Quinone Reduction in Ionic Liquids for Electrochemical CO2 Separation

We report the redox activity of quinone materials, in the presence of ionic liquids, with the ability to bind reversibly to CO2. The reduction potential at which 1,4-naphthoquinone transforms to the quinone dianion depends on the strength of the hydrogen-bonding characteristics of the ionic liquid solvent; under CO2, this transformation occurs at much lower potentials than in a CO2-inert environment. In the absence of CO2, two consecutive reduction steps are required to form first the radical anion and then the dianion, but with the quinones considered here, a single two-electron wave reduction with simultaneous binding of CO2 occurs. In particular, the 1,4-napthoquinone and 1-ethyl-3-methylimidazolium tricyanomethanide, [emim][tcm], system reported here shows a higher quinone solubility (0.6 and 1.9 mol·L–1 at 22 and 60 °C, respectively) compared to other ionic liquids and most common solvents. The high polarity determined through the Kamlet–Taft parameters for [emim][tcm] explains the measured solubility of quinone. The achieved high quinone solubility enables effective CO2 separation from the dilute gas mixture that is contact with the cathode by overcoming back-diffusive transport of CO2 from the anodic side.