**1. Introduction**

CO2 emission is an urgent issue due to its main contribution to global warming [1]. It has been evidenced that CO2 capture is a promising route to mitigate CO2 emissions, and in general, cost, energy demand, and environmental impact need to be considered when for selecting the potential CO2 capture technologies [2]. At present, the absorption technology with 30 wt % MEA aqueous solution is the commercialized one. However, this technology with the corresponding solvent has the drawbacks of high energy demand (4.2 GJ/t CO2), high cost (\$0.19–1.31/t CO2), low thermal and chemical stability, and high volatility and corrosion [3–6], which underlines the necessity for developing greener and more efficient solvent for CO2 capture.

Compared with the traditional amine-based solvent, the emerging new absorption solvents of ionic liquids (ILs) and deep eutectic solvents (DESs) have attracted more and more attention due to the merits of recyclability, good solvent stability, low energy demand, and environmentally friendly nature [7]. However, some of the ILs (e.g., [P4442][Cy-Suc], 2567 mPa·s at 303.15 K) [8] and DESs (e.g., [MTPP][Br]-GLY 1:4, 1658 mPa·s at 298.15 K) [9] have high viscosities that influence the rate of absorptions, inhibiting their industry applications. To cope with this disadvantage, hybrid ILs or DESs with water have been strongly recommended [9–14]. For example, Zhang et al. investigated the mass transfer feature in [BMIM][NO3] + H2O, evidencing that mass transfer increases with the increase of water content, e.g., the mass transfer of [BMIM][NO3] (95 wt %) + H2O (5 wt %) is 0.55 <sup>×</sup> 10<sup>5</sup> m·s−1, while it is 0.64 <sup>×</sup> <sup>10</sup><sup>5</sup> <sup>m</sup>·s−<sup>1</sup> for [BMIM][NO3] (90 wt %) <sup>+</sup> H2O (10 wt %), which may due to the decrease of viscosity that from 35.74 ([BMIM][NO3] (95 wt %) + H2O (5 wt %)) to 12.65 mPa·s ([BMIM][NO3] (90 wt %) + H2O (10 wt %)) [13]. Sarmad et al. studied the viscosity of DES, finding that a small amount of water has a significant effect on DES viscosity [9]. For instance, at 298 K, the viscosity of [TEMA][Cl]-GLY (1:2) is 236.59 mPa·s, while it is 72.75 mPa·s for [TEMA][Cl]-GLY-H2O (1:2:0.055).

Except for adding water to decrease the viscosity, organic solvents (e.g., PEG, TEG, and TG) can substitute water totally or partially to decrease the viscosity or to overcome high energy demand in IL + H2O [15–17]. The experimental result indicates that TG can significantly decrease the viscosity of [P66614][4-Triz], especially at low temperatures, i.e., the viscosity of [P66614][4-Triz] is 4640 mPa·s at 278.15 K, while it is 163 mPa·s for [P66614][4-Triz] + TG (58.2 mol%) [18]. Liu et al. found that the addition of a certain amount of PEG200 to [Cho][Gly] + H2O not only decreases the viscosity but also enhances the CO2 solubility, as well as decreases the desorption enthalpy [17].

To further take the benefits of both neoteric and conventional solvents, IL–amine-based and superbase–amine-based DES hybrid absorbents have also been proposed and developed [19–25]. These hybrid solvents possess certain advantages of low energy demand, low water evaporation, and high CO2 solubility compared to the commercialized MEA aqueous solution [26]. For example, Yang et al. reported that [BMIM][BF4] (40 wt %) + MEA (30 wt %) + H2O (30 wt %) has higher CO2 solubility than MEA (30 wt %) + H2O (70 wt %), and the energy demand reduced by 37.2% with respect to MEA (30 wt %) + H2O (70 wt %) [21]. The CO2 solubilities of four functionalized ILs in MDEA aqueous solution were investigated and compared with MEA + MDEA aqueous solution [27], showing that the CO2 solubility of [N2222][Lys] (15 wt %) + MDEA (15 wt %) + H2O (70 wt %) (0.74 mol/mol) > [N1111][Lys] (15 wt %) + MDEA (15 wt %) + H2O (70 wt %) (0.69 mol/mol) > [N2222][Gly] (15 wt %) + MDEA (15 wt %) + H2O (70 wt %) (0.64 mol/mol) > [N1111][Gly] (15 wt %) + MDEA (15 wt %) + H2O (70 wt %) (0.56 mol/mol) > MEA (15 wt %) + MDEA (15 wt %) + H2O (70 wt %) (0.36 mol/mol). For DES hybrid solvent of [Ch][Cl]-MEA 1:2 + DBN with volume ratio of 1:1, its CO2 solubility improved from 3.29 to 5.11 mol/kg compared with [Ch][Cl]-MEA 1:2 at 298.15 K [25].

To develop the potential IL/DES-based hybrid solvents for CO2 capture, CO2 solubility (in accordance with Henry's constant for physical absorption) and viscosity are two key properties. Furthermore, the selectively for physical absorption and the CO2 absorption enthalpy for chemical absorption are other concerns in development, while the research is still limited, especially when compared to those for CO2 solubility and viscosity. Several reviews for IL/DES-based hybrid solvents from the aspect of CO2 solubility, Henry's constant, and viscosity have been published. Babamohammadi et al. summarized the viscosities of IL + H2O and IL + MEA/EG + H2O until 2014 and the CO2 solubilities of ILs (imidazolium- and ammonium-based ILs)-amine hybrid solvents since 2008 [28]. Gao et al. summarized the CO2 solubility of 18 kinds of IL-amine based hybrid solvent until 2015. Huang et al. reviewed the advantages and disadvantages of five kinds of IL–hybrid solvents (i.e., IL–organic, normal IL–amine, normal IL aqueous–amine, functionalized IL–amine, and functionalized IL aqueous–amine) until 2016 for CO2 capture [26], finding that IL–hybrid solvents can significantly reduce the viscosity. Lian et al. introduced the ILs–hybrid processes for CO2 capture and compared the CO2 solubilities for IL–DEA/DMEE/ethanol, indicating that IL–ethanol has the highest solubility of 2.3 mol/mol [29]. Recently, more IL-based hybrid solvents have been developed

combined with property measurements, making it necessary to update the latest research progress. Meanwhile, to the best of our knowledge, there is no review article for the DES-based hybrid solvents.

To fulfill this gap and to promote the technology development on CO2 capture in IL/DES-based hybrid solvents, this review summarizes the CO2 solubilities (including Henry's constants) and viscosities of IL-based hybrid solvents since 2016 and DES-based hybrid solvents since 2014 to avoid the repetition of the published reviews. Finally, the best candidates for IL/DES-based hybrid solvents were obtained and compared with each other.

#### **2. ILs-Based Hybrid Solvents**

Regarding the CO2 solubilities for 73 kinds of IL–H2O and 37 kind of IL–organic-based hybrid solvents since 2016, 28 types of IL–amine hybrid solvents since 2018 together with 62 Henry's constants have been reported. The results were collected and summarized in Tables 1 and 2. The viscosities for 30 kinds of IL–H2O and 121 IL–organic/organic aqueous solution hybrid solvents since 2016, and 15 kinds of IL–amine/amine aqueous solution hybrid solvents since 2018 have been determined, and these are listed in Table 3. The full names of ILs-based hybrid solvents are displayed in Table S1.
