Investigating the Influence of Alkali Chloride Salts and Hydration on the Direct Air Capture Capacity of Polyethylenimine Films with Quartz Crystal Microbalance and Infrared Spectroscopy.

With carbon dioxide (CO2) concentrations on the rise, direct air capture (DAC) technologies to remove carbon dioxide from the atmosphere are attracting increased attention. One popular approach is to use aminopolymer sorbents, such as polyethylenei mine (PEI), to absorb the CO2 and capture it through chemical reactions with the amine groups. In practice, the CO2 capture efficiency in aminopolymer sorbents strongly depends on the relative humidity, where the theoretical CO2 capture efficiency can change by 2-fold in going from a dry to a humid environment through the formation of either an ammonium carbamate or carbamic acid capture product, respectively. In this study, we utilize monovalent chloride salts with increasing cation size (LiCl, NaCl, KCl, or CsCl) to modulate water uptake and thus product formation, quantified using both gravimetric and spectroscopic methods. We utilize tandem quartz crystal microbalance with dissipation (QCM-D) and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) to decouple the individual uptake of CO2 and H2O from the total sorption mass. The added salts were found to alter the H2O sorption levels of the PEI films, thereby impacting the CO2 uptake. We complement QCM-D/PM-IRRAS measurements using a high-throughput attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy cell that conditions PEI films under varying relative humidities at a constant CO2 concentration. The carbamate/carbamic acid formation and H2O absorption were quantified by measuring the intensity of their characteristic vibrational signatures, informed by the QCM-D/PM-IRRAS experiments. Overall, we find that the addition of salt additives enables tuning of H2O and CO2 sorption at various humidity levels, with the identity of the salt cation directing the relative component uptake. These observations provide insight into the effectiveness of additives, especially salts, in modulating the CO2 uptake of aminopolymer sorbents being deployed in strategic carbon capture applications.