A cost-effective, salt-resistant and environmentally stable solar evaporator with a wetting-gradient bilayer structure for long-term seawater desalination

Solar-powered water evaporation, as an environmentally friendly method, offers a way of alleviating the global shortage of clean water. However, salt accumulation due to rapid seawater evaporation, poor environmental stability, and high preparation costs have greatly limited the promotion of its application in practice. In this study, novel PDMAA+CNTs/P(AM-co-AA) (PCs) hydrogels with wetting-gradient bilayer structure are prepared by a two-step in-situ polymerization method, which are further used for the development of highly efficient solar-powered desalination devices. The relatively hydrophobic top layer is made of PDMAA+CNT composite hydrogel with highly efficient broadband solar energy absorption and high photothermal conversion efficiency. The hydrophilic bottom layer is designed with P(AM-co-AA) hydrogel, which have excellent mechanical properties and an interpenetrating porous structure that could rapidly replenish water by capillary action and accelerate the rate of water transfer. The evaporation rate is as high as 2.11 kg·m^-2·h^-1 under 1 sun irradiation, and the photothermal conversion efficiency could be up to 92.22%. After 72 h of continuous evaporation of a 15 wt% salt solution under 2 solar irradiations, the PCs evaporator demonstrate stable photothermal performance and excellent salt stability. In addition, this novel PCs evaporator exhibit outstanding durability and environmental stability that kept its initial water transport capacity even after being treated under harsh conditions for 30 days, providing an attractive platform for cost-effective ($9.18 m^-2 of total materials cost), salt-resistant, environmentally stable and sustainable solar-driven water management.