Design Of Defects-rich Three-dimensional Doped Porous Carbon Materials And Its Application In Oxygen Reduction Reaction

In response to energy shortages and environmental pollution caused by the massive consumption of traditional fossil fuels,researchers have been exploring new energy storage and conversion technologies.Among them,fuel cells and metal-air batteries have attracted much attention due to their high energy efficiency and environmental friendliness,which are expected to replace traditional fossil energy sources as the next most promising new energy battery.However,its energy efficiency is still limited by the slow kinetics of the cathodic oxygen reduction reaction(ORR).The overpotential of the reaction is high,and the energy loss is serious in practical applications.Developing highly efficient ORR reaction catalyst is the key to the development of those new energy devices.Pt-based precious metal catalysts are currently the most important catalysts for ORR.However,high cost,low reserves,and poor stability have seriously limited their commercial applications.Therefore,there is an urgent need to develop cost-effective non-precious metal or non-metal electrocatalysts to replace Pt-based precious metal catalysts.Carbon materials are considered to be one of the most promising ORR electrocatalysts due to their advantages such as low cost,rich sources,and high stability.However,there is still a big gap from commercial applications,and it is still necessary to further improve the electrocatalytic properties of carbon materials.Developing advanced material preparation methods to increase the activity and density of catalytic active sites,and constructing high-efficiency catalytic electrode interface structures to improve the utilization efficiency of active site are the scientific challenges for carbon-based material electrocatalysts.This thesis aims to comprehensively design and optimize carbon materials by regulating intrinsic activity of carbon material and constructing high-efficiency electrode interface.A new material preparation method was developed,and a carbon-based ORR catalyst with abundant intrinsic active sites and catalytic electrode interface structure was obtained,which would provide a new sight to optimize the catalytic activity of carbon materials for its efficient applications in electrochemical energy storage and conversion technology.This paper has carried out the following research work:(I)In this paper,a nitrogen-doped,defect-rich three-dimensional porous carbon-based electrocatalyst was prepared by salt-sealing pyrolysis method.The salt-sealed metal-organic framework(MOF)was firstly prepared by ball milling,then pyrolysis at high temperature.Since the MOF is encapsulated in a solid salt reactor,the interrupted organic ligand degraded during calcination would undergo a rearrangement reaction to form carbon nanosheet on the surface of the MOF-derived carbon polyhedron,causing a three-dimensional network structure.The synthesis method has a high carbon conversion yield,the obtained material is defects-rich,and the nitrogen doping amount is high,which provides more catalytic active sites for the ORR reaction.At the same time,the three-dimensional porous network structure of the material is beneficial to fully expose the active sites,improve the mass transfer process and enhance the electron transfer rate.Abundant active sites,efficient mass transfer channels,and good electrical conductivity synergis tically promote the material to have good ORR electrocatalytic activity.The primary zinc-air battery constructed with this material also exhibits excellent performance.(II)The salt-sealed iron-containing MOF was prepared by ball milling using zinc oxide,ferrous acetate,2-methylimidazole and sodium chloride as raw materials.After high temperature carbonization,a three-dimensional Iron and nitrogen co-doped carbon carbon(Fe–N_x-C)nanomaterials was obtained.Sodium chloride crystal can be used as a pore-forming agent and a closed nanoreactor can not only promote the effective incorporation of Fe to form Fe-N_x active sites,but also promote the production of three-dimensional rich defects and high nitrogen-doped structures to expose active sites and promote mass transfer,while this three-dimensional interactive network structure can accelerate electron transfer.The unique structure can promote the effective contact of the three-phase interface during the reaction,which greatly improves the utilization efficiency of the Fe-N_x active site.Therefore,although the doping amount of Fe is not very high,the ORR electrocatalytic performance can be greatly improved.It is also proved that this synthetic strategy is appropriate for the preparation of other defect-rich three-dimensional transition metal-nitrogen-carbon materials.