Computational Study On The Electrocatalytic Carbon Dioxide Reduction Reaction Of Single-atom Materials With Bimetallic Centers

With the implementation of the strategic goal of"emission peak and carbon neutrality",the development of electrochemical carbon dioxide reduction technology powered by renewable electricity is the key to solving energy and environmental problems and promoting social sustainable development.However,the strong C=O double bond makes CO₂ molecules inert.Therefore,designing high-efficiency electrocatalysts for CO₂ reduction reaction（CO₂RR）is a major challenge to achieve the goal of“two-carbon”in China.Among the currently reported CO₂RR electrocatalysts,single-atom catalysts with bimetallic centers have aroused the considerable attention of surface science researchers by their high metal utilization and enhanced catalytic performance.However,to further realize the reasonable design of bimetallic catalysts with appropriate electronic properties and obtain specific reduction products with high selectivity,the following problems still exist:（1）the reaction mechanism and structure-function relationship of CO₂ reduction catalyzed by bimetallic catalysts are lacking;（2）the influence of coordination environment and bimetallic species of the active site of bimetallic catalysts on the catalytic performance is unclear.Therefore,in this work,two substrates,graphene and graphitic carbon nitride,are selected to anchor the single atom of the bimetallic center.Through density functional theory（DFT）,the mechanisms of electrocatalytic CO₂ reduction of a series of single-atom-nitrogen-carbon materials with bimetallic centers and bimetallic center single-atom materials anchored on graphitic carbon nitride were systematically studied,the structure-function relationship between the atomic-structures of the active site（including coordination environment and bimetallic species）and electrocatalytic CO₂ reduction was explored.The specific research contents are as follows:1.Computational study on the electrocatalytic CO₂ reduction reaction of single-atom-nitrogen-carbon materials with bimetallic centers:the CO₂RR catalytic performance of dual-metal-and nitrogen-doped graphene catalysts（M₁/M₂-N₆-Gra,Model 1,Model 2,and Model 3）with different bimetals and different coordination environments at active sites（the coordination number of metal and nitrogen ranges from 2 to 4）has been systematically studied through DFT.The calculated formation energies indicate that high-coordinated catalysts are thermodynamically stable.By calculating the Gibbs free energy changes of the CO₂RR reaction pathway and the selectivity of hydrogen evolution reaction（HER）,we determine that the coordination environment of the active sites of the catalysts strongly affects the activity and selectivity of CO₂ reduction to CO.The low-coordinated catalysts have strong*COOH and*CO adsorption capacity,and CO is difficult to desorb from the catalyst surface,resulting in the poor catalytic activity of CO₂RR;the high-coordinated catalysts have moderate*COOH and*CO adsorption capacity,and have excellent catalytic activity for reducing CO₂ to CO.In addition,the CO₂RR reduction activity of most heteronuclear M₁/M₂-N₆-Gra has been enhanced due to the synergy between heterometallic metals,and the synergy is effectively improved with the increase of the coordination number.In particular,using the limiting potential difference of CO₂RR and HER as the selective descriptor,we have screened out four catalysts,including Co/Zn,Fe/Zn,Mn/Zn,and Fe/Mn-N₆-Gra-Model 3（the coordination number of metal and nitrogen is 4）,and they have low limiting potentials of-0.20,-0.24,-0.26,and-0.26 V,respectively,which are much lower than those of Cu（211）surface（-0.46 V）.This work provides theoretical insights for the rational design of efficient dual-metal-nitrogen-carbon materials for CO₂RR.2.Computational study on electrocatalytic CO₂ reduction reaction of bimetallic-centered single-atom materials anchored on graphitic carbon nitride:the activity and selectivity of nine M₁M₂-C₃N₄ catalysts for electrocatalytic CO₂RR to C₁ and C₂ products were investigated by DFT method.By calculating the energy barriers of C-C coupling processes and the key steps that determine the C₁,C₂ product selectivity of CO₂RR,we determine that the bimetallic center is capable of C-C coupling,and clarified the catalysts（Cu Cu,Cu Ni,and Ni Mn-C₃N₄）coupled via*CO+*CO have C₂ product selectivity,and catalysts（Cu Mn,Cu Fe,Cu Co,Ni Fe,Ni Co,and Ni Ni-C₃N₄）coupled via*CHO/*COH+*CO have C₁ product selectivity.Detailed C₁ reaction pathway studies on catalysts with C₁ product selectivity show that Cu Mn-C₃N₄exhibits CH₃OH selectivity,Cu Fe-C₃N₄ has HCOOH selectivity,and Cu Co,Ni Fe,Ni Co,and Ni Ni-C₃N₄ have CH₄ selectivity.In particular,based on the previously reported CO₂RR and HER limiting potentials of Cu-C₃N₄ and Cu（111）,we screen out five promising bimetallic systems for CO₂RR,namely,Cu Cu-C₃N₄（-0.45 V）and Ni Mn-C₃N₄（-0.66 V）are the most promising electrocatalysts for the production of C₂ products,Cu Mn-C₃N₄（-0.63 V）can be used for the production of CH₃OH,Ni Ni-C₃N₄（-0.50 V）and Ni Fe-C₃N₄（-0.56 V）can be used to produce CH₄,and these catalysts have high thermal stability at 500 K.This work provides a theoretical basis for the experimental study of CO₂RR catalyzed by M₁M₂-C₃N₄.