Preparation And Performance Of Iron And Nitrogen Doped Carbon-Based Catalysts For Oxygen Reduction Reaction

The slow cathodic oxygen reduction reaction（ORR）in proton exchange membrane fuel cells（PEMFC）is one of the major factors limiting their large-scale scale-up.The Pt-based noble metal catalysts with the most desirable catalytic performance are currently facing problems such as high cost,low storage capacity,susceptibility to poisoning and poor stability,so the development of highly active,low-cost non-precious metal oxygen reduction catalysts is crucial.In this thesis,atomic-scale iron-based nitrogen-doped carbon non-precious metal catalysts（Fe/N/C）with high density of monodisperse active sites were prepared to improve the oxygen reduction performance of the catalysts through structural modulation to optimise the pore structure,specific surface area and active site density.By introducing acetate and melamine into the precursor preparation process,respectively,two Fe/N/C catalysts with high density of active sites were prepared.The two synthetic strategies provide a new reference method for the design and construction of non-precious metal oxygen reduction catalysts with high-density active centres and porous structures.Fe/N/C catalysts derived from zeolitic imidazole framework compounds（ZIF-8）are considered to be one of the most promising non-precious metal catalysts for the development of proton exchange membrane fuel cells.However,their ORR mass activity is still far below that of Pt-based noble metal catalysts.A molecular scissors"acetate"shearing strategy was used to significantly improve the ORR mass activity of Fe/N/C catalysts,and catalysts with a graded porous structure（Fe@MNC-OAc）were prepared.The acetate increased the Fe doping content of the Fe@ZIF8 precursor,thus increasing the Fe N₄ active site density in the Fe@MNC-OAc catalyst.In addition,as a porogenic agent,acetate increased the porosity of the catalyst,exposing more active sites.The Fe@MNC-OAc catalyst was used as the cathode for PEMFC with a peak power density(P_max)of up to 903 m W cm^-2 at H₂-O₂（1 bar）.The catalytic activity of Fe/N/C catalysts for oxygen reduction in PEMFC is constrained by low Fe N_x active site density（SD）and insufficient accessibility.On this basis,catalysts with a high density of atomically dispersed active centres（Fe@NC-MA）were prepared by direct pyrolysis of Fe@ZIF8-MA precursors using a melamine soft-moulded version co-firing strategy.The melamine not only compensated for the nitrogen loss due to high temperature pyrolysis,but also effectively corroded the carbon substrate and increased the specific surface area and porosity of the catalyst.When the mass ratio of melamine to Fe-ZIF8 was 1:1,the synthesised Fe-@NC-MA catalysts had a rich three-dimensional interconnected structure,high specific surface area and high density of Fe-N_x active sites,exhibiting excellent ORR activity.