Structural Regulation And Performance Of Fe-N-C Catalysts For Oxygen Reduction Reaction

With the continuous depletion of fossil fuels,new energy conversion devices such as Zn-air batteries and fuel cells have attracted widespread attention.However,the sluggish kinetics of oxygen reduction reaction（ORR）restricts the performance of these devices.The commonly used Pt-based catalyst is currently scarce,expensive and unstable,which makes it difficult to meet the needs of commercial applications.Therefore,it is urgent to develop a non-precious metal catalyst to take the place of Pt-based materials.Among various non-noble metal catalysts,iron-nitrogen co-doped carbon-based catalysts（Fe-N-C）have become the focus of research due to their outstanding ORR activity.However,Fe-N-C catalysts still face the problems of low loading of active sites,poor intrinsic activity and relatively simple pore structure.In this thesis,a series of Fe-N-C catalysts with excellent ORR performance were prepared.We optimized the structure and composition of Fe-N-C catalysts from the perspectives of increasing the loading of active sites,optimizing the electronic structure of Fe centers and constructing hierarchical porous structures,and the relationships between catalytic activity and structural composition was further studied.The Fe-ZIF（C）precursor was synthesized in the aqueous solution by solvent environment engineering,and the Fe-N-C（C）catalyst with densely Fe-N_x sites was prepared by one-step pyrolysis.In addition,the control groups were synthesized with methanol and water/methanol mixed solution as solvent,respectively.The influence of solvent during the synthesis process of precursors on morphology,structure and active site loading of catalyst was investigated.The results revealed that the content of iron in precursors could be affected by changing the solvent environment during the process of Fe-ZIF synthesis,so as to realize the controllable adjustment of the Fe-N_x site density in catalysts after pyrolysis.Compared with methanol or water/methanol mixed solution,aqueous solution could provide a more moderate Fe content for the ZIF precursor,which improved the loading of Fe-N_x sites in catalysts while avoiding metal agglomeration.Therefore,the Fe-N-C（C）catalyst prepared in an aqueous solution had the highest loading of Fe-N_x sites among the prepared samples.The optimized Fe-N-C（C）revealed satisfactory catalytic activity with the half-wave potentials(E_1/2)of 0.902 V and 0.792 V,as well as good stability,in alkaline and acidic electrolytes,respectively.The peak power density of the Fe-N-C（C）-based liquid electrolyte Zn-air battery and gel electrolyte Zn-air battery reached up to 131 mW cm^-2 and 78 mW cm^-2,respectively.The sulfur-modified Fe-N-S-C catalyst was prepared using Fe SO₄ as the iron source and sulfur source by impregnation and pyrolysis with ZIF-8 and Phen.The influence of sulfur doping on morphology,composition of catalysts and electronic structure of active sites was investigated.The results revealed that the doping of sulfur atoms in Fe-N-S-C not only increased the specific surface area and introduced thiophene-sulfur sites,but also facilitated the formation of more Fe-N and pyridinic-N species.The EXAFS results revealed that the configuration of the active center was the atomically dispersed Fe-N₃S site.Theory calculations elucidated that the introduction of sulfur atoms could adjust the d-band electronic structure of singe-atom Fe center,thus optimizing the adsorption strength of Fe center to OH intermediate and improving the ORR kinetics process.Compared with traditional Fe-N₄ site,the ORR on Fe-N₃S site exhibited a lower energy barrier and overpotential,which proved that the Fe-N₃S site had a higher intrinsic catalytic activity.Therefore,the obtained Fe-N-S-C catalysts with Fe-N₃S sites exhibited outstanding ORR activity in alkaline media with a E_1/2 of 0.915 V,as well as good acidic catalytic performance(E_1/2=0.797 V).The peak power density of the Fe-N-S-C-based liquid electrolyte Zn-air battery and gel electrolyte Zn-air battery reached up to 146 mW cm^-2 and 83 mW cm^-2,respectively.The HPFe-N-C catalyst with hierarchically micro-mesopores structure was designed and prepared by employing the dual melt-salt-mediated templating strategy（i.e.Zn Cl₂ and Na Cl as templates）to assist the pyrolysis of adenine precursors,followed by iron salt impregnation and secondary pyrolysis.The hierarchical porous structure was beneficial for the exposure of Fe-N₄ sites and mass transfer process.The effect of dual melt-salt templates on morphology,pore structure and active site of HPFe-N-C catalyst was investigated.The results revealed that the construction of hierarchical porous structure benefited from the synergistic effect between Zn Cl₂ and Na Cl templates:Zn evaporated at 900℃,resulting in the formation of a large number of micropores;The Na Cl template was employed to promote the transformation of partial micropores（generated by the evaporation of Zn）to mesopores.In addition,dual melt-salt templates also provided protection for adenine precursors,effectively alleviated the loss of nitrogen source in the pyrolysis process,thus increasing the amount of N-doping and the loading of Fe-N₄ active sites in the obtained catalysts.Therefore,the obtained HPFe-N-C catalysts exhibited outstanding ORR activity in alkaline media with a E_1/2 of 0.911 V,as well as good acidic catalytic performance(E_1/2=0.802 V).The peak power density of the HPFe-N-C-based liquid electrolyte Zn-air battery,gel electrolyte Zn-air battery and fuel cell reached up to 160 mW cm^-2,109 mW cm^-2 and 0.619 W cm^-2,respectively.