Study On Electrocatalytic CO₂ Reduction Of Transition Metal-Covalent Organic Framework Hybrid Materials

Electrochemical carbon dioxide reduction（CO₂RR）has the potential to convert CO₂ into value-added chemicals or fuels using"green power"and is one of the most important ways to achieve the carbon cycle,with the conversion of CO₂ to carbon monoxide（CO）considered the most economically viable technology option.Noble and non-precious metals and carbon materials have been reported as CO₂RR catalysts,but they still generally have disadvantages such as low reaction efficiency and poor product selectivity,which are far from meeting the needs of commercial applications.Covalent organic frameworks（COFs）have a large application potential in CO₂RR due to their tunable structures and porous properties,but the currently reported ones are mainly porphyrin/phthalocyanine based COFs,which suffer from cumbersome synthesis process and limited monomer selection.Thus,the development of efficient,high product selective and stable COF-based CO₂RR electrocatalysts is of great academic significance and application prospects.In this thesis,Ag@COF hybrid materials with different functional group modifications were prepared by a facile impregnation method using typical imine COFs as carriers.The CO₂RR catalytic activity was investigated in a flow cell to achieve highly selective preparation of CO at large current densities.The main studies and results are as follows:（1）Three kinds of COF-R（R=–H,–OCH₃,–OH）modified with different functional groups were prepared by the solvothermal method and impregnated with silver acetate（Ag OAc）to obtain three Ag@COF hybrid materials.The effects of the crystallinity,pore structure and N and O contents of COF and chemical properties on the CO₂ adsorption capacity,size and distribution of Ag species of Ag@COF hybrid materials were investigated.（2）The CO₂RR catalytic activity of three Ag@COFs was investigated in 1M KOH using flow cell.Impressively,Ag@COF-OCH₃ showed the greatest catalytic activity with a high FE_CO of 93.0%with a high j _CO of 213.9 mA cm^–2at–0.87 V（vs.RHE）.Besides,it exhibited long-term durability at 100 mA cm^–2 for 30 h.（3）In situ Raman spectroscopy show that Ag（0）formed by in situ electrochemical reduction of Ag OAc occurs as the active site for electrolysis CO₂RR.They differ significantly in the electronic effect of the COF wall,the distribution of Ag OAc size,and the mass transfer of CO₂ within the hybrids,which thereby influences both the activity and the selectivity of electrolytic CO₂-to-CO transformation.