Studies Of Structural Regulation On ZIFs-based Core-shell Catalysts For Oxygen Reduction Reaction

The cathode of proton exchange membrane fuel cells（PEMFCs）needs to use catalysts to improve its reaction rate.At present,Pt-based catalysts are widely used,but Pt-based precious metals have problems such as high cost and insufficient stability,which limit their commercial development.Therefore,the research and development of highly performance non-precious metal catalysts（NPMCs）are critical to PEMFCs.Zeolitic imidazolate frameworks（ZIFs）,as a subclass of metal-organic frameworks（MOFs）,are excellent precursors for NPMCs due to their high stability,high specific surface area,high porosity,and adjustable pore sizes.In this paper,ZIFs core-shell materials were obtained based on the preparation of ZIFs materials.Then,ZIFs core-shell catalysts with high ORR catalytic performance were prepared by adjusting the structure of ZIFs core-shell materials.Furthermore,MN/Z₈@Z₆₇catalysts were prepared using transition metal salts,1,10-phen and ZIF-8@ZIF-67 as catalysts precursors.The role of metal species and contents on the catalytic activity of MN/Z₈@Z₆₇catalysts,as well as the synergistic effect between transition metals on Fe MN/Z₈@Z₆₇（M=Co,Cu,Ni）catalysts were investigated.The main research results are as follows:1.On the basis of the different ORR catalytic activities between Z₈@Z₆₇and Z₆₇@Z₈,ZIFs core-shell materials were prepared by seed-mediated epitaxial growth of Co-based ZIFs materials（ZIF-9,ZIF-67）on the surface of Zn-based ZIFs materials（ZIF-7,ZIF-8）.Z₈@Z₆₇（7:1）catalyst showed the best ORR catalytic activity and stability when the Zn to Co molar ratio was 7:1.The suitable content of Co₃C,large specific surface area and pore volume,abundant carbon nanotubes（CNTs）,pyridine-N and graphite-N in Z₈@Z₆₇（7:1）catalyst are the possible reasons for its best ORR performance.2.MN/Z₈@Z₆₇catalysts（M=Mn,Cu,Fe）were prepared by one-pot method and wet ball milling.The Fe N/Z₈@Z₆₇catalyst prepared by wet ball milling underwent one pyrolysis in Ar has better ORR catalytic activity than the one that underwent second pyrolysis in NH₃.The Fe N/Z₈@Z₆₇-OP-NH₃-1 catalyst prepared by the one-pot method underwent second pyrolysis in NH₃at 900°C for 15 min and this catalyst showed good ORR catalytic activity.The rich CNTs,metal-N,Fe₃C,and the possible synergistic effect of Co-Fe in the catalyst are the possible reasons why these catalysts had good ORR catalytic activities.3.After 16000 cycles of accelerated degradation tests（ADTs）,the half-wave potential(E_1/2)of Fe N/Z₈@Z₆₇-OP-NH₃-1 and Fe N/Z₈@Z₆₇catalysts decreased 53 m V and 46 m V,respectively.Both catalysts showed unsatisfactory stability due to the destroyed structure in catalysts resulting from the Fenton effect between Fe²⁺,Fe³⁺and H₂O₂.The Fe N/Z₈@Z₆₇catalyst prepared by wet ball milling under one pyrolysis in Ar showed better ORR stability than Fe N/Z₈@Z₆₇-OP-NH₃-1.4.Fe MN/Z₈@Z₆₇（M=Co,Cu,Ni）catalysts prepared by wet ball milling had lower ORR catalytic activities but excellent ORR stabilities compared to Fe N/Z₈@Z₆₇catalyst.After 16000 cycles of ADTs,Fe Ni N/Z₈@Z₆₇catalyst exhibited the best stability and catalytic activity with negatively shifted only 16 m V of E_1/2.The abundant nitrogen species and the synergistic effect between Fe and Ni are the possible reasons for Fe Ni N/Z₈@Z₆₇catalyst’s best stability and catalytic activity after ADTs.