Controllable Synthesis Of Atomically Dispersed Metal Catalysts For Electrocatalytic CO₂ Reduction

China has put forward the goal of peaking carbon dioxide emissions by 2030 and striving to achieve carbon neutrality by 2060,the effective capture and transformation of CO₂has become an urgent task.Electrocatalytic CO₂ reduction to high-value-added products is an effective way to solve this problem.However,as the CO₂ molecule is very stable,and the reduction process is accompanied with competing hydrogen evolution reaction,the design of efficient and stable catalysts has become the most critical topic in electrocatalytic CO₂reduction.Atomically dispersed metal catalysts,such as single-atom catalysts,have received extensive attention in recent years by virtue of their well-defined structure,100%atom utilization,and high catalytic activity.Researchers have adjusted the metal centers and coordination structures of single-atom catalysts to achieve higher activity,and a series of progress has been achieved.Nevertheless,the CO₂reduction reaction involves multiple steps and multiple electron transfer processes,requiring two or more metal sites as catalytic centers for synergic catalytic.Dual-atom catalysts feature adjacent metal sites as active centers,so that the adjacent sites of the catalyst can cooperate while maintaining atomic dispersion and stability,so as to improve the catalytic activity.Although dual-atom catalysts are still in their infancy thus far,it is still reasonable to believe that they will be a very important class of catalysts.In this paper,a series of atomically dispersed metal catalysts have been synthesized with different structures for electrocatalytic CO₂ reduction.With the results obtained from theoretical calculations,the structure–activity relationship of the catalysts was investigated.The main contents of this paper are as follows:1.By regulating the metal centers of molecular catalysts and loading them onto conductive carbon nanotubes throughπ–πinteractions,four single-atom catalysts with well-defined structures were synthesized（namely,Co（qpy）/CNTs,Fe（qpy）/CNTs,Ni（qpy）/CNTs,and Cu（qpy）/CNTs）for electrocatalytic CO₂ reduction,to study the effect of different metal centers on the electrocatalytic CO₂ reduction.The experimental results show that Co（qpy）/CNTs exhibits the highest electrocatalytic activity for CO₂ reduction compared with other M（qpy）/CNTs.The Co（qpy）/CNTs can efficiently reduce CO₂ into CO over a wide potential range with the Faraday efficiency reaching 98%and with excellent stability.Theoretical calculations show that the metal center has a strong interaction with the intermediate*COOH,which lowers the free energy change for the formation of*COOH,thereby enhancing the catalytic activity.2.By using the precursor preselection method,a pyrrolic-type nitrogen-coordinated dual-atom Ni catalyst Ni₂/N-CNTs was synthesized,with a dinuclear Ni complex and carbon nanotubes as the precursor.For comparision,the pyrrolic and pyridinic N coordinate single atom catalysts Ni/N-CNTs and Ni/N-CNTs-1000 were also synthesized,and the catalyst was tested for CO₂ reduction.The experimental results show that pyrrolic N coordinate dual-atom Ni catalyst Ni₂/N-CNTs show a higher activity for CO₂ reduction than pyrrolic and pyridinic N coordinate single-atom counterpart Ni/N-CNTs and Ni/N-CNTs-1000.The maximum j _COand TOF_CO are 2 and 2.7 times higher than that of single-atom Ni/N-CNTs catalyst,respectively.Combined with DFT calculations,it is shown that the*CO-absorbed catalyst Ni₂/N-CNTs-*CO optimizes the free energy of*COOH formation and CO desorption during CO₂ reduction due to the synergistic between diatomic Ni₂ sites,thus enhances the activity of Ni₂/N-CNTs for CO₂ reduction.3.Using nickel phthalocyanine and iron phthalocyanine as metal sources,a bimetallic catalyst Ni Fe/N-CNTs supported on nitrogen-doped carbon nanotubes was prepared by co-pyrolysis with dicyandiamide.Compared with single-atom catalysts Ni/N-CNTs and Fe/N-CNTs,Ni Fe/N-CNTs exhibits lower overpotentials and higher activity for CO₂RR.The experimental result showed that the Ni Fe/N-CNTs can efficiently reduce CO₂ into CO with a Faradaic efficiency higher than 90%over a wide potential range of-0.5 V to-1.2 V（vs.RHE）,reaching the maximum j _CO of 88.9 m A cm^-2,1.8 and 4.9 times higher than that of Ni/N-CNTs and Fe/N-CNTs,respectively.Experiments and theoretical calculations show that the free energy of*COOH formation on Ni Fe/N-CNTs is significantly reduced,which further enhanced the activity of CO₂ reduction.