Designing Moisture-Swing CO2 Sorbents through Anion Screening of Polymeric Ionic Liquids

Polymeric ionic liquids (PILs) are promising CO2 sorbents, as their behaviors are tunable by assembling ion pairs. This work aims to design CO2 sorbents with unique moisture-swing adsorption performance by assembling different anions on quaternary-ammonium-based PILs. Two aspects of the sorbent design were studied: the suitability of the CO2 affinity for different applications (e.g., direct air capture or flue gas capture) and capability for moisture-swing adsorption. Carbonate, fluoride, and acetate were chosen as counteranions, as they are representative anions with different basicity, valence, and water affinity. CO2 affinity was found to positively correlate with the pKa value of the counteranion, except for fluoride, which has an intrinsic character of attracting protons. The moisture swing capacity is determined by the difference between the hydration energies of the reactant and product after CO2 adsorption and followed the order carbonate > fluoride > acetate. Further investigations revealed that the repulsion between the two quaternary ammonium cations could promote the dissociation of hydrated water, which results in the lowest activation energy for CO2 adsorption for the PIL with carbonate. Therefore, the PIL with carbonate is potentially a desirable candidate for air capture and moisture-swing regeneration, while the PIL with acetate is suitable for CO2 capture under high partial pressure and regeneration through conventional approaches. This study provides a quantitative microscopic insight into the role of the anion in CO2 adsorption and paves the way toward the optimal PIL structure for CO2 capture under specific circumstances.