Research On The Systems Of Electrode And Electrolyte Solution For Electrochemical Reducing CO₂ To Prepare CH₃OH

In the last century,the massive consumption of fossil fuels increased CO₂emissions,A large amount of CO₂enrichment in the air has caused"greenhouse effect"and the emergence of natural disasters such as glacier melting.In order to alleviate these adverse effects,CO₂is needed to be transformed into fuels or chemicals to realize the transformation of energy from today’s"fossil fuel economy"to sustainable"carbon dioxide economy".Electrochemical reduction of CO₂provides a cleaner,more sustainable and cost-effective way for fuels and chemicals.In the early stage of our research group,CH₄was electrochemically oxidized to CH₃OH.In order to cooperate with the oxidation of CH₄and the reduction of CO₂to prepare CH₃OH in the same electrolytic cell to effectively use electrical energy and to improve CH₃OH product yield,C-Cu₂O composite electrode,Cu₂O thin film electrode,HBr modified Sn electrode and HCl modified brass electrode were prepared as cathode working electrode in this thesis.The morphological characteristics of each electrode was observed by the scanning electron microscope（SEM）,the composition of elements on the surface of each electrode was analyzed by the energy dispersive spectrometer（EDS）,and the surface characteristics of the electrode was analyzed by X-ray diffraction（XRD）.It showed that that the preparation of the electrode was successful.Based on the C₂H₅OH solution containing KOH as electrolyte,the effects of C₂H₅OH concentration,KOH concentration,CO₂flow rate in electrolyte and electrolytic potential to each electrode on the preparation of CH₃OH by CO₂reduction were explored by the orthogonal experiments and range analysis.The optimal process conditions corresponding to each electrode were determined.（1）C-Cu₂O composite electrode.The oxide Cu₂O of Cu has two characteristics:intermediate hydrogen super-potential and CO adsorption.The electrode surface was composed of C-Cu₂O and C-Cu₂O spherical particles almost covered the whole surface.The network structure on the electrode surface greatly increased the micro surface area and had more abundant CO₂adsorption sites for the catalytic reduction of CO₂to improve the Faraday efficiency of the catalytic reduction of CH₃OH by CO₂.Using C-Cu₂O composite electrode as cathode and the electrolyte was KOH/C₂H₅OH solution.The reaction of electrochemical reduction of CO₂was carried out in a three electrode one chamber electrolytic cell.The results showed that the Faraday efficiency of CH₃OH was as high as 82%in the KOH/C₂H₅OH base electrolyte of 90 vol.%C₂H₅OH,2 g·L^-1KOH,10 m L·min^-1flow rate of CO₂and-1.0 V_vs.SCEelectrolytic voltage.（2）Cu₂O thin film electrode.Due to the high adsorption energy of CO₂,Cu in Cu₂O directly reduced CO₂to CH₃OH through electron capture.The surface coverage of Cu₂O thin film particles was very high and theoretically has the potential of good electrolytic performance.The experiments of electrochemical reduction of CO₂to CH₃OH were carried out with Cu₂O thin film electrode as cathode.The results showed that in the KOH/C₂H₅OH based electrolyte of 90 vol.%C₂H₅OH and 2 g·L^-1KOH,the Faraday efficiency of CH₃OH was 89.6%when the flow rate of CO₂was10 m L·min^-1and the electrolytic voltage was-0.7 V_vs.SCE.（3）Sn electrode modified by HBr.Among the numerous catalysts for electrochemical reduction of CO₂,Sn catalyst has the advantages of high hydrogen evolution potential and single product.Previous experiments proved that Sn electrode could reduce CO₂to methanol,but Faraday efficiency was relatively low.In this thesis,the Sn electrode was treated with HBr to increase the micro working area and electrode activity.The electrochemical reduction of CO₂to CH₃OH was carried out with HBr modified Sn electrode as cathode,the results showed that the Faraday efficiency of CH₃OH was 78.1%in the KOH/C₂H₅OH based electrolyte of 90 vol.%,C₂H₅OH and 2 g·L^-1KOH,when the flow rate of CO₂was 15 m L·min^-1and the electrolytic voltage was-1.7 V_vs.SCE.（4）Brass electrode modified by HCl.Brass copper is an alloy of copper and zinc.In the previous research on electrochemical reduction of CO₂by our research group,brass electrode was a metal materials that can efficiently catalyzed the reduction of CO₂,the brass electrode had good stability in the electrocatalytic process and could reduce CO₂to CH₃OH,but the Faraday efficiency of CH₃OH produced by untreated brass electrode was low.In this thesis,the brass electrode was treated with thermal HCl solution to improve the Faraday efficiency of CH₃OH.The experimental results showed that the Faraday efficiency of CH₃OH formation was as high as 87.3%in the KOH/C₂H₅OH based electrolyte of 90 vol.%C₂H₅OH and 2 g·L^-1KOH,when the flow rate of CO₂was 10 m L·min^-1and the electrolytic voltage was-1.4 V_vs.SCE.