Steering The Local Microstructure And Mass Transfer Capability Of Fe Single-atom Oxygen Reduction Electrocatalyst Via Ligand Engineering

Zinc-air batteries（ZABs）are considered as one of the promising clean and sustainable energy conversion devices due to the advantages of high theoretical energy density,excellent safety,and low cost,creating momentum for green energy development.Nevertheless,the kinetics of the sluggish oxygen reduction reaction（ORR）in the currently relevant technologies is one of the critical elements hindering the widespread commercialization of ZABs.Although Pt-based catalysts have been well developed and have shown excellent ORR activity,the high cost,scarcity,and poor long-term stability of Pt have greatly limited their large-scale application.Therefore,the exploration of non-precious metal catalysts with high catalytic activity,high stability and low cost is essential for the further development of ZABs.Among various non-precious metal catalysts,atomically dispersed metal-nitrogen-carbon（M-N-C）catalysts have received great attention because of their maximum atom utilization efficiency,tunable electronic configuration,and excellent catalytic performance.Especially,Fe-N-C catalysts,with remarkable ORR activity and selectivity,are considered as one of the most promising non-precious metal catalysts.However,to satisfy practical applications,the performance of Fe-N-C catalyst needs to be further improved.This thesis focues on enhancing the ORR performance of Fe-N-C catalysts by modulating the local microstructure to increase the intrinsic activity and improve the material transport channels towards practical application in ZABs.The details are as follows:1.A metal organic framework ligand engineering strategy was used to adjust the ratio of 2-methylimidazole and methimazole ligands to prepare the precursor（Fe-ZIF-S）,and Fe-N-C based coupling catalysts containing Fe S nanoparticles（denoted as Fe S/Fe NSC）were successfully prepared after high temperature pyrolysis.High-angle annular dark-field scanning transmission electron microscopy（HAADF-STEM）maps and X-ray absorption fine structure data（XAFS）indicate that the obtained Fe S/Fe NSC catalysts have abundant Fe-N₄ sites and Fe S nanoparticles embedded on N/S-doped carbon.In this unique structure,Fe S nanoparticles and oxidized sulfur synergistically induce electron redistribution thereby modulating the electronic configuration of Fe-N₄ sites.Thus,the Fe S/Fe NSC catalyst shows superior ORR performance with a half-wave potential of 0.91 V,better four-electron selectivity and lower H₂O₂ yields.Moreover,thanks to the Fe S nanoparticles encapsulated by the in situ generated carbon nanotubes,excellent stability is provided for the catalyst.Further applying the catalysts to the air-cathode catalyst layer of liquid ZABs,the Fe S/Fe NSC-based ZABs exhibit favorable power density(256.06 m W cm^-2)and high specific capacity(807.54 m A h g^-1),which can be stably cycled for more than 600 h at a current density of 20 m A cm^-2.Meanwhile,the Fe S/Fe NSC-based solid-state flexible ZABs also display outstanding performance with power density up to 85.06 m W cm^-2 and maintain cycling stability for 60 h at a current density of 10 m A cm^-2.2.The zinc ethylxanthate ligand（Zn（S₂COEt）₂）and the iron ethylxanthate ligand（Fe（S₂COEt）₃）were prepared by precipitation method to form coordination compounds with 2-methylimidazole and triphenylphosphine,and Ketjen Black（KB）was used as carbon carrier.Zn（S₂COEt）₂ was used as an in situ self-sacrificial template in combination with KB to not only construct hierarchical porous structures but also to prevent metallic iron agglomeration.X-ray photoelectron spectroscopy（XPS）data show that the Zn-N_x and Fe-N_x structures coexisting in Zn/Fe-NSPC-1000.Modulation of the central metal atom microstructure by N/S/P co-doping and construction of a hierarchical porous structure to optimize the material transport channels result in the final Zn/Fe-NSPC-1000 catalyst exhibiting excellent catalytic performance(E_1/2=0.91V)and four-electron selectivity.Furthermore,Zn/Fe-NSPC-1000 shows favorable stability in accelerated durability test（ADT）.The Zn/Fe-NSPC-1000 based ZABs provide excellent power densities of 280.15 and 230.19 m W cm^-2 in liquid and all-solid flexible configurations,respectively.Meanwhile,the liquid ZABs have a lifetime of600 h at a current density of 5 m A cm^-2.