Structural Modulation Of Zn-polyazole Carboxylic Acid Heterocyclic Framework Group N,O-doped Porous Carbon And Its Performance In Oxygen Reduction Reaction

Fuel cells are highly efficient energy conversion devices,which can alleviate the problems of environmental pollution and energy shortages.However,the kinetics of oxygen reduction reaction（ORR）on the cathode is very slow,and there is an urgent need to study metal-free carbon catalysts with high activity,high stability and low cost.The metal-organic frameworks（MOF）with adjustable structure,large specific surface area and high porosity are the most suitable self-sacrificial precursor for metal-free carbon catalysts.However,the carbon material prepared by MOF synthesized from Zn2+and nitrogen-containing ligands has a high micropore content,which limits the mass transfer in ORR.In order to solve this problem,Zn-MOF synthesized with Zn2+and polyazole carboxylic acid heterocyclic ligand was used as the precursor to prepare carbon-based electrocatalysts for ORR with hierarchical porous structure and high-performance by adjusting its structure.The main research contents are as following:（1）Based on the structural regulation of Zn-MOF synthesized from Zn2+and 5carboxybenzotriazole by NaCl,a high-performance N,O co-doped lamellar hierarchical porous carbon-based ORR electrocatalyst（NOHPC）was prepared.Zn-MOF@NaCl was prepared by adding Zn-MOF into saturated NaCl solution.After carbonization,the template was removed to obtain NOHPC.FESEM and TEM analysis showed that after using NaCl,the morphology of the catalyst changed from a rod-like shape to a porous nano-sheet shape.This is because NaCl had the dual functions of a template and an intercalator.Compared with the N,O co-doped porous carbon material（NOPC）derived from Zn-MOF,NOHPC has a high content of meso/macropores,graphitic carbon and graphitic nitrogen.The onset potential（Eo）of NOHPC in acidic,neutral and alkaline solutions is 0.82 V,0.88 V,0.97 V vs.RHE,the half-wave potential（E1/2）is 0.69 V,0.71 V,0.87 V vs.RHE,and the limiting current（JL）is 4.25 mA cm-2,4.56 mA cm-2,5.1 mA cm-2,respectively.Its performance is surpassing that of NOPC and close to that of Pt/C,NOHPC has better stability and methanol resistance than Pt/C.The excellent catalytic performance of NOHPC is attributed to the influence of NaCl on its structure（2）Based on the adjustment of the molar ratio of Zn2+to 4,5-imidazole dicarboxylic acid,Zinc-based zeolite-like metal organic framework（Zn-ZMOF）of bicrystal was prepared by hydrothermal method,which was pyrolyzed with a one-step process to prepare high-performance N,O co-doped porous carbon-based（NOPC）ORR electrocatalyst.XRD shows that the structure of Zn-ZMOF changes with the molar ratio of H3IDC to zinc acetate,and the 1:1 Zn-ZMOF has the bicrystal phase.The study found that the NOPC derived from the bimorph Zn-ZMOF has the structure of micro/mesopores,large specific surface area,a high contents of graphite/pyridine nitrogen and surface oxygen groups and high degree of graphitization,and its catalytic performance is the best.Eo of NOPC in acidic,neutral and alkaline solutions is 0.84V,0.91 V,1.0 V vs.RHE,E1/2 is 0.65 V,0.75 V,0.87 V vs.RHE,and JL is 5.27 mA cm-2,5.2 mA cm-2,5.38 mA cm-2,respectively.Its performance is similar to that of Pt/C in alkaline and neutral electrolytes,and is close to that of Pt/C in acidic electrolyte.NOPC has better stability and methanol resistance than Pt/C.The excellent ORR catalytic performance is attributed to the unique morphology and structure of NOPC derived from bicrystalline Zn-ZMOF.This work uses unmodified Zn-ZMOF to prepare pH-universal ORR electrocatalyst,which has the advantages of green and simple preparation process.