Mo-Bi-S@CNTs Composite Catalysts Electrocatalytic Reduction Of CO₂ To Methanol In Room Temperature Ionic Liquid Composite Electrolyte

Due to the extensive use of fossil energy for a long time,it has caused serious climate and environmental problems,resulting in obvious“greenhouse effect”.The chemical conversion of carbon dioxide to synthesize high value-added chemicals is generally considered to be an ideal way for the comprehensive utilization of carbon dioxide.Therefore,the capture and utilization of carbon dioxide have attracted great attention all over the world.According to the research results at home and abroad in recent years,the preparation of hydrocarbons and alcohols by electrocatalytic reduction of carbon dioxide is an important research direction.However,the reported research results are far from practical application,and there are many theoretical and technical problems to be solved.The main problems are the lack of electrocatalyst activity and selectivity,the lack of compatibility between catalyst and electrolyte,and the high reactor tank voltage.In order to make a breakthrough in this research field,it is necessary to select electrolytes with high cocatalysis based on the construction of high activity and high selectivity electrocatalysis materials,systematically consider the main influencing factors and interaction rules of electrocatalytic reduction of CO₂,and then optimize the electrochemical reduction of CO₂reactor system.In this thesis,chemically bonded molybdenum bismuth bimetallic nano sulfides（Mo-Bi-S）and their carbon nanotube supported nanocomposites（Mo-Bi-S/CNTs）were prepared by hydrothermal method.The relevant research results have important theoretical and practical values in the development of electrocatalytic materials,electrolytes and reactors.TEM,XRD,and XPS were used for characterization.The electrochemical properties of the electrocatalytic materials were investigated in the electrolytes composed of different ionic liquids such as[Emim]BF₄-water composite electrolyte.By comparing the experimental results and theoretical analysis,the main influencing factors and action rules of the electrocatalytic reduction of CO₂were revealed.The results are summarized as follows:1.Mo-Bi-S nanocomposites were prepared by hydrothermal method at different reaction temperatures.The samples were characterized by XRD,SEM,TEM and XPS.The results showed that the microstructure of the composites is lamellar structure,and the average particle size is about 20 nm.The test results of electrocatalytic reduction of CO₂in[Emim]BF₄+H₂O（40wt%）showed that the sample prepared at hydrothermal temperature of200℃possessed the best performance,the peak current density reaches 11.1 m A·cm^-2,and the main reduction reaction products are CH₃OH and CO.The reduction current density can basically remain stable after 6h potentiostatic electrochemical test and the Faraday efficiency of CH₃OH reaches 56.4%.2.Mo-Bi-S@CNTs composite materials with different carbon nanotube content and different atomic ratio of Bi and Mo were prepared by hydrothermal method at 200℃.The composite materials were characterized by XRD,TEM and other technologies.The prepared Mo-Bi-S@CNCTs nanoparticles showed layered structure with an average particle size of about 12 nm,and were uniformly anchored on the surface of carbon nanotubes.3.Taking Mo-Bi-S@CNTs as the working electrode catalyst and[Bmim][PF₆],[Bmim][BF₄],[C₄mpy][Tf₂N]and[Mmim][Me SO₄]+H₂O as the electrolytes,the electrochemical performance test results showed that the molecular structures of anions and cations of ionic liquids possessed a significant effect on the electrochemical reduction kinetics of CO₂.The order of significance of cocatalysis is:[BF₄]^->[Me SO₄]^->[Tf₂N]^->[PF₆]^-;The influence of cations is slightly weak,and the significance of cocatalysis is as follows:[Emim]⁺>[Bmim]⁺.Moreover,the addition of water molecules is conducive to the improvement of ionic liquid cocatalysis.The main reason is that the addition of water molecules makes the anion and cation pairs in ionic liquid easy to be separated,the volume of solvent ions increases,the free space increases,the electrostatic attraction between anion and cation decreases greatly,the viscosity and resistivity of electrolyte decrease,and the electrochemical reduction kinetic environment of CO₂improves.4.The composite electrolyte system was composed of[C₄NH₂mim][BF₄],[C₂NH₂mim][BF₄]and[Emim][BF₄]with H₂O and CH₃CN,Mo Bi-S@CNTs（15%）catalyst material was used to prepare working electrode,and the performance of electrocatalytic reduction of CO₂was tested by electrochemical method.The test results showed that:the polarization reduction peak current of potential linear scanning increases gradually with the increase of water content in[Emim][BF₄]+H₂O（x%）electrolyte.In the test of[Emim][BF₄]+H₂O（40%）electrolyte,the Faraday efficiency of CH₃OH increased first and then decreased in the range of-0.2 V～-0.6 V,with the highest value of 81.2%,and the Faraday efficiency of CO decreased gradually;In the test of[Emim][BF₄]+H₂O（40%）electrolyte,the addition of CH₃CN can improve the current density of potential linear scanning voltammetric reduction peak,improve the Faraday efficiency of CO,but the Faraday efficiency of CH₃OH was reduced relatively.5.In the tests of[C₂NH₂mim][BF₄]+H₂O（x%）and[C₄NH₂mim][BF₄]+H₂O（x%）electrolytes,the polarization reduction peak current of potential linear scanning gradually increases with the increase of water content,the Faraday efficiency of CH₃OH increased first and then decreased in the range of-0.2 V～-0.6 V and the Faraday efficiency of CO decreased gradually.The order of the effect of electrolyte on the promotion of CO₂electrochemical reduction rate is:[Emim][BF₄]+H₂O>[Emim][BF₄]+H₂O+CH₃CN>[C₄NH₂mim][BF₄]+H₂O>[C₂NH₂mim][BF₄]+H₂O;According to the significance of improving the selectivity of CH₃OH products:[Emim][BF₄]+H₂O>[C₄NH₂mim][BF₄]+H₂O≈[C₂NH₂mim][BF₄]+H₂O.Overall,[Emim][BF₄]+H₂O（40%）electrolyte is a cocatalyst with strong promotion of CH₃OH generation rate and selectivity.It is an excellent electrolyte formula.