Oxygen-Stable Electrochemical CO₂ Capture and Concentration with Quinones Using Alcohol Additives

Current methods for CO₂ capture and concentration (CCC) are energy intensive due to their reliance on thermal cycles, which are intrinsically Carnot limited in efficiency. In contrast, electrochemically driven CCC (eCCC) can operate with much higher theoretical efficiencies. However, most reported systems are sensitive to O₂, precluding their practical use. In order to achieve O₂-stable eCCC, we pursued the development of molecular redox carriers with reduction potentials positive of the O₂/O₂^- redox couple. Prior efforts to chemically modify redox carriers to operate at milder potentials resulted in diminished CO₂ binding. To overcome these limitations, we used common alcohol additives to anodically shift the reduction potential of a quinone redox carrier, 2,3,5,6-tetrachloro-p-benzoquinone (TCQ), by up to 350 mV, conferring O₂ stability. Intermolecular hydrogen-bonding interactions with the dianion and CO₂-bound forms of TCQ were correlated to alcohol pK_a to identify ethanol as the optimal additive, as it imparts beneficial changes to both the reduction potential and CO₂-binding constant, the two key properties of eCCC redox carriers. We demonstrated a full cycle of eCCC in aerobic simulated flue gas using TCQ and ethanol, two commercially available compounds. Based on the system properties, an estimated minimum of 21 kJ/mol is required to concentrate CO₂ from 10 to 100% or twice as efficient as state-of-the-art thermal amine capture systems and other reported redox carrier-based systems. Furthermore, this approach of using hydrogen-bond donor additives is general and can be used to tailor the redox properties of other quinone/alcohol combinations for specific CO₂-capture applications.