*2.2. Linear Sweep Voltammetry Measurements*

Linear sweep voltammograms (LSV) curves are recorded to study the effect of the alkyl chain length of the imidazolium cations on the electroreduction of CO2, and the results are shown in Figure 2. For comparison, the LSV curve of [Bu4N]BF4 with the same anion BF4 − as catholyte is also recorded. In agreement with previous reports [49], the onset potential for CO2 reduction was defined as the potential that results in a current density of 0.6 mA/cm2. As shown in Figure 2, though all CV curves show similar profiles, the onset potentials shift anodically when ILs replace [Bu4N]BF4 as supporting electrolytes, indicating that ILs play a promoting role in the process of CO2 electrochemical reduction. Furthermore, it can be observed that the onset potentials change with the alkyl chain length of imidazolium cations. Among these five studied ILs, the [Bmim]BF4 gives the most positive onset potential of <sup>−</sup>1.702 V (vs. Fc/Fc+), which shifted positively by 133 mV compared with that in [Bu4N]BF4/PC solution (−1.835 V (vs. Fc/Fc+)).

**Figure 2.** Linear sweep voltammograms (LSV) curves in CO2-saturated propylene carbonate (PC) solution of ionic liquids (ILs) and [Bu4N]BF4.
