Preparation Technology And Oxygen Reduction Performance Of Fe-Based Catalysts

Electrocatalytic oxygen reduction?ORR?can be used as an electrochemical reaction technology to convert chemical energy into electrical energy.,which is widely used in fuel cells,metal-air batteries and other electrochemical energy conversion devices.Platinum and other noble metal catalysts easily adsorbs the reactants to form an intermediate"active compound"on its surface,which results in excellent catalytic activity,selectivity and stability,and is considered to be the most important ORR catalyst material at present.However,the precious metal reserves are small and expensive,which limits the large-scale application and promotion of the catalyst.Compared with noble metal electrocatalysts,inexpensive non-precious metal electrocatalysts also have excellent electrocatalytic properties,especially iron alloys and compounds?oxides,nitrides,carbides,etc.?,which are considered to be one of the most promising catalysts to replace noble metal catalysts.In this study,the preparation process of iron-based catalysts was studied,especially the heat treatmentprocess.Generally,the heat treatment of precursors requires atmosphere protection,high temperature heating and other conditions.Compared with the traditional preparation method,we studied the preparation of iron-based catalysts and their ORR properties by carbon bath and microwave-assisted carbon bath.The carbon bath method and microwave-assisted carbon bath method can not only effectively improve the heat treatment efficiency of catalyst precursor,but also achieve the reduction of materials,element doping,etc.,and can also prepare highly dispersed and small-sized catalysts,which provides a new idea for the efficient preparation of catalyst materials.The details are as follows:?1?Synthesis and properties of Fe₃O₄/Fe₃C@NC catalyst by carbon bath method.We have developed a muffle furnace carbon bath method to successfully synthesize Fe₃O₄ nanoparticles embedded in Fe₃C nanoparticles encapsulated in N-doped carbon shell.Moreover,the proper amount of Fe₃O₄ nanoparticles adjacent to the Fe-N_x active site changes the activity of the catalyst,and having a synergistic effect with the special core-shell structure for improving the ORR performance.In the alkaline medium,the initial potential and limiting current density of the catalyst were 0.966 V vs.RHE and 5.59 mA cm^-2,respectively,which was superior to the Pt/C catalyst.?2?Synthesis and properties of Fe/Fe₃C@NC catalyst by microwave-assisted carbon bath method?MW-CBM?.We developed a microwave radiation technique for the carbon bath method,successfully prepared Fe embedded in the in-situ N-doped graphite layer wrapped Fe₃C nanoparticles at 750W for 35minutes.The presence of Fe nanoparticles in the vicinity of the Fe-N_x site improved the adsorption and reduction of oxygen on the catalyst surface,while the increase in specific surface area and good mesoporous structure not only exposed more catalytically active sites,but also promote electron transport,and synergized with the special core-shell structure to increase the ORR activity of the catalyst.The catalyst had a half-wave potential of 0.87 V vs.RHE and a limiting current density of 6.87 mA cm^-2.It also had good stability and methanol resistance over commercial Pt/C.?3?Synthesis and properties of FeCo@NC catalyst by microwave-assisted carbon bath method.TheN-doped graphite-coated FeCo alloy particles were successfully synthesized by MW-CBM method at a power of 900 W for 10 min,and the wrapped graphite carbon layer was peeled off by dielectric barrier discharge plasma treatment,which reduced ORR reaction resistance and facilitated charge transfer.It also causes an increase in the active sites of internal defects of the catalyst,which is favorable for the ORR reaction.At the same time,the plasma treatment increased the content of pyridine N and graphite N which was favorable for the ORR reaction.These characteristics together improved the activity of the catalyst,and the catalyst showed excellent onset potential and half-wave potential?0.95 V vs.RHE and 0.88 V vs.RHE,respectively?in alkaline medium.The stability of the catalyst was tested by i-t method.After running for 32,000 seconds,the catalyst still maintained 75.6%initial current density,which was better than 20 wt%commercial Pt/C under the same conditions.