Nitrogen Doped Carbon Materials As Bifunctional Oxygen Electrocatalysts For Metal-Air Batteries

The rapid economic development and industrialization have raised concerns about energy availability and environmental pollution,and the sustainable supply of energy requires new clean energy technologies.As the most promising electrochemical energy storage device,metal air batteries have gained great interest due to their high energy density and conversion efficiency.The oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)are the central reactions of oxygen electrode in metal air batteries,but the slow kinetic processes make the metal air batteries inefficient.Therefore,the improvement of the cathode catalyst is still a challenge for its application.The bifunctional catalysts for oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)can effectively improve the performance of metal air batteries.Therefore,there is in urgent need of efficient and suitable bifunctional catalysts to promote the development of metal air batteries.Nitrogen-doped carbon materials exhibit excellent electrocatalytic activity in alkaline electrolyte and are considered to be the promising catalysts.Based on this,several nitrogen-doped carbon materials have been synthesized and the following work has been carried out on the subject of bifunctional catalysts:(1)Preparation of silver/nitrogen-doped carbon sheets and their catalytic properties in metal-air batteriesSilver/nitrogen-doped carbon sheets(Ag/NC)were prepared via a simple one-step and in-situ pyrolysis using urea and sucrose as carbon and nitrogen sources.The structural characterization analysis revealed that Ag and N were successfully incorporated in the sample.In alkaline media,electrochemical tests showed that Ag/NC exhibited superior ORR catalytic activity with a 4-electron(4e-)redox pathway due to its large electrochemically active area(ECSA),as well as synergistic effect between silver particles and pyridine nitrogen species.In addition,Ag/NC catalyst showed better methanol resistance and stability than commercial Pt/C.(2)Synthesis,catalytic performance of Co@Co3O4-N/C as a bifunctional catalystThe Co@Co3O4-N/C catalyst was synthesized by direct high-temperature carbonization and in-situ oxidation using urea and formaldehyde polycondensation products as substrates.XRD,TEM,XPS and other characterizations showed that Co@Co3O4 had a unique core-shell structure.Electrochemical tests revealed that the sample Co@Co3O4-N/C had remarkable catalytic activity for ORR and OER,as well as excellent ORR stability and good methanol resistance.The superior bifunctional catalytic activity is attributed to a large number of active sites and synergistic effect between Co@Co3O4 and the carbon-doped nitrogen atoms.Morever,the ?E value of Co@Co3O4-N/C is smaller than Co-N/C,indicating that it has better oxygen electrocatalytic activity.(3)Synthesis of Co Fe-N/C bifunctional catalyst and their catalytic performance.Cobalt-iron bimetallic doped nitrogen/carbon catalysts(Co Fe-N/C)at different temperatures were synthesized by hydrothermal and high temperature pyrolysis,using stable sodium chloride as template and ethylenediaminetetraacetic acid disodium as precursor.Nitrogen adsorption and desorption test showed that the material obtained by pyrolysis at 700 oC had a large specific surface area and a mesoporous structure.Electrochemical tests showed that the Co Fe-N/C-700 catalyst showed better ORR and OER catalytic performance in alkaline condition,which can be attributed to the combination of metal Co,Fe and N to form a large number of active sites,the synergistic effect between metallic Co,Fe and nitrogen species.The Co Fe-N/C-700 catalyst was more prominent in the performance of methanol resistance and stability.In addition,the ?E value of Co Fe-N/C-700 is the smallest among all catalysts,that is to say,Co Fe-N/C-700 poesses superior oxygen electrocatalytic performance.