PH Swing Cycle for CO2 Capture Electrochemically Driven through Proton-Coupled Electron Transfer

Abstract

We propose and perform a thermodynamic analysis of the energetic costs of CO2 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 CO2 is absorbed; subsequent electrochemical oxidation of the reduced molecules results in the acidification of the solution, triggering the release of pure CO2 gas. We examined the effect of buffering from the CO2-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 CO2 capture-release cycle. The thermodynamic minimum work input varies from 16 to 75 kJ/molCO2 as throughput increases, for both flue gas and direct air capture, with the potential to go substantially lower if CO2 capture or release is performed simultaneously with electrochemical reduction or oxidation. These values are compared with those for other separation methods. We also discuss the properties required of molecules that would be suitable for such a cycle, and preliminary experimental data from a cycle run using one such molecule.