Preparation Of Fe-N/C Porous Materials By Non-solution Method For Catalytic Reduction Of Oxygen

Metal-organic framework（MOFs）is an open three-dimensional network of matrix materials,with a high specific surface area,the structure controlled,coordination and species diversity and so on.Porous carbon materials have been extensively studied in recent years by pyrolysis of MOFs,especially zeormimidazole（ie,ZIFs,an important branch of MOFs）as precursors.Previous studies have shown that porous carbon materials obtained from either ZIF-8（Zn（MIm）,MIm=2-methyl imidazolium）or ZIF-67（Co（MIm））are not ideal oxygen reduction reaction（ORR）electrocatalysts.Among them,the carbon material obtained after the pyrolysis of ZIF-8 has a high surface area but lacks the catalytic active sites such as Fe/Co and has a low degree of graphitization.The pyrolysis products of ZIF-67 contained Co-N_xactive sites,but there is a large number of metal nanoparticles aggregation,greatly reducing the catalytic activity and specific surface area.Therefore,on the basis of keeping the high specific surface area of the carbon material generated after the pyrolysis,it is crucial to increase the dispersion of the M-N_xactive center of the metal.Based on these considerations,a new type of ZIF framework material is selected in this dissertation,that is,ZIF with rho topological structure with large internal cavity（about 18.1?）assembled with Zn and nitrogen-containing organic ligand2-ethylimidazole is the main framework.Metallotetraphenylporphyrin（M-TPP）as a guest molecule was successfully encapsulated into the inner cavity of ZIF by the non-solution method with mechanical milling to form M-TPP?rho-ZIF complex.The non-solution method has the advantages of simple synthesis steps,green environmental protection,non-precious metals,cost saving,batch synthesis and so on,which is simple and highly efficient;Finally,a high specific surface area metal/N-doped porous carbon material（M-N/C）was obtained as a precursor of oxygen reduction reaction,ORR electrocatalyst after being calcined at high temperature.Among them,iron-iron porphyrin iron formed Fe-N/C material performance is the most prominent.Compared with the pyrolyzed product of pure framework rho-ZIF and composite Fe-TPP?rho-ZIF,the oxygen reduction performance is greatly improved,which indicates that the addition of iron porphyrin is the key to the high catalytic activity of the composites after pyrolysis.Fe-N/C materials have excellent ORR properties both in alkaline and acidic conditions:in 0.1M KOH solution,the half-wave potential measures at 0.895 m V exceeded commercial Pt/C by about 53 m V;In 0.5 M H₂SO₄solution,the half-wave potential is found to be 0.744 m V lower than commercial Pt/C about 70 m V.Analyzing the catalyst activity so well is attributed to the following:（1）the high specific surface area of carbon material reaches 1219.2 cm²g^-1,which makes the Fe-N₄active site fully expose to participate in the reaction;（2）the distribution of highly ordered hierarchical porous structure（micropore/mesopore）facilitates the diffusion and mass transport of O₂during oxygen reduction;（3）Iron porphyrin wrapped into nitrogen-rich rho-ZIF porous framework can ensure the formation of Fe-N₄active sites and atomic distribution;（4）choosing optimization catalytic conditions at 950℃with a high degree of graphitization,electrical conductivity has been significantly improved;（5）the oxygen reduction reaction shows excellent cycling stability after 30000 s in the time-current curve.No significant fluctuations in methanol resistance were tested in 3M aqueous methanol.Rotary ring disk electrode（RRDE）test to determine the reaction path is mainly in accordance with 4 electron transfer process.