Study On The Formation Mechanism Of Porous Carbon Derived From Imidazole Coordination Polymers And Oxygen Reduction Reaction

With the rapid development of industries around the world,the excessive use of traditional fossil energy has caused a global energy crisis and environmental crisis.The development of renewable energy technologies such as fuel cells and metal-air batteries has become the inevitable way for the current industrial development.The oxygen reduction reaction(ORR)involved in these new energy technologies is a kinetically sluggish process,so the development of efficient oxygen reduction catalysts is the key to the development of these technologies.Pt-based catalysts have a high ORR catalytic activity.However,unfavorable factors such as too low earth reserves,excessively high.prices,and instability in practical applications limit their further development.Porous carbon materials with low price,high chemical stability,and good conductivity are deemed as the most promising materials for replacing precious metal catalysts.However,the existing methods for preparing heteroatom-doped and 3D porous carbon materials are relatively complicated and the preparation processes pollute environment and consume energy.Therefore,it is necessary to further solve these problems.In this dissertation,imidazole coordination polymers are taken as the starting point.The specific research contents include the following aspects:(1)In order to explore the formation mechanism of porous carbon derived imidazole coordination polymers,porous ZIF-8 and non-porous dia(Zn)are first directly carbonized.The specific surface area and pore structure of the prepared carbon material were characterized by BET.The results show that the specific surface area and the micro and mesopores of the zinc-containing imidazole-based polymer-derived carbon materials will increase with the incraese of temperature.At the same time,the carbonization time also has a great impact on the specific surface area of the porous carbon material.During the carbonization,process,the porous ZIF-8 undergoes a process of structural collapse and subsequent volatilization of organic components and zinc,which mainly acts as carbon precursors.The pores are simultaneously formed by the pyrolysis of organic compounds and the zinc volatilization,which is proved by studying the pyrolysis of the imidazole-based coordination pioymers containing cobalt with non-volatile property.This study shows that porous precursors are not a prerequisite for the preparation of porous carbon materials.This work open up a new direction for the preparation of porous carbon by imidazole-based coordination polymers.(2)We prepare highly porous heteroatom-doped ultrathin carbon nanosheet networks directly starting from mechanically grinded mixtures of 2-methylimidazole/2-mercaptoimidazole and zinc nitrate without any additional template.Evaporation of crystalline water and coordination interaction are proposed to be responsible for the formation of the 3D ultrathin carbon nanosheet networks.The doped heteroatoms change the surface charge density and spin density of adjacent carbons leading to the increase of ORR catalytic activity.In addition,the hierarchical porous structure of the carbon material exposes to more active sites,increases contact area with the electrolyte,and accelerates the mass transfer process.The heteroatom-doping was easily achieved through changing the ligand.The N/S-doped carbon nanosheet networks possess high surface area with micro-and macropores,ultrathin wall structure,and effective N/S-doping,which show outstanding ORR catalytic performance.This work not only provides highly active ORR electrocatalysts via an operationally simple and green process and also demonstrates a general method to prepare 3D ultrathin carbon nanosheet networks without any additional template and solvent.(3)The iron carbide/N-doped porous carbon material was prepared starting from mechanically grinded mixtures of 2-methylimidazole,zinc nitrate and ferrous chloride.2-Methylimidazole provides carbon and nitrogen sources during the pyrolysis process.And Fe-Nx structure and Fe3C nanoparticles are generated because of the presence of ferrous chloride and 2-Methylimidazole.At the same time,the presence of iron and carbon sources promotes the in situ growth of bamboo-like carbon nanotubes,which can provide more edge sites.When the molar ratio of zinc nitrate to ferric nitrate in the metal salt mixture is 30:1,the resulting catalyst exhibits comparable activity with Pt/C.