Understanding and predicting CO2 capacity loss in activated carbons under humid conditions

This study demonstrates that water isotherm measurements can reliably predict moisture tolerance in activated carbons (ACs), reducing the need for extensive CO 2 isotherm testing under varying humidity and guiding the design of hydrophobic, high-capacity adsorbents for post-combustion and direct-air capture. We investigate how water vapor adsorption influences CO 2 capture by combining systematic AC synthesis, detailed textural and chemical characterization, and mechanistic modeling of water isotherms. ACs were produced from wood charcoal via mechanochemical KOH activation at different ratios (AC1.5–AC4), yielding materials with increasing surface areas (≈1700–2900 m 2 g −1 ) and pore volumes (0.74–1.21 cm 3 g −1 ), spanning ultramicroporous to supermicroporous/mesoporous structures. Two commercial high-surface ACs were also included in this study. Their water vapor isotherms showed S-shaped cooperative adsorption and were accurately described by the Do and Do (DD) model. For the first time, DD parameters were linked to AC physicochemical properties: initial water affinity scales with surface heteroatom density (N + O per surface area), while pore-filling parameters correlate with pore volume and micropore width. Under humidity, ACs exhibited decreasing CO 2 capacity, and we show that a 50% capacity loss can be predicted directly from the pure water isotherm.