Study On Structure Construction And Catalytic Mechanism For Oxygen Reduction Reaction Of Transition Metal Sulfides And Carbides

Platinum was now considered to be the best catalyst for oxygen reduction reaction.But the low reserves and high cost of platinum limited the wide application of platinum catalysts for fuel cells.Therefore,developing high activity,stability and low-cost catalysts for oxygen reduction reaction was the key to the large-scale application of fuel cells.It had been reported that the transition metal carbides,nitrides,sulfides,etc.exhibited excellent catalytic performance and application potential for oxygen reduction reaction.Here,the preparation,structure and catalytic performance of iron-based metal sulfide,titanium-based carbide and nitrogen-doped cobalt-based carbide for oxygen reduction reaction were investigated in this thesis.The conclusions are as follows after the investigation:1.FeS₂@rGO and FeS@rGO for oxygen reduction reaction had been investigated in this theis,which was prepared by annealing and one-step hydrothermal.The octahedral structure of FeS₂ and the hexagonal layered structure of FeS were confirmed by X-ray diffraction,scanning electron microscope,X-ray photo-electron spectroscopy and transmission electron microscope tests.Cyclic voltammetry,Tafel,linear sweeping voltammetry,rotating disc electrode and rotating ring disc electrode tests showed that the onset potential of FeS₂@rGO for ORR was-0.142 V?vs Hg/Hg₂Cl₂?,which was close to the commercial Pt/C?-0.114 V vs Hg/Hg₂Cl₂?catalyst.The excellent catalytic performance could be attributed to the enhanced oxygen adsorption and the increased cloud density of S-S bond in FeS₂.Compared with the monosulfide bond in FeS,the oxygen molecules could be absorbed and broken easily by S-S bond.2.A facile one-pot method for in-situ synthesis of co-axial structure titanium carbide?TiC?had been developed with the catalysis of iodine and sodium,while CNT was the carbon source and titanium was the titanium source.X-ray diffraction,scanning electron microscopy and transmission electron microscopy showed that TiC was coated on the surface of the CNT,which confirmed the co-axial structure.Cyclic voltammetry,Tafel,linear sweeping voltammetry,rotating disc electrode and rotating ring disc electrode tests showed that the TiC@CNT performed high catalytic performance for ORR in both alkaline and acidic electrolytes.The oxygen reduction raction mainly happens through 4-electron pathway.The CNT plays the role of electron conductor,while the active centers for oxygen reduction reaction were formed on the surface of TiC.The high catalytic performernce of TiC@CNT should be attributed to the synergistic effect between the high conductivity of CNT and excellent catalytic performance of TiC.3.A high performance core-shell-structured nitrogen-doped CoC_x/FeCo@C/reduced graphene oxide?rGO?hybrid catalyst had been synthesized for oxygen reduction reaction,which was derived from Fe-doped Co₃[Co?CN?₆]₂ MOFs.Scanning electron microscope and transmission electron microscope tests showed that the carbon shell of CoC_x/FeCo and raphene support were bonded to each other,which offered a large number of surface area and more active sites for CoC_x/FeCo.Cyclic voltammetry and linear sweeping voltammetry tests showed that N-doped CoC_x/FeCo@C/rGO hybrid exhibits superior onset potential?1.0183V vs RHE?and excellent long-term durability for ORR than that of Pt/C?1.0174V vs RHE?in0.1 M KOH electrolyte.The catalyzed oxygen reduction reaction mainly happened through4-electron pathway.Density functional theory confirmed that the enhanced positive charge of Fe atoms would be more conducive to the adsorption of O₂.With the increasing charge transfer,the bond length of O-O was increased,which finally promoted the the activity of ORR.