Synthesis Of Fully Conjugated 3D Covalent Organic Frameworks And Their Oxygen Reduction Properties

Covalent Organic Framework（COF）materials have been widely used in various photoelectric chemical fields due to their designability of molecular blocks,flexibility and adjustability of pore structure,as well as excellent thermal and chemical stability.Three-dimensional COF（3D-COF）,especially fully conjugated three-dimensional COF,has more abundant redox active sites and more expandableπ-conjugated structure than most two-dimensional COF,so it is more conducive to electron transport when applied to photoelectrocatalysis.In this paper,based on the fully conjugated 3D-COF,the cyclooctatetraentetrathiophene monomer（COTh P-CHO）as the basic building unit,two kinds of 3D-COFs with different bond types were designed and synthesized by traditional solvothermal method,and their electrocatalytic oxygen reduction characteristics were studied respectively,which expanded the application of organic semiconductor electrocatalysts.1.Based on the saddle-type structure of COTh P-CHO and4,5,9,10-Pyrenetetrone,the fully conjugated 3D COF,BUCT-COF-7 with imidazole unit linkage was synthesized by solvothermal polycondensation.Due to its extendedπ-conjugate structure and long-range order,BUCT-COF-7not only has a large number of active sites and semiconductor activities,but also has good structural and chemical stability,which can selectively undergo2 e^-oxygen reduction reaction,and has high H₂O₂production rate.Between0.6 and 0.7 V（vs.RHE）,the average electron transfer number is 2.4,and the H₂O₂yield can reach 82.1%.When applied to devices,The H₂O₂yield reached 1680.39 mmol g^-1h^-1by electrolysis at 50 m A cm^-2.A relatively high H₂O₂yield of 1090 mmol/g,with an average yield of 363.3 mmol/g/h and a faraday efficiency of about 80%,can be produced after 3 hours of discharge at a low current density of 10 m A/cm².Through fenton reaction principle,H₂O₂produced can be effectively used for wastewater degradation,which has potential application value in environmental materials.This is the first reported fully conjugated 3D COFs connected by imidazole units for electrosynthesis of H₂O₂.2.Due to the unique redox properties and semiconductor properties of porphyrin molecules and their metal derivatives,porphyrin derivatives of chelating metal Co were synthesized,and COTh P-CHO was effectively combined through Schiff base reaction.It is proved that C=N bonded fully conjugated 3D porphyrin-based COFs（BUCT-COF-6-M,M=2H,Co）have been successfully synthesized.The unique metal characteristics and photoelectric application characteristics make it have excellent potential for electrocatalytic oxygen reduction,so it is applied to oxygen reduction test.Experiments show that BUCT-COF-6 can selectively undergo 4-electron oxygen reduction reaction and is successfully applied to zinc-air batteries.The initial potential of BUCT-COF-6-Co was 0.86 V vs.RHE,the half-wave potential was 0.7441 V vs.RHE,and the power density of the zinc-air batteries was 197 m W cm^-2,which is a good catalyst for oxygen reduction test.