High-Capacity, Reversible, and Energy-Efficient Water Vapor Sorption by Hierarchical Micro-to-Macroporous Carbon Aerogels from Polybenzoxazine and Polybenzodiazine

Quantitative, reversible water vapor sorption is in rising demand for applications ranging from home appliances to atmospheric water harvesting. However, most sorbents suffer from low water-uptake capacities, nonreusability, and especially high regeneration temperatures. This study addresses this challenge by introducing polybenzoxazine- (PBO) and polybenzodiazine-derived (PBDAZ) carbon aerogels as reversible high-capacity desiccants. PBO and PBDAZ aerogels were prepared from structurally related monomers via HCl-catalyzed ring opening polymerization. Both types of aerogels were first aromatized at 200-240 °C under air or O₂ and then were carbonized at 800 °C under Ar. These as-prepared carbon aerogels were further etched at 1000 °C under flowing CO₂. Both as-prepared and CO₂-etched carbon aerogels were characterized with CHN elemental analysis, XPS, and gas (N₂ and CO₂) sorption porosimetry. Their water-uptake capacity was assessed at 273, 298, and 313 K. The long-term performance and cycling stability of these aerogels were studied by switching their surrounding environment between a highly humid and a dry atmosphere (99% and 10% relative humidity, respectively), staying for 24 h in each environment. No performance deterioration was detected after 50 full cycles (100 days). Carbon aerogels from both PBDAZ and PBO showed significant water-uptake capacities (43% and 42% w/w, respectively). However, CO₂-etched PBDAZ- and PBO-derived carbon aerogels showed among the highest water uptake capacities reported in the literature, reaching 117% w/w and 140% w/w at 298 K, respectively (all values at 298 K). Water uptake started with hydrogen bonding to the O and N lining the concave surfaces of the pores and continued until micropores and mesopores were filled with water. Adsorbed water was released quantitatively at room temperature just by reducing the relative humidity of the environment.