Study On Bimetallic Oxygen Reduction Catalysts With Precise Design Of Atomic Structures

Providing clean,efficient,and economical new energy to meet the growing demand for sustainable development of human society is the biggest challenge in the21st century.Among many new energy sources,fuel cell,as the driving device of energy vehicles,has the advantages of high efficiency,high energy conversion rate and zero harmful gas emission by directly converting chemical energy into electric energy.However,the fuel cell cathode oxygen reduction reaction is the bottleneck to improve the performance and improving the oxygen reduction reaction performance is beneficial to the fuel cell energy conversion with high efficiency and environmental protection.At present,platinum-based precious metal catalysts are still the most important materials to promote cathodic oxygen reduction.Only by finding low-cost and high efficiency electrocatalysts can we solve the problem of high price of fuel cells.Non-precious metal catalysts are composed of carbon,nitrogen,and transition metal elements with large contents in the earth's crust,among which metal–nitrogen ring complexes(M–N–C,M=Fe,Co,etc.)are the most promising non-precious metal catalysts,and related research is attracting wide attention.Although thanks to the vigorous development of nanomaterials and nanotechnology,the design and synthesis of a series of efficient metal–nitrogen–carbon catalysts has become a reality,the understanding of the active sites of these catalysts has not yet been agreed.In the experimental preparation,there are many factors affecting the catalytic activity of M–N–C,and the composition of M–N–C synthesis products is complex,which brings problems to the understanding of the function-performance-relationship of oxygen reduction catalysts and the precise regulation of active sites.Therefore,the systematic study of the active site structure of the catalyst and the construction of the atomic structure and electronic structure of the catalyst are very important to study the reaction path of the catalytic reaction,clarify the function-performance-relationship and reveal the catalytic mechanism.Based on the above analysis,this paper adopts the idea of‘model prediction–experimental characterization–interpretation mechanism?guiding design',combined with density functional theory(DFT)calculation based on first principles and synchrotron radiation X-ray absorption fine structure(XAFS)characterization methods to accurately design bimetallic oxygen reduction active sites,which provides guidance for further study of function-performance-relationship of catalyst materials.This research work is carried out from the following three aspects:Firstly,taking M–N–C with Fe and Co as metal centers as the research object,the effects of nitrogen and carbon coordination environment of metal atoms on the structural stability,electronic structure and oxygen reduction activity of Co–N–C monatomic catalysts were discussed through theoretical calculation.Then,aiming at the bimetallic active sites,six kinds of M₂N_x model catalysts were designed to explore the effects of different metal centers,different nitrogen coordination numbers and the pre-adsorption of functional groups of the reaction intermediates on the activity of the catalytic sites.The regulation of metal electronic state by the change of active site structure and the change of catalytic activity by pre-adsorbed oxygen-containing ligands were revealed.Secondly,taking the Fe-based bimetallic nitrogen–carbon active sites as the research object,the three most representative 3d transition metals near the top of the‘volcano'curve were selected to construct Fe MN₆ active sites.The prediction order of catalytic activity of Fe Co N₆,Fe Ni N₆ and Fe Mn N₆,was designed by DFT calculation,and then the catalysts of corresponding active sites were synthesized by the same method,and the existence of active sites was verified by synchrotron radiation XAFS.Finally,combined with the oxygen reduction activity test,the catalytic activity of the catalyst was confirmed to be consistent with the predicted catalytic activity,and the accurate synthesis of the catalyst under the guidance of theoretical prediction was completed.Thirdly,Co-based bimetallic catalyst was designed,and the effect of high positive valence metal vanadium into Co–N–C catalyst was discussed.Vanadium-doped Co–N–C catalysts were prepared by two-step pyrolysis.The effect of different V content on the oxygen reduction activity of Co-based catalysts was adjusted.The form of vanadium-doped Co–N–C active sites was revealed by SEM,TEM,XPS and XAFS.