Study On Electrocatalytic Carbon Dioxide Reduction Of Metal Phthalocyanine/defective Graphene Material

At present,with the deepening of human dependence on fossil energy in the production process,excessive carbon dioxide emissions have been caused.According to reports,in recent decades,the amount of carbon dioxide stored in nature has a significant positive correlation with global warming,which seriously affects natural climate change and the world’s ecological balance,and threatens the long-term sustainable development of human society.In recent years,new energy sources such as electric energy,hydropower,wind energy,solar energy and biomass energy have been continuously explored by human beings.However,these new energy sources are limited by various external conditions in the application process.In addition,some new energy sources also have technical defects,so they cannot completely replace fossil energy at present.Therefore,capturing carbon dioxide to achieve its direct or indirect catalytic transformation,and then using it as part of the circular economy to achieve the goal of reducing dependence on fossil energy and carbon emission is one of the effective strategies at present.Among them,the electrocatalytic reduction of carbon dioxide（CO₂RR）to value-added products is considered as a very promising method to curb greenhouse effect because it can be carried out continuously and pollution-free.However,due to the inert chemistry of CO₂and the presence of competitive hydrogen evolution reaction（HER）,the CO₂RR process requires catalysts to lower the CO₂RR reaction energy barrier and avoid HER generation.Among a variety of catalytic materials,single-atom catalysts（SACs）,as one of the most studied CO₂RR electrocatalysts.Unfortunately,the preparation and characterization of SACs are often complex and difficult to control,which makes it difficult to fabricate and apply SACs materials on a large scale.Transition metal phthalocyanine（MPc）,with typical SACs active catalytic center metal-nitrogen site（M-N）,has been considered as one of the ideal CO₂RR electrocatalysts.However,MPc is prone to aggregate and conceal the active site and has poor conductivity.In addition,the low electron density of the valence d orbital,which is not filled with electrons,leads to the unsatisfactory performance of MPc electrocatalysis of CO₂RR.In this paper,by introducing a single layer of graphene（DG）with defects,the MPc/DG composite was prepared byπ-πstacking and coupling,and the catalytic performance of the material on CO₂RR was studied.The main work contents are as follows:（1）Preparation of formic acid from CO₂electrocatalyzed by Cu Pc/DG composite..Selective electrochemical conversion of CO₂to formic acid（HCOOH）is achieved by regulating the electronic structure of the active center of Cu-N through the defect site of DG.The Cu Pc/DG composite showed a Faraday efficiency（FE）of44.6%in the reaction.According to the density functional theory（DFT）differential charge calculation results,the defects in DG can effectively promote the charge redistribution of dispersed Cu Pc,and the electrons are transferred from the defect sites to Cu Pc,forming an electron-rich environment around the Cu sites,making the center of the Cu d-band close to the Fermi level.By enhancing the adsorption of HCOOH intermediate*OCHO and reducing the reaction energy barrier of CO₂to HCOOH,the Cu Pc/DG composite exhibits excellent HCOOH selectivity and stability.（2）Preparation of carbon monoxide from CO₂electrocatalyzed by Co（Fe）Pc/DG composite.In CO₂RR,there is a close relationship between the catalytic activity of MPc with different metal centers and the redox potential,and low-potential metal ions such as Fe should be selected to improve the catalytic activity of the phthalocyanine complex.However,under the working conditions of electrocatalysis,Fe Pc is less stable than Co Pc.Due to the moderate binding energy of CO-N₄site to the reaction intermediate*CO,Co Pc exhibits better carbon monoxide（CO）formation activity than other MPc,which is a necessary process for electrocatalysis of CO₂RR.Therefore,based on the optimization of the electron cloud density of phthalocyanine metal center and the catalytic advantages of Co Pc and Fe Pc.Among them,Co Pc significantly enhanced the adsorption of*CO,Fe Pc decreased the reaction overpotential of CO₂RR,DG optimized the charge redistribution of the active sites of two phthalocyanine metals,and finally the Co（Fe）Pc/DG composite showed96.2%FE in the CO₂RR-CO reaction,its catalytic activity was significantly better than that of Co Pc(FE_CO=89%),Fe Pc(FE_CO=49%)monomer catalyst.