Electrodeposition of Magnesium in Deep Eutectic Solvents Based on Magnesium Chloride Hexahydrate and Choline Chloride with Urea Added

Magnesium (Mg) alloy has small density, large elastic modulus, good heat dissipation and corrosion resistance to organic matter and alkali. At present, magnesium alloy is more and more used in automotive industry, medical devices and aerospace industry. However, the traditional preparation method of Mg has the disadvantages of high investment, high labor intensity and great environmental pollution. Therefore, it is of great significance to develop simple, environment-friendly methods of the magnesium. In this study, urea was added to adjust the electrochemical property of the deep eutectic solvent (DES) mixed by choline chloride (ChCl) and magnesium chloride hexahydrate (MgCl2·6H2O). Cyclic Voltammetry (CV) curves reveals that the addition of urea made the reduction potential of Mg shifted from -0.9 V to -1.3 V. Among the CV curves, one was proposed as the “dividing line”, which shows that the electroactive species in the two DESs, ChCl-MgCl2·6H2O and urea-MgCl2·6H2O, are different due to the changes of the component of the DESs. Fourier transform infrared (FTIR) data shows the type of hydrogen bond had been changed with the increase of urea content. Furthermore, the Raman spectra indicates that Mg2+ was coordinated with urea chains, which did not exist in ChCl-MgCl2·6H2O. Moreover, it was found that urea changed the electrochemical performance of the ChCl-Urea-MgCl2·6H2O by changing the hydrogen bond in the system and coordination form of the electroactive species, rather than adsorbing onto the electrode surface. Combined with geometry calculations at the B3LYP/6-311++G (d, p), the most probable mechanism of electrodeposition process was deduced.


Introduction
Ionic liquids (ILs) have received extensive interests for electrochemical applications in electrodeposition [1][2][3][4], batteries [5][6][7]and catalysis [8,9].Currently, most studies focus on using imidazolium or pyridinium salts mixed with aluminum chloride for metal deposition [10][11][12], because ILs have unique properties such as extremely low vapor pressure, high ionic conductivity, and wide electrochemical window [13,14] for electrodeposition.However, finding non-toxic and excellent performance IL is still attractive and challenging [15].Recently, the DESs [16] have been studied for electrodeposition not only because they are easily prepared, but also they are water and air insensitive in contrast to the AlCl3-containing ILs.Moreover, the affordable price makes them a promising electrolyte for metal finishing [17][18][19][20].
Mg and its alloys are requisites for structural materials.It has a density of 1.738 g cm -3 [21], which is only 65% compared to aluminum, and 25% for iron [22].However, Mg coatings cannot be electrodeposited in aqueous solutions due to the large negative standard potential of Mg (II)/Mg couple (−2.375V vs. NHE).Nowadays, the primary method to obtain Mg coatings is electrodeposition in high temperature molten salts [23], but high temperature molten salt is highly corrosive, and the investment cost of the electrolytic cell is too high.So far, many researchers focused on Mg electrodeposition in non−aqueous solutions at low temperature [24][25][26].Among them, electrodeposition of Mg in DESs has many advantages such as low operating temperature, high ionic conductivity, and wide electrochemical window.
Previous studies shown DESs can be formed by mixing quaternary ammonium salt, metal chloride, and metal chloride hydrate with hydrogen bond donors, which can be divided into four types [27].Among these DESs, a wide variety of metal chloride hydrate mixed with choline chloride have been reported, including CaCl2• 6H2O, CrCl3• 6H2O, LaCl3• 6H2O, CoCl2• 6H2O [28], which can make the metal finishing easier due to the reduced process of metal salt dehydration.Here we only review some typical systems for Mg electrodeposition.For example, Mg deposition behaviors were demonstrated from ionic liquid analogues based on dimethylformamide (DMF) with MgCl2• 6H2O [26].In addition, a homogeneous, colorless ionic liquid analogous containing ChCl and MgCl2• 6H2O [29] was investigated by FTIR, and its physical and chemical properties such as melting point, viscosity, conductivity, and density were also studied.However, the electroactive species in Mg DESs remain unknown, and Mg deposits obtained from DESs are non-uniform and highly insufficient in thickness.Thus, a detailed and systematic study of the electrochemical behavior of DESs for Mg electrodeposition is necessary.
Therefore, in this study, six kinds of DESs were prepared using MgCl2• 6H2O source for electrodeposition of Mg, which were ChCl−MgCl2• 6H2O, ChCl−urea (mole ratio = 1:0.5,1:1, 1:1.5, 1:2) −MgCl2• 6H2O and urea−MgCl2• 6H2O (figure 1).In order to modify the electrochemical behavior and electroactive species, urea was added into the system.The electrochemical behavior of the system was determined by CV, and the electrochemically active species were verified by FTIR and Raman spectroscopy.Finally, the electrodeposition mechanism of this system was speculated by the experiment data combined with Density functional theory (DFT) calculations.

Cyclic Voltammetry
CVs were conducted in a conventional three-electrode cell in the glove box, and data were collected by an electrochemical workstation (CHI660D, CH Instrument, USA).A Teflon-sheathed Ag disk electrode was used as the working electrode (WE).Prior to experiments, we were polished with aqueous slurry of 0.3 μm alumina, cleaned in an ultrasonic bath for 5 min, and then dried with cold N2.While Mg ribbon was employed as the counter electrode (CE), and Pt wire was used as the reference electrode (RE).The potential range was from 0.1 V of open circuit potential (OCP) to -2V, with a constant sweep rate of 50mV s-1, and all CV curves were conducted at 80℃.

Raman Spectroscopy
Raman samples were sealed well in a quartz container prior to tests in the glove box.They were measured with the laser at an excitation wavelength of 784.9 nm.The laser was focused onto the sample using a 50×super-long working distance objective lens with a numerical aperture of 0.5.The data was collected by Horiba Jobin-Yvon LabRAM HR800 UV-vis μ-Raman equipped with a CCD detector.For the regions of interest, measurements were made with a 600 nm grating from 100 to 2000 cm−1.Spectra were obtained for 10 s five times for adequate signal-to-noise levels.All Raman spectra were acquired at room temperature.

FTIR Spectroscopy
FTIR was carried out on a Nicolet Nexus 380 FTIR spectrometer (Thermo Electron Corporation, Madison, USA) with a spectral range from 500 cm-1 to 4000 cm-1.The DESs samples were prepared as thin liquid films using KBr salt tablets.

Computational Details
All geometry calculations were performed by the Gaussian 09 programs [30] within the IEF-PCM formalism [31,32].Meanwhile, the geometries of all structures were fully optimized with Becke's three-parameter functional with the Lee-Yang Parr correlation (B3LYP) [33] in conjunction with 6-311++G (d, p) basis set [34].DFT has taken the electronic correlation into account, so this method has advantages in dealing with systems containing metals and has become a powerful method to study the chemical reaction pathway and electrochemical process of metals.Density functional calculation was applied in various studies [35][36][37] with a reasonable measure of uniformity between the theoretical and experimental data.

Cyclic Voltammogram
Figure 2 shows CVs of an Ag disc electrode in the ChCl-MgCl2• 6H2O containing various concentrations of urea at 80℃.In this figure, a small peak at ca.-0.9 V is attributed to the reduction potential of Mg2+ in ChCl-MgCl2• 6H2O.As the concentration of urea increases, the initial deposition potentials shift more negatively, and their peaks are gradually obvious from curve "b" to curve "e" until curve "f" shifts to a reduction potential at ca. -1.3V.The increasing concentration of urea causes electrochemical polarization.For metal deposition, a proper electrochemical polarization could gain a high enough over-potential for nucleation, which is required for dense and smooth surface metal coating [38].
Abbott et al. have stated that there are usually two ways for the additive to act in an aqueous system [39].The first one is that organic species as additives are adsorbed onto the surface of the cathode, blocking cluster nucleation and hindering the growth.In another situation, the additives form the coordination complex with the metal ions, which lead to the reduction potential decrease, and makes it more difficult to nucleate.Both ways are beneficial for the nucleation and electrodeposition.Combining the results of CV in figure 2, it can be concluded that the electrochemical polarization is caused by addition of urea, which were similar to the principle of the additives in aqueous solution [40].
According to a previous study of Abbott et al., it can be confirmed that the concentration of urea influences the type of donor ligands in the coordination shells in an ILs system [15].Therefore, ChCl and urea are formed different complexes with MgCl2• 6H2O, and their electroactive species are different as well.Based on figure 2, the trend of the dull curve "a" is similar to curve "b", both curves are smooth and their current density peaks are relatively broad, and the peak occurs at ca.-0.9V.On the other hand, the trend of the curves d, e, f is roughly the same as well, because their current density peaks shift negatively to ca.1.08V,1.12V, and 1.28V respectively, and these peaks also become sharper due to the effect of additional urea.Owing to the difference of electrochemical polarization from curve a to f, the curve "c" is regarded as the "dividing line".From the above results, it is revealed that the concentration of urea has a significant influence on the coordination complex of metal ions.That is to say, urea acts the latter way, and changes the coordination complex of metal ions to obstruct the cluster nucleation.Simultaneously, we assume that the primary form of Mg in the curve "a" and curve "b" below the dividing line is MgClm(H2O)n.Then, as the proportion of urea increases in samples d, e, f above the dividing line, the main form of Mg gradually changed into MgClm(urea)n.
On the whole, when urea was added gradually, the reduction potential of Mg has shifted negatively, and the types of complexes in the system changed.Eventually, one CV curve is proposed as the "dividing line," which obviously shows that the types of complexes are different.Therefore, urea could be used as a complexing agent, improving the cathode polarization and the coating properties.

FTIR Spectroscopy
Figure 3a displays FTIR spectra of urea, MgCl2• 6H2O, urea-MgCl2• 6H2O, respectively.By comparison, a broad peak of absorption band between 3000 and 3600 cm-1 can be attributed to the characteristic absorption of hydrogen bond vibration in urea-MgCl2• 6H2O.The hydrogen bond of O-H from hydrate molecule are the most significant contributors for this peak.Other vibrations of hydrogen bond derived from urea in this system are N-H...N and N-H...O, which also increases the signal, and broads the peak.Compared with the spectra of urea, the frequency of ν C=O in the urea-MgCl2• 6H2O changed from two peaks (1620 and 1680 cm-1) to a broad peak (1640 cm-1), indicating that the hydrogen bond C=O…H-O is formed.
Based on the results, it can be concluded that the DES is formed, because of a significant amount of hydrogen bond.The melting points for urea, MgCl2• 6H2O, urea-MgCl2• 6H2O (mole ratio = 1:1) are 132.7℃,117℃, and 80℃ respectively.The last one was reduced, when the former two have been mixed.Because the formation of the hydrogen bond can reduce the lattice energy when urea and MgCl2• 6H2O complex into a homogeneous liquid mixture.However, a homogenous mixture cannot form just by adding MgCl2 to urea with six molecular equivalents of water, which shows that the hydration from crystal water is critical to controlling the coordination of the magnesium center.
As shown in figure 3b, ChCl-MgCl2• 6H2O also has a broad peak at about from 3100 to 3600 cm-1.The reason is the same as explained above that hydrogen bond exist in ChCl-MgCl2• 6H2O, and the types of bond should be O-H...O and O-H...Cl.Therefore, types of hydrogen bond are different in two DESs.The absorption band at 1480 cm−1 is assigned to the CH3 of ChCl, and the peak at 957 cm−1 is due to νC-C in ChCl-MgCl2• 6H2O.By comparing ChCl-MgCl2• 6H2O with ChCl, the frequency of νC-C rarely changed, indicating that the Ch+ structure is not destroyed [29].In general, compounds that can either donate or accept electrons or protons to form hydrogen bond show high solubility in DES like ChCl-urea [41].FTIR spectroscopy of ChCl-urea (mole ratio = 1:1)-MgCl2• 6H2O and ChCl-urea (mole ratio = 1:2)-MgCl2• 6H2O are shown that the Ch+ structure and hydrogen bond still exist (figure 3c).However, the band shape of IR absorption at 3100 cm-1-3600 cm-1 in ChCl-urea (mole ratio = 1:1)-MgCl2• 6H2O is significantly broader than that in ChCl-urea (mole ratio = 1:2)-MgCl2• 6H2O due to the increasing of urea.
Since a large amount of hydrogen bond is contained in the mixture, it means the MgCl2• 6H2O is well dissolved.Moreover, urea has a significant effect on the hydrogen bond formation, the type of hydrogen bond varies with the concentration of urea.

Raman Spectroscopy
Raman spectroscopy was used to study the species of Mg, as well as to understand the influence of urea on various chemical bonds in the urea-MgCl2• 6H2O and ChCl-urea-MgCl2• 6H2O electrolytes.Regions containing the characteristic peaks of MgCl2• 6H2O and urea are analyzed respectively.By adding into different concentration of urea, the characteristic peaks change in both regions, and new peaks emerge.Two particular regions of interest are discussed in detail.
In figure 4a, the Mg-Cl stretching is observed at 194 cm-1 in MgCl2• 6H2O, which is consistent in various ratios of MgCl2• 6H2O coordinated by urea except the one of the ratio 1:0.  Figure 4b shows the Raman spectra of free and coordinated modes of urea change as the mole fraction of MMgCl2• 6H2O is varied, which can be found in the range of 980-1080 cm-1.The band of urea can be attributed to non-coordinating urea chains, which contains the N-C-N symmetric stretching mode observed at 1022 cm-1 (figure 4b) [42,43].The dominant peak of 1022 cm-1 is also observed in urea-MgCl2• 6H2O (mole ratio = 1:0.5).However, as the concentration of MgCl2• 6H2O increases, Mg 2+ coordination makes the intensities of peaks become weak gradually in that the vibration stretching from unbounded N-C-N groups decreases.This mode is similar to C-O-C stretching modes of the PEG-IL systems [44].When the mole fraction of MgCl2• 6H2O changes, colored lines also show the variations of coordinate peaks from low to high value.The peak appears at ca.1046 cm-1, which does not exist in ChCl-MgCl2• 6H2O (the Raman spectra of ChCl-MgCl2• 6H2O is not listed).However, there is a trend of decreases of coordinated peaks (ca.1046 cm-1) in the urea-MgCl2• 6H2O solutions, while the fraction of MgCl2• 6H2O increases.It may be caused by the instability of the solution when the molar ratio of urea-MgCl2• 6H2O is 1:2 at room temperature.
Figure 5 shows the Raman spectra of the cases of ChCl-MgCl2• 6H2O with adding different amount of urea.It is known that C=O stretching in urea usually locates between 1580-1650 cm-1 [45].As the concentration of urea increases, the intensity of free C=O (peak e) is constant, but the hydrogen bonded C=O group (peak d) increase gradually.It is noted that the peak d occurs at around 1603 cm-1 by the addition of urea, which could be assigned as ChCl-urea-Mg 2+ species.A shoulder peak c appears at 1050 cm-1, which should attribute to the urea-Mg 2+ (see figure 4b).The intensity of the shoulder peaks changes a little, which may be due to the presence of ChCl.As for other peaks a and b, it is strongly suggested that the intensity of ChCl-urea is increased when the urea component is added gradually.Based on the above data, Mg coordination with urea through both O and N cause these modes change, which is a good indication of the complexation of urea.Additionally, the difference in intensity may suggest the concentration of Mg-Cl bonds change in the electrolyte.Some species in the spectrum are difficult to identify.However, the peaks suggest the species of MgClm(urea)n are present due to various changes in coordination by urea and MgCl2• 6H2O.Furthermore, it was found that coordination form of the electroactive species in urea-MgCl2• 6H2O electrolyte is not the same with ChCl-MgCl2• 6H2O.That is to say, the coordination form of ChCl-urea-MgCl2• 6H2O was altered after adding urea.

Mechanism of Electrodeposition Process
DESs synthesized by urea and metal salts have strong hydrogen bond and coordination species [46].Therefore, the interaction in the DESs is more complex, making it harder to understand on a molecular level.A roughly relative stabilization of the urea coordination to MgCl2• 6H2O has been obtained by the Raman spectra, which is [MgClm (H2O) n(urea)p]2-m.In order to further determine the complex species inferred from the above data, a theoretical study is utilized because it is a good tool for understanding the structure and banding of ILs [47].By analysis of the stable conformational structure obtained through DFT method, a reasonable explanation for the electrochemical processes and mechanisms was obtained.Moreover, the interaction between the urea and MgCl2•6 H2O are mainly formed by hydrogen bond from H2O or urea and coordination bond from urea.Therefore, these results demonstrate that hydrogen bond and coordination bond are indeed formed in the Mg DESs.Formula (1) describes the possible substances contained in the synthetic solution.
Based on the data of the experiment and theoretical calculations of the urea-MgCl2• 6H2O electrolyte, possible electrodeposition mechanism of Mg in urea-MgCl2• 6H2O is proposed as the following Formula (2).As the system contains H2O, the reaction at the cathode gives priority to H2 and OH -.A large amount of Cl -in the solution is oxidized to Cl2.Simultaneously, the hydrogen in the urea may be reduced on the cathode [48].The most likely electroactive species of urea-MgCl2• 6H2O electrolyte is [MgCl(urea)2] + and the chemical structural formula of the urea is NH2-CO-NH2.

2[MgCl(NH2-CO-NH2
The system of ChCl-urea-MgCl2• 6H2O form numerous hydrogen bond and coordination bond rather than a simple combination of the component ChCl and urea solutions.The cationic Mg complexes are very likely [MgCl(urea)2] + , [Mg(H2O)(urea)2] 2+ , [MgCl(H2O)(urea)] + as evidenced by a strong contribution in the DFT calculations.At the same time, FTIR spectra indicate that the types of hydrogen bond in ChCl-MgCl2• 6H2O and urea-MgCl2• 6H2O are different, and the coordination form of Mg has changed referring to the Raman spectra.It can be seen that the electroactive species of ChCl-MgCl2• 6H2O is varied from urea-MgCl2• 6H2O, and cationic complex of Mg is coordinate with water and chlorine.When adding urea gradually in ChCl-urea-MgCl2• 6H2O, the reduction potential of Mg has shifted negatively, and the types of complexs in the system changed.It is also found that the FTIR and Raman spectra have a regular change of intensitiy and shape of peaks when increasing the concentration of urea from ChCl-urea (mole ratio = 1:0.5)-MgCl2•6H2O to ChCl-urea (mole ratio = 1:2)-MgCl2• 6H2O.Thus, the adding of urea changes the electrochemical performance of the ChClurea-MgCl2• 6H2O system by changing the hydrogen bond and coordination form of the electroactive species, rather than adsorption onto the electrode surface.

Conclusions
DESs were expected as promising electrolyte for electrodeposition of Mg, so a series of ChCl-urea-MgCl2• 6H2O DESs were synthesized to explore the effect of urea on the electrochemical behavior of the system.CV was used to investigate the reduction potential of the system, and the results revealed that the addition of urea played a polarization role and favored the electrodeposition of Mg.Hydrogen bond was essential to form DESs of ChCl-urea-MgCl2• 6H2O.FTIR and Raman spectra showed that the electroactive species were altered by adding urea, which were attributed to that the addition of urea made the hydrogen bond and coordination form of the system change basically.Furthermore, theoretical calculations further proofed tha the possible electroactive species of Mg were [MgCl(urea)2] +, [MgCl(H2O)(urea)]+, [Mg(H2O)(urea)2]2+.Herein, urea has a significant impact on the electrochemical activity of Mg speciation in a bulk electrolyte, and which is helpful for electrodeposition of Mg.
5. The shapes of peaks of MgCl2• 6H2O coordinated with urea change, comparing with the sole MgCl2• 6H2O.These changes are attributed to the changing of the Mg-Cl concentration in solutions.