Synthesis Of Two-dimensional MOF Materials And Their Carbon-based Electrocatalysts For Oxygen Reduction Reaction

The electrochemical oxygen reduction reaction（ORR）is the most important cathode reaction for low temperature fuel cell.The sluggish kinetic process of ORR greatly limits the commercial application of fuel cell.Until now,Pt-based noble metal catalysts have proven to be the most efficient catalysts for ORR.However,the exorbitant price,scare reserves,and poor stability of Pt severely restrict its large-scale commercialization.Therefore,it is of great theoretical value to develop efficient non-noble metal electrocatalysts to replace noble metal electrocatalysts.MOFs-derived carbon-based catalysts have been widely used in the field of electrocatalysis due to their high specific surface area,high conductivity and abundant active sites.In order to solve the problems of low utilization of active sites of three-dimensional MOFs-derived carbon materials and slow mass transfer of reactants,this paper designed and prepared two-dimensional MOFs-derived electrocatalysts.Adjust the structure and active site of catalyst by studying the structure-effect relationship of the catalyst to further improve the performance of catalyst.Using water as the solvent and methylimidazole as the ligand,the hydrogen-bonding strategy was used to induce the three-dimensional structure of ZIF-8 to two-dimensional flake structure of ZIF-L.The flake ZIF-L structure was obtained when the molar ratio of zinc nitrate to imidazole ligand was 1:8.On this basis,the Co Zn-ZIF-L was prepared by in-situ doping of Co,and the cobalt-embedded nitrogen-doped mesoporous carbon nanoleaves（Co-N/C）were obtained by pyrolyzing Co Zn-ZIF-L.The specific surface area of the catalyst is 283.2 m² g^-1,and the pore volume reaches 0.421 cm³ g^-1.Co-N/C can reach a good activity and a half-wave potential of0.825V,and a limiting current density of 5.13 mA cm^-2.Inorganic acid was used to regulate the active site of Cu N_x.The two-dimensional flake structure of Cu Zn-ZIF-L was synthesized by using zinc nitrate and copper nitrate as metal sources,methylimidazole as ligands,and water as solvent.The copper-embedded nitrogen-doped mesoporous carbon nanoleaves（Cu-N/C）were obtained by pyrolyzing Cu Zn-ZIF-L.H₂SO₄ and HNO₃ solutions were used to regulate the active sites in Cu-N/C.X-ray absorption fine structure spectrum（XAFS）revealed that Cu⁺-N₂ active site was found in Cu-N/C catalyst treated with H₂SO₄,while Cu²⁺-N₄ active site was found in Cu-N/C catalyst treated with HNO₃.Electrochemical test results show that Cu-N/C containing Cu⁺-N₂ possesses optimal performance catalytic property with a half-wave potential of 0.860V and a limiting current density of 5.19 mA cm^-2.The electrocatalytic stability is excellent,and the half-wave potential has almost no negative shift after 5000 cycles.Modification of ultrathin nitrogen-doped graphene nano-mesh（NGM）with small molecules of iron phthalocyanine（Fe Pc）by adsorption method.Fist,flake ZIF-L was further“thinned”by molten salt exfoliating to obtain NGM with high specific surface area,high porosity,and rich nitrogen doping and defective structures.Then,small molecules of Fe Pc are introduced as molecular-level catalytic units to improve the oxygen reduction performance of NGM.By optimizing the content of Fe Pc and comparing the specific surface area and pore diameter distribution of the catalyst before and after modification,it was found that Fe Pc were adsorbed in the micropores of NGM.This catalyst possesses a high-performance catalytic property with a half-wave potential of 0.90V and a limiting current density of 5.32 mA cm^-2.