Solvent-free Preparation And Electrocatalytic Oxygen Reduction Performance Of Iron-based Nitrogen-doped Carbon Materials

Fuel cells have become one of the most promising clean energy equipment by virtue of their high energy conversion efficiency and environmental-friendly feature.However,the large-scale commercialization of fuel cell is predominantely determined by the slow kinetics of cathode oxygen reduction reaction（ORR）.As a consequence,it is urgent to develop appropriate catalyst to accelerate the cathode oxygen reduction to promote the conversion efficiency.Until now,platinum（Pt）-based catalysts have been widely used as the most efficient catalysts to catalyze the inherently sluggish ORR.While the scarcity,high cost and poor stability of Pt limited its wide application.Among many candidate materials for ORR catalysis,transition metal-nitrogen-carbon catalysts have received extensive attention because of their low cost and relatively good stability.Since the intrinsic activity of this type material is lower than that of commercial Pt/C,it is main problem to improve its ORR catalytic activity of transition metal-nitrogen-carbon catalysts.In addition,the reported synthetic process of transition metal-nitrogen-carbon catalytic material is complicated.Therefore,it is necessary to find a simple method to prepare transition metal nitrogen carbon materials with high catalytic activity.Based on that,we developed three efficient Fe-N-C ORR catalyst through a solvent-free synthetic method using abundant and low-cost chemicalsas the starting materials in our study.By optimizing the synthesis conditions such as reaction materials and pyrolysis temperature,the iron-based nitrogen-doped carbon materials with different microstructures were prepared.Then,the affect of catalyst structure and composition on the electrocatalytic performance has been systematically investigated.The research includes the following three contents.（1）The porous Fe-N-C catalysts were synthesized by using different iron,nitrogen and carbon sources.XRD,SEM and Raman were used to characterize the composition,morphology and graphitization degree of the samples,and their electrochemical properties were tested in alkaline solution.The Fe-N-C nanoparticles that was prepared using iron nitrate as iron source,melamine as nitrogen source,PEG4000 as carbon source exhibits the best ORR activity than other semples,which may originate from its uniform size,good crystallinity,high graphitization and good electrical conductivity.The onset potential of Fe-N-C is 0.96 V,the half-wave potential is 0.82 V and the limiting current density is 5.4 m A cm^-2.（2）The two-dimensional porous Ni/Fe-N-C（NFCN-MP4000）was prepared by pyrolysis of the prussian blue analogues Ni₃[Fe（CN）₆]₂that was synthesized via a simple solid-sate chemical reaction and served as a metal source,the melamine as nitrogen source and the PEG as carbon source,which may own to the synergistic effect of melamine and PEG in heat treatment process and the in situ formed Ni Fe alloy in the carbon skeleton as the pore template.Due to the abundant pore and two-dimensional lamellar structure,the NFCN-MP4000 catalyst has a large specific surface area,which can provide more active sites for ORR reaction.Thus the NFCN-MP4000 catalyst with high-density M-N_x active sites exhibits comparable ORR acitivity,superior long-term stability,better tolerance to methanol than that of commercial Pt/C catalyst.The onset potential,half-wave potential and limiting current density of NFCN-MP4000 are 0.94 V,0.80 V and 6.1 m A cm^-2,respectively.After 18000 s reaction in the same condition,the current density of the NFCN-MP4000 remained 92%.（3）The one-dimensional core-shell Fe₃C@NCNTs nanotubes（FFCN-MP4000）were constructed by controlling the pyrolysis temperature of prussian blue Fe₄[Fe（CN）₆]₃,melamine and PEG.Fe₃C nanoparticles and Fe-N_x sites of NCNTs can cooperate to catalyze the reduction of oxygen,and the special encapsulated structure can effectively protect the Fe₃C active sites,FFCN-MP4000 exhibits superior oxygen reduction performance,as well as good long-term stability,better tolerance to methanol than that of commercial Pt/C catalyst.The onset potential,half-wave potential and limiting current density of FFCN-MP4000 is 0.96 V,0.83 V and 6.63 m A cm^-2,respectively.After 10000 s reaction in the same condition,the current density of the FFCN-MP4000 remained 84%.