Modeling and Experimental Analysis of pH Swing-Based Approaches to Electrochemical CO2 Capture

Abstract

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/molCO2 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/molCO2 will be discussed, as well as our efforts toward experimental demonstration of this concept. References Renfrew, S. E.; Starr, D. E.; Strasser, P., Electrochemical Approaches toward CO2 Capture and Concentration. ACS Catalysis 2020, 10 (21), 13058-13074.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.