Gradient Heating Effect Modulated by Hydrophobic/Hydrophilic Carbon Nanotube Network Structures for Ultrafast Solar Steam Generation

The emerging interface solar-thermal water evaporation has been widely studied to solve fresh water shortage because of its high solar-thermal conversion efficiency, environmental friendliness, and low cost. However, traditional water evaporation systems inevitably lose heat to the environment, which not only greatly affects the water evaporation rate but also hinders their practical applications. In this work, an interface solar-thermal water evaporation system with enhanced heat localization, which is combined by a hydrophobic carbon nanotube (CNT) film (heating layer) and hydrophilic polyvinyl alcohol (PVA)/CNT foam (evaporating layer), is demonstrated. Under solar irradiation, the temperature of the hydrophobic CNT film is higher than that of the hydrophilic PVA/CNT foam due to the differences in wettability, so the thermal energy in the CNT film can be continuously transferred to the PVA/CNT foam evaporator, forming a gradient heating effect and greatly increasing the water evaporation rate. As a result, the water evaporation rate can reach 4.2 kg m^-2 h^-1 under a solar illumination of 1 kW m^-2, which is among the highest water evaporation rate levels. More importantly, this water evaporation system structure is simple, can be easily scaled up, and has gradient applicability to other photothermal materials, which provides a route to improve the interfacial solar steam evaporation rate.