Preparation Of Transition Metal Single Atom Catalysts For Electrocatalytic CO₂ Reduction

Electrocatalytic CO₂reduction reaction involves complicated mechanisms due to multiple electron-proton transfer processes,resulting in various intermediates.It is still difficult to achieve high selectivity for a specific reduction product,due to the competing reaction of hydrogen reduction.It is thus of urgent necessity to develop highly active electrocatalysts so as to deal with these challenges.Single atom catalysts have been widely studied as promising electrocatalysts for CO₂reduction reaction,which is due to their 100%atomic utilization efficiency and highly active sites.In addition,the catalytic performance is closely related to the coordination configuration of metal atoms and the structure of supports in single atom catalysts.In this paper,a series of single atom catalysts have been synthesized to elevate the CO₂reduction reaction performance and explore catalytic mechanism,by developing an approach of accurately control the coordination configuration of metal atoms,optimizing the structure of support loaded single atom sites and co-introducing single atom sites and cluster sites on the support.The details are as follows:1.A general strategy was developed to regulate the coordination configurations of metal atoms in single atom catalysts precisely.The vacancy-defect Ni-N₃-V SAC was successfully obtained and its catalytic performances in CO₂reduction reaction were assessed.Specifically,a N/O mixing coordinated Ni-N₃O was synthesized through high-temperature pyrolysis method,by using Ni Cl₂and organic ligands containing nitrogen and oxygen as precursors.Ni-N₃-V SAC was obtained by further pyrolysis of Ni-N₃O due to the difference in the coordinating ability between nitrogen,oxygen atoms with nickel atoms.X-ray absorption structure spectroscopy and HAADF-STEM images confirm that the Ni atoms with vacancy-defect Ni-N₃are atomically dispersed on the support.The theoretical calculations reveal that the Ni-N₃-V has the optimal free energy for the activation of CO₂and desorption of CO,compared with Ni-N₃and Ni-N₄.In the H-type electrolyzer,the Ni-N₃-V SAC shows outstanding selectivity for CO at high current density,achieving high current density(65 m A cm^-2),as well as a record turnover frequency of 1.35×10⁵h^-1at-0.9 V（vs.RHE）.The mass activities of Ni-N₃-V are four times than those of Ni-N₄.2.In acidic medium,the catalytic performance of single atom catalysts for CO₂reduction was improved by decorating pyrenyl graphdiyne on the support.Co Pc@G/CNT and Co Pc@CNT were synthesized throughπ-πinteractions between Co Pc and carbon nanotubes（CNT）with or without decorating pyrenyl graphdiyne（G）on,respectively.X-ray absorption structure spectroscopy and HAADF-STEM images confirm that the Co atoms are atomically dispersed on the support.In the acidic medium containing different alkali cations,the catalytic performance of Co Pc@CNT increasing in the following order:Li⁺<Na⁺<K⁺<Cs⁺,shows an obvious cation effect for electrocatalytic CO₂-to-CO.However,the catalytic performance of Co Pc@G/CNT is no longer related to the type of cations.In-situ Raman spectra suggest that there is specific adsorption between the alkyne bond and alkali metal cations due to the red shift of alkyne bond under cathode potential.ICP test confirms that the adsorption capacity to alkali metal cations of Co Pc@G/CNT is more than three times that of Co Pc@CNT.Co Pc@G/CNT shows outstanding CO partial current density of 164 m A cm^-2,which is obviously better than Co Pc@CNT(127 m A cm^-2).This work opens a new window for design and optimization of microenvironment for electrocatalytic CO₂reduction in acidic medium.3.We have successfully constructed a bifunctional catalyst including single atoms and clusters for electrocatalytic reduction of CO₂.Co Pc-Cu@G/CNT was synthesized by introducing Co atoms and Cu clusters on CNT modified by pyrenyl graphdiyne（G）.In the H-type electrolyzer,the Co Pc-Cu@G/CNT shows outstanding electrocatalytic performance for CO₂reduction,achieving high faradaic efficiency over 90%across a wide potential range from-0.65 to-1.15 V,and up to 98%at-1.05 V（vs.RHE）.In the gas diffusion cell,CO partial current density for Co Pc-Cu@G/CNT is 560 m A cm^-2,about 100 m A cm^-2more than Co Pc@CNT.Experimental and theoretical calculations reveal that Co atoms is the active sites for CO₂reduction.The Cu clusters could promote the formation of COOH^*by catalyzing water dissociation.This work has guidance significance for designing efficient synergistic catalysts for electrocatalytic CO₂ reduction.