Preparation And Performance Research Of Porous Sponge-based Solar Interface Evaporation Materials

With the rapid growth of population,the rapid development of industrialization and the aggravation of water pollution,the shortage of fresh water resources has become one of the great challengesfacing mankind.Although 70%of the earth is covered by water,97.5%of that is seawater and brackish water,which cannot be used directly.In recent years,solar desalination technology has attracted extensive attention of researchers.Compared with the traditional desalination methods,solar desalination technology has the advantages of environmental protection,energy saving and low cost,which is a promising desalination technology.Therefore,two kinds of low-cost,stable and efficient solar desalination evaporation materials were prepared by spraying method.The evaporation performance and salt resistance of the two evaporation materials were systematically studied.The main research contents and results are as follows:Firstly,carbon nanotubes（CNTs）after acid modification were dispered into ethanol/n-hexane solution containing polydimethylsiloxane for preparingthe suspension,then it was sprayed onto the top surface of the melamine sponge（MS）to obtain CNTs-MS evaporation material.The study found that the evaporation materialhas excellent light absorption capacity and light-to-heat conversion performance,absorption of sunlight in the range of 200-2500 nm is as high as 98%.The evaporation rate is 1.44kg m^-2h^-11 and the evaporation efficiency is 85%under 1sun.Moreover,the average water evaporation rate is 1.44kg m^-2h^-11 in 30 evaporation experiments,indicating that the evaporation performance of the system is stable.This is mainly because of stable photothermal conversion of CNTs,consecutive water supply of porous structure of MS and the adheresion between CNTs and MS.Moreover,the evaporation rate and photothermal conversion efficiency are 1.40 kg m^-22 h^-11 and 80%in 3.5 wt%NaCl solution for 30 days evaporation test,respectively.Only a spot of salt precipitated on the top surface of photothermal conversion coating,and then the system was immersed into water to remove precipitated salt.The evaporation material remianed stable evaporation rate and photothermal conversion efficiency.This is because the precipitated salt could be dissolved due to hydrophilic and macroporous 3D network of MS.Additionally,the system has excellent mechanical properties and chemical stability.No collapse obviously occurred in microstrcture of the evaporation material after 60%compressive strain for 300 cycles.Moreover,no changes occurred in appearance of the evaporation material after immersing into strong acid（2 mol/L H2SO4）,strong base（saturated NaOH）and high concentration salt（10 wt%NaCl soultion）for 30 days.Secondly,to further enhance the evaporation rate and salt-resistance stabiliy of the system,silicone porous structure prepared by sol-gel method was constructured on the MS skeleton,forming composite sponge material with multi-level porous structure（SMS）.Then,the above CNTs suspension was sprayed onto the top surface of the complex sponge to obtain CNTs-SMS evaporation material.The outside surface of the evaporation systerm was changed from superhydrophobic to superhydrophlic via O2-plasma treatment.Afterwards,solar evaporative performance test of CNTs-SMS evaporation material with O2-plasma treatment for 150 s,the evaporation rate is 1.53kg m-2h-1 and the evaporation efficiency is 90%under 1 sun.Moreover,the average evaporation rate remain 1.53 kg m-2 h-1 during 30 days tests.The evaporation rate for3.5 wt%NaCl solution is 1.53 kg m^-22 h^-1,and photothermal conversion efficiency is90%.No salt was precipitated on the surface of the system.This is due to fast water supply from superhydrophilic outside surface and salt exchange in the porous structure.In addition,the salt could not proliferate into the inner of the systerm because of superhydrophobic inner,enhancing the salt-resistance of the system.Moreover,the system has good chemical stability.No change occurred in the appearance of the system after immersing into strong acid（2 mol/L H2SO4）,strong base（saturated NaOH）and high concentration salt（10 wt%NaCl soultion）for 30days,and the evaporation performance is still stable.The CNTs-SMS evaporation material with O2-plasma treatment for 360 s was tested for solar water desalination.The evaporation rate of the CNTs-SMS evaporation material in 3.5 wt%NaCl solution was 1.70 kg m-2 h-1,and the evaporation efficiency was 96%.In the 10 solar seawater desalination and evaporation experiments,the average evaporation rate was1.70 kg m^-22 h^-1,and during the 12 h continuous solar water desalination process,there was no salt precipitation on the surface of the CNTs-SMS evaporation material.In addition,the salt resistance of the CNTs-SMS evaporation material was tested in 7wt%,10 wt%,15 wt%,and 20 wt%NaCl solutions,and there was no salt precipitation on the surface.The research of the paper successfully solves the problems of high cost,complex technology,poor stability and salt tolerance of most of the evaporation materials,and provides a new idea for the preparation of solar desalination devices.