Construction Of Hollow Structured Metal Oxide Induced By Carbon Nanotubes

Owing to the peculiar structural advantages,hollow micro-/nanostructured materials have been proved with fascinating physical and chemical properties for electrochemical energy-related applications,such as hybrid supercapacitors?HSCs?,lithium-ion batteries?LIBs?,hydrogen evolution reaction?HER?and oxygen evolution reaction?OER?.Classical synthesis of hollow structured materials usually involves the utilization of hard templates and soft templates.However,the template approach usually encounters time-consuming complex synthesis procedures that may hardly be overcome.Therefore,direct synthesis of hollow structured materials with no need of additional templates is preferred in practical applications.A variety of template-free methods have been developed in construction of hollow structures in the light of different mechanisms,such as the Ostwald ripening,Kirkendall effect,Surface-protected etching,and Galvanic replacement.Nevertheless,each mechanism of template-free method only works in some specific systems and irregular.On the other hand,the unfavourable electron conductivity of most metal oxides and hydroxide still remain unavoidable challenge to optimize the performance of hollow micro-/nanostructured metal oxide/hydroxide materials,especially when being employed as electrode materials.With these limitations in mind,induced by functionalized multi-walled carbon nanotubes?MWCNTs?,three hollow sphere metal oxide and hydroxide/multi-walled carbon nanotubes composites have been successfully prepared for the first in this paper.A universal method for the preparation of conductive hollow metal oxides/hydroxide composites with hollow structures induced by carbon materials is developed.Furthermore,the electrochemical properties of three composites electrode have been investigated.In the end,the prominent asymmetric supercapacitor were encapsulated using the above hollow sphere metal oxide and hydroxide/multi-walled carbon nanotubes composites.The main results are outlined as following:1.Induced by functionalized multi-walled carbon nanotubes?MWCNTs?,rationally designed hollow sphere CeO₂/multi-walled carbon nanotubes composite?HS-CeO₂/MWCNTs?has been successfully prepared for the first time via facile one-pot template-free hydrothermal reaction followed by calcination.The appropriate functional groups density on the surface of MWCNTs can lead to the in-situ growth of hollow sphere CeO₂.The HS-CeO₂/MWCNTs composite featuring a novel 3D interpenetrating structure of hollow metal oxides and MWCNTs display advanced electrochemical performance:delivering excellent specific capacitance,brilliant cycling stability of 90.1%retention after 5000 cycles,and prominent rate capability of86.5%even at 10 A g^-1.It is worth noting that the newly developed method equally well applies for the preparation of hollow sphere CeO₂/activated carbon,hollow sphere CeO₂/graphene oxide.It is worth noting that hollow nickel hydroxide and iron oxide can also be induced by multi-wall carbon nanotubes,and graphene oxide and activated carbon have the same induction effect,proving the universality of hollow structure formation induced by carbon materials.2.By employing modified MWCNTs,rationally designed hierarchical hollow microsphere???Ni?OH?₂/MWCNTs?HHM-???Ni?OH?₂/MWCNTs?has been successfully prepared using the same simple templatel-free one-pot hydrothermal synthesis.Be different from The HS-CeO₂/MWCNTs,the newly prepared HHM-???Ni?OH?₂/MWCNTs composite featuring a novel 3D hierarchical hollow microsphere structure of???Ni?OH?₂ and MWCNTs displays advanced electrochemical performances:delivering high specific capacitance of 1540.8 F g^-1 at1 A g^-1,brilliant cycling stability of 85.3%retention after 5000 cycles,and prominent rate capability of 58.8%even at 20 A g^-1.3.Hollowsphereferroferric oxide/MWCNTs?HS-Fe₃O₄/FMWCNTs?was produced through the same method as above and then investigated for its potential in supercapacitor.When used as supercapacitor electrodes in 6 M KOH electrolyte displays advanced electrochemical performances,delivered a high capacitance of 235.7 F g^-1 at 1 A g^-1,prominent cycling stability of 93.3%retention after 5000 cycles,and excellent rate capability of 68.7%even at 20 A g^-1.Moreover,an asymmetric supercapacitor device synthesized using HHM-???Ni?OH?₂/MWCNTs as the cathode and HS-Fe₃O₄/FMWCNTs as an anode showed a high energy density of up to 43.5 Wh kg^-1 at a power density of 810 W kg^-1.Formation mechanism,controlled synthesis and charge storage mechanism of three hollow sphere metal oxide and hydroxide@multi-walled carbon nanotubes composites as electrode materials for asymmetry supercapacitor are systematically analysed.The experimental results showed that,when the number of functional groups on MWCNTs is in moderation,the functional groups on the surface of MWCNTs anchor metal oxide crystal seeds.In such a situation,the number of functional groups on the surface of MWCNTs and metal oxide crystal seeds is limited.Therefore,the crystal seeds continue to grow on 1D MWCNTs and form large diameter hollow sphere.This paper not only provides a simple and universal approach to prepare high-performance electrode but also offers new evidence for in-depth understanding the formation mechanism of hollow structure from template-free method.Developing MWCNTs based hollow nanostructured composites with highly capacitive capability and excellent cycling stability will open up a new route for the development of advanced electrode materials for supercapacitor and other energy store systems.