Same materials, bigger output: A reversibly transformable 2D–3D photothermal evaporator for highly efficient solar steam generation

Using solar-thermal energy to drive seawater desalination via interfacial solar steam generation is a sustainable strategy for clean water supply. Since photothermal materials and evaporators serve as key platforms for efficient light-to-heat conversion and water evaporation during solar steam generation, the rational design of the structure of photothermal materials and evaporators is important. In this work, a reduced graphene oxide and cellulose sponge-based transformable photothermal evaporator which can reversibly switch between a 2D flat and a 3D spiral structure is designed. Although the mass and volume of the materials are identical, simple structural transformation from a 2D to a 3D evaporator significantly enhances the evaporation rate due to an increase in the evaporation surface area and optimized water transportation. In addition, numerical simulations demonstrate that the 3D spiral structure is able to effectively take advantage of convective flow to fully activate and enhance evaporation on its surfaces, resulting in a much higher evaporation rate (up to 4.35 kg m−2 h−1), which is 185.9% of the evaporation rate of the 2D flat structure. Furthermore, the 3D spiral structure can be easily transformed back to the 2D flat structure for easy storage. Thus, this work presents an effective strategy to minimize the use of photothermal materials while simultaneously achieving higher evaporation rates for practical clean water production.