Integrating Latent Heat Storage and Photothermal Conversion in Waste Polyurethane Sponges for All-Weather Solar Water Purification

Solar-driven evaporation is a promising green technology for freshwater production, yet its practicality is hampered by the intermittent nature of sunlight. Integrating phase-change thermal storage with photothermal systems presents a viable strategy to regulate energy flow and achieve efficient, all-weather evaporation. A multifunctional composite material was designed by integrating efficient photothermal conversion with latent heat storage. Using a waste polyurethane (PU) sponge as the skeleton, a synergistic photothermal layer of polydopamine (PDA) and carbon nanotubes (CNTs) was constructed on its surface via in situ polymerization. Subsequently, the phase change material polyethylene glycol (PEG) was encapsulated into the modified porous network using a melt impregnation method. This design aims to synchronously achieve efficient light absorption, optimized thermal management, and enhanced water transport. The results indicate that successful encapsulation of PEG endowed the PU/PDA/CNTs/PEG composite with a high phase change enthalpy (130 J/g) and excellent photothermal capability, providing a basis for thermal management. A surface temperature of 78 °C was achieved under 1 sun irradiation. Thanks to the thermal energy storage capability of PEG and its improvement in water transport, the PU/PDA/CNTs/PEG achieved a high evaporation rate of 2.4 kg/(m²·h) and a photothermal conversion efficiency exceeding 85% in simulated 1 sun indoor pure water evaporation tests. In a 10-hour outdoor test under real environmental conditions using 3.5 wt% NaCl solution, a stable evaporation rate of 1.4–1.8 kg/(m²·h) was maintained, accompanied by excellent resistance to salt crystallization. Its overall performance was significantly superior to that of the control material without PEG. This study integrates photothermal conversion, thermal energy storage, and interfacial regulation into a single material. Furthermore, the successful fabrication of large-area macroscopic samples demonstrates the scalability of this composite, and its effectiveness in purifying complex natural water bodies, such as river water and municipal reclaimed water, highlights its strong potential for practical, large-scale solar-driven desalination and water purification applications.