Rational Preparation Of Graphene-based Oxygen Electrocatalysts For High-performance Zinc-air Battery

With the unique features of large energy density and good safety,the zinc-air batteries are attracting increasing attention.However,the sluggish kinetic processes of oxygen evolution reaction and oxygen reduction reaction involved in the charge and discharge process on the air electrode result in the poor performance of zinc-air batteries.In comparison with the high cost of precious metal-based electrocatalysts,the development of efficient and low cost non-precious bifunctional oxygen electrocatalysts is highly desirable to improve the performance of zinc-air batteries.This thesis focuses on the rational design of graphene-based oxygen electrocatalysts,through regulation and optimization of experimental conditions to adjust the microstructure and electronic properties of electrocatalysts for improving the performance of zinc-air batteries.The key works are summarized below:1.With tannic acid as dispersing and chelating agent,cobalt ions were uniformLy dispersed on the surface of graphene oxide,which enabled the formation of nitrogendoped graphene electrocatalyst with well-dispersed cobalt nitrige nanoparticles via the one-step nitridation process at an elevated temperature.In the thermal process of nitridation,the in-situ formation of highly porous graphene embedded with cobalt nitride nanoparticles can be achieved due to the etching effect of cobalt nitride.The obatiend electrocatalyst showed excellent electrocatalytic activities for oxygen reduction and evolution reactions as well as hydrogen evolution reaction.The fabricated rechargeable zinc-air battery with a liquid electrolyte exhibited an open circuit potential of 1.45 V,large power density of 120.7 mWcm-2 and excellent cycling stability for over 330 h.The assembled solid-state zinc-air batteries also showed good cycling stability with good flexibility.Furthermore,the water splitting driven by the zinc-air batteries exhibited good gas generation rates by using the as-prepared electrocatalysts.2.To further improve the dispersion of cobalt species,folic acid was used as the nitrogen source and complexing agent for the dispersion of cobalt ions on graphene oxide.The subsequent pyrolysis activation led to the formation of nitrogen doped graphene sheets with trace cobalt(～1.7 wt%).The good bifunctional catalyties of the electrocatalyst to both of ORR and OER in alkaline electrolyte would be contributed to the synergic effect of the highly dispersed cobalt species on the nitrogen doped graphene sheets with large surface area.When used as electrocatalysts in air electrode,rechargeable zinc-air batteries displayed the the maximum power density of 148.4 mW cm-2),large specific capacity of 746.4 mAh gZn-1 and good cycling stability over 1000 cycles.