Design And Synthesis Of A New Covalent Organic Framework-Based Material And Its Energy Storage Performance

Due to the advantages of high power density and long cycle life,supercapacitors are widely used in hybrid electric vehicles,power grid energy storage and national defense equipment.However,the energy density of supercapacitors is several orders magnitude lower than secondary batteries,which limits their further development.Therefore,on the premise of maintaining the inherent performance advantages of supercapacitors,increasing its energy density is important research topic.Covalent Organic Frameworks?COFs?is a new type of porous crystalline polymer composed of light elements?H,B,C,N,O,etc.?.Compared with other inorganic materials,COFs has a highly ordered skeleton structure and excellent chemical stability.Researchers can design COFs materials with different functions by changing the building blocks.Although COFs exhibit obvious structural advantages,its low intrinsic conductivity makes energy storage potential drowned.In view of this,by adjusting the proportion of reaction solvent and applying external force to the nucleation process of COFs,this paper achieves the precise control of COFs morphology.At the same time,in order to give full play to the energy storage performance of anthraquinone-based COFs,we combined COFs with different types of carbon-based materials?graphene and multi-walled carbon nanotubes?through two synthesis strategies?covalent functionalization and non-covalent functionalization?.The main contents are summarized as follows:?1?The formation of COFs is mainly based on the dynamic covalent reversible reactions.Therefore,in order to obtain COFs materials with high crystallinity,it is important to accurately adjust the thermodynamic/kinetic parameters?such as reaction pressure,reaction temperature,reaction solvent ratio and material growth rate,etc.?during reaction process.In this part,we first adjusted the crystallization rate of COFs by changing the ingredient of the reaction solvents to prepare uniform-sized Dahlia flower COFs?COFs-F?.In addition,we use grinding step to affect the nucleation of COFs.This strategy can apply a certain pressure to the reactants,which is conducive to the stack of COFs conjugated?planes along the Z axis.After solvothermal reaction of the above products,the COFs showed the morphology of rod clusters with high crystallinity?COFs-R?.Electrochemical tests for two materials?COFs-F and COFs-R?show that the unique layered structure of COFs-F leads to a large specific capacitance.Due to the strong interlayer forces,COFs-R material has good cycle stability and high capacitance contribution rate.In addition,the calculation of density functional theory proves that COFs-R has a lower band gap and better electron transport performance.?2?As the macroscopic conductivity of COFs is far lower than the actual standard of energy storage materials(<<10^-1 S m^-1),its application in supercapacitors is limited.In this part of the work,we selected amino-functionalized carbon nanotube?NH₂-CNTs?as the conductive substrate.The amino group grafted on the surface of carbon nanotubes can be used as covalent anchor positions of COFs to synthesize composite materials?COFs-C-CNTs?with enhanced electrochemical properties.In the three-electrode system test,the specific capacitance of the COFs-C-CNTs is 211 F g^-1,which is 10 times as high as that of pure anthraquinone-based COFs.The asymmetric supercapacitor COFs-C-CNTs//GH was assembled by using COFs-C-CNTs as negative electrode and graphene hydrogel?GH?as positive electrode?cell voltage 1.7 V?.At a power density of 0.85kw kg^-1,the energy density of COFs-C-CNTs//GH is 13.1wh kg^-1.?3?Covalent functionalization can provide a stronger connection mode for composites.However,the sp² carbon atoms of the conductive substrate during the reaction tend to produce defect sites during the reaction,resulting in reduced conductivity for the composite.In this part,COFs was modified on the surface of graphene sheets using a non-covalent strategy.This process is easy to operate and does not destroy the original?-conjugated system of graphene.Electrochemical tests show that at a scan rate of 5 mV s^-1,the specific capacitance of rGO-COFs is 335 F g^-1,which is 26 times as high as the specific capacitance of pure anthraquinone-based COFs.The asymmetric supercapacitor rGO-COFs//AC was assembled with rGO-COFs and activated carbon?AC?as the negative and positive electrodes respectively?the test voltage is 0-1.6V?.When the power density is 0.8 kW kg^-1,the energy density of the capacitor is 15.4 Wh kg^-1.