Surfactants on Microstructure and Electrochemical Properties of Nickel Cobalt Oxide Prepared by Hydrothermal Calcination Method
As a new type of energy storage device,super capacitor has the advantages of high charge and discharge power,long cycle life and large working range,and has been widely used in various industries.As the performance of super capacitors depends largely on electrode materials,exploring suitable electrode materials will become a critical step in the development of super capacitors.Among various materials,nickel cobalt oxide has received extensive attention due to its excellent electrical conductivity,high capacity,relatively friendly price and minor environmental pollution.At present,the methods for synthesis of nickel cobalt oxide mainly include hydrothermal method,sol-gel method and electrodeposition method.Among them,hydrothermal method has been commonly used due to its simple operation and relatively uniform product distribution.Surfactants are substances that can change the properties of liquids,therefore the application of surfactant in hydrothermal environments can optimize the morphology of products and improve their electrochemical performance.Here the hydrothermal method was applied to synthesize nickel cobalt oxide,which was formed on nickel foam by using cobalt nitrate and nickel nitrate as raw materials,and different types of additives including polyethylene glycol(PEG),hexadecyl trimethyl ammonium bromide(CTAB),and sodium dodecyl sulfate(SDS)were used as surfactants.The aim was to investigate the effects of surfactants on the phase,morphology and electrochemical properties of the synthesized materials.The preparation of nickel cobalt oxide mainly included pretreatment of foam nickel,synthesis of precursor and calcination of the precursor to obtain the final product.The raw material of cobalt nitrate and nickel nitrate were dissolved in deionized water and urea was added to adjust the pH value of the solution,then the surfactant was added to the above solution.After fully stirred,the solution was transferred to the hydrothermal reactor,in which the nickel foam was placed beforehand and acted as the matrix for the growth of reaction product.After that the hydrothermal reactor was sealed and heated in the oven at 120 ℃ for 6 h to obtain the precursor material.Then the precursor was washed,dried and calcined at 300 ℃ for 2 h to get nickel cobalt oxide.The sample was characterized by X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS)and scanning electron microscopy(SEM)to determine the phase,molecular structure and atomic valence,and to observe the micro structure.The electrochemical properties of the sample were analyzed by cyclic voltammetry(CV),galvanostatic charge-discharge(GCD)and electrochemical impedance spectroscopy(EIS).The results showed that the nickel cobalt oxide prepared without surfactant had the morphology of nanowires,but the nanowires distribute unevenly and some of them even aggregate to some extent.Compared with the sample prepared without surfactant,nickel cobalt oxide prepared with PEG(NC-PEG)had relatively slender nanowires,but the distribution of nanowires was relatively sparse;the morphology change of the sample prepared with CTAB(NC-CTAB)was not obvious;the sample prepared with SDS(NC-SDS)showed regular and dense nanowires,with the average length of the nanowires of about 500 nm,the diameter of 30 nm,meanwhile the nanowires grew in the same direction.And the nickel cobalt oxide prepared with SDS also has a larger specific surface area and pore volume.As an anionic surfactant,SDS could provide a template for the growth of nickel cobalt oxide precursor in a hydrothermal environment,ensuring uniform and regular growth of the nanowires.When nickel cobalt oxide was used as electrode material for super capacitor,the regular structure of samples prepared with SDS would conducive to sufficient contact between the electrode material and the electrolyte,therefore promoting the redox reaction and improving the electrochemical performance of the material.Through electrochemical tests,it was found that electrochemical properties of the nickel cobalt oxide samples prepared with surfactant were improved,the improvement degree depending on the type of the surfactant.The specific capacitance of nickel cobalt oxide prepared without surfactant was 356 F·g-1,and the capacitance retention rate after 1000 cycles at 20 A·g-1 was 80％.And dynamic analysis for this sample showed that diffusion control played a major role in the charging and discharging process.For each sample,the ability for charge storage increased with the following order:nickel cobalt oxide prepared without surfactant(NC),NC-CTAB,NC-PEG and NC-SDS,and the specific capacitance of the above samples were 352,378,564 and 1102 F·g-1 respectively.Nickel cobalt oxide prepared with SDS showed bigger improvement in electrochemical performance than with PEG and CTAB.For the sample prepared with SDS,the specific capacitance increased to 1102 F·g-1,the capacitance retention rate increased to 84％after 1000 cycles at a high current of 20 A·g-1,and the charge transfer resistance was 0.47 Ω.The addition of SDS could improve the electrochemical performance of the prepared nickel cobalt oxide,which would provide a way for the preparation of high-performance electrode materials.
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