Construction And Electrocatalysis Performance Of Fe/Co-doped Hierarchically Porous Carbon

The rapid consumption of traditional fossil fuels and the increasingly serious environmental pollution have become urgent problems in modern society.The research of efficient and pollution-free energy devices such as fuel cells and metal-air batteries has become a targeted solution.The development of low-cost and high-efficiency non-noble metal electrocatalysts is of great practical significance to reduce the cost of devices and accelerate the process of their industrialization.As an important branch of non-noble metal electrocatalysts,hierarchically porous carbon-based materials have been widely studied by domestic and foreign scientific research institutions for their nanostructure design and electrocatalytic performance optimization,but most of the catalysts have limited activity,low utilization of active sites,and complex process,which can not meet the commercial requirements.In order to further promote the development of hierarchically porous carbon（HPC）,based on the construction of hierarchically porous carbon,the electronic structure of carbon-based materials was changed by doping nitrogen atoms in the carbon matrix,and the efficient electrocatalytic sites were formed by doping Fe and Co with unsaturated d orbital,which further improved the catalytic performance.In this paper,a series of transition metal-doped hierarchically porous materials with high catalytic activity and durability were prepared by templating method and Kirkendall effect based on the coordination of 2-methylimidazole and polyvinylimidazole with a variety of transition metals.The main research results of this paper are as follows:1.Given the cost,pollution,safety,and other problems caused by the traditional organic solvent during the preparation of ZIF materials,this paper uses deionized water as the solvent to synthesize the same rhomboid dodecahedral ZIF-8 precursor as the traditional organic solvent preparation by changing the proportion of 2-methyl imidazole and metal salt.Iron phthalocyanine and Si O₂ templates were introduced in the preparation process,and Fe-doped porous carbon（Fe/N-HPC）was obtained after high-temperature carbonization and template removal process,the original spaces occupied by Zn and silicon dioxide were transformed into micropores,mesopores,and macropores.The hierarchically porous structure can improve the multi-scale mass transport in the electrochemical reaction process and meet the requirements of quick mass transfer under a large current.Therefore,the half-wave potential of Fe/N-HPC in acidic and alkaline media is 0.90 V and 0.77 V,respectively.It shows better oxygen reduction activity and durability than Pt/C in alkaline electrolyte and Zn-air batteries.2.Cheese has a porous shape inspired design cheese-like ZIF-derived materials,cubic bimetallic Co/Zn ZIF precursors were synthesized in deionized water solvent using CTAB and silicon dioxide as structure-directing agent and hard templates,respectively.After the high-temperature carbonization and template removal process,the cubic cheese-like hierarchically porous ZIF-derived catalyst was successfully prepared.CTAB restricts the growth of specific crystal surfaces during the growth of ZIF precursors and plays a role in regulating the morphology and size of materials.Silicon dioxide plays an important role in the formation of mesopores.The bimetallic Co and Zn perform their respective functions,the volatilization of Zn brings a large number of micropores after carbonization,while Co participates in the construction of active sites for oxygen reduction reaction.Compared with the microporous electrocatalyst without using silicon dioxide,the hierarchical porosity greatly improves the catalytic activity of oxygen reduction reaction in alkaline,neutral,and acidic electrolytes.Moreover,the peak power density of the microbial fuel cell assembled with the hierarchically porous material as the cathode catalyst was 1.17 times that of Pt/C,and it could work stably for nearly 200 hours.3.Template itself and the cost of additional removal process makes templating-free method to build hierarchically porous structure is of great realistic significance,in this paper,the use of innovative hydrothermal prepared a high yield rate of polyvinyl imidazole-Zn²⁺coordination polymer,as carbon source and nitrogen source of the materials,the high-temperature carbonization zinc volatilization in the process of producing a large number of pores,the specific surface area is as high as 2249 m² g^-1.In the preparation process of Zn-based coordination polymers,Fe sources with different qualities were introduced,while iron species act not only as a part of Fe-N active site,but also as the structure-directing agent to form mesopores and macropores due to the nanoscale Kirkendall effect.The hierarchically porous structure with high specific surface area can be constructed without using templates,which further enhances the catalytic activity and stability of the catalyst.The measurement results show that the iron-nitrogen-carbon prepared with an appropriate amount of iron source exhibits half-wave potentials of 0.91 V and 0.78 V under alkaline and acidic electrolytes,respectively.The Zn-air battery fabricated by such attractive electrocatalyst as air cathode displays a high peak power density,which 1.39 times that of Pt/C.4.Low-cost bifunctional oxygen catalysts for rechargeable metal-air batteries have important practical value in accelerating their commercialization.In this paper,a Co/Zn bi-metallic coordination polymer was formed using polyethylimidazole as an organic linker combining with the coordination of Co²⁺and Zn²⁺.In the carbonization process,the volatilization of Zn leads to a large number of micropores,while Co species act not only as a part of the active site,but also as the structure-directing agent to form large-size mesopores and macropores due to the nanoscale Kirkendall effect.This hierarchical porosity can improve the mass transport efficiency in the gas-involved electrochemical reaction.We studied the effects of different Co/Zn ratios on the catalytic activity of oxygen reduction and oxygen evolution under alkaline conditions.The results show that optimal Co/Zn ratios can bring the best activity of oxygen reduction and oxygen precipitation at the same time.E_1/2 and E_j=10 in alkaline electrolyte are 0.86 V and 1.65 V,respectively.When used such material as the air electrode for the Zn-air battery,the charge-discharge polarization curve and the charge-discharge cycle stability is better than Pt/C+RuO₂.