Nitrogen-Doped Carbon Nanotube-Grafted-Nanosheet Frameworks Encapsulating Fe-Co-Ni Nanoalloys As Bifunctional Oxygen Electrocatalysts For Rechargeable Zinc-Air Batteries

Owing to the merits of high energy density,high safety,environmental friendly,inexpensive and easy-to-operate,the rechargeable zinc-air batteries(ZABs)are promising as new-generation energy storage device,especially using for large-scale energy storage.However,the commercial application of rechargeable ZABs is still limited.Because the electrical efficiency,power density and cycling performance of the rechargeable zinc-air batteries are required to be improved.The air cathode is a vital component of the rechargeable zinc-air batteries.Improving the performance of air cathode is the key to solve the problems of rechargeable ZABs.The oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)on the air cathode have large overpotential.Developing highly efficient bifunctional oxygen electrocatalysts highlight the pathway to reduce the overpotential of ORR and OER.Although the precious-metal-based oxygen electrocatalysts exhibit superior ORR or OER activity.However,the using of expensive precious-metal-based oxygen electrocatalysts will largely increase the cost of rechargeable ZABs.Transition-metal-based oxygen electrocatalysts have many advantages over precious-metal-based oxygen electrocatalysts,such as inexpensive,convenient and robust,are considered as ideal substitutes for the precious-metal-based oxygen electrocatalysts.However,boosting the bifunctional activities towards both ORR and OER is still challenging for transition-metal-based oxygen electrocatalysts.In this thesis,we forcus on the morphological and structural designing strategy.An efficient nitrogen-doped carbon nanotube-grafted-nanosheet frameworks confining Fe-Co-Ni nanoalloys bifunctional oxygen electrocatalyst(Fe Co Ni-NC)has been developed.The nanotube-grafted-nanosheet frameworks with hierarchical porosity can largely increase both the specific surface area(398.5 m² g^-1)and the electrochemical active area(244.8 m² g^-1),provide ample accessible active sites,and improve the electron transfer and mass diffusion of the as-prepared electrocatalyst.The Fe-Co-Ni nanoalloys are protected by stable nitrogen-doped carbon layers,thus the as-prepared electrocatalyst exhibits excellent anticorrosion.Systematic charaterizations,electrochemical experiments and theoretical calculations have proved that the synergistic effect between rich pyridinic-nitrogen-doped carbon layers and encapsulated Fe-Co-Ni nanoalloys can significantly improve the ORR and OER performance of the as-prepared electrocatalyst.The synergistic effect can regulate the charge densities of the active sites,therefore the binding strength of the ORR and OER oxygen-containing intermediates to the active sites are optimized,reducing the overpotentials of ORR and OER.The Fe Co Ni-NC exhibit superior bifunctional electrocatalytic performance towards ORR and OER.The half-wave potential for ORR is only 0.89 V,the potential to reach the OER current density of 10 m A cm^-2 is only 1.54 V and their difference is only 0.65 V.The electrocatalytic performance towards ORR and OER of Fe Co Ni-NC exceed commercial Pt/C and Ru O₂ catalysts.Moreover,the rechargeable ZABs using the as-prepared electrocatalyst display peak power density of 315.2 m W cm^-2,specific capacity of 803.78 m Ah g^-1 at 100 m A cm^-2,and stable charge-discharge cycling over 100 h at 50 m A cm^-2.This thesis demonstrates a new guidence to synthesizing precious-metal-free bifunctional electrocatalysts for energy strorge technologies.