Electrochemical Behaviour of Copper (II) Chloride in Choline Chloride-urea Deep Eutectic Solvent ANA-MARIA POPESCU1*, VIRGIL CONSTANTIN1, ANCA COJOCARU2, MIRCEA OLTEANU1 Romanian Academy, „Ilie Murgulescu“ Institute of Physical Chemistry, Laboratory of Molten Salts, 202 Splaiul Independentei, 060021, Bucharest, Romania 2 University Politehnica of Bucharest, Department of Applied Physical Chemistry and Electrochemistry, 132 Calea Grivitei, 010737, Bucharest, Romania 1
Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were used for studying both cathodic and anodic processes on Pt electrode in a choline chloride containing ionic liquid with copper (II) ions. The investigations were conducted at 343K and 353K temperatures in choline chloride – urea (1:2 molar ratio) deep eutectic solvent in which CuCl 2 (anhidrous) salt was dissolved. The electrodeposition processes of copper ions at different concentrations (0.05-0.38M) and scan rates (10200mVs-1) were evidenced by CV technique. Supplementary measurements were performed for determining the electrochemical window for background electrolyte. Experiments with either CuCl salt or CuCl2 salt introduced in the same choline chloride-urea deep eutectic solvent have shown identical shapes of voltammograms, proving a disproportionation process between copper (I) and copper (II) ions. From the CVs of copper (II) ion, using the cathodic peak currents for the Cu2+/Cu+ couple, the diffusion coefficient of Cu2+ was calculated by two procedures. EIS investigations carried out in the choline chloride-urea ionic liquid with CuCl2 (0.05M) also evidenced both Cu2+/Cu+ and Cu+/Cu0 couples. Keywords: ionic liquids, deep eutectic solvent, choline chloride, electrodeposition, copper chloride
Before the discovery of liquid salts that can be used at room-temperature, i.e. room- temperature ionic liquids (RTILs), the starring role in the chemistry of liquids salts was related to high temperature ionic liquids (HTILs) that are based on molten inorganic salts. In general, all ionic liquids exhibit attractive physical and chemical properties and such features have created many technological opportunities. Ionic liquids based on choline chloride are of great interest for last years. The preparation and applications of this new class of ionic liquids containing a quaternary ammonium salt [choline chloride (ChCl) chemical compound is 2-hydroxy-N,N,N-trimethylethanaminium chloride, HOC2H4N(CH3)+3Cl-] mixed with hydrogen bond donor organic species, as amides, carboxylic acids, etc., were described first under the name “deep eutectic solvent” (DES) [1-3]. The deep eutectic phenomenon occurs for a mixture of choline chloride and urea in a 1:2 mole ratio, respectively. Choline chloride has a melting point of 3020C and that of urea is 1330C. The eutectic mixture melts as low as 12 0C. Among the electrochemical applications, these new ionic liquids can be used for the deposition of a large range of metals (Zn, Cu, Cr, Sn, Ag) with high current efficiency [4]. These DESs share many characteristics of ionic liquids and are known to be less-toxic, air and moisture stable, biodegradable and economically viable to large-scale processes [2]. Our team has a great experience in electrodeposition of metals and metal compound in molten salts and start working on electrodeposition from choline chloride based ionic liquids [5, 10]. Recently, we reported some data [11] and demonstrated that the copper electrodeposition process in ChCl-urea ionic liquid represents an environmental friendly alternative for the classic procedures in aqueous solutions which are used in present at industrial scale. This new proposed technique is an ecological one because the used ionic liquid based on choline chloride is air and
moisture stable and its components are both common chemical compounds: ChCl is used for chicken feed as vitamin B4 and urea is a common fertilizer. The DES are also shown to be good solvents for metal oxides and chlorides which could have potential application for metal electrodeposition [12].For electrodeposition in choline chloride media it is usually used the chloride hydrated salts [13]. The electrodeposition of copper in deep eutectic solvents was before evaluated using CuCl [14, 15] and CuO [16]. The present paper aimed to clarify the mechanism of electro-deposition of copper from choline chloride-urea ionic liquid with different additions of CuCl2(anh). For this purpose a systematic cyclic voltammetry study was performed and the diffusion coefficients were calculated for the Cu 2+/Cu + redox couple. The data obtained by electrochemical impedance spectroscopy investigation were also reported. Experimental part All reagents used were purchased from Sigma Aldrich. The choline chloride (ChCl) (>99%), urea (>99%) and anhydrous copper (II) chloride (CuCl2(anh)) (>97%) were used as purchased, without recrystallization or drying, in order to simulate a more appropriate technological process for future industrial use. The supporting electrolyte ChClurea was prepared by mixing and heating at ~353 K the two components in 1:2 molar ratio, until homogenous and colorless liquid is formed. This DES is also known as Ethaline 200. Then, the CuCl2(anh) (in concentration of 0.05 M, 0.1 M and 0.38 M) was dissolved under stirring and a yellow-brown liquid was obtained. It is important to mention that the anhydrous salt, and not the hydrated salt, is used for the first time in electrodeposition of metals from ionic liquids assuming that it will be enough water in the electrolyte from the choline chloride. Figure 1 presents the
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Fig.1 The aspects at room temperature of ChCl-urea 1:2 M (DES) (a), the prepared ChCl-urea-CuCl2 0.38M ionic liquid (b), and the ChCl-urea-CuCl2 mixture after electrolysis (c).
obtained DES and ionic liquid with CuCl2. We did not pay attention to removal of water content as it is already demonstrated that in the electrodeposition process from ionic liquids water does not disturb the process and even in many cases low concentration of water is advantageous [17]. In order to calculate the solution molarities a density value of 1.1729 g . cm-3, for ChCl-urea determined in our laboratory at 353K was used. The electrical conductivity (with a WTW-350i-Germany multi-parameter instrument provided with a conductivity cell Tetra Con 325, k=0.475 cm -1) and viscosity (with Ubbelohde viscometer Jenaer Glaswerk Schott & Gren with A=1.026, fixed in a special thermostated bath) were measured at 353K for the ChCl-urea (1:2) deep eutectic solvent. Cyclic voltammetry measurements were carried out using a PS3 potentiostat or a Zahner IM6e potentiostat, connected to a PC for data acquisition and control. A threeelectrode system was used consisting of a platinum wire/ foil (0,3 cm2 and 0,5 cm2) as stationary working electrode (WE), a glassy carbon rod / or a platinum gauze as a counter electrode (CE) and a silver wire as a quasi-reference electrode (QRE) (Johnson Mattews Ag wire >99%). The working electrode was polished with alumina paste, rinsed and dried prior to all measurements. Cyclic voltammograms were recorded at 343 K and 353 K and at various scan rates in the range 10 - 200 mVs-1. The electrochemical impedance spectroscopy (EIS) investigations were carried out using with an Autolab PGSTAT 302 potentiostat. The impedance was measured under potentiostatic conditions with a sinusoidal potential perturbation of the peak to peak amplitude equal to 10 mV at frequencies from 1 MHz to 0.01 Hz. For fitting the impedance data a Zview 2.80 software (Scribner Assoc.inc.) was used. All electrochemical experiments were carried out in a quiescent aerated ionic liquid. Results and discussions The definition of a reference potential in ionic liquids is difficult due to unknown liquid junction potential. Most studies have used either ferrocene solution as internal standard or a silver wire as quasi-reference electrode. With the deep eutectic solvent based on choline chloride, the latter approach is used for two reasons: ferrocene is largely insoluble in this ionic liquid and reference potential of the silver wire in ionic liquid environments is likely to be dominated by the activity of chloride ions [14, 15, 18]. Preliminary study on the physical properties of ChCl-urea (1:2) ionic liquid shows, at 353K, values for viscosity of 0.036 Pas and for electrical conductivity of 0.6130 Sm-1. This DES is highly electrical conductive confirming that the ionic species are in the liquid phase and can move REV. CHIM. (Bucharest) ♦ 62♦ No. 2 ♦ 2011
independently. The viscosity values recommend this DES as a good electrolyte. Cyclic voltammetry results In comparison with other ionic liquids the potential window of the ChCl-urea deep eutectic solvent was found to be relatively narrow (-1.2 to +1.2V vs.Ag quasireference), as shown in figure 2.
Fig.2 Cyclic voltammogram for Pt electrode (0.3 cm2) in ChCl-urea mixture (1:2) at 353 K; 200 mVs-1 scan rate
In this background electrolyte, during the cathodic scan the current density value was lower than 0.5mAcm-2. As can be seen, on the cathodic branch of the voltammograms two consecutive reduction waves appear at ~ -0.20V and ~ -0.91V, respectively, with current amplitude that increase with the scan rate. We assumed that the wave at –0.91V is due to the presence of low concentrations of H+ ions in the binary ionic liquid, resulted by dissociation of H 2O molecules, protons that are present as impurities, being known that choline chloride is a ver y hygroscopic substance. The wave at –0.20V is difficult to be explained being probably a reduction process of either choline chloride or of urea. By scanning in the anodic direction, the current is almost zero (in the potential range between -0,8V to +0.91V) and exhibits a continuous increase at more positive potentials (more than +0.95V) which may be due to the evolution of chlorine gas. In fact, knowing that ChCl may form with urea a complex, similar with that formed with ethylene glycol or malonic acid, it is probably for this anodic process to be more complicated. However, we assumed that all waves that occur in the background voltammogram do not interfer with the waves of the electroactive copper species. The electrodeposition of copper using ChCl-urea + CuCl2 ionic liquid was investigated at 343K on a Pt working
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electrode in aerated electrolyte, with additions of 0.05 M, 0.1 M, 0.35 M and 0.38 M CuCl2. During cyclic voltammetry experiment we noticed for 10 cycles at 200 mVs-1 scan rate that the consecutive voltammograms were completely overlapped and such a good reproducibility was also seen at all scan rates. Figure 3 shows cyclic voltammograms recorded for 0.05M CuCl 2 at 343K with Pt working electrode, in the potential range of -1.4V ÷ +1.0V. It was found that the peak potentials for Cu(II) ions reduction are ver y similar using either Pt or glassy carbon as counterelectrodes.
Fig.4. Comparative cyclic voltammograms for Pt electrode (0.3 cm2) in ChCl:urea 1:2 with 0.05 M CuCl2 and with the background electrolyte (ChCl-urea, 1:2) : GC counterelectrode, T=353 K.
Fig.3 Cyclic voltammograms with various scan rates for Pt electrode (0.5cm2) in ChCl-urea (1:2) containing 0.05 M CuCl2, Pt counterelectrode, T=343K
The cyclic voltammograms reveal two pairs of peaks as two distinguishable steps of reduction and oxidation representing Cu(II)/Cu(I) and Cu(I)/Cu(0) redox couples. For the cyclic voltammogram with scan rate of 10mV s -1 the following steps and potential values were determined: step I corresponds to the Cu(II)/Cu(I) redox couple with cathodic and anodic peak potentials of Epc(I) = +284mV and Epa(I) = +445mV, respectively; the peak separation ΔEp(I) =161mV indicates a quasi-reversible behaviour; step II corresponds to the Cu(I)/Cu(0) redox couple with cathodic and anodic peak potentials of Epc(II) = -166mV and Epa(II) = -546mV, respectively, with ΔEp(II) = -380mV. The latter process results in metallic copper deposition during cathodic scan with a characteristic stripping response on the anodic scan for the copper dissolution. Figure 3 shows cathodically a supplementary wave (a shoulder) at very negative potentials (-1.2 ÷ -1.4 V) which may be attributed to reduction of H+ ions. Figure 4 shows also very clearly that the less pronounced waves occurred on the cathodic region of voltammogram for the ChCl-urea background electrolyte do not interfere with any peak obtained on the voltammograms for ChClurea-CuCl2 mixture. As in the case of the supporting electrolyte ChCl-urea, we attributed the large increase of anodic current on the ChCl-urea-CuCl2 voltammograms, between 1.0V and 1.5V, to the irreversible oxidation due to the conversion of Cl- ion into Cl2 gas. All cyclic voltammetry results for Cu(II)/Cu(I) and Cu(I)/ Cu(0) couples obtained in the described conditions are in very good agreement with literature [15,19]. Thus, it is confirmed that copper deposition on Pt electrodes in both aqueous chloride solutions and ionic liquids occurs by electroreduction of Cu(II) ions through the well known twostage mechanism, although the anodic and cathodic peak potentials and currents are significantly different. The cathodic reactions in Ethaline 200 + CuCl2 mixture are 208
certainly attributed to the consecutive reduction of Cu(II) to Cu(I) and Cu(I) to Cu(0) respectively. This suggestion is also supported by the change of yellow-brown colour of ChCl-urea-CuCl2 ionic liquid in green or red brown colour during electrolysis. It is surely that by introducing copper (II) ions in ionic liquid new bonds are formed with the complexes already existing in ChCl-urea mixture or with either choline chloride or urea molecules remained noncomplexated. This behavior is similar with that of anhydrous copper (II) chloride salt during dissolution in aqueous concentrated chloride solution to give the yellow or red brown colour of the halide complexes of the formula [CuCl2+X]x-. We noticed that the concentrated ionic liquid with CuCl2 appears even green (fig. 1c) because of the combination of various formed complex chromophores. It is most likely that, similarly to CuO dissolved in ChClurea ionic liquidS [16] the CuCl2 salt dissolved in the same ionic liquid will form also a complex. This complex is similar to the [Cu2O . m(NH2)CO . nCl]n- and in fact this kind of complex species is taking part in the electrochemical reactions observed in the solutions containing CuCl and CuCl2. In order to confirm the supposition of simultaneous existence of cuprous and cupric ions during electrodeposition of Cu(II) in the ChCl-urea ionic liquid, we performed comparative cyclic voltammetry measurements in ChCl-urea with addition of either CuCl2 or CuCl salts. The same shape and evolution of the voltammograms (fig. 5 a,b) were obtained. The results suggest that whatever mono- or divalent copper salt is introduced in the deep eutectic solvent two reduction peaks and two oxidation peaks appear; the existence of Cu(II) ions in ionic liquid with dissolved CuCl salt was attributed to a disproportionation process: 2 Cu+ → Cu2+ + Cu
(1)
Regarding to the voltammetry of copper ions in the ionic liquid ChCl-urea + copper (II) ions, it is worth to note that for every Cu(II) ion concentration, the increase of scan rate led to the increase of both cathodic peaks and anodic peaks, as figure 6 shows. However, it was remarked by increasing the scan rate and copper (II) ion concentration a very small and gradual shift of peak potentials in cathodic and anodic direction, respectively i.e. an increase of ΔEp peak difference. An explanation could be the increase of IR ohmic drop within the ionic liquid, owing to the gradual diminishing of electrical conductivity of the ionic media by adding CuCl2
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Fig.5 Comparative cyclic voltammograms for Pt electrode (0.3 cm2) in ChCl:urea (1:2) with addition of either 0.35 M CuCl or 0.35 M CuCl2, Pt counterelectrode T=353 K, potential scan rates (a) 10mVs-1; (b)150mVs-1
ChCl-urea-CuCl2 . 2H2O ionic liquid in similar temperature working conditions. We also used an alternative procedure for diffusion coefficient determination, described in the followings. The procedure consists in comparison of voltammetry data obtained in aqueous solutions with those obtained by us in ionic liquid. Using the same approach as Abbott [20,21]. and according to the Walden’s rule [22], one may write: (2)
and (3)
Fig.6. Influence of CuCl2 concentration on the voltammograms obtained on Pt electrode (0.08cm2) in ChCl-urea-CuCl2 mixture, at T=353 K, v =100 mVs-1, GC counterelectrode)
amounts in ChCl-urea. More information should be obtained if the conductivity measurements on ChCl-urea-CuCl2 system will be performed. Regarding the Cu(I)/Cu0 couple process it was found to be a quasi-reversible reaction with an anodic stripping after metallic copper deposition. The evidence of the potential shift of the cathodic peak potential may be attributed to the adsorption of complex species present in the DES on the Pt electrode surface. The voltammograms exhibit a clearly thermodynamic reversibility for Cu(II)/Cu(I) redox process. For this couple, the anodic/cathodic peak current ratios are essentially independent on scan rate, although the potential peak separation value is somewhat larger than might be expected for a one-electron Nerstian process. However this deviation from theoretical ΔEp for one-electron transfer is common for resistive organic electrolytes, and is expected for the experiments were the solution resistance is not compensated. Both cathodic and anodic peak currents from the reversible Cu(II)/Cu(I) couple were plotted against the square root of potential scan rate (iPv1/2 plots not presented) and good linear correlations were obtained, proving a diffusion control of electrode process. From these plots the mean value of diffusion coefficient for Cu 2+ species (D Cu 2+) was determined using the Randles-Sevcik equation [15]. A value of 1.39×10-8 cm2s-1 at 343 K was calculated for DCu2+ in good agreement with the value of 1.35 . 10-8 cm2s-1 obtained by Abbott [15] for REV. CHIM. (Bucharest) ♦ 62♦ No. 2 ♦ 2011
where D is the diffusion coefficient of Cu(II) in the two solvents, water and ChCl-urea, η is the viscosity of water and ethaline 200, and ipc is the cathodic peak currents in aqueous solution and ethaline 200, respectively. Equation (3) supposes that all other parameters involved in RandlesSevik equation are quite similar (temperature, ion concentration, scan rate) Inserting eq.(3) in eq.(2), a new equation is obtained: (4)
Using values of ηwater=0.653cP [14] and ηethaline= 437cP (measured by us), both at 313K temperature, the value of ipc,water/ipc,ethaline ratio was found to be 25.86 .This result is in very good agreement with the experimental ratio of cathodic peak currents, ipc,water/ipc,ethaline of ~27, although ipc,ethaline was taken at another temperature (343K) and copper ion concentration in ionic liquid was three times higher than in aqueous medium. This finding also indicates that the effect of viscosity accounts largely to the differences in cathodic peak currents in both solvents. Making the same assumptions as in [14] that eq.(3) is valid in our conditions and using the value of DCu(II),water= 1.29×10-5 cm2s-1 at 313±1 K, obtained by these authors, it was possible to estimate the diffusion coefficient of Cu(II) in ethaline 200, DCu(II),ethaline. The value of diffusion coefficient was found to be approximately DCu(II),ethaline=1.9×10-8 cm2s1 at 313K temperature. Comparing this result with literature data [14] in glyceline 200 ionic liquids (ChCl-glycerol mixture) at the same temperature, where DCu(II),glyceline =7.41×10-8 cm2 s-1, we can conclude that the difference in
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Fig.7 Impedance data of Pt electrode (0.5 cm2) for ChCl-urea-CuCl2 ionic liquid with 0.05 M Cu(II) concentration at 353 K temperature: (a) high frequency part of Nyquist diagram; (b) Bode diagram
Fig.8. The proposed equivalent circuit for Pt/ChCl-urea-CuCl2 system (the significances of circuit parameters are given in the text).
the values of diffusion coefficients in ethaline 200 and glyceline 200 is due to the difference in viscosity of the two solvents. This result allows an interpretation of a slower diffusion of copper ions in ethaline 200 (ChCl-urea, 1:2M) than in water, due to viscosity differences between the two solvents. We mention that this last procedure of estimation of DCu(II),ethaline was done for the first time in literature. Electrochemical impedance characterization of copper deposition EIS measurements were performed in ChCl-urea-CuCl2 ionic liquid with Pt electrode in order to evaluate the properties of copper deposit. Figure 7 presents the obtained results as Nyquist (a) and Bode (b) spectra. The part of Nyquist diagrams corresponding to high frequencies consists in the beginning of a semicircle with variable diameter (curves 2, 5 and 6), a fact that is characteristic for an electrode process under charge transfer control. Further lowering of frequency, it results a linear increase of impedance as a straight line, showing a mass transfer control through copper film. Depending on the roughness or on homogeneity of the copper film, this straight line can deviate from its ideal 45 degrees slope [24]. However, the Nyquist diagrams recorded at electrode potentials denoted as 1, 3 and 4, shows straight lines, only; the explanation is the absence (or very diminished process) of Faradaic process of copper ions at these potentials, as CV curves indicate. The Bode curves illustrate the same characteristics of electrode processes of copper ions. The impedance data were fitted with the equivalent circuit presented in figure 8, where Rs is the solution resistance (ohmic drop) of ionic liquid electrolyte, CPE1 -a constant phase element (see equation (5)) simulating the behaviour of double layer capacitance, Cdl, R1 – the charge transfer resistance and W1 – the Warburg impedance.
The impedance (Z) of CPE element is defined by two component values, a capacitance CPE-T and an exponent CPE-P. If CPE-P equals 1, the equation (5) is identical to that of a pure capacitor. In fact, a capacitor is actually a constant phase element with a constant phase angle of 90 degrees. A rough or porous surface can cause a doublelayer capacitance to appear as a constant phase element with a CPE-P value between 0.9 and 1. If CPE-P equals 0.5, a 45 degree slope of straight line is recorded on the Nyquist diagram. Similarly, the Warburg impedance has two components, a resistive part (W1-R) and an inductive part consisting in W1-T component and W1-P exponent. A Warburg element occurs when charge carrier diffuses through a material (liquid, solid). Lower frequencies correspond to diffusion deeper into the material. If the material is thin, low frequencies will penetrate the entire thickness (Finite Length Warburg element). If the material is thick enough so that the lowest frequencies applied do not fully penetrate the layer, it must be interpreted as Infinite Length Warburg. The values of parameters determined by fitting the results with the proposed model are summarized in table 1. It can be seen an averaged value of solution resistance of cca. 27Ω and values of CPE1-T of 100-200 μF representing the capacitance of the double layer, which is in good agreements with other measurements in ionic liquids. Also, smaller charge transfer resistances (R1) of 4-5 Ω and 0.030.07Ω were obtained for electrode potentials corresponding to the Cu2+ /Cu+ and Cu +/Cu 0 couples, respectively. The Warburg inductance (W1-R) shows also minimum values for the same ranges (of reduction couples). It is worth to mention that our EIS data for ChCl-ureaCuCl2 ionic liquid were presented for the first time in literature.
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Table 1 IMPEDANCE DATA OF Pt / ChCl-UREA - CuCl3 SYSTEM OF Pt ELECTRODE 0.5 cm2, 80oC
Conclusions The CV observations confirm that Cu(II) ions existing in choline chloride-urea ionic liquid can be electrochemically reduced up to metallic copper by two consecutive steps.. From cyclic voltammetry data the diffusion coefficient for the Cu2+ ion was calculated using two procedures. The electrochemical impedance spectroscopy (EIS) technique is sensitive to the change of surface properties caused by the deposition of copper at very negative potentials. This study demonstrates an alternative to the classical copper electrodeposition in aqueous solutions, by using choline chloride based ionic liquids, which are now a key to new green technologies. The described electrodeposition process in ChCl-urea ionic liquid may be also used for recycling the wastes containing cuprous or cupric compounds that exist in metallurgical industry of nonferrous metals. Acknowledgements: The financial support within the Romanian Ministry of Education and Science, Partnership Programm – Project nr. 31066 / 2007 is gratefully acknowledged.
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