Electrochemically Induced CO2 Capture Enabled by Aqueous Quinone Flow Chemistry

Electrochemically driven CO2 capture processes utilizing redox-active organics in aqueous flow chemistry show promise for nonflammability, continuous-flow engineering and the possibility of being driven at a high current density by inexpensive, clean electricity. We show that deprotonated hydroquinone–CO2 adducts, whose insolubility limits the utility of the quinone–hydroquinone redox couple, become soluble when alkylammonium cations are introduced. Consequently, we introduced alkylammonium groups to anthraquinone via covalent bonds, making the resulting bis3-(trimethylammonio)propylanthraquinones (BTMAPAQs) soluble. We report the first aqueous quinone flow chemistry-enabled electrochemical CO2 capture/release process, which occurs at ambient temperature and pressure, and show that it proceeds via both pH-swing and nucleophilicity-swing mechanisms. 1,5-BTMAPAQ reaches the theoretical capture capacity of two CO2 molecules per quinone from 1-bar CO2–N2 mixtures, for which the CO2 partial pressure is as low as 0.05 bar, or the applied current density is as high as 100 mA/cm2, or the organic concentration is as high as 0.4 M. The energetic cost ranges from 48 to 140 kJ/mol CO2. In a crude simulated flue gas composed of 3% O2, 10% CO2, and 87% N2, 1,5-BTMAPAQ electrolyte reversibly captured and released 50% of the theoretical capacity during an exposure of over 4 h. It outperforms its isomeric counterparts 1,4-, and 1,8-BTMAPAQ in capture capacity and O2 tolerance, demonstrating a substituent position effect on the reactivity of isomers with CO2 and O2. The results provide fundamental insight into electrochemical CO2 capture with aqueous quinone flow chemistry and suggest that the oxygen tolerance of reduced quinones may be significantly advanced through molecular engineering.