Preparation And Performance Of MOFs Derived Metal And Nitrogen Co-Doped Carbon-Based Catalysts For Oxygen Reduction

As a new generation of energy technology,the large-scale commercial application of the fuel cell can’t be realized because of the reliance on expensive platinum catalysts in cathodic oxygen reduction reaction(ORR).Transition metals and nitrogen co-doped carbon-based materials(M/N-C)are considered to be one of the most promising materials to replace platinum-based catalysts as the cathode oxygen reduction catalysts for fuel cells,profiting from the advantages of low price,simple preparation,high catalytic activity and good stability.Metal-Organic Frameworks(MOFs)are applicable to prepare M/N-C catalysts because of their regularity of structure,controllability of composition and structure.However,the MOFs have some inherent defects:(1)The pure MOFs mostly are nano-level dispersed particles,resulting to poor electrical conductivity for derived carbon materials;(2)The microporous property of MOFs leads to similar porous strctures for derived carbon materials,significantly limiting exposure of active sites;(3)The MOFs particles are easily bonded together so that the service life of the catalyst is seriously affected.To solve the above problems,we adopt the method of regulating synthesis step by conditioning agent and adding carbon supports in this paper to prepare nitrogen doped crosslinked carbon-based materials and transition metal,nitrogen co-doped one-dimensional and two-dimensional carbon-based materials.The catalysts prepared have the equal catalytic performance to commercial platinum carbon due to improved conductivity and highly exposed active sites.The specific research programmes are as follows:(1)Crosslinked zeolimidazole skeleton structure material ZIF-8-L from zinc nitrate and2-methylimidazole was achieved by one-step controlled preparation method with assist of salicylic acid.And N-C-L catalysts with crosslinked structure were obtained after carbonization.The addition of salicylic acid changed the growth process of ZIF-8 and produced crosslinking structure,where effectively connected the individual catalyst particles and reduced the interface resistance between particles.The catalytic performance of crosslinked N-C-L catalyst is superior to the N-C catalysts from pure ZIF-8 and its initial potential and the limit current density have reached up to 0.93 V and-5.7 mA/cm~2 respectively.But the catalytic performance of crosslinked N-C-L catalysts still has a certain gap with the commercial platinum carbon because of the lack of reactive metals.(2)ZIF-67 was in situ synthesized on carbon nanotubes from cobalt nitrate,2-methylimidazole and acidizing carbon nanotubes,and then was carbonized to generate in situ doped one-dimensional Co/N-CNTs nano-catalyst materials.By controlling the growth time of ZIF-67,the homogeneous doping of Co elements was realized and the Co particle agglomeration was avoided,so the Co/N-CNTs possesses comparable catalytic activity and selectivity,but superior durability to commercial 30 wt%Pt/C catalyst in alkaline media,profiting from the characteristic of high conductivity and openness of carbon nanotubes simultaneously.Its initial potential and the limit current density have reached up to 0.95 V and-5.2 mA/cm~2 respectively.(3)On the basis of experiment(2),in situ doped mesoporous two-dimensional flake Co/N-GO catalyst materials were prepared assisting by surfactant polyvinyl pyrrolium copper(PVP),using zinc nitrate,cobalt nitrate,2-methyl imidazole and graphene oxide(GO)as the precursors.The two-dimensional laminar structure and rich mesoporous materials provide a large surface area for the material,and the homogeneous doping of Co and N provides a rich CoN_x active site,so the 5%Co/N-GO catalyst materials delivered better catalytic activity and stability than commercial platinum carbon that its initial potential and the limit current density have reached up to 0.97 V and-5.8mA/cm~2 respectively and showed excellent four electron selectivity under alkaline condition.