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 mol_CO2⁻¹ 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 mol_CO2⁻¹. This result suggests that, in the presence of a 0.465 bar/1.0 bar inlet/outlet pressure ratio, a 1.9 kJ mol_CO2⁻¹ thermodynamic penalty should add to the measured value, yielding an energy cost of 48.9 kJ mol_CO2⁻¹ in the low-current-density limit. This result is within a factor of two of the ideal cycle work of 34 kJ mol_CO2⁻¹ 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.