(439e) Energetic Benefits and Kinetic Drawbacks of Simultaneous Electrochemical CO2 Capture Sorbent Regeneration and CO2 Absorption

Electrochemical alternatives to the temperature-swing CO₂ capture process have gained attention in recent years due to their potentially lower energy demands. Many of these alternatives use a sorbent whose affinity for CO₂ is tied to the electrochemical potential of the system. As a result, electrochemical cycling between oxidizing and reducing conditions will alternate the system between conditions that capture CO₂ and those that regenerate the sorbent. Experimental studies have generally operated in a 4-stage configuration: (1) electrochemical sorbent regeneration, (2) CO₂ capture, (3) electrochemical sorbent inactivation, and (4) CO₂ release. However, multiple computational process intensification studies have predicted that the energy demands can be lowered by using a 2-stage configuration in which the gas transfer stages are combined with their immediately preceding electrochemical stage. A key issue, however, with using a 2-stage configuration is that the rate of CO₂ transport across the vapor-liquid interface may not be fast enough for the solution to reach equilibrium with the gas stream instantaneously. This means that the 2-stage configuration may only be realized at unreasonably low current densities.

Here, we assessed the impact of the number of stages on the idealized energy demands and absorber process times using our previously constructed model of electrochemical CO₂ capture using pH swings driven by proton-coupled electron transfers. The model takes inputs of solution chemistry properties, such as the sorbent pK_a values, to generalize results to the set of possible solution compositions beyond what have been measured experimentally. For this study, the ideal energy demand was defined as the thermodynamic lower limit assuming all charge is transferred at the Nernst potential, while the idealized absorber process time was defined as the minimum process time for CO₂ absorption based on the van Krevelen and Hoftijzer film model for reaction-enhanced gas absorption. In the 4-stage configuration, the flux from this model was combined with the maximum amount of CO₂ captured to obtain an estimate for the process time. In the 2-stage configuration, this flux was converted into a current density using the ratio of CO₂ captured per Coulomb of charge transferred and subsequently converted to a process time using the total charge transferred.

We used an adaptive sampling method to determine the Pareto frontiers describing the trade-offs between low energy demand and low process times because lower energy conditions often required longer process times and vice versa. Our calculations of the Pareto frontiers of these configurations indicated that while changing from a 4-stage to a 2-stage configuration caused the minimum energy demand to decrease by about 30%, the CO₂ capture rate decreases sufficiently such that the absorber residence time would need to be approximately one order of magnitude larger. This is in large part because bounding the CO₂ fugacities of the combined reduction-absorption stage leads to smaller CO₂ concentration gradients and thus slower fluxes, which in turn require lower current densities to be current-limited. Looking at the size of the domain of solution chemistries that can achieve both lower residence times and lower energies than temperature-swing benchmark estimates, the 4-stage configuration is more forgiving: 40% of possible solution chemistries predicted better performance for both metrics in the 4-stage configuration, but this reduces to only 13% for the 2-stage configuration. This is in large part because high concentrations of sorbent require more Coulombs of charge, leading to longer absorber residence times in the 2-stage configuration and causing an upper bound concentration that the 4-stage configuration does not have. Overall, we find that for electrochemical CO₂ capture, the energy benefit associated with combining the gas transfer and electrochemical stages is small compared to the drawbacks associated with the slower rate, and as a result, better energetics should not be the sole reason for combining the electrochemical sorbent regeneration with CO₂ absorption.