| The development of new energy conversion devices represented by metal-air batteries is an effective means to alleviate the energy crisis,and the development of cheap,efficient and stable ORR/OER bifunctional catalysts is one of the key issues in the development of metal-air batteries.Carbon encapsulated transition metal catalysts are considered to be the most promising OER/ORR bifunctional catalysts due to their unique structure and properties.However,carbon encapsulated transition metals catalysts still face the problems of complex preparation process and poor catalytic activity.In response to these problems,this paper proposes a highly efficient and universal way to prepare carbon cladding materials,and further optimizations of material components to improve the ORR/OER bifunctional catalytic activity of the materials.Meanwhile,the influence of carbon layer structure and core metal on catalytic activity is discussed.The specific results of the paper are as follows:1.In this paper,Co@C is prepared by arc discharge method.Subsequently,oxygen is introduced through heat treatment to construct the oxygen doped graphene encapsulated Co/Co3O4 composite catalyst.The amount of oxygen introduced is changed by precisely controlling the temperature and time of the heat treatment.The experimental results show that the oxygen doped graphene encapsulated Co/Co3O4 composite obtained by heat treatment for 500 min at 200℃ in air atmosphere shows the best catalytic activity and stability.In alkaline solution,ORR performance test show that the half-wave potential of oxygen-doped graphene coated Co/Co3O4 was 0.80 V.OER performance tests show that the overpotential at current density of 10 mA cm-2 is only 300 mV.The potential of ORR/OER bifunctional catalysis is only 0.73 V,which is better than Pt/C catalyst.The main source of the excellent performance is the synergistic effect of oxygen-doped graphene,metal Co and Co/Co3O4 heterogeneous interface,which effectively regulates the electronic structure of graphene and improves its adsorption energy for reaction intermediates,thus optimizing the catalytic performance.2.In this paper,nitrogen-doped carbon nanofiber loaded transition metal alloy composites are prepared by a simple electrostatic spinning method combined with high temperature carbonization.The relationship between alloy composition and catalytic properties is studied through precise control of alloy composition and ratio.The results show that nitrogen-doped carbon catalysts with a ratio of 1:1:1 nickel,cobalt and iron show the best catalytic activity and stability.In the basic solution,the ORR performance tests show that the half-wave potential of NiCoFe@N-CNFs is 0.81 V.OER performance tests show that when the current density reaches 10 mA cm-2,the overpotential is only 270 mV.The potential of ORR/OER bifunctional catalysis is only 0.69 V,which is better than Pt/C catalyst.It is verified by theoretical calculation that the change of alloy composition can effectively adjust the position of the center of the d-band,thus improving the adsorption energy of the catalyst to the reaction intermediate and optimizing the catalytic activity.In addition,a large number of nitrogen doping sites formed in situ also further optimize the electronic structure of the carbon layer and improve the catalytic activity. |
