Preparation Of MOFs-derived Transition Metal Carbon-based Catalysts And Their Oxygen Reduction Processes

New energy sources to replace the depleting fossil energy sources are an inevitable trend in energy development,and the core of new energy technology development is the design and construction of energy storage and conversion devices.Proton exchange membrane fuel cells（PEMFC）and zinc-air cells（Zn-air）are highly efficient energy storage and conversion devices that suffer from slow and unstable cathodic oxygen reduction reactions（ORR）in practice,while commercial platinum-based catalysts are costly,poorly methanol-resistant and prone to deactivation,which hinders their large-scale application.Highly efficient,low-cost,and highly stable non-platinum-based catalysts are essential for solving these problems and improving energy storage and conversion efficiency.In this paper,a series of MOFs-derived transition metal carbon-based catalysts were prepared with the aim of enhancing the oxygen reduction process at the cathode of fuel cells,exploring the intrinsic link between their composition,structure,and oxygen reduction activity,and exploring methods and means to enhance the ORR process,with a view to obtaining new ways of oxygen reduction catalyst preparation.The specific research contents and results are as follows:（1）The star-shaped cobalt-nitrogen co-doped porous carbon material（S-CoxNPC）was successfully prepared by mechanical stirring and high-temperature pyrolysis using Co（NO₃）₂·6H₂O,and 2-methylimidazole as raw materials,and the morphology and structure of th e catalyst were characterized by SEM,TEM,Raman,XRD,and XPS tests.The results show that the S-Co1NPC sample has a large specific surface area（1043.45 m2·g-1）and an excellent pore size structure,which facilitates the exposure of active sites and promotes the diffusion and electron transfer of reactants.Electrochemical test results showed that S-Co1NPC exhibited significant ORR activity with initial and half-wave potentials of 0.95 V and 0.83 V（vs.RHE）respectively,both higher than that of commercial Pt/C catalysts.In addition,S-Co1NPC exhibited good stability and methanol tolerance and played an important role in the enhanced oxygen reduction process through the formation of a special cobalt-nitrogen coordination bond following the modulation of the Zn/Co ratio.（2）The cobalt-nitrogen co-doped nanocomposites（CoNC-NaCl-T）were prepared by molten salt impregnation and high-temperature carbonization using ZIF-67 as the precursor,and the samples exhibited excellent ORR performance with the high onset and half-wave potentials（Eoneset=0.91 V and E1/2=0.82 V）in 0.1 M KOH solution,and also showed superior stability and resistance to methanol tolerance.Further analysis shows that the molten salt impregnation procoptimizesises the pore size distribution of the catalyst and enhances the diffusion process of the reactants and that the introduction of NaCl not only increases the pore size and specific surface area of CoNC but also improves the physicochemical properties of the composite,resulting in a unique structure with high pyridine-N and graphite-N content.（3）Based on the metal-organic skeleton-derived ZIF-67,3D nanostructures with zinc as the core and cobalt as the shell were designed and constructed,and furthermore,3D cobaltnitrogen co-doped hollow porous carbon materials（CoNHPC）were obtained by salt coating and one-step calcination.The samples exhibit a high ORR initial potential（Eoneser=0.97 V）,a half-wave potential（E1/2=0.87 V）and an excellent limiting current density（5.41 mA·cm-2）and belong to a near four-electron reaction path.Analysis of the strengthening mechanism shows that the construction of a core-shell structure assisted by salt coating not only enhances the dispersion of the metal active species but also promotes the production of carbon nanotubes in the hollow structure,resulting in a cross-linked conductive carbon network that strengthens the electrical conductivity of the composite.