Design And Study Of Zif-derived Co-N-C Oxygen Reduction Catalysts

Fuel cell is a clean energy conversion device with high energy conversion efficiency and no pollution to the environment.It has great development potential in hydrogen energy conversion devices.The catalytic layer is the place where the electrochemical reaction discharge is carried out,which determines the discharge performance of the fuel cell.The cathodic oxygen reduction reaction（ORR）process has a higher reaction overpotential,and the reaction rate is not as fast as the anodic hydrogen oxidation reaction（HOR）rate.Therefore,the ORR catalyst performance of the cathode determines the overall performance of the fuel cell.At present,Pt/C catalysts with high ORR catalytic activity are still the mainstream catalysts for the cathode catalytic layer of fuel cells.However,the high cost and scarcity of the precious metal Pt hinder the large-scale commercial application of fuel cells.Therefore,there is a need to continuously develop efficient and cost-effective non-noble metal catalysts to replace commercial Pt/C catalysts to reduce the overall cost of fuel cells.Zeolite imidazolate framework（ZIFs）are metal-organic frameworks（MOFs）with high porosity and large specific surface area.The three-dimensional ZIF catalyst precursor prepared with metal nodes such as Co,Fe,Mn and N-rich ligands（such as methylimidazole）as the linker,after high temperature activation,formed abundant M-N_x active sites.These active sites serve as the active center of ORR and show good ORR catalytic activity.ZIF-derived transition metal nitrogen-doped carbon catalysts（M-N-C,M=Fe,Co,Mn…）materials have received extensive attention from researchers due to their cost-effectiveness and high natural abundance.Therefore,the development of non-precious metal catalysts is of great significance to the development of fuel cells.In this paper,four Co-N-C catalysts with Co/Zn molar ratios of 1:99,5:95,10:90and 15:85 were designed to explore the effect of different Co/Zn molar ratios on the microscopic morphology,active sites and pore structure of Co-N-C catalysts.The performance differences of the four Co-N-C catalysts in acidic and alkaline electrolytes were explored by rotating disk electrode（RDE）,respectively.The study found that the performance difference of Co-N-C catalysts under acid-base conditions is closely related to the active site density and pore structure of the catalysts.Catalysts with more micropores have more active sites,while mesopores are more conducive to mass transfer.In an acidic environment,the 0.05Co-N-C catalyst with more micropores and larger specific surface area exhibits better ORR performance.The catalyst with the best performance under alkaline conditions is 0.10Co-N-C,the half-wave potential reaches0.91 V vs RHE,which is better than the most non-precious metal catalysts,in which micropores and mesopores coexist,and a large number of mesopores facilitate the mass transfer of macromolecules（such as OH^-）.The effect of Si O₂protection strategy on the structure and performance of Co-N-C catalysts was investigated by two Co-N-C catalysts with Co/Zn molar ratios of 10:90and 15:85.The study found that for ZIF precursors with a low Co/Zn molar ratio,the protective effect of Si O₂ can inhibit the migration of Co atoms and significantly reduce the number of mesopores.However,for ZIF precursors with a high Co/Zn molar ratio,Si O₂ can reduce the excessive agglomeration of Co atoms and protect the mesopores in the catalyst,which is beneficial to improve the active site density（high Co/Zn molar ratio）and mass transfer efficiency（abundant mesopores）.In summary,this paper firstly explores the development of catalysts with reasonable pore structures for acid-base specific ORR processes.This work highlights the differences in the ORR process of Co-N-C catalysts in acidic and basic environments,and have some guiding significance for the design of cathode non-PGM porous ORR catalysts in PEMFC and AEMFC.Secondly,the Si O₂ protection strategy provides ideas for the design of high metal content ORR catalysts,which can increase the active site density of alkaline fuel cell cathode catalysts and reduce the thickness of the catalyst layer,which has certain guiding significance for the subsequent design of non-precious metal ORR catalysts for alkaline fuel cells.