Synthesis And Oxygen Reduction Properties Of Heteroatom-Doped Fully Conjugated Three-Dimensional Covalent Organic Framework

COFs（Covalent Organic Frameworks）is a new type of porous material after MOFs（Metal Organic Frameworks）,which not only has the characteristics of high specific surface and crystallinity of MOFs,but also has stable covalent bonds and low density framework.So it is widely used in gas adsorption and separation,photoelectric sensors,semiconductors,etc.Among them,the application of 2D COFs in semiconductor is limited due to the charge transfer bottleneck between layers.Full-conjugated three-dimensional COFs have the advantages ofπconjugation extension in space,network structure,more open sites,etc.,which can solve the problems of two-dimensional COFs.In addition,heteroatom doping will affect the electronic structure of COFs,thus improving the performance of oxidation and reduction.Therefore,in this thesis,we designed two new three-dimensional conjugated ligands doped with thiophene and furan,and then synthesized COFs doped with different heteroatoms by solvothermal method.1、The thiophene-rich ligand COTh Th-CHO is used as the building unit to dehydrate and condense with p-phenylenediamine to form fully conjugated3D COFs（BUCT-COF-11）.The characterization of BET and XPS proves the pore structure of COFs and the formation of organic covalent bonds.The Hall effect of BUCT-COF-11 was also tested,and it was found that the conductivity of BUCT-COF-11 reached 2.97×10^-5 S m^-1 is ten times of BUCT-COF-1.The power density of anion exchange membrane fuel cells（AEMFCs）assembled with BUCT-COF-11/CNT and Pt Ru/C at 1280 m A cm^-2 can reach 493 m W cm^-2,which is much higher than the power density of BUCT-COF-1(317 m W cm^-2).DFT calculations reveal that thiophene introduction in the COF not only improves the conductivity but also optimizes the electronic structure of the sample,which therefore boosts the ORR performance.2.Inspired by the synthesis of thiophene-rich ligands,I designed and synthesized a cyclooctene-tetrathiophene-tetrafuran ligand COTh Fn-CHO,which also formed full-conjugated 3D COFs（BUCT-COF-12）rich in furan by Schiff base condensation reaction with p-phenylenediamine.The successful synthesis of BUCT-COF-12 was proved by some basic characterization.In addition,the electrochemical oxygen reduction（ORR）performance of BUCT-COF-12 was tested,and it was found that it had the best doping performance with single-walled carbon nanotube conductive agents,with a half wave potential of 0.74 V.