Thickness Dependent CO₂ Adsorption of Poly(ethyleneimine) Thin Films for Direct Air Capture

Mesoporous silica impregnated with polyethyleneimine (PEI) has been shown to be a suitable material for the direct air capture (DAC) of CO₂. Factors such as CO₂ concentration, temperature, and amine loading impact overall capture capacity and amine efficiency by altering diffusional resistance and reaction kinetics. When studied in the impregnated 3-dimensional sorbent material, internal diffusion impacts the evaluation of the reaction kinetics at the air/amine interface. In this work, we designed a novel tandem quartz crystal microbalance with dissipation (QCM-D) and polarization modulation infrared reflective absorption spectroscopy (PM-IRRAS) instrument. CO₂ adsorption kinetics of the PEI-based amine layer in a 2-dimensional geometry were studied at a variety of film thicknesses (10 nm to 100 nm), temperatures (25 °C to 80 °C), and CO₂ concentrations (5 % and 0.04 % by mole fraction). Total CO₂ capture capacity increased with film thickness but decreased amine efficiency, as additional diffusional resistance for thicker films limits access to available amine sites. The capture capacity of thick films (>50 nm) is shown to be limited by amine availability, while capture of thin films (<50 nm) is limited by CO₂ availability. A 50 nm PEI film was shown to be optimal for capture of 0.04 % (400 ppm) CO_2. The adsorption profiles for these conditions were fitted to pseudo-first order and Avrami fractional order models. The reaction process switches between a diffusion limited reaction to a kinetic limited reaction at 80 °C when using 5 % CO₂ and 55 °C when using 0.04 % CO₂. These results offer accurate analysis of adsorption of CO₂ at the air/amine interface of PEI films which can be used for the design of future sorbent materials.