1、对用于电化学介导胺再生（EMAR）CO₂捕集技术的最具应用前景的两种溶剂（MEA和EDA）进行了较全面的对比分析。

2、利用电化学和统计学的方法对两种溶剂从动力学和热力学方面进行了系统的比较。

3、为电化学介导胺再生（EMAR）CO₂捕集技术最佳溶剂的筛选提供了一种方法。

• Two most promising solvents (MEA, EDA) used in EMAR were comprehensively discussed.

• Systematic comparison from kinetics to thermodynamics conducted by electrochemistry and statistics methods.

• Providing a method for the selection of best solvents to be used in the EMAR process.

根据Mauna Loa Observatory的观测数据，在过去的60年里大气中二氧化碳(CO₂)的浓度从315 ppm上升到近420 ppm，引发了一系列全球性的环境危机。其中，人为CO₂排放占很大比例，化石燃料的持续消耗是主要来源。然而，化石燃料作为能源的主要来源，在未来几十年仍将被广泛使用。因此，部署碳捕集与封存(CCS)技术对于减少CO₂排放关重要。胺基化学吸收法CO₂捕集技术是目前最成熟、最适用于集中点源的碳捕集技术。但是，由于胺再生过程需要较高的温度(>120℃)，使得传统的胺基CO₂捕集过程需要消耗大量的能量。此外，较高的温度也会降低吸收剂的稳定性。这些都导致运行成本大幅度增加，严重制约了该技术的大规模应用。因此，迫切需要一种新的方法来降低传统胺基CO₂捕集过程的能耗。

电化学介导的胺再生技术(EMAR)是一种有望取代传统的胺基CO₂捕集技术。EMAR过程依赖于CO₂和金属离子(如Cu²⁺)与胺分子(如单乙醇胺(MEA)，乙二胺(EDA))之间的竞争性配位。溶液中的铜离子(Cu²⁺)会取代CO₂，与MEA、EDA结合形成Cu(II)-MEA/EDA配合物。与传统胺基CO₂捕集过程一样，EMAR过程也由吸收和解吸两个阶段组成，其高温解吸阶段被电解池取代，进行电化学解吸，可以在较低的温度和较高的压力下工作。更重要的是，电力驱动的特性使该技术有望与电力驱动的二氧化碳还原反应(CO2RR)相结合，为CO₂捕获以及将CO₂转化为碳中性碳氢化合物燃料提供有前景的途径，从而实现大规模的CO₂清洁捕集。在EMAR过程中，合适的金属和胺的组合对系统的整体性能有显著影响。铜因其较好的抗氧化性和与胺较强的竞争配位性而被认为是EMAR过程中最适用的金属。在胺的选择上，MEA和EDA是EMAR过程中讨论最多的吸收剂。但它们也有缺点，如MEA的溶解度较低，EDA的挥发性较高。因此，MEA和EDA在EMAR过程中的综合性能比较，对于EMAR技术的进一步实际应用具有重要意义。

本文对EMAR过程中讨论最多的MEA和EDA溶剂进行了系统的比较，以求对该技术的实际推广应用提供指导。首先，我们进行了CO₂吸收性能比较，研究铜离子的存在对MEA和EDA吸收性能的影响。其次，从热力学角度比较了配合物的稳定性和介导反应发生的倾向性。然后，为了进一步了解MEA和EDA溶液中阴极和阳极的电化学过程，分别采用循环伏安法(CV)和电化学阻抗谱(EIS)进行了定量和定性分析。最后，系统的研究了铜在MEA和EDA中的循环性能，并观察了电化学解吸后电极表面的微观结构，以证明其在实际CO₂捕获过程中的适用性。

According to the observation data of Mauna Loa Observatory, in the past 60 years, the concentration of carbon dioxide (CO₂) in the atmosphere has risen from 315 ppm to nearly 420 ppm, which has resulted in a series of global environmental crisis. Among them, anthropogenic CO₂ emissions contributed a significant proportion, and continued consumption of fossil fuels is the main source. However, as the primary source of energy, fossil fuels will continue to be widely used for decades to come. Therefore, the deployment of mitigation technologies such as Carbon Capture and Storage (CCS) is imperative to cut down the carbon footprint of the sustained utilization of fossil fuels. The amine-based chemical absorption CO₂ capture process is the most suitable carbon capture technology for large-scale point-source emitters. Due to the high temperature (>120 ℃) required during amine regeneration, the conventional amine-based CO₂ capture process requires a large amount of energy. In addition, higher temperatures also reduce the stability of the absorbents. The large amount of energy required for the amine-based CO₂ capture process dramatically increases the operating cost, which severely restricts the large-scale application of this technology. Therefore, a new strategy is urgently needed to reduce the energy consumption of the traditional amine scrubbing CO₂ capture process.

Electrochemically-mediated amine regeneration (EMAR) for CO₂ capture is a promising substitute for conventional amine-based CO₂ capture. The EMAR process relies on the competitive binding between CO₂ and a suitable metallic species (e.g., Cu²⁺) to an amine molecule (e.g., monoethanolamine (MEA), ethylenediamine (EDA)). The cupric ions (Cu²⁺) in the solution will undergo coordination reaction with the carbamate, produced by the reaction of CO₂ with MEA and EDA, replacing CO₂ and binding with MEA and EDA to form Cu(II)-MEA/EDA complexes. Like the amine-based CO₂ capture process, the EMAR process also consists of absorption and desorption stages, but the high-temperature desorption stage is replaced with an electrochemical cell for electrochemical desorption, which can operate at a lower temperature and higher pressure. What’s more, the electric-powered feature makes the technology is expected to be combined with electricity-powered CO₂ reduction reactions (CO2RR) to offer promising routes for the solution of CO₂ capturing as well as conversing CO₂ into carbon neutral hydrocarbon fuels, and enable larger-scale clean CO₂ capture. In the EMAR process, a suitable combination of mentals and amines has a significant effect on the overall performance of the system. Copper was considered the best metal in the EMAR process because of its stability in resisting oxidation and competitive coordination with amines. As for the selection of amines, MEA and EDA are the most discussed absorbents to be used in the EMAR process. However, they also have shortcomings, such as the lower solubility of MEA and the higher volatility of EDA. Hence, it is meaningful to comprehensively compare the performance of MEA and EDA in an EMAR process and promote its further enlarged application.

In this work, a systematic comparison of the most discussed MEA and EDA solvents in the EMAR process was conducted to provide a guidance for their practical application. First, we performed a CO₂ absorption performance comparison to study the effects of the presence of cupric ions on MEA and EDA absorption properties. Next, the stability of complexes and the probability of mediated reactions occurring were compared from the perspective of thermodynamics. Then, to further understand the electrochemical processes of cathode and anode in MEA and EDA solutions, the electrochemical measurements of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used for quantitative and qualitative analysis, respectively. Finally, copper cycling performance in MEA and EDA have been carefully studied, and the microstructure of the electrode’s surface after electrochemical desorption was observed in MEA and EDA to show its applicability in a practical CO₂ capture process.

1、CO₂吸收性能比较

在MEA和EDA溶液中，CO₂吸收负荷和CO₂吸收速率均随着铜负荷(L_Cu(Ⅱ))的增加而显著降低。CO₂吸收容量几乎是铜负荷(L_Cu(Ⅱ))的线性递减函数。MEA和EDA溶剂的拟合线分别为 y=0.55068-1.02033x 和 y=1.0013-1.59307x 。这意味着1摩尔的铜离子倾向于与约2摩尔的MEA或1.6摩尔的EDA络合。总体而言，EDA比MEA表现出更好的CO₂吸收性能(CO₂吸收容量更大，吸收速率更高，络合能力更好)。这意味着在EMAR过程中，当处理相同的气体时，选择EDA作为吸收剂时，所消耗的吸收剂量更少，吸收时间也更短。

The CO₂ loading and the rate of CO₂ absorption decrease significantly with increasing copper loading (L_Cu(Ⅱ)), the same trend is offered in both MEA and EDA solvents. The CO₂ capacity is almost a linearly decreasing function of L_Cu(Ⅱ). The fitting line for MEA and EDA solvents are y=0.55068-1.02033x and y=1.0013-1.59307x separately. This means that 1 mole of cupric ions tends to be complex with about 2 moles of MEA or 1.6 moles of EDA. Overall, EDA exhibits better CO₂ absorption performance (larger CO₂ absorption capacity, higher absorption rate and better coordination ability) compared with MEA. It means when dealing with the same flue gas, the number of absorbents consumed and the absorption time are less when EDA is chosen as absorbent in an EMAR process.

2、热力学性能比较

配合物Cu(II)-EDA的稳定常数大于配合物Cu(II)-MEA，说明EDA与Cu²⁺形成的配合物比MEA与Cu²⁺形成的配合物更稳定。综合∆用来表示不同吸收剂的平均竞争力，其定义为不同种类配合物的百分比与其∆(∆=log₁₀(β)-n_ligandlog₁₀(K_CO2)) 值的乘积。综合∆的值越大，说明该配合物越容易形成并越容易释放吸收的CO₂。得到EDA和MEA的综合∆分别为8.68和6.11。可以发现，与MEA相比，EDA仍然更容易与Cu²⁺形成稳定的配合物，这意味着在EDA中被吸收CO₂的更容易被解吸。因此，从热力学角度看，EDA具有更强的耦合性能，更适合在EMAR系统中使用。

The stability constant of Cu(II)-EDA is larger than that of Cu(II)-MEA, which represents the complexes formed by EDA and Cu²⁺ are more stable than those formed by MEA and Cu²⁺. The comprehensive ∆, which defined as the product of distribution of different kinds of complexes (%) and their values of ∆(∆=log₁₀(β)-n_ligandlog₁₀(K_CO2)), is used to show the average competitiveness in different absorbents. The larger the value of comprehensive ∆ is, the complex is more likely to form and release the absorbed CO₂. We can obtain the comprehensive ∆ of EDA and MEA as 8.68 and 6.11 respectively. It can be found that EDA is still easier to form stable complexes with Cu²⁺ compared with MEA, which means EDA is more conducive to the release of absorbed CO₂. Thus, from the perspective of thermodynamics, EDA has a more robust mediated coupling performance and is more suitable for practical application in an EMAR system.

3、电化学性能比较

通过循环伏安(CV)测试的定性分析，我们发现在MEA溶液中铜的还原反应比在EDA溶液中更难发生，在MEA溶液中氧化反应比在EDA溶液中复杂，说明在考虑EMAR过程中的电化学反应时，EDA是更好的选择。通过电化学阻抗谱(EIS)对电化学系统中各部分阻抗的定量分析，发现铜离子在EDA中的反应阻抗更小，还原反应比在MEA中更容易进行，但由于扩散阻力更显著，反应强度不如MEA。因此，建议在EMAR过程中使用EDA，并需要施加适当的扰动或较高的温度来提高铜离子还原反应的强度。

Through qualitative analysis by cyclic voltammetry (CV) test, we can find that the reduction reaction of copper is more difficult to occur in MEA than in EDA solutions, and the oxidation reaction in MEA solution is more complex than that in EDA solution, suggesting EDA is a better choice when considering the electrochemical reactions in the EMAR process. Through the quantitative analysis of the impedance of each part in the electrochemical system by electrochemical impedance spectroscopy (EIS), it was found that the reaction impedance of copper ion in EDA is smaller, and the reduction reaction of cupric ions in EDA is easier to carry out than in MEA, but the intensity of the reaction is not as good as in MEA because of the more significant diffusion resistance. Therefore, EDA is suggested to be used in the EMAR process, and it is necessary to apply appropriate disturbances or higher temperatures to improve the intensity of copper ions reduction reactions.

4、CO₂解吸性能比较

在不同电流密度下，MEA的实际能耗始终高于EDA。我们可以看到，即使在300 A/m²的大电流密度的情况下，EDA的能耗也只有42.4 kJ_e/mol CO₂。已发表的文献中解吸能耗为35 ~ 100 kJ_e/mol CO₂，电流密度相对较小(小于150 A/m²)，与其相比本实验所得到的结果具有很强的竞争力和实际指导意义。除电流密度为300 A/m²时的阴极法拉第效率（MEA为76.69%，EDA为57.44%）外，EDA溶液中阳极和阴极法拉第效率均高于MEA溶液。这表明，在相同条件下，在EDA溶液的电子利用率更高，EMAR过程中可以转化更多的铜离子，从而可以解吸出更多的CO₂。通过观察在MEA和EDA溶液中电化学解吸后电极表面的微观结构，发现与MEA溶液相比，EDA溶液中具有更好的铜循环性能。

Under different current densities, the energy requirement in MEA is always higher than that in EDA. We can see that even in the case of a large current density of 300 A/m², the energy consumption of EDA is only 42.4 kJ_e/mol CO₂. Compared with the published work, the desorption energy consumptions are 35-100 kJ_e/mol CO₂, and the current density is relatively small (less than 150 A/m²), the results in this work are very competitive and practically instructive. The Faraday efficiency of anode and cathode electrodes in EDA solution is higher than that in MEA solution, except that the cathode Faraday efficiency at a current density of 300A/m², which is 76.69% in MEA and 57.44% in EDA. This shows that the electron utilization rate in EDA is higher, more cupric ions can be converted for the EMAR process, and a more considerable amount of CO₂ can be desorbed under the same conditions. By observing the microstructures of the electrodes surface after electrochemical desorption in MEA and EDA solvents separately, it was found that EDA shows better copper cycling performance compared with that of MEA solvent.