A Study On The Preparation And Supercapacitor Performance Of Novel Layered Nanocomposite Materials

The design and development of layered composite materials is a frontier issue in the current supercapacitor field.Layered nanomaterials have been widely studied among supercapacitor electrode materials.They have a high in-plane electron transfer rate,a short transmission distance of electrolyte ions from the host solution to the electrode surface,and a low resistance of ion diffusion.In addition,they also have other advantages,such as high reactivity and short charge transfer path.This thesis is mainly focused on the design,fabrication,supercapacitor performance and mechanism of layered nanomaterials.Combining the research accumulation oflayered compounds over the years by our group,four new layered composite nanomaterials were obtained through exfoliation and intercalation,and further hybridized with highly conductive or electrochemically active materials.The structure,morphology,supercapacitor performance and corresponding mechanism of the obtained composites were throughly investigated.The main research contents are summarized as follows:1.Amorphous vanadyl phosphate/graphene nanohybrids is successfully synthesized by first exfoliating bulk layered vanadyl phosphate?VOPO₄·2H₂O?into nanosheets,and then hydrothermal treatment with graphene oxide?GO?.The electrochemical properties of the resulted materials are systematically investigated.As supercapacitorelectrode material,the amorphous VOPO₄/graphene composite exhibits a high specific capacitance(508 F·g^-1 at 0.5 A·g^-1),an excellent rate capability(359 F·g^-1 at 10 A·g^-1),and a good cycling stability(retention 80%after 5000 cycles at 2 A·g^-1).Particularly,it simultaneously has a greatly enhanced energy density of 70.6 Wh·kg^-1 with a power density of 250 W·kg^-1 The outstanding energy storage performance mainly originates from the generation of amorphous VOPO₄ phase that facilitates ion transport by shortening ion diffusion paths and provides more reversible and fast faradic reaction sites,the hybridization with graphene that greatly improves the electric conductivity and structure stability,and the unique layer-on-sheet nanohybrid structure that effectively enhances the structure integrity.2.A novel asymmetric supercapacitor device in aqueous electrolyte is fabricated using vanadyl phosphate/carbon nanocomposite as the positive electrode and polypyrrole-derived carbon nanowire as the negative electrode.The vanadyl phosphate/carbon nanocomposites are synthesized by a simple two-step approach in which layered VOPO₄·2H₂O is first intercalated by dodecylamine and then annealed at high temperature,leading to the in-situ carbonization of the intercalated dodecylamine.It is found that the sample in which the incorporated carbon with a high degree of graphitization exhibits a high specific capacitance of 469 F·g^-1 at a current density of 1 A·g^-1 and an excellent rate performance(retained 77%capacitance at 10 A·g^-1).Polypyrrole-derived carbon nanowire is synthesized by direct carbonization of nanowire-shaped polypyrrole,revealing a rough surface of nanowire-like frameworks and good electrochemical behaviors.Taking advantages of both positive and negative materials,the assembled asymmetric supercapacitor device exhibits a high energy density of 30.6 Wh·kg^-1 at a high power density of 813 W·kg^-1 in a wide voltage region of 0-1.6 V,as well as a good electrochemical stability?84.3%capacitance retention after 5000 cycles?.3.MoO₃/C nanocomposites are synthesized by first intercalating alkylamines with different chain lengths into layered MoO₃ and then in-situ calcinating at high temperature.The possible arrangements of alkylamines in the interlayers of MoO₃ and the interaction between inorganic host and organic guest are discussed.The resulted MoO₃/CDA nanocomposite with a high degree of graphitization exhibits a high specific capacitance of 335 F·g^-1 at a current density of 1 A·g^-1 and an excellent rate performance(retained 70%capacitance at 10 A·g^-1).An asymmetric supercapacitor device is fabricated by using MoO₃/CDA as a positive electrode material and expanded graphite as a negative electrode.These two materials can be matched well to maximize the specific capacitance and to extend the potential window,giving rise to the increased energy density of the cell.The resulted MoO₃/CDA//expanded graphite asymmetric supercapacitor could be cycled reversibly between 0 and 1.6 V with a specific capacitance of 88 F·g^-1 at 1 A·g^-1 and an energy density of 31.3 Wh·kg^-1 at 838.4 W·kg^-1.Moreover,this supercapacitor exhibits an excellent cycling behavior with no more than 13.5%capacitance loss after 5000 cycles at 1 A·g^-1.This work may pave a novel intercalation way to fabricate active electrode materials for high energy devices.4.Hierarchically porous holey-graphene/PANI/graphene nanocomposite architecture is synthesized by a facile two-step approach in which PANI/graphene composite is first prepared by a hydrothermal method and then wrapped with a layer of holey graphene.The resulted sandwich-like structure can not only promote electronic transmission and ionic diffusion,but also contribute to an excellent rate performance.The assembled symmetric supercapacitor device based on holey-graphene/PANI/graphene nanocomposite exhibits a maximum specific capacitance of 437 F·g^-1 at a current density of 1 A·g^-1,a high energy density of 55 Wh·kg^-1 at a power density of 0.9 kW·kg^-1,as well as an ultrahigh-rate performance(retained 43%capacitance at 100 A·g^-1)and a good electrochemical stability(84%capacitance retention after 5000 cycles at a high current density of 10 A·g^-1).