Study On Carbon-based Non-precious Metal Oxygen Reduction Catalyst

As a common cathodic reaction in new energy sources such as fuel cells and metal-air batteries,studies of oxygen reduction reaction（ORR）is of great significance for improving the current energy structure and developing new green energy sources.However,ORR is a multi-electron and-proton transfer reaction,and multiple reaction intermediates and reaction paths are involved in this complex process.Currently,noble metals such as Pt are still popularly considered as the best catalysts for this reaction.However,high cost,low selectivity,poor stability and susceptibility to impurity toxicity can severely hinder the application of this type of catalysts.Therefore,development of efficient and low-cost catalysts is particularly critical.Among many non-noble metal ORR catalysts,carbon-based non-noble metal ORR catalysts are highly favored by researchers due to their low cost,high conductivity,and structural versatility.At present,to further improve the catalytic activity of carbon-based non-noble metal catalyst,to identify the catalytic sites and analyze their catalytic mechanisms and to establish the"structure-efficiency relationship"of carbon-based catalysts are the three tough but key tasks in the research.In this paper,we synthesized three different types of non-precious metal ORR catalysts,namely cobalt-nickel bimetallic MOFs-derived carbon-based materials,nitrogen-doped coal-based porous carbon materials and cobalt-nitrogen co-doped composite carbon-based materials.The structural composition,morphology,specific surface area,and valence information of the materials were characterized with transmission electron microscopy（TEM）,X-ray diffraction（XRD）,Raman spectroscopy（Raman）,N₂adsorption and desorption（BET）,X-ray photoelectron spectroscopy characterization techniques（XPS）and in-situ Fourier infrared（In situ FTIR）.The effects of these factors on the ORR catalytic activity of the materials and the catalytic mechanism were investigated in the electrochemical lab test system.Co Ni bimetallic MOFs-derived carbon-based materials were prepared by a one-step solvothermal method using Zn as a"sacrificial template".The relationship between the catalyst performance and the elemental composition as well as the structure of the materials was analyzed by comparing with the Zn-free and monometallic MOFs-derived carbon-based materials.The results show that the limiting current density of Co Ni-950（Zn）is 6.27 m A cm^-2,which is higher than that of Pt/C(5.92m A cm^-2).At the same time,Co Ni-950（Zn）half-wave potential is 0.817V（vs RHE）,and the starting potential is 0.91 V（vs RHE）.Moreover,the catalytic stability and anti-methanol toxicity of the material are better than that of Pt/C electrode.This excellent ORR activity of the material is closely related to the formation of Co Ni alloy,large specific surface area,good micro/mesoporous hybrid structures and high N content.Finally,the active sites and catalytic mechanism of the catalyst were also obtained by acid leaching and KSCN poisoning procedure.The material completes O₂four-electron transfer reduction through a continuous two-step two-electron reduction process.The anthracite coal was treated with strong acid to obtain clean pretreated coal sample.Then the nitrogen-doped coal-based porous carbon material was obtained by high temperature activation using clean coal as the carbon source,dicyandiamide as the nitrogen source and KOH as the activator.The nanostructure,specific surface area,degree of graphitization,nitrogen content and valence of the materials were precisely regulated by controlling the pyrolysis temperature and the mass of dicyandiamide.NCOH5-900 has a half-wave potential of 0.846 V（vs RHE）and a starting potential of 0.94 V（vs RHE）.Based on the electrochemical test results,the relationship between these factors and the ORR catalytic activity of the materials was obtained.Finally,the ORR catalytic mechanism of the material was studied by in-situ FTIR.The material completes O₂four-electron transfer reduction through a continuous two-step two-electron reduction process.The nitrogen-doped coal-based porous carbon material prepared to obtained a composite material with ZIF67.Then the cobalt-nitrogen co-doped composite carbon-based material was obtained by low temperature annealing at 600°C.The Co₃O₄@NC-5 half-wave potential is 0.862 V（vs RHE）,which is only 11 m V different from the Pt/C electrode（0.873 V）.Finally,the material was oxidized,and the valence state of Co,N elements in the material was regulated.A high-performance cobalt-nitrogen co-doped ORR catalyst was successfully prepared,and the effects of Co,N elemental valence and catalyst phase structure on the catalyst ORR catalytic performance were also investigated.Finally,the ORR catalytic mechanism of the material was studied by in-situ FTIR.This four-electron transfer reduction process was completed through a continuous two-step two-electron reduction procedures.