Design Of Single/Dual-Atom Sites Embeded In Nitrogen-Doped Carbon Materials For Oxygen Electrocatalysts

Nitrogen-doped carbon-loaded transition metal atomic-level catalysts（M-N-C,M being the transition metal）have been the subject of much research in recent years in the field of catalysts for electrocatalytic reactions[including Oxygen Reduction Reaction（ORR）and Oxygen Evolution Reaction（OER）]at oxygen electrodes because of their well-defined active sites and high catalytic potential.Generally speaking,the active center of M-N-C type catalysts is MN_x（x=1～4）,and its composition and geometric structure play a decisive role in the activity of the catalyst.In this paper,based on density functional theory calculations,the coordination element types,quantity control and axial functional group modification of homonuclear dual-atom Zn/Zn-N-C and single-atom Co-N-C catalysts were carried out,and the change trend of catalytic activity and the rule of product selectivity were studied.The main research content and results are summarized as follows:（1）The homonuclear Zn₂N_x（x=1～6）active center was designed and the 4e^-ORR/OER catalytic activity was predicted.The calculation results show that the active centers of Zn₂N₃（I）,Zn₂N₃（III）,Zn₂N₄（I）,Zn₂N₄（III）,Zn₂N₄（VI）,Zn₂N₅（I）,and Zn₂N₅（II）have excellent ORR catalytic activity and theoretical overpotential（η）,The sequence are0.41 V,0.37 V,0.42 V,0.41 V,0.44 V,0.38 V,and 0.43 V,which are lower than commercial Pt/C;Further considering the effects of hydroxyl modification and protonation processes,it was found that Zn₂N₆（*OH）（η=0.39 V）.The catalytic performance of the active center was significantly improved.However,as the adsorbed hydroxyl group increases,the metal zinc falls off from the carrier,and the demetallization process occurs.On the other hand,the active center of Zn₂N₄（I）（*H）has dual functional catalytic activity of ORR and OER,with overpotentials of 0.39 V and 0.21 V,respectively;Finally,the influence of the co-coordination environment between heteroatoms with different electronegativity and nitrogen on catalytic activity was studied.Research found that Zn₂P₂N₄（I）（η=0.38 V）active center has excellent ORR activity,while Zn₂S₁N₅（I）,Zn₂S₁N₅（II）,and Zn₂S₃N₃（I）have excellent OER catalytic activity with theoretical overpotentials of 0.37 V,0.40 V,and 0.43V,respectively.（2）The effects of axial ligand modification on the catalytic activity of Co N₄ active centre and the selectivity of oxygen reduction products were studied.According to theoretical calculations,axial ligand（*X）modification can improve the ORR activity of Co N₄ active centers,and 16 active centers with excellent 4e^-ORR activity have been selected,including Co N₄（*HSO₄）（η=0.42 V）,Co N₄（*CH₃COO）（η=0.41 V）,Co N₄（*SCN）（η=0.42 V）,Co N₄（*py）（η=0.39 V）,Co N₄（*SH）（η=0.33 V）,Co N₄（*CIO₂）（η=0.25 V）,Co N₄（*CIO）（η=0.22 V）,Co N₄（*OH）（η=0.23 V）,Co N₄（*NH₂）（η=0.27 V）,Co N₄（*O）（η=0.30 V）,Co N₄（*CI）（η=0.35 V）,Co N₄（*OCH₃）（η=0.37 V）,Co N₄（*HCO₃）（η=0.35 V）、Co N₄（*H₃PO₂）（η=0.38 V）,Co N₄（*COOH）（η=0.39 V）,Co N₄（*Br）（η=0.41 V）,theoretical overpotentials have a lower than commercial Pt/C.By comparing the adsorption energies of*OOH and*HOOH intermediates,the effect of ligand modification on the selectivity of 4e^-and 2e^-ORR products was further discussed.Four active centers with excellent 4e^-ORR catalytic performance were calculated and selected,namely Co N₄（*OH）,Co N₄（*py）,Co N₄（*HSO₄）,and Co N₄（*CH₃COO）.More importantly,the Co N₄（*OH）active center has excellent 4e^-ORR/OER bifunctional catalytic activity,with overpotentials of 0.23 V and 0.34 V,respectively.