Mechanistic Study of the Electrochemical Reduction of CO₂ in Aprotic Ionic Liquid in Air

The capture and electrochemical conversion of dilute CO₂ in air is a promising approach to mitigate global warming. Aiming to increase the efficiency of the electrochemical reduction of CO₂, we fabricated electrodes and developed a custom-designed sealed electrochemical reaction system to study the mechanism of this conversion. The performance of three metal electrodes, Ag, Cu, and SUS 316 L, was compared in an aprotic ionic liquid as the electrolyte to monitor the CO₂ concentration and chemical reactions using a CO₂ sensor and diffuse reflectance infrared Fourier transform spectroscopy and Raman spectroscopy in CO₂/N₂ (400 ppm CO₂ and 99.96 % N₂) or synthetic air (400 ppm CO₂, 21 % O₂, and 79 % N₂). The CO₂ concentration decreased at negative potentials and was more drastic in synthetic air than in CO₂/N₂. At negative potential in synthetic air, IR revealed carbon monoxide, carbonate, or peroxydicarbonate on the Ag, Cu, or SUS 316L electrodes, respectively. Reaction intermediates were identified using Raman spectroscopy. Superoxide (O₂⋅^-), produced by the reduction of O₂ on each electrode, promotes the electrochemical reduction of CO₂ whose reduction potential is higher on the negative side than that of O₂. This research deepens our understanding of the electrochemical capture/release and conversion of dilute CO₂.