Investigation On The Design And Preparation Of Single-atom Fe-N-C Catalysts For ORR

The positive reaction of the fuel cell is the oxygen reduction reaction（ORR）,and the efficient ORR catalyst has become a research hotspot.In this thesis,through the density functional theory（DFT）calculation,a highly efficient ORR catalyst structure with Fe-N-C single atom active sites is designed and selected.In order to optimize the designed catalyst structure,the molecular tailoring-self-assembly-reconstruction vapor deposition（TSRVD）technology independently developed by the research group was used to synthesize an efficient and stable ORR catalyst.The morphology and structure of the catalyst were characterized by SEM,TEM,XRD,Raman,XPS and XAFS.Using LSV,CV,Tafel,EIS and i-t and other electrochemical test methods to study the performance of the synthesis catalyst.Through DFT calculation,the catalyst models with different Fe-N_xactive sites embedded in carbon six-membrary ring structure were established,and the electronic structure and ORR activity were calculated.The results show that E_fdecreases with the increase of N atomic number.The adsorption free energy of the key oxygen-containing species（*O,*OH,and*OOH）involved in ORR is proportional.The theoretical overpotential of Fe-2N-2C-pen is 0.652 V,and the ORR rate determined step is the transformation from*O to*OH.Moderate N-doping level and structure can enhance ORR activity of Fe-N coordination structure in double-vacancy,while N-atom doping has no effect on ORR activity of single-vacancy.The ligand effect and surface tension effect caused by N atom doping are helpful to optimize the d-band center of Fe atom.The Fe-N₅active site structure designed for DFT,using iron phthalocyanine（Fe Pc）as a precursor,using TSRVD technology to synthesize nanowire catalyst Fe-N-C/G-120 with excellent ORR performance on graphene substrates.The studies show that the synthesized nanowire catalyst owned a layered crystal structure completely different from the precursor Fe Pc,in which Fe（Ⅱ）is coordinated with four N atoms on the same plane and one N atom in the adjacent layer to form Fe-N₅active site.According to the analyses,the Fe Pc molecules undergone dehydrogenation,self-assembly and reconstruction processes to form the crystal structure of the catalyst.The ORR of Fe-N-C/G-120 catalyst in 0.1M KOH solution at 25℃is an efficient four-electron process.Fe-N-C catalysts with nano-columnar,nano-wire and nano-particle shapes were synthesized under different temperature and pressure conditions using TSRVD technology.The high pressure in the tube furnace significantly inhibited the sublimation of Fe Pc molecules,making nucleation difficult and resulting in linear growth of the catalyst along the substrate surface and poor crystallinity.In contrast,low pressure promotes the sublimation of Fe Pc molecules and subsequent nucleation,resulting in the catalyst growing in a columnar direction perpendicular to the substrate surface and high crystallinity.In addition,the growth of the catalyst at low pressure depends on the temperature.Low temperature will reduce the energy and mobility of molecules,thereby limiting the growth of small particles and forming particulate catalysts.