Synthesis And Electrochemical Properties Of Nickel Oxide Hollow Spheres And Composite Oxides

Energy storage has emerged as a key technological challenge in the 21st century.Due to the rapid depletion of fossil fuels and deteriorating environmental pollution,the increasing demand for renewable and clean energy sources has stimulated intense research on energy storage.Supercapacitors,as one of the power sources and energy-storage devices,attract great interest because of fast energy delivery,high power capability,and long durability.The self-assembled flowerlike NiO,NiO/Co₃O₄ core/shell composites,NiO/MnO₂core/shell composites for supercapacitor material have been fabricated by hydrothermal process.The characterizations of the nanoparticles were investigated by X-ray diffraction?XRD?,transmission electron microscopy?TEM?,scanning electron microscopy?SEM?and N₂ adsorption–desorption isotherm.Electrochemical properties were characterized by cyclic voltammetry,galvanostatic charge/discharge measurements,and electrochemical impedance spectroscopy.Three samples were prepared by CO₂ soft template:NiO hollow spheres,Ni O/Co₃O₄composites and NiO/NiO composites.The crystallinity of the samples is well,and great changes have taken place in the size of samples,there are very few scattered debris.The capacitances of the NiO hollow spheres,NiO/Co₃O₄ composites and NiO/NiO composites are269 F·g^-1,280 F·g^-1and 456 F·g^-1 respectively.The charge transfer resistances of the samples are 1.2?,1.5?and 1?respectively.Furthermore,the samples showed excellent cyclic performance after 1000 cycles,and the NiO/NiO composites showed high cycle stability and good reversibility;For the NiO hollow spheres prepared by NH₃ soft template:it is important for the reaction temperature and reaction time.The optimum reaction time is 24 h,and the optimum reaction temperature is 170 ^oC.The NiO hollow spheres are uniform and the average size of the sample is 3-5?m,the electrode displayed superior capacitive performance with large capacitance.The charge transfer resistance is 4?and the total resistance for the sample is 1.9?;After 1000 cycles,the specific capacitance only decreased by 17%.For the NiO/Co₃O₄ core/shell composites:the XRD results indicated the formation of NiO/Co₃O₄ composites;the SEM and TEM images showed that the NiO hollow spheres were intimately decorated by Co₃O₄ nanowires,and the nanowires are composed of interconnected nanoparticles.The average size of NiO/Co₃O₄ composites is 4-7?m,and the size of Co₃O₄nanowires is 1-2?m.The electrode was a reversible process,and had good electrochemical reversibility;the electrode displayed superior capacitive performance with large capacitance(510 F g^-1).Furthermore,the charge transfer resistance is 3.3?and the total resistance for the sample is 0.6?;After 1000 cycles,the specific capacitance only decreased by 8%.For the NiO/MnO₂ core/shell composites:the XRD results indicated the formation of NiO/MnO₂ composites;the SEM and TEM images showed that the NiO hollow spheres were intimately decorated by MnO₂ nanosheets,and the nanosheets are composed of interconnected nanoparticles.The size MnO₂ nanosheets of is about 100 nm.The electrode was a reversible process,and had good electrochemical reversibility;the electrode displayed superior capacitive performance with large capacitance(568 F g^-1).Furthermore,the charge transfer resistance is 0.5?and the total resistance for the sample is 0.5?;After 1000 cycles,the specific capacitance only decreased by 9%.For the self-assembled flowerlike NiO:the XRD results indicated that the P123 inhibits grain growth of NiO;The SEM and TEM images showed that the P123 plays a critical role in the formation of flowerlike NiO.N₂ adsorption-desorption results indicated that the type of pore was type IV and the isotherm displayed a H2 hysteresis loop for NiO/P and H3 hysteresis loop for NiO/WP,and the electrode displayed superior capacitive performance with large capacitance(619 F g^-1),which is 1.8 times of the sample without P123.Furthermore,the charge transfer resistance is 0.25?and the total resistance for the sample is 0.05?;After1000 cycles,the specific capacitance only decreased by 5%.