(401m) Towards a Versatile Direct Air Capture Process: Carbon Recovery Performance of Supported Ionic Liquid Membranes under various Humidity Conditions

Direct Air Capture (DAC) has emerged as a promising negative emissions technology, offering a direct approach to removing CO₂ from the atmosphere. However, the performance of liquid-based DAC systems is strongly influenced by ambient humidity, which varies widely across regions. In particular, managing water balance in aqueous absorbent systems remains a key challenge. Under low-humidity conditions, water readily evaporates from the absorbent, concentrating the solution, increasing viscosity, and imposing additional pumping load, while also raising the operational cost of water replenishment in arid environments. Conversely, under high-humidity conditions, moisture ingress from the air can dilute the absorbent, reducing its CO₂ capture capacity. Therefore, maintaining the optimal water balance in the absorbent is essential for stable and efficient DAC operation across diverse humidity conditions. Membrane-based DAC (m-DAC) systems have attracted increasing attention for their potential advantages in energy efficiency, scalability, and design flexibility [1]. Recent studies have also explored the use of supported ionic liquid membranes (SILMs) in DAC-related gas separation applications, showing promise in terms of chemical stability and selectivity [2]. However, their ability to regulate vapor–liquid equilibrium—specifically, to suppress solvent evaporation and moisture ingress while maintaining CO₂ permeability—under realistic ambient humidity conditions remains insufficiently characterized. This study aims to bridge that gap and demonstrate the potential of SILM-based architectures toward a versatile DAC process adaptable to diverse climates.

In this study, we conducted CO₂ absorption experiments from atmosphere under various humidity conditions using supported ionic liquid membranes (SILMs) and aqueous potassium hydroxide (KOH) as the absorbent. KOH solution was selected due to its non-volatility, high CO₂ affinity, and resistance to chemical degradation under ambient conditions. The membrane used was a hydrophobic SILM supported on a porous polymer substrate. To examine the membrane's tolerance to atmospheric humidity, we compared its performance against a hydrophilic SILM and a blank (support-only) membrane. Importantly, all membranes used the same support material PTFE to isolate the effects of the liquid phase and surface characteristics. To minimize the impact of external fluctuations, we measured the ambient CO₂ concentration for each run and corrected the observed absorption performance based on those values [3].

Consequently, the raw CO₂ absorption performance exhibited no noticeable trends with respect to type of ionic liquids, making it difficult to interpret the effects of humidity and ionic liquid type. To eliminate the influence of ambient CO₂ concentration fluctuations, the absorption rates were normalized by the inlet CO₂ concentration measured during each experiment. After applying this correction, the differences in CO₂ absorption performance among membranes with different humidity levels and ionic liquids largely disappeared. These results indicate that, under the conditions tested, membrane hydrophobicity and atmospheric humidity were not dominant factors affecting CO₂ absorption performance, and that proper correction for environmental variables is essential for fair membrane evaluation. We attribute the observed uniformity in CO₂ absorption performance to the use of aqueous KOH, which functioned as a chemically stable, non-volatile absorbent that enabled clear comparison across membrane types and humidity conditions. Its insensitivity to water vapor dilution made it particularly well-suited for isolating membrane-specific effects.

The fact that CO₂ absorption performance remained consistent across different humidity levels and membrane types suggests that these SILMs can offer reliable operation even in challenging ambient conditions by suppressing solvent evaporation while maintaining CO₂ capture performance. Also, these findings highlight the importance of carefully selecting test conditions when evaluating membrane performance and suggest that, under corrected and controlled environments, the role of membrane hydrophobicity may be less significant than previously assumed. Ultimately, this work contributes to the development of DAC systems that are robust to environmental variability, expanding their potential applicability to a broader range of climates.