A heterogeneous nanocomposite architecture with contrasting thermal conductivity and hydrophilicity for synergistic solar-thermal storage and evaporation

Solar-driven evaporation is an eco-friendly and cost-effective freshwater production technique. It is essential to maintain continuous evaporation under intermittent sunlight for practical application. Integrating solar-thermal storage with evaporation is a promising solution. However, existing designs struggle to balance high evaporation rates with effective thermal energy storage in a single device due to conflicting thermal conductivity and hydrophilicity requirements for the two functions. Here, we develop a heterogeneous 3D graphene architecture featuring a hydrophilic gradient hydrogel evaporator (GHE) encircled by a hydrophobic thermal storage composite (TSC). The thermally conductive and hydrophobic TSC made from 3D graphene and paraffin wax enhances solar-thermal conversion and storage, while the thermally insulative and hydrophilic GHE featuring radiating channels with gradient pores facilitates efficient heat localization and water transport. This structurally and compositionally separated design leverages contrasting thermal and hydrophilic properties, achieving a high evaporation rate of 3.6 kg m^-2 h^-1 under direct sunlight and extending the evaporation at a rate of 2.7 kg m^-2 h^-1 for 30 minutes even when sunlight dims. The integrated device produces twice as much water as the hydrogel evaporator alone under intermittent lighting. This work presents an effective strategy for extending water generation capabilities under intermittent sunlight.