Study And Application Of B,N Co-doped Graphite Carbon(BCN) And BCN Matrix Composites As Oxygen Electrode Catalysts

Rechargeable lithium-air battery has attracted much attention because of its ultrahigh theoretical specific capacity.The key to the successful stable charging-discharging cycling of the Li-O₂ battery is that the highly reversible Li₂O₂ could form and decompose on cathode during the cycle.Due to the slow kinetic rate of positive electrode reaction of lithium-air battery,convenient synthesis,low cost and high catalytic efficiency catalysts have attracted extensive research and attention.Graphene-like materials are used widely in photocatalysis,material chemistry and electrochemistry.Its special physical and chemical structures exhibit many special properties.B,N dual-doped 2D graphitic carbon?BNC?was prepared by introducing B and N atoms into carbolic inert?system,separately.As a result,it breaks the electroneutrality of sp₂ carbon to create charged sites,which is favorable for O₂adsorption and to activate carbon?electrons for effective utilization by O₂.Amount of N atoms and B atoms doped in carbon supplied free electrons and the sp_z empty orbit to form O₂^-in the battery reaction easily.In battery testing under the current density of 500mA g^-1_cat and the cutoff capacity of 1000 mA h g^-1_cat,the BNC-supported cathode can maintain a stable cycling for 160 cycles within a voltage window of 2.2-4.8 V,whereas the s-BNC catalyst supported cathode only lasts for 20 cycles.Moreover,the capacity retention rate of BNC was great,which was significant to the practical application of the Li-air battery.For composite materials,good substrate material is important.In chapter 4,carbon-based catalysts with different doping ratios of nano-Co₃O₄ and BNC were synthesized by hydrothermal synthesis.The performance of the composite catalyst is the best than that of the two pure phase catalysts for the unique properties of the transition metal oxides.The ORR and OER properties of the composite material with different doping ratios were tested by using disk electrode.The optimum doping ratio was selected as follows:the mass ratio of Co₃O₄:BNC was 3:1 called 25%BNC.When the current density and voltage range are limited,the performance of the lithium-air battery loaded with the catalyst is measured.The first discharge capacity of 25%BNC is as high as 12985 mA h g^-1,and the overvoltage of the first charge and discharge is about 1.02 V.When the current density is 500 mA g^-1 and the cutoff capacity is 1000 mA h g^-1,voltage range is 2.2-4.5V,the number of cycles is 58.The discharge capacity of the battery increased,the overvoltage decreased,the number of cycles increased,and the overall performance of the battery was greatly improved.In the end,the results of this study shed lights on rational design of cathode catalyst for Li-air and other metal-O₂ batteries.