Elucidation of the Roles of Ionic Liquid in CO2 Electrochemical Reduction to Value-Added Chemicals and Fuels

Abstract

The electrochemical reduction of carbon dioxide (CO₂ER) is amongst one the most promising technologies to reduce greenhouse gas emissions since carbon dioxide (CO₂) can be converted to value-added products. Moreover, the possibility of using a renewable source of energy makes this process environmentally compelling. CO₂ER in ionic liquids (ILs) has recently attracted attention due to its unique properties in reducing overpotential and raising faradaic efficiency. The current literature on CO₂ER mainly reports on the effect of structures, physical and chemical interactions, acidity, and the electrode-electrolyte interface region on the reaction mechanism. However, in this work, new insights are presented for the CO₂ER reaction mechanism that are based on the molecular interactions of the ILs and their physicochemical properties. This new insight will open possibilities for the utilization of new types of ionic liquids. Additionally, the roles of anions, cations, and the electrodes in the CO₂ER reactions are also reviewed.

Keywords: CO2 electrochemical reduction; co-catalyst; electrolyte; ionic liquids.

Figure 1

Carbon dioxide reduction cycle using renewable and green source of energy.

Figure 2

Illustration of an electrochemical H-cell for CO₂ reduction.

Figure 3

Illustration of the flow cell for CO₂ reduction. Reprinted with permission from [20]. Copyright 2014, American Chemical Society.

Figure 4

Different electrolyzer configurations for CO₂ERR [18].

Figure 5

Chemical structure of the most common IL cations and anions.

Figure 6

Faradaic efficiencies of CO₂ERR products on different metal electrodes in 0.1 M KHCO₃ solution at 18.15 ± 0.05 °C saturated with carbon dioxide. * The total value contains C₃H₅OH (1.4%), CH₃CHO (1.1%), andC₂H₅CHO (2.3%) in addition to the tabulated substances. The total value contains C₂H₆ (0.2%) [53].

Figure 7

Kinetic volcano at 0.35 V overpotential for CO propagation from the transition metal (211) stage. The transition metals follow a linear trend that does not cross over the peak of the volcano. The noble metals, on the other hand, reach the trend line’s optimum. The specific CO generation current from the ChCODH II and MbCODH enzyme models is comparable to, if not superior to, that of the noble metals. Reprinted with permission from [54]. Copyright 2014, American Chemical Society.

Figure 8

Mechanisms proposed regarding the role of IL as co-catalysts in the CO₂RR. Left: covalent activation. (A) Generation of a covalent bond between CO₂ and the imidazolium IL. (B) Im IL as a hydride/proton donor reducing CO₂. Right: non-covalent activation. (C) Stabilization of CO₂^•− by hydrogen bonding. (D) Stabilization of CO₂⁻ through the adjustment of the local electric field. Reprinted with permission from [104]. Copyright 2014, American Chemical Society.

Figure 9

A schematic of how the free energy of the system changes during the reaction of CO₂ + 2H⁺ + 2e⁻ → CO + H₂O in water or acetonitrile (solid line) or in the IL [Emim][BF₄] (dashed line) [45].

Figure 10

Reaction pathways for the electrochemical reduction of CO₂ in (A) the absence and (B,C) presence of [Emim][Tf₂N] at a Pb electrode in acetonitrile. Reprinted with permission from [85]. Copyright 2014, American Chemical Society.

Figure 11

The interaction between the active imidazolium protons and CO₂. Reprinted with permission from [87]. Copyright 2014, American Chemical Society.

Figure 12

Pyrazolium cations used in the Vasilyev study [88]. Reprinted with permission from [88]. Copyright 2018, American Chemical Society.

Figure 13

The CV values to illustrate the differences in the stability of several substituted Pz cations under Ar (Ag polished disk electrode, 0.1 M NBu₄PF₆ in dry acetonitrile, and 0.02 M IL additive). Reprinted with permission from [88]. Copyright 2018 Chemical Society.

Figure 14

Qualitative molecular orbital diagram of carbon [16,120,121]. Reprinted with permission from [16]. Copyright 2018, American Chemical Society.