Preparation Of Bimetallic/nitrogen-doped Carbon Composites And Their Electrocatalytic Performance For Oxygen Reduction Reaction And Oxygen Evoluton Reaction

Metal-air batteries have attracted much attention due to their extremely high specific energy density in recent years.However,the sluggish oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)decreases the efficiency of metal-air batteries,thus hindering their practical applications.It is of great significance to develop non-noble metal ORR/OER catalysts with excellent performance and low price for the development of metal-air batteries.Among the many transition metal catalysts,the cobalt-based compounds have good catalytic properties of ORR and OER,and they are abundant reserves and low in price,especially after doping have more excellent stability.In this thesis,three series of bimetallic/nitrogen-doped carbon composites of Co-SrCO3/NC,Co-Nd/NC and Co-Ce/NC were successfully synthesized by heating reflux and subsequent high-temperature calcination.The composites were characterized by XRD,Raman,XPS,TEM and N2 adsorption-desorption isotherms.The performances of ORR and OER were evaluated through electrochemical tests.Their structure-activity relationship was investigated.The Co-SrC03/NC series composites were prepared by calcining bimetallic precursors at different temperatures with transition metal cobalt and alkaline earth element strontium.According to the characterization results,the metal cobalt was wrapped in nitrogen-doped carbon graphite shell,the SrC03 nanorods were embedded in the graphite shell and in contact with the metal cobalt.The results showed that Co-SrCO3/NC series composites had excellent ORR/OER catalytic activity in 02-saturated 0.1 M KOH electrolyte.In particular,Co-SrC03/NC-600 had a superior stability over 20 wt%Pt/C and Ir02.The air electrode prepared by Co-SrCO3/NC-600 also showed excellent discharge performance in zinc-air battery.During the deep discharging at 10 mA·cm-2 with the cutoff voltage of 0.6 V,the flat voltage plateau at about 1.10 V is achieved by Co-SrCO3/NC-600.For 20 wt%Pt/C,the discharging voltage plateau is maintained at 1.17 V.The corresponding discharge capacity normalized to the mass of consumed Zn is as high as 486.31 mAh·gZn-1 for Co-SrCO3/NC-600 air cathode,which is higher than that of 20 wt%Pt/C(446.99 mAh·gZn-1).The Co-Nd/NC series composites prepared by using transition metal cobalt and rare earth element neodymium had large specific surface area and mesoporous structure.In O2-saturated 0.1 M KOH electrolyte,Co-Nd/NC-700 exhibited a potential of 1.65 V(vs.RHE)at 10 mA·cm-2,OER activity exceeded that of Ir02.The half-wave potential of Co-Nd/NC-800 was 0.78 V(vs.RHE)for ORR,which was only 0.04 V lower than that of 20 wt%Pt/C.The Co-Ce/NC series composites had stable multi-stage pore structure and large specific surface area.Among them,Co-Ce/NC-600 showed the most excellent ORR/OER bifunctional catalytic activity and stability in 02-saturated 0.1 M KOH electrolyte.Co-Ce/NC-600 possessed a stable multistage pore structure and large specific surface area.The potential value at 10 mA·cm-2 was 1.70 V(vs.RHE)and the OER activity was close to that of IrO2.The half-wave potential for ORR reached 0.79 V(vs.RHE),and the stability exceeded that of noble metal catalysts.