Preparation And Electrochemical Properties Of Well-Aligned And Hierarchical Carbon-Based Composites By Template Method

In order to meet the needs of modern wearable and portable electronic products and modern electric equipment,it is urgent to develop energy storage devices with high performance,which can provide high power and energy density and maintain excellent cycle stability.As one of the frontier research directions in the field of energy storage devices,supercapacitors are ahead of traditional battery devices due to their high power density,excellent cycle life and green safety.However,the low energy density restricts its applications.Therefore,how to maintain high power density and improve energy density of supercapacitors is the research interests in this field.Electrode material is a key factor in determining the capacitance performance of supercapacitors.The preparation of a hierarchical nanomaterial with a multi-component synergistic effect can effectively improve the charge storage capacity of the electrode material.Among the various methods of material preparation,the template method has become one of the effective ways to prepare hierarchical nanomaterials due to its simple operation and controllable structure.In this dissertation,we used polyacrylic acid?PAA?as the soft template to uniformly distribute the high pseudocapacitive amino-functional graphene quantum dots?NH₂-GQDs?on graphene hydrogel?GH?nanosheets to prepare a hierarchical structure NH₂-GQD/GH-PAA composite electrode material.Using Si O₂as a hard template,nickel-cobalt layered double hydroxide nanosheets?Ni-Co LDH?were uniformly grown on silica nanospheres.At the same time,in order to improve the electrical conductivity,carbon microspheres were interspersed in the middle of the sheet to form Ni-Co LDH@CS pomegranate-like nanosphere structure.We assembled NH₂-GQD/GH-PAA symmetric supercapacitor and fabracated asymmetric supercapacitor employing Ni-Co LDH@CS as the negative electrode,and the electrochemical performance was investigated.The research content of this dissertation includes the following aspects:1.Preparation and characterization of amino functionalized graphene quantum dot/polyacrylic acid?PAA?grafted graphene hydrogel?NH₂-GQD/GH-PAA?composite.PAA was used as a soft template to mix with GO?GO?.After adding NH₂-GQDs,the hierarchical structure NH₂-GQD/GH-PAA composite was prepared by hydrothermal synthesis.The composite material was analyzed and characterized by SEM,TEM,XRD,Raman and XPS.The results show that NH₂-GQDs are evenly distributed on the graphene sheet under the PAA as soft template,while there is a large amount of agglomerated NH₂-GQDs in NH₂-GQD/GH composite without PAA.The NH₂-GQD/GH-PAA hierarchical structure maintains the original three-dimensional network structure of GH with the specific surface area of 684.5 m²g^-1,forming a structure conducive to the rapid transport of electrolyte ions.Most importantly,the uniformly dispersed NH₂-GQDs effectively increases the active site of the electrode material and gives full play to the high pseudocapacitive activity of quantum dots.2.Electrochemical performance of NH₂-GQD/GH-PAA symmetric supercapacitor and all-solid flexible supercapacitor.The specific capacitance of the symmetric supercapacitor assembled by the NH₂-GQD/GH-PAA electrodes reaches 436 F g^-1,and provides a high density of 15.07 Wh kg^-1at the power density of 500.02 W kg^-1.After 10000 cycles,the capacity retention rate is 87.2%,which displays excellent cycle stability.The voltage window of the all-solid supercapacitor based on NH₂-GQD/GH-PAA can be stretched to 1.3 V.At a current density of 1A g^-1,the device shows a high specific capacitance of up to 326 F g^-1and delivers an energy density of 19.5 W h kg^-1at a power density of 650.03 W kg^-1.Moreover,after 8,000cycles,the device has a capacity retention rate of 84.4%and the Coulomb efficiency is about 99.5%,showing good cycle stability.The capacity of the flexible supercapacitor has almost no attenuation under bending and folding deformation,which indicates excellent cyclic stability and flexibility.3.Preparation and characterization of nickel-cobalt layered double hydroxide nanosheets@carbon microspheres?Ni-Co LDH@CS?pomegranate-like nanosphere composites.First,the functionalized Si O₂nanospheres were prepared as a hard template by the sol-gel method.Then the well-aligned Ni-Co LDH nanosheets were grown in situ on the Si O₂nanospheres by the solvothermal method,while glucose-derived carbon microspheres were added and interspersed in the middle of the lamellar layer.Finally,after removing the Si O₂hard template,Ni-Co LDH@CS pomegranate-like nanospheres were obtained.This hierarchical structure not only helps to make full use of the dual active sites of Ni-Co LDH,but carbon microspheres can act as a conductive medium for electrons,which is conducive to improving the conductivity of the composite.The structure and morphology of the composite materials were characterized by SEM,TEM,XRD,XPS analytical method.4.Electrochemical performance of Ni-Co LDH@CS//N,S-porous nanosheets ?Ni-Co LDH@CS//N,S-PCP?asymmetric supercapacitors.In the three-electrode system,the capacitance of the Ni-Co LDH@CS is 1240 F g^-1at the current density of1 A g^-1with 1 M KOH electrolyte.Asymmetric supercapacitors were assembled with N and S co-doped porous nanosheets as the positive electrode and Ni-Co LDHS@CS as the negative electrode.The asymmetric supercapacitor has a wide voltage window of 0-1.6 V with the specific capacity of the device is 101.1 F g^-1at the current density of 1 A g^-1.When the current density is 20 A g^-1,the specific capacity still retains 60.4F g^-1with 60%the capacity retention rate,which shows a good rate performance.ASC device can deliver the energy density of 35.93 W h kg^-1at the power density of863.96 W kg^-1.After 5000 cycles,the capacitance retention and columbic efficiency of the device are 88.2%and 99.9%respectively,indicating excellent electrochemical performance of Ni-Co LDH@CS//N,S-PCP asymmetric supercapacitor.