Sustainable sponge-like composite hydrogel evaporator for highly efficient solar steam generation

Solar steam generation (SSG) using a hydrogel-type solar evaporator has emerged as one of the most expedient approaches for producing clean water to address the issue of water scarcity at a global level. In this work, we developed a stable two-dimensional (2D) spongy hybrid hydrogel using partially hydrolyzed polyvinyl alcohol (PVA) and δ-MnO2 nanosheet (a solar absorber) in a simple and cost-effective method. Water-uptake capillaries in the 2D hydrogel have been effectively modified by optimizing the volume of the cross-linker, which in turn facilitates a balanced water transport speed and the heat localization ability of the solar absorber and hence the evaporation rate. Simultaneously, the 2D planar hydrogel is transformed into a three-dimensional (3D) spiral and cylinder-shaped evaporator, and its evaporation efficiency was tested employing a water contactless double-layer interfacial evaporating system. A prototype SSG system showed an evaporation rate of 1.56 kg/m2/h, 3.33 kg/m2/h, and 3.63 kg/m2/h corresponding to a solar to vapor conversion efficiency of 78%, 166%, and 181% respectively, for a 2D planar, 3D spiral and cylinder-shaped evaporator under 1 sun. The surface temperature was always below the environmental temperature during the illumination period, which helped the 3D systems absorb energy from the environment. This led to cold vaporization with zero energy loss and thus enhanced the efficiency of the 3D configurations compared to 2D. Furthermore, the designed double-layer evaporator maintained consistent long-term performance with antifouling and salt rejection properties and ensured clean water production from sewage and seawater.