Effect of hydrothermal and deep eutectic solvent solvothermal pretreatments on the structural evolution of...

Agricultural residues are promising precursors for sustainable porous carbons, but their activation efficiency strongly depends on precursor chemistry. Here, corncob-derived activated carbons were prepared through three routes: direct KOH activation, hydrothermal carbonization followed by KOH activation, and acidic deep eutectic solvent (DES) solvothermal carbonization followed by KOH activation. The acidic choline chloride/phosphoric acid DES pretreatment substantially transformed the lignocellulosic precursor by promoting dehydration, deoxygenation, aromatic condensation, mineral removal, and precursor stabilization. Compared with direct activation and hydrothermal pretreatment, DES pretreatment produced a more carbon-rich and thermally stable precursor, enabling more effective KOH activation and ultramicropore development. The resulting DES-derived activated carbon showed the highest BET surface area of 3084 m² g^-1, total pore volume of 1.35 cm³ g^-1, and micropore volume of 1.08 cm³ g^-1. This structure delivered 3.08 wt% H2 uptake at 77 K and 1 bar, along with CO₂ adsorption capacities of 8.98, 7.27, and 5.74 mmol g^-1 at 273, 288, and 298 K, respectively. CO₂ isotherm modeling, isosteric heat analysis, and adsorption-desorption cycling confirmed heterogeneous ultramicropore-filling physisorption with favorable reversibility. These results demonstrate that acidic DES-assisted precursor engineering is an effective strategy for converting corncob waste into high-performance activated carbon for hydrogen storage and carbon capture.