Activation Temperature-Dependent Dynamic Water Vapor Sorption in Chestnut Shell-Derived Carbons

Water vapor sorption in porous activated carbons (PACs) is governed by a complex interplay of pore architecture and surface functionality and often exhibits pronounced adsorption–desorption hysteresis. In this work, chestnut-shell-derived carbons were synthesized via a two-step thermal route—pyrolysis at 550 °C for 120 min followed by KOH activation at either 600 °C or 800 °C for 240 min—and evaluated using a dynamic vapor sorption analyzer to quantify water uptake, hysteresis, and temperature-dependent energetics. Both materials exhibit sigmoidal Type V isotherms, characteristic of cooperative water clustering on hydrophobic carbon surfaces with localized polar sites. At 25 °C, The PAC sample prepared at 800 °C shows a sharper uptake transition and higher total capacity (~0.45 g/g at 90% RH), compared to the broader, more gradual isotherm of the 600 °C sample (~0.17 g/g). Temperature-dependent isotherms collected between 25 °C and 45 °C were fit using the Dubinin–Serpinsky (DS-4) model, yielding good agreement (R2 ≈ 0.997) and enabling mechanistic interpretation of primary site adsorption and cooperative cluster growth. Clausius–Clapeyron analysis of ln P versus 1/T at fixed loadings yielded isosteric heats of adsorption (ΔH) decreasing from approximately 45.4 kJ mol−1 at low uptake (0.02 g g−1) to ~43.8 kJ mol−1 at intermediate loading, followed by a slight increase to ~44.2 kJ mol−1 at higher coverage (0.35 g g−1). This trend reflects the transition from strong adsorption at high-energy surface sites to cooperative water clustering and confinement effects within the pore network. These findings highlight the role of activation temperature in modulating sorption mechanisms and energetics, offering practical guidance for tuning biomass-derived carbons for atmospheric water harvesting applications.