Research On Non-platinum And Low-platinum Loding Catalysts Derived From Zeolitic Imidazolate Framework

The cathode oxygen reduction reaction is an important part of the two-electrode reaction of fuel cells.Although the precious metal platinum catalyst has good catalytic activity,due to the slow cathode reaction kinetics,the external output of fuel cells are limited,and the scarcity of platinum resources on the earth leads to high catalyst costs,and the catalyst of fuel cells account for a large proportion of its cost,which hinders the large-scale application of fuel cells.In order to reduce the cost of the catalyst,two types of oxygen reduction catalysts,non-platinum and low-platinum are prepared.The pyrolysis of Fe-doped Zeolitic Imidazolate Frameworks（ZIFs）is used to produce pore-rich materials,and the structure improvement scheme makes the prepared Fe NC catalyst have good electrocatalytic performance for oxygen reduction;Nitrogen-doped porous carbon derived from ZIF-8 was used to support platinum nanoparticles.The effect of different morphology of supports was explored,and then the oxygen reduction performance of these two types of catalysts was studied.The main research contents and achievements are as follows:Using ZIF-8 as precursors to prepare Fe NC catalysts through pyrolysis,the Fe-doped ZIF-8 precursor（Fe-ZIF-8）is prepared by solvothermal method,and iron acetylacetonate is added to the precursor solution.In the self-assembly process of ZIF-8 with Zn²⁺as the central node and 2-Methylimidazole as the ligand,small iron-containing molecules are confined in its pores,Fe NC catalysts with dodecahedral structure are obtained by pyrolysis,it can inherit the porous structure of ZIF and produce dense active sites on its surface.The Fe-ZIF-8 is mixed with different proportions of g-C₃N₄ and then co-pyrolyzed.The instability of g-C₃N₄ at high temperature will corrode Fe-ZIF-8,which can further improve the pore structure of the material.The effects of direct pyrolysis and staged pyrolysis on the structure of the derived catalysts were investigated.Under the same proportion,the framework of Fe-ZIF-8 could be effectively retained by using the staged pyrolysis scheme which would not lead to complete decomposition of Fe-ZIF-8.X-ray diffraction,transmission electron microscopy,scanning electron microscopy and other characterization methods are used to investigate the morphology,composition and phase of the catalyst material.The results show that Fe element well dispersed in Fe-ZIF-8/g-C₃N₄ mixed co-pyrolysis product Fe NC,and produce a hollow structure.The oxygen reduction performance of Fe NC catalyst materials are explored under alkaline conditions.The best performance of Fe NC-0.5 with a onset potential 1.14 V and half-wave potential 0.91 V both of which are better than 20%Pt/C catalysts.The number of electron transfer is close to 4 through rotating ring disk electrode test,and the hydrogen peroxide yield is close to 10%,in addition,the stability of the catalyst is evaluated,after 50000 seconds of test,the current remained 93%.After adding 0.1mol/L methanol solution in 0.1 mol/L KOH solution with oxygen saturated,the current remained stable,showing good methanol resistance.ZIF-8 is prepared by solvothermal method,the porous carbon obtained by pyrolysis of ZIF-8 is used as support,chloroplatinic acid was reduced by H₂+N₂,sodium borohydride and microwave ethylene glycol.Platinum nanoparticles were loaded on the surface of nitrogen doped porous carbon suppots derived from ZIF-8.Among the three reduction schemes,the gas reduction scheme has a relatively complicated process and is prone to produce large platinum nanoparticles,which is not conducive to catalyzing the oxygen reduction reaction.The catalysts prepared by the sodium borohydride and microwave ethylene glycol methods have similar performance.The microwave glycol method is more convenient,and this method is chosen as the chloroplatinic acid reduction scheme in the following study.ZIF-8 and g-C₃N₄ are mixed and co-pyrolyzed to make the derivatized porous carbon support have a rougher surface and provide more anchor locations for platinum.XRD and scanning electron microscopy are used to analyze the influence of pyrolysis process with different proportions of g-C₃N₄.Pt nanoparticles was loaded on different supports by microwave ethylene glycol method,and the catalytic performance of the catalyst was tested.Among them,the catalyst with NC-0.125 as a support showed the best performance,which derived from the pyrolysis of a mixture of g-C₃N₄ and ZIF-8 at a ratio of 0.125,with a half-wave potential of 0.85 V and a limiting current density of 5.70 m A/cm².After 5000 cycles of potential,the half-wave potential lost only 3mV.