The use of renewable electricity to drive large-scale electrochemical CO₂ removal and concentration from point sources, air, and seawater is receiving considerable interest as one strategy for mitigating climate change.¹ The low concentration of CO₂ in air however makes rapid and energy-efficient CO₂ capture challenging. Understanding and optimizing the energetic cost of CO₂ separation at practically reasonable throughputs is a prerequisite for engineering devices that can be widely deployed.

I will review our past and current efforts at developing electrochemical pH-swing-based CO₂ separation (EPCS) using a combination of modeling and experiments. In EPCS, CO₂ is captured from a mixture of gases into an aqueous electrolyte in the form of (bi)carbonate ions when its pH is increased from acidic to strongly alkaline conditions, and then released as a pure gas when the pH is reversed. We have shown that CO₂ separation is experimentally possible for less than 100 kJ/mol_CO2 using such a pH swing cycle that is driven by proton-coupled electron transfer (PCET).² Thermodynamic modeling shows that the minimum work input for electrochemical CO₂ separation is the sum of exergy losses incurred from differences in CO₂ partial pressure between the CO₂ source/exit streams and the electrolyte. This framework rationalizes minimum work inputs for pH-swing and redox-mediator-based CO₂ separation cycles, and motivates the measurement or estimation of the aforementioned CO₂ partial pressures in future experimental studies.

More recently, we have begun to consider a new design for EPCS in which PCET-active species are conformally attached to the internal surface area of a highly porous electrode, and effect a reversible pH swing in an adjacent layer of aqueous electrolyte upon redox cycling. We have formulated a continuum reaction-transport model that simulates reactive CO₂ capture into and release from the electrolyte layer, as well as transport of (bi)carbonate species, and protons, in response to an applied current or voltage. Possible pathways toward achieving an energetic cost of direct air capture of CO₂ less than 100 kJ/mol_CO2 will be discussed, as well as our efforts toward experimental demonstration of this concept.

References

1. Renfrew, S. E.; Starr, D. E.; Strasser, P., Electrochemical Approaches toward CO2 Capture and Concentration. ACS Catalysis 2020, 10 (21), 13058-13074.
2. Jin, S.; Wu, M.; Gordon, R. G.; Aziz, M. J.; Kwabi, D. G., pH swing cycle for CO2 capture electrochemically driven through proton-coupled electron transfer. Energy & Environmental Science 2020.