Structure Modulation On Fe Single Atom Site Of Nitrogen-Doped Carbon Based Materials And Their Oxygen Reduction Properties

Electrochemical energy conversion and storage devices such as fuel cells and metal-air batteries are the key technologies to change from fossil fuels to renewable energy and have attracted considerable attention due to their high conversion rate,low pollution,and portability.However,presently,fuel cells and metal-air batteries are limited by the sluggish kinetics of the cathodic oxygen reduction reaction(ORR),which requires electrocatalysts to promote the electrode ORR reaction process.The noble metals platinum,ruthenium,and iridium are commonly used as electrocatalysts,such as Pt/C for ORR,and iridium and ruthenium for oxygen evolution reaction(OER).Howerer,the high cost of noble metal catalysts greatly limits their large-scale applications.Therefore,the development of low-cost,high-activity,and remarkablestability electrocatalysts is essential for efficient energy conversion.In recent years,transition metal nitrogen-doped carbon materials(M-N-C,M=Fe,Mn,Ni,Co,etc.)have achieved tremendous success in energy development as nonprecious metal ORR electrocatalysts due to their high activity,low cost,and abundant reserves.So far,SACs,which contain isolated metal/nitrogen coordinated active sites supported by nitrogen-doped carbon matrixes,show outstanding ORR performance on account of their fully exposed active sites and the maximum atom-utilization efficiency.Particularly,Fe-N-C materials have been regarded as one of the most promising candidates to replace Pt-based ORR catalysts.However,the activity and stability of such catalysts are still not as good as expected due to the weak adsorption strength of single metal to ORR intermediates.In this thesis,we reported two strategies(metal nanoparticle/single-atom coexistence method and heterometallic doping method)to tune the binding strength of Fe SA sites and ORR reaction intermediates in Fe-N-C materials,thereby narrowing the energy barriers of the rate-determining steps(RDS),improving the ORR performance of Fe-N-C materials.The specific work is as follows:(1)Metal nanoparticle/single-atom coexistence method:We designed a series of Fe-N-C materials(Fe/Meso-NC-T)with different SA active sites first by gaseous acidmediated pyrolysis then followed by a secondary heat-treatment at various temperatures(700-1000℃).As the temperature increases,the ORR activity of asprepared Fe-N-C materials improves.And,Fe/Meso-NC-1000 shows outstanding ORR performance with a half-wave potential of 0.885 V,remarkable durability,and excellent methanol tolerance in alkaline media,outperforming the commercial Pt/C catalyst.Xray absorption fine structure(XAFS)results reveal the significant role of coordinated atoms of SA and metallic Fe nanoparticles(NPs)in altering the electronic structure of isolated Fe-N-C sites.High-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM)and electron energy loss spectroscopy(EELS)results confirm the strong interaction between isolated Fe-N-C sites and adjacent Fe NPs,which can change the geometric structure of isolated Fe-N-C sites.From the experimental results and density functional theory(DFT)calculations,we can conclude that optimal regulation of the electronic and geometric structure of isolated Fe-N-C sites by the coexistence of Fe NPs narrows the energy barriers of the rate-determining steps of ORR,thereby resulting in the outstanding ORR performance.(2)Heterometallic doping method:A single-atom catalyst Fe/Mn-HNCS with bimetallic sites embedded in hollow nitrogen-doped carbon spheres(HNCS)was synthesized by tuning the adsorption of Fe2+and Mn2+precursors.Compared with the single-atom Fe/Fe-HNCS and Mn/Mn-HNCS catalysts,the ORR activity of the bimetallic single-atom Fe/Mn-HNCS catalyst is significantly improved.It exhibited outstanding ORR performance with a half-wave potential(0.895 V),remarkable durability,and excellent methanol tolerance in alkaline media,outperforming the commercial Pt/C catalyst.Furthermore,the Fe/Mn-HNCS material achieved a high power density of 140.3 mW cm-2 when used as a cathode catalyst for Zn-air batteries,outperforming most reported non-noble metal catalysts.The doping of the heterometallic Mn optimized the adsorption strength of the active sites for ORR reaction intermediates,thereby narrowing the reaction energy barrier of the ORR ratedetermining step,improving the ORR performance of Fe-N-C materials.