Regulating ORR/OER Performance Of M-N-C Single-atom Catalysts Based On Curvature Effect And Bimetallic Combination

There has been an upsurge in research towards the development of sustainable energy conversion and storage technologies,such as fuel cells and metal-air batteries,to address issues with energy and environment.The most widely used catalysts for oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）are noble metals,but their high cost and element scarcity prevent their widespread commercialization.Consequently,it is crucial to reduce the loading of noble metals and investigate abundant and inexpensive elements as highly active bifunctional ORR/OER catalysts.Single-atom catalysts（SACs）with metal-nitrogen-carbon（M-N-C）coordination have recently captured the interest of scientists due to their high activity,100%metal usage,and well-defined model.A similar coordination environment also underlies the intrinsic electronic structure of the active sites;this makes it difficult to break the linear relationship between the catalytic activity and the adsorption energies of reaction intermediates in multi-step reactions,thus limiting the possibility of improving their activities.In this thesis,utilizing density functional theory（DFT）calculations and experimental verification,two feasible methods,curvature effect and bimetallic combination,to optimize ORR/OER activities of M-N-C as well as the source of the activity enhancementare are investigated.The main finding are as follows:Understanding the effect of surface curvature on ORR and OER activities and the underlying connection between the structural information of electrocatalysts and their activities is critical for the regulation of several essential electrochemical processes.DFT results indicate that Fe N₄and Co N₄catalysts have the excellent ORR activity and the activity may be further increased by creating a specific curvature K.Co N₄and Ni N₄have the good OER activity.Co N₄exhibits good bifunctional activity with a suitable K.Studying on the enhancement mechanism reveals that the curved structure can weaken the interaction between the metal atom and the substrate and alter the electronic structure of the metal atom,resulting in the upshifted d-band center and the increased adsorption strength of intermediates,which modifies the OER and ORR activities further.Moreover,a descriptor C=N_d×（χ_M+4χ_N）/χ_O×（1-0.4K）is developed to evaluate the bifunctional activities of MN₄catalysts based on their directly obtained parameters,such as the surface curvature,the number of d electrons of the metal element,and the electronegativity of the metal atom and its coordination atoms.A catalyst with the C between 22.5 and 27.6 is projected to be the candidate for the bifunctional ORR and OER.The descriptor reasonably links the structure of catalysts to their catalytic performance,enabling the rapid development of high-performance catalysts.A strategy for optimizing ORR/OER activities of M-N-C SACs is developed by the introduction of additional metal single-atoms to produce bimetallic sites.DFT simulations suggest that among the three metal sites（Fe,Co,and Ni）,Fe and Co are typically the most active for ORR and OER,respectively.The combination of Co and Fe to generate CoFe bimetallic sites not only increases Fe’s ORR and Co’s OER activities,but also greatly enhances Co’s ORR and Fe’s OER activities.In contrast to SACs,the synergistic effect of bimetallic sites is a result of the altered charge density of active sites brought about by the addition of another metal single-atom,thereby altering the adsorption energies of intermediates and enhancing the catalytic activities.Based on these theoretically revealed guidelines,CoFe-N-C is synthesized by a two-step pyrolysis process and demonstrates remarkable ORR(E_1/2=0.90 V)and OER(E_j=10=360 m V)activities in alkaline media,exceeding Fe-N-C and Co-N-C SACs as well as commercial Pt/C-Ir O₂.CoFe-N-C also outperforms Pt/C-Ir O₂with a high open-circuit voltage（1.49 V）,energy density(917.4 Wh/kg _Zn),power density(142.1 m W cm^-2),rate capability and cycling stability in rechargeable Zn-air batteries.Based on the preceding study,a series of Ru M-N-C catalysts with bimetallic sites are construted to further investigate the cause of the activity enhancement and the synergistic influence of bimetallic sites on the reaction mechanism.DFT results suggest that combing 3d transition metals from the left to the right with Ru modifies the charge density and downshifts d-band center of metal sites,resulting in weaker adsorption energies of reaction intermediates and altered catalytic activity.Meanwhile,the introduction of Ru to construct Ru Fe（Co）-N-C may greatly enhance the bifunctional ORR/OER activities of Fe（Co）sites,surpassing Fe-N-C and Co-N-C.Furthermore,the presence of Fe（Co）can enhance the bifunctional ORR/OER activities of Ru sites in comparison to Ru-N-C.Utilizing a bimetal MOF as precursor,Ru Fe-N-C is synthesized in accordance with these theoretical guidelines.Ru Fe-N-C exhibits outstanding ORR(E_1/2=0.92 V)and OER(E_j=10=320 m V)activities in alkaline media with a smallΔE of0.63 V,outperforming Fe-N-C,Ru-N-C,and even commercial Pt/C-Ru O₂.Ru Fe-N-C also exhibits a high open-circuit voltage（1.52 V）,energy density(916.1 Wh/kg _Zn),power density(139.9 m W cm^-2),rate capability and cycling stability in rechargeable Zn-air batteries.Our results offer a comprehensive evaluation of bimetallic sites based on correlations between electronic structure and overall catalytic activity,as well as a novel strategy for developing high-performance bimetal catalysts with synergistic effect.