*2.3. Tafel Analysis*

Tafel plots of CO2 reduction in PC solution with different ILs and [Bu4N]BF4 as electrolytes are further conducted, and the results are shown in Figure 3. It can be determined from Figure 3 that the equilibrium potential of CO2 reduction in [Emim]BF4/PC, [Bmim]BF4/PC, [Hmim]BF4/PC, [Omim]BF4/PC, [Dmim]BF4/PC and [Bu4N]BF4/PC solution is −1.512 V, −1.275 V, −1.561 V, −1.633 V, <sup>−</sup>1.672 V, <sup>−</sup>1.751 V (vs. Fc/Fc+), respectively. In consistency with the results of LSV characterization, the [Bmim]BF4 IL has the higher catalytic promotion effect than other ILs and the [Bu4N]BF4 salt. The equilibrium potential of CO2 in [Bmim]BF4/PC solution shifted positively 476 mV by comparison with that in [Bu4N]BF4/PC solution. The kinetic parameters of CO2 reduction in different electrolytes were further calculated according to Equations (1) and (2) [63], and the results are listed in Table 1.

$$
\eta = a + b \log |i| \,\tag{1}
$$

$$a = -2.303RT \log i\_0 \text{(}\text{(}\text{ry}\text{)}\text{)}, b = 2.303RT \text{(}\text{(}\text{ry}\text{)}\text{)},\tag{2}$$

where η is the overpotential, *a* is the Tafel constant, *b* is the Tafel slope, γ is the charge transfer coefficient to indicate the symmetry of the energy barrier, *i*<sup>0</sup> is the exchange current density, *n* is the number of electrons transferred, *F* is the Faradaic constant, *T* is the absolute temperature, and *R* is the gas constant.

**Figure 3.** Tafel plots of CO2 reduction in CO2-saturated PC solution of ILs and [Bu4N]BF4.

**Table 1.** Electrochemical kinetic parameters of CO2 reduction in [Emim]BF4/PC, [Bmim]BF4/PC, [Hmim]BF4/PC, [Omim]BF4/PC, [Dmim]BF4/PC, and [Bu4N]BF4/PC electrolyte solution.


It can be seen from Table 1 that, for Tafel constant *a* and Tafel slope *b*, these two parameters in all ILs/PC solution are smaller than that in [Bu4N]BF4/PC solution. From Equation (2), the smaller value of *a* and *b* means that the activation over potential η will also be lower at the same current density. Moreover, the charge transfer coefficient γ and the exchange current density *i*<sup>0</sup> in all ILs/PC solution are higher than that in [Bu4N]BF4/PC solution. Therefore, these kinetic parameters derived from Tafel characterization confirm that the electrolytes of imidazolium-based ILs can lower the activation energy, and then is expected to enhance the electrochemical conversion efficiency of CO2. Furthermore, [Bmim]BF4 electrolyte gives the smallest value of *a* (0.791 V) and *b* (0.160 V/dec), and on the other side, has the largest value of <sup>γ</sup> (0.185) and *<sup>i</sup>*<sup>0</sup> (1.14 <sup>×</sup> <sup>10</sup>−<sup>5</sup> <sup>A</sup>/cm2), which means that [Bmim]BF4 would have the highest catalytic activity among all the studied ILs.

## *2.4. Electrochemical Impedance Analysis*

Based on LSV and Tafel characterization, it has been confirmed that ILs, especially [Bmim]BF4, showed an enhanced ability for activating CO2 in the electrochemical conversion process. To further study the underlying role of ILs in CO2 reduction on the Ag electrode, we further performed electrochemical impedance spectroscopy (EIS) analysis. The EIS results in different ILs/PC and [Bu4N]BF4/PC solutions are shown in Figure 4. As shown in Figure 4a, Nyquist plot in [Bu4N]BF4/PC electrolyte shows a semicircle in the region of high frequency, which can be ascribed to the Faradaic electron transfer process, and a nearly straight line can be observed in the low-frequency region, which resulted from the diffusion-control process. The deviation of the line from the typical 45◦ may be due

to the rough surface of the Ag electrode. However, different from that in the [Bu4N]BF4/PC electrolyte, the Nyquist plots in all ILs/PC electrolytes (Figure 4b) show an additional big semicircle. The apparent differences of the Nyquist plot between ILs/PC systems and [Bu4N]BF4/PC electrolyte mean that the reduction mechanism of CO2 is changed when IL is used as the electrolyte.

**Figure 4.** Nyquist plots of electrochemical impedance spectroscopy (EIS) in 0.1 M CO2-saturated [Bu4N]BF4/PC (**a**) and different ILs/PC (**b**) electrolyte solutions.

 

The equivalent circuits of EIS results were further derived and were shown in Figure 5. Figure 5a is a typical Randles circuit that can well feature the Nyquist plot in [Bu4N]BF4/PC electrolyte (see Figure 4a), in which *Rs* is the solution resistance, *R*ct is the charge transfer resistance, *C* is the double-layer capacitance, and *W* is the Warburg impedance. The diagram in Figure 5b represented the equivalent circuit of the Nyquist plot in ILs/PC solution. The presence of two semicircles in the Nyquist plot (see Figure 4b) means that there exist two different time constants. Since each time constant is related to an RC component [64], an additional RC component should be added in the equivalent circuit. When ILs/PC are used as the electrolyte in CO2 reduction, CO2 molecules have to pass through the absorbed ILs film layer to reach electrode surface, and then anticipate into an electroreduction reaction. Therefore, the additional RC circuit should be connected in parallel with another RC circuit [64]. So, in the equivalent circuit diagram of Figure 5b, the *R*<sup>1</sup> represents the migration resistance of CO2, through the adsorbed layer of IL. In addition, the second semicircle in Figure 4b is an arc and deviated obviously from the semicircular trajectory. This deviation is due to a dispersion behavior of the real capacitive component. Therefore, a constant phase element (CPE) has been commonly introduced and defined as in Equation (3) [65,66]:

$$Z\_{\text{(CPE)}} = \left(j\omega\right)^{-\mathfrak{n}} / \mathcal{Y}\_0 \tag{3}$$

where *Y*<sup>0</sup> is the constant phase coefficient, *n* is the dispersion coefficient (if *n* = 0, it is equivalent to a resistance; if *n* = 1, it is equivalent to a capacitance), *j* is the imaginary unit, and ω is angular frequency.

**Figure 5.** The equivalent circuit diagram of CO2 in [Bu4N]BF4/PC (**a**), [Bmim]BF4/PC (**b**) electrolyte. *Rs* is the solution resistance, *R*ct is the charge transfer resistance, *C* is the double layer capacitance, *W* is the Warburg impedance, *R*<sup>1</sup> is the migration resistance of CO2 through the ionic liquid adsorption layer, and constant phase element (CPE) is the constant phase element.

According to the equivalent circuit shown in Figure 5, the impedance parameters are further obtained, and the results are shown in Table 2. For example, in [Bmim]BF4/PC solution, the value of the dispersion coefficient *n* of the CPE is 0.8325. This means its deviation from a pure capacitance, indicating that [Bmim]BF4 formed a film layer on the surface of the Ag electrode and then caused a dispersion behavior [67].


**Table 2.** Parameter values of equivalent circuit components.

It can be seen from Table 2 that the Rct values of all studied ILs/PC systems are also smaller than that of [Bu4N]BF4/PC solution, confirming the promotion effect of ILs on the electrochemical reduction of CO2. This promotion effect of ILs/PC system is expected because of the catalytic role of the absorbed IL for the activation of CO2 through the complexation and stabilization of the CO2 •− radical anion [7,31,47]. In addition, the Rct values of ILs/PC systems have the following trend: [Bmim]BF4/PC (43.07 Ω) < [Hmim]BF4/PC (47.32 Ω) < [Emim]BF4/PC (49.07 Ω) < [Omim]BF4/PC (53.38 Ω) < [Dmim]BF4/PC (55.09 Ω). Among all the studied ILs with different chain lengths, [Bmim]BF4 has the smallest *R*ct value, indicating its highest catalytic effect for CO2 reduction. This result of EIS characterization is consistent with that obtained by LSV and Tafel analyses.

Furthermore, from the above electrochemical characterizations, it was found that, although the exact promotion effect of ILs on the electrochemical reduction of CO2 is not very clear, previous reports have generally indicated that the ILs adsorbed on the electrode can complex with CO2 •−reactive intermediate [45,46,54,68–71], resulting in the reduction of the overpotential and the ultimate facilitation of the CO2 conversion. From this point of view, the adsorption behavior of ILs on the electrode can influence the interaction between the imidazolium cation and CO2 •−, and plays a decisive role in the course of CO2 electrochemical conversion [7,29,47]. However, the adsorption behavior (i.e., the adsorption strength, the spatial structure, and the density) is influenced by many factors, such as the material and the potential of electrode, the solvent, the concentration of ILs, and the type of anion. In our study, when decreasing the chain length at N1-position of imidazolium cation from octyl to butyl, the catalytic activities of ILs increase, which can be ascribed to the higher adsorbed quantity with a lower steric hindrance of shorter chain length. However, further deceasing the chain length from butyl to ethyl, the adsorbed [Emim]<sup>+</sup> may further increase, and cause the film layer too dense to let CO2 molecules diffuse across, which, on the contrary, is detrimental to CO2 conversion.

#### *2.5. The Catalytic Performance and the Catalytic Mechanism*

After the electrochemical characterizations, we then evaluated the CO2 conversion performances in [Bu4N]BF4/PC and [Bmim]BF4/PC electrolyte solution, and the results of Faradaic efficiency (FE) and current density are shown in Figure 6. It can be seen from Figure 6a that the current density in both [Bmim]BF4/PC and [Bu4N]BF4/PC electrolytes increase as the applied potential decrease, and at each potential, the current density enhanced when ILs of [Bmim]BF4 replace [Bu4N]BF4 as the electrolyte. For example, at <sup>−</sup>1.90 V (vs. Fc/Fc<sup>+</sup>), the current density in [Bmim]BF4/PC (8.2 mA/cm2) was about three times that in the traditional [Bu4N]BF4/PC (2.7 mA/cm2). The FE results in Figure 6b show that the FE of CO in [Bmim]BF4/PC is much higher than that in [Bu4N]BF4/PC at each applied potential. Correspondingly, the FE of byproduct H2 is reduced when the [Bmim]BF4/PC is alternatively used as

the electrolyte solution. More importantly, it can be seen that the FE as high as 98.5% can be obtained in [Bmim]BF4/PC solution when the applied potential is at <sup>−</sup>1.90 V (vs. Fc/Fc+).

**Figure 6.** The current density (**a**) and Faradaic efficiency (FE) of CO and H2 (**b**) for electrocatalytic CO2 conversion in [Bu4N]BF4/PC and [Bmim]BF4/PC electrolyte solution.

Based on the above electrochemical characterization, the performance results and previous reports, the reduction mechanism of CO2 at the presence of [Bmim]BF4 is proposed, and the corresponding schematic diagram is shown in Figure 7. Firstly, the imidazolium cation [Bmim]<sup>+</sup> adsorbs on the surface of the Ag electrode and forms a film layer of ILs [43]. Subsequently, the CO2 molecules diffuse through the film layer of ILs and reach the Ag electrode. Then, the CO2 molecule obtains one single electron, resulting in the formation of radical CO2 •−, and the generated CO2 •− interacts with [Bmim]<sup>+</sup> and forms a [Bmim-CO2]ad complex intermediate [45,46,68–70], in which the cation of [Bmim]<sup>+</sup> plays the role of stabilizing the radical CO2 •−, and in consequence, reduces the required activation energy for the overall reduction of CO2 [7,31,47]. The formed [Bmim-CO2]ad intermediate is further combined with another electron and two protons H<sup>+</sup> to ultimately produce CO. It should be noted here that the H<sup>+</sup> was supplied from the anodic electrolyte (sulfuric acid) and passed through the Nafion N-117 membrane to reach the cathode cell, to participate into the CO2 reduction reaction. At last, the generated CO diffuses into the solution and overflows the liquid surface.

**Figure 7.** Schematic diagram of electroreduction of CO2 in [Bmim]BF4/PC solution with Ag as working electrode.
