Electrochemical CO2 reduction (CO2RR) is one of the most promising methods for decreasing the concentration of CO2, meanwhile, converting them into the high value-added chemicals, which has been more and more investigated and developed recently. Imidazolium ionic liquids (ILs) have been widely used as electrolytes in CO2RR and shown satisfactory performance. While the function of ILs is still unclear. Besides, the economic feasibility and potential of CO2RR with ILs-based electrolytes as well as the environmental effects are also unclear. Therefore, this work focuses on the technology evaluation and exploration for CO2RR-to-CO with ILs-based electrolyte.

Firstly, a literature review about CO2RR to CO, CH4, CH3OH, and syngas (H2/CO=1:1 and 1:2) in ILs-based electrolytes was conducted. Then the processes to obtain these C1-products were analyzed from both economic and environmental aspects based on the state-of-the-art technology and the rationally hypothetical future cases. The results show that CO is the most valuable product considering both the economic benefits and environmental impact, which will be more lucrative in the future with the improvement of CO2RR performance. 

Then, based on 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), a series of imidazolium ILs with various proton in the group (-CH3, -CH3OH and -SH, noted as [BMMIM][PF6], [BMOHIM][PF6] and [BMSHIM][PF6], respectively) at C2 site of the imidazole ring were synthesized and used as electrolyte to perform CO2RR over a commercial Ag foil. As a result, the more inert the active proton, the more favorable for the production of CO. Notably, nearly 100% CO was obtained when [BMMIM][PF6] with the most inert proton. This confirms that the C2-H of the imidazole ring has an important influence on CO2RR performance and may be involved in the reaction.

Finally, [BMMIM][PF6] and [BMIM][PF6] were selected as the electrolytes to conduct CO2RR over a bimetallic catalyst. As a result, the product from 99.69% HCOOH switched into 98.85% CO only via changing the electrolyte from [BMIM][PF6] into [BMMIM][PF6]. Mechanistic studies reveal that the CO2 adsorption configuration on the surface of the catalyst was altered when switching to another IL with a different CO2 active site, resulting in two distinct pathways for the generation of HCOOH and CO, respectively.