Synthesis Of Metal-free Difluorenylidene-based Covalent Organic Frameworks And Study Of Their Electrocatalytic Oxygen Reduction Properties

Covalent organic frameworks are a class of crystalline porous organic polymers with periodic frameworks that are composed of covalent linkages,and the structures of this class of materials can be pre-designed and tailored to impart specific chemical or physical properties to them.Over the years,various covalent organic frameworks with excellent properties have been developed and applied in scientific research fields such as gas adsorption/separation,fluorescence sensing,and photocatalysis.However,covalent organic framework materials for oxygen reduction electrocatalysts are still not sufficiently studied,and although transition metal-doped covalent organic framework materials have been utilized,the related research on metal-free covalent organic framework materials is still in the initial stage.To develop more non-metallic ORR electrocatalysts,in this paper,covalent organic framework materials with 9,9′-bifluorenylidene as the main skeletal structure were synthesized by Schiff base reaction,and a series of detailed characterization and analysis of these materials were carried out to explore the electrocatalytic oxygen reduction performance among different materials.The study contains two main parts as follows.1.9,9′-bifluorenylidene is a recognized non-fullerene-type electron conductor with a dimeric fluorophore structure,and its derivatives are widely used in solar energy storage and other fields.Therefore,COF-JLU82 was successfully synthesized by Schiff base reaction based on the existing organic synthesis by introducing this structure into the covalent organic framework.The crystal structure,spectral data and important information related to the electrocatalytic oxygen reduction performance of the material were obtained by using powder X-ray diffraction,infrared/ultraviolet spectroscopy,scanning/transmission electron microscopy,and electrochemical tests for characterization.It was demonstrated that COF-JLU82 exhibited higher ORR activity than classical pyrene-based COF-JLU23,It was demonstrated that COF-JLU82exhibited higher ORR activity than the previous classical pyrene group COF-JLU23:the TOF value of COF-JLU82 was 0.00439 s^-1,which was higher than that of COF-JLU23(0.00391 s^-1),indicating that COF-JLU82,due to the introduction of 9,9’-difluorenylene resulted in a catalyst with each active site Se has a higher intrinsic activity,which is more favorable for the interphase diffusion of O₂ and products on the catalyst,as well as the adsorption and desorption on the catalyst surface.In addition,COF-JLU82 exhibited greater mass activity(2.41 A g^-1)compared to COF-JLU23(2.35 A g^-1),indicating that COF-JLU82 has higher active site utilization,proving that9,9′-bifluorenylidene can enhance the intrinsic activity related to electrocatalytic oxygen reduction of covalent organic frameworks.2.Judging from the experimental results according to the previous chapter,9,9′-bifluorenylidene is an excellent structure that can improve the electrocatalytic activity of the material,so we used this structure to expand two more new covalent organic frameworks with different thiophene units,and named them COF-JLU83 and COF-JLU84,respectively.through a series of characterizations,both materials are highly Both materials are highly crystalline porous materials.In order to further understand the electrocatalytic oxygen reduction performance of the two materials,the electrocatalytic oxygen reduction performance of the two materials was characterized under the same experimental conditions.Assuming that each S-atom in the structure is used as an active site,the transition frequency（TOF）of both materials at 0.7 V was calculated from the data of LSV curves as a comparison,where the TOF value of COF-JLU83 is 0.00187 s^-1,which is much higher than that of COF-JLU84(0.00121 s^-1),which indicates that COF-JLU83 has a higher active site utilization.Furthermore,it was calculated that COF-JLU83 exhibited a greater mass activity than COF-JLU84(COF-JLU83:2.72 A g^-1;COF-JLU84:2.17 A g^-1),indicating that COF-JLU83 has a better ORR catalytic activity,proving that the COF-JLU83 with the structure of benzo[1,2-b:4,5-b′]dithiophene showed a higher performance than the COF-JLU84 with the structure of thieno[3,2-b]thiophene,it is demonstrated that the introduction of conjugated aromatic ring can also improve the electrocatalytic oxygen reduction performance of covalent organic frameworks materials.