Study On Fe-N-C Catalyst For Oxygen Reduction Reaction At The Cathode Of Fuel Cells

With the increasing attention to the environment and energy,the development of new green battery energy conversion technologies has also become the focus.The commercial development of energy conversion technologies such as fuel cells and metal-air batteries can be limited by slow reaction kinetics of oxygen reduction reaction（ORR）,and Pt/C catalysts,currently the most widely catalysts,used are limited by the challenges of high cost and low reserves.Therefore,it is of great significance to design and research non-noble metal-based catalysts with low cost and high performance in order to reduce the cost of ORR catalysts.Based on the background,according to the current research status of Fe-N-C catalysts,and aiming at the problems of low performance and few active sites that Fe-based catalysts are currently facing,leaf zeolite imidazolate framework material（ZIF-L）can be designed as a precursor,which was co-pyrolysis with Fe（NO₃）₃·9H₂O to synthesize Fe-N-C catalyst.The electron transfer rate and mass transfer rate of the catalyst can be improved by introducing an additional carbon source,and the oxygen vacancy concentration can be controlled by optimizing the synthesis conditions.The ORR electrochemical performance of the as-synthesized catalysts was tested and the potential of the catalysts in practical applications was verified by assembly of Al-air batteries.The specific work is as follows:（1）ZIF-L was prepared with Zn（NO₃）₂·6H₂O and 2-methylimidazole in aqueous solvent by stirring method,and Fe-N-C catalyst（named Fe-N-900）prepared by co-pyrolysis with Fe（NO₃）₃·9H₂O.Then,the changes of catalyst can be tracked by adopting SEM,XRD,XPS and other physical characterization methods and the formation process of the catalyst crystal structure s explored.The ORR electrocatalytic performance of the catalyst was tested by a rotating disk electrode device,and the effect of the change of the crystal structure of the catalyst on the electrochemical performance was explored.The results showed that the Fe-N-900 catalyst prepared with ZIF-L as a precursor had excellent electrocatalytic performance,with a half-wave potential of 0.867 V vs.RHE compared to the Pt/C catalyst.（2）By compounding the additional carbon source with Fe-N-900,the conductivity of the catalyst can be increased,specific surface area and the number of accessible active sites enhanced,the electrocatalytic activity and electron transfer rate of the catalyst improved,and the purpose of optimizing the catalyst achieved.It can be found by using physical characterizations such as SEM,XRD,BET that the cross-linked structure including larger Fe-containing active particles and one-dimensional CNTs-wrapped Fe/Fe₂O₃ nanoparticles（named as Fe-N-CNTs-900）can be prepared when carbon nanotubes（CNTs）were used as the carbon source,which can greatly improve the mass-charge transport rate and the accessibility of active sites of the catalyst.As the results of ORR,it is found that its half-wave potential is higher about 15 m V than that of the Pt/C catalyst.（3）By changing the synthesis conditions——Zn（NO₃）₂·6H₂O concentration of Fe-N-CNTs-900,catalyst grain size can be controlled and the oxygen vacancy concentration on the catalyst surface optimized.The concentration of oxygen defects can be qualitatively and quantitatively,characterized by characterizations such as TEM,Raman and XPS,and it was found that the concentration rule of oxygen vacancies followed:Fe-N-CNTs-900-0.2<Fe-N-CNTs-900-0.15<Fe-N-CNTs-900-0.6<Fe-N-CNTs-900-1.The test of ORR electrochemical performance results,caused by the existence of oxygen defects,showed that electrochemical performance decreases with the increase of oxygen defect concentration because the increase of the surrounding electron cloud density and difficult of absorption-desorption rate of O₂,which can result to decrease in electrochemical performance.Al-air batteries were assembled with Fe-N-CNTs-900-0.2 and Pt/C catalysts,and results said that The Al-air battery of Fe-N-CNTs-900-0.2 exhibits high power density(55.43 m A·cm^-2)and specific capacity(463.16 m Ah·g^-1).The implementation of this paper can provide an efficient catalyst for ORR,experimental basis for the preparation of oxygen-deficient catalysts,and application prospect for Al-air batteries.