Synthesis Of Metal-defected Manganese Oxide And Cobalt Oxide For Oxygen Reduction Reaction

Fuel cells have attracted much attention because of their high energy storage efficiency and good selectivity.Oxygen reduction reaction happening on the cathode of fuel cells is extremely important for whole system.So far the most effective cathode materials for fuel cells are Pt group materials,but the expensive price and rare storage of Pt have limited its wide application.Transition metals,with their low price and high content in nature,have attracted more and more attention.With the development of materials science and nanotechnology,the transition metal materials show good ORR activity through regulation of their composition and structure.The main purpose of this thesis is to explore new and efficient ORR catalyst based on metal-defected manganese and cobalt oxides to replace Pt/C.Firstly,metal defected Mn3O4 powder is synthesized by solvothermal-calcination method.The characterization results of XPS,EELS and positron annihilation show that the catalyst contains a lot of metal defects.XRD spectra show that the the crystal phase of Mn3O4 does not change.The magnetic characterization shows that the catalyst exhibits room temperature ferromagnetism.Defected Mn304 powder is coated on the electrode to test its ORR activity,the result shows its onset potential is 0.94 V(vs.RHE)which is better than normal Mn3O4(0.90 V),but it is still lower than commercial 20 wt.%Pt/C(0.96 V).The ratio of Mn3+/Mn2+ in defected Mn304 is 2.34,which is higher than that in normal Mn3O4(1.91),proving that defected Mn3O4 contains more active sites(Mn3+)than normal Mn3O4.In addition,Mn-doped Co3O4(denoted as MnxCo3-XO4)powder is synthesized by solvothermal-calcination method in order to obtain higher ORR activity.It is confirmed that Mn ion is embedded into C03O4 lattice by XRD spectra.The EDS、XPS and EELS results show that Mn has a uniform distribution in both surface and bulk phase of Co3O4,and the doped Mn act as electron-donor to transfer electrons to Co which leads to the high content of Co2+.MnxCo3-xO4 powders are coated on the electrode to test their ORR activity,the result shows that the catalyst has the best activity when the ratio of Co/Mn is 1:1(denoted as Mn1.5Co1.5O4).Its onset potential is almost the same with Pt/C(0.96 V)and it has higher half wave potential(0.82 V)than Pt/C(0.80 V).The limited current and stability are better than Pt/C.Mn1.5Co1.5O4 has the highest ratio of Co2+/Co3+(1.16),proving that it contains the most active sites(Co2+),so it has the best activity.