Synthesis Of Dual Metal Atom Dispersed Catalysts For Oxygen Reduction Reaction

Owing to single atomic dispersed catalysts（SACs）have high atomic utilization rate,which exhibit excellent oxygen reduction reaction（ORR）activity to replace precious metal Pt catalysts.Recently,bimetallic single atomic dispersed catalysts（DACs）as an extension kind of SACs have attracted extensive attention.Compared with SACs,DACs can expose much more active sites and further improve the ORR electrocatalytic activity.In addition,the coordinated structures of two metal atoms can effectively adjust the d-band center,so as to optimize the adsorption and activation energy of the active center of DACs for reactants and intermediates.However,DACs is still faced with many problems,such as difficult to achieve the controllable synthesis and insufficient understanding of catalytic mechanism.ZIF-8 has the advantages of large specific surface area,uniform morphology and controllable coordination with other metals.After high temperature thermal decomposition of ZIF-8,which remain a portion of Zn single atoms and combined with other metals to prepare different DACs.In view of above,this paper is used ZIF-8 as the precursors to develop an effective strategy for controllable synthesis of various DACs,which has excellent ORR activity and stability.Operando X-ray absorption spectroscopy（XAS）and density functional theory（DFT）are used to elucidate the synergistic effect and ORR mechanism of DACs.The main research contents of this paper are as follows:Firstly,predict the structural stability of Fe–N_xC_y（x=0～4,y=4–x）by using DFT calculations,and then synthesize the asymmetric structure of Fe/Zn–NC–950 catalysts by controlling the pyrolytic temperature of Fe-doped ZIF-8 precursor.XAFS analyses shows Fe–N₃C and Zn–N₄ coordination structures exist in this catalyst.The Fe–N₃C/Zn–N₄ catalyst shows a boosted ORR activity under alkaline conditions,with a half-wave potential(E_1/2)of 0.91 V and superior long-term stability（no degradation after 10000 CV cycles）,outperforming Pt/C and state-of-the-art noble metal-free electrocatalysts.The catalytic mechanism of the asymmetric Fe–N₃C for ORR was investigated by DFT calculations.It is indicated that the Fe valence in Fe–N₃C is higher than that of the symmetric Fe–N₄,which can enhance the electron delocalization.In addition,the residual Zn atom derived from ZIF-8 gives its d-orbital electrons to the Fe atom,and the synergistic interaction between Fe–N₃C and Zn–N₄ leads to enhance adsorption-activation of oxygen-containing intermediates and increases the ORR activity.Developing Cr/Zn–NC bimetallic catalyst with metal sites containing Cr is anchored in the carbon matrix derived from ZIF-8 precursor.The prepared Cr/Zn–NC catalyst shows excellent electrocatalytic ORR activity with an initial potential of 0.95 V and half-wave potential of 0.82 V,no degradation can be observed after 20000 CV cycles.It also shows excellent methanol resistance.This work opens a way for the rational design of DACs catalysts with customized electronic structure and uniform geometric configurations.A bimetal single atom dispersed catalyst composed of Cu–N₄ and Zn–N₄ on the N-doped carbon support（Cu/Zn–NC）.It exhibits high-efficient ORR activity with an onset potential of 0.98 V and a half-wave potential of 0.83 V,excellent stability（no degradation after 10000 cycles）and superior tolerance to methanol crossover,surpassing the state-of-the-art Pt/C and great mass of previously reported Pt-free catalysts.DFT indicates that the presence of Zn atom can regulate the d orbital electron distribution of Cu atom,which benefits to the stretch and cleavage of O–O on Cu active center,decreasing the rate determining step energy of OOH*formation.Operando XAFS further demonstrates that the Cu–N₄ active center experiences the transform from atomic dispersion to Cu–N₂ clusters during ORR process,while the Zn–N₄ consistently keep the initial state.Manganese（Mn）has the advantages of multiple valence states and low price.In this work,we design and synthesis of ZIF-8 as a template,synchronous introduction of Mn atoms and further high temperature calcination to obtain carbon-based DACs of Mn-Zn-NC.XAFS indicates that the catalyst has the coordination structures of Mn–N₄ and Zn–N₄.The Mn–Zn–NC catalyst displays high onset potential of 0.99 V and half-wave potential of 0.85 V for ORR in alkaline condition.The dynamic evolution from Mn–N₄ to Mn–N₃C and Mn–N₂C₂ during ORR reaction was determined by operando XAFS and DFT calculations.Driven by an applied electric field,this process occurs simultaneously with the reduction of Mn⁴⁺to Mn³⁺.The oxidation state of Mn decreases from 3.88 to 3.02 with the progress of the reaction,indicating that Mn–N₄ is the real active site for ORR.DFT calculations further prove that the formation of O* step for Mn–N₃C and Mn–N₂C₂ are enhanced in ORR process.At the same time,the synergy between Zn and Mn atoms can adjust the catalytic performance of the intermediate adsorption reaction free energy to oxygen.Due to the accurate optimization of the valence state and d-band center of the oxygen-containing intermediate,an appropriate adsorption energy was obtained.