Study Of FeNS/C As Cathode Electrocatalyst Toward Oxygen Reduction Reaction In Proton Exchange Membrane Fuel Cell

Fuel cell is an environmentally friendly and efficient mobile energy conversion power generation device.Proton exchange membrane fuel cell?PEMFC?,as one of the important representatives of fuel cells,has the advantages of low operating temperature,fast start-up speed,convenient operation,long service life and high energy conversion efficiency,which shows abroad application prospect intransportation,military and other aspects.However,the current comercial scale of the battery development has been hampered by a number of factors.Among them,the high production cost caused by using a large number of rare precious metal Pt as cathode oxygen reduction catalyst is one of the important factors restricting the promotion and application of PEMFC.Therefore,in order to promote the further development of PEMFC,we urgently need to find a cheap non-noble metal catalyst with excellent performance of oxygen reduction?ORR?to replace the Pt based catalyst and reduce the loss of Pt.After years of research,it has been found that the oxygen reduction catalytic performance of FeN/C catalysts is good,and it is one of the most promising non-noble metal electrocatalytic materials.However,the activity and stability of the catalysts were poor under strong acid conditions.In recent years,a number of electrocatalysts with excellent oxygen reduction properties have been prepared by doping carbon based materials with heteroatoms?B,N,P,S,F,etc.?.The principle is to regulate the surface electronic structure of carbon materials through doping by using the difference between heteroatomic and carbon atomic electronegativity,thus changing the electrocatalytic performance of the catalyst.Due to the unique atomic structure of S and the strong synergistic effect between N and S,carbon based catalysts co-doped by N and S usually show better oxygen reduction activity than other hetero-doped materials.Therefore,in this paper,a series of FeNS/C catalysts were prepared by doping different sulfides on the basis of there search on FeN/C catalysts in the early stage,and the effects of sulfur sources,nitrogen sources and heat treatment conditions in the precursor on the oxidation-reduction activity of the catalyst were investigated,and the stability of the catalyst with better activity was preliminarily explored.At the same time,physical characterization of the catalyst was carried out by means of surface area and pore size distribution test?BET?,transmission electron microscope?TEM?,X-ray diffraction?XRD?and thermogravimetric analysis?TGA?to explore the structure-activity relationship between the structure of the catalyst and the catalytic performance of oxygen reduction.Main research contents are as follows:1.FeNS/C catalyst was prepared by using FeCl₃ as the iron source,1,10-phen as the nitrogen source,and Black Pearl 2000?BP?as the carrier,respectively using thiourea,ammonium thiocyanate and ammonium persulfate as the sulfur source,and changing the content of each sulfur source.The results showed that when thiourea was used as the sulfur sourceand the sulfur content in the catalyst precursor was 1%,the prepared catalyst had larger surface area,more ORR active sites and better catalytic activity.The catalyst was treated at high temperature under NH₃ atmosphere,its specific surface area was obviously increased,the crystalline structure was improved,and ORR activity was also enhanced.Therefore,sulfur sources and heat treatment conditions in the precursor have important influence on ORR activity of the catalyst.2.Based on the above experiment,fixed thiourea as sulfur source?the catalyst precursor contains 1wt.%S?,changed the mass ratio of carbon and nitrogen source to BP/phen,and prepared FeNS/C catalyst to investigate the effect of nitrogen content in the precursor on the activity of catalyst ORR.The results showed that when phen was used as nitrogen source,BP/phen was selected as 6/4,and the best activity of catalyst ORR was obtained after NH₃ secondary heat treatment.Through calculation of K-L equation,the main choice of this catalyst in the electrochemical reaction process is the 4e^-path of directly generating H₂O through catalytic O₂ reduction.After 10000 cycles of rapid cyclic scanning,the activity of ORR of the catalyst was significantly reduced,and its stability was worse than that of FeN/C catalyst studied earlier.This has an important relationship with the agglomeration of catalyst particles and the increase of particle size during aging.