Design,synthesis And Electrocatalytic Active Site Study Of Crystalline Porous Framework Materials

With the growing energy demand and rapid depletion of fossil fuels,the exploitation and utilization of clean energy has become an important approach to achieve sustainable development.As a key technique of clean energy utilization,electrocatalysis can be used in multiple energy conversion devices,such as water-splitting,fuel cell,and metal-air battery.The activities of electrocatalysts determine the energy conversion efficiencies of these devices.Currently,the most active electrocatalysts are inorganic nanomaterials,with activity mainly derived from unsaturated metal sites exposed on the surface,while the metals in bulk phase are inactive,resulting in low utilization of metal sites.Therefore,how to improve the utilization of metal atoms and achieve efficient electrocatalysis has become a hot issue in the field of energy materials.Recently,the development of conductive metal-organic frameworks（conductive MOFs）and metal-covalent organic frameworks（M-COFs）has provided an opportunity for the exploitation of novel and efficient electrocatalysts.As an emerging class of metal-organic frameworks（MOFs）,conductive MOFs materials not only possess atomically dispersed metal sites,but also exhibit superior conductivity,which is beneficial for the enhancement of electrocatalytic activity.As an important part of covalent organic frameworks（COFs）,M-COFs also possess attractive features,such as well-defined structures,stable frameworks,and tunable functions.Meanwhile,the various metal sites in M-COFs provide the basis for enhancing catalytic activity of the materials.In this thesis,a series of conductive metal-organic frameworks and metal-covalent organic frameworks were designed and synthesized through metal site introduction and substitution strategies,and then applied to the study of electrocatalytic oxygen reactions.Furthermore,theoretical calculations were used to reveal the electronic modulation and activity enhancement mechanism of the active sites,and the structure-activity relationships of the materials were initially uncovered.The research contents of this thesis are outlined below:1.A series of conductive MOFs with dual metal sites（M-N₄ and M′-O₄）were synthesized by introducing phthalocyanine ligands.Electrochemical study confirms the highest electrocatalytic activity of Ni Pc-Ni for oxygen evolution reaction（OER）.Turnover frequency（TOF）calculation indicates that the intrinsic activity of Ni-O₄ is higher than that of Ni-N₄ site in the materials.Theoretical calculations indicate that the electronic interactions between the Ni-O₄ and Ni-N₄ sites enhance their intrinsic activity.The present work modulates the electronic structure of the materials by introducing extra metal sites,further enhances the catalytic activity and reveals the structure-activity relationship of the dual metal sites.2.A series of bimetallic conductive MOFs were synthesized by replacing some Ni-O₄sites in Ni Pc-Ni with Fe-O₄ sites.TOF results show that Ni Pc-Ni Fe_0.09 has the best OER intrinsic activity(TOF=1.943 s^-1 at overpotential=300 m V),and the intrinsic activity of bimetallic MOFs decreases with the increasing Fe-O₄ ratio.Theoretical calculations reveal that the change of Fe-O₄ ratio affects the electronic structure of MOFs,further changes the catalytic activity.This work may provide guidelines for the electronic modulation of conductive MOFs.3.A series of Ni-Cu bimetallic conductive MOFs were synthesized by replacing some Ni-N₄ sites in Ni₃（HITP）₂ with Cu-N₄.Electrochemical studies indicate that Ni_1.5Cu_1.5（HITP）₂ exhibits highest oxygen reduction reaction（ORR）activity,which is superior to the monometallic and other bimetallic conductive MOFs.Theoretical calculations show that the d-band centers of the materials are linearly related to the Cu-N₄contents.Introducing a moderate number of Cu-N₄ sites into MOFs moves the d-band center away from the Fermi level,which decreases the binding strength between metal sites and oxygen intermediates,further enhances the intrinsic activity.Otherwise,the introduction of excessive Cu-N₄ sites causes unstable adsorption of oxygen intermediates on the material,leading to decreased intrinsic activity.The present work realizes the electronic modulation and activity enhancement of ORR electrocatalysts by metal site substitution strategy,showing the universality of this strategy in the structural design of electrocatalysts.4.COF-112Co and COF-112Fe were synthesized using metal center variation strategy,and the relationship between the metal center and the ORR activity was investigated.COF-112Co possesses superior ORR activity(E_1/2=0.76 V vs.RHE,n=3.86).Experimental and theoretical studies demonstrate that the carbon bridge site beside the metal center is the active site for ORR.Free energy calculations reveal that the presence of cobalt metal center reduces the ORR free energy barrier and enhances the ORR intrinsic catalytic activity.In addition,COF-112Co was used as the cathode catalyst for a Zn-air battery with an open-circuit voltage of 1.354 V,indicating the certain application values of COFs in metal-air batteries.This work explores the application of woven COFs in ORR electrocatalysis for the first time,highlighting the great potential of M-COFs in the field of energy conversion.5.Based on the imine-pyridine moieties formed by pyridine units and their adjacent imine groups,additional Co-N₂ sites were introduced into porphyrin COFs to synthesize Co-Por Bpy-Co.Due to the presence of numerous unsaturated Co-N₂ sites,the material exhibits superior ORR activity(E_1/2=0.79 V vs.RHE,n=3.75).Theoretical calculations and experimental studies indicate that the introduced Co-N₂ sites reduce the band gap of the material,enhance the charge transfer ability,and can serve as the ORR catalytic active sites.In addition,the intrinsic activity of Co-N₂ sites is much higher than that of Co-N₄,demonstrating the great potential of unsaturated metal sites in material design and activity modulation.