Preparation And Oxygen Reduction Performance Of Fe-N-C Single Atom Catalysts

Nowadays,with the advent of the global energy crisis and the increasingly serious environmental pollution problem,the development and utilization of clean and renewable energy technology have attracted more and more attention.Fuel cell is a direct power generation device that converts the chemical energy in the fuel into electrical energy through an electrochemical reaction.Fuel cell has become one of the most important and promising clean energy technologies due to many advantages such as high efficiency,cleanliness,low noise,high reliability,as well as environmentally friendly.At present,platinum（Pt）or Pt-based alloy is widely used as an electrocatalyst for oxygen reduction reaction（ORR）.However,many disadvantages including scarce reserves,high cost,poor stability,deactivation and other issues,limiting the utilization of Pt-based catalysts in large-scale commercial application of fuel cells.Atomically dispersed Fe-N₄ moieties in porous carbon material（Fe-N-C）displays the most promising non-precious metal catalyst to replace Pt-based catalysts due to its good catalytic activity and low cost.However,how to further improve the catalytic performance of Fe-N-C catalysts by increasing single-atom Fe loading,site density of active sites and modification of electronic structure of active sites is still a challenge.Based on this,in this thesis,the ZIF-8 was used as the precursor to study how to deveople Fe-N-C catalysts.The effects of nitrogen and iron sources on the microscopic morphology,porosity and single-atom structure of the catalyst have been studied.The correlation of single-atom structure with ORR activity has been established.The approaches for synthesis of Fe-N-C catalyst with high site density of active sites was developed.Finally,the applications based on ORR such as znic-air battery and hydrogen fuel cell were inversitigated.（1）The Fe-doped ZIF-8（Fe-ZIF-8）prepared by the chemical doping method was used as the precursor.The phenanthroline as a secondary nitrogen source was adsorbed into Fe-ZIF-8 by a wet impregnation process.The precusors were converted into atomically dispersed Fe atoms on nitrogen-doped porous carbon catalyst（AD-Fe-NPC）catalyst after a heat treatment at 1000°C.The effect of phenanthroline content on the microscopic morphology,porous structure and content of single Fe atom has been revealed.The results showed that phenanthroline can provide additional nitrogen elements to stablize Fe atoms,resulting in the formation of Fe-N₄ sites with a high loading of 1.07 at%for AD-Fe-NPC with utilization of 50 mg phenanthroline.The specific surface area is up to 1011.2 m² g^-1.The catalyst exhibits excellent ORR catalytic performance in 0.1 M KOH solution,including a high half-wave potential(E_1/2)of 0.91 V（vs reversible hydrogen electrode potential,RHE）and a high kinetic current density（J_k）at 0.85 V of 31.2 m A cm^-2,which is approximately 6.5 times higher than that of Pt/C(4.8 m A cm^-2).The zinc-air battery with AD-Fe-NPC as an air electrode displays an excellent performance with a higher power density of 201 m A cm^-2 than that of the commercial Pt/C(136 m A cm^-2),indicating a promising application of AD-Fe-NPC catalyst in zinc-air batteries.（2）In-situ trapping method using was demonstrated to introduce secondary nitrogen sources（such as melamine（MA）,dicyandiamide（DCD）and phenanthroline（Phen））into precursors during the synthesis of Fe-ZIF-8 nanocrystals.Synchrotron radiation-based X-ray absorption spectroscopy（XAS）analysis revealed that the MA introduced in-situ can participate in the coordination of Fe ions to form Fe-N₆complexs.Compared with Fe-N₄ structure,this structure has higher stability which can be converted into atomically dispersed Fe-N₄ moieties after high-temperature pyrolysis.The aberration-corrected transmission electron microscopy and XAS analysis showed that single Fe atoms with Fe-N₄ structure were highly dispersed on the carbon layer in the Fe-N-C catalyst.The content of single-atom Fe reached 3.5 wt%.In contrast,the Fe NC catalyst obtained without using a secondary nitrogen source contained a large number of Fe clusters and nanoparticles.In addition,the secondary nitrogen source is also conducive to generating more microporous and mesoporous structures,leading to a high specific surface area of 1160 m² g^-1,thereby increasing the density of active sites.As a result,Fe-NC/MA-200 exhibited an improved ORR activity in 0.5 M H₂SO₄ with a higher E_1/2 of 0.83 V（vs RHE）,compared to that of Fe NC prepared without using secondary nitrogen source（0.79 V）.The proton exchange membrane fuel cell with Fe-NC/MA-200 as the cathode catalyst display a good performance.Under 1 bar H₂/air conditions,the current density of the fuel cell at voltages of 0.80 and 0.60 V are 0.080and 0.370 A cm^-2,respectively.The peak power density can be calcuated as high as 0.21W cm^-2.（3）A combination of chemical doping and in-situ encaupslation is demonstrated to increase the content of Fe-N₄ active sites in the ZIF-8 precursor,thereby improving the density of active sites after pyrolysis.Specifically,Fe ions are coordinated with 2-methylidazole to form the Fe-N₄ tetrahedral structure in the framework of ZIF-8,while the hemin with well-defined Fe-N₄ strucutre are encapsulated into micropores of ZIF-8.The spatial confinement effect of micropores and stable chemical structure of ZIF-8can be fully utilized to limit the migration of Fe atoms,leading to maintaining the single-atom Fe-N₄ structures in porous carbon.The loading of single Fe atoms reached4.5 wt%.XAS analysis revealed that the coordination number of Fe in the Fe-N-C catalyst prepared by this method is caculated to be 6 and the valence of Fe is+3.According to the calculation of the X-ray absorption near-edge structure,the active center of the catalyst may be the Fe-N₄ structure in the graphene and the axially coordinated with O₂,that is,Fe-N₄-O₂.The resulting catalyst has excellent ORR catalytic performance with a high E_1/2of 0.85 V（vs.RHE）in 0.5 M H₂SO₄,which is comparable to commercial Pt/C catalysts（0.85 V）.The good performance in H₂/air fuel cell also indicated an excellent ORR catalytic activity for Fe-N-C.