Electrochemically induced CO2 capture enabled by aqueous quinone flow chemistry

Climate change caused by the accumulation of anthropogenic CO2 emissions motivates the development and deployment of cost-effective, scalable, and energetically efficient techniques to capture CO2 from point or diffuse sources. 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 high current density by inexpensive, clean electricity. We show that the deprotonated hydroquinone-CO2 adducts, whose insolubility limits the utility of the quinone-hydroquinone redox couple, become soluble when alkylammonium cations are introduced. Consequently, we introduce alkylammonium groups to anthraquinone via covalent bonds, making the resulting bis[3-(trimethylammonio)propyl]-anthraquinones (BTMAPAQs) soluble. We report the first aqueous quinone flow chemistry-enabled electrochemical CO2 capture process, which occurs at ambient temperature and pressure, and show that it proceeds via both a pH-swing and a nucleophilicity-swing mechanism. 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/molCO2. 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 hr. 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 oxygen tolerance of reduced quinones may be significantly advanced through molecular engineering.