The Preparation Of Iron,Nitrogen And Phosphorus Doped Graphene And Its Application For Oxygen Reduction Reaction

Fuel cell is an environment-friendly electrochemical power generation device with high energy efficiency.However,it suffers from a high cathodic activation potential which causes a serious voltage loss.How to reduce cathodic oxygen reduction reaction(ORR)potential and accelerate the ORR reaction are important for the commercial application of fuel cell.Developing low cost,highly active and stable cathode catalyst is crucial to address the above problems for ORR cathode reaction.As a two-dimensional carbon material,graphene has many excellent properties such as high electrical conductivity,large specific surface area,and chemical stability,which open up the possibility for ORR application.However,pristine graphene is lack of ORR active sites,and consequently has a low catalytic activity.In this work,we develop a ternary-doped graphene material with iron,nitrogen and phosphorous,which shows an improved catalytic activity than the common iron and nitrogen co-doped graphene materials because of the synergistic effect of iron,nitrogen,phosphorous and carbon atoms.The ternary-doped graphene material was prepared by high temperature heat treatment under an inert gas atmosphere using graphene as basal material,ferric nitrate and ferric chloride as iron source,cyanide diamine as nitrogen source,and phytic acid as phosphorus source.Raman,SEM,TEM,XPS,XRD and nitrogen isothermal adsorption measurements were carried out to characterize the composition and structure of the samples.The oxygen reduction performances were measured by cyclic voltammetry,rotating-disk electrode and ring-disk electrode techniques.In particular,we studied the influence of heat treatment temperature,acid leaching and the second heat treatment methods,and different doping elements on the chemical composition,microstructure and catalytic performance of the material,including the onset potential(Eonset),half wave potential(E1/2),diffusion limit current density,the yield of peroxide,the average electron transfer number and catalytic stability.The main conclusions are as follows:(1)The ORR catalytic properties of the iron,nitrogen and phosphorus co-doped graphene material strongly depend on the heat treatment temperatre.Among the samples obtained by 800?,900 ? and 1000 ? treatment,the Fe/N/P-G-9 sample obtained by 900 ? heat treatment shows the best catalytic performance,close to that of commercial Pt/C catalyst.(2)Compared with Fe/N/P-G-9,the Fe/N/P-G-9-H-9 sample obtaind by further acid leaching and the second heat treatment shows a big improvement in specific surface area(mainly mesopores),nitrogen and phosphorus doping quantities,highly reactive nitrogen content and proportion of P-O bands.Moreover,Fe/N/P-G-9-H-9 sample has wrinkle,curly structure and very uniform Fe,N and P doping.As a result,it shows an improved ORR catalytic erformance than Fe/N/P-G-9.(3)After performing a secondary nitrogen doping during the second heat treatment process,the product,Fe/N/P-G-9-H-9-N,shows a better catalytic activity than Pt/C.The Eonset and E1/2 of Fe/N/P-G-9-H-9-N are increased by 31 mV and 39 mV,respectively,than those of Pt/C.Morevoer,it follows almost a four-electron-transfer pathway,and has a low rate of peroxide products and higher catalytic stability than Pt/C.(4)The ORR performance comparison of nitrogen and phosphorus,iron and phosphorus,and iron,nitrogen dual doped graphene and iron,nitrogen and phosphorous ternary-doped graphene shows that(i)N contributes most to the catalytic activity of ORR,followed by Fe and P in sequence,and(ii)on the basis of two elements dual doping,the introduction of a third element can further improve the catalytic performance because of the improved synergistic effects between different elements.