HUMIDITY-PROGRAMMED CO2/H2O CO-ADSORPTION IN FLUOROALKYL-SHIELDED AMINE COVALENT ORGANIC FRAMEWORKS FOR DIRECT AIR CAPTURE

Objective: Here we develop a design-and-analysis framework for fluoroalkyl-shielded amine COFs (denoted F-COF-NH₂) in which perfluorobutyl (–C₄F₉) groups tune the local hydrophobicity around amine sites. Method: A humidity-programmed CO₂/H₂O co-adsorption protocol at DAC-relevant conditions (400 ppm CO₂, 25 °C), coupled to adsorption microcalorimetry and operando spectroscopy, is used to map the enthalpic signatures of carbamate–bicarbonate switching and water-cluster nucleation. Results:
A medium fluoroalkyl density (F-COF-NH₂-M) yields an optimum humidity window near 40–50% relative humidity (RH) with a humidity enhancement factor of ≈2.1, the lowest hysteresis index of the series, and 94.8% CO₂-capacity retention over 100 cycles, whereas the unshielded framework is suppressed above ~50% RH by uncontrolled hydration. Novelty: We formulate a controlled-hydration design rule for humidity-tolerant DAC sorbents.