Preparation And Electrochemical Properties Of Porous Organic Frameworks

In recent years,porous organic frame materials have been widely used in sensing,gas adsorption,supercapacitors and other fields due to their advantages of stable structure,high specific surface area,highly ordered pore structure,adjustable pore size,easiable functionalization and variety.Porous organic frame materials mainly include metal-organic frame materials(MOFs),covalent organic frame materials(COFs),hydrogen bonded organic frame materials and supramolecular organic frame materials.In particular,MOF and COF materials show great potential in the field of energy storage.However,due to poor conductivity,poor stability and exclusive pore structure,the rich pore and high specific surface area of the material are not fully utilized,and the structural advantages are not fully exploited,leading to poor electrochemical performance.In this paper,to improve the electrochemical performance of organic frame materials,graphene is introduced to improve their conductivity and mesoporous structure is introduced to optimize their pore structure.The main results are as follows:(1)Study on graphene/covalent organic frame composites.Graphene was used as carrier and COF crystals were prepared by in-situ synthesis on its surface to obtain novel graphene/covalent organic framework(GO/COF)hybrid nanocomposites.The sheet-like COFs crystals are uniformly attached to the GO surface.Due to good conductivity of graphene oxide,high specific surface area of COF and their synergistic effect,the GO/COF composite exhibits good electrochemical performance.At 1 A g^-1,the specific capacity of GO/COF is 73.75 F g^-1,which is much higher than those of graphene oxide(48.06 F g^-1)and COF(56.38 F g^-1).When the current density increases from 1 A g^-1to 10 A g^-1,the specific capacity is 46.15 F g^-1,and the capacitance retention rate is 62.6%.(2)Study on GO/template composites.GO/template composites with different morphologies and structures were prepared by self-assembly of F127 and PS-b-PEO on the surface of graphene oxides.The effects of solvent types and dosage,concentration of templates,p H value and polymerization degree of block copolymer on the structure and morphology of GO/template composites were studied.Results are as follows:1)With the increase of concentration of templates,the morphology of single spherical micelle will change.Aggregation of spherical micelles or coexistence of multiple micelles would be formed with the increased concentration in further.When the concentration of PS-b-PEO increased,a large number of spherical micelles were tightly packed.When continuous increase in concentration of PS-b-PEO,a small number of spherical micelles and a large number of short rod micelles were observed.With the increase in concentration of F127,the number of spherical micelles increased.When the concentration is too high,aggregation or coexistence of multiple micelles would be formed.2)The type and ratio of solvents affect the micelle structure.In DMF/water mixed system,when the water content is low(0.1%),a large number of spherical micelles of PS-b-PEO were formed on GO surface;When the increase in water content,micelles changed from spherical to mixed micelles;In DMF/methanol system,when the volume ratio of DMF:methanol was 2:1,regular spherical micelles of F127 were formed on the GO surface.When the volume of methanol increased,the morphology of micelles changed into multiple micelles structure.3)The p H value affects the morphology of micelles.With the decrease of p H value,the micelles of F127 on the surface of GO changed from single spherical structure to spherical micelles and wormlike micelles;When the p H value decreased to 8.5,the micelles became mixture of spherical,vermicular and vesicular structure.4)In the system of THF/aqueous solvent,when the degree of polymerization of hydrophilic segments in PS-b-PEO decreased,the micelle morphology changed from spherical to wormlike and then to vesicular structure.(3)Study on porous MOF heterostructure composites.Two dimensional Ni-MOF was synthesized on the surface of the GO/template composites by solvothermal method.Then,the template was removed and porous MOF heterostructure composites were obtained.The good conductivity of graphene endows the porous MOF heterostructure composites with enhanced conductivity.The introduction of mesopores in two-dimensional MOF endows the composite with hierarchical pore structure,and the abundant pores provide convenient condition for the diffusion and transport of electrolyte ions in active materials.At the same time,the large specific surface area was obtaind.The specific surface area values of GO/Ni-MOF-F and GO/Ni-MOF-P are 133.95 m²g^-1and 211.57 m²g^-1,respectively.The large specific surface area and high mesoporous volume accelerate ion transport,leading to excellent electrochemical properties of porous MOF heterostructure composites.When the current density is 1A g^-1,the specific capacitance values of Ni-MOF,GO/Ni-MOF-F and GO/Ni-MOF-P are 63.64 F g^-1,151.52 F g^-1and 245.71 F g^-1,respectively.The specific capacitance of GO/Ni-MOF-P is nearly five times higher than that of Ni MOF.When the current density increased from 1 A g^-1to 10 A g^-1,the specific capacity decreased from 245.71 F g^-1to 142.86 F g^-1,and the capacitance retention rate is 58.14%.The cycle stability of GO/Ni-MOF-P composite electrode is good.When the current density is 1A g^-1,the capacitance retention rate is 70%after1000 cycles.