Synthesis And Oxygen Reduction Reaction Performance Of Fe-N-C Catalysts

Recent year,fuel cells with high energy density and friendly environment have become global concerned focus due to the depletion of fossil resources and environmental degradation.The development of efficient and stable ORR catalyst is a pivotal basic problem that needs to be solved due to the sluggish kinetics of ORR.In this paper,a series of Fe-N-C catalysts with different structures were designed and prepared to solve these activity and stability problem.The mechanism of ORR activity was explored by combining modern spectroscopy and electrochemical methods.The main contents include the following three parts:?1?Fe-N-CNT@GN catalyst was prepared through hydrothermal and calcination method,using nitrogen-enriched melamine,ferric chloride and disodium edentate as nitrogen,iron and graphene precursors,respectively.Physical characterization results show that the iron species of Fe-N-CNT@GN formed Fe3 N and evenly dispersed on CNT as well as were covered in graphitic carbon layer with the thickness of 2-3 nm.The onset and half-wave potential of Fe-N-CNT@GN is-0.13 V and-0.25 V.Chronoamperometric current-time responses show a little attenuation with a current loss of 5%?Pt/C of 17%?at the potential of-0.4 V.Accelerated aging tests show that the ORR curves of this catalyst is almost no change after 8000 continuous cycles scanning tests?the half-wave potential of Pt/C negative shift 20 mV?,suggesting that this catalyst possesses superior stability to the Pt/C.Such excellent ORR activity mainly results from the formation of Fe3 N species,and the superior stability can be attributed to N-doped graphitic carbon layers.?2?Fe/FeCx@NC catalysts were prepared via a facile two-step anneal strategy using glucose,aniline and FeCl₃ as precursors,and SiO₂ nanospheres as template.The Fe/FeCx@NC catalyst possesses abundant mespores,large specific surface area and core-shell structured metal species,which composed of Fe/FeCx core and N-doped carbon shell.The onset and half-wave potentials of the optimized Fe/FeCx@NC-800-0.5 are 0.010 V,-0.109 V,respectively.Accelerated aging tests demonstrate that the ORR curves of the Fe/FeCx@NC-800-0.5 catalyst is almost no change after 8000 continuous cycles scanning tests?the half-wave potential of Pt/C negative shift 20 mV?,suggestting that the Fe/FeCx@NC-800-0.5 catalyst shows more high stability than that of the Pt/C.By regulating secondary anneal time,temperature and precursors,the formation of core-shell structure was mainly due to the different diffusion rates of Fe,C during the secondary pyrolysis process and the formation of iron carbide species.In addition,it is concluded that the excellent ORR activity mainly comes from the synergistic effect between Fe/Fe Cx core and N-doped carbon shell.?3?FePc-CNT catalyst was synthesized by wet ball-milling,and the ORR activity was tested in KOH electrolyte containing EDTA^2-ion.Physical characterization results show that the lattice space of iron species increased after ORR in 0.05 M EDTA^2-+ 0.1 M KOH,and the congfiguration and state of surface N,Fe have shifted positively.By analyzing the ORR activity of various Fe-N-C catalysts in 0.05 M EDTA^2-+ 0.1 M KOH,it can be found that the onset potential was increased about 100 mV for the Fe-containing catalyst while the ORR activity of ketjen black has no change.It can be concluded that EDTA^2-ion not only adjusts pH value of electrolyte,but also might be reacted with iron ions of the catalyst surface.The mechanism of ORR need to be further explored.