Structure Control And Oxygen Reduction Reaction Electrocatalytic Performance Of Iron And Nitrogen Co-doped Carbon Materials Prepared By In-situ Pyrolysis Method

Oxygen reduction reaction(ORR)is an important electrode reaction in fuel cell and other energy conversion devices.Electrocatalysts could play a substantial role in increasing kinetic rate and reaction efficiency in the energy conversion technologies.However,most state-of-the-art electrocatalysts are based on precious metals(e.g.,platinum)with extremely high price and low yield.It is of great practical significance to develop low-cost and highly efficient non noble metal catalysts.In this thesis,we demonstrate an effective in-situ pyrolysis method by introducing several oxygen-contained ligands and nitrogen-contained ligands to prepare a series of 2-D FeNC electrocatalysts.The density and dispersity of the active site are significantly increased,and excellent ORR catalytic activity,electrochemical stability and fuel selectivity are shown in both acid and alkaline media.First,a series of oxygen-containing ligands based g-FeNC-PA catalysts are prepared by in-situ pyrolysis and the effects of the experimental conditions on the morphology,structure and catalytic properties of the catalysts are investigated.Moreover,the structural modification of ligands by halogen further controls the precipitation of metal nanoparticles.The g-FeNC-PABr catalyst performs the excellent ORR activity in alkaline medium with half wave potential of 0.86 V vs.RHE,which is 60 mV higher than that of t-FeNC catalyst,and better than commercial Pt/C.Secondly,in order to further enhance the catalytic activity of catalysts in acidic medium,a series of FeNC catalysts are prepared by using different nitrogen-contained ligands.Because of the strong complexing effect,the metal precipitation in the thermal condensation process is suppressed,and the catalyst inherits 2-D structure from the g-C3N4 intermediate.The g-FeNC-PT catalyst with phenanthroline hydrochloride as the ligand exhibits the atomic-level dispersion of the active site by optimizing the pyrolysis conditions.In alkaline electrolyte,the half wave potential is 80 mV and 30 mV more positive than that of t-FeNC and Pt/C,respectively.Meanwhile,the half wave potential of the g-FeNC-PT is 0.77 V vs.RHE in acidic electrolyte,70 mV more positive than that of t-FeNC prepared without ligand and very close to Pt/C(0.82 V vs.RHE).Finally,we reveal the mechanism of ligand on the ORR catalytic activity through the analysis of the intermediate products,the carbonization process and the active site.Research shows that the interaction of ligand and Fe3+ and g-C3N4 weakens the interaction and increases the distance between Fe3+,so that the Fe element is dispersed uniformly and the process of metal thermal reduction is delayed.That avoids the formation of the structure of non active iron nanoparticles coated with carbon nanotubes.The catalysts retain the Fe-Nx active sites with high density and high dispersion after the pyrolysis,and finally achieve the excellent ORR catalytic activity both in acid and alkaline media.