Carbon-based Porous Materials For Continuously High-efficiency Interfacial Solar Desalination

With the fast development of modern society and the rapid increase of the world's population,the shortage of freshwater resources has become one of the most serious global challenges in the 21st century.In today's society,more than three billion people are seriously affected by the shortage of freshwater.Interfacial solar water evaporation,as a green and sustainable solar desalination technology,is an important strategy to solve this problem.However,the existing systems are always accompanied by salt deposition in the evaporation process,which will weaken the light absorption capacity of the system,clog the water transportation channels and reduce continuous evaporation efficiency.Therefore,the reasonable design and performance improvement of the interfacial desalination materials are great challenges to realize continuously high-efficiency solar desalination.Herein,on the basis of carbon-based materials with wide spectrum absorption,strong photothermal conversion and low thermal conductivity,multi-stage porous structure and spatial dimensions are synergized to enhance the light absorption capacity,promote water transportation,avoid salt deposition,gradually improve evaporation efficiency and ensure sustainability.This thesis provides novel ideas for promoting the practical application and industrialization of interfacial solar desalination technology.The main contents are as follows:(1)Self-floating biomass porous carbon was prepared by the direct combustion of low-cost and large-scale Elymus stalks.It has the plant natural characteristics of the low density,multistage microporous structure and superhydrophilicity.It can meet the evaporation conditions of the interface,effectively capture the sunlight,provide sufficient water supply,and show good salt rejection ability.Under one solar(1 k W m^-2)irradiation,the water evaporation rate and the solar energy conversion efficiency were 1.41 kg m^-2 h^-1 and 89.9%,respectively.The result is at the leading level in the field of interfacial solar desalination.Continuously high-efficiency solar desalination has been achieved in simulated seawater,even in the high-salinity brine.(2)In order to further improve the light absorption ability,a self-floating CNT-PVDF-PVP porous nanofiber mat was prepared using CNTs with stronger light absorption ability as the photothermal material,PVDF as the support material,and PVP as the pore forming agent.The exposure of hydrophilic CNTs on the nanofiber surface and the three-dimensional network porous structure of the nanofiber mat ensure efficient light absorption,provide water transport channel and avoid salt deposition.Under one solar irradiation,the water evaporation rate and the solar energy conversion efficiency were respectively increased to 1.45 kg m^-2 h^-1 and 91.1%,which remained long-term stable in simulated seawater,strong acid,strong alkali and high salt water.(3)In order to further improve the utilization of sunlight and break through the upper limit of light absorption of two-dimensional nanofiber membrane,self-floating CNT-PVDF porous microspheres were prepared.The two-dimensional interface is upgraded to three-dimensional space,which enhances the light absorption capacity and expands the effective evaporation area.The water evaporation rate and solar energy conversion efficiency were increased to 1.50 kg m^-2 h^-1 and 94.2%,respectively.The excellent salt rejection ability of the porous microspheres enables the continuously high-efficiency solar desalination.In order to further optimize the pore structure,self-floating CNT-PVDF-PVP porous microspheres were prepared by introducing PVP as pore forming agent.The microspheres not only inherit the dimensional advantages of three-dimensional space,but also improve the three-dimensional interconnection of multistage pore structure.It leads to increase the exposure of hydrophilic CNTs,further improve the light absorption capacity,ensure sufficient water supply,avoid salt deposition,and realize continuously high-efficiency solar desalination.Under one solar irradiation,the water evaporation rate reached up to 1.67 kg m^-2 h^-1,and the solar energy conversion efficiency reached up to 104.8%,breaking through the theoretical upper limit of the solar energy conversion efficiency.