Synthesis Of Non-noble Metal-based Catalysts For Oxygen Reduction Reaction And The Origin Of High Activity

Fuel cell is an energy conversion device with high conversion efficiency,long life and high-power density,which fully meets the requirements of China's current new energy conversion policy.Oxygen reduction is an significant cathode process in the fuel cell devices.At present,Pt/C is often used as a catalyst for promoting oxygen reduction reaction in industrial production.However,since Pt has a small amount of reserves in the earth's crust,the use of Pt is high,and it is prone to dissolution and agglomeration,resulting in poor stability.These problems above seriously restrict the practical application of fuel cells in new energy vehicles.Therefore,the design and development of a highly efficient non-Pt material as a cathode catalyst is significant to China's vigorous implementation of energy transformation policy.In this paper,a carbon-supported cobalt tetraoxide composite catalyst was used as the research object.The composition of the composite catalyst was modified from the cobalt oxide nanoparticels,carbon support and interfaces.The origin of activity enhancement was also revealed by XAS and UPS measurements.It will provide a new direction for the design and synthesis of transition metal oxide catalysts.The results of this research are as follows.(1)Co3O4/C was used as the research object,and the interface was designed by nitrogen doping.One of the key factors for controlling the ORR activity is C-O-Co and C-N-Co bonds formed on the interface.The electrochemical performance test showed that the catalytic activity of N-doped Co3O4/C was significantly improved compared with Co3O4/C,and the half-wave potential was improved by nearly 30 mV.(2)Using melamine as a reducing agent to partially reduce the Co3O4/C catalyst,a thin shell Co3O4_shell@Co_core composite catalyst was prepared.The thickness of the thin layer was adjusted,and it was found that when the thickness of the thin layer of Co3O4 was 1.1 nm,it showed the most excellent activity,and the half-wave potential exceeded Pt/C 10 mV.According to UV and UPS tests,the activity enhancement is attributed to the change in the energy level of Co3O4 caused by oxygen vacancies in the thin layer of Co3O4.(3)Nitrogen-doped carbon nanotubes(CNT/CB)with high length to diameter ratio were prepared by using melamine as carbon source and Co3O4/C as catalyst.The CNT/CB composite catalyst exhibits excellent catalytic activity,and its half-wave potential exceeds that of physical blending CNT-CB catalyst 14 mV,while the mass specific activity is 15 times that of the CNT-CB catalyst.Its high ORR activity is mainly related to the synergistic effect of nitrogen atoms doped on the surface of carbon nanotubes and internal metal Co.(4)A pure phase Fe-N-C catalyst was prepared by a simple Si02 confinement method,in which Fe and N coordinated to form a non-planar Fe(II)-N-C structure.Compared with the traditional pyrolysis method,SiO2 can effectively avoid the generation of Fe-based impurity nanoparticles.Electrochemical tests show that Fe(II)-NC is closely related to the activity of the catalyst.When the Fe(II)-NC content is 26.1%,the catalyst exhibits the best ORR activity,and its half-wave potential exceeds that of Pt/C by 20 mV.