Electrochemical Reduction Of Carbon Dioxide In Ionic Liquids Medium

Carbon dioxide?CO₂?is the main component of greenhouse gases.At the same time,it is also a natural,abundant and renewable C₁ resource.The electrochemical reduction of CO₂ into high value-added chemicals and fuel can be realized by distributed energy under mild conditions.The electroreduction of CO₂ in conventional aqueous solutions has the disadvantages of serious hydrogen evolution and low current density.As a new type of medium composed of anions and cations,ionic liquids?ILs?are widely used in CO₂ electroreduction as an excellent electrolyte because of the unique properties for high efficient CO₂ dissolution and activation,wide electrochemical window,and high conductivity,etc.It has become a research hotspot at home and abroad.In this work,aromatic ester functionalized and conventional ILs were designed and synthesized,and the effect of ILs on CO₂ electrochemical reduction performance was investigated,and the interaction and activation mechanism between ILs and CO₂ were revealed.The main research contents are as follows:?1?Establishing a reaction method for electrochemical reduction of CO₂ in IL medium.A three-electrode system and H-type double-chamber electrolytic cell with good sealing effect,stability and reliability were built for the research of CO₂electrochemical reduction.The conventional analysis methods of linear sweep voltammetry test and constant potential electrolysis were used to monitor the CO₂reduction process,and a complete analysis method for different products of CO₂electrochemical reduction was established.?2?Design and synthesis of aromatic ester functionalized IL for CO₂electrochemical reduction to oxalic acid.The aromatic ester functionalized IL,4-methoxycarbonylphenol tetraethylammonium?[TEA][4-MF-Ph O]?was synthesized by a one-step method.The physical properties of IL,including viscosity and density were determined.The CO₂ absorption capacity of[TEA][4-MF-Ph O]was investigated,and it was much higer than that of the conventional imidazole-based ILs.The electrochemical reduction of CO₂ to oxalic acid in the aromatic ester functionalized IL?[TEA][4-MF-Ph O]?medium on Pb electrode was investigated.It was found that the exceptionally high partial current density(9.03 m A·cm^-2)for oxalic acid with appreciable faradaic efficiency?FE?of 86%on Pb electrode were reached in anhydrous[TEA][4-MF-Ph O]?0.9 M?-Ac N cathodic electrolyte,and the oxalic acid production rate reaches 168?mol·cm^-2·h^-1.Through quantitative calculation,it was found that the double active sites of phenoloxy group and ester group in[TEA][4-MF-Ph O]can efficiently activate CO₂ and form[TEA][4-MF-Ph O]-COOH intermediate,and finally dimerizes it into oxalic acid?H₂C₂O₄?.The results demonstrate[TEA][4-MF-Ph O]has significant catalytic effect on electrochemical reduction of CO₂ to oxalic acid.?3?Carrying out research on the electrochemical reduction of CO₂ to CO in imidazole-based IL.The process of CO₂ electrochemical reduction in conventional imidazole-based IL?[Bmim][OTF]?medium on Ag electrode was investigated.The partial current density for CO₂ electrochemical reduction to CO is as high as 65 m A·cm^-2 with the FE of 97%in[Bmim][OTF]?0.9 M?-Ac N/H₂O?10 wt%?ternary electrolyte system at a potential of-2.2 V?vs.Ag/Ag⁺?.Through quantitative calculations,it is revealed that CO₂ dispersed on the surface of the Ag electrode easily forms[Bmim]-CO₂-[OTF]intermediate with[Bmim][OTF],which effectively reduce the reduction overpotential of CO₂ electroreduction reaction,and inhibit the self-polymerization of CO₂·^-radical anion and the occurrence of hydrogen evolution reaction,and improve the selectivity of CO₂·^-reduction to CO.