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近年来由于化石能源的大量使用，二氧化碳（CO2）在大气中的含量不断增加，已成为全球关注的环境和社会问题。另一方面，CO2又是廉价、且可再生的C1资源，可用于生产醇、酸、酯、烃类等重要化工产品，其资源化利用对环境保护和能源安全都具有重要的意义。电化学还原CO2可利用自然界广泛存在的分布式能源（太阳能、风能、地热、潮汐能等）产生的电能实现CO2在温和条件下的转化，将电能以化学能的形式储存起来，以CO2为能量转换的载体可实现“碳平衡”的目标。但CO2分子特有的结构决定了其热力学高度稳定和动力学惰性，极难被活化，在常规水溶液体系中存在CO2电还原电流密度低、析氢严重的难题。作为一种仅由阴阳离子组成、酸碱可调、结构可高度设计的新型介质，离子液体（Ionic Liquid, IL）具有极低的挥发性、宽的电化学窗口、高的电导率和优异的热-电化学稳定性，同时对CO2有较高的溶解度和活化作用，为CO2的电还原提供了机遇，成为国际关注热点。本文主要设计合成了常规和功能化离子液体，并研究了其对CO2电化学还原反应的影响规律，揭示了离子液体介质中CO2的活化-催化转化机制和电极界面的CO2电还原的微观反应过程，主要的研究结论如下：（1）采用一步法和两步法合成并纯化了系列咪唑、季鏻和氨基功能化等离子液体，系统研究了离子液体的基本物性，包括粘度、密度、热稳定性和电化学稳定性。结果发现，功能化离子液体的粘度整体高于常规离子液体，对于常规咪唑基离子液体，粘度随着阳离子侧链长度增长而升高；稳定性实验表明，合成的离子液体具有较高的热、电化学稳定性。对比了不同离子液体对CO2的吸收能力，对于常规咪唑离子液体，阴离子对CO2吸收容量的影响顺序为[Tf2N]- > [PF6]- > [BF4]- > [DCA]- > [NO3]-，离子液体的阴离子相同时，咪唑阳离子上碳链越长，CO2吸收容量越高。合成了阴阳离子氨基功能化离子液体，CO2吸收容量是常规咪唑离子液体的10倍左右，此外，在阴离子引入了与CO2作用的双化学位点吸收CO2，合成的季鏻离子液体[P66614][2-OP]每摩尔吸收容量达到1.37 mol CO2。（2）采用H型三电极电解池，系统研究了Ag电极上咪唑基常规离子液体介质中的CO2电化学还原过程。复配溶剂乙腈（AcN）的引入可显著降低离子液体的粘度，提高CO2在本体溶液中的传质效率，从而提高CO2电还原电流密度，同时在复配溶剂中加入少量水（5%）可以显著提高电解液反应体系的稳定性。对比不同种类的离子液体电还原CO2结果，发现阳离子为咪唑基的离子液体对CO2的电化学还原具有明显的促进作用，咪唑离子液体在Ag电极表面和·CO2-自由基形成了[Cnmim]-CO2-[BF4]-的络合物中间体，中间体的形成可以降低·CO2-的能量状态，从而大大降低CO2电还原的反应过电位，在[Bmim][BF4]/AcN-H2O三元体系中，可以在高的电流密度下（22.52 mA·cm-2）将CO2电化学还原为CO，且法拉第效率高达90.2%，咪唑基离子液体中阳离子上碳链越短，其对CO2电化学还原的反应越有利。（3）CO2在Au/Ag电极上功能化离子液体介质中电化学还原的研究。系统研究了电极种类、离子液体功能基团对CO2电化学还原行为和产物分布的影响规律。采用电化学阻抗谱方法，揭示了咪唑基离子液体对CO2电还原过程的催化作用机制。结果表明，在相同离子液体介质中，与Ag电极相比，Au电极上电还原CO2的起始还原电位更正，阴极电流密度更大，而Ag电极上CO2还原产物的选择性更高。采用不同CO2吸收能力的季鏻离子液体，发现CO2电化学还原性能与吸收容量之间并无直接关系，而与阳离子大小有关，阳离子体积越小，CO2电化学还原性能越好。对咪唑基的阳离子进行氨基功能化可以显著提高其CO2电催化还原性能，电流密度提高了1倍，起始还原过电位降低0.15 V，这是由于氨基官能团和CO2之间存在较强的相互作用，提高了CO2在电极界面的浓度。（4）设计合成了系列常规和阴离子功能化咪唑基离子液体用于CO2电化学还原制甲酸。相较常规离子液体，CO2在功能化离子液体[Bmim][124Triz]介质中还原电位由常规离子液体中的-1.97 V正移到-1.78 V（vs. Ag/Ag+），同时在[Bmim][124Triz]介质中电化学还原制甲酸的电流密度可到24.5 mA·cm-2，法拉第效率高达95.2%，反应效果远优于常规离子液体。通过量化计算分析了功能化离子液体对CO2的活化作用，在[Bmim][124Triz]中稳定的CO2分子结构得到了有效活化：其中CO2分子的O-C-O键角由180o变为136o，C原子杂化轨道由sp杂化类型向sp2过渡，杂化轨道类型接近甲酸中C的杂化类型，且电中性的CO2分子携带了-0.546 e的负电荷，从而降低了CO2电还原为·CO2-自由基的反应电位。在功能化离子液体介质中，·CO2-活化态分子距反应活性位点距离（2.3 ?）远低于常规离子液体，且界面的双电层电荷转移电阻Rct（41.8 Ω·cm2）和吸附分子膜层电阻Rf（0.7 Ω·cm2）远小于常规离子液体介质中，说明在功能化离子液体介质中电极表面离子和反应分子的传输速率更快。 ;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·molIL-1 is obtain in [P66614][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 Au/Ag 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 136o, 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.
|冯建朋. 基于离子液体介质的CO2电化学还原过程研究[D]. 中国科学院大学,2019.|
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