Structural Design And Photothermal Application Of Graphene Aerogel

With the current rapid growth of the population and water pollution problems in the world,mankind is facing a crisis of shortage of fresh water resources.Solar water desalination is one of the effective ways to solve the water crisis.In recent years,researchers have developed a method of solar steam generation by localized heating,which greatly improves the evaporation efficiency.Solar light absorber is the most critical part to this system.This paper introduces graphene aerogel as a carbon material,which has wide and high light absorption,low thermal conductivity,excellent mechanical properties,rich pores,and self-floating properties.Therefore,graphene aerogels are expected to become a promising candidate as solar absorber.However,the inherently hydrophobic nature of graphene aerogels has prevented its application in solar evaporation systems.Although the existing structural design can achieve hydrophilicity,it affects the mechanical properties of the material and is not conducive to practical application.Based on the excellent properties of graphene aerogels,this paper uses the method of nitrogen doping to improve its hydrophilic properties.Nitrogen-doped graphene aerogels were prepared by hydrothermal,freeze-drying and thermal annealing.Hydrophilic properties of graphene aerogels were changing at different annealing temperatures(200-1000oC).The results show that 600oC is a better annealing temperature,and the graphene aerogel achieves both hydrophilic properties and excellent mechanical elasticity.The intrinsic mechanism of nitrogen doping to improve the hydrophilic properties of graphene aerogels was further explored by carefully designed experiments and simulation calculations.The results indicate that nitrogen doping increases the polarity of graphene,which results in active sites,forming hydrogen bonds with water molecules in humid air,and forming a water film on the surface,preventing the adsorption of alkane and thus it results in super hydrophilicity.Nitrogen-doped graphene aerogels were then used for photothermal evaporation.Under one solar light condition(1 k W m^-2),the evaporation rate reaches 1.41 kg m^-2h^-1,with the evaporation efficiency reaching 85.2%.In addition,in this thesis,the light absorption performance of graphene aerogel was further improved by compounding it with molybdenum disulfide.Graphene-based aerogels with high photothermal energy conversion efficiency was prepared and applied to solar photothermal evaporation systems.Due to the synergistic effect of graphene and molybdenum disulfide,the light absorption performance is improved,especially in the near infrared region.The average absorbance(275-2500 nm)is increased from 83.2%to 92.6%.Due to nitrogen doping,Mo S₂/GA also has super wettability after high temperature annealing.Mo S₂/GA aerogel is then applied to photothermal evaporation.Under a solar light condition(1 k W m^-2),the evaporation rate is as high as 1.53 kg m^-2h^-1,and the evaporation efficiency is as high as95.4%.Based on the excellent properties of graphene aerogels,this dissertation designs two new types of graphene aerogels for solar desalination systems in terms of improving their hydrophilicity and light absorption,respectively.Through the above structural design,this paper improves the evaporation efficiency of graphene aerogels in solar evaporation,and explores the performance and underlying mechanism of graphene aerogels as photothermal materials in locally heated solar seawater desalination systems.The basic theoretical results of this thesis will be beneficial to facilitate the application of graphene aerogels in the field of solar evaporation.