m, reducible zinc species seem to be present. Table 1 shows the concentrations of Zn(OAc)2 in www.chemelectrochem.org

Zn(OAc)2 mole [%]

[a] Density of [EMIm]OAc = 1.027 g mL¢1 at 25 8C. Molar mass Zn(OAc)2 = 183.48 g mol¢1. Molar mass [EMIm]OAc = 170.21 g mol¢1. IL = ionic liquid

Figure 1. Cyclic voltammograms of neat [EMIm]OAc on gold at RT and 100 8C. Scan rate: 10 mV s¢1.


Zn(OAc)2/IL molar [mol L¢1]

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Articles anion interactions of the ionic liquid ( + C < C.CH···A¢), in which the C¢H moiety of the imidazolium ring interacts with the acetate ion. The vibrational bands at n˜ = 125 and about 250 cm¢1 can be related to the out-of-plane bending mode of the ethyl [CH3CH2¢(N)] and methyl [CH3¢(N)] groups attached to the imidazolium ring, respectively.[32, 33] The bending mode of the ethyl group in the imidazolium cation is not clearly observed because of its relative low intensity.[32] The vibrational band at n˜ = 170 cm¢1 can be assigned to the bending mode of Zn¢O. This band intensity increases upon increasing the concentration of Zn(OAc)2.[34] The Raman spectra at five different concentrations of Zn(OAc)2 in [EMIm]OAc are shown in Figure 5. The vibrational bands at n˜ = 600 and 703 cm¢1 are related to stretching modes of the methyl and ethyl groups of the imidazolium cation. Bending of the ethyl group is found at around n˜ = 1383 cm¢1, and upon increasing the Zn(OAc)2 concentration the intensity of this band gradually decreases. The intensity of the band for the antisymmetric CO stretching mode, observed at n˜ = 1569 cm¢1, slightly decreases by adding Zn(OAc)2 to the electrolyte.

Figure 3. a) The chemical structure of Zn(OAc)2 and its reaction with [EMIm]OAc. b) Comparison of the IR spectra of Zn(OAc)2, [EMIm]OAc, and three different concentrations of Zn(OAc)2 in [EMIm]OAc.

The oxygen atom in the C=O group of OAc¢ coordinates with zinc, and as a result, the shoulder at n˜ = 1640 cm¢1 shifts to n˜ = 1606 cm¢1; the intensity also visibly increases upon increasing the concentration of Zn(OAc)2 in [EMIm]OAc. A comparison of the vibrational bands in the far-IR spectra of the neat ionic liquid with varying concentrations of Zn(OAc)2 in [EMIm]OAc between n˜ = 50 and 260 cm¢1 is shown in Figure 4. The band at n˜ = 133 cm¢1 can be attributed to cation and

Figure 5. Raman spectra of different concentrations of Zn(OAc)2 in [EMIm]OAc between n˜ = 600 and 1600 cm¢1.

The band at n˜ = 635 cm¢1 is correlated with the bending of COO¢ and/or the stretching mode of the C¢C bond. It was reported that the signal of “free” acetate (OAc¢) is found at roughly n˜ = 900 cm¢1,[34] but the position of this band can change if a complex is formed. The vibration of the C¢C stretching mode is a sensitive probe.[35] It was shown that this stretching is suitable and more sensitive than that of the COO group in understanding the complexation of the carboxylate group. Upon adding zinc acetate, a band develops at about n˜ = 920 cm¢1, and the intensity of this band increases upon increasing the concentration of Zn(OAc)2 ; this can be due to the coordination of Zn2 + with the acetate anion, as shown in Figure 6. To get more insight into the coordination of Zn2 + in [EMIm]OAc, the Raman spectra in Figure 7 a–c show Voigt fits at different concentrations of Zn(OAc)2.

Figure 4. Far-IR spectra of Zn(OAc)2, [EMIm]OAc, and their mixtures at three different concentrations between n˜ = 50 and 260 cm¢1.


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Articles anion. From the curve fit, the average number of OAc¢ anions around the Zn2 + cations can be calculated by Equation (1):[16, 36]

N¼

Aco= At x

ð1Þ

in which Aco is the intensity integral of the Zn2 + ion coordinated with the acetate anion, At is the total area of the coordinated and free OAc¢ anions at n˜ = 900 and 922 cm¢1, respectively, and x is the mole fraction of Zn2 + ions to total OAc¢ . At 0.5 m Zn(OAc)2, the coordination number calculated with Equation (1) is 5.79, and consequently, each Zn2 + is roughly coordinated with six (OAc)¢ to give a complex of the form [Zn(OAc)6]4¢. Upon increasing the concentration of Zn(OAc)2 further to 1 and 2 m, the coordination numbers decrease to 5 and 4, which results in the formation of [Zn(OAc)5]3¢ and [Zn(OAc)4]2¢, respectively. Upon adding 4 m Zn(OAc)2, the band at n˜ = 900 cm¢1 almost disappears and only the band at n˜ = 920 cm¢1 develops, from which we cannot seriously calculate the coordination number, as the intensity of the free acetate ion is almost zero. Thus, to obtain assessable data for 4 m concentration, the Raman spectra between n˜ = 1540 and 1640 cm¢1 related to antisymmetric

Figure 6. Raman spectra of [EMIm]OAc with different concentrations of Zn(OAc)2 between n˜ = 880 and 940 cm¢1.

The fit curve deconvolutes the spectra into two curves, of which the first band is related to the free OAc¢ anion and the second one is from Zn2 + coordinated with the free OAc¢

Figure 7. Deconvolution of the Raman spectra by the Voigt function for various concentrations of Zn(OAc)2 in [EMIm]OAc: a) 0.5 m, b) 1 m, c) 2 m, and d) 4 m.



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Articles stretching of the carbonyl group were fitted by Voigt functions, which is shown in Figure 7 d. The coordination number is around 1.8, which is indicative of the formation of a Zn(OAc)2/Zn(OAc) + mixture. As a consequence, it is more facile to reduce Zn2 + at higher concentrations (> 4 m) than at lower concentrations of Zn(OAc)2 (0.2–2 m). At concentrations of Zn(OAc)2 of 4 m and higher, the viscosity increases remarkably (Figure 8) from 0.08 Pa s for 0.2 m to 3.1 Pa s for 4 m, and

Figure 9. Cyclic voltammograms at concentrations of Zn(OAc)2 > 4 m in [EMIm]OAc recorded on gold at 100 8C. Scan rate: 10 mV s¢1.

Figure 8. Viscosity of different concentrations of Zn(OAc)2 in [EMIm]OAc at RT.

this hinders low-temperature experiments. Therefore, all experiments were performed at a higher temperature of 100 8C at concentrations of Zn(OAc)2 > 4 m. Potentiostatic electrolysis was performed to deposit Zn from 4 m Zn(OAc)2 in [EMIm]OAc at ¢0.3 V, from 5 m Zn(OAc)2 at ¢0.4 V, and from 6 m Zn(OAc)2 at ¢0.5 V for 1 h at 100 8C on the gold substrate. The reduction wave in Figure 9 is due to the bulk deposition of zinc, and the corresponding oxidation occurs at around + 0.6 V. From the cyclic voltammograms shown in Figure 9, it is evident that the onset of Zn electrodeposition occurs at around ¢0.1 V. The electrodeposited zinc was analyzed by XRD and SEM. As shown in Figure 10, the XRD patterns show peaks at 2 q = 36.3, 39.0, 43.2, 54.3, 70.05, 70.66, and 82.18, which are related to the (002), (100), (101), (102), (103), (110), and (112) planes of hexagonal zinc, respectively (JCPDS File No. 04-0831). The diffraction peaks observed at 2 q = 22.4, 25.1, 27.6, 32.0, 41.2, and 46.08 are related to the AuZn3 alloy (JCPDS File No. 50-1336), which indicates the growth of AuZn3 along with Zn crystals. Zinc growth takes place preferentially along the (002), (100), and (112) planes, and the average crystallite size is about 40 nm according to the Scherrer equation.[37] Agglomerated Zn crystals can be seen in the SEM images, which shows that these microcrystals seem to consist of nanocrystals (Figure 11).

Figure 10. Comparison of the XRD patterns of electrodeposited Zn from Zn(OAc)2 in [EMIm]OAc at 100 8C for 1 h: a) 4 m at ¢0.3 V, b) 5 m at ¢0.4 V, and c) 6 m at ¢0.5 V.

([EMIm]OAc) was studied. Raman and IR spectroscopy analysis showed the coordination of Zn2 + with OAc¢ . It was observed that upon the addition of Zn(OAc)2, the intensity of the band at n˜ = 900 cm¢1 for the free acetate anion decreased and the new band at n˜ = 922 cm¢1 intensified with increasing Zn(OAc)2 concentration. It was revealed by Raman spectroscopy at a concentration of 0.5 m Zn(OAc)2 that the calculated coordination number is 5.79, which implies that coordination of Zn2 + with OAc¢ takes the form [Zn(OAc)6]4¢. At lower concentrations of Zn(OAc)2 in [EMIm]OAc (0.2–2 m), [Zn(OAc)6]4¢ and [Zn(OAc)5]3¢ are formed, and they are difficult to reduce to Zn. Upon increasing the concentration to 4 m Zn(OAc)2, the coordination number decreases to 1.8, and consequently, a mixture of Zn(OAc)2 and Zn(OAc) + species is present in the solution. A minimum concentration of 4 m Zn(OAc)2 in [EMIm]OAc is required to observe bulk Zn deposition clearly. XRD analysis indicated the presence of AuZn3 along with Zn crystals, which de-

3. Conclusions In this paper, the electrodeposition of Zn from various concentrations of Zn(OAc)2 in 1-ethyl-3-methylimidazolium acetate ChemElectroChem 2016, 3, 598 – 604


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Articles platinum wires (Alfa, 99.99 %) were used as reference and counter electrodes, respectively. The electrochemical measurements were performed by using a PARSTAT 2263 potentiostat/galvanostat controlled by PowerCV and PowerStep software. After deposition, the samples were rinsed with 2-propanol to remove the ionic liquid and were subsequently dried under vacuum. A high-resolution SEM equipped with EDX (Carl Zeiss DSM 982 Gemini) was used to characterize the surface morphology and the composition of the deposited films. X-ray diffraction patterns were recorded at room temperature by using a PANalytical Empyrean Diffractometer with CuKa radiation. The IR measurements were performed with a Bruker VERTEX 70 spectrometer at the Institute of Inorganic Chemistry with the ability of an extension for measurements in the far- and mid-IR regions with multilayer beam splitter, equipped with DLaTGS detector with a preamplifier, polyethylene (PE) windows, and potassium bromide (KBr) windows for the internal optical path of far- and mid-IR, respectively. The Raman spectra were measured with a Raman module FRA 106 (Nd:YAG laser, 1064 nm) attached to a Bruker IFS 66v interferometer. For Raman measurements (Institute of Inorganic Chemistry), samples were sealed in a glass capillary under an argon atmosphere, and data were recorded at an average of 250 scans with a resolution of 2 cm¢1. A Rheometer AR 2000 (TA instruments Ltd) equipped with a stepped disposable Peltier plate (plate/plate 4 cm) at 10 revolutions s¢1 was used at the Institute of Polymer Materials and Polymer Technology to measure the viscosity of the samples.

Figure 11. SEM images of Zn on gold from Zn(OAc)2 in [EMIm]OAc: a) 4 m at ¢0.3 V, b) 5 m at ¢0.4 V, and c) 6 m at ¢0.5 V.

creased with an increase in the concentration of Zn(OAc)2, and the SEM images showed that these microcrystals were composed of nanostructures on top of them. The results of this work showed that [EMIm]OAc as an environmentally friendly ionic liquid can be a promising electrolyte for the electrodeposition of zinc.

Acknowledgements Financial support by the Bundesministerium fìr Bildung und Forschung (BMBF) projects LuZi (03SF0499A) and AlSiBat (03SF0486A) are gratefully acknowledged, and the authors would like to thank Mrs. Karin Bode (Institute of Inorganic Chemistry) and Mrs. Drçttboom (Institute of Polymer Material and Polymer Technology) for help with the IR/Raman spectroscopy and viscosity measurements, respectively.

Experimental Section The ionic liquid (Figure 12 a), 1-ethyl-3-methylimidazolium acetate ([EMIm]OAc), was purchased from Io-Li-Tec with a purity of 98 %. The ionic liquid was dried under vacuum at 40 8C to a water content of 0.19 % and was stored in a closed bottle in an argon-filled glove box (OMNI-LAB). As the pure ionic liquid tended to decompose at high temperatures, we could not achieve a lower water concentration. Zinc acetate [Zn(OAc)2] powder (Figure 12 b) was procured from Sigma–Aldrich with 99 % purity and was used as the zinc source. Various concentrations of Zn(OAc)2 (from 0.2 to 6 m) in [EMIm]OAc were prepared and used for the electrochemical experiments. The cyclic voltammetry and electrodeposition experiments were performed in the glove box at RT and at 100 8C. Gold substrates (gold on glass) from Arrandee, Inc., were used as working electrodes. The gold substrates were cleaned by heating at reflux in 2-propanol, and they were then heated in a hydrogen flame to remove surface contaminations. Zinc (Alfa, 99.99 %) and

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Figure 12. Chemical structures of a) 1-ethyl-3-methylimidazolium acetate and b) zinc acetate.



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Manuscript received: October 11, 2015 Accepted Article published: December 16, 2015 Final Article published: January 12, 2016

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