| Currently,the earth is facing a severe shortage of fresh water resources,and thus it is of great importance to convert seawater into fresh water.Solar-driven interfacial evaporation,a new technology for water purification,can transform light into thermal by the porous photothermal materials to efficiently heat the interfacial water and generate water steam for desalination.However,most porous photothermal materials have an excessively fast water supply rate during evaporation,and the excess water will fill the internal pores,reducing the evaporation area,causing energy loss,and lowering the interfacial evaporation rate.Furthermore,the high cost and tedious preparation process limit the development and application of interfacial evaporation technology.In this study,to solve the abovementioned problems,low-cost and versatile tannic acid（TA）-based photothermal coating and commercial sponge were utilized to design highly efficient photothermal evaporators.By adjusting the surface properties of the photothermal coating and changing the preparation method,the current problems of excessive internal water supply rate and tedious,time-consuming preparation process of porous photothermal materials are solved.The following are the main innovative points and works in this paper.Firstly,the concept of confinement capillary is proposed to significantly enhance the solar-driven interfacial evaporation.Currently,porous photothermal materials have the problem of excessive water supply during evaporation,and the evaporation area is limited to the outer surface of the material,which greatly limits the evaporation rate.Forming ultrathin water layers on the inner and outer surfaces of porous photothermal materials to form abundant water-air interfaces is an effective way to greatly improve the solar-driven water evaporation rate,but it is still a huge challenge.Herein,this study proposes the confinement capillary,which can form ultra-thin water on the inner and outer surfaces of the porous photothermal sponge,significantly improving the evaporation area and the water evaporation rate.Confinement capillary action is achieved by a thin photothermal hydrophilic coating on the sponge skeleton.Based on tannic acid,a large number of black hydrophilic nanoparticles（BHNPs）are aggregated on the sponge skeleton and formed nanochannels thin coating,generating strong capillary forces for water transport.By cooperatively controlling the water supply rate at the bottom of the photothermal material,water can be filled only in the pores of the accumulation of photothermal spheres on the surface of the sponge skeleton,rather than in the large pores of the porous sponge.Therefore,without precise water control,an ultra-thin water layer of about 2-5 microns is formed on the inner and outer surfaces of the porous sponge.The ultra-thin water layer not only increases the evaporation surface and generates more water vapor escape channels,but also prevents the heat loss for heating water.Thus,the water evaporation rate is greatly improved.And the maximum evaporation rate can reach 3.2 kg m-2 h-1 at one solar light intensity with the introduction of confinement capillarity,which is about 2.2 times higher than that of the sponge without the introduction of confinement capillarity(1.45 kg m-2 h-1).The above-mentioned confined capillary action can greatly increase the evaporation rate of the photothermal sponge,and it has a better effect when treating seawater with lower concentrations in a discontinuous state,but it is prone to salt formation on the surface when it continuously treats seawater for a long time.Compared with other commonly used photothermal materials,porous hydrogel photothermal materials have been shown to have higher water evaporation rates,but their interconnected porous structures require a time-consuming and energy-intensive freeze-drying process or sacrificial template method to prepare it.It is unsuitable for the creation and use of porous photothermal hydrogels.Inspired by porous sponge organisms in the ocean,the second work of this study developed a new strategy for efficient preparation of“porous photothermal hydrogel sponges”.The hydrogel sponge is prepared by in-situ growth of a thin layer of photothermal hydrogel on the surface of the polyurethane（PU）sponge skeleton,and the preparation process does not require freeze-drying.The sponge skeleton can not only give ideal mechanical stability to the evaporator,but also provide connecting channels for water transport.The tannin-based black hydrogel coated on the surface of the sponge skeleton can achieve high-efficiency photothermal conversion,and its thickness can be controlled to retain high porosity for water transport.The water evaporation rate of the prepared hydrogel evaporator under one sun can reach 2.8 kg m-2 h-1,which has the advantages of high cost-effectiveness,long-term cycle stability,and large-scale preparation. |
