Preparation Of Graphene Photothermal Materials And Their Application In Seawater Desalination

Solar desalination technology is one of the most promising technologies to use sustainable solar energy in solving the shortage of fresh water.Aerogels,especially three-dimensional graphene based aerogels,show unique advantages in solar water evaporation due to their high porosity,light weight,adjustable light absorbance and low thermal conductivity.However,the preparation conditions of three dimensional（3D）graphene based aerogels are complex,especially for higher drying requirements,such as freeze-drying and supercritical drying,resulting in high cost of preparation,which limits the large-scale application.In this work,atmospheric drying instead of freeze-drying and supercritical drying was adopt by optimizing the material preparation process,so as to reduce the preparation cost of materials.Atmospheric drying technology is operated under environmental pressure without complicated and expensive equipment conditions.It has low energy consumption,simple operation and short production cycle.It is a more promising method to prepare graphene based aerogels.Graphene aerogels（GA）were prepared by one-step freezing and two step hydrothermal reduction under ambient pressure.The effects of the concentration of graphene oxide,the ratio of height to diameter of aerogel materials and p H on the formation of aerogels and the rate of evaporation of solar water were explored.The light absorbance,internal pore structure,surface functional groups and elemental composition were characterized and analyzed.The water evaporation properties of the materials were studied with deionized water,artificial seawater and natural seawater as the evaporating objects,and the comprehensive evaluation of the photothermal conversion properties of aerogels was carried out.On the basis of GA,in order to enhance the absorbance of the material,an ultra black material carbon nanotube was added to prepare graphene composite aerogel（GA-CNT）.The average absorbance of the prepared aerogel in the full spectrum（250-2500nm）range from 82%to 94%.The solar thermal evaporation experiment was carried out under the simulated sunlight of xenon lamp in the laboratory.An infrared camera was used to monitor the surface temperature of the material in real time,and the temperature before and after water evaporation is measured by thermocouple.Under the sunlight intensity of 1.6 k W.m^-2,the evaporation rate of pure graphene aerogel（GA）in deionized water,natural seawater and artificial seawater is 2.4551 kg.m^-2h^-1,2.1666 kg.m^-2h^-1 and 2.1243 kg.m^-2h^-1,respectively,and the corresponding photothermal evaporatiom efficiency is 94.38%,82.09%,80.29%.The evaporation rate of GA-CNT in natural seawater is 2.294 kg.m^-2h^-1.The photothermal conversion efficiency of GA-CNT is 87.52%.The addition of carbon nanotubes improves the photothermal evaporation efficiency of the material.Finally,the performance of the material was evaluated.After a photothermal experiment in natural seawater,the GA material was soaked and cleaned in natural seawater.It was found that the pores of the material were intact after cleaning,and there was no obvious salt blockage.Only a small amount of crystalline salt was found on the graphene layer.At the same time,the material was placed in natural seawater for ten times,and the evaporation efficiency decreased by only 15%.In the process of seawater desalination with GA-CNT,the removal rate of cations after desalination is more than 99.8%.