**4. Conclusions**

In summary, the ILs/PC mixture solution was constructed and investigated as the electrolyte for electrochemical reduction of CO2 on the Ag plate electrode. The investigation on the alkyl length of imidazole-based ILs indicates that the IL of [Bmim]BF4 gives the lower onset potential and Tafel slope, as well as higher exchange current density, compared to other studied ILs and the traditional [Bu4N]BF4 salt. The EIS characterization further indicated that the imidazolium cation could absorb on the electrode surface and reduce the overpotential through complexing with anion radical (CO2 •−) and stabilizing them. The [Bmim]BF4 IL gives the highest performance for the conversion of CO2. The performance tests show that in [Bmim]BF4/PC electrolyte solution, FE for CO as high as 98.5% and current density of 8.2 mA/cm2 can be achieved at <sup>−</sup>1.9 V (vs. Fc/Fc+).

**Author Contributions:** Conceptualization, F.J. and W.L.; Funding acquisition, W.L.; Synthesis, characterization, performance test and writing—original draft preparation, F.J. and J.Z.; Formal analysis and writing—review and editing, F.J., W.L. and J.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China, grant number 21673067.

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

#### **References**


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