Preparation And Supercapacitor Performance Of Hollow Carbon Sphere Based Nanomaterials

With the increase of energy consumption and environmental pollution,the development and utilization of new energy is the primary problem facing humanity.Currently,the use of green energy such as solar or wind energy is growing rapidly,and further developments in this area require intelligent and versatile energy storage devices,such as lithium ion batteries,sodium ion batteries,lithium sulfur batteries and supercapacitors.Therefore,the development of electrode materials has become one of the key issues in energy utilization.Transition metal oxides or sulfides have attracted extensive attention as electrode materials for supercapacitors due to their high theoretical capacitance.However,due to the relatively small electrochemical surface area,poor conductivity and less electrolyte ion channels of the transition metal oxide or sulfide,its practical application is hindered.In order to improve the utilization of electrode materials,nanostructured design and carbon matrix recombination are two means to increase the capacitance.As a member of the carbon family,hollow carbon spheres?HCS?have the advantages of high specific surface area,low density,excellent electrical conductivity,adjustable porosity and good mechanical strength.They are one of several excellent carbonaceous substrates.The transition metal oxides/sulfides are combined with the hollow carbon spheres to achieve nano-structure and conductivity of the electrode material structure,thereby achieving the purpose of increasing the capacitance.We prepared a series of hollow carbon sphere?HCS?based electrode materials by direct growth,chemical bath deposition and template-engaged methods.The synthetic mechanism of the composites was explored and the microstructure was studied for the effect of capacitor performance.The main research contents and conclusions are as follows:?1?A composite of manganese oxide?MnO₂?and N-doped hollow carbon sphere?NHCS?was fabricated by a facile two-step process for supercapacitor electrodes.The MnO₂-NHCS composite had a NHCS core and a shell composed of hierarchical birnessite-type MnO₂ nanoflakes.NHCS in the composite not only serves as the template for the growth of MnO₂ nanoflakes,but also as the electrically conductive channel.The physicochemical and electrochemical properties of the MnO₂-NHCS composite were significantly enhanced as compared with those of MnO₂ hollow spheres?MnO₂ HS?.The asymmetric supercapacitors?ASCs?assembled with MnO₂-NHCS anode and NHCS cathode exhibited a high energy density of 26.8 Wh kg^–1 at a power density of 233 W kg^–1,which is superior to that of the ASCs assembled with MnO₂ HS anode and NHCS cathode(13.5 Wh kg^–1 at 229 W kg^–1).The MnO₂-NHCS ASCs also show superior cycling stability for 2000 cycles.The enhanced electrochemical performance of MnO₂-NHCS makes it a promising electrode material for application in supercapacitors and potentially other energy storage devices.?2?Nickel?II?oxide?NiO?nanosheet grown on N-doped carbon hollow spheres?NiO/NCHS?with hierarchical pore structure are obtained via facile chemical bath deposition followed by calcination at 350°C under nitrogen atmosphere.Phase structure measurements indicate that the material is composed of NiO and N-doped carbon.The NiO/NCHS composite exhibits a unique flower-like morphology,where ultrathin NiO nanosheets are vertically grown on the surface of NCHS.This hierarchical nanostructure is beneficial for facilitating electron and electrolyte ion transport and accelerating the reversible redox reaction.The specific capacitance of the NiO/NCHS composite(585 F g^–1 at 1 A g^–1)is higher than that of pure NiO particle(453 F g^–1 at 1 A g^–1).Meanwhile,the NiO/NCHS composite exhibits excellent rate performance and superior cycling stability over 6000 cycles.The enhanced supercapacitive performance of the NiO/NCHS nanocomposite indicates that it can be an appealing candidate electrode material for supercapacitors.?3?The unique hierarchical N-doped hollow carbon sphere@NiO with unique core-shell structure?NiO/NHCS?has been synthesized by template-engaged method and calcination treatment.Well-dispersed Ni?OH?₂ nanosheets on the NHCS surface are first prepared by using N-doped carbon/SiO₂ spheres as template,where N-carbon shells and SiO₂ cores are served as non-sacrificed and sacrificed templates,respectively,followed by a calcination process to fabricate NiO/NHCS.In view of the hierarchical structure and the introduction of NHCS,the NiO/NHCS composite exhibits a high specific capacitance of 686 F g^–1 at 1 A g^–1.Moreover,a hybrid supercapacitor constructed by NiO/NHCS as positive electrode and activated carbon as negative electrode shows enhanced electrochemical properties with high energy density(30.5 Wh kg^–1 at 193 W kg^–1)and outstanding cycle ability?100%retention after 5000 cycles?.The well-structured and enhanced-performance NiO/NHCS is regarded as an alternative electrode material for supercapacitors.?4?Co₃O₄/Nitrogen-doped carbon hollow spheres?Co₃O₄/NHCS?with hierarchical structures have been synthesized by virtue of a hydrothermal method and subsequent calcination treatment.NHCSs,as a hard template,can aid the generation of Co₃O₄ nanosheets on its surface;while SiO₂ spheres,as a sacrificed-template,can be dissolved in the process.The prepared Co₃O₄/NHCS composites are investigated as the electrode active material.This composite exhibits an enhanced performance than Co₃O₄ itself.A higher specific capacitance of 581 F g^–1 at 1 A g^–1 and a higher rate performance of 91.6%retention at 20 A g^–1 have been achieved,better than Co₃O₄ nanorods(318 F g^–1 at 1 A g^–1 and 67.1%retention at 20 A g^–1).In addition,the composite is employed as positive electrode to fabricate an asymmetric supercapacitor.The device can deliver a high energy density of 34.5 Wh kg^–1 at the power density of 753 W kg^–1 and display a desirable cycling stability.All of these attractive results make the unique hierarchical Co3O4/NHCS core-shell structure a promising electrode material for high-performance supercapacitors.?5?Designing electrode materials with high specific capacitance is crucial for further improving the energy storage performance of supercapacitors.Nickel sulfide?NiS?nanosheets anchored on the surface of N-doped hollow carbon spheres?NiS/NHCS?were rationally designed and synthesized via a multistep transformation approach.Uniform nickel silicate nanosheets were first deposited on the surface of NHCS,followed by chemical conversion into NiS nanosheets.The NiS/NHCS composite shows excellent electrochemical property as an electrode material for supercapacitors.The NiS/NHCS electrode can deliver a high specific capacitance of1150 F g^–1 at 1 A g^–1 and show outstanding cycling performance with a retention rate of 76%over 4000 cycles,which are much higher than those of pure NiS hollow spheres?NiS-HS?(400 F g^–1 and 63%,respectively).Moreover,the energy density of a hybrid supercapacitor assembled with NiS/NHCS as cathode and activated carbon electrode as anode delivers a high energy density of 38.3 Wh kg^–1 at a power density of 160 W kg^–1 and has an excellent cycle performance with the retention rate of 96%after 5000 cycles.The exceptional electrochemical performance of the NiS/NHCS composite indicates its potential application in high-performance supercapacitors.Therefore,the introduction of carbonaceous substrates and the design of nanostructures are two important strategies for improving the conductivity and the utilization of effective active materials of transition metal oxides or sulfides.Hollow carbon sphere-based nanomaterials were prepared by different synthesis strategies.The different preparation parameters and process conditions have impacts on the composition,morphology,structure and properties of hollow carbon shell-based nanocomposites,to reveal the relationship between micro-structures and their electrochemical energy storage.