Passive solar desalination towards high efficiency and salt rejection via a reverse-evaporating water layer of millimetre-scale thickness

Although solar-driven reverse distillation integrated with thermal localization has recently shown attractive solar-to-water conversion efficiency, effective salt rejection/discharge approaches are scarce for achieving sustainable passive solar desalination. Here we elaborately fabricated solar distillation devices based on reverse-evaporating water layers of millimetre-scale thickness and successfully realized simultaneous high efficiency and salt rejection during solar desalination processes. Two passive operation modes (gravity mode and discharge mode) were developed for sustainable salt rejection, which showed solar-to-water conversion efficiencies of 59.1% and 60.6%, respectively, with 3.5 wt% brine. More notably, the device fabricated also showed excellent capacity (47.4% efficiency) to continuously desalt high-salinity (21 wt%) water without salt crystallization. For a wide application level, we discussed and tested ten-stage desalination devices based on reverse-evaporating water layers. A total efficiency of 354% was achieved alongside the success of salt rejection in each stage, indicating a new pathway for passive solar high-efficiency and salt-rejection desalination.