Preparation And Oxygen Reduction Performance Of Iron-doped ZIF-67-derived Carbon Catalyst

Nowadays,owing to the increasing environmental pollution and energy crisis caused by the huge consumption of fossil fuels,the clean energy-generation technologies play an indispensable role in the energy-consumption fields.Recently,proton exchange membrane fuel cells?PEMFCs?,especially direct methanol fuel cells?DMFCs?,have been studied as the efficient energy conversion technologies that can energetically derive the electricity from the oxidation of methanol.However,the large scale application of DMFCs is inevitably hampered by the low activity of catalysts and the sluggish dynamics of catalytic oxygen reduction reaction?ORR?.⁴ ORR is a complicated 4-electron?4e^-?or 2e^-?or 4/2 mixed?reaction involving the fracture of the double bonds of O₂ and the constitution of 4 O-H bonds,which govern the performance of DMFCs.In this paper,two ORR catalysts with oxygen reduction performance close to Pt/C,but better durability and methanol tolerance than Pt/C were prepared.Scanning electron microscope?SEM?,X-ray photoelectron spectroscopy?XPS?and other six methods were used to perform physical and chemical characterization of the prepared catalyst,and linear voltammetry scanning test?LSV?and rotating ring disk electrode test?RRDE?were used Six methods were used to test the ORR electrochemical performance and methanol tolerance of the prepared catalyst.In this study,a dodecahedron Co@N doped carbon graphitized?Co@DNC?composite is synthesized by simple hydrothermal method with ZIF-67 as template.Then,Fe S₂ nanoshells were deposited uniformly on the surface of the porous Co@DNC composite material?core?to prepare Co@DNC@Fe S₂ composite material,which was used as a cathode ORR catalyst for DMFC under alkaline conditions.Then adjust the loading of Fe Cl₃·6H₂O to study its effect on the morphology and performance of Co@DNC@Fe S₂ catalyst.Co@DNC@Fe S₂-0.5?Co@DNC-to-Fe Cl₃·6H₂O mass ratio of 0.5?catalyst shows a comparable ORR catalytic activity?onset potential of 0.942 V and half-wave potential of 0.846 V,vs.RHE?to commercial Pt/C?20 wt.%?in alkaline media.The core@shell structure and highly-active moieties of Co@DNC@Fe S₂-0.5contribute to the high ORR activity.DNC-wrapped Co/Co O?originated from the oxidation of surface Co⁰?possesses the positive charges to prioritize the adsorption of O₂ on the Co–N_x?Co atom binds to pyridinic N?active sites and then the S–S bonds in Fe S₂ with multivalent redox activity can supply/transfer the electrons to Co–N_x to boost the ORR via a 4e^-pathway.Moreover,DNC@Fe S₂ shell with porous structure can protect the active sites on the Co cores and accelerate the mass transfer of ORR-related species?O₂,OH^–,etc.?to the active sites.This study provides a novel strategy to enhance ORR activity using the core@shell structured catalysts originated from ZIFs.In addition,we also used ZIF-67 as a template to prepare a dodecahedral Co Fe₂O₄@NC@Fe₂O₃?CFO@NC@Fe₂O₃?composite material by hydrothermal method and high temperature carbonization method,which was used as an ORR catalyst for DMFC under alkaline conditions.It has been found through research that CFO@NC@Fe₂O₃-1 exhibits oxygen reduction performance equivalent to Pt/C,as well as excellent durability and methanol resistance.This is related to the fact that Co Fe₂O₄is an inverse spinel structure with considerable electrical conductivity and surface redox centers due to the electron hopping between different valence states of metals in Oct-sites.Moreover,Co O(from the oxidation of Co²⁺)with high density positive charge promotes the chemical adsorption of O₂ molecules and the breaking of O-O bonds,thus promoting ORR activity.In addition,the presence of?-Fe₂O₃ forms a protective membrane that successfully protects the active sites(Co²⁺and Fe³⁺),thus improving the stability and methanol resistance of the catalyst.In conclusion,the synergistic effect described above shows the good performance of CFO@NC@Fe₂O₃-1 catalyst.