From Fungus to Water: A Low-Cost Biocompatible Composite Film for Solar-Driven Atmospheric Water Harvesting

Abstract Water scarcity poses a growing threat to sustainable development and human well-being. Despite a substantial amount of freshwater being stored in the atmosphere, this resource remains largely unexploited. Atmospheric water harvesting (AWH), especially using hygroscopic materials, represents a promising solution for retrieving atmospheric moisture. However, existing materials often exhibit limited biocompatibility and high production costs, whereas biocompatible alternatives require external heating, have low solar-to-water efficiency, and exhibit slow sorption–desorption rates without an additional photothermal layer. In this study, an inexpensive, biocompatible, and easily fabricated hygroscopic composite film was developed by functionalizing naturally available black wood ear (Auricularia auricula-judae/black fungus) with lithium chloride (LiCl) for passive AWH under sunlight. The porous structure and inherent dark pigmentation of wood ear (WE) make it an excellent water reservoir and photothermal scaffold for efficient AWH. The WE–LiCl composite film demonstrates substantial moisture uptake across a wide range of relative humidity (RH) (0.76 g g–1 at 25% RH, 1.57 g g–1 at 50% RH, and 2.39 g g–1 at 70% RH, 23 °C, 28 h) and effective release of moisture at low temperatures (83.5% of sorbed moisture at 40 °C and 94.9% at 50 °C within 12 h) potentially due to the presence of natural melanin in its backbone. The film also exhibits high cycling efficiency, maintaining stable moisture sorption performance over repeated cycles. Under ambient conditions (40 ± 13% relative humidity and 945 ± 95 W m–2 solar flux), the film collected 0.305 ± 0.0927 g g–1 day–1 of water using sunlight. These findings demonstrate the potential for cyclic atmospheric moisture harvesting and indicate the film’s potential applicability for low-cost AWH in remote environments.