Oxygen Reduction Activity And In Situ Electron Microscopic Study Of MOFs Derived Pt-based Electrocatalysts

As a key step in the development of clean energy,oxygen reduction（ORR）reaction is needed in both metal-air batteries and fuel cells.However,as a commercial oxygen reduction catalyst,the high price of 20%Pt/C limits its mass production in industry.Therefore,the key to solve the energy problem is to reduce the platinum load and improve the stability of catalyst.Among the many new oxygen reduction catalysts developed at present,platinum-based catalyst is most likely to replace 20%Pt/C as a commercial catalyst and has attracted much attention.In order to study the more stable and durable oxygen reduction electrocatalysts with lower platinum content,two different oxygen reduction electrocatalysts were prepared by impregnation method and high temperature annealing using Pt-Snand Pt-La as models and the nitrogen doped carbon structure derived from ZIF-8-NH₂as the support.The formation mechanism of Pt-Snbimetallic catalyst was studied by in situ electron microscopy,and the application prospect of the catalyst in alkaline oxygen reduction reaction was explored.Specific research contents are as follows:1.Electrocatalysts（SnO_x@PtSnNC）with SnO_xas shell and PtSnas core have been synthesized.The core-shell structure of the catalyst was characterized by spherical correction transmission electron microscopy,and the coordination environment of Sn-O-Sn/Ptwas characterized by X-ray photoelectron spectroscopy and synchrotron radiation.The content of Ptin the catalyst was only 1.11%by plasma emission spectrometer（ICP-OES）.The oxygen reduction performance of the catalyst was studied under alkaline conditions.The half-wave potential(E_1/2)was up to 0.9V,which is higher than 20%of commercial Pt/C catalysts and most platinum-based catalysts reported in recent years.After 20000 cycle stability tests,the catalyst only attenuates 17 m V in E_1/2,showing good cycle durability.The structure and morphology of the samples did not change significantly after long cycle test.The real-time structure evolution of PtSn@ZIF-8 under high temperature annealing in different atmospheres was studied by in situ heating and in situ gas techniques.It was found that Snnanoparticles underwent"reverse sintering"in both atmospheres,which transformed into highly dispersed Snclusters/monatom,and the reverse sintering temperature in nitrogen was higher than that in vacuum environment.With the further increase of temperature,Snand aggregated Ptnanoparticles alloyed to form Pt-Snbimetallic structure.3.Pt-LaNC catalyst was synthesized,and Pt/La₂O₃catalyst was doped with La element to form a strong metal-support interaction（SMSI effect）,which prevented the sintering of nanoparticles during high temperature annealing,and induced the formation of PtZn intermetallic compounds by mixed gas annealing.The active structures of Pt/La₂O₃and PtZn intermetallic compounds were revealed by spherical correction electron microscopy.The effects of different annealing atmospheres on the oxygen reduction activity of the catalyst were studied.The oxygen reduction performance of the catalyst was tested under alkaline conditions.The half-wave potential of the Pt-LaNC catalyst annealed in mixed atmospheres reached 0.91V,which exceeded the oxygen reduction activity of 20%commercial platinum carbon.In order to investigate the commonality of rare earth element doping,we also investigate the effect of annealing atmosphere on the oxygen reduction properties of Pt-Ce@ZIF-8,and provide a possibility for the application of rare earth element doping in electrocatalytic oxygen reduction reactions.In this paper,two types of platinum-based supported catalysts were constructed with core-shell nanoparticles and intermetallic compounds as active sites,and the formation process of a bimetallic catalyst based on reverse sintering induction was studied under different atmospheres by advanced in situ electron microscopy technology,and a new synthesis mechanism of platinum-based catalyst was proposed.It provides a new idea for the synthesis of more stable and reliable low platinum catalysts in the future.