Study Of The Preparation Of Carbon-Supported Non-Noble-Metal-Based Single Atom Catalysts And Oxygen Reduction Reaction

Oxygen reduction reaction（ORR）is one of the most important electrocatalytic reactions in the field of new energy and chemical engineering.Currently,due to the high price of precious metals,the large-scale application of commercial Pt-based ORR catalyst is greatly restricted.Single-atom catalyst（SAC）refers to the atomic-level dispersion of metal active components on a specific substrate,exhibiting extremely high atom utilization（close to 100%）and excellent catalytic performance,which is a potential alternative to traditional Pt-based ORR catalysts.In addition,the well-defined electronic and chemical geometric structure of the SAC active center can help people to understand the mechanism of heterogeneous catalysis.The catalytic activity of carbon-based SAC（abbreviated as C-SAC）can be compared with commercial Pt/C in the ORR field.However,the currently reported C-SAC generally has the problems of complex preparation method,cumbersome steps and high cost.At the same time,important scientific issues such as the structure-activity relationship between the structure of the C-SAC catalytic center and ORR performance,as well as the regulation strategy of ORR activity,are still lacking in systematic research.Accordingly,this thesis started from the synthetic strategy,by means of molten salt-assisted pyrolysis to effectively solve the problem of easy agglomeration of metal atoms during the preparation of C-SAC;on this basis,the structure of the catalytic site was adjusted to further improve the C-SAC catalytic activity.In addition,the structural reconstruction of the C-SAC active center was achieved by post-modification,and the ORR reaction was successfully regulated from the 4-electron（4e^-）path to the 2-electron（2e^-）path,and we further clarified the structure-activity relationship between the C-SAC active center and the catalytic performance.The details are as follows:Firstly,based on the cumbersome steps of the current preparation method,a new synthesis strategy,molten-salt-assisted one-step pyrolysis,was proposed to prepare C-SAC.Its advantages include:（i）In the molten state,the strong polar force of molten salt can break metal bonds and prevent metal atoms from agglomeration;（ii）The molten salt environment has a well confinement effect on organic small molecules（carbon precursors）,which can avoid direct sublimation or thermal decomposition of small molecular precursors during high-temperature heating,so that carbonization can be carried out more effectively.This strategy greatly expands the selection range of metal and carbon precursors,and has good universality.Two C-SACs were prepared by molten-salt-assisted pyrolysis under an inert atmosphere and 900℃,with Na Cl/Zn Cl₂as the molten salt,adenine as the carbon precursor and Fe₂O₃ and Co₂O₃ as the metal source.With the help of high-angle annular dark field-scanning transmission electron microscope imaging（HAADF-STEM）,synchrotron X-ray absorption spectroscopy（XAFS）and other characterization methods,the electronic and chemical geometric structures of two C-SACs active centers were revealed to be Fe N₄and Co N₄,respectively.Furthermore,the ORR performance of the two samples in alkaline and acidic media was systematically investigated.The results showed that the catalytic activity of Fe-based C-SAC in alkaline media exceeded that of commercial Pt/C（20 wt%）,and the catalytic activity in acidic media was comparable to that of Pt/C.In addition,Fe-based C-SAC exhibited good stability both in alkaline or acidic media.Secondly,based on the catalytic inertness of the Cu-N₄ structure,the active sites were regulated through molten-salt-assisted pyrolysis to improve catalyst ORR catalytic activity(abbreviated as Cu N₂₊₂/C).The sample exhibited obvious porosity and rich edge effects.The structure of metal active center was studied through comprehensive characterization of TEM,HAADF-STEM,XAFS,XPS,etc.The results showed that the Cu-N bond length in the sample（1.93（?））exceeded the Cu-N bond in the conventional Cu-N₄ structure（1.91（?））and the symmetry of the active center was also different from the typical Cu-N₄ structure.Therefore,it was inferred that Cu and N in the sample formed a Cu N₂₊₂ coordination structure.The electrochemical performance test results displayed that Cu N₂₊₂/C had excellent ORR catalytic activity in alkaline medium with a half-wave potential of 0.88 V,which was 30 m V higher than that of commercial Pt/C,and had excellent stability and resistance to methanol.Theoretical calculations proved the source of Cu N₂₊₂/C activity lies in the perfect match between the d orbitals of Cu atoms in the Cu N₂₊₂ coordination structure and theπ*orbitals of the adsorbed O₂ molecules,thereby reducing the energy barrier required to activate O₂ molecules.At the same time,the upward shift of the d orbit reduced the energy barrier required for the ORR rate-determining step.Assembling Cu N₂₊₂/C into a Zn-air battery（the electrolyte is liquid）,the open circuit voltage（OCP）reached 1.51 V,and the highest power density was 228 m W cm^-2;assembling into a flexible solid-state battery,the OCP and highest power density reached 1.46V and 150 m W cm^-2,respectively,and the flexible solid-state battery can still maintain a constant charge and discharge potential difference even after 169h of stability charge and discharge test.Finally,the C-SAC active center was restructured by molten salt-assisted pyrolysis combined with ozone post-modification to adjust the selectivity of the catalyst.A Co-based C-SAC（abbreviated as Co N/C）was prepared by the molten-salt-assisted pyrolysis.On this basis,a new C-SAC（Co NO/C）was prepared by means of ozone post-treatment through which oxygen-containing functional groups were introduced and the chemical structure of the metal active center was reconstructed.XPS and XRD test results showed that ozone post-treatment significantly increased the O content in the sample,the OH group was attached to the C atom adjacent to pyridine N,and the degree of graphitization of the sample was increased;the BET results pointed that the ozone post-treatment oxidized the bulky amorphous carbon outside the carbon shell,reducing the specific surface area of the sample from 1716 m² g^-1 to 77 m² g^-1;combined with the comprehensive characterization of HAADF-STEM,XAFS,XPS,etc.,Co atoms existed in Co N₄Cl configuration and Co N₂Cl configuration in Co N/C as well as Co N₂configuration in Co NO/C.The electrochemical performance demonstrated that the 4e^-path was dominant when Co N/C catalyzed ORR reaction and the H₂O₂ selectivity was less than 30%within a wide electrochemical window;in contrast,with the synergy of Co single-atom sites and OH groups,the 2e^-path was dominant when Co NO/C catalyzed ORR reaction and the selectivity of H₂O₂ was as high as 90%,and it also had good catalytic stability.