Rational Design Of Covalent Organic Frameworks For Constructed Oxygen Reduction Reaction Electrocatalysts

Covalent organic frameworks（COFs）are constructed from small organic molecules linked through reversible covalent bonds,the formation of crystalline porous organic network of COFs is controlled by thermodynamics.With using different building units,the porosities and functions of COFs are well tailed.In addition,the periodic units in the frameworks of COFs provides abundant and uniform catalytic sites.Given that these features,COFs have shown promise advantages in establishing active sites directly and constructing precursors of catalysts.Oxygen reduction reaction（ORR）is the key reaction in green energy system for energy storage and conversion,that is also important to green chemical synthesis.The activity and selectivity for ORR depend on the binding strength of the catalytic site with intermediates such as*OOH,*O,and*OH.The too strong or too weak binding ability of these intermediates hinder the catalytic process and limit the activity and selectivity of ORR.How to achieve highly active catalysts together with high selectivity in 2e^-or 4e^-is critical but challenging.In this thesis,the COFs were used to regulate the d bands of metal and construct the ultra-close single atom sites;the interface engineering and edge site engineering of COFs were explored,respectively.As a result,the activity and selectivity for the catalysts from noble metal to noble metal-free and metal-free catalysts were optimized and modulated towards ORR.The structure-activity relationship of the constructed catalysts were demonstrated via theory calculation and experiment.The achievements were shown as followed:（1）The ideal transition metal catalysts for ORR required the d-band structure with appropriate adsorption strength.The adsorption strength is dependent on the state of antibonding orbital of metal molecule orbital with the adsorbate.Pt show the superior activity of ORR to many other metals,but the strong adsorption strength of intermidates limited the activity.Herein,we design an electrocatalyst by confining Pt nanoparticles on a conjugated nitrogen-rich COF,and the XAFS confirmed the electronic states of Pt and revealed the large aera contact between metal and COF.The lower d-band center of Pt reduced the adsorption strength,thus promoting oxygen reduction process.The catalyst exhibited ultrahigh ORR activity with an onset potential of 1.05 V versus reversible hydrogen electrode（vs RHE）and a half-wave potential of 0.89 V,which are more positive than those of commercial Pt/C and other reported catalysts.（2）Transition metal single atom catalysts are considered as one of the most promising catalysts to replace precious metal Pt due to their low metal consumption,extreme atomic utilization rate and special electronic structure.Although isolated Fe atomic sites have displayed outstanding performance in ORR,the high*OH desorption energy hinder their activity further improvement.Herein,a sp² carbon linked COF was employed to confine the Fe ions during pyrolysis via the one-dimensional channels.We have successfully fabricated 2D carbon nanorods and ultra-close atomic sites.The obtained catalyst showed high catalytic activity for ORR.The theoretical calculation revealed the close Fe N₄ sites achieved a lower*OH desorption energy than isolated Fe N₄ sites.This work provides a new insight into developing single atom catalysts from COFs.（3）It is well known that the N-doped carbon materials were the highest ORR activity of metal-free single element doped carbon.We constructed a uniform type nitrogen contained fully conjugated microporous COF by thermodynamic polymerization,but the activity was low due to its low conductivity and weak oxygen adsorption ability.We composited the COF with carbon nanotubes to form COF-CNT interface via fully conjugated structureπ-πstacking.This interface not only improves the conductivity of COF but also modifies the electronic state of the catalytic sites.The X-ray absorption spectra revealed that pyrazine nitrogen of COF-CNT improved the electron density,which increased the adsorption capacity of adjacent carbon atoms to oxygen molecules.The interface engineering made the catalyst achieved the turnover frequency（TOF）increases by two orders of magnitude,and improved the halfwave potential to 0.79 V and limited current density of 5.5 m A cm^-2.More importantly,the electron transfer number for oxygen reduction changed from 2e^-to4e^-.This work provides us a new insight in designing COFs for electrocatalysis.（4）The traditional method for assembling and decorating catalytic sites usually at COFs’backbone.We demonstrate a novel tactic to construct catalysts with controllable density and location of active sites and well-defined active ability by edge-defect engineering of COFs.Catalysts with optimized density of edge carbonyl sites showed remarkable activity and selectivity in catalyzing oxygen reduction in 2e-pathway.Density functional theory calculations further revealed that the carbons next to carbonyl group on the edges enhance the binding ability of OOH*,which contributes to the high activity and selectivity.This work provides a general insight of designing H₂O₂ electrosynthesis catalysts through regulating the edge-defective properties of COFs.