Design And Catalytic Performance Of Transition Metal/nitrogen Co-doped Porous Carbon Catalysts

Environmental pollution and energy crisis have become two major issues in morden society,and it is thus urgent to explore and ultilize clean energy sources.Rechargeable flexible solid Zn-air battery,with a high theoretical energy density,great safety,low cost and zero carbon emissions,attract tremendous recent research interests.Nevertheless,the practical application is greatly hindered by the sluggish oxygen reduction reaction（ORR）kinetics on the air electrode.Therefore,the development of high-efficient ORR catalysts is greatly desired but still challengeable.Currently,the most effective catalysts are Pt-based materials,but its high price and scarce reserves limit the large-scale application.Transition metal/nitrogen co-doped carbon（M-N-C）materials are a potential substitute for Pt-based catalysts due to their low price and high activity.This thesis chooses M-N-C catalyst as the research object.Three different types of M-N-C were prepared by optimizing the inherent mass transfer process of the catalysts,increasing the density of catalyst active sites,and enhancing the intrinsic activity of the catalytic active sites.The application of these M-N-C catalysts in ORR and zinc-air batteries was also explored:1.A size-induced strategy is proposed to optimize the inherent mass transfer process of ORR catalyst.The core@shell structure materials of ZIF-67@ZIF-8 with different particle sizes were prepared in a controllable manner.After pyrolysis,uniform Co and N co-doped porous carbon materials are formed.These materials have similar surface areas and adjustable particle sizes ranging from 10 nm to 2 μm.Through systematic kinetic studies,the strategy of size-induced inherent mass transfer is proved,and the 700 nm material exhibits excellent performance in both ORR and rechargeable Zn-air batteries.During the electrocatalytic ORR process of CS Co@NC-700,the half-wave potential reaches 0.86 V.A Zn-air battery constructed with CS Co@NC-700 as a catalyst can achieve a specific capacity of 757.3 mAh g-1 and a peak power density of 221 mW cm-2.2.A new type of dense metal-organic framework（ZIF-EC1）material was prepared,and the new structure was discovered from the mixed phase by continuous rotating electron diffraction.The pure phase ZIF-EC1 was successfully prepared,which was constructed from mononuclear and dinuclear Zn clusters and contained high-density N and Zn sites.Furthermore,ZIF-EC1 is converted into N-doped carbon material（NC-ZIF-EC1）.The precursor ZIF-EC1 has a high active site density,and the dense non-porous structure is also transformed into a porous structure after being converted to NC-ZIF-EC1.The NC-ZIF-EC1 shows the best catalytic performance,and the half-wave potential of electrocatalytic ORR can reach 0.86 V and the limiting current density is about 5 mA cm-2.After 10 hours of electrolysis,the current density of NC-ZIF-EC1 only dropped by 7.4%,proving the excellent stability.3.Chiral N-stearyl-L-glutamic acid was selected as the template,and pyrrole（Py）was used as the nitrogen-doped carbon material precursor.Because of the strong electrostatic interaction,polypyrrole（PPys）can effectively form a helical structure at the molecular level through subsequent molecular assembly of chiral surfactants.The helical structure and the strong electron-withdrawing ability of N atoms enable the helical PPy to anchor atomically disperse metal ions and stabilize these structures.In this work,Fe（acac）3 is loaded into the spiral polypyrrole as a metal source,and then the thiophene is refluxed and mixed with Fe atoms to form PPy@Fe-S.The CCNT@Fe-S obtained after pyrolysis was used as ORR catalysts and exhibit great activity and stability both in 0.1 M KOH and 0.1 M HClO4 conditions.The half-wave potential is as high as 0.91 V in alkaline electrolytes and the limiting current density is about 6 mA cm-2.After 10 hours of electrolysis,the current density of PPy@FeS only dropped by 11%.The half-wave potential is 0.78 V in acid electrolytes and the limiting current density is about 5.8 mA cm-2.After 10 hours of electrolysis,the current density of PPy@Fe-S only dropped by 15.0%.