Study On The Electrocatalytic Reduction Of CO₂ In Nonaqueous System Of Ionic Liquids

In recent years,in the face of climate and environmental problems caused by the increasing content of greenhouse gas CO₂,researchers have adopted the development concept to reduce CO₂into value-added chemicals and fuels using clean energy,which can simutaneiously solve the environmental problem and the energy shortage crisis.However,due to the high thermodynamic stability and kinetic inertia of CO₂molecules,and the low solubility of CO₂in aqueous electrolyte solution,the electrocatalytic reduction of CO₂faced the problems of high overpotential,low current density and low selectivity of specific products.In order to solve the problem of low solubility of CO₂in traditional aqueous solution,as well as the competition between hydrogen evolution from water reduction and CO₂reduction,the electrocatalytic reduction of CO₂using ionic liquid as absorption medium and electrolyte has widely been studied.However,due to the high cost of using pure ionic liquid as electrolyte solution and the high viscosity of most ionic liquids at room temperature,it is necessary to find a suitable organic solvent to form with ionic liquids as electrolyte solution.Therefore,in this thesis,the electrocatalytic reduction of CO₂in ionic liquid system using propylene carbonate（PC）as solvent was firstly studied,and the catalytic mechanism of ionic liquid in this new system was further investigated.On this basis,we have further developed new noble metal catalysts based on their plasmon properties for the electrocatalytic reduction of CO₂.The main research contents and conclusions of this thesis are as follows:1.A new electrolyte solution with low viscosity,high conductivity and high CO₂solubility was prepared with imidazolium-based tetrafluoroborate as supporting electrolyte and PC as solvent.The electrocatalytic reduction of CO₂was studied with Ag as working electrode.The catalytic mechanism of ionic liquid in the electrochemical reduction of CO₂was studied by linear voltammetry,Tafel and electrochemical impedance spectroscopy.The results show that1-butyl-3-methylimidazolium tetrafluoroborate[Bmim]BF₄has the lower initial reduction potential and higher exchange current density than the traditional electrolyte[Bu₄N]BF₄and other ILs with different carbon chain length.When[Bmim]BF₄is used as electrolyte,the current density of CO₂conversion can reach8.2 m A/cm²and the Faraday efficiency of CO can reach 98.5%at-1.9 V（vs.Fc⁺/Fc）.The results of electrochemical impedance show that the cation[Bmim]⁺can adsorb on the Ag electrode,and the intermediate[Bmim-CO₂]_adis formed between the cation and CO₂^·-,which reduces the energy barrier for the formation of CO₂^·-and the activation energy required for the whole reaction in the reduction process of CO₂,thus improving the conversion performance of electrocatalytic CO₂.2.The Ag nanocubes were prepared by glycol reduction method.The electrocatalytic reduction of CO₂was carried out by using Ag nanocubes as electrocatalysts and light absorber,and[Bmim]BF₄/PC as electrolyte solution.Firstly,the effects of wavelength and intensity of light on the electrochemical reduction of CO₂were studied by means of electrochemical characterization.The results of electrochemical characterization show that the combination of plasmon resonance and electrochemistry makes the transfer of electrons and holes more directional,the decreased recombination of electrons and holes.Thereofe this method improves the kinetics of CO₂reduction reaction,and finally speeds up the reaction rate and improves the selectivity of product of CO.At last,we evaluated the CO₂conversion performance under the condition of light excitation.The results show that the Faraday efficiency of CO can reach 100%when the light intensity is more than 60m W/cm²under the light condition of 420 nm.3.On the basis of the synthesis of hollow Ag nanospheres,the Ag@Pd catalysts with core-shell structure were successfully prepared,and the conversion performance of the composite catalysts for electroreduction of CO₂was studied.The results show that this hybid plasmon catalysts can reduce the reduction potential of the electrode and increase the current density in the electrocatalytic process.The results show that under illumination with light of 520 nm and 60 m W/cm²,the current density of electrocatalytic reduction of CO₂can reach 20 m A/cm²,and the FE of product CO can reach 96.2%.