Preparation And Active Site Structure Of Transition Metal Iron/cobalt-nitrogen-carbon Subnanocarbon Layer Catalys

Fuel cells have been consider as an ideal solution for the growing energy shortage and environmental pollution problems in the 21st century,and to achieve"carbon emissions peak"and"carbon neutrality"due to their advantage of high energy density,simple operation,and environmental friendliness.However,the heavy reliance on the expensive Pt-based catalysts to facilitate cathodic oxygen reduction reactions（ORR）hinder their widespread application.Therefore,the development of high-performance,low-cost non-precious metal based catalysts and getting rid of the dependence on Pt-based catalysts is the key to advancing the commercialization of fuel cells.Among various non-precious metal catalysts,transition metal-nitrogen-carbon（M-N-C）catalytic materials（M=Fe,Co,etc.）were regarded as one of the most promising materials to replace Pt-based catalysts.In order to understand the active site structure and surface properties of M-N-C catalyst to guide the design and preparation of high performance M-N-C catalyst.In this study,a precursor with well-defined surface structure was first synthesized by molecular design,then the precursor was pyrolyzed to obtain an M-N-C catalyst with a uniform distribution of catalytic active sites on the surface,and finally the active site structure and formation mechanism were investigated by microscopic,spectroscopic and electrochemical methods.The main contents are as follows:Surface-open polymeric hollow microspheres（PHMs）were prepared by chemical oxidative polymerization.The PHMs@Fe-N and PHMs@Co-N precursors were obtained by directly coordinating transition metal ions to the surface of PHMs using the strategy of coordinating Fe³⁺and Co²⁺with N-containing groups on the surface of PHMs.The surface composition and structure of the precursors,and the metal-centred atomic coordination structure were explored using XPS,TOF-SIMS and XAS spectroscopy and theoretical calculations（DFT）,demonstrating that Fe³⁺coordinates only with quinoneimine（-N=）on the surface molecular chain of PHMs to form Fe N₈structure,and Co²⁺coordinates with quinoneimine（-N=）to form Co N₂and Co N₄structures.Highly active NHMs@Fe catalysts with Fe single atoms dispersed on N-doped carbon surfaces were obtained by pyrolysis of PHMs@Fe-N precursors with well-defined surface structures.Electrochemical test results showed that NHMs@Fe catalyst have high half wave potential of 0.912 V in alkaline conditions,which is 61 m V higher than that of commercial Pt/C catalysts and 152 m V higher than NHMs catalysts,respectively.the kinetic mass activity reached 4.2 A/mg _Feat 0.9 V,which was 84 times that of the Pt/C catalyst(0.05 A/mg _Pt).Further,the catalyst also showed excellent ORR activity and stability in acidic conditions.The chemical composition of the top-near-subsurface layer on NHMs@Fe catalysts was analyzed in depth direction by TOF-SIMS,AES,XPS and SRPES,and the microstructure of the catalyst surface was observed in combination with HRSEM,HRTEM and AC-STEM microscopy,which confirmed that Fe single atoms were only distributed in the sub-nanosphere region of the catalyst surface,but that Fe and N did not form an Fe N_xstructure.Microscopic and spectroscopic methods were used to characterize the evolution of the surface chemical composition,morphology and microstructure of PHMs@Fe-N catalyst at different pyrolysis temperatures.The results show that the formation temperature of Fe atoms and the removal of N atoms at high temperature will introducing micropores and defects on the surface of the catalyst,and atomic-sized Fe species was dispersed in the surface carbon defects.Based on this,a new active site structure of the NHMs@Fe catalyst and corresponding formation mechanism are proposed,i.e.,the Fe atoms reduced from carbon at high temperature are anchored by the vacancies generated by the removal of adjacent N to form stable Fe single atoms.Further,DFT theoretical calculations demonstrated that the carbon defect-anchored Fe single atom is the effective ORR active site for the NHMs@Fe catalyst.High-performance NHMs@Co catalysts were obtained by pyrolysis of PHMs@Co-N precursors.In alkaline conditions,the half-wave potential of NHMs@Co catalyst reached 0.865 V,which revealed a better ORR activity than that of Pt/C catalyst.The NHMs@Co was applied to the air electrode of an aqueous sodium-air battery and achieved a peak discharge power density of 7.94 m W/cm²,which was 2.1 times higher than that of the Pt/C catalyst（3.77 m W/cm²）,and showed excellent cycling stability.The composition and microstructure of the catalyst surface were characterised by spectroscopic and microscopic means,confirming that Co single atoms are distributed within the sub-nanosphere region of the catalyst surface and that the Co N₂structure formed by individual Co atoms anchored by pyridine-N is the main active site of the NHMs@Co catalyst.This work elucidates the key role of atomically dispersed metal atoms in promoting oxygen reduction and the active site structures and formation mechanisms of NHMs@Fe and NHMs@Co catalysts at the molecular/atomic level,providing theoretical guidance for the design and preparation of high-performance M-N-C catalysts.