Fabrication And Properties Of Three-dimensional Porous Salt-blocking Seawater Evaporation Material

Interfacial solar evaporators generate steam below the boiling point of water by concentrating the solar energy resulted from photothermal conversion at the air/water interface on the solar absorber.Currently,most of the seawater evaporation materials are mainly hydrophilic or hydrophobic,which might cause salt deposition on the material surface during long-term evaporation.Moreover,rich water entering the solar absorption layer might cause heating of bulk water,resulting in energy loss and decreasing evaporation rate.Additionally,long-time exposure of solar evaporation materials to rain flushing,dust accumulation would damage the microstructure of the materials,thereby causing attenuated photothermal performance and decreased life span.This research aims to fabricate superhydrophilic/superhydrophobic Janus interfacial seawater evaporation material with wetting gradient by using carbon nanotubes(CNTs)as photothermal conversion materials,polyvinyl alcohol(PVA)as hydrophilic material,and threedimensional(3D)porous polyurethane(PU)foam as substrate via spraying and impregnation.Utilizing the asymmetric wettability gradient of the evaporation material to control the water transport rate,the amount of water entering into the solar absorption layer was decreased with the benefit of decreasing heat loss,increasing evaporation rate and reducing salt deposition.The superhydrophobic self-cleaning property prevents the seawater evaporation material from contamination,which is favorable for maintaining well the photothermal conversion performance for long-term seawater evaporation.The main contents are as follows:(1)Hydrophilic/superhydrophobic Janus 3D porous seawater desalination interfacial evaporator with salt resistance performance was prepared by firstly dipping PU foam in PVA solution for modification,followed by using the carbon nanotubes/polydimethylsiloxane(CNTs/PDMS)dispersion to treat the upper surface of the modified PU foam.The surface wettability was measured by a video optical contact angle measuring instrument(OCA).The surface morphology of the evaporator was observed by scanning electron microscopy(SEM).UV-Vis-Nearinfrared spectrophotometer(UV-Vis-NIR)was used to measure the reflectance and transmittance of the evaporator surface.A self-made device was built to evaluate the evaporation performance and salt-blocking performance of the evaporator.The influence of self-cleaning property of the photothermal conversion performance of the evaporator was investigated.The results showed that the evaporator exhibited superhydrophobicity of the upper surface with a water contact angle of 165.9°and hydrophilicity of the bottom surface with contact angle of 44°,which demonstrating an asymmetric wetting gradient.The surface temperature of the evaporator reached 59.0℃ under 1 kW/m2 illumination of simulated sunlight for 5 minutes,and the evaporation rate of the evaporator was 1.42 kg·m-2·h-1.The salt resistance test showed that only a small amount of salt crystals appeared on the surface of the evaporated material in the high-salinity salt solution(20 wt%NaCl solution)after evaporation for 6 h under 1 kW/m2 simulated sunlight illumination.The superhydrophobic self-cleaning performance ensures that the photothermal conversion performance of seawater evaporation materials were not affected during the long-term seawater evaporation process.(2)A 3D porous salt-blocking interfacial evaporator with a wettability gradient of superhydrophobic-hydrophobic-hydrophilic-superhydrophilic was fabricated by spraying the CNTs/PDMS composite dispersion onto the upper surface of PU foam and using the PVA solution to treat the bottom surface of PU foam,finally placing the PVA treatment surface of the modified foam in the PVA solution.The influences of CNTs,PDMS and PVA dosage on the surface morphology,wettability,optical properties and surface temperature of the evaporator were investigated.The effects of salinity on the stability of evaporation rate and salt resistance were investigated.It was found that the evaporator got a high contact angle of 175.0° at the upper surface and contact angle of 0° at the bottom.Moreover,the solar reflectance and transmittance of the evaporator was 1.0%and 0.85%,respectively,The surface temperature of the evaporator reached 71.4℃ when exposed to 1 kW/m2 simulated sunlight for 5 minutes,and the evaporation rate of the evaporator was 2.26 kg·m-2·h-1.The salt resistance test showed that there is no salt deposition on the surface of the evaporator after 6 h evaporation test,and only slight appears at the adge.In summary,the project used commercial hydrophobic PU foam as the porous skeleton for water transmission.Nanostructure CNTs and low surface energy PDMS were used to combine with porous strcutres of the PU foam to endow the upper surface with superhydrophobicity and obtain a photothermal conversion layer.The hydrophilicity of PVA was used to form excellent water transmission channels at the bottom and inside of the material.The evaporation rate was greatly increased and the salt deposition was reduced by designing an asymmetric wettability gradient in the evaporator,which can control the water delivery rate and steam evaporation rate with an equilibrium.The superhydrophobic selfcleaning property keeps the seawater evaporation material away from pollution,which ensure the photothermal conversion performance of the evaporation material in long-term application,and prolong their lifespan.which lay a foundation of fabricating long-lasting seawater evaporation material.