**1. Introduction**

The unprecedented increase of CO2 concentration in the atmosphere has led to many concerns about global warming, and even predictable environmental disasters, which make us feel urged to limit the CO2 emission and effectively utilize them [1–4]. The catalytic transformation of CO2 into value-added chemicals and fuels has been regarded as one of the most promising ways to realize the valorization of CO2 [5]. In this context, several strategic options, including electrochemical [6–12], thermochemical [13–15], photocatalytic [16–18] and photoelectrochemical [19–22] approaches, have been developed to undertake the CO2 conversion. Among them, the electrochemical reduction of CO2 is regarded as the most prospective way, because it allows one to combine with carbon capture and storage technology, and to utilize renewable energy (such as solar energy and wind energy), as inputting energy and water as a reductant to reduce CO2 into various carbon-based fuels and chemicals (e.g., CO, HCOOH, CH4, C2H4, and CH3OH) in a modular electrochemical reactor under ambient temperature and pressure [7–11]. However, the linear CO2 molecule is thermodynamically stable and kinetically inert to be reduced, due to its low electron affinity and large energy gap between its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) [23]. It has been reported that the main hurdle in CO2 electroreduction lay in the first-step one-electron reduction of CO2 to form an anion radical (CO2 •−), because this activation step requires a much high reduction potential of −1.9 V (vs. NHE) [24,25]. Therefore, to accelerate this process, considerable

efforts have been devoted to study the electrocatalysts, because the structure of electrocatalysts provides active sites to activate the reactant of CO2, and great progress has been witnessed in recent years [26].

While the electrocatalysts are important in research efforts, the electrolyte on the other side plays an equally pivotal role in catalysis, by interacting with the reactant and intermediate species, ultimately influencing the overall reduction reaction [8,27]. In this aspect, ionic liquids (ILs), which are composed of relatively large organic cations and small inorganic anions, have shown their advantages in the reduction of CO2, and have been extensively studied in recent years [6,7,28–34]. The first consideration is that ILs have a high capacity for CO2 capture [35,36] and unique electrochemical properties [37–40], such as wide electrochemical windows, high conductivity, and high stability. More importantly, ILs could interact with CO2 or the reaction intermediate species, and eventually improve the catalytic activity and influence the product selectivity [7,34,41–47]. For example, by using the 18 mol% 1-ethyl-3-methylimidazolium tetrafluoroborate [Emim][BF4] aqueous solution as the cathode electrolyte, Rosen et al. [45] reported that Faradaic efficiency (FE) greater than 96% for CO from CO2 could be achieved at very low overpotential. In the subsequent work [48], they proposed that the formation of an adsorbed CO2–[Emim]<sup>+</sup> complex provided a low-energy pathway for CO2 conversion to CO, accounting for the enhanced performance at the presence of ILs. In another ILs assistant CO2 conversion work, Sun et al. [49] reported that the cation of 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [Emim][NTf2] played the role of stabilizing effect to prevent the close approach and dimerization between two CO2 •− to form oxalate, and thus switched the C-derived product to CO. Furthermore, they proposed that the formation of an imidazolium carboxylate through the coordination of [Emim]<sup>+</sup> with CO2 •− appeared as a feasible pathway for the CO2 conversion to CO [49]. However, due to the vast number of ILs through different combinations of various cations and anions, an understanding of the interactions between ILs electrolyte and CO2 molecule or intermediate species, as well as the behind decisive role of ILs is necessary for properly selecting the ILs to enhance the efficiency for CO2 electroreduction.

Because of the high cost and high viscosity, ILs are often mixed with different molecular solvents of water or organic solvent, and then used as supporting electrolytes, as well as active co-catalysts [50–54]. For water solution, the hurdles for efficient electrochemical conversion of CO2 stem from the low solubility of CO2 in aqueous solution, the complicated CO2 species in water, and the competitive H2 evolution from H2O reduction [55]. Compared with water, organic solvents have high solubility of CO2 and have been alternatively investigated as solvents of ILs for the conversion of CO2 [10,50,51]. However, these widely investigated organic solvents, such as acetonitrile (AN), *N*,*N*-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), have some severe shortages in practical use. AN is unsuitable for practical use because of its high toxicity and volatility. DMF is also not an appropriate candidate, since it is prone to hydrolysis in water. As for DMSO, owing to its low melting point (18.4 ◦C), its utilization is highly restricted at ambient conditions. Therefore, the seeking of suitable liquid solvents that can be utilized in CO2 conversion is still a challenging and urgent task. In recent years, propylene carbonate (PC), as a polar aprotic liquid, has attracted much attention and has been regarded as a promising and "green" sustainable alternative solvent in various chemical and electrochemical transformations fields [56–58]. This is mainly due to its wide electrochemical window and unique physicochemical properties, such as the low toxicity and vapor pressure, as well as the non-corrosive and biodegradable nature of PC. More importantly, PC also has a high capacity of CO2, which makes PC a promising alternative solvent to overcome the afore-mentioned problem faced in CO2 conversion. Despite these advantages, PC has only been seldom used as a solvent in the electrocatalytic conversion of CO2 [59,60].

Inspired by these signs of progress, in this study, the electrocatalytic conversion of CO2 was performed in imidazolium-based ILs/PC solution, in which the ILs act as active component and electrolyte, and PC as the solvent. First, the onset potential and the main kinetic parameters of CO2 reduction in PC solution of 1-alkyl-3-methyl imidazolium tetrafluoroborate with different chain length were measured using linear voltammetry (LSV) and Tafel characterization, respectively. Then, the electrochemical impedance spectroscopy (EIS) and equivalent circuit analysis were carried out to

investigate the catalytic role of ILs in the course of CO2 reduction. At last, the catalytic performance in the presence of ILs and tetrabutylammonium tetrafluoroborate salt was evaluated and compared.
