Boosting Oxygen Reduction Electrocatalytic Performance Of Fe-N-C Catalysts By Optimizing The Electronic Structure And Utilization Of Active Sites

As an emerging green energy source,zinc-air batteries（ZABs）have attracted much attention because of their high theoretical energy density,high safety,and low-cost.However,the wide commercial applications of ZABs are still limited due to their low power density and poor cycle stability.One of the main factors limiting their development is the slow kinetics of cathodic oxygen reduction reaction（ORR）.Therefore,in order to accelerate the ORR kinetics of ZABs,a highly active and stable catalysts at the cathode reaction are required.Although the precious metal catalysts（such as Pt/C）show excellent ORR performance.However,the using of precious metals increases the cost of ZABs due to their scarce reserves and high prices.Among the non-precious-metal-based catalysts,single-atom iron-nitrogen-carbon（Fe-N-C）catalysts have many advantages,such as low cost and high catalytic activity,which are considered as promising alternatives to precious-metal-based catalysts.However,the intrinsic activity and active site utilization of Fe-N-C catalysts are still unsatisfactory.Therefore,this thesis focuses on regulating the microelectronic structure of Fe-N-C catalysts and constructing carbon support with large specific surface area and hierarchical pore structure to improve the utilization of active sites and jointly improve the performance of Fe-N-C oxygen reduction electrocatalysts.The structure and electrochemical properties of the catalysts were investigated by systematic characterizations,and the obtained catalysts were applied in ZABs.The details are as follows:（1）N/S co-doped porous carbons are rationally prepared and have been verified with rich Fe-active sites,including atomically dispersed Fe-N₄ and Fe nanoclusters(Fe SA-Fe _NC@NSC),according to aberration-corrected high-angle annular darkfield scanning transmission electron microscopy and X-ray absorption fine structure analyses.Experimental and theoretical calculations unraveled that the iron single atom and Fe nanoclusters on N/S co-doped carbon matrix modulate d-band center of Fe by tandem effect,synergistically optimize the adsorption/desorption free energies of oxygen intermediates and lower the energy barrier,thus significantly enhances the intrinsic ORR activity of Fe SA-Fe _NC@NSC.Meanwhile,the unique hollow structure and the large specific surface area enhance the mass and charge transport of the as-prepared electrocatalyst.Consequently,benefiting from the rational structure design and the electronic structure modulation,the Fe SA-Fe _NC@NSC exhibits excellent ORR performance(E_1/2=0.90 V),superior methanol tolerance,and durability,outperforming commercial Pt/C catalyst.Furthermore,the Fe SA-Fe _NC@NSC catalyst was employed as the cathode catalyst in ZABs,which show excellent power density of 259.88 m W cm^-2,specific capacity of 811.03 m A h g^-1 at 50 m A cm^-2,and stable charge-discharge cycling over 100 h at 10 m A cm^-2.Moreover,the all-solid-state ZAB using the as-prepared electrocatalyst exhibits a power density up to 55.86 m W cm^-2,better than Pt/C-based device.（2）Hierarchical porous N/S co-doped carbon materials with atomically dispersed Fe（Meso/Micro-Fe NSC）were prepared by using a nanoemulsion polymerization self-assembly strategy.X-ray absorption fine spectroscopy analysis shows that atomically dispersed iron atoms exist in Fe-N₄ structure.Systematic experiments demonstrate that the large specific surface area and the interconnected hierarchical porous structure facilitate the exposure of active sites and increase the utilization of active sites.Therefore,the Meso/Micro-Fe NSC catalyst shows excellent ORR activity(E_1/2=0.91 V)and electrochemical active surface area(173.24 m² g^-1).Notably,Meso/Micro-Fe NSC catalyst exhibits high kinetic mass activity,which is superior to catalysts dominated by microporous structures or with smaller specific surface areas.Moreover,as cathode catalysts for ZABs,Meso/Micro-Fe NSC outperformed Pt/C in terms of maximum power density(264.34 m W cm^-2),specific capacity(814.09 m A h g^-1).