Electrochemical Reduction Process Of Carbon Dioxide Based On Ionic Liquids

The increasing emission of carbon dioxide?CO₂?caused by the unrestrained consumption of fossil fuel in recent hundred years has caused globally ecological,environmental,and social problems.On the other hand,CO₂ is also a cheap,abundant and renewable C1-feedstock which can be converted into alcohols,ethers,acids and other chemicals,thus it is a promising solution to the aforementioned environment problems and energy crisis.The electrochemical reduction of CO₂ into value-added chemicals can be realized under mild conditions by using geographical,seasonal,and intermittent energy?e.g.,solar,wind,geothermal and tide?,and the energy can be stored and released with CO₂ as the energy-carrier chemical and eventually achieve the goal of "carbon balance".CO₂ activation and reduction has been proven to be a significant challenge due to its high thermodynamic stability and kinetic inertness,as a matter of fact,quite a lot of essential problems are confronted of CO₂ electrochemical reduction in aqueous solution,such as low current density and serious hydrogen evolution reaction?HER?.As a novel type of liquid salt composed of cations and anions,ionic liquids?ILs?have been attracted more attention in CO₂ electrochemical reduction reaction?CO₂RR?,due to their basic physicochemical properties.Such as high intrinsic ionic conductivity,strong electrostatic field,wide electrochemical potential windows and excellent thermo-electrochemical stability,as well as high solubility of CO₂,are beneficial for CO₂ activation and reaction.In this work,conventional and functionalized ILs were designed and prepared,and the performance of CO₂ reduction,activation and conversion mechanism of CO₂ in ILs were studied and revealed.The main research contents and results of the dissertation are as follows:?1?A series of imidazolium,phosphonium and amino-functionalized ILs were synthesized and purified by one-step and two-step methods.The basic physicochemical properties of ILs,including viscosity,density,thermal stability and electrochemical stability,were systematically studied.For conventional imidazolium ILs,the viscosity increases with the increase of side chain length on the cation,and the viscosity of functionalized ILs is much higher than that of conventional ionic liquids.The stability results show that the synthesized ILs have high thermal and electrochemical stability.The order of imidazolium anions on CO₂ absorption capacity is[Tf2N]->[PF6]->[BF4]->[DCA]->[NO3]-,and a higher CO₂ absorption capacity will be obtained while increasing the side chain length on the cation of imidazolium ILs.Compared with the conventional ILs,the anionic and cationic amino-functionalized ILs exhibit 10 times higher capacity of CO₂ due to the reaction between CO₂ and amine.At the same time,the maximum absorption capacity of 1?37 mol CO₂·?mol IL?-1 is obtain in[P666]4][2-OP],the great enhancement in CO₂ capacity mainly derive from the multiple-site cooperation interactions on the anion.?2?The electrochemical reduction performance and mechanism of CO₂ in conventional ILs on Ag electrodes by a self-developed H type electrolysis cell.The presence of acetonitrile has great impacts on reducing viscosity of ILs and improving CO₂ transfer in bulk solution,thus increasing the current density of CO₂ reduction,the trace water?5%?in the mixed solvent can significantly improve the stability of the electrolyte reaction system simultaneously.It is found that the imidazolium ILs show obvious promotion on the electrochemical reduction of CO₂,with respect to the mechanism,it is indicated that the[Cnmim]-CO₂-[BF4]-complex intermediates on the surface of Ag electrode formed by[Cnmim][BF4]and CO₂-free radicals could reduce the energy state of CO₂-.In the[Bmim][BF4]/AcN-H2O ternary electrolyte,it exhibits exceptional performance for CO₂ electrochemically reduction to CO with a high current density?22.52 mA.cm-2?,and the Faraday efficiency is as high as 90.2%,and the performance of CO₂ electrochemical reduction in imidazolium ILs mainly attributes to the size of cation.?3?Investigation of electrochemical reduction of CO₂ in functionalized ILs on Ag/Au electrodes.Anionic and cationic imidazolium,quaternary phosphonium and amino functionalized ILs were designed and synthesized,and the effects of electrode and ILs on CO₂ electrochemical reduction behavior and product distribution were systematically studied,the catalytic effect mechanism of imidazolium ILs towards electroreduction of CO₂ was investigated by electrochemical impedance spectroscopy?EIS?.Comparing with the Ag electrode,Au shows better performance with lower overpotential and higher cathode current density in CO₂ reduction,while with a lower Faradaic efficiency for CO.Moreover,it was found that there was no direct relationship between the electrochemical reduction performance of CO₂ and the absorption capacity based on the phosphonium ILs,but the smaller size of cation,the better performance of CO₂ electrochemical reduction in ILs.The catalytic activity of cationic amino-functionalized ILs is much higher than that of conventional ILs and anionic amino-functionalized ILs.Because of the strong interaction between the amino group and CO₂,the concentration of CO₂ at the electrode interface is increased significantly.Hence,the current density of CO₂ electrochemical reduction is as twice as that of the conventional ILs,and the initial reduction potential of CO₂ shift anodically by 0.15 V.?4?The design and synthesis of conventional and anionic functionalized ILs for CO₂ reduction to HCOOH.Compared with the conventional ILs,the initial reduction potential of CO₂ in functionalized ILs[Bmim][124Triz]is shifted from-1.97 V to-1.78 V?vs.Ag/Ag+?,the partial current density for formic acid in[Bmim][124Triz]medium can reach 24.5 mA·cm-2,as well the Faraday efficiency is as high as 95.2%.which are much higher than that of conventional ILs.Due to the strong interaction between CO₂ and[Triz]-,the stable CO₂ molecule tend to a bent form with a O-C-O bond angle of 136°,calculation results indicate that the rehybridzation of CO₂ is from sp to nearly sp2 character,and the hybridization of C in CO₂ is close to that in HCOOH,and the net charge of neutral CO₂ molecule turns to-0.546 e.In this vein,the anion[124Triz]-CO₂-proposed above may provide a low energy pathway for CO₂ reduction to HCOOH.Furthermore,activated CO₂ can be easily transferred to the cathode surface due to the lower resistance in the electric double layer at the Pb electrode interface,therefore leading to a higher current density and Faradaic efficiency.