Preparation And Electrochemical Properties Of Reduced Graphene-based Transition Metal Electrodes

The development of new energy industry is booming,new energy vehicles and new energy power plants emerge like bamboos after rain.It is inevitable that the electrochemical performance of energy storage devices will be more demanding:the energy storage devices with only high energy density or high power density will be phased out.One of the most important factors determining the performance of energy storage devices is the electrode material.In transition metals,the theoretical pseudo-capacitance performance of cobalt oxide/hydroxide is very good,and it is a favorable candidate for high energy density and high power density electrode materials.1.electrochemical deposition of cobalt hydroxide electrode materials.Co(OH)2 nanocrystalline electrode materials with stacked petals were prepared by electrochemical deposition on nickel foam.Firstly,the mechanism of electrochemical deposition of cobalt hydroxide was investigated by cyclic voltammetry in 0.1 mol·L-1 cobalt nitrate solution.The deposition potential at-0.6 V?-1.0 V was used to prepare electrode material of supercapacitor by electrochemical deposition cobalt hydroxide on nickel foam.Then the influence of two variables(deposition potential and concentration of deposition solution)on the deposition mechanism was investigated by controlling the changes of them.The sediments were characterized by scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD)and X-ray photoelectron spectroscopy(XPS).It was found that the resulting sediments are getting thicker and thicker,with the deposition potential gradually shifted negative from-0.60 V to-0.95 V(interval 0.05 V)and the concentration of deposition solution increased gradually from 0.1 mol·L-1 to 0.9 mol·L-1(interval 0.2 mol·L-1)and the deposited material was cobalt hydroxide.Combined with the results of cyclic voltammetry and the model derived by Scharifker-Hills,it is found that the electrochemical deposition of cobalt hydroxide nucleates on foamed nickel by instantaneous nucleation,and the subsequent growth is controlled by both deposition potential and deposition concentration.High potential and high concentration of Co(NO3)2 can promote the nucleation and growth of Co(OH)2,but decrease the specific surface area of Co(OH)2.The electrochemical measurements of the electrode materials were also carried out by cyclic voltammetry and constant current charge-discharge test.The results show that,The specific surface area of Co(OH)2/NF electrode prepared in 0.1 mol·L-1 Co(NO3)2 solution and-0.65 V was the highest.This special nano-reticular lamellar morphology made the contact surface between the active material layer of electrode material and electrolyte increased exponentially.The specific capacitance of cobalt compounds was measured by cyclic voltammetry and constant current charge-discharge measurements.The results were as follows:Ccv=836 F·g-1 and CGCD=725 F·g-1,respectively.When the electrode material was used for the cycling performance test,it was immersed in 2 mol·L-1 KOH solution and went through 1000 cycles.The specific capacitance increased from 836 F·g-1 to 1116 F·g-1.This is due to the removal of Co(NO3)2(increasing the active center)and the conversion of ?-Co(OH)2 to ?-Co(OH)2(increase in electrode conductivity).2.Preparation of composite electrode materials by electrodeposition.In order to make up the disadvantage of poor electrical conductivity of cobalt hydroxide electrode material,a layer of graphene oxide was deposited on foamed nickel by electrophoretic deposition method,and precursor is obtained by reduced to reduced graphene oxide with thermal reduction at 300?.Then cobalt hydroxide was deposited on the precursor by electrochemical deposition.It has been verified by many experiments that the optimum deposition potential and the concentration of deposition solution were different due to the introduction of reduced graphene oxide coating.Finally,we choose to deposit cobalt hydroxide active nano-coatings on the precursor under the deposition potential of-0.75 V,-0.85 V and-0.95 V in 0.5 mol·L-1 Co(NO3)2 solution.A mesh connected by ultrafine nanosheets electrode material of cobalt hydroxide was prepared.SEM,XRD and XPS is used to characterize sediments.It was found that the introduction of graphene increased the specific surface area and resulted in the refinement of nanocrystalline grains.As a result,the specific surface area is greatly increased.The results of cyclic voltammetry and constant current charge-discharge show that the specific capacitance can reach 1503 F·g-1 at the scanning rate of 3 mV·s-1.After 2000 cycles of cyclic voltammetry,the specific capacitance can still be kept at 81%.The electrochemical performance is excellent.