Design And Application Of Iron-Based Nanoparticle/Heteroatom-Doped Carbon Electrocatalysts

Globally,the energy consumption situation is dominated by fossil fuels,high carbon emissions from non-renewable energy sources,and depletion of resources,raising concerns about environmental security and resource sustainability.Therefore,it is imperative to develop a sustainable green energy technology.The economic development of green and sustainable energy is inseparable from the new generation of sustainable energy technologies such as fuel cells and metal-air batteries.Among them,rechargeable zinc-air batteries are considered as a promising technology due to their environmental friendliness,high safety and high theoretical energy density.However,the poor reaction kinetics and high during charging and discharging greatly hinder the large-scale application of zinc-air batteries.So far,Pt and Pt-based alloys are the dominant electrocatalysts for oxygen reduction reaction（ORR）,and Ir and Ru are the dominant catalysts for oxygen precipitation reaction（OER）.However,their scarcity and high cost have seriously hindered their large-scale application in commerce.For this reason,considerable efforts have been invested to develop non-precious metal catalysts to replace the precious metal-based catalysts.Carbon-carrier transition metals are ideal replacements due to their low cost and high activity in facilitating the electrolytic process.Therefore,in this paper,a series of ORR,ORR/OER electrocatalysts with high specific surface area and high intrinsic active sites are prepared by doping different iron-based carbon materials with heteroatoms around the transition metal Fe.And they are designed and assembled in Zn-Air batteries for practical applications,and the studies are as follows:1.A carbon nanofiber precursor with Fe nanoparticles encapsulated in Si O₂hollow spheres embedded in Si O₂was prepared by electrostatic spinning using Si O₂and Zn Cl₂zinc as dual templates,Fe Pc as metal source and partial nitrogen source.The Fe-doped and N-doped graded porous nanofibers(Fe-N@CNFs_{（Zn Cl2+NH3）})electrocatalysts were obtained by pre-oxidation,carbonization,and etching of the template agent.The material is etched with different templating agents to produce a hierarchical porous structure with different pore sizes.The large specific surface area allows more exposure of the active sites and full contact with the electrolyte solution,which is characterized by fast mass transfer and electron transport.The catalytic efficiency of Fe-N structure is better than that of metal nanoparticle catalysts.Meanwhile,the high-temperature pyrolysis solves the problem of easy demetallization of phthalocyanine-based metals and improves the stability of the catalyst.the Fe-N@CNFs_{（Zn Cl2+NH3）}catalyst shows excellent ORR activity and in alkaline media with a half-wave potential of 0.859 V,which is better than the commercial noble metal Pt/C electrocatalyst.2.Porous carbon nanosheet catalysts（Fe S₂/Fe C₃@NSC）co-doped with Fe,N,and S were prepared by high temperature carbonization with iron p-toluenesulfonate as the metal source and S source,tannic acid as the carbon precursor,and melamine as the self-sacrificing template through simple mixing.Nynic acid as a carbon source,in which the O-diphenol group can complex well with transition metal ions,is an ideal carbon source used to dope excessive metal ions.Melamine as a templating agent was thermally condensed to graphitic carbon nitride（g-C₃N₄）at 500-600℃,and then rapidly decomposed to produce a large amount of gas to generate porous carbon nanosheets when the temperature was raised above 700℃again.This structure has a high specific surface area,which can fully expose the active sites and improve the catalytic activity of the catalyst.The use of iron p-toluenesulfonate as the metal source can be accompanied by S doping,which can increase the positive charge density of carbon adjacent to nitrogen,which is beneficial to enhance the chemisorption of O₂and weaken the O-O bond,thus promoting the reaction from O₂to OH-,improving electron transfer and enhancing the catalytic activity.the Fe S₂/Fe₃C@NSC catalyst has a high ORR activity with a half-wave potential up to 0.868 V.The performance is better than that of commercial Pt/C,and it also has high stability and methanol resistance.3.A bifunctional oxygen electrocatalyst（Fe Ni@NC）loaded with Fe Ni alloy on N-doped porous carbon substrate was prepared by using Fe Pc and Ni Pc as metal sources and organometallic skeleton（ZIF-8）as carbon precursors,compounding the metal sources with ZIF-8 by solvent and dual-solvent methods,and homogeneously distributing them on the carbon skeleton by high-temperature pyrolysis.The use of Fe Pc and Ni Pc as metal sources can make a large amount of nitrogen uniformly attached to the organic ligand,and then the N doping can be effectively carried out by high-temperature pyrolysis.Meanwhile,Fe Ni alloy was successfully synthesized and immobilized in the carbon substrate during the high-temperature pyrolysis of ZIF-8,and the synergistic effect of Fe Ni alloy resulted in a higher bifunctional catalytic activity compared with the monometallic doping.In addition,the Zn nodes were reduced and volatilized during the pyrolysis（Zn,boiling point=907°C）,which promoted the conversion of organic ligands into porous graphitic carbon substrates,making the carbon substrates have a larger specific surface area,which is favorable for the exposure of active sites,electron transfer and mass transport during the electrocatalysis.The optimized catalyst achieves an ORR half-wave potential of 0.85 V,which is better than that of commercial Pt/C.Among the OER,an overpotential of only 330 m V is required to drive a current density of 10 m A cm^-2,which is close to that of commercial Ru O₂,exhibiting good bifunctional catalytic activity.In addition,the designed catalyst has a higher peak power density and better cycling stability in rechargeable Zn-Air batteries.