Preparation And Electrocatalytic Performances Of ZIF-67-derived Nitrogen-doped Co-based Carbon Materials

Oxygen evolution reaction?OER?and oxygen reduction reaction?ORR?are the critical reactions of energy storage and conversion devices such as metal-air batteries and regenerative fuel cells.Electrocatalysts with high activities are important for the development of these energy devices.However,there is still a lack of low-cost,high-efficient and stable bifunctional electrocatalysts for both OER and ORR.Metal-organic frameworks?MOFs?are porous materials assembled by metal ions/clusters and organic ligands through chemical coordination bonds,featuring with high porosity,large specific surface areas and tailorable structures.MOFs have been widely employed as precursors and templates to prepare transition metal-carbon materials,which possess porous structures,enhanced electrical conductivity,high specific surface areas,and abundant active sites,in favor of electrocatalytic reactions.Based on the above premiss,a series of N-doped cobalt-based carbon materials have been prepared by the pyrolysis of cobalt-containing ZIF-67 and further applied as OER/ORR bifunctional electrocatalysts in this thesis.The relationships between the structures and electrocatalytic activities of the catalysts have also been investigated.The main contents and results of this thesis are as follows:In this thesis,a simple KCl-assisted pyrolysis strategy was employed to construct a series of hollow polyhedra assembled with Co nanoparticles embedded in N-doped carbon nanotubes?NCNHP?.The OER/ORR performances of the as-obtained hybrid architectures were evaluated subsequently.The effects of pyrolysis temperature?T?and the mass ratio of KCl/ZIF-67?r?on the structures and compositions of the ZIF-67 derived materials were investigated.The optimal NCNHP-1-500 possessed a well-defined N-doped hollow carbon polyhedral framework,where the formed Co nanoparticles catalyzed the in-situ growth of CNTs on it and the Co nanoparticles were encapsulated on the top of CNTs.The hollow and hierarchically porous structures,and high BET surface areas could endow the NCNHP-1-500with significantly improved mass transfter and more accessible Co-N_x active sites.By virtues of the unique structures and the synergistic effect between the N-doped CNTs and cobalt nanoparticles,NCNHP-1-500 exhibited the best bifunctional OER/ORR performance in the alkaline media,with the lowest potential difference of 0.712 V?vs.RHE?between OER and ORR.For OER,NCNHP-1-500 required a small overpotential of 310 m V to reach the current density of 10 m A cm^-2,far surpass the commercial Ir O₂.For ORR,NCNHP-1-500 showed a half-wave potential comparable to commercial Pt/C and superior stability and methanol tolerance to Pt/C.Our strategy might open up a new avenue for the rational design and frabrication of MOFs derived porous metal-based carbon materials with novel structures.