pH swing cycle for CO2 capture electrochemically driven through proton-coupled electron transfer

Abstract

We perform a thermodynamic analysis of the energetic cost of CO2 separation from flue gas (0.1 bar CO2(g)) and air (400 ppm CO2) using a pH swing created by electrochemical redox reactions involving proton-coupled electron transfer from molecular species in aqueous electrolyte. In this scheme, 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 the cycle, and thereby on the open-circuit potential of an electrochemical cell in an idealized four-process CO2 capture-release cycle. The minimum work input varies from 16 to 75 kJ molCO2−1 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. We discuss the properties required of molecules that would be suitable for such a cycle. We also demonstrate multiple experimental cycles of an electrochemical CO2 capture and release system using 0.078 M sodium 3,3′-(phenazine-2,3-diylbis(oxy))bis(propane-1-sulfonate) as the proton carrier in an aqueous flow cell. CO2 capture and release are both performed at 0.465 bar at a variety of current densities. When extrapolated to infinitesimal current density we obtain an experimental cycle work of 47.0 kJ molCO2−1. This result suggests that, in the presence of a 0.465 bar/1.0 bar inlet/outlet pressure ratio, a 1.9 kJ molCO2−1 thermodynamic penalty should add to the measured value, yielding an energy cost of 48.9 kJ molCO2−1 in the low-current-density limit. This result is within a factor of two of the ideal cycle work of 34 kJ molCO2−1 for capturing at 0.465 bar and releasing at 1.0 bar. The ideal cycle work and experimental cycle work values are compared with those for other electrochemical and thermal CO2 separation methods.