Study On The Preparation And Oxygen Electrocatalysis Performance Of The Transition Metal/Nitrogen/Carbon Hybrid Materials

With the continuous depletion of fossil energy,new energy technology has attracted people’s attention.Zn-air battery and fuel cell technology have been widely studied and gradually become the hot spot in the field of new energy conversion and storage.One of the most critical technologies in such devices is the development of low cost non-noble metal cathode electrocatalysts with high activity and durability.In this paper,a series of transition metal-nitrogen-carbon（M-N-C）electrocatalysts with good porous structure and high active site density were prepared in order to improve the intrinsic activity of catalysts and boost the utilization of catalytic activity.Firstly,based on the research idea that the synergistic effect between encapsulated transition metal-based nanoparticles and graphitic carbon shell can effectively improve the electrocatalytic activity of materials,S-N co-doped mesoporous carbon electrocatalyst with encapsulated Co_xS_y nanoparticles was prepared by combining in situ polymerization of dopamine and pyrolysis strategy.The effects of pyrolysis temperature on the active site and nanopore structure of the catalysts were studied by using various physical and chemical characterization methods.Additionally,the relationship of the prepared catalysts between microstructure and property was evaluated.The results of the half-cell test show that the half-wave potential of the optimized catalyst（Co_xS_y/SNC-800）towards ORR in alkaline electrolyte is 0.80 V,and the electron transfer number n is 3.7-3.9;The overpotential at 10 m A cm^-2 towards OER is 350 m V.Moreover,Co_xS_y/SNC-800excellent ORR and OER stability.Besides,Co_xS_y/SNC-800 shows excellent charge/discharge performance and cycle stability in advanced Zn-air batteries.Secondly,to improve the utilization rate of active sites,a template-assisted method is described that was used to design Fe,S,and N co-doped hierarchical porous carbon（Fe SN-HPC）.The in situ-formed templates and sulfur source（thiocyanate）can facilitate the formation of numerous micro-/meso-/macro-pores with an expected large surface area(1247 m² g^-1)and abundant defects.More importantly,numerous active sites are dispersed in Fe SN-HPC,including Fe^II–N₄,pyridinic N,and graphitic N.These ideal characteristics endow Fe SN-HPC with high electrocatalytic activity and excellent stability towards ORR in alkaline media.In addition,as an air electrode catalyst,Fe SN-HPC shows good application potential in both primary liquid and flexible Zn-air batteries.Finally,in order to improve the intrinsic activity of the catalyst,highly efficient carbonous hybrid is developed based on a facile and effective strategy via template-assisting and optimized post-pyrolysis process.On the one hand,the prepared catalyst（Fe@NC-700）has ideal high specific surface area and multi-stage pore structure,which is conducive to the mass transfer process in ORR and can effectively expose the catalytic active sites.On the other hand,the catalyst contains abundant active sites such as pyridine N,Fe^II-N₄ graphite N and Fe@NC.These active sites make good synergistic effect and jointly improve the electrocatalytic activity of the catalyst.In other words,the synthesized catalyst has dual advantages of desirable microstructure and high effective active sites.Therefore,the half-wave potential of the prepared material towards ORR in alkaline electrolyte is 0.865 V.More importantly,after 10,000 cycles of cyclic voltammetry test,the half-wave potential only negatively shifts 14 m V,which is better than that of the commercial Pt/C.In addition,when Fe@NC-700 is used as the cathode catalyst,the maximum power density of the Zn-air battery is 160 m W cm^-2,and it shows excellent stability.