Electrodeposition of Aluminum on Cathodes in Ionic Liquid Based Choline Chloride / Urea / ALCL 3

Electrodeposition of aluminum on metal at low temperatures, with the participation of the ionic liquid electrolyte will contribute to solving many technical issues in critical industries such as automobiles, aircraft, electronics ... the industry uses aluminum and aluminum coated materials most.Using ionic liquids based on choline chloride and urea to electrodeposition of aluminum is more published in scientific journals in the world. In this article, we present the preliminary results of electrodeposition of aluminum on cathodes as copper, iron and especially glassy carbon. The nature of electrolytes based on choline chloride and urea were analyzed and identified. The solubility of aluminum chloride in this mixture also surveyed. The results showed that only choline chloride mixture / urea with a mole ratio of 1:1 solution is to create transparent and recrystallized at room temperature. The nature of electrolytes based on choline chloride and urea were analyzed and identified. The solubility of aluminum chloride in this mixture also surveyed. The results showed that only choline chloride mixture / urea with a mole ratio of 1:1 solution is to create transparent and recrystallized at room temperature. Solubility of aluminum chloride in this system is 5% by weight. So in the study of aluminum deposited on the cathode, we use only liquid choline chloride / urea / AlCl3 at the rate of 1 mol / 1 mol + 5% by weight of AlCl3. The electroplating are made with temperatures from 90-120 ° C, the time is 2 hours. Distance between two poles is 2 cm, the maximum of surface area 3 cm voltage between the two poles from 1-3 V and amperage stable at 100 150 mA. Nitrogen environment are set to limit the influence of oxygen and hydrogen. The results of the analysis on the cathode structure of aluminum films by XRD, SEM, XRF, EDS ... showed similarities with the aluminum anode. The purity of the aluminum film obtained depending on the materials used as the cathode. The purity of aluminum on the GC-cathode is 92.42 % atom, on iron is 70.33 % atom and on copper is 82.63 % atom.


Introduction
Research using ionic liquids for electrodepositing of aluminum was Hurley FH and Wier TP, published in 1951 [Hurley F. H., Wier T. P.].Accordingly, the authors have proposed the use of low-temperature molten salt, formed by the combination of aluminum chloride anhydrous salt with organic compounds such as 1-ethyl-3-methylimidazolium chloride [EMim]Cl as the electrolyte for aluminum electroplating process at low temperatures.Ionic liquids based on AlCl 3 , can be considered as the first generation of ionic liquids.
The ionic liquid has the advantage over the previous generation using ionic liquids as Tetrafluoroborate anion [BF4] ¯ or hexafluorophotphat [PF6] ¯ hydrophobic by nature and their stability.In addition, during electrodeposion do not appear toxic HF gas for environmental.
In the early years of the 20-th century, a new ionic liquids based on mixtures of quaternary ammonium salt-(2-hydroxy-ethyl-trimethyl ammonium chloride) with a number of substances such as hydrogen bonding amide, glycol or acid carboxylic acid (mono-diacid) -to form a solvent capable of dissolving the metal salt and oxide, low cost and ease of recycled or biodegradable without causing pollution.
This new ionic liquids are called DES (deep eutectic solvent).The first publication of this ionic liquid system is of Abbott et al from 2001 to present.In some applications of DES, for electroplating applications are particularly interested in research, which plated silver, copper, zinc, nickel, chromium ... as well as their mixtures have been published in the past 5 years.The results of this study showed that ionic liquids DES system is not affected by air and humidity should stabilize in the electroplating process.In addition, the use of DES does not need to add the strong inorganic acids as the other ionic liquid system -factors affecting the environment.It can be seen that the potential of the use of DES in the metal electrodepositing are positive and some areas have been able to the industry.However, the use of DES for aluminum electrodepositing almost many unresolved issues, the number of related works have not been published, whether or not the works mentioned in the present.
The preliminary results of the study we aim to use DES for aluminum plating on the surface of various materials shown appearance aluminum layer structure similar to the structure of the aluminum anode.

Experimental Methodology
All the experimental works of this research were carried out in a natural environment of the laboratory except the electrodeposition process is performed in nitrogen atmosphere.

Synthesis and Characterization of DESs
The eutectic solvents formed by following the method previously described in many publications [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30].In this study, choline chloride and urea are mixed in a glass flask according to the molar ratio, respectively, 1: 1; 1: 2 and 2: 1.The flask was mounted with a magnetic stirrer and heated by oil bath.Nitrogen is added to the flask to remove water vapor and air.Determine the temperature and the time is completely dissolved and the flask put out oil bath, retain nitrogen environment for at least 1 hour.Then, the mixture was dried overnight at 100 °C in a vacuum oven to obtain the final product and prior to the next experiment.

Measurement of Solubility's and Physicochemical Properties of Electrolytes of AlCl3 in DES
Measurement of the solubility of AlCl 3 in the DES solution was performed by stirring 5% (each 0.015 mol) of AlCl 3 in 40 g 0f DES (each 0.2 mol) and gradually increase the temperature until dissolved AlCl 3 in DES.Let cool at room temperature and measuring the physical properties of electrolyst.Same as above with AlCl 3 concentration of 10%, 15% and 20% (corresponding to 0.03 mol, 0.045 mol and 0.06 mol AlCl 3 )

Study on Cyclic Voltammetry of Electrolyte
In the determination of cyclic voltammograms were performed using an electrochemical workstation Potential -Galvan stat ((PGS-10-Institute of chemistry of Vietnam academy of science and technology-VAST).Platinum working electrode and platinum plate counter electrode were used.A non-aqueous Ag+/Ag electrode, containing the 0.1 mol.L −1 solution of AgNO 3 in KCl was employed as reference electrode (RE).As for the electrodeposition experiments, the working, counter and reference electrodes were iron, copper foil, and glassy carbon plate, respectively.Cyclic voltammograms are investigated with Choline Chloride -urea 1:1, and choline chloride / urea / AlCl 3 at a temperature of 30 o C, amperage of 50 mV/s.

Electrodeposition of Aluminum on Cathode
The electrodeposition experiments of aluminum on the surface of foil copper, iron foil and plate GC was conducted by galvannostatic method.Electrolyte (cholinechloride / urea / AlCl 3 ) is heated by oil bath at a temperature of 80 -95°C, the electrical current is 80 -95 mA and time was 2 h.After electroplating, the cathode was washed with acetonitrile, deionized water and finally dried and determined the weight.Aluminum layer deposited on the cathode was analyzed and determined by methods such as XRD, EDS, SEM.

Soluble Choline Chloride and Urea
-Soluble Choline Chloride and Urea at the rate of 1:1 in mole ratio is at 60 o C temperature, dissolved time of 5 minutes.The mixture was transparent and did not crystalized when cooled to room temperature.
-Soluble Choline Chloride and Urea at the rate of 2:1 in mole ratio is at 60 o C temperature, dissolved time of 5 minutes.The mixture was transparent and did not crystalized when cooled to room temperature.
-Soluble Choline Chloride and Urea at the rate of 1:2 in mole ratio is at a temperature of 75 o C, time is 8 minutes completely dissolved.The mixture was transparent, and crystallized fastly when the temperature starts to decrease (less than 75 o C temperature).
Through the survey results of physical properties of the solution depends on the rate of choline chloride as table 1, we chose the DES solution with a choline chloride / urea is 1:1 to carry out research steps next.

Solubility
AlCl 3 (99.99%)was dissolved slowly in Choline Chloride/Urea with stirring at room temperature in a glove box under nitrogen atmosphere and heated to complete dissolution.F i g u r e 1(a, b, c) are pictures sowed solution after dissolving AlCl 3 in DES (1:1) 24 hours and let cool at room temperature.
Choline chloride 1:1 + 5% AlCl 3 solution: after dissolving milky-colored liquid, allow to cool at room temperature after 24 hours with no salt crystallization, the solution remains opaque white (Figure 1 a).
Choline chloride 1:1 + 10 % AlCl 3 solution after dissolving milky-colored liquid, allow to cool at room temperature after 24 hours with no salt crystallization, the solution remains opaque white (Figure 1b).

Choline chloride 1:1 + 10 % AlCl 3 solution
Salt AlCl 3 start fading into Cho-urea solution (1:1) at temperatures up to 120 o C still not completely dissolved salt, heated to 140 o C continue to remain undissolved salt added.Let cool at room temperature after 24 hours, the mixture separated layers (Figure 1c).When the presence of AlCl3, Cycle voltammograms picture on the working electrode ( WE ) of electrolyte changes considerably better for GC , worse for cupper and iron ( figure 3 ).
While limits of GC cathode pulled down to -3.5 V, the limit of copper cathode and iron only -0.43 V and -0.875 V, respectively.On the other side, the anode limit of the GC reduced at 2 V compared with 1.5 V and of Iron from -0.2 V to -0.625 V, respectively.Meanwhile, copper for WE is occurring phenomenon increased from -0.6 V to -0.42 V.The results of electrolyte ECW on making the presence of 5 % AlCl 3 increased from 4 V to 5 V for the GC, was reduced from 0.8 V to 0.1 V for copper and from 0.8 V to 0.1 V for iron (Table 4 ).The results showed that, for choline chloride electrolyte / urea 1:1 + 5% AlCl 3 can be used for electro-deposition of alumina on the aforementioned electrodes with very little amperage.This is consistent with some previous studies for other ionic liquids.Osteryoung and Welch proved that the aluminum deposition process can be reversed (in an ionic liquid butylpyridinium chloride/AlCl 3 ) [31].Deposition occurred at -0.43 V and oxidation was observed at -0.22 V. Lay and Skyllas-Kazacos [32] studied the electrodeposition of aluminum on GC electrode in liquid imidazolium with tetrachloroaluminate at -0.2 V.The same phenomenon also on the iron electrode was observed at 0.2 V.

Electrodeposition of Aluminum on Cathode
Aluminum electro-deposition process was carried out in equipment as described in section 2.5.Some technical indicators of the experiment are shown in Table 5 below.The spectrum showed the highest peak of aluminum and clear.On the cathode surface is GC, because no other metals to purity of the aluminum coating can reach 92%.For iron and copper cathode, these metals covered a part of aluminum layer, should result is decreased purity of aluminum.In addition, we also found a number of  SEM was measured on Ultra-high Resolution Scanning Electron Microscope S-4800 (HITACHI).In Figure 9a is SEM image of cathode copper, taken at 20 micron.Although it has been carefully cleaned but still see the lines on the surface.In Figure 9b is the SEM image of copper cathode taken at 300 nm. Figure 9c shows the aluminum anode surface taken at 300nm.There the similarity of the surface entirely made of aluminum catode (9b) with aluminum layer deposited on a copper cathode (9e-300nm).Figure 9d show aluminum layer deposited on the copper cathode, taken at 20 micron. Fig

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Figure 6.Aluminum coated on Fe electrode

Figure 8 .
Figure 8. XRD pattern of the Al anode and Aluminum electrodeposit on copper cathode

Table 1 .
Physical properties of the solution depends on the rate of choline chloride / urea

Table 5 .
The data of electro-deposition aluminum process on the different electrodes -Also, aluminum oxide can form on the surface of the cathode, where the presence of oxygen or water.This suspicion can be removed because of the water content in the electrolyte solution our very low (Table2) and nitrogen gas was bubbled directly into the solution during electroplating.The EDS analysis results can indicate the following things.

Table 7 .
Component elements exist in aluminum deposited layers on three cathode