Research On ZIF Precursor-based Derived Carbon Materials And ORR Properties

With the decrease of traditional fossil energy,fuel cell technology comes into being in order to save energy and improve the utilization efficiency of energy.Oxygen reduction reaction(ORR)is a key part of fuel cell reaction because it is more difficult to react.This has always been an important constraint in fuel cell technology,and the use of ORR catalyst can make ORR norrmal and efficient reaction to break this limitation.However,the catalysts that can be used in practice are mainly expensive and less stable Pt based catalysts.It is a hot topic to find an ORR catalyst with good performance,low price and stable performance to replace it.In a large number of studies,N modified carbon materials are favored for their good stability,conductivity,low cost and easy access.The preparation of materials with MOF as precursor in different doped nitrogen materials is a hot research direction at present.In this paper,nitrogen-doped carbon nanotube clusters were obtained by pyrolyzing ZIF-67 with different pyrolysis processes,and the catalytic activity of different activation sites was investigated.The main research contents and results are as follows:(1)The first part explores the influence of different pyrolysis processes on ORR performance.ZIF-67 was prepared by room temperature statics.The subsequent ZIF-67 was pyrolyzed by different pyrolysis processes.According to the pyrolysis process.The doped carbon material obtained by adding urea pyrolysis and changing the heating curve has the best ORR performance(diffusion limit current density(Jd)=5.3 mA cm-2,half wave potential(E1/2)=0.84 V).Which is close to the performance of Pt/C catalyst(Jd=5.0 mA cm-2 at 0.4 V,E1/2=0.85 V),The increase in the relative content of pyrrolic nitrogen caused the half wave potential to shift further to the right.It indicated that pyrrolic nitrogen was the most important ORR activation site.In addition,the addition of a new 350℃ thermal insulation process resulted in the formation of more carbon nanotube clusters,thus increasing the diffusion limit current density.(2)The second part explores the effect of urea on the structure and ORR properties of catalytic materials.ZIF-67 was prepared by room temperature statics.Then,urea and its pyrolysis products diurea,melamine and NH3 were pyrolysis with zif-67 respectively.Through characterization and testing,the doped carbon materials obtained from the pyrolysis of melamine with ZIF-67 have the best ORR performance(diffusion limit current density(Jd)=5.2 mA cm-2,half wave potential(E1/2)=0.84 V)and the highest pyrrole nitrogen relative content.The second is the doped carbon material obtained by the pyrolysis of biuret and ZIF-67,while the doped carbon material obtained by the pyrolysis of NH3 and ZIF-67 is worse than the former two.Biuret and melamine are deamination polymers of urea,which indicates that urea is more likely to exist in the carbon ring in the form of pyrrole nitrogen after thermal release of ammonia,thus improving the ORR performance of pyrolysis ZIF-67.(3)The third part explores the influence of transition metal ion-dope on the ORR performance of ZIF-67 derived carbon material catalyst.ZIF-67 doped with Fe and Ni was prepared by room temperature static method.The obtained doped zif-67 was then pyrolyzed.Ni doping could not improve ORR activity,and ORR activity of Fe doping was very poor.The reason for the poor activity of ORR of doping Fe is that it is difficult to crystallize due to the different valence state of ions,which makes it difficult to obtain better carbon materials after pyrolysis.