Elucidation of the Roles of Ionic Liquid in CO2 Electrochemical Reduction to Value-Added Chemicals and Fuels

The electrochemical reduction of carbon dioxide (CO2ER) is amongst one the most promising technologies to reduce greenhouse gas emissions since carbon dioxide (CO2) can be converted to value-added products. Moreover, the possibility of using a renewable source of energy makes this process environmentally compelling. CO2ER in ionic liquids (ILs) has recently attracted attention due to its unique properties in reducing overpotential and raising faradaic efficiency. The current literature on CO2ER mainly reports on the effect of structures, physical and chemical interactions, acidity, and the electrode–electrolyte interface region on the reaction mechanism. However, in this work, new insights are presented for the CO2ER reaction mechanism that are based on the molecular interactions of the ILs and their physicochemical properties. This new insight will open possibilities for the utilization of new types of ionic liquids. Additionally, the roles of anions, cations, and the electrodes in the CO2ER reactions are also reviewed.


Introduction
Since industrialization, fossil fuels have been chiefly used as the primary source of energy, which has led to global environmental problems due to the carbon dioxide (CO 2 ) emissions to the atmosphere. Thus, the mitigation of CO 2 emissions has become a serious issue globally [1,2]. Carbon capture, storage, and utilization (CCSU) is a pivotal way for reducing emissions in industries. Several CO 2 -utilization technologies using biological and chemical methods such as thermal reduction [3], electrochemical reduction [4], and photoelectrochemical reduction [5] have been investigated. The thermal reduction of CO 2 thermal reduction is far from use in real-world applications because of the high threshold requirements for the temperature, the available materials, and the system configurations [6]. Many techniques have been studied methods to reduce the threshold temperature and to improve the conversion efficiency of the CO 2 reduction reaction, such as the through the utilization of heterogeneous catalysts [7]. The development of heterogenous catalysts have focused on modifying the structure and the composition, the study of reaction pathways considering dimension-geometry effects, bifunctional processes, ligand impacts, and lattice strain [6]. Several metal catalysts have recently been designed that have shown CO 2 methanation at low temperatures and at low atmospheric pressure [8]. However, the thermal reduction of CO 2 thermal is still a big challenge.
The major interest in adopting the electrochemical reduction of CO 2 is its potential integration with renewable energy such as wind and solar energy, as shown in Figure 1. Moreover, it can be operated under ambient conditions, and the reactions can be easily controlled by adjusting external parameters such as the electrolytes, the type of electrodes, and the applied voltages. Moreover, several studies have reported the use of solar energy for CO 2 electrochemical reduction (CO 2 ER) both directly and indirectly through photocatalytic chemistry [9][10][11][12], photo-electrochemical [13][14][15], and electrochemical systems [16,17]. Different configurations have been used as a reactor for the CO 2 electrochemical reduction reaction, which have been inspired by water electrolyzers (liquid phase, solid oxide, and gas phase) [18]. In each type of reactor, the CO 2 electrochemical reduction reaction (CO 2 ERR) occurs on the cathode side, while the water oxidation reaction takes place on the anode side. In liquid-based electrolytes, the typical CO 2 reduction cells are traditional H-cells, as depicted in Figure 2, and flow cells, as illustrated in Figure 3 [19].
In the H-cell configuration, the cell consists of an immersed anode and cathode in an electrolyte that have been separated from each other by an ion-exchange membrane. The membrane only allows hydrogen ions to flow into the cathode side, where it prevents the product that is produced in the cathode side from flowing to the anode side and from being oxidized again. Furthermore, the ion-exchange membranes prevent the evolved O 2 in the anode side from passing across the cathode and consuming the electrons for an oxygen reduction reaction (ORR) that could otherwise be utilized for CO 2 ERR. On the anode side, the water oxidation reaction occurs, producing the hydrogen ions and electrons that will be transferred to the cathode side where the CO 2 reduction reaction takes place. In the flow cell (Figure 3), the liquid electrolyte is in a flow-through configuration to increase CO 2 solubility, minimize mass transport limitations, and inhibit hydrogen evolution reactions (HER) [19,20].  Other design configurations are shown in Figure 4 [18]. The liquid-phase electrolyzer has three flow streams: CO 2 , catholyte, and anolyte each have their own stream. The catholyte and gas channel are separated by a gas diffusion electrode. The electrolyte is in contact with a catalyst layer, which is located on the liquid-facing front part of the gas diffusion electrode. In contrast, from the back of the electrode, the CO 2 gas stream is continuously supplied. This configuration enables the accurate control and enhancement of the reaction environment. A liquid electrolyte system can be advantageous; however, it can also be a source of system instability due to contaminant accumulation on the catalyst and the potential permeation of the contaminant into the gas diffusion electrode; it may also cause flooding, which is a widespread type of failure. In gas-phase reactors, a solid polymer electrolyte separates the cathode and anode (ion-exchange membrane). In a zero-gap configuration, the cathode catalyst is pressed up against the ion-exchange membrane. The humidity must be supplied to the system via a liquid electrolyte on the anode and/or humidification of the gas inlet stream to ensure that the membrane is hydrated while it is in use. The advantage of gas-phase electrolyzers over liquid-phase configurations is that they have less ohmic losses, less electrolyte pumps and flow fields are used, and they can easily be pressurized. In addition, the absence of the catholyte eliminates multiple sources of instability, such as the accumulation of electrolyte impurities on catalysts and the electrolyte inundating the gas diffusion electrode. On the other hand, liquid products can accumulate in the gas diffusion electrodes, obstructing gas diffusion [18].
The solid oxide flow cell consists of a solid cathode, anode, and electrolyte. Lanthanum gallate-based oxides, zirconia-based oxides, or ceria-based oxides are used as solid electrolytes. This configuration requires a high temperature (above 600 • C) in order to produce a C 1 gas product (CO/CH 4 ). This setup is highly stable, and it runs at la ow voltage and at high current densities [18]. However, the product's limited range and the excessive temperature demand limits it from being widely used [21][22][23][24].
The processing of carbon dioxide into more complex molecules through chemical catalysis is conducted on a grand scale by photosynthesis using enzymes and radiation. This effective natural system has stimulated a massive amount of work to develop catalysts to perform as enzyme mimics [25][26][27][28][29][30][31][32]. The electrochemical reduction of carbon dioxide (CO 2 ) to produce various organic compounds such as methane (CH 4 ) [33], methanol (CH 3 OH) [34,35], carbon monoxide (CO) [36], ethylene(C 2 H 4 ) [37], and formic acid (HCOOH) [38] have been reported. During CO 2 reduction, different products can be obtained on the cathode side, which is based on the number of electrons that is required at the respective reduction potentials shown in Table 1. Moreover, the hydrogen ions that have been dissolved in the catholyte might also be converted to hydrogen (H 2 ), causing a hydrogen evolution reaction (HER), which is a competitive reaction to CO 2 ERR. This also affects the selectivity, which primarily depends on the number of electrons and catalytic active sites as well as the reactants/intermediates adsorption/desorption properties [39]. Thus, the practical potentials needed for CO 2 ERR are more negative than the equilibrium potentials. Thermodynamically, a high-energy barrier is required for the initiation of the CO 2 radical (CO 2 •− ) with an equilibrium potential of −1.90 V vs. a standard hydrogen electrode (SHE), pH = 7), which influences the CO 2 ERR significantly [40]. In CO 2 ERR, the faradaic efficiency must be considered since it represents the efficiency with which charges (electrons) are transported in a system, promoting an electrochemical reaction. On the other hand, the current density, which is defined as the rate of charge passed across a specified cross-section unit area per unit time [42], is affected by the conductivity and thickness of the thin film on the electrode [43]. The main problem associated with this technology is finding a catalytic system that can efficiently convert CO 2 to the desired value-added product at a low energy requirement. This is due to the linear structure of CO 2 , which makes it electrochemically stable. This results in a process with low overpotential, high faradaic efficiency, high current density, high selectivity towards a specific product with a good yield, and high CO 2 solubility in the electrolyte.
Many reviews have reported on the role of the electrode as a catalyst employing metals such as copper [44], silver [45], gold [46], lead, and indium [47], alloys at different scales (nano, micro), structures, and morphology [48][49][50], and coated metals [51] as well as metal oxides [52]. Hori et al. reported the effects of different metals on CO 2 ERR and highlighted the electrode's role in terms of selectivity and reducing the overpotential [53]. The difficulties that are faced during the adsorption of CO 2 on the electrode surface are the reason for the increased amount of energy that is required for the activation of the reaction [39,54]. On the other hand, in terms of the electrolytes, potassium bicarbonate salt has been widely employed [55,56]. Dunwell and Lu [57] reported that bicarbonate salt has a significant function in the CO 2 ERR instead of merely acting as a pH buffer or proton donor. They suggested that the bicarbonate raises the CO 2 ERR rates by increasing the effective reducible CO 2 concentration in a solution. A growing interest in ionic liquids as electrolytes has been reported, as they can enhance the catalytic activity of the system in terms of lowering the overpotential of the reactions and of increasing the faradaic efficiency toward a specific product.

Introduction to Ionic Liquids
Ionic liquids are substances that are entirely composed of ions and that have a melting point below 100 • C [58]. ILs have attracted attention in many applications due to their unique properties, such as thermal stability [59], chemical stability [60], conductivity [61], a relatively high CO 2 absorption capacity [62], electrochemical stability [63], negligible volatility [64], and an ability to be used as electron transmission mediators for redox catalysis [65]. The only disadvantage of ionic liquids is their relatively high cost compared to conventional organic solvents [66]. However, the cost limitation can be minimized by using ionic liquids as additives in organic solvents as well as by recycling ionic liquids. Due to these distinguished properties, ionic liquids have recently gained attention in a variety of applications, such as in biological applications, electrolytes, separation process, heat storage, and catalysis [67]. Figure 5 shows the chemical structure of the most common IL cations and anions.
In CO 2 electrochemical reduction, ILs have shown good catalytic performance in producing various products such as methanol (CH 3 OH) [68], carbon monoxide (CO) [45], dimethyl carbonate (C 3 H 6 O 3 ) [69], formic acid (HCOOH) [38], and ethanol [70]. In this paper, the roles of the electrode, the anions, and the cations of the ionic liquids in the CO 2 ER reactions are reviewed.

The Roles of the Electrode in CO 2 ERR
Many studies have described the effect of the electrode geometry, size, interface area between the electrode and the electrolyte, and the effect of the electrical double layer on CO 2 ERR [71][72][73]. In this review, the roles of the major components are briefly highlighted, as many existing reviews have discussed the work on electrode materials extensively [74,75]. The cathode material plays a primary role of a catalyst for electroreduction in the system. Hori et al. reported the effects of different metals on CO 2 ERR, which are summarized in Figure 6 [53]. From Figure 6, it can be observed that the potential of the zinc (Zn) and indium (In) electrodes are approximately similar, but the faradaic efficiencies of the products are different. For the production of CO, the highest faradaic efficiency is 79.4% for the Zn electrode, while it is just 2.1% on the In surface. However, for HCOOH production, the highest faradaic efficiency is 94.9% on the In electrode, while it is 6.1% on the Zn surface. It can be observed that the Sn, Hg, Pb, and In metals produce HCOOH favorably, while the group of Au, Ag, Zn, and Pd metals produce CO favorably. Among all of the elements in Figure 6, copper (Cu) has the highest tendency to produce various types of products. This property makes Cu and its derivatives attractive materials for study [76][77][78]. Hansen reported that the selectivity of the product principally depends on the binding energy of CO 2 •− on the metal surface [54]. He concluded that the rate in the noble metals gold (Au) and silver (Ag) is limited by CO 2 activation because CO binds loosely. For the Pd, Ni, Pt, and Rh electrodes, CO 2 activation and conversion to adsorbed CO is easy, and the rate is restricted by CO desorption due to massive binding. As shown in Figure 7, Cu is positioned at the center and is shown to be binding intermediately with CO and COOH compared to the other materials, which may explain the reason why a variety of products can be produced by this electrode.
It can be concluded that the electrode materials play a major role in product selectivity. Therefore, a new way to increase the catalytic activity of the electrodes is by introducing molecules on the electrode surface that can reallocate the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO), as reported by Yadav and Singh [79].  (211) stage. The transition metals follow a linear trend that does not cross over the peak of the volcano. The noble metals, on the other hand, reach the trend line's optimum. The specific CO generation current from the ChCODH II and MbCODH enzyme models is comparable to, if not superior to, that of the noble metals. Reprinted with permission from [54]. Copyright 2014, American Chemical Society.

Ionic Liquids as Co-Catalyst for CO 2 ERR System
Many studies have been performed to elucidate the role of ILs in CO 2 ERR. Table 2 shows a non-exhaustive list of ionic liquids used in CO 2 ERR, with the list of IL abbreviations being shown in Table 3. Although various ionic liquids have been reported, the role of ILs and the exact mechanism behind the reactions remain an open question. The ionic liquids that are incorporated into the liquid electrolyte aid in increasing CO 2 solubility while enhancing the catalytic system at the same time. As ionic liquids are composed of cations and anions, the effects of each component are discussed separately for a better understanding.

The Effect of Anions on the CO 2 ERR System
Many research studies have reported on the solubility of CO 2 in ILs using prediction and experimental methods [92][93][94][95][96][97]. Aki and Mellein [89]  show that the solubility of carbon dioxide is mainly dependent on the anions, and it is higher in anions that contain fluoroalkyl groups such as [Tf 2 N] and [methide]. They attributed this to the acid/base interactions of CO 2 with anions. They also noted that the CO 2 solubility slightly increases when the alkyl chain length of the cation increases due to a higher free volume in the ILs with longer alkyl chains.
On the other hand, Anthony and Anderson [98] studied the solubility of various gases (carbon monoxide, oxygen, carbon dioxide, ethylene, ethane, nitrous oxide, and benzene) in butyl-methyl pyrrolidinium bis(trifluoromethylsulfonyl) imide, tri-isobutyl-methyl phosphonium p-toluenesulfonate, and methyl-tributylammonium bis(trifluoromethylsulfonyl) imide, where they observed that carbon dioxide and nitrous oxide have strong interactions with ILs followed by interactions with ethylene and ethane. However, oxygen has demonstrated very poor solubility and limited interaction with ILs. For carbon monoxide, the authors could not detect the solubility because of limitations with their apparatus. They concluded that the ILs with the bis(trifluoromethylsulfonyl) imide anion had the highest CO 2 solubility, regardless of whether the cation was tetraalkylammonium, pyrrolidinium, or imidazolium. Their study highlighted that anions have the most significant effect on gas solubilities. Similarly, Almantariotis and Stevanovic studied the absorption of carbon dioxide, nitrogen, ethane, and nitrous oxide by 1-alkyl-3-methylimidazolium (C n mim, n = 2,4,6) tris(pentafluoroethyl)trifluorophosphate ionic liquids (FAP). They observed that the ILs containing highly fluorinated anions (tris(pentafluoroethyl)trifluorophosphate [FAP]) recorded the highest CO 2 solubility among the ILs for the same cations [91]. They noted that amino acid-based ILs have high CO 2 solubility due to the interaction between carbon dioxide and amino functional groups [99,100]. Snuffin 3 Cl], which has a C 3 -symmetric tetrahedral structure. They noted that the dative B-Cl covalent bond is not as strong as the ordinary B-F covalent bond. The B-Cl bond-length was 0.173 nm, which is 33% greater than the B-F bond length of 0.130 nm. They reported that the dissolved CO 2 molecules may replace the C l atoms in the BF 3 Cl by forming BF 3 -CO 2 adducts. On the other hand, the oxygen atoms of CO 2 are Lewis bases; therefore, the BF 3 can form a Lewis acid-base adduct with CO 2 . They studied [Emim][BF 3 Cl] IL as an electrolyte for the electrochemical reduction of CO 2 at 1 atm and 25 • C on a platinum electrode and discovered that the reaction occurred at −1.  [101][102][103]. They explained that the increase in the catalysis performance occurred through the reduction of the overpotential, which was due to the formation of the Lewis acid-base adduct BF 3 -CO 2 . It can be concluded that the anion affects both the catalytic and solubility; however, its highest effect is on the solubility, whereas the influence in reducing the overpotential is less important because of its negative charge, making it far apart from the cathode.

The Effect of the Cations on the Activity of CO 2 ERR System
The low adsorption of CO 2 on the electrode surface and the linear stability of the CO 2 structure and its hybridization are the reason behind the difficulty in activating CO 2 . Thus, a high overpotential is required to reduce the large energy barrier for CO 2 •− radical initiation, which is considered to be the reaction-determining step. More attention has been given to the cations since they have been reported as the central active part of catalytic performance [87].
Vasilyev et al. [104] summarized the role of imidazolium-based cations and highlighted that the obtained results are diverse and conflicting and that they can be divided into two major groups. The first group suggests that a covalent bond is formed between the IL and CO 2 (as shown in Figure 8A, carboxylic adduct formation) or that there is a hydride transfer ( Figure 8B); the second assumption is based on non-covalent interactions (stabilization of the complex through hydrogen bonds or changes in the local CO 2 environment, as seen in Figure 8C,D, respectively). We observed that this diversity in the results is due to the CO 2 absorption type (physical or chemical), which is determined by the dominant molecular force between the CO 2 and the imidazolium-based group.
Rosen et al. and Masel performed many experiments to understand the mechanism of CO 2 ERR [45,[105][106][107]. They observed that ILs have a good ability to reduce the overpotential (i.e., merely 200 mV higher than the equilibrium potential) with a high faradaic efficiency of 96% on the silver cathode (Ag) [45]. Moreover, the authors reported that the ILs showed a high ability to suppress the competing H 2 generation reaction by building a monolayer on the electrode. They highlighted that the [Emim] cations are mostly located at the metal catalyst surface of the electrode throughout the electrolysis and suggested that CO 2 is not directly adsorbed onto the metal catalyst surface of the electrode but that it is the [Emim] bound to the CO 2 complex ([Emim]-CO 2 ) that is adsorbed on the electrode surface, which is the reason for the enhancement observed in the catalytic activity in terms of lowering the overpotential, as illustrated in Figure 9. They also noted that the addition of water into the IL promotes catalytic performance, which shows that the hydrogen bond interaction promotes the catalyst efficiency. By comparing their results with those obtained in the study by Li et al. [108], who concluded that the addition of water narrows the electrochemical window of ionic liquids, we can observe that the addition of water in Rosen's experiments also narrows the electrochemical stability of the electrolyte by increasing the reduction potential. It should be noted that the oxidation potential (loss of electrons) or reduction potential (gain of electrons) of a molecule is closely related to the ionization energy, which is the amount of energy that is required to remove an electron from the highest occupied molecular orbital (HOMO) and the electron affinity, which is the amount of energy that is released when an electron is added to the lowest unoccupied molecular orbital (LUMO), respectively. This decreases the LUMO values since the reduction is related to the LUMO energy level, whereby the HOMO and LUMO values can be approximated using Equation (1) and Equation (2), respectively [109]: where E HOMO and E LUMO correspond to the energy levels of HOMO and LUMO, E ox(onset) is the onset of the oxidation potential, E red(onset) is the onset of the reduction potential, 4.8 is the reference energy fpr ferrocene (4.8 eV under the vacuum level), and E FOC is the potential of ferrocene/ferrocenium (FOC/FOC + ) versus Ag/AgCl. Based on this approximation, it can be deduced that the LUMO energy values could be the reason behind the electroreduction reactivity. Moreover, the interaction between the OH − anion from the added water in the cathode side and the H + from the anode side can obstruct the competing H 2 generation reaction through the attraction force, which suppresses the competitive HER. Reprinted with permission from [104]. Copyright 2014, American Chemical Society. Sun et al. and Ramesha [85] studied the selectivity of the reaction towards CO, where they used Pb as the cathode and [Emim][Tf 2 N] as the electrolyte, and they found that the production of CO can be attributed to the interaction between CO 2 and the cation [Emim] + monolayer at the cathode surface. The results indicated that the CO 2 •− and [Emim] + adsorbed on the cathode surface could direct the CO 2 •− reaction toward forming CO rather than oxalate. They concluded that [Emim] + stabilized the CO 2 •− radical, which shifted the reaction in the direction of forming CO rather than oxalate, as shown in Figure 10. Hu et al. studied CO 2 reduction by using MoO 2 as the cathode and different imidazolium-based ILs as electrolytes at several temperatures and pressures [86]. The IL 1-butyl-3-methylimidazolium hexafluorophosphate [Bmim][PF 6 ] recorded the highest activity results at low temperatures. They demonstrated that the IL cations play the role of the co-catalyst in enhancing catalytic activity. Several studies [110,111] have been conducted to enhance catalysis performance. Nonetheless, the design of a high-performance catalysis system remains a challenge. Many researchers [112][113][114][115] have studied the thin area between the electrode and the electrolyte where the electrochemical reaction takes place (electrical double layer region). They concluded that the electrical double layer could be the reason behind the problem of the system having a high over potential. Lau and Vasilyev [87] studied the different structures of imidazolium-based ILs supported by a 0.1 M solution of tetrabutylammonium hexafluorophosphate [TBA][PF 6 ] in anhydrous acetonitrile. They evaluated the performance of different structures as cocatalysts for the CO 2 ERR on a silver (Ag) electrode. It was reported that the catalytic activity mainly originates in the cation and that protons at C 4 and C 5 are critical, as shown in Figure 11. We deduced that this is due to the π-bond, which enhances the acidity and therefore also enhances the acid/base interaction between CO 2 (which is relatively considered to be a Lewis base) and the cations (which is considered to be a Lewis acid).
Vasilyev and Shirzadi [88] investigated the CO 2 reduction in pyrazolium ILs (Pz-IL) on a silver electrode, where they observed a significant decrease in the onset potential for the reduction (approximately 500 mV). They proceeded with the electrochemical conversion in acetonitrile-based electrolytes containing Pz-IL as co-catalysts with faradaic efficiencies (FE) of nearly 100% for CO over a range of at least 0.5 V. The molecular structures of the pyrazolium that were used are shown in Figure 12, and the potentials for the different structures are shown in Figure 13. They suggested that the co-catalyst effect comes from ion pair stabilization, ion interactions, and modifications in the local microenvironment. They reported that the nitrogen domain in the pyrazolium cation is the most charged region and that substitutions at the C 3 and C 5 positions change the system more than when substitutions are made at the C 4 position. They assumed that the CO 2 •− intermediate interacts with the positively charged region of the Pz ring, which helps to stabilize the intermediate and to reduce the reaction overpotential. Figure 11. The interaction between the active imidazolium protons and CO 2 . Reprinted with permission from [87]. Copyright 2014, American Chemical Society.  [88]. Reprinted with permission from [88]. Copyright 2018, American Chemical Society.
The high stability of CO 2 comes from its molecular orbital, as shown in Figure 14. The molecular orbitals (MO) with the highest relevance to the reactivity are the 1π g and 2π u orbitals, which play the role of HOMO and LUMO, respectively. The doubly occupied nonbonding 1π g MOs are mainly localized at the terminal oxygen atoms, whereas the empty antibonding 2π u orbitals are mostly centered on the carbon atom. Therefore, CO 2 can be considered to be an amphoteric molecule, where the oxygen atoms display a Lewis basic character and where the carbon atom behaves in a similar manner to a Lewis acid center with a slightly negative electron affinity (Ea) of about −0.6 eV [116] and a first ionization potential (I P ) of about 13.8 eV [117]. Carbon dioxide is more of an electron acceptor than it is an electron donor. The molecule's reactivity is regulated by the electrophilic characteristics of the carbon atoms rather than the weak nucleophilic properties of the oxygen atoms. The resulting lowest energy state corresponds to a curve geometry when the LUMO orbitals of CO 2 are occupied with electrons. The CO 2 •− radical anion, for example, is a twisted molecule with a balanced angle of 134 • [118]. Any carbon dioxide interaction with an electron-rich metalcore would also induce a loss of linearity [119].   [16,120,121]. Reprinted with permission from [16]. Copyright 2018, American Chemical Society.
Buijs and Witkamp [122] used quantum chemical calculations to predict the electrochemical window of ILs with reduction-resistant anions. They conducted their experiments using the Spartan'10 molecular modeling program suite. They optimized the structures at the B3LYP level using the corresponding PM3 structures as input. They highlighted that the lowest unoccupied molecular orbital (LUMO) of the ILs containing reductionresistant anions is entirely concentrated on the cation. They concluded that the order in resistance against reduction is piperidinium > pyrrolidinium > quaternary phosphonium > quaternary ammonium > imidazolium > pyrazolium pyridinium [122]. This sequence is consistent with the experimental data [123][124][125].
From this review, by comparing the performance of imidazolium-based ILs [87] with the pyrazolium-based ILs [88] and by taking the lowest unoccupied molecular orbital (LUMO) values [122] into consideration, it can be stipulated that the cation structures with the lowest LUMO energy levels have a higher hydrogen bond interaction value [126,127] and higher catalytic efficiency in terms of reducing the overpotential. Since the complex of (CO 2 •− cation) formed is adsorbed on the catalyst, the CO 2 properties could change in terms of the LUMO energy levels, which may be due to the acid/base interaction between the CO 2 (which is relatively considered to be a Lewis base) and the cations (which is considered to be a Lewis acid). The other possibility is that due to the low LUMO value, the high hydrogen bonding energy can disturb the strong linearity of the CO 2 and can decrease the strength of the covalent bond. Consequently, this will reduce the activation energy that is required and will reduce the overpotential. To determine which hypothesis is more dominant, a study considering the type of CO 2 absorption (physical or chemical) is recommended.
It is noteworthy to highlight the effect of the alkyl chain length of the cations on the solubility, as reported by Safavi and Ghotbi [128]. They studied the solubility of carbon dioxide and hydrogen sulfide in the 1-alkyl-3-methylimidazolium hexafluorophosphate ([C n mim][PF 6 ]) IL at different temperatures and pressures. They found that when the cation alkyl chain length becomes longer, the solubility of CO 2 and H 2 S in the ionic liquid increases. It can be concluded that the reason behind the improvement of the solubility is due to the increase in the non-polarity since CO 2 is nonpolar, thus behaving in the same way that it would when dissolving. They also concluded that the solubility of carbon dioxide increased when the pressure increased and decreased when the temperature increased. The high temperature reduces the risk of overpotential because of the higher activation energy, it but decreases the solubility of carbon dioxide, which is not very high (0.033 mol/dm 3 at standard temperature, 1 atm and pH = 7) in aqueous solutions [129]. Moreover, the reaction components that are adsorbed on the electrode are also lowered. The use of an organic solvent can help to solve this problem in terms of increasing the solubility of CO 2 [130]. In non-protic solvents, an increase in the temperature changes the products of CO 2 reduction, and CO is favored over oxalate formation at higher temperatures [131].
From this review, ionic liquids have good potential as co-catalyst for CO 2 ER because of the numerous possible combinations between the cations and anions that lead to a diverse suite of characteristics that serve the application. For instance, in CO 2 electrochemical reduction, the cations must have low LUMO values while also maintaining the electrochemical stability of the ILs. This can be achieved through molecular interaction by selecting the suitable types of anions [132].

Conclusions
For CO 2 ERR, ionic liquids play an essential role due to their unique physical chemistry properties, CO 2 •− stabilization, their ability to lower the overpotential, and their ability to increase the faradaic efficiency and current density. The main role of IL anions is to exhibit high CO 2 absorption on the catalysis surface as well as a co-catalyst in terms of reducing the overpotential if it forms a Lewis acid-base adduct. The IL cation that is closer to the electrode plays the main role as a co-catalyst in terms of reducing overpotential, and the formation of the cation-CO 2 complex could change the properties of the system in terms of the hydrogen bond interaction energy, the electrical double layer region, and the relocation of the HOMO and LUMO values. Most of the research highlighted that the strong hydrogen interaction could be the reason why the catalytic efficiency is enhanced; however, since the LUMO values and hydrogen interaction energy are related, we concluded that the main reason for this could be the relocation of the LUMO values. Moreover, we highlighted that the determination of the nature of the absorption (physical/chemical) of CO 2 in the ionic liquids is required to further understand the mechanism. Cations also play a secondary role in increasing the solubility of CO 2 by increasing the alkyl chain length. The material of the electrode plays the main role of product selectivity, according to the binding energy between the electrode and CO 2 radical. Furthermore, the catalytic activity of the electrode could be enhanced by introducing molecules on the electrode surface that could reallocate the lowest unoccupied molecular orbital and increase the catalytic performance.

Conflicts of Interest:
The authors declare no conflict of interest.