Electrochimica Acta 178 (2015) 617–623

Electrochemical Kinetics of Reduction of Zinc Oxide to Zinc Using2:1 Urea/ChCl Ionic Liquid

Haoxing Yanga, Ramana G. Reddyb,*aDepartment of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, Alabama, 35487, USAb Faculty of Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, Alabama, 35487, USA

A R T I C L E I N F O

Article history:Received 22 May 2015Received in revised form 9 August 2015Accepted 11 August 2015Available online 13 August 2015

Keywords:ZincUrea/ChCl Eutectic MixtureElectrowinningCurrent efficiency

A B S T R A C T

Electrochemical reduction of zinc oxide to zinc was investigated in 2:1 urea/choline chloride electrolyteat applied cell potentials of 3.2 - 3.5 V. Concentration polarization was observed at the beginning ofelectrochemical reduction process. The diffusion coefficient of zinc species in the electrolyte wascalculated to be 7.85 �10�13m2/s at 363 K. An increase in the current density was observed after a sharpinitial decrease due to activation polarization, in which case, a linear relationship was establishedbetween log (current density) and over-potential. The electrochemical kinetic parameters wasinvestigated in the steady-state current region. The transfer coefficient was calculated to be 0.2 indicatingan irreversible charge transfer process. The XRD analysis confirmed that the electrodeposits consists ofhigh purity zinc. The SEM analysis showed the formation of dendrites at higher over-potential (�0.65 V),which is comparable to the calculated critical over-potential for Zn dendrite formation. The estimatedenergy consumption varied from 3.04 to 3.66 kWh/kg and current efficiency from 78.5 to 88.9%.

ã 2015 Elsevier Ltd. All rights reserved.

Contents lists available at ScienceDirect

Electrochimica Acta

journa l home page : www.e l sev ier .com/ loca te /e le cta cta

1. Introduction

Zinc is commonly recognized as an effective coating materialfor reactive metal surfaces due to its anti-corrosive property [1].The long-established method for deposition of zinc from sulfuricacid electrolyte baths involve complicated processes that posesvarious problems and therefore, require further improvements.Hydrogen evolution is inevitable and it results in low currentefficiency and embrittlement of the products. In addition, difficultsecondary purification processes are required due to highsensitivity of Zn deposition to the electrolyte impurities thatoccur as byproducts of electrowinning in sulfuric acid [2].

To overcome the major difficulties involved in the zinc sulfateelectrolytic process, it is important to find an alternativeelectrolyte that is eco-friendly and highly efficient. Recently, ionicliquids (ILs) have been widely investigated as novel green solventsfor metal extraction and deposition. Ionic liquids have manyattractive properties such as: (1) non-corrosive, (2) non-flamma-ble, (3) low vapor pressure, (4) wide electrochemical window, (5)good thermal stability, (6) high electrical conductivity, etc. [3,4].However, large-scale industrial application of ionic liquids arelimited mainly due to handling issues and relatively high costs.

* Corresponding author. Tel.: +1 205 348 4246.E-mail address: rreddy@eng.ua.edu (R.G. Reddy).

http://dx.doi.org/10.1016/j.electacta.2015.08.0500013-4686/ã 2015 Elsevier Ltd. All rights reserved.

More recently, deep eutectic solvents (DES) have attractedmany scientists due to their low price, non-toxicity, and highstability in air and moisture. The most commonly used componentof DES is choline chloride (ChCl), a biodegradable quaternaryammonium salt. ChCl can easily form DES at very low temperaturesthrough its hydrogen bond donor functional groups [5]. Theeutectic mixture of urea and choline chloride (2:1 molar ratio) isconsidered for the current study because of its stability in ambientenvironment, affordability, and availability [6]. Also, the lowmelting point (285 K) of urea/ChCl mixture indicates a potential forenergy savings [7]. Reddy et al. have reported a relatively largeelectrochemical window (2.5 V) for Urea/ChCl mixture [8].Moreover, the capability of eutectic urea/ChCl mixture toselectively dissolve metal oxides greatly simplifies the purificationprocedure. It is reported that only PbO2, Cu2O, and ZnO showappreciable solubility, whereas other metal oxides, such as Al2O3

and CaO, have negligible solubility in the studied eutectic solvents[7]. In our previous studies, successful deposition of Zn and Pb fromcorresponding metal oxides was carried out in urea/ChCl, and inparticular, ZnO showed great solubility (�1.23 mol/L) at 373 K.Electrochemical measurements, using cyclic voltammetry (CV),confirmed that the onset reduction potential for Zn2+ to Zn occursat � �1.05 V via a one step, two electron transfer process. Inaddition, from CVs with different scan rates, the charge transferprocess is characterized as an irreversible, diffusion-controlled,over-potential driven, and nucleation mediated process [9]. The

Fig. 1. X-ray diffraction pattern of deposited Zn on copper electrode.

618 H. Yang, R.G. Reddy / Electrochimica Acta 178 (2015) 617–623

earlier studies on ZnO solubility in urea/ChCl and electrochemicalmeasurements of Zn deposition provides the preliminary data forfurther studies on electrowinning of zinc. In the current study, athorough investigation was carried out on electrowinning processof zinc in eutectic mixture of urea/ChCl. Specific objectives of theresearch are: (i) to study the kinetic parameters of zincelectrowinning process, (ii) to examine the effect of over-potentialon the surface morphology of zinc deposits, (iii) to calculate andcompare the current efficiency and energy consumption during thezinc electrochemical reduction process.

2. Experimental procedure

2.1. Preparation of ionic liquid and materials

Urea (CO(NH2)2) (Alfa Aesar, 99.3%) and ChCl (C5H14ClNO) (AlfaAesar, 98%) were dried under vacuum at 373 K for 2 hours beforeelectrolyte preparation. The eutectic mixture was prepared bycombining urea and ChCl (2:1 in molar ratio) at a temperature of363 K for 12 hours until a homogeneous colorless solution wasobtained. One gram of Zinc oxide (Fisher Scientific, certified ACS, �99.0%) was dissolved in 30 mL of 2:1 urea/choline chloride mixturewith constant stirring for 12 hours at a temperature of 363 K.Concentration of ZnO additions was fixed throughout theexperiment.

2.2. Electrodeposition set-up

All experiments were conducted in a 100 mL glass beaker. Cuplate (0.0200 thickness) was used as the cathode and a controlledarea (1.92 cm2) was immersed in the ionic liquid. Platinum wire(0.00400 in diameter) and silver wire (0.00400 in diameter) were usedas the anode and quasi-reference electrode respectively. Thepurpose of reference electrode was to measure the cathodic andanodic potentials using a multimeter (Keithley 2000 Multimeter).The constant cell potential difference was applied betweencathode and anode using EG&G PARC model 273A potentiostat/galvanostat. The instrument was controlled from a desktopcomputer using Power Suite (Princeton Applied Research)software to conduct the polarization studies. A thermometerwas inserted in the solution to monitor the temperature [10–13].

2.3. Characterization of the deposits on the substrate

The characterization of zinc deposits on the cathode wereanalyzed using the X-ray diffraction pattern, obtained from aPhillips MPD XRD that utilizes a monochromatic Cu ka radiation.The resulting pattern was compared with the standard ICDD card.The morphology and elemental analysis of the deposits werecarried out using Scanning Electron Microscope (SEM), model JEOL7000, equipped with an Energy Dispersive Spectrometer (EDS).The SEM images were obtained by setting the accelerating voltageat 20 kV, and a working distance of 10.0 mm.

3. Results and discussions

3.1. Electrochemical reduction of Zn in Urea/ChCl eutectic mixture

The potentiostatic electrochemical reduction of Zn was con-ducted at 363 K for one hour from 3.2 to 3.5 V. ZnO was dissolved inthe electrolyte to form predominant [ZnO�Cl�urea]� species, whichwas also confirmed from the previous literature [9,14,15]. Theisolated Ch+ ions reacts with [ZnO�Cl�urea]� species and the anodesupplies two electrons needed for the reduction of Zn2+ to Zn onthe cathode. Therefore, the possible electrode reactions are shownas follows:

Dissolution reaction: ZnO + Cl� + urea ! [ZnO�Cl�urea]� (1)

Anodic reaction: [ChCl�O�urea]2�! Ch+ + Cl� + 2e� + 0.5O2 (2)

Cathodic reaction: Ch+ + [ZnO�Cl�urea]� + 2e�! Zn(s) + [ChCl�O�urea]2� (3)

Overall reaction: [ZnO�Cl�urea]�! Zn(s) + 0.5O2 + Cl� + urea (4)

A typical XRD pattern of the electrodeposits is shown in Fig. 1.The XRD peaks of electrodeposits matches with the standard ICDDdiffraction peaks of Zn, except for a peak at �50.4�, which is due toCu substrate (ICDD 00-004-0836), and thus, verifying that the zincdeposits obtained are of high purity.

Current output was recorded at regular time intervals and thecurrent density was calculated by dividing the current with thesurface area of zinc deposited on the cathode. Fig. 2 shows thecurrent density (A/m2) as a function of time (s) at applied cellpotential differences from 3.2 to 3.5 V and at a temperature of363 K. A typical current density against time curve is obtained forall applied cell potentials and the plot is divided into three sections.A steep decrease marked as section A is observed during the first50 seconds. To further analyze the sharp decrease of the curve, achronoamperometry study was conducted using a three electrodesystem, which consisted of Cu as the cathode, Ag as a quasi-reference, and Pt as the anode. Cathodic potential has beendelicately set from �1.09 to �1.37 V and the conversion to cellpotentials (3.2 to 3.5 V) can be realized from the measurementsbetween the cathode and anode using a multimeter. Fig. 3 isplotted for the first six seconds of the electrochemical reductionprocess. A short rising trend of the curve, which occurred in thefirst 0.1 s, can be attributed to the nucleation process: the numberof nuclei increases with the increase in electroactive area of theelectrode [16]. More clearly, it is then followed by a decreasingportion from 0.5 to 6 s. The decaying current density is plotted as alinear function of t�1/2 (Fig. 4), which is in agreement with theCottrell equation given below [17]:

j ¼ nFC D=pð Þ1=2t�1=2 ð5Þ

Fig. 2. Variation of current density with time during electrowinning of Zn in ChCl/2Urea eutectic mixture, at fixed temperature of 363K at different applied cellpotential differences: (&) 3.2 V, (�) 3.3 V, (~) 3.4 V, (5) 3.5 V; ZnO concentration is0.41 mol/L.

Fig. 3. Current-time transient for the first 6s at applied cell potential differences:(&) 3.2 V, ( ) 3.3 V, ( ) 3.4 V, ( ) 3.5 V, at fixed temperature of 363K. (Forinterpretation of the references to colour in this figure legend, the reader is referredto the web version of this article.)

Fig. 4. Variation of current density as a function of t�1/2, data plotted for thedecreasing portion of current-time transients at 3.2 V, at fixed temperature of 363K.

Fig. 5. Cathodic polarization curve exhibiting the variation of current density as afunction of cathodic over-potential in the B section showed in Fig. 2 at differentapplied cell potential differences (&) 3.2 V, ( ) 3.3 V, ( ) 3.4 V, ( ) 3.5 V, at fixedtemperature of 363 K. (For interpretation of the references to colour in this figurelegend, the reader is referred to the web version of this article.)

H. Yang, R.G. Reddy / Electrochimica Acta 178 (2015) 617–623 619

where j is the current density (A/m2), t is the time (s), C is theconcentration of the solution, n is the number of exchangeelectrons, D is the diffusion coefficient, F is the Faraday’s constant.The diffusion coefficient determined from the slope of Fig. 4 is7.85 �10�13m2/s, which is in agreement with the previouslyreported value (1.89 � 10�12m2/s at 373 K), and is in reasonablerange compared to other literatures [9,18]. The diffusion controlledprocess occurring in section A is kinetically driven, meaning thatthe rate of reaction is much higher than the rate of diffusion, thuscreating a concentration gradient between electrode and solution.

Hence, the concentration polarization is the main reason that leadsto the sharp decrease in current density.

It is then followed by an increasing portion which is shown insection B of Fig. 2. The gradual increase in current density is due tothe activation polarization at the cathode. Fig. 5 is shown toconfirm that the activation polarization occurred in this region.Cathodic over-potential is calculated using eq. (6):

hc ¼ Ec � E� ð6Þ

where Ec is the cathodic potential (V) measured between cathodeand reference and E� is the equilibrium potential (V) obtained inliterature as –1.05 V vs. Ag [9]. Due to the use of the quasi-referencesilver electrode in the measurement, over-potentials can only beroughly estimated and are subjected to experimental error. As seenin Fig. 5, the cathodic current density is plotted as a function of

Fig. 6. Tafel plots obtained from steady-state Zn electrochemical reduction processin temperatures from 353 to 373K.

Table 1Temperature dependence on equilibrium potential and exchange current density

Temperature (K) E� (V) log jo jo (A/m2)

353 �1.0290 0.7687 5.8708363 �0.9775 1.0147 10.3442373 �0.8958 1.2740 18.7931

Fig. 7. Cathodic polarization plots during steady-state electrochemical reductionprocess at various temperatures: (1) 353 K, (2) 363 K, (3) 373 K.

620 H. Yang, R.G. Reddy / Electrochimica Acta 178 (2015) 617–623

over-potential. The observed linear relationship indicates that theactivation polarization causes current density to increase in sectionB [13].

A constant current density over time in section C of Fig. 2 is asign of steady-state electrochemical reduction. To investigate thekinetics of the electrochemical reduction process, Tafel plots(Fig. 6) were obtained at temperatures of 353–373 K, using a threeelectrode system after reaching the steady-state of electrowinningin the section C. By extrapolating the extended cathodic and anodiclinear Tafel region, the intercepts gives the equilibrium potential aswell as exchange current density and the data is listed in Table 1.The absolute value of equilibrium potential decreases, while theexchange current density increases with the increase in tempera-ture. Based on the equilibrium potentials of Table 1, the cathodicpolarization potentials are plotted in Fig. 7 for the linear Tafelregion (i.e., �1.15 to �1.27 V of Fig. 6) at temperatures of 353 -373 K. Generally, the linear relationship between cathodic over-potential and current density is represented by Tafel equation andis expressed as follows:

logj ¼ logjoþac � n � F2:303RT

� �� h ð7Þ

where h is the cathodic over-potential (V), j is the cathodic currentdensity (A/m2), R is gas constant, T is the temperature (K), jo is theexchange current density (A/m2), ac is the cathodic charge transfercoefficient, n is the number of transferred electrons, and F is theFaraday's constant. The average transfer coefficient obtained fromtemperatures 353 to 373 K is approximately 0.2. The irreversibilityof a given electrochemical reaction is quantitatively measuredfrom the value of charge transfer coefficient. The charge transfercoefficient for a completely reversible reaction is a = 0.5 and for anirreversible reaction the value of a lies between 0.2 and 0.8 [19].The calculated a is approximately 0.2, which is very likely suggestsan irreversible charge transfer process; similar observation has

been made in other literatures [9,20] [21]. This phenomenon iscommonly observed with ionic liquid electrolytes and takes placemainly because of the passivation of Zn surface in the electrolyteunder investigation.

3.2. Effect of applied cell potential difference on microstructure

The effect of applied potential difference on the microstructureof Zn is shown in Fig. 8. At lower applied potentials, as shown inFig. 8(a), the deposited layer is dense and uniform with an averageparticle size of 1 mm. During this stage, a reduction-controlledgrowth can be visualized i.e., the loss of Zn2+ on the electrodesurface can be quickly compensated by diffusion. As a result, thenewly formed crystals are surrounded by sufficient Zn2+ ions thatreadily undergoes reduction and forms morphology with mini-mum surface energy. As the cell potential reaches a more negativevalue of 3.3 V (Fig. 8(b)), a significant grain growth is observed forthe typical hexagonal chunks that lie above the dense layer, with anapproximate size of 10 mm. In Fig. 8(c), agglomeration of Znparticles is observed without any further growth in grain size.However, it is seen that the individual particles split into smallplate-like structures that are aligned perpendicular to the surfaceof the substrate. Moreover, as the cell potential increases to 3.5 V,the perpendicular growth of the platelets is significant whichchanges the morphology from a granular structure to dendritic.The cross-sectional SEM micrographs are shown in Fig. 9 to providea direct comparison of before and after forming dendritic zinc. Thisis a unique feature observed for a hexagonal close packed (hcp)lattice, since the growth along a high-index axis is much moresignificant than low index, leading to an extended growth from aone-dimensional nucleation mechanism [22]. The over-potentialwas �0.65 V when the applied cell potential was kept at 3.5 V. Thisover-potential is considered to be above the critical over-potential(hcrt) at which dendritic growth initiates. When the extra over-potential is provided, a depletion zone is formed on the surface ofthe electrode. This is due to the consumption of ions is morequickly than replenishment by mass transfer, thus suggesting thegrowth of the particles is governed by diffusion [23]. Furthercalculation of critical over-potential (hcrt) was carried out using eq.

Fig. 8. The effect of applied cell potential differences on deposits surface morphology: (a) 3.2 V, (b) 3.3 V, (c) 3.4 V, (d) 3.5 V, at fixed temperature of 363 K.

Fig. 9. Cross section morphology of zinc deposition on copper cathode: (a) 3.5 V; (b) 3.4 V, at fixed temperature of 363 K.

H. Yang, R.G. Reddy / Electrochimica Acta 178 (2015) 617–623 621

Table 2Comparison of critical over-potential of dendrite formation for different metaldeposits

Metals Electrolytes hcrt Temp D Ref

Al AlCl3-EMIC �0.536V 363 K 6.89 � 10�11m2/s [13]Zn Urea/ChCl �0.62V 363 K 7.85 �10�13m2/s This workCu H2SO4 �0.65V 298 K – [26]Zn H2SO4 �0.173V 298 K – [27]

Fig. 10. Variation of current efficiency with applied cell potential differences at atemperature of 363 K.

Table 3Summarization of current efficiency and energy consumption in different appliedcell potential differences, at fixed temperature of 363 K

Applied potential (V) E (%) E(kWh/kg)

3.2 86.2 3.043.3 88.9 3.053.4 87.7 3.183.5 78.5 3.66

622 H. Yang, R.G. Reddy / Electrochimica Acta 178 (2015) 617–623

(8) [24]:

hcrt ¼ þ RTaF

� �In

jLdjoh

� �ð8Þ

where h is the average height of the growing dendrite measuredfrom Fig. 9(a) that is equal to 314.3 mm, d is the diffusion layerthickness, jo is the exchange current density which is obtainedfrom Table 1 to be 10.3442 A/m2, a is equal to 0.2, and temperature,T, is 363 K. The product of jL�d can be calculated from eq. (9) [25]:

jL ¼nFDCd

ð9Þ

where n is the number of transferred electrons, D is calculated to be7.85 �10�13m2/s, F is the Faraday constant, and C is bulkconcentration of electro-active species which is equal to0.41 mol/L. The minimum over-potential for initiation of dendriticformation is given when h is equal to d. Combining eq. (8) and (9)yields hcrt, calculated as �0.62 V at a temperature of 363 K. It is lessthan the measured over-potential of �0.65 V, which explains whydendrites form beyond the critical point at this temperature. Thecalculated critical over-potential for dendrite formation of differ-ent metals are given in Table 2 [13,26,27]. The critical over-potential is dependent on (1) temperature, (2) dendrite size, (3)

exchange current density, and (4) diffusion rate. Although a largerdiffusion coefficient was obtained in AlCl3-EMIC system, whichcould be due to the more viscous nature of the deep eutecticsolvent, hcrt of Zinc is smaller than that of Aluminum at giventemperature, suggesting an easier formation of Al dendrite. This isdue to the difference in the dendrite size as well as the exchangecurrent density. In addition, the critical value for zinc is only�0.173 V at a temperature of 298 K reported by Popov et al.Aqueous solutions are subjected to higher diffusion ratescompared to deep eutectic solvents; therefore, low temperatureshould be the main factor that causes easier dendrite formation,since a much larger critical over-potential was observed.

3.3. Effect of applied cell potential difference on current efficiency andenergy consumption

Cathodic current efficiency (h) is defined as the percentage ofthe actual obtained amount of zinc deposits to theoreticallycalculated zinc amount. The experimental weight gain (DW) canbe obtained by measuring the weight difference of cathode, beforeand after electrowinning (DW = Wfinal– Winitial). The theoreticalweight gain (Wt) of deposited zinc is calculated by using Faraday'slaw:

Wt ¼ jtaAnF

ð10Þ

where j is the current density that passes through electrodes (A/m2), t is the time (s), a is the atomic weight of the deposits (g/mol),A is the area of electro-active region on the electrode (m2), n is thenumber of transferred electrons which is equal to 2 in the studiedsystem, and F is Faraday constant. The product of j�t is calculatedfrom the total area under the current density against the timecurve shown in Fig. 2. The current efficiency can be calculated fromthe equation as follows:

h ¼ Dwwt

� 100% ð11Þ

The energy consumption in the electrowinning process isdetermined using eq. (12):

E ¼ VQh

ð12Þ

where V is the applied cell potential difference, Q is the total chargerequired to get specific amount of deposits on the basis of Faraday'slaw, and h is the current efficiency. The calculated energyconsumption under applied cell potentials of 3.2 - 3.5 V variesfrom 3.04 to 3.66 kWh/kg, which falls within reasonable rangereported in literature [28]. Fig. 10 shows the variation of thecurrent efficiency with applied potential difference. The currentefficiency lies between 78.5 and 88.9% and is also listed in Table 3.The calculated efficiency and power consumption values fall closein lower cell potentials (from 3.2 to 3.4 V). However, an obviousdecrease in current efficiency as well as an increase in energyconsumption was observed at higher cell potential of 3.5 V. At thehigh potential, total charges that pass through the electrochemicalcell increases and thus, a simultaneous gain in the theoreticalweight was also observed. Nevertheless, the actual weight of thedeposited zinc is lower than the expected value, as majority of thecurrent is lost during the electrolysis process due to strong effect ofpolarization and co-deposition of hydrogen.

4. Conclusions

Zinc oxide was successfully reduced to zinc using 2:1 urea/choline chloride ionic liquid. The presence of a deposited zinc layer

H. Yang, R.G. Reddy / Electrochimica Acta 178 (2015) 617–623 623

in high purity on a copper substrate was confirmed by XRDanalysis. The variation of current density and time was plotted invarious applied potential differences. A decreasing portion in thefirst 50 seconds was attributed to the concentration polarization.The average diffusion coefficient was confirmed to be7.85 �10�13m2/s at 363 K. A gradual increase in current densitywas due to activation polarization and the linear relationshipbetween current density and over-potential was confirmed. Thekinetics study of the electrochemical reduction process was carriedout in the steady-state region. Tafel plots were obtained intemperatures from 353 to 373 K. The average transfer coefficientwas calculated from the slope of the linear Tafel region and thevalue was approximately 0.2, which suggests an irreversible chargetransfer process. The effect of applied potential differences onmicrostructure was investigated. Dendrites of Zn were formed at ahigher over-potential of �0.65 V, beyond the calculated criticalvalue of �0.62 V. Current efficiency varied from 78.5 to 88.9% andthe energy consumption increased from 3.04 to 3.66 kWh/kg withchange in cell potentials from 3.2 to 3.5 V. A significant drop in theefficiency was observed at 3.5 V, which is because of strongpolarization and co-deposition of hydrogen.

Acknowledgements

The authors are pleased to acknowledge the financial supportfrom National Science Foundation Grant No. DMR-1310072 andACIPCO for this research project. We also thank the University ofAlabama for providing the experimental and analytical facilities.

References

[1] H. Geduld, Zinc Plating, ASM International, Teddington, London, 1988.[2] D.J. Mackinnon, J.M. Brannen, R.C. Kerby, The Effect of Lead on Zinc Deposit

Structures Obtained from High Purity Synthetic and Industrial Acid SulphateElectrolytes, J. Appl. Electrochem 9 (1979) 55–70.

[3] R.G. Reddy, Ionic liquids: How well do we know them, J. Phase. Equilib. Diff 27(2006) 210–211.

[4] A.P. Abbott, K.J. Mckenzie, Application of Ionic Liquids to the Electrodepositionof Metals, Phys. Chem. Chem. Phys 8 (2006) 4265–4279.

[5] Q. Zhang, K. Vigier, S. Royer, F. Jerome, Deep Eutectic Solvents: Syntheses,Properties and Applications, Chem. Soc. Rev 41 (2012) 7108–7146.

[6] A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, V. Tambyrajah, Novel SolventProperties of Choline Chloride/Urea Mixtures, Chem. Commun 0 (2003) 70–71.

[7] A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, P. Shikotra, SelectiveExtraction of Metals from Mixed Oxide Matrixes Using Choline-Based IonicLiquids, Inorg. Chem 44 (2005) 6497–6499.

[8] H. Yang, R.G. Reddy, Fundamental Studies on Electrochemical Deposition ofLead from Lead Oxide in 2:1 Urea/Choline Chloride Ionic Liquids, J.Electrochem. Soc 161 (2014) D586–D592.

[9] H. Yang, R.G. Reddy, Electrochemical Deposition of Zinc from Zinc Oxide in2:1 Urea/Choline Chloride Ionic Liquid, Electrochim. Acta 147 (2014) 513–519.

[10] V. Kamavaram, D. Mantha, R.G. Reddy, Recycling of Aluminum metal-matrixcomposites using Ionic Liquids: Effect of Process Variables on currentefficiency and deposit Characteristics, Electrochim. Acta 50 (2005)3286–3295.

[11] D. Pradhan, R.G. Reddy, A. Lahiri, Low-Temperature Production of Ti-Al AlloysUsing Ionic Liquid Electrolytes: Effect of Process Variables on Current Density,Current Efficiency, and Deposit Morphology, Metall. Trans B 40 (2009)114–122.

[12] D. Pradhan, R.G. Reddy, Electrochemical Production of Ti–Al Alloys UsingTiCl4–AlCl3–1-butyl-3-methyl Imidazolium Chloride (BmimCl) Electrolytes,Electrochim. Acta 54 (2009) 1874–1880.

[13] D. Pradhan, D. Mantha, R.G. Reddy, The Effect of Electrode Surface Modificationand Cathode Overpotential on Deposit Characteristics in AluminumElectrorefining Using EMIC–AlCl3 Ionic Liquid Electrolyte, Electrochim. Acta54 (2009) 6661–6667.

[14] T. Tsuda, L. Boyd, S. Kuwabata, C.L. Hussey, Electrochemical Behavior of Copper(I) Oxide in Urea-Choline Chloride Room-Temperature Melts, ECS Trans. 16(2009) 529–540.

[15] A.P. Abbott, G. Capper, K.J. McKenzie, K.S. Ryder, Electrodeposition of Zinc TinAlloys from Deep Eutectic Solvents Based on Choline Chloride, J. Electroanal.Chem 599 (2007) 288–294.

[16] Y. Lin, I. Sun, Electrodeposition of Zinc from a Mixture of Zinc Chloride andNeutral Aluminum Chloride-1-Methyl-3-ethylimidazolium Chloride MoltenSalt, J. Electrochem. Soc 146 (1999) 1054–1059.

[17] D. Grujicic, B. Pesic, Electrodeposition of Copper: The Nucleation Mechanisms,Electrochim. Acta 47 (2002) 2901–2912.

[18] P. Chen, C.L. Hussey, The Electrodeposition of Mn and Zn–Mn alloys from theRoom-temperature Tri-1-butylmethylammonium Bis((Trifluoromethane)sulfonyl) imide Ionic Liquid, Electrochim. Acta 52 (2007) 1857–1864.

[19] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamental andApplications, John Wiley&Sons, New York, 200087–230.

[20] A. Gomes, M.I. da Silva Pereira, Zn Electrodeposition in the Presence ofSurfactants: Part I. Voltammetric and Structural Studies, Electrochim. Acta 52(2006) 863–871.

[21] P. Chen, I. Sun, Electrodeposition of Cobalt and Zinc Cobalt Alloys from a LewisAcidic Zinc Chloride-1-Ethyl-3-Methylimidazolium Chloride Molten Salt,Electrochim. Acta 46 (2001) 1169–1177.

[22] K.I. Popov, S.S. Djokic, B.N. Grgur, Fundamental Aspects of Electrometallurgy,Springer, 200278–79.

[23] C.M. Lopez, K. Choi, Electrochemical Synthesis of Dendritic Zinc FilmsComposed of Systematically Varying Motif Crystals, Langmuir 22 (2006)10625–10629.

[24] D. Pradhan, R.G. Reddy, Dendrite-Free Aluminum Electrodeposition fromAlCl3-1-Ethyl-3-Methyl-Imidazolium Chloride Ionic Liquid Electrolytes,Metall. Trans B 43 (2012) 519–531.

[25] S.H. Zhang, S.B. Lyon, The Electrochemistry of Iron, Zinc and Copper in ThinLayer Electrolytes, Corr. Sci 35 (1993) 713–718.

[26] N. Nikolic, K. Popov, L. Pavlovic, M. Pavlovic, Determination of CriticalConditions for the Formation of Electrodeposited Copper Structures Suitablefor Electrodes in Electrochemical Devices, Sensors 7 (2007) 1–15.

[27] K. Popov, M. Pavlovic, M. Spasojevic, V. Nakic, The Critical Overpotential forCopper Dendrite Formation, J. Appl. Electrochem 9 (1979) 533–536.

[28] H. Yang, R.G. Reddy, Electrodeposition of Zinc from Zinc Oxide using Urea andCholine Chloride Mixture: effect of [BMIM]HSO4, Temperature, Voltage oncurrent Efficiency, Energy Consumption, and Surface Morphology, Rare MetalTechnology, TMS (2014) 21–26.