We propose and
perform a thermodynamic analysis of the energetic costs of CO<sub>2</sub>
separation from flue gas using a pH swing created by electrochemical
redox reactions involving
proton-coupled electron transfer from molecular species in aqueous
electrolyte. Electrochemical reduction of these molecules results in the
formation of alkaline solution, into which CO<sub>2</sub> is absorbed;
subsequent electrochemical oxidation of the reduced molecules results in the
acidification of the solution, triggering the release of pure CO<sub>2</sub>
gas. We examined the effect of buffering from the CO<sub>2</sub>-carbonate
system on the solution pH during this pH swing cycle, and thus on the
open-circuit potential of a hypothetical electrochemical cell in a 4-step CO<sub>2</sub>
capture-release cycle. The thermodynamic minimum work input varies from 16 to
75 kJ/mol<sub>CO2 </sub>as throughput increases, for both flue gas and direct
air capture, with the potential to go substantially lower if CO<sub>2</sub>
capture or release is performed simultaneously with electrochemical reduction
or oxidation. These values are compared with those for other separation methods. We discuss the properties
required of molecules that would be suitable for such a cycle.
Standard Electrode Potentials for the Reduction of CO2 to CO in Acetonitrile–Water Mixtures Determined Using a Generalized Method for Proton-Coupled Electron-Transfer Reactions