Research On Preparation And Performance Study Of MOF-derived Carbon-bansed Oxygen Reduction Electrocatalytic Materials

Facing the energy crisis and environmental problems caused by traditional fossil fuels,sustainable and clean energy is urgently needed to improve these situations.Fuel cells and metal-air cells have received much attention by virtue of their high energy conversion efficiency,high safety and environmental friendliness.The oxygen reduction reaction is the key half-reaction in such energy devices,but its kinetics is very slow.Platinum-based catalytic materials can effectively improve the efficiency of the oxygen reduction reaction,but platinum is expensive and scarce for large-scale applications.It is evident that there is a need to develop new efficient,highly stable and inexpensive nonprecious metal catalysts.Metal organic framework materials(MOFs)are a series of 3D porous materials formed by metal ions or ion clusters and organic ligands through coordination bonding,which have the advantages of rich pore structure,large specific surface area,adjustable components and stable structure.In this paper,a series of studies have been carried out.(1)The influence of pyrolysis conditions on the ORR performance of catalytic materials is crucial,but there are few systematic studies on their effects.In this section,the effect of different temperatures,different ramp rates and different pyrolysis methods(one-step and two-step pyrolysis)on the catalytic activity of Zeolitic imidazolate framework-8(ZIF-8)prepared by solvothermal method was investigated.It was found that decreasing the ramp-up rate was beneficial to improve the carbonation integrity and thus the catalytic activity of the material.In addition,distributed pyrolysis is an effective method to improve the catalytic activity of the material.The use of lower ramp rates in the low temperature stage and relatively higher ramp rates in the high temperature stage can lead to catalysts with better activity,which can reach the half-wave potential of 0.82V(vs.RHE)in alkaline media.(2)The effect of bimetallic co-doping on the catalytic performance of the material was also studied.Fe Co co-doped ZIF-8 was prepared by introducing cobalt nitrate hexahydrate and iron acetylacetonate as metal sources inside ZIF-8,and finally the derived carbon materials containing Fe Co alloy were obtained by high temperature pyrolysis.By adjusting the doping ratio of Fe to Co,it was found that when Fe:Co=1:1,the catalyst had the morphology of coexisting ZIF framework and carbon nanotubes and obtained a high degree of graphitization,which enabled Fe Co-NC to obtain a good activity and exhibited the onset potential of 0.91 V(vs.RHE),the half-wave potential of0.82 V(vs.RHE),and the current retention percentage could reach 82% in the stability test.(3)The catalytic properties of Co-ZIF and graphene hybrid materials were also investigated.Co-ZIF/r GO composites were successfully prepared by changing the growth mode of ZIF particles and growing Co-ZIF in situ on reduced graphene oxide(r GO)containing various oxygen-containing groups on the surface.By adjusting the addition ratio of r GO,it was found that the distribution of Co-ZIF particles changed from agglomerated to discrete state with the increase of r GO.When the r GO addition ratio reached 1.5,Co-ZIF could be distributed uniformly on the surface of r GO,resulting in the best activity and stability of Co-N-C/r GO-1.5 with the half-wave potential of 0.825 V(vs.RHE)in alkaline medium,and the final holding percentage of current density in the stability test was better than that of Co-N-C and Pt/C.