Research On Microenvironment Engineering Of Pyrolysis-Free Polymer-Based Oxygen Reduction Electrocatalysts And Corresponding Structure-Performance Relations

Zinc-air batteries（ZABs）leverage their semi-open structure to sustainedly provide economical,safe and efficient energy output,however the sluggish kinetics of cathodic oxygen reduction reaction（ORR）significantly limits the efficiency of ZABs.The commercial precious metal-based materials have been identified as the best ORR catalysts,despite several inherent flaw,including scarcity and poor long-term durability,stimulated the exploration on efficient and cheap non-noble metal-based catalysts.Transition metal and nitrogen codoped carbon（M-N-C）based catalysts have emerged as promising ORR catalysts by virtue of their suitable orbital arrangements and easily controllable electronic configurations.However,their difficulty in precise control of site configuration hinders the study of structure-performance relationship for electrocatalysis.In this regard,this thesis focuses on pyrolysis-free transition metal-based covalent organic polymers,with the versatile tunability properties of structural components,the local electronic microenvironments on metal centers are systematically and precisely customized.More importantly,by combining theoretical calculations and cutting-edge characterization techniques,the catalytic structure-performance relationships and interfacial ORR dynamic evolution mechanisms on M-N-C sites during catalysis were proposed.The details are as follows:1.It is widely accepted that the classical planar symmetric M-N4 site configuration is not the optimal structure for the oxygen adsorption and activation.This thesis precisely prepared Co porphyrin-based covalent organic polymer with distinct Co-N5 site configuration through constructing an axially coordinated asymmetric fifth ligand on planar Co-N4 site.This penta-coordination strategy induces charge localization on Co center and promotes the O=O bond cleavage of chemisorbed oxygen,thus enhancing the ORR towards the 4-electron pathway on Co-N5 sites.The Co-N5 catalyst exhibits superior half-wave potential in alkaline solutions compared to Co-N4 catalyst,also the smaller Tafel slope and higher normalized kinetic current density indicate the superior intrinsic activity of Co-N5 catalyst.The Co-N5 catalyst-based ZAB demonstrates outstanding power density and long-term cycling durability（over 160 hours of cycling）.This thesis not only proposes a pyrolysis-free penta-coordination strategy to achieve atomic-scale control on site microenvironments,but also provides an excellent platform for understanding the catalytic enhancement mechanisms.2.The linear relationship between the adsorption energies of various ORR intermediates aids in understanding and predicting the ORR intrinsic activity trends of catalysts.However,there is still a lack of well-defined M-N4 catalytic configuration to systematically verify the Sabatier volcano activity plots from the experimental perspective.This thesis primarily proposes an activity descriptor based on the adsorption behavior of different oxygen intermediates（*OOH,*O,and*OH）on Co-N4 site and derives Sabatier volcano plots for activity prediction,thereinto the electron-deficient Co center is likely to approach the volcano top.Resultingly,a series Co porphyrin-based polymers were prepared,and the secondary sphere microenvironment customization strategy was proposed to promote the catalytic efficiency through customizing the Co center microenvironment.More detailedly,Co porphyrin-based polymers with electron-withdrawing carboxyl groups exhibits the best intrinsic ORR activity.Systematic in-situ spectroscopic techniques and in-situ electrochemical characterization attribute the high turnover frequency(4.6 e s^-1 site^-1)of the optimal carboxyl groups-substituted catalyst to the high achievable active site density(7.3×10¹⁹ sites g^-1)and rapid interfacial charge transfer/outward migration kinetics.This thesis clarifies the structure-activity relationship on site microenvironments and provides valuable guidance for designing high-efficiency atomically dispersed catalysts for practical applications.3.The adsorbed H2O molecules on catalysts surface are the sole proton supply source for alkaline ORR,and the activation energy for proton outward transfer is extremely high.Therefore,the continuous efficient water capture/dissociation and timely OH^-diffusion ensures the maximal ORR efficiency.Considering this fact,a dual active site configuration was constructed by coupling Co phthalocyanine（Co-N4）and Co Salphen（Co-N2O2）structural components to facilitate the ORR proton-coupled electron transfer conversion.Systematic electrochemical tests testify that,benefiting from the Lewis acid-type Co-N2O2 site as the proton donor,the main Co-N4 site exhibits significantly superior half-wave potential（0.91 V）and a smaller Tafel slope(29.2 m V dec^-1).Kinetic isotope effects confirm that the Co-N2O2 site accelerates proton-coupled transfer and reduces the energy barriers for proton generation in ORR.Theoretical calculations further elucidate that the Co-N2O2 site efficiently promotes water dissociation to generate active protons,while the main Co-N4 site facilitates O=O bond cleavage to accelerate rapid ORR conversion,synergistically optimizing ORR kinetics/thermodynamics.Furthermore,the dual-site catalyst enhances the efficiency of ZAB device with high power density(232.2 m W cm^-2).This thesis lays a solid foundation for exploring the ORR dynamic transformation mechanisms on catalytic interfaces and understanding the deep principles of ORR intrinsic proton-coupled electron transfer.