Study On Electrodeposition Of Zinc And Zinc Alloy In ChCl-Urea Deep Eutectic Solvent

Ionic liquid is an emerging green solvent from the beginning of the 21st century,and is widely used in the field of electrodeposition.As a kind of ionic liquid,the deep eutectic solvent has advantage of higher resource interest rate,less pollution than traditional water solvent,simple preparation conditions,reusable and so on.Therefore,the studies on electrodeposition of zinc and zinc alloys in a deep eutectic solvent system are of great significance.In this paper,the studies on electrodeposition of Zn and Zn-Ni alloys were carried out in a deep eutectic solvent system.Firstly ChCl-Urea deep eutectic solvent was successfully prepared by physical mixing and the structure and physicochemical properties of ChCl-Urea deep eutectic solvent were studied by conductivity test,viscosity test and Fourier transform-infrared spectroscopy（FT-IR）.Then the electrochemical behavior and electrocrystallization mechanism of Zn and Zn-Ni alloys in deep eutectic solvents were studied by cyclic voltammetry（CV）and chronoamperometry（CA）.In addition the effect of process conditions such as temperature,time,deposition potential and Zn/Ni ratio on Zn and Zn-Ni electrodeposits were investigated.The microscopic morphologies and elemental analysis of the Zn and Zn-Ni electrodeposits were obtained by Scanning electron microscopy（SEM）and energy dispersive spectrometer（EDS）.The compositions of the Zn and Zn-Ni electrodeposits were analyzed by X-ray diffraction（XRD）.The main conclusions were drawn as follows.（1）ChCl-Urea deep eutectic solvent can be successfully prepared by physical mixing,which is verified by the result of FT-IR.The conductivity of ChCl-Urea deep eutectic solvent increases with increased temperature,and the viscositydecreases with the increased of temperature.The electrochemical window of ChCl-Urea deep eutectic solvent is 2.04V.（2）The CV curves of ChCl-Urea-ZnCl₂ deep eutectic solvent show that the zinc ion is one-step reduction.The electrode reaction is an irreversible process and controlled by the diffusion step.Ignoring the change in the surface area of electricity due to electrodeposition,the diffusion coefficient can be calculated to be 6.99×10^-6cm²/s.The electrocrystallization process of zinc conforms to the Scharifker-Hill three-dimensional nucleation model,while the test results experimental data does not fit either the progressive or the instantaneous.（3）Temperature plays an important role on the morphology of the electroplated zinc deposit.With the increased temperature of the plating solution,the concentration polarization and the electrochemical polarization reduce to some extent,which in turn causes the deposited particles to become larger.And as time increases,the zinc coating gradually thickens,the deposited particles gradually become larger,and the plating layer is more uniform.（4）Optimum process parameters of zinc electrodeposition in ChCl-Urea deep eutectic solvent are obtained by orthogonal experiments,which is 70°C,deposition potential 1V,ChCl-Urea-（0.5mol/L Zn Cl₂,0.24mol/L Zn O）.The deposition rate of zinc coating using the above optimum process parameters in ChCl-Urea deep eutectic solvent is higher than that aqueous solution.Compared with that in aqueous solution,the corrosion resistance of zinc coating deposited in ChCl-Urea deep eutectic solvent is almost the same,while the morphologies of the electroplated zinc coating are different.（5）The Zn-Ni alloy can be prepared by electrodeposition in ChCl-Urea-Zn Cl₂-Ni Cl₂·6H₂O deep eutectic solvent.There are two reduction peaks,one at-0.80V～-0.90V which refers to the reduction of Ni²⁺,and the other one at-1.10V～-1.25V which refers to the reduction of Zn²⁺.The Zn-Ni co-deposition reaction is an irreversible process and controlled by diffusion.The electrochemical dynamic diffusion coefficient is 2.288×10^-3 cm²/s.The electrocrystallization mechanism of the Zn-Ni alloy in this system follows three-dimensional continuous nucleation growth.（6）Temperature has large effect on the morphology of Zn-Ni co-deposition coating.The morphologies of Zn-Ni coating become more uniform and compact with the increase of temperature.The composition of Zn-Ni co-deposition coating at 50℃,60℃and 70℃are the same,which is Ni₃Zn₂₂.And XRD patterns show the same strong peak（111）preferred orientation at the above temperatures.（7）The SEM analysis show different microscopic morphologies of Zn-Ni electrodeposits obtained under different Zn/Ni ratio condition.The surface of Zn-Ni electrodeposits presents similar features of uniform and compact particles under Zn/Ni1/2 and Zn/Ni 1/1.While some fine cracks appear on the surface of Zn-Ni electrodeposits under Zn/Ni 2/1 and Zn/Ni 5/1.The XRD analysis also show different components of Zn-Ni electrodeposits obtained under different Zn/Ni ratio condition.The diffraction peaks of the deposited layers correspond to the intermetallic compound Ni₃Zn₂₂under Zn/Ni 1/2 and Zn/Ni 1/1.The diffraction peaks of the deposited layers correspond to the intermetallic compound Ni₅Zn₂₁under Zn/Ni 2/1 and Zn/Ni 3/1.While the diffraction peaks of the deposited layers correspond to the intermetallic compound Cu_0.81Ni_0.19under Zn/Ni 5/1.（8）Microscopic morphologies of Zn-Ni electrodeposits obtained under Zn/Ni 1/2at 60℃change with deposition time.The uniform and compact particulaters have covered the surface within 10 min.The particle size grows up gradually with deposition time.When the deposition time reaches 90 min,the surface of the deposit looks like cauliflower structure and some fine cracks appear.The changes of morphologies of Zn-Ni electrodeposits accord with the electrocrystallization mechanism of three-dimensional continuous nucleation growth.The EDS analysis show that content of Zn in the Zn-Ni electrodeposits increase with time.The diffraction peaks of the deposited layers obtained under Zn/Ni 1/2 at 60℃for 60 min and 90 min both correspond to the intermetallic compound Ni₃Zn₂₂ with the same peak（111）,（200）and（220）.