Studies Of ZIFs-based M-N/C Catalysts As Non-noble Metal Catalysts For Oxygen Reduction Reaction

With the continuous development of the society,the depletion of non-renewable energy sources and environmental due to the large scale use of coal,oil and natural gas.Proton Exchange Membrane Fuel Cell?PEMFC?attracted much attentions by researchers for its low operating temperature,cleanness and high efficiency.In PEMFC,the platinum-based catalyst used for cathode is one of the main reasons limited its large-scale commercial application due to the high price,easy to poison and limited storage of Pt.Therefore,the research on non-noble metal catalysts with high activity,high stability and low cost is one of the current research hotspots.There are many factors that affect the structural and performance of the catalysts,such as carbon support,metal precursor,nitrogen precursor,heat treatment,etc.Among them,carbon support can affect the structure,conductivity and stability of the catalyst directly.In types of carbon materials,Zeolitic imidazolate frameworks?ZIFs?have many advantages,such as large specific surface area,high thermal stability,chemical stability,structural and functional adjustability,etc.It can be used as carbon support to prepare Fe N/ZIFs catalyst.In this works,Fe N/ZIF-8 catalysts were prepared by the process of wet impregnation,ball milling and pyrolysis using ZIF-8 as the carbon precursor.The effects of iron precursors and iron content,as well as second pyrolysis temperature and time on the structure and activity of the catalysts are investigated.On these basis,FeN/ZIFs catalysts were prepared using four kinds of ZIFs as carbon precursor respectively.In this paper,ZIF-8 was used as the carrier,and the effects of secondary heat treatment,secondary heat treatment time and temperature were investigated during the preparation of catalyst.Then,the influence of iron source and iron content on the catalyst was investigated.Then,the influence of ZIFs carrier on the catalyst was investigated.Thirdly,the catalyst was treated with acid before and after the second heat treatment,and the effect of acid treatment on the stability of the catalyst was investigated.Finally,the influence of iron and cobalt co-doping on the catalyst was preliminarily investigated.Linear sweep voltammetry was used to test the activity and stability of the catalyst.Brunauer-Emmet-Teller?BET?,transmission electron microscope?TEM?,thermogravimetric analysis?TGA?,X-ray diffraction?XRD?and other methods were used to characterize the structure and composition of the catalyst.The main results are as follows:1.Using ZIF-8,1,10-phenanthroline and FeSO₄·7H₂O as catalyst precursor,the influence of second pyrolysis on the catalyst structure and activity were investigated.The catalyst underwent second pyrolysis at 950? for 15 minutes in ammonia atmosphere showed best activity.The secondary pyrolysis has great influence on the specific surface area,particle size and surface nitrogen content of the catalyst.Then,using ZIF-8 and 1,10-phenanthroline as catalyst precursor,the effects of iron sources and iron contents on the catalyst were studied.the influence of second pyrolysis on the catalyst structure and activity were investigated.The catalysts have various specific surface area,pore size distribution and particle size with different iron source and content.The Fe3 C presented in catalyst using FeSO₄·7H₂O as iron precursor may the reason for its better activity.2.Different catalysts were obtained by using four kinds of ZIFs as carbon sources.The catalyst using ZIF-8 as the carbon precursor has highest catalytic activity with an initial potential of about 0.94 V.The specific surface area of FeN/ZIF-8-Ar-NH₃ catalyst is 526.744 m²·g^-1,the catalyst with larger specific surface area has better catalytic activity.3.The acid leaching were used to remove excess of iron in catalysts which using ZIF-8,1,10-phenanthroline and FeSO₄·7H₂O as catalyst precursors.The influence of acid leaching on catalyst activity and stability were studied by ADTs.The results show that the activity and stability of the catalyst obtained by acid leaching before the secondary pyrolysis is improved,with the initial potential of about 0.95 V,and 15 m V initial potential shift after 25,000 cycles of ADTs.4.The catalysts co-doped with iron and cobalt were prepared and the effect of metal sources and the mass ratio of iron to cobalt were investigated.The catalyst using cobalt sulfate as cobalt precursor with mass ratio of 1:3 show better activity with an initial potential of about 0.94 V.The synergistic effect of iron and cobalt is beneficial to the formation of catalytic active sites.