Preparation And Electrochemical Performance Of Carbon-based NiO/MnO₂ Composite Electrode Materials

The increasing demand for environmentally sustainable energy supplies is presently a topical and urgent issue that requires addressing.In this context,supercapacitors(SCs)have attracted growing attention as alternative energy storage and conversion devices due to their high power density,outstanding reversibility,safe operation,environmentally friendliness,excellent pulse charge-discharge properties,and long cycle lives.With the rapid development of solar energy,wind energy,nuclear energy,ocean energy and geothermal energy,supercapacitors will be an indispensable energy storage device in the future.Firstly,a NiO/Ni/rGO electrode material was prepared by sol-gel method combinated with calcination in nitrogen atmosphere.Complexing agent oxalic acid and citric acid were adopted to prepare two different precursors of NiO/Ni/rGO,respectively.The electrochemical properties of material under different calcination temperature and the influence of adding different amount of graphene under the optimal calcination temperature was investigated.The structure and composition of prepared materials were characterizated by scanning electron microscopy(SEM),X-ray powder diffraction(XRD)and Fourier infrared spectrum(FTIR).Also,cyclic voltammetry(CV),constant current charge and discharge(GCD),ac impedance(EIS)and cycle stability test were used to study the electrochemical performance.Results show that the optimal calcination temperature is 400? using oxalic acid as complexing agent.The specific capacitance of as prepared sample is 1264 F·g-1.Furthermore,the specific capacitance of the composite after adding 5%graphene could reach as high as 1540 F·g-1 with an increased of about 20%.After 1000 cycles of stability test,the retention rate of capacitance increased from 50%to 82%.Meanwhile,using citric acid as complexing agent,the optimal calcination temperature is 350?.The specific capacitance of as prepared sample is 848 F·g-1.After adding 2%graphene,the capacitance of composite could reach as high as 1428 F·g-1 with an increased of about 68%.After 1000 cycles of stability test,the capacitance is still 1157 F·g-1.In general,the electrochemical properties of NiO/Ni/rGO samples prepared with oxalic acid as complexing agent were better than those prepared by citric acid as complexation agent.Secondly,phenolic resin spheres synthesize by the Stober method was activated to prepare carbon spheres after carbonization.The effect of activation time of water vapor and HNO3 treatment on the electrochemical performance of activated carbon spheres was investigated.Then,the composite materials of carbon sphere and MnO2 were prepared by hydrothermal method,using KMnO4 and MnSO4 as manganese source.The influence of mole ratio of KMnO4 and MnSO4 on the structure and electrochemical property of MnO2/carbon sphere compound was investigated.The morphology and composition of various samples were analyzed by thermogravimetric analysis(TGA),energy dispersive spectrometer(EDS),X-ray photoelectron spectroscopy(XPS)and N2 adsorption/Adsorption.The electrochemical performance was investigated by cyclic voltammetry(CV),constant current charge and discharge(GCD),ac impedance(EIS)and cycle stability test.The results show that the spherical activated carbon after water vapor activation of 3 h has the specific surface area of 1432.6 m·g-1.The specific capacitance could reach 145 F·g-1,Furthermore,the specific capacitance of spherical activated carbon after nitric acid treatment increases to 187 F·g-1?Although,MnO2/carbon sphere composites of different morphologies can be used for supercapacitor electrode materials,we found that the core-shell structured MnO2 petal nanosheet@carbon sphere composite exhibited better capacitance performance than other composites.Furthermore,the composite exhibites an excellent electrochemical performance with a specific capacitance of 231 F·g-1 under a current density of 0.5 A·g-1,and also exhibited 96%of their initial capacitance after 1,000 cycles at 10 A·g-1 in a 1 M Na2SO4 aqueous electrolyte.Finally,an in situ nitrogen-doped activated carbon anode material was prepared using the nitrogen-rich precursor.The effect of activation time of water vapor on the electrochemical properties was investigated.And then,the prepared positive electrode material and negative electrode materials were assembled into asymmetric capacitor for electrochemical performance testing.The results show that the nitrogen-doped activated carbon has 5%nitrogen element on the surface after water vapor activation of 2 h.The specific surface area of nitrogen-doped activated carbon could reach 1306.4 m2·g-1,and the specific capacitance is 189 F·g-1,To enhance the energy density of supercapacitor,an asymmetric supercapacitor was prepared by well-matched MnO2 petal nanosheet@carbon sphere as cathode material and nitrogen-doped activated carbon as anode material.The assembled asymmetric supercapacitor delivers a remarkable energy density of 13.0 Wh·Kg-1 at power density of 2862 W·Kg-1.Also,it has a high potential window of 1.9 V as well as nearly 100%of the charge and discharge efficiency.