Electrochemical behavior and electrodeposition of cobalt from choline chloride-urea deep eutectic solvent

Accepted Manuscript Electrochemical behavior and electrodeposition of cobalt from choline chlorideurea deep eutectic solvent

Xiaozhou Cao, Lulu Xu, Yuanyuan Shi, Yaowu Wang, Xiangxin Xue PII:

S0013-4686(18)32404-6

DOI:

10.1016/j.electacta.2018.10.163

Reference:

EA 32962

To appear in:

Electrochimica Acta

Received Date:

27 July 2018

Accepted Date:

26 October 2018

Please cite this article as: Xiaozhou Cao, Lulu Xu, Yuanyuan Shi, Yaowu Wang, Xiangxin Xue, Electrochemical behavior and electrodeposition of cobalt from choline chloride-urea deep eutectic solvent, Electrochimica Acta (2018), doi: 10.1016/j.electacta.2018.10.163

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ACCEPTED MANUSCRIPT Electrochemical behavior and electrodeposition of cobalt from choline chlorideurea deep eutectic solvent Xiaozhou Cao1, Lulu Xu1 Yuanyuan Shi2, Yaowu Wang1, Xiangxin Xue1 1School

of Metallurgy, Northeastern University, Shenyang, 110819, China

2Chinalco

Shenyang Non-ferrous Metal Processing Co., Ltd, Shenyang, 110108,

China E-mail address: [email protected] Abstract The electrochemical behavior and electrodeposition of cobalt was investigated in the choline chloride-urea deep eutectic solvent containing cobalt chloride. Cyclic voltammetry was used to study the effect of temperature and concentration on the electrochemical behavior of cobalt. Chronoamperometry results show that as the applied potential increases, the electrocrystallization of cobalt on the tungsten electrode gradually tends to three-dimensional instantaneous nucleation. SEM observation of the microstructure of the coating shows that the deposited films obtained in this experimental condition (-0.85V ~ -1.0V; 343K ~ 373K) are dense and uniform with fine crystals. As the deposition potential increases, the contours of the particles are become clear and the edges of the particles gradually join. As the temperature rises, the coating particles increase to become gradually uniform, the plush contour gradually disappears, and the particles are no longer connected with the other particles. EDS and XRD analyses show that pure cobalt coating was obtained. Keywords

ACCEPTED MANUSCRIPT Cobalt; Nucleation; Electrodeposition; Cyclic voltammetry; Chronoamperometry 1. Introduction Cobalt is a silver-gray shiny metal with many excellent properties, high temperature resistance, corrosion resistance, and is one of the few metals that can maintain its magnetic properties after magnetization. The preparation techniques for high-purity cobalt include extraction, ion exchange, membrane separation, electrolytic refining, vacuum sintering (controlled atmosphere) degassing and vacuum melting [1-6].Cobalt is widely used in aerospace, machinery manufacturing, electrical and electronic, chemical, ceramic and other industrial fields. It is one of the important raw materials for manufacturing high-temperature alloys, hard alloys, ceramic pigments, catalysts and batteries [7]. The addition of cobalt coating on the surface of the material can effectively improve the properties of the material [8]. Electrodeposition is an effective method for obtaining cobalt coating, and good surface morphology plays an important role in ensuring the quality of the coating. The surface morphology of the coating is affected by electrodeposition conditions including temperature, electrolyte concentration, deposition potential and deposition time [9, 10]. Due to the different electrodeposition conditions and the nature of the metal, electrodeposition can obtain strong coating as well as dendritic metal powder (in addition to dendrites, there are flakes or needles, fibrous or sponge, cauliflowerlike forms, etc.) [11-13]. Cobalt is an inert metal with high melting point, low exchange current density and low hydrogen evolution overpotential. Therefore, the electrodeposition of cobalt in aqueous solution occurs together with the hydrogen

ACCEPTED MANUSCRIPT evolution reaction (HER). As alternative to aqueous solutions, ionic liquids (ILs) have been used to electrodeposited metals and alloys as electrolytes due to their a series of advantages such as wide electrochemical window (up to 4 V~6V in some systems), good solubility (can dissolve organic compounds, organometallic compounds, inorganic compounds), high conductivity, thermodynamic stability, non-flammable, low vapor pressure (close to zero) [14]. Thus it has a wide range of applications in the battery, electrodeposition, capacitors and other electrochemical fields [15] However, recent studies have shown that the issues of ILs such as toxicity, availability and cost may limit ionic liquid practical use for larger scale applications of metals [16]. A new class of ILs, the named deep eutectic solvent (DES) was proposed by Abbott et al [17]. The deep eutectic solvent is a eutectic mixture which is similar in physicochemical properties to ionic liquids, and is composed of a quaternary ammonium salt (mostly choline chloride) and a metal salt or a hydrogen bond donor (amides, glycols, caboxylic acids) at a certain stoichiometric ratio by hydrogen bonding or the like [18]. Compared with the most of the common room temperature ILs, raw materials for synthesizing deep eutectic solvents are inexpensive, widely available, safe, non-toxic, and greater stability towards moisture and oxidation, and the preparation process is quick and convenient. Most eutectic solvents are biocompatible and biodegradable and therefore considered a green solvent [19, 20]. The electrodepostiton of cobalt from DES and ILs has been reported. Yang et al. have studied the electrochemical behavior of pure cobalt in 1-ethyl-3-methylimidazolium chloride (EMIC) and ethylene glycol. The cyclic voltammetry (CV) test indicated that

ACCEPTED MANUSCRIPT the growth process of Co is a diffusion controlled quasi-reversible process and the chronoamperometric curve indicated the three-dimensional progressive nucleation mechanism of Co electrodeposition [21, 22]. Katayama et al. obtained the cobalt nano-particles with a diameter of 2~10nm on the platinum mesh electrode in 1-nbutyl-1-methylpyrrolidinium

bis(trifluoromethylsulfonyl)imide

(BMPTFSI),

and

found that the overpotential of Co electrodeposition decreased obviously with the temperature increasing [23, 24]. Li et al. investigated the electrochemical behavior of cobalt in choline chloride and urea, which showed that the reduction of cobalt ion is irreversible and the two electrons transfer process is done in one step[25]. Anca Cojocaru et al. investigated the cathode reduction process of cobalt ion in a series of choline-chloride-based DES. The reduction mechanisms of Co2+ in ChCl-urea/EG is quasi-reversible and diffusion controlled process. The diffusion coefficient in ChClurea is computed up to two orders of magnitude lower than in aqueous solutions，a better cobalt film obtained by electrodeposition in ChCl-urea/EG [26]. Manh et al. prepared different cobalt crystal structure in ChCl-based DES, which showed that the large current density as the driving force play the important role to facilitate the change of crystal structure [27]. Although, the deposition of cobalt using various ILs and DES has been studied, the electrochemical behavior, the influences of parameter on the morphology of Co electrodeposition and the cobalt species structure in ChClurea DES still need more investigation. The present work aims to clarify the reduction kinetics and the nucleation mechanism of cobalt electrodeposition and the structure of cobalt species in ChCl-urea. The

ACCEPTED MANUSCRIPT effects of temperature, electrolyte concentration on the electrochemical behavior of cobalt were investigated. The main research methods used in this study include cyclic voltammetry and chronoamperometry. The microstructure of the deposit was analyzed by scanning electron microscopy with energy dispersive spectroscopy. The crystallinity of the deposit was evaluated by X-ray diffraction. The structure of ChClurea-CoCl2 was investigated by UV-visible spectrum and fourier transform infrared spectroscopy. 2. Experimental 2.1. Electrolyte preparation Urea (99%; Sinopharm Chemical Reagent Co., Ltd) and choline chloride (ChCl) (98%; Aladdin Reagent Co.) used in the preparation of DES were dried under vacuum at 80ºC for 24 hours by using the vacuum drying oven. Then quickly weighed appropriate amount of ChCl and urea (molar ratio of 1: 2) mixed and sealed in a round bottom flask, stirred at 80ºC until a colorless and transparent liquid formed. Cobalt chloride (98%; Aladdin Reagent Co.) was used as the cobalt source directly added in the DES to obtain the selected concentration. 2.2. Spectral experiment In order to infer the internal structure of ChCl-urea-CoCl2, the UV-visible spectrum and Fourier transform infrared (FTIR) spectroscopy were carried out. The UV-Vis spectra of the electrolytes containing different concentrations of Co(II) was measured by UV-Vis spectrophotometer (UV-2550, SHIMADZU). UV spectra were obtained using quartz cell with a light path length of 1.0cm. FTIR measurements were

ACCEPTED MANUSCRIPT performed using a Bruker Vertex70v Spectrometer in transmission mode coupled， potassium bromide (KBr) windows were used to encapsulate the samples，in the region of 4000-400 cm-1 wave length, and 2 cm-1 resolution. The infrared spectra of the DES containing different concentrations of Co(II) were recorded on the room temperature. 2.3. Electrochemistry and electrodeposition experiments All

electrochemical

electrochemical

experiments

workstation.

A

were

carried

three-electrode

out

using

system

was

solartron used

in

1287 the

electrochemical measurement and electrodepostion experiment. The counter electrode was a platinum plate (1 × 1cm) and the reference electrode was a silver wire (r = 0.5mm), the working electrode was a tungsten wire (r = 0.5mm) for cyclic voltammetry (CV) and chronoamperometry experiments. All the electrodes were inserted into the solution under 1cm. Before used the electrodes should be pretreated as follows: polished with fine sandpaper, ultrasonic cleaned with anhydrous ethanol, washed with deionized water, finally dried at low-temperature in vacuum. The influences of potential scan rate, temperature and electrolyte concentration on electrochemical behavior of Co were investigated by CV, all cyclic voltammetric curves were recorded starting in the cathodic direction. The nucleation and growth mechanism of Co in ChCl-Urea were monitored by chronoamperometry test under different potentials. Electrodeposition

experiment

was

also

conducted

using

solartron

1287

electrochemical workstation in a three electrode system. Copper foil (0.5cm2) as

ACCEPTED MANUSCRIPT substrate was used to obtain bulk electrodeposits under potentiostatic control. The effect of potential and termperature on the electrodeposits was investigated. After electrodepostion experiments, the products were cleaned with distilled water and acetone to remove the rest electrolyte and dried under vacuum. 2.4. Characterization The surface morphology of the cobalt electrodeposit was characterized by scanning electron microscopy (SEM) (Zeiss Germany) with 3-6kV accelerating voltage. Elemental compositions are analyzed by energy dispersive spectroscopy (EDS). The phase composition of the cobalt electrodeposit on copper substrate was analyzed with X-ray diffracrtion (XRD) (model X’Pert Pro, Panalytical) with Cu Kα radiation. Typical operating conditions were 45 KV, 40 mA scanned between 10° and 90°with 0.02° increments. 3. Results and discussion 3.1. Infrared spectroscopy and UV-Vis spectroscopy analysis Electrolytes containing different concentrations of cobalt ions in ChCl-urea are shown in Fig. 1. The color of electrolytes are transparent blue solutions, and the color gradually become darker as the concentration of Co(II) increases from 0.001M to 0.3M. The blue color observed may be due to transitions between the d orbitals of the transition metal Co in the coordination compounds [28]. The complex anion is the dominate existence from of Co(II) after enhanced dissolution of CoCl2 in ChCl-urea DES [29]. This color change indicating the increased concentration of cobalt complex anion.

ACCEPTED MANUSCRIPT

The UV-Vis spectra of the electrolytes containing different concentrations of Co(II) were measured and the spectrum was shown in Fig. 2. It can be observed that the electrolytes have two absorption peaks in the ultraviolet-visible region between 200 nm and 800nm. The most intense bands in the range 200 nm-320nm with a peak at 235.5nm (λ1) is possibly due to the conjugate effect formed between carbonyl and amino in urea. The visible spectra of the cobalt complexes exhibit a d-d broad band peak at 638 nm (λ2) may due to the formation of [CoCl3(OD)]- . Hartley reported data obtained from EXAFS spectra indicating that Co(II) exists in reline solution as [CoCl3(OD)]- complex (OD=Oxygen band donor, urea) [30]. The position of the absorption peak remains invariable when the concentration of CoCl2 in DES is changed. A significant increase in the intensity of the absorption band with the Co(II) content increase. A strong linear dependence between absorbance and Co(II) content is observed in complete agreement with the beer-Lambert law for both bands. These observations allow us to assert that the nature and the composition of the Co(II) complexes remain unchanged in the examined range of the concentration [31]. The FTIR spectra of ChCl-urea and the electrolytes containing different concentrations of Co(II) were measured and the spectrum are shown in Fig. 3. The standard stretching and bond vibration IR frequencies of different functional groups compiled by Reddy et al. are shown in Table 1[32]. The bands with strong intensity at around 3205 to 3347 cm-1 associated with N-H stretching vibration of amide group. The bands with strong intensity at 1622 cm-1 and 1668 cm-1 corresponded to amide N-

ACCEPTED MANUSCRIPT H scissoring vibration and amide C=O stretching vibration. The band at 955 cm−1 is due to the asymmetric stretching vibrations of CCO bond, which shows that the structure of Ch+ is not destroyed in ChCl–urea system [33]. The infrared absorption of ChCl-urea is basically consistent with the literature. The infrared absorption of ChClurea-CoCl2 electrolyte basically showed an increase with the increase of Co(II). The cobalt-containing electrolyte showed an absorption peak different from that of the ChCl-urea at a wave number of 2208cm-1, which may be formed complex anion, where urea is bound to the cobalt metal through an O-M bond. Theophanides et al. found that the metal-urea complexes may be formed by the bidentate coordination of urea with metal atoms [34]. 3.2. Cyclic voltammetry The effects of scanning rate, temperature and electrolyte concentrations on cyclic voltammetric behavior of Co(II) in ChCl-urea were investigated. 3.2.1. The effect of scanning rate Typical cyclic voltammetry of electrolyte containing 0.05M Co(II) was performed at different potential scan rates at 373K, with a potential range of -1.2 V to 0.3V (Fig.4). As shown in Fig. 4, there is only one pair of redox peaks in the cyclic voltammetry curve, the cathodic current peak observed at -0.9V is attributed to the reduction of Co(II) to the metal Co, and an anodic current peak observed at 0.1V is due to the stripping of the electrodeposited Co. A crossover loop occurs during the reversal sweep which means that the reduction of Co(II) occurs via an overpotential-driven nucleation and growth process, and the characteristics associated with a nucleation

ACCEPTED MANUSCRIPT followed by diffusion-limited growth process [35]. This behavior is consistent with the results reported by Sakita et al [36]. Theoretically, the peak potential of the reversible electrode reaction does not change with the potential scan rate. As can be seen from Fig. 4, as the potential scan rate increases, the peak current increases and the reduction peak potential shifts to more negative potential. The difference between the cathodic peak and half-peak potentials |Ep-Ep/2| increase with the increase of scan rate. The minimum value of |Ep-Ep/2| is 77mV, which is larger than the theoretical value for the reversible process (35mV, 373K). It can be concluded that the reduction of Co(II) in ChCl-Urea system is quasireversible. The relationship between the square root of scan rate(ν1/2 )and the corresponding cathodic peak current (Ip) was shown in Fig. 5. It can be seen that the plot displays an excellent linear relationship, indicating a diffusion controlled process as reported by Manh et al [37]. These characteristics showed that the reduction of cobalt is quasi-reversible and follows a one-step two-electron transfer process [38]. The formula for calculating the irreversible reaction transfer coefficient is given in Eq. (1). Formula for calculating the diffusion coefficient of irreversible reaction is shown in Eq. (2) [39], which is also applicable to the quasi-reversible charge transfer process [40].

|Ep - Ep 2| = 1.857RT (αnF) 12 1

(1) 12

2 0 jp = 0.4958nFc (αnF RT) D v

(2)

where Ep is the cathodic peak potential, Ep/2 is the cathodic half-peak potential, R is the ideal gas constant, α is the transfer coefficient, n is the electron transfer number, F

ACCEPTED MANUSCRIPT is the faraday constant, jp is the cathode peak current density, c0 is the concentration of metal ions, D is the diffusion coefficient, v is the potential scan rate. The average transfer coefficient of 0.05 M Co(II) in ChCl-urea at 373K calculated from Fig. 4 and Eq. (1) is 0.191. The diffusion coefficient of Co(II) in ChCl-urea is calculated by Fig. 5 and Eq. (2) to be 1.2178×10-7 cm2∙s-1, which is similar with the value of 2.7×10−7 cm2∙s-1 reported by Manh et al. in the same DES [27]. Temperature has the greatest effect on diffusion coefficient and is the easiest of the factors to change. The diffusion coefficient is smaller than the value in Urea-NaBr-KBr-CoCl2 (2.5×10-6 cm2∙s-1) [41] and larger than the value in BMIMBF4-Co(BF4)2 (1.76×10-8 cm2∙s-1) [42], which may due to the different viscosity of different ionic liquids and different cobalt-complex anion [43]. The diffusion coefficient of Co is smaller than the value in NH4Cl aqueous solution (7×10-5 cm2∙s-1) due to the lower viscosity of the aqueous solution than DES and ionic liquid [44]. 3.2.2. The effect of temperature The cyclic voltammetry curves of electrolyte with 0.05 M CoCl2 at different temperatures (343K, 353K, 363K, 373K, 383K) are shown in Fig. 6. It can be seen from Fig. 6 that the peak current increases and the reduction peak potential moves towards positive potential as the temperature increases. This is because the viscosity of the DES decreases and the electrical conductivity increases as the temperature rises, resulting in the increase of current. At the same time, the increase of temperature leads to the increase of the kinetic energy and the acceleration of thermal movement of the electroactive substances. And thus the deposition

ACCEPTED MANUSCRIPT potential of cobalt ions is less affected by the polarization, leading to the positive shift of reduction peak [45]. 3.2.3. The effect of cobalt chloride concentrations The CVs of electrolyte solutions with different concentrations of cobalt chloride (0.01 M, 0.05 M, 0.1 M) at 373K are shown in Fig. 7. As can be seen from Fig. 7, with the cobalt ion concentration increases, the value of reduction peak current becomes larger, and the reduction peak potential slightly positive shift. This also indicates that the concentration polarization of cobalt ions in the vicinity of the cathode also has an important effect on the deposition potential. The higher concentration of cobalt ion will improve the supplement of the cobalt ions during the cathodic discharge progress. And as the ion concentration increases, the electron transfer rate increases significantly, which causes the peak potential to increase [46]. 3.3. Chronoamperometric analysis In order to analyze the nucleation mechanism of cobalt, the chronoamperometric test was performed. Typical current-time curve obtained for the deposition of Cobalt by stepping the potential from an initial value of -0.70V, where no faradaic process occurred, to potentials sufficiently negative to initiate nucleation and growth of cobalt in ChCl-urea-CoCl2 (0.05M) at 373K are shown in Fig.8. It can be clearly seen from Fig. 8 that the current density curves present three stages corresponding to the cobalt nucleation process. The first part of the rapid decline in current corresponding to the decay of the current during the charging of the electric double layer [47]. The second part of the current gradually rises corresponding to the

ACCEPTED MANUSCRIPT formation and growth of Co nuclei. In this latter region, the current maximum increases while the time required to reach the current maximum decreases with increasing applied potential as a consequence of the higher nucleation densities [48]. The third part of the current decays slowly from the peak to the steady due to the increased diffusion layer thickness and overlap of diffusion zones [49]. The curves in Fig. 8 are in accordance with the regularity of the potentiostatic current transient for three-dimensional multiple nucleation with diffusion controlled growth summarized by Scharifker et al. They proposed two limiting model, the instantaneous nucleation (IN)and the progressive mucleation (PN), to indentify the nucleation nucleation/growth process of metal deposition [50]. Performed the following non-dimensional treatment of the relationship between transient current and time [51]:

Instantaneous nucleation:

Progressive nucleation:

I

2 2

Im I

=

2 2

Im

=

1.9542 t

{

tm

{

1.2254 t

tm

2

[

( )] }

[

( t ) ]}

1 - exp - 1.2564

1 - exp - 2.3367

t

tm

(3)

2 2

t

m

(4)

In this experiment, the same non-dimensional treatment on the experimental data obtained by the chronoamperometry and compare the obtained curves with the theoretical model are shown in Fig. 9. It can be observed that as the applied potential increases negatively, the curve gradually approaches instantaneous nucleation curve. This shows that at lower potential, the nucleation of cobalt is between instantaneous nucleation and continuous nucleation. When the applied potential increases to a certain extent, the nucleation process of cobalt becomes instantaneous nucleation.

ACCEPTED MANUSCRIPT 3.4. Electrodeposition and characterization of Co In order to explore the influence of potential and temperature on the morphology of the coating, cobalt was electrodeposited for 7200s at different potentials (according to the reduction peak potential in the cyclic voltammetry curve) and temperatures, and the microscopic morphology was observed by scanning electron microscopy (SEM). Fig.10 is the scanning electron micrographs of cobalt electrodeposits obtained at different cathodic potentials at 373K. It can be seen from the Fig. 10 that the coatings obtained under the electrodeposition conditions are dense and uniform, and the particle diameters are all below 0.5 μm. The electrodeposition potential is the main driving force for the formation of Co and can change the nucleation-growth process of Co [52]. At lower potentials, the contours of the particles are not clear. As the cathodic potential increases, the edges of the particles gradually join and the coating image magnified 20000 times at the deposition potential of -1V shows a knit pattern (Fig.10 d), which due to the increase of the electrodeposition rate and the number of growth sites. Fig. 11 is the scanning electron micrographs of cobalt electrodeposits at different temperatures at -0.9V. It can be observed that as the temperature increase, the particles size increase to become gradually uniform, the plush contour gradually disappears, and the particles are no longer connected with the other particles. At 373K (Fig.11 c), the particles of the electrodeposits grow up to form larger on the substrate which due to the viscosity of electrolyte decrease at the higher temperature, the charge transfer rate and the growth of nuclei accelerate.

ACCEPTED MANUSCRIPT The EDS spectrum of the electrodeposits obtained at -0.9V，343K is shown in Fig. 12. The energy spectrum shows that the electrodeposits is mainly cobalt (96.4 wt%), which indicates that the cobalt ion has been successfully electrodeposited and the main chemical constituent is cobalt. In addition, a small amount of oxygen is present, and it is presumed that the sample is oxidized in the air. Fig. 13 shows the XRD pattern of the electrodeposits obtained at -0.9V and 343K. The peaks in the XRD pattern are the characteristic diffraction peaks for cobalt and copper substrates, no other peaks are observed. It is shows that the electrodeposited films are pure cobalt. Combined with SEM results, the smaller diffraction peak intensity of cobalt may be related to the small grain size. 4. Conclusions The electrochemical behavior of cobalt in choline chloride-urea was investigated by using cobalt chloride as cobalt source. The Cobalt electrodeposition is a diffusioncontrolled quasi-reversible and follows one step two electron transfer process in ChCl-urea based on the CV results. The diffusion efficient of Co(II) was 1.2178×10-7 cm2∙s-1 at 373K. The UV-Vis and FTIR spectra indicating that Co(II) species exist as [CoCl3(OD)]- complex in ChCl-urea-CoCl2 solution. The complex inhibited the cobalt deposition and decreased the diffusion efficient. The results of chronoamperometry showed that the nucleation of cobalt is between the three-dimensional instantaneous nucleation and continuous nucleation at lower applied potential. When the applied potential is higher than -0.9V, the nucleation of cobalt shows the three-dimensional

ACCEPTED MANUSCRIPT instantaneous nucleation. SEM image indicated that the morphologies of the deposits were effect by the potential and temperature. The deposits became densely distributed with smaller size at more negative potentials. The particles gradually grow up to form larger grain size with the increase of temperature. XRD and EDS spectrum confirmed that the deposits on Cu substrate were pure Cobalt. Acknowledgments The authors would like to acknowledge the financial support from the National Natural Science Foundation of China (No. 51204043). References [1] M. Isshiki, Y. Fukuda, K. Igaki, Preparation of high purity cobalt, J. Less Common Metals, 105 (1985) 211-220. [2] N.B. Devi, K.C. Nathsarma, V. Chakravortty, Separation and recovery of cobalt(II) and nickel(II) from sulphate solutions using sodium salts of D2EHPA, PC 88A and Cyanex 272, Hydrometallurgy, 49 (1998) 47-61. [3] V.G. Glebovsky, N.S. Sidorov, E.D. Stinov, B.A. Gnesin, Electron-beam floating zone growing of high-purity cobalt crystals, Mater. Lett., 36 (1998) 308-314. [4] N.V. Thakur, S.L. Mishra, Separation of Co, Ni and Cu by solvent extraction using di-(2-ethylhexyl) phosphonic acid, PC 88A, Hydrometallurgy, 48 (1998) 277-289. [5] J. Gega, W. Walkowiak, B. Gajda, Separation of Co(II) and Ni(II) ions by supported and hybrid liquid membranes, Sep. Purif. Technol., 22-23 (2001) 551558.

ACCEPTED MANUSCRIPT [6] F.J. Alguacil, M. Alonso, Separation of zinc(II) from cobalt(II) solutions using supported liquid membrane with DP-8R (di(2-ethylhexyl) phosphoric acid) as a carrier, Sep. Purif. Technol.,41 (2005) 179-184. [7] G. Leon, G. Martinez, L. Leon, M.A. Guzman, Separation of cobalt from nickel using novel ultrasound-prepared supported liquid membranes containing Cyanex 272 as carrier, Physicohem. Probl. Miner. Process., 52 (2016) 77-86. [8] H.A. Ching, D. Choudhury, M.J. Nine, N.A.A. Osman, Effects of surface coating on reducing friction and wear of orthopaedic implants, Sci. Technol. Adv. Mater., 15 (2014) 1-21. [9] K.I. Popov, S.S. Djokić, B.N. Grgur, Fundamental aspects of electrometallurgy, Kluwer Academic/Plenum Publishers, New York, 2002. [10]V.M. Maksimovic, N.D. Nikolic, V.B. Kusigerski, J.L. Blanusa, Correlation between morphology and magnetic properties of electrochemically produced cobalt powder particles, J. Serb. Chem. Soc., 80 (2015) 197-207. [11]N.D. Nikolic, V.M. Maksimovic, G. Brankovic, P.M. Zivkovic, M.G. Pavlovic, Influence of the type of electrolyte on the morphological and crystallographic characteristics of lead powder particles, J. Serb. Chem. Soc., 78 (2013) 13871395. [12]V.D. Jović, N.D. Nikolić, U.Č. Lačnjevac, B.M. Jović, K.I. Popov, Morphology of different electrodeposited pure metal powders, in: S.S. Djokić (Ed.) Electrochemical Production of Metal Powders, Springer US, Boston, MA, 2012, pp. 63-123.

ACCEPTED MANUSCRIPT [13]M.G. Pavlović, Lj.J. Pavlović, V.M. Maksimović, N.D. Nikolić, K.I. Popov, Characterization and morphology of copper powder particles as afunction of different electrolystic regimes, Int. J. Electrochem. Sci., 5 (2010) 1862-1878. [14]S. Baldelli, Surface structure at the ionic liquid-electrified metal interface, Acc. Chem. Res., 41 (2008) 421-431. [15]M. Armand, F. Endres, D.R. MacFarlane, H. Ohno, B. Scrosati, Ionic-liquid materials for the electrochemical challenges of the future, Nat. Mater., 8 (2009) 621-629. [16]A. Abo-Hamad, M. Hayyan, M.A. AlSaadi, M.A. Hashim, Potential applications of deep eutectic solvents in nanotechnology, Chem. Eng. J., 273, (2015) 551-567. [17]A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, V. Tambyrajah, Novel solvent properties of choline chloride/urea mixtures, Chem. Commun., 1 (2003) 70-71. [18]E.L. Smith, A.P. Abbott, K.S. Ryder, Deep eutectic solvents (DESs) and their applications, Chem Rev, 114 (2014) 11060-11082 [19]Y. Yu, X. Lu, Q. Zhou, K. Dong, H. Yao, S. Zhang, Biodegradable naphthenic acid ionic liquids: synthesis, characterization, and quantitative structurebiodegradation relationship, Chem. Eur. J., 14 (2008) 11174-11182. [20]K.D. Weaver, H.J. Kim, J. Sun, D.R. MacFarlane, G.D. Elliott, Cyto-toxicity and biocompatibility of a family of choline phosphate ionic liquids designed for pharmaceutical applications, Green Chem., 12 (2010) 507-513. [21]P.X. Yang, M.Z. An, C.N. Su, F.P. Wang, Fabrication of cobalt nanowires from

ACCEPTED MANUSCRIPT mixture of 1-ethyl-3-methylimidazolium chloride ionic liquid and ethylene glycol using porous anodic alumina template, Electrochim. Acta, 54 (2008) 763-767. [22]P.X. Yang, M.Z. An, C.N. Su, F.P. Wang, Electrodeposition Behaviors of Pure Cobalt in Ionic Liquid, Acta Phys-Chim Sin., 24 (2008) 2032-2036. [23]Y. Katayama, R. Fukui, T. Miura, Electrodeposition of cobalt from an imide-type room-temperature ionic liquid, J. Electrochem. Soc., 154 (2007) D534-D537. [24]Y. Katayama, R. Fukui, T. Miura, Electrochemical preparation of cobalt nanoparticles in an ionic liquid, Electrochemistry, 81 (2013) 532-534. [25]M. Li, Z.W. Wang, R.G. Reddy, Cobalt electrodeposition using urea and choline chloride, Electrochim. Acta, 123 (2014) 325-331. [26]A. Cojocaru, M.L. Mares, P. Prioteasa, L. Anicai, T. Visan, Study of electrode processes and deposition of cobalt thin films from ionic liquid analogues based on choline chloride, J. Solid State Electr., 19 (2015) 1001-1014. [27]T.L. Manh, E.M. Arce-Estrada, M. I.Mejía-Caballero, J. Aldana-González, M. Romero-Roma, M.Palomar-Pardavé, Electrochemica synthesis of cobalt with different crystal structures from a deep eutectic solvent, J. Electrochem. Soc., 165 (2018) 285-290. [28]M.A.M. Ibrahim, R.M. Al Radadi, Noncrystalline cobalt coatings on copper substrates by electrodeposition from complexing acidic glycine baths, Mate. Chem. Phy., 151 (2015) 222-232 [29]L.H. Skibsted, J. Bjerrum, Studies on cobalt(II) halide complex fornation. II. cobalt(II) chloride complexes in 10M perchloric acid solution, Acta Che. Scan. A,

ACCEPTED MANUSCRIPT 32 (1978) 429-434. [30]J.M. Hartley, C.M. Ip, G.C.H. Forrest, K. Singh, S.J. Gurman, K.S. Ryder, A.P. Abbott, G. Frisch, EXAFS study into the speciation of metal salts dissolved in ionic liquids and deep eutectic solvents, Inorg. Chem., 53 (2014) 6280-6288. [31]A.A. Kityk, D.A. Shaiderov, E.A. Vasil'eva, V.S. Protsenko, F.I. Danilov, Choline chloride based ionic liquids containing nickel chloride: Physicochemical properties and kinetics of Ni(II) electroreduction, Electrochim. Acta, 245 (2017) 133-145. [32]H. Yang, R.G. Reddy, Electrochemical deposition of zinc from zinc oxide in 2:1 urea/choline chloride ionic liquid, Electrochim. Acta, 147 (2014) 513-519. [33]D.Y. Yue, Y.Z. Jia, Y. Yao, J.H. Sun, Y. Jing, Structure and electrochemical behavior of ionic liquid analogue based on choline chloride and urea, Electrochim. Acta, 65 ( 2012) 30-36. [34]T. Theophanides, P.D. Harvey, Structural and spectroscopic properties of metalurea complexes, Coordin. Chem. Rev., 76 (1987) 237-264 [35]J.Q. Ni, K.Q. Han, M.H. Yu, C.Y. Zhang, The influence of sodium citrate and potassium sodium tartrate compound additives on copper electroceposition, Int. J. Electrochemi. Sci., 12 (2017) 6874 -6884. [36]A.M.P. Sakita, R.D. Noce, C.S. Fugivara, A.V. Benedetti, On the cobalt and cobalt oxide electrodeposition from a glyceline deep eutectic solvent, Phys. Chem. Chem. Phys., 18 (2016) 25048-25057. [37]T.L. Manh, E.M. Arce-Estrada, M. Romero-Romo, I.Mejía-Caballero, J. Aldana-

ACCEPTED MANUSCRIPT González, M.Palomar-Pardavé, On wetting angles and nucleation energies during the electrochemical nucleation of cobalt onto glass carbon from a deep eutectic solvent, J. Electrochem. Soc., 164 (2017) 694-699. [38]A.J. Bard, L.R. Faulkner, Electrochemical methods: fundamentals and applications, John Wiley & Sons, New York, 2nd edn, 2001. [39]J.T. Hinatsu, F.R. Foulkes, Electrochemical kinetic parameters for the cathodic deposition of copper from dilute aquesous acid sulfate solutions, Can. J. Chem. Eng., 69 (1991) 571-577. [40]R. Nagaishi, M. Aeisaka, T. Kimura, Y. Kitatsuji, Spectroscopic and electrochemical properties of europium(III) ion in hydrophobic ionic liquids under controlled condition of water content, J. Alloys Compd., 431 (2007) 221225 [41]Q.Q. Yang, K.R. Qiu, D.R. Zhu, L.C. Sa, Electrodeposition of cobalt and rare earth-cobalt in urea-NaBr-KBr melt, Electrochem., 1 (1995) 274-277 [42]C. Su, M. An, P. Yang, H. Gu, X. Guo, Electrochemical behavior of cobalt from 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid, Appl Surf Sci, 256 (2010) 4888-4893. [43]T. Tsuda, L.E. Boyd, S. Kuwabata, C.L. Hussey, Electrochemistry of copper(I) oxide in the 66.7-33.3 mol% urea-choline chloride room temperature eutectic melt, J. Electrochem. Soc., 157 (2010) 96-103. [44]L.H. Mendoza Huizar, C.H. Riosreyes, M. Rivera, Electrodeposition of cobalt nanoclusters from ammonical chiloride solutions onto hopg electrodes. A

ACCEPTED MANUSCRIPT kinetical and morphological study, J. Chil. Chem. Soc., 62 (2017) 3621-3626. [45]G. erkiewicz, F. Perreault, Z.R.Hrapovic, Effect of temperature variation on the under-potential deposition of copper on Pt(111) in aqueous H2SO4, J. Phys. Chem. C, 113 (2009) 12309-12316. [46]M. Lovrić, M. Hermes, F. Scholz,The effect of the electrolyte concentration in the solution on the voltammetric response of insertion electrodes, J. Solid State Electr., 2 (1998) 401-404. [47]A. Kahoul, F. Azizi, M. Bouaoud, Effect of citrate additive on the electrodeposition and corrosion behaviour of Zn–Co alloy,Trans. IMF, 95 (2017) 106-113. [48]L.P. Zhou, Y.T. Dai, H. Zhang, Y.R. Jia, Nucleating and growth of bismuth electrodeposition from alkaline electrolyte, Bull. Korean Chem. Soc., 33 (2012) 1541-1546 [49]Y.F. Lin, I.W. Sun, Electrodeposition of zinc from a Lewis acidic zinc chloride1-ethyl-3-methylimidazolium chloride molten salt, Electrochim. Acta, 44 (1999) 2771-2777. [50]S. Salomé, M.M. Pereira, E.S. Ferreira, Tin electrodeposition from choline chloride based solvent: influence of the hydrogen bond donors, J. Electroanal. Chem., 703 (2013) 80-87. [51]B. Scharifker, G. Hills, Theoretical and experimental studies of multiple nucleation, Electrochim. Acta, 28 (1983) 879-889. [52]X.L. Xie, X.L. Zou, X.G. Lu, C.Y. Lu, H.W. Chen, Q. Xu, Electrodeposition of

ACCEPTED MANUSCRIPT Zn and Cu-Zn alloy from ZnO/CuO precursors in deep eutectic solvent, Appl. Surf. Sci., 385 (2016) 481-489

Fig. 1. Electrolyte containing mixtures of ChCl-urea and CoCl2 with various Co(Ⅱ) concentrations. (room temperature)

5

λ1=235.5nm

λ1

4

Absorbance

Absorbance

4

3

2

0 0.0002

λ2 0.0004

0.0006

0.0008

0.0010

-1

C(Co(Ⅱ ))/molL

2

0.00100mol/L Co(Ⅱ ) 0.00075mol/L Co(Ⅱ ) 0.00050mol/L Co(Ⅱ ) 0.00025mol/L Co(Ⅱ ) λ2=638nm

1

0 200

300

400

500

600

700

800

Wavelength/nm

Fig. 2 UV-Vis absorption spectra of electrolytes containing cobalt ions at various concentrations

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Table 1. Standard stretching and bond vibration IR frequencies of different functional groups of choline chloride and urea. Functional Group

Characteristic Absorption Frequency(cm-1)

Intensity

-CCO

955

Strong

-R4N+

970-1360

Medium

-CH3

1360-1474

Strong

-RNHC=ONHR

1620-1668

Strong

-NH2

3202-3444

Strong

100

2208

Transmittance/%

80 1281

60

865

787

40

1165 1080

956

20

534

1450

587

0.30mol/L Co(Ⅱ ) 0.20mol/L Co(Ⅱ ) 0.05mol/L Co(Ⅱ ) Cobalt-free DES

0 500

1000

1500

1668 1622

3205

3347

2000

2500

Wavenumber/cm

3000

3500

4000

-1

Fig. 3. Infrared absorption spectrum of choline chloride-urea blank system and electrolytes

ACCEPTED MANUSCRIPT containing cobalt ions at various concentrations

0.003

0.002

j/Acm

-2

0.001

a-40mV/s b-50mV/s c-60mV/s d-80mV/s e-100mV/s f-120mV/s

0.000

-0.001

-0.002 -1.4

starting point

a b c d e f -1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

E vs. Ag/V

Fig. 4. Cyclic voltammograms of electrolytes containing CoCl2 at various scan rates. (T=373 K, c(CoCl2)=0.05 M)

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0.0022

0.0020

-jp/A cm

-2

0.0018

0.0016

0.0014

0.0012 0.18

0.20

0.22

0.24

0.26 1/2

0.28

0.30

0.32

0.34

0.36

-1

v (v/V s )

Fig. 5. The curve of cathodic peak current densities against the square root of the scan rates. (T=373 K, c(CoCl2)=0.05 M)

0.003

a-343K b-353K c-363K d-373K e-383K

0.002

j/A cm

-2

0.001

0.000

a b c d e

-0.001

stating point

-0.002 -1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

E vs. Ag/V

Fig. 6. Cyclic voltammograms of electrolytes containing CoCl2 at various temperatures. (scan rate=50 mV s-1, c(CoCl2)=0.05 M)

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0.002

a-0.01mol/L b-0.05mol/L c-0.10mol/L

j/A cm

-2

0.001

0.000

a -0.001

starting point

b c

-0.002 -1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

E vs. Ag/V

Fig. 7. Cyclic voltammograms of electrolytes containing CoCl2 at various concentrations. (T=373 K, scan rate=50 mV s-1)

0.005

-0.96V -0.94V -0.92V -0.90V -0.86V -0.70V

0.004

-i/A

0.003

0.002

0.001

0.000

0

2

4

6

8

10

t/s

Fig. 8. Potentiostatic chronoamperometric curves of electrolytes containing CoCl2 on the tungsten electrode. (T=373 K, c(CoCl2)=0.05 M)

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1.0

0.8 Continuous nucleation (i/im)

2

0.6 -0.70V -0.86V -0.90V -0.92V -0.94V -0.96V

0.4 Instantaneous nucleation

0.2

0.0 0.0

0.5

1.0

1.5

2.0

t/tm Fig. 9. Dimensionless (i/im)2-t/tm plot of ChCl-urea-CoCl2 electrolyte on tungsten electrode. (T=373 K, c(CoCl2)=0.05 M)

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Fig. 10.Scanning electron micrographs of cobalt electrodeposited obtained at different cathodic potentials: (a) -0.85 V, (b) -0.90 V, (c) -0.95 V and (d) -1.00 V. (T=373 K, c(CoCl2)=0.05 M) (The upper right corner of the image is a 20000× magnification image of the sample)

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Fig. 11.Scanning electron micrographs of cobalt electrodeposits obtained at different temperatures: (a) 343 K, (b) 363 K, and (d) 373 K. (electrodeposition potential=-0.90 V, c(CoCl2)=0.05 M) (The upper right corner of the image is a 20000× magnification image of the sample)

Fig. 12. EDS spectrum of the cobalt electrodeposits obtained at -0.9 V and 343 K

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

Intensity



 

 — Co

 — Cu 

 

Ref. Pattern: Copper, 00-004-0836 Ref. Pattern: Cobalt, 01-089-4308

20

30

40

50

60

70

80

90

2θ/degree Fig. 13. XRD pattern of the cobalt electrodeposit obtained at -0.9 V and 343 K

100