Form-stable cellulose nanofiber/alginate double-layer aerogel prepared by freeze-assisted structuring and ambient-pressure final drying for solar desalination

Sorghum distillers' grains (SDG) are abundant by-product of sorghum liquor production but are often underutilized. In this study, we report a form-stable, carbohydrate-based double-layer aerogel (DLA) fabricated via directional freezing followed by ambient-pressure drying, which avoids energy-intensive supercritical freeze-drying routes. Cellulose nanofibers (CNF) extracted from SDG via TEMPO-mediated oxidation were combined with sodium alginate to form a Ca²⁺-crosslinked skeleton, and an rGO/g-C₃N₄ photothermal layer was integrated on top to construct the DLA evaporator. By tuning the rGO/g-C₃N₄ ratio, A₃ (0.6/0.4, w /w) exhibited markedly enhanced dimensional stability during cycling, showing only 3.2% projected-area shrinkage after 10 days, compared with severe shrinkage in the control samples. Meanwhile, A₃ maintained efficient solar evaporation with an evaporation rate of 1.43 kg·m⁻²·h⁻¹ and a photothermal conversion efficiency of 93.14% under simulated irradiation. Outdoor tests further confirmed robust freshwater production across seasons (maximum 2.74 kg·m⁻²·h⁻¹), and the collected condensate satisfied WHO ion-concentration guidelines. This work demonstrates an ambient-pressure-drying-enabled, form-stable CNF aerogel platform for scalable solar desalination and SDG valorization.