The Design,Synthesis,and Capacitive Performances Of Transition Metal Nanocomposites Materials

Supercapacitors represent a kind of green and efficient electrochemical energy storage devices with superior advantages such as high power densities,rapid charging-discharging rates,long cycling lifetimes,wide operation temperature range,superior safety and environmentally friendly,which can be potentially applied in very wide energy facilities related to spower output and charging-discharging rate,including auxiliary power sources,start-up device etc.However,one of major challenges in supercapacitors lies in the lower energy density relative to traditional secondary batteries.Therefore,the design of high specific capacitance materials to achieve great power density and energy density without sacrificing cycle life has become the focus in the field of supercapacitors.In this thesis,polydopamine-reduced graphene oxide composite substrate?PDA-RGO?was constructed as host framework for depositing transition metal compounds,in the intention to optimize the specific capacitance,rate capability and cycle performance through the modulation of composite microstructure.Furthermore,hybrid supercapacitors were constructed based on the Faradic composites and activated carbon?AC?,high energy densities and excellent cycling stabilities were achieved by balancing the specific capacitance and widening the voltage window accompanied with the high power density.The main contents of this thesis are summarized as follows:1.Graphene oxide was served as basic platform for the self-polymerization of dopamine in weak alkali medium to afford PDA-RGO.The model of PDA-RGO structure is optimized through DFT using the Gaussian 09 package.The N atoms of-NH₂ in dopamine almost vertically graft onto the RGO surfaces.The anchoring of electroactive PDA onto RGO can effectively improve the specific capacitance of the resultant composite.The PDA-RGO electrode can offer a higher specific capacitance of 368 F g^-1 at a current density of 0.5 A g^-1.2.The interactions of Ni²⁺with hydroxyl groups in PDA-RGO can facilitate the subsequent hydrothermal conversion toward Ni?HCO₃?₂ nanoparticles.In the resultant ternary Ni?HCO₃?₂-PDA-RGO composite,PDA triggers the well-distributed Ni?HCO₃?₂ nanoparticles,and enlarges the Ni²⁺site/electrolyte interface area and ensures efficient electrons/ion migration channels,which are beneficial for the Faradic reversibility and stability.The Ni?HCO₃?₂-PDA-RGO composite herein can offer high specific capacitance(1740 F g^-1 at 0.5 A g^-1),good rate and cycle stability.Within the voltage window of1.7 V,the assembled Ni?HCO₃?₂-PDA-RGO//AC asymmetric supercapacitor?ASC?can deliver a specific capacitance of 113 F g^-1 at 0.5 A g^-1.The maximum energy density of ASC reaches to 45.3 W h kg^-1 at the power density of 425 W kg^-1,and the specific capacitance of ASC can still retain 90.5%after 3,000successive charge-discharge cycles,showing the good energy density,power density and cycling performance.3.3D Fe₂O₃@PDA-RGO nanocomposite was prepared by a simple and efficient hydrothermal route.The 3D PDA-RGO framework can govern the growth and distribution of the formed Fe₂O₃ nanoparticles to ensure high specific surface area.When employed as Faradic negative electrode in supercapacitor,the Fe₂O₃@PDA-RGO offered a Faradic capacitance of 609 F g^-1 at 1 A g^-1.The assembled AC//Fe₂O₃@PDA-RGO ASC device also shows excellent electrochemical performance,the specific capacitance achieves 100.6 F g^-11 at 1 A g^-1,and the energy density reaches 35.8 W h kg^-1 at the power density is 800 W kg^-1,and the high cycling stability could also be maintained simultaneously,showing the great potential in high performance energy storage devices.4.The manipulation of defects in materials is an effective strategy to optimize the surface microstructure and electronic configuration of metal oxide to improve the physicochemical properties of metal oxides.The Co₃O₄@PDA-RGO composite containing Co₃O₄ nanorods and rich oxygen vacancies was prepared via a facile hydrothermal method.In this composite,oxygen vacancies were created by partial reduction of Co₃O₄ by PDA-RGO,and increased by 14.8%as compared with that of Co₃O₄.The Co₃O₄@PDA-RGO composite can offer a specific capacitance of 1562 F g^-1 at 0.5 A g^-1.The assembled Co₃O₄@PDA-RGO//AC ASC device shows a specific capacitance of 129.7 F g^-1 at 1 A g^-1,and a high energy density of 46.1 W h kg^-1 at the power density of 800 W kg^-1.The capacity retention rate of the ASC is 91%after 10 000 continuous cycles.The resulted Co₃O₄@PDA-RGO//AC ASC delivered excellent specific capacitance,rate performance and cycling stability,showing the good capacitive performance from the oxygen vacancies in Faradic materials.5.NiWO₄ nanowires were synthesized in ethylene glycol solvent,its high specific surface area and efficient electrons/ions migration channels facilitate the sufficient and rapid Faradic reactions.When used as Faradic electrode,NiWO₄ nanowires can offer a high specific capacitance of 1190 F g^-1(at 0.5 A g^-1)and a high rate capability(61.5%capacitance maintaining ratio within 0.5¹0 A g^-1).The assembled NiWO₄//AC ASC displays the widened voltage window of 1.7 V and a high specific capacitance of 160 F g^-1(at 0.5 A g^-1),and hence delivers a high energy density up to 64.2 W h kg^-11 and an initial capacitance retaining ratio of 70.8%after 20,000 charge-discharge cycles.These good capacitive performances highlight the potential of the as-prepared NiWO₄ nanowires as high performance Faradic material.