Sandwich-structured wettability foam for highly efficient, cost-effective, salt-resistant, and durable solar desalination

Solar desalination via interfacial solar-driven vapor generation is an eco-friendly method for purifying seawater and wastewater into freshwater. In this study, we designed a polypyrrole (PPy)-coated sandwich wettability structure to achieve highly efficient, cost-effective, salt-resistant, and durable solar desalination. The structure consists of a top hydrophobic layer coated with the photothermal polymer PPy, a hydrophilic melamine foam (MF) interlayer with high porosity and mechanical strength, and a bottom layer with variable hydrophobicity. The PPy polymer efficiently absorbs broadband solar energy and retains heat in situ, while the strategically designed hydrophobic top layer, combined with a hydrophilic middle layer, effectively mitigates salt accumulation. The cost-effective MF network contains interconnected microporous channels that facilitate water supply, form confined water clusters, and reduce evaporation enthalpy. The tunable hydrophobicity of the bottom layer in the central region controls the water transport rate, balancing water supply and evaporation. Meanwhile, the hydrophobic bottom layer at the edges provides self-floatability, minimizing heat loss and enhancing solar vapor conversion efficiency. This sandwich wettability structure achieved an exceptional evaporation rate of 2.71 kg m² h⁻¹ under 1 sun irradiation. Moreover, it consistently desalinated and purified brine and seawater under natural outdoor lighting conditions. The proposed solar desalinator, featuring a simple, low-cost, and long-lasting design with effective salt rejection, represents a promising technology for highly efficient freshwater production, leveraging sustainable energy to help address the global freshwater crisis.