Preparation And Electrocatalytic Oxygen Reduction Properties Of Graphene Supported Iron Based Nanomaterials

In recent years,with the aggravation of energy and environmental crisis,the development of new energy storage and conversion technology has become a research focus.Fuel cell has aroused wide public concern due to its high utilization efficiency,high energy density and environmental friendliness.Exploring high performance electrocatalysts for the oxygen reduction reaction（ORR）at cathode of fuel cells is the key to promoting the rapid development and application of fuel cells.For a long time,the precious metal platinum（Pt）and its alloys have been the most efficient ORR electrocatalysts for industrial application.However,their low reserves,high price,low stability and weak oxidation resistance limit their large-scale application.Therefore,it is particularly important to find economical,sustainable and efficient ORR electrocatalyst to replace the expensive Pt-based catalyst.The transition metal iron（Fe）is abundant and cheap.Due to the advantages of abundant ORR active site,large specific surface area and good electric conductance,graphene supported iron based nanomaterials have become one of the research hotspots of high-performance ORR catalytic materials.However,high-temperature heat treating method for the preparation of iron-based graphene composites still suffers from the drawbacks of poor Fe active phase dispersion,easy aggregation into large particles,and serious agglomeration of active phase carrier graphene sheets.Based on this,this paper starts with the development of high-performance Fe-based non-noble metal catalysts.A series of iron-based graphene composites were constructed by introducing organic ligands and nitrogen-doped functional components.The aim was to anchor Fe through the coordination of metals and ligands to improve the dispersion of Fe active phases and reduce Fe nanoparticles size,and to reduce graphene agglomeration.The ORR catalytic activity of the prepared materials were studied.The specific research contents and conclusions are as follows:（1）H₃BTC,Fe（NO₃）₃ and GO were used as raw materials.First,H₃BTC and Fe（NO₃）₃ were mixed to obtain the precursor of the complex,and then mixed with GO.The mixture was conventionally dried and then subjected to high-temperature heat treatment to prepare the iron-based oxides reduced graphene oxide（r GO）composite.XRD,Rama,SEM and XPS were used to analyze the composition and structure of the materials.The types of iron were determined.The ORR catalytic activity of Fe₂O₃@r GO was tested under alkaline condition.The results showed that the optimized Fe₂O₃@r GO catalyst had more positive initial potential(E_onset=0.85 V)and half-wave potential(E_1/2=0.66 V),the limiting current density was up to 4.6 m A cm^-2,and the Tafel slope was close to that of commercial Pt/C catalyst.The results of RDE and RRDE showed that the ORR process is dominated by four-electron path and had a low H₂O₂ yield（4.48%）.Compared to commercial catalyst（Pt/C）,Fe₂O₃@r GO had better cycle stability and methanol oxidation resistance.In addition,the prepared material was further assembled into zinc air batteries,which had an open circuit voltage of 1.45 V and specific capacity of 777.5 m Ah g _Zn^-1.The specific capacity was slightly better than that of Pt/C+Ru O₂(750.6 m Ah g _Zn^-1).（2）In order to further improve the ORR catalytic performance of the material,the nitrogen doping strategy was adopted on the basis of the previous work.A sheet like Fe₃C@NG（nitrogen-doped graphene）composites were prepared by using ammonium chloride as the nitrogen source.The gas could be generated by the decomposition of ammonium chloride in the heat treatment process to weaken the agglomeration of graphene.The composition and structure of the samples were characterized.The types of iron were determined.The ORR electrochemical performance showed that the optimized Fe₃C@NG composite had more positive initial potential（0.98 V）and half-wave potential（0.81 V）,a larger limiting current density(6.0 m A cm^-2),a near four electron transfer process（n=3.99）,and better resistance to methanol oxidation and durability than Pt/C.The prepared sample was assembled into a zinc air battery.The open circuit voltage（1.53V）was better than that of commercial Pt/C+Ru O₂（1.39 V）,and the specific capacity was898.7 m Ah g _Zn^-1.（3）In this chapter,green amino acid was selected as additional ligand.Fe-based complex precursor was prepared using H₃BTC,Fe（NO₃）₃ and histidine as raw materials,which was then dispersed in GO.Then,urea was used as a heteroatom source to prepare a series Fe NC@NG materials.Among them,urea not only served as a nitrogen source,but also used as a template to weaken the agglomeration of graphene with the help of the g-C₃N₄ that formed during heat treatment.The composition and structure of the materials were characterized.The types of iron were determined.The ORR electrochemical performance test results showed that Fe NC@NG composites had more positive initial potential（0.98 V）and half-wave potential（0.83 V）,better limiting current density(5.0m A cm^-2)and near four electron transfer process（n=3.98）,as well as better methanol resistance and durability than commercial Pt/C.The prepared material was assembled into a zinc air battery.The open circuit voltage（1.54 V）was better than that of Pt/C+Ru O₂（1.39 V）,and the specific capacity was 880.4 m Ah g _Zn^-1.The superior catalytic performance of the material could be attributed to the following aspects:（a）The large specific surface area(399 m² g^-1)and hierarchical pore structure,which was conducive to electrolyte penetration and ion migration;（b）The high nitrogen content（6.2 at.%）with the pyridine N of 19.3%,graphite N of 19.3%and Fe–N_x of 24.6%was beneficial for providing more active sites,which had positive effects on the improvement of initial potential and limiting current density;（c）The higher O_ad content（32.5%）in the sample further increased the defect sites,as a result,accelerated the mass transfer and electron transfer,and ensured the rapid exchange kinetics of O^2-/OH^-;（d）Reversible redox of Fe²⁺/Fe³⁺optimized the bond energy between Fe center and ORR intermediate and improved the reactivity.