Oxygen Stable Electrochemical CO2 Capture and Concentration through 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 at 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 a loss in CO2 binding. To overcome these limitations, we used common alcohols 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 to the dianion and CO2-bound forms of TCQ were correlated to alcohol pKa 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 for eCCC redox carriers. We demonstrate 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 quinones/alcohol combinations for specific CO2 capture applications.