Preparation Of Hierarchical Porous Fe-N-C Materials And Their Study Of Oxygen Reduction Reactuon

The evolution of clean energy conversion and storage technologies including fuel cells and metal-air batteries has attracted extensive research interests as a result of the rapid growing energy consumption and severe environmental issues.These technologies are inevitably associated with the sluggishly complex cathodic oxygen reduction reactions(ORR),which directly affects the overall performance and efficiency of the equipment.At present,platinum(Pt)-based materials are deemed to be state-of-the-art ORR catalysts,whereas high cost and scarcity dramatically restrict their commercial applications on a large scale.Therefore,developing cost-effective ORR electrocatalysts is urgently desired for the popularization of fuel cells and metalair batteries.Among various promising alternatives,Fe-N-C materials have been regarded as a formidable competitor to supersede the Pt-based ORR catalysts thanks to their relatively high intrinsic activity and low cost.Nevertheless,hierarchical pores and defects of hierarchical porous Fe-N-C materials all impede the transmission of electrons and leading to the decrease of conductivity,which markedly weakens the electrocatalyst activity.In this paper,a new type of crystalline carbon modified hierarchical porous Fe-N-C material was designed and synthesized using two synthesis processes: template method and in-situ growth method.The structural characterization and electrochemical performance tests were conducted to explore the mechanism of crystal carbon modification to enhance the oxygen reduction activity.The hierarchical porous Fe-N-C was first prepared by template method,and Fe Pc was added during the second calcination to catalyze the reaction to obtain the crystalline carbon modified hierarchical porous Fe-N/C-800.The prepared Fe-N/C-800 material has the clear hierarchical porous nanostructure and crystal lattice fringes consistent with crystalline carbon.The Fe-N/C-800 exhibits the excellent ORR activity,half-wave potential is 0.87 V,K-L equation fitted electron number n = 3.9,Tafel slope is 59 m V dec-1,and it shows the excellent cycle stability and resistance to methanol toxicity.The ZIF-8 functional body was anchored on the CNTs matrix with in-situ growth method to prepare the crystalline carbon modified hierarchical porous Fe-N/C-6.The Fe-N/C-6 sample contains the hierarchical porous structure and more crystalline carbon.The oxygen reduction half-wave potential is 0.85 V,the K-L equation fitted electron number n = 3.9,the Tafel slope is 56 m V dec-1,and it shows the good resistance to methanol toxicity and fine cycle stability.Replacing ZIF-8 with ZIF-67,the OER function was expanded,and the OER overpotential is 390 m V,showing the good ORR and OER performances.Combined with Mathhissen's rule,it was determined that the reason for the increase of ORR activity of the prepared hierarchical porous Fe-N/C-m material was that the modification of the crystalline carbon provides a faster channel for electron transport in the hierarchical porous carbon-based Fe-N/C-m material,and improves the conductivity of the hierarchical porous carbon.The crystalline carbon modified hierarchical porous carbon-based Fe-N/C-m materials guarantee the balance of the hierarchical porous structure and crystalline carbon,as well as the good mass transfer rate and good electrical conductivity.This is the fundamental reason why crystalline carbon modified hierarchical porous Fe-N/C-m materials exhibits excellent ORR activity.