Preparation And Properties Of Iron-Based Nitrogen Doped Porous Carbon Catalysts For Oxygen Reduction Reaction

The sluggish kinetics of the cathode oxygen reduction reaction（ORR）limits the reaction rate in hydrogen fuel cells.At present,platinum-based catalysts are the most commercially used cathode catalysts with outstanding performance.However,the low natural abundance of platinum,high price,and poor cycle stability hinder the wide application of platinum-based catalysts in industry.It is imperative to develop the inexpensive,efficient and stable non-precious metal cathode catalysts.Among them,non-noble metal-doped carbon materials have attracted lots of interests due to their efficient ORR catalytic activity.There are many factors,including the type and content of metals,specific surface area,elemental composition and graphitization degree of carbon materials,play crucial influence on their catalysis performanceBased on the above considerations,a series of iron-based nitrogen-doped porous carbon materials,including Fe₃C-encapsulated iron-nitrogen co-doped carbon nanotube composites and concave N-doped carbon cubes structure adorned with high-density Fe-N_x were prepared.Prussian blue analog（Zn-FePBA）with tunable composition and uniform structure was selected as the iron source.The structure-activity relationship between the structural components of the catalysts and the electrocatalytic oxygen reduction performance was systematically explored.The main contents of the dissertation are as follows:（1）Fe₃C-encapsulated iron-nitrogen co-doped carbon nanotubes were prepared by direct pyrolysis of Zn-FePBA with external carbon and nitrogen sources.In the experiment,2-methylimidazole and dicyandiamide were used as external carbon and nitrogen sources.2-methylimidazole and Zn-FePBA were mixed physically and then placed in a tube furnace with dicyandiamide.After carbonization in N₂ atmosphere,acid treatment and secondary calcination were performed to prepare the Fe₃C-encapsulated iron-nitrogen co-doped carbon nanotubes（Fe₃C/Fe-N-CNT）.The proportion of iron in the precursor affected the size of iron nanoparticles,which catalyzed the growth of carbon nanotubes,thereby regulating the diameter of carbon nanotubes.The half-wave potential of Fe₃C/Fe-N-CNT-3 catalyst could reach 0.851V,which was comparable to commercial Pt/C.In addition,this dissertation explored the effect of pyrolysis conditions on the oxygen reduction performance of Fe₃C/Fe-N-CNT.Finally,the optimal calcination conditions were determined as 2℃min^-1 heating to 400℃for 2 hours,then heating to 900℃for 2 hours,and then acid treatment for secondary calcination.The final product obtained was Fe₃C/Fe-N-CNT900-3-slow with a half-wave potential as high as 0.858V,which exceeded Pt/C by 20m V.（2）The precursor with core-shell structure（ZIF-8@PBA）was prepared after coating Zn-FePBA with ZIF-8,and concave N-doped carbon cubes structure adorned with high-density Fe-N_x was prepared by pyrolysis.The surface curvature of NC@Fe-N-C can be effectively regulated by changing the shell thickness,and the NC@Fe-N-C-2 derived from the ZIF-8@PBA-2 precursor had the largest curvature,thereby realizing the surface maximize point activity.In addition,the strong interaction between the core-shell restricted the movement of iron atoms during pyrolysis,thereby preventing the generation of iron nanoparticles and increasing the Fe-N_x active site density,resulting in efficient and stable electrocatalytic performance.The catalyst had excellent performance in both acidic and alkaline electrolytes.The half-wave potential in alkaline solution was 0.86V,and the half-wave potential in acidic solution was 0.75V.After 5000 cycles in alkaline solution,the performance had no obvious attenuation.