Study On The Electrodeposition Of Metals From Amide Based Room Temperature Molten Salts

Recently,the room temperature molten salts have received much attention because of their cheap price,low melting point,high solubility of metal chloride,and high conductivity.Therefore,the room temperature molten salts can be used as potential substitutes for high temperature molten salts,which allows the metals or alloys can be obtained at room temperature.All these advantages have allowed the room temperature molten salts to be an ideal supporting electrolyte for metal electrodeposition.Among various room temperature molten salts,ionic liquids used for the electrodeposition of metals or alloys have been limited due to their high price,low solubility of metal chloride,and potential toxicity.In addition,the electrodeposition of reactive metals from the deep eutectic solvents has not been found.Based on the above mentioned shortcomings of ionic liquids and deep eutectic solvents as mediums for electrodeposition,two kinds of novel room temperature molten salts,urea-acetamide-LiBr and AlCl₃/amide,have been developed and used for both inert metals and aluminum electrodeposition for the first time in this paper.The electroreduction and nucleation mechanisms of cobalt,nickel,and aluminum were investigated by cyclic voltammetry and chronoamperometry techniques.Meantime,the liquidus temperature,electrical conductivity,density,viscosity,and electrochemical window of the room temperature molten salts were also studied.The urea-acetamide-LiBr,urea-acetamide-LiCl,and urea-acetamide-LiF systems,with the following composition:36.6mole%urea,52.8mole%acetamide,and 10.6 mole%lithium halide?LiBr,LiCl,and LiF?,have liquidus temperatures of 40.5?,45.7?,and 65.5?,respectively.Within the temperature range of 313K to 373K,electrical conductivities of urea-acetamide-LiBr?36.6mole%-52.8mole%-10.6mole%?and AlCl₃/amide systems increase with increasing temperature,and the data are fit to a second order polynomial equation.Densities of urea-acetamide-LiBr?36.6mole%-52.8mole%-10.6mole%?and AlCl₃/amide systems also increase with increasing temperature,and the data are fit to a linear equation.In addition,the electrical conductivity and density of AlCl₃/acetamide system increase with increasing molar ratio of AlCl₃/acetamide.Within the temperature range of 313K to 353K,the viscosity of urea-acetamide-LiBr?36.6mole%-52.8mole%-10.6mole%?system decreases with increasing temperature,and the In? increases linear with increasing T^-1.The neat urea-choline chloride?66.7mole%-33.3mole%?,urea-acetamide-LiBr,urea-acetamide-LiCl,urea-acetamide-LiF,and urea-propionamide-LiBr?36.6mole%urea-52.8mole%amide-10.6mole%lithium halide?LiBr,LiCl,and LiF??systems have electrochemical windows of approximately 2.5V,2.2V,2.5V,2.6V,and 2.2V,respectively,at 353K.They have the identical cathodic limit due to the same reaction:R-CO-NH₂+e^-?1/2H₂+R-CO-NH^-?R=NH₂,CH₃?.However,the anodic limits depend on the oxidation potential of anions,and they will increase with the oxidation potential of anions become more positive.AlCl₃/amide and urea-amide-LiX?X=F,Cl,Br?-AlCl₃?16.9 mole%urea-24.4mole%amide-4.9mole%lithium halide?LiBr,LiC₁,and LiF?-53.8 mole%AlCl₃?systems have the identical cathodic limit due to the same reaction:4 Al₂Cl₇-+3e^-?Al+7 AlCl₄^-.Among them,AlCl₃/amide and urea-amide-LiX?X=F,Cl?-AlCl₃ systems have electrochemical window of approximately 2.5V at 333K,whose anodic limits depend on the oxidation potential of AlCl₄-,while urea-amide-LiBr-AlCl₃ systems have electrochemical window of approximately 1.9V at 333K,whose anodic limits depend on the oxidation potential of Br-.Urea-choline chloride?66.7mole%-33.3mole%?and urea-acetamide-LiBr?36.6 mole%-52.8mole%-10.6mole%?systems were used as electrolytes to study the electrodeposition of cobalt on a tungsten electrode.Co???was introduced into the salts by the addition of CoCl₂.Cyclic voltammetry studies demonstrated that the electrode reactions are CoCl_x^2-x+2e^-?Co+xCl^-,[CoCl₂·xBr]x^-+2e^-?Co+xBr-+2Cl^-,respectively.Chronoamperometry experiments revealed that the electrodeposition of cobalt from the two systems proceed via three-dimensional?3D?progressive nucleation,instantaneous nucleation,respectively.The average values of the diffusion coefficient of Co?II?ions in urea-choline chloride and urea-acetamide-LiBr obtained from the Cottrell equation,1.1×10^-6cm²/s,2.2×10^-6cm²/s,are in good agreement with the data measured from nucleation data.The values of the saturation number density and the number density of nucleation of Co???ions at different potentials can be calculated using the corresponding equations.The results showed that both of them increase with increasing overpotential.Urea-choline chloride?66.7mole%-33.3mole%?and urea-acetamide-LiBr?36.6 mole%-52.8mole%-10.6mole%?systems were used as electrolytes to study the electrodeposition of nickel on a tungsten electrode.Ni???was introduced into the salts by the addition of NiCl2.Cyclic voltammetry studies demonstrated that the electrode reactions are NiCl_x^2-x+2e^-?Ni+xCl^-,[NiCl₂·xBr]^x-+2e^-?Ni+xBr^-+2Cl^-,respectively.Chronoamperometry experiments revealed that experimental plots of urea-choline chloride system deviate from the limit cases of instantaneous and progressive nucleation with diffusion-controlled growth model,displaying a behavior intermediate to instantaneous and progressive model,while the electrodeposition of nickel from urea-acetamide-LiBr system proceeds via three-dimensional?3D?progressive nucleation.The average value of the diffusion coefficient of Ni???ions in urea-acetamide-LiBr obtained from the Cottrell equation,1.6×10^-6cm²/s,is in good agreement with the data measured from nucleation data.The values of the saturation number density of nucleation of Ni???ions at different potentials can be calculated using the corresponding equations.The results showed that the saturation number density increases with increasing overpotential.AlCl₃/amide and urea-acetamide-LiBr-AlCl₃ systems were used as electrolytes to study the electrodeposition of aluminum on a tungsten electrode.The homogeneous,transparent,yellow or brown colored,and free flowing AlCl₃/acetamide mixtures were formed only within the molar ratio range of 1.0 to 1.5.Cyclic voltammetry studies indicated that the reduction potentials of Al???ions are dependent on the molar ratio of AlCl₃/acetamide,AlCl₃/?urea+acetamide?,respectively.Within the molar ratio range of 1.1 to 1.5,Al???can be reduced to Al,and the reduction onset potentials of Al???ions shift toward more positive values with increasing molar ratio of AlCl₃/acetamide and AlCl₃/?urea+acetamide?,indicating that Al?III?ions are easier to be reduced.The cathodic reaction is 4 Al₂Cl₇^-+3e^-?Al+7 AlCl₄^-.Chronoamperomet-ry experiments revealed that the electrodeposition of aluminum from urea-acetamide-LiBr-AlCl₃ system,with the following composition:16.9mole%urea,24.4mole%acetamide,4.9mole%LiBr,and 53.8mole%AlCl₃,proceeds via three-dimensional?3D?instantaneous nucleation.Raman spectra results showed that the complex structures are dependent on the molar ratio of AlCl₃/acetamide.At a molar ratio of 1.0,only AlC₁₄ complex is found in the AlCl₃/acetamide system.Within the molar ratio range of 1,1 to 1.5,both AlCl₄^-and Al₂Cl₇^-complexes are found in the AlCl₃/acetamide system,the content of AlCl4-will decrease and Al₂Cl₇^-will increase with increasing molar ratio of AlCl₃/acetamide.Similar results have been reported in the AlCl₃/EMImCl ionic liquid.In addition,the Raman shifts corresponded to main vibration peaks for AlCl₄ and Al₂Cl₇^-are located in 347cm^-1,310cm^-1,respectively.The SEM and XRD analysis revealed that silver colored,compact,adherent,and high-purity aluminum layers were obtained from systems in the absence of urea?such as AlCl₃/acetamide,AlCl₃/propionamide,and AlCl₃/butyramide systems?,while black colored,porous,and non-adherent aluminum layers were obtained from systems in the presence of urea?such as AlCl₃/urea,urea-acetamide-AlCl₃,and urea-acetamide-LiBr-AlCl₃ systems?.It is worth mentioning that urea-acetamide-LiBr and AlCl₃/amide systems are typical but not the only developed amide based room temperature molten salt.By varying the compound of the molten salt,such as amide and lithium halide,various room temperature molten salts can be developed.In addition;,besides the investigated metals of this paper,the amide based room temperature molten salts can be used as mediums for various inert and reactive metals electrodeposition.The detailed discussion is shown in the "future work" section of this paper.