The Research For MOFs-derived Transition Metaland Nitrogen Co-doped Carbon Based Catalyst For Efficient Oxygen Reduction Reaction

The cathode reaction in the fuel cell is oxygen reduction reaction(ORR),which needs effective catalyst to improve the sluggish kinetic process.At present,commercial Pt/C catalyst ranks one of the most excellent.However,the costly price and poor durability severely hindered its wide application.Therefore,developing alternative non-precious oxygen reduction reaction catalysts with low cost,high activity and excellent durability is of great importance.Zeolitic imidazolate frameworks(ZIFs)that possess periodic arrangement of transition metal-nitrogen coordination structure,three-dimensional pore structure and high specific surface area,were adopted as precursors to prepare hierarchically porous transition metal and nitrogen co-doped carbon(TM-N-C)catalysts.The influences of catalysts' pore structure and elemental composition on ORR performance were fully studied.What's more,the controllable preparation of TM-N-C catalysts was realised by rationally designing the composition and structure of ZIFs precursor in order to tune the pore structure and active sites of derived catalysts.First of all,we synthesized ZIF-67 with a mean diameter of about 300 nm through solvothermal method at room temperature,and then prepared cobalt and nitrogen co-doped carbon(Co-N-C)catalyst by pyrolysis of ZIF-67.The effects of pyrolysis temperature on pore structure,specific surface area,element doping and dispersion of metal nanoparticles were studied.It was disclosed that Co-N-C catalyst pyrolyzed at 700? possessed the biggest specific surface area of 320.1 m2 g-1,of which micro-pore proportion came up to 55.8%and meso-pore proportion was 18.3%,and the highest N content of 8.33 at.%.What's more,Co nano particles aggregated in the Co-N-C catalyst.Electrochemical studies showed that the ZIF-67 derived Co-N-C catalyst possessed good ORR performance close to commercial Pt/C in alkaline electrolyte.In order to further improve the dispersion of Co nanoparticles and optimize the pore structure to enhance the catalytic activity of Co-N-C,core-shell structured ZIF-8@ZIF-67 with an average diameter of 1 ?m was synthesized by seed-mediated method at room temperature,through which ZIF-8 was used as core and ZIF-67 as shell.Cobalt and nitrogen co-doped carbon nano-polyhedra with hierarchically pore structure(Co,N-HCNP)was prepared by pyrolysis of ZIF-8@ZIF-67.The outcomes showed that when tuning the Co/Zn ratio to be 0.1,the prepared Co,N-HCNP catalyst possessed a specific surface area of 632.5 m2 g-1,of which micro-pore occupied 69.4%,the pore structure in which was hierarchical with inner micro-pore and outer meso-pore.It was found that ZIF-8 core was beneficial for the uniform pyrolysis of ZIF-67 shell,contributing to the uniform dispersion of Co nano particles.Electrochemical studies showed that Co,N-HCNP catalyst possessed excellent ORR performance,the half wave potential of which was 0.855 V,surpassing that of Pt/C.Meanwhile,the methanol resistance and durability of Co,N-HCNP were also superior to those of Pt/C.Finally,we doped iron into ZIF-67 and prepared iron,cobalt and nitrogen co-doped carbon nano-polyhedra(Fe,Co,N-HCNP)catalyst with Fe-Nx and Co-Nx dual active sites,greatly enhancing ORR performance.We synthesized bi-metal Fe,Co-ZIF with regular nano polyhedra structure for the first time through the coordination between Fe2+,Co2+ and 2-methylimidazole.Fe,Co,N-HCNP was prepared by pyrolysis of Fe,Co-ZIF.It was found that metal based nano particles were spread more uniformly and the micro-pore proportion was elevated with the increase of Fe/Co ratio and the subsequent Fe,Co,N-HCNP catalyst possessed the most outstanding ORR performance when Fe/Co ratio was 0.3.In alkaline electrolyte,the onset potential and half wave potential were 38 mV and 29 mV more positive than those of Pt/C.In the meanwhile,the half wave potential reached 0.764 V in acidic electrolyte,very close to Pt/C(0.814 V)...