Coordination Structure Optimization Of Atomic Dispersed Metal Sites For CO₂ Hydrogenation Electrocatalysts

The increase in global fossil fuel consumption has led to a sharp increase in the concentration of carbon dioxide（CO₂）in the atmosphere.The electrocatalytic CO₂ reduction reaction（CO₂RR）coupled with electricity powered by renewable energy presents a clean and sustainable way to mitigate the greenhouse effects and yield value-added feedstock.However,one of the keys to this technology depends on the development of high-efficient and low-cost electrocatalyts to confront the challenges of the thermodynamic stability of CO₂,large mass transfer resistance,sluggish reaction kinetics and poor product selectivity.In this work,we prepared single-atom catalysts（SACs）and diatomic catalysts（DACs）with non-noble metal centers,the well-defined active sites of SACs provide an ideal platform to probe the structure-activity relationships.The electronic effects induced from the coordination beween metal atoms and C/N atoms and from the heterogeneous neighboring metal sites were systematically investigated through modulating the coordination environment of metal centers,and thus effectively optimize the reaction pathways of CO₂RR and promote the CO₂-to-CO conversion.This work offers new insight in designing outstanding electrocatalysts from an atomic level for CO₂RR and the main contents are as follows:The sublimation-cavity trap strategy is proposed to synthesize single-atom Fe catalysts.As the sublimated ferrocene captured by the surface cavities of ZIF-8,the atomic dispersed high-efficient iron sites are favorably anchored on the outer layers of carbon matrix,which shortens the mass transfer pathway during CO₂ electroreduction.Verified by XAFS and XANES fitting results,the isolated iron sites form the coordination of Fe N₅ moities.Electrochemical measurements show that Fe N₅ sites achieve CO Faradaic efficiency(FE_CO)of over 90%at the potential range of-0.4～-1.0 V vs.RHE,presenting superior catalytic performance compared with Fe nanoparticles（Fe-NP/C,less than 1%）and porous N-doped carbon（83%）.DFT calculations further provide synergistic mechanism between pyridine N and Fe N₄ in Fe N₅ sites for catalyzing CO₂RR.Compared with Fe N₄ moieties,Fe N₅ sites favorably enhance the thermodynamic stability,down-shift the d-band center of Fe atoms,thus facilitating*CO desorption.Based on the effect of the coordination environment of metal centers on the catalytic behavior in the previous chapter,a series of Co SACs(Co-N_5-xC_x,x=1,2,3)were synthesized with different ratios of hybrid Co-N and Co-C coordination.The difference in electronegativity of the coordinated N and C atoms is applied to adjust the electron density of central Co atom,which reduces the energy barrier for*COOH formation and promotes intrinsic catalytic activities.As a result,the FE_CO boosted considerably from 54%to 76%and 92%at-0.8 V vs.RHE for Co-N₄C₁,Co-N₃C₂,and Co-N₂C₃,respectively,with the increase ratio of Co-C coordination.Accordingly,the CO partial current density increased from 4.2 m A cm^-2to 6.7 m A cm^-2 and 8.3 m A cm^-2.Also based on the intense p-d orbital interactions between central metal atoms and coordinated atoms,a series of Cu single-atom catalysts with tunable isolated Cu-N_x（x=3,4）sites were proposed.Benefitting from the electron withdraw effect of N atioms,the chemical state of Cu was effectively enhanced,which featured by balancing the binding strength of*COOH and*CO with the optimized coordination environment of Cu-N₄ sites.Therefore,overcome the drawbacks of wide products distribution of Cu nanoparticles and the generally low catalytic activity of Cu SACs.In this case,the FE_CO and CO partial current desity of Cu-N₄ achieves 97%and-4.8 m A cm^-2 at-0.8 V vs.RHE,respectively,superior to that of the reported Cu single-atom catalysits for CO₂-to-CO conversion.In contrast,the FE_CO and CO partial current desity of Cu-N₃ only reach 21%and-0.1 m A cm^-2.To further boost the intrinsic catalytic activities of monodispersed metal sites,we proposed the ZIF-8 in-situ Cu²⁺doping coupled with Ni（acac）₂ cavity confinement strategy.Through introducing isolated Ni sites in the adjacent of Cu sites with similar atomic size,thereby accurately construct diatomic Ni/Cu catalysts,which mitigates the limitation of structural simplicity of SACs toward the complexity multi-step CO₂RR.XAFS results demonstrated that diatomic Ni/Cu catalysts present the coordination structure of both Ni N₄and Cu N₄ sites,which narrow the bandgap between HOMO and LUMO orbitals and endow Ni/Cu-N-C with enhanced electron conductivity.Besides,the stronger adsorption of*COOH intermediates lowers the overall reaction barriers,promotes the intrinsic catalytic activity and ability to suppress hydrogen evolution reaction（HER）by the synergisitic effect of Ni/Cu dual sites.At-0.79 V vs.RHE,the FE_CO of Ni/Cu diatomic catalyst reaches the maximum value of 99.2%with CO partical current density of 29.9 m A cm^-2.Its CO turnover frequency（TOF）achieves 8778 h^-1（-1.09 V vs.RHE）,which is 1.45 and 3.51 times than that of Ni-N-C and Cu-N-C,respectively.Herein,by constructing uniform catalytic sites,the coordination environment of the metal active center was adjusted from an atomic level.Meanwhile,the influence of the metal local structure on charge density and d-band center was systematically investigated,which reveals the chemical adsorption properties of the reaction intermediates towards catalytic performances.In this case,it provides a new approach to design highly active CO₂electroreduction catalysts at the atomic/molecular level.