Preparation Of Three-dimensional Carbonbased Photothermal Aerogel And Its Photothermal Performance

With the rapid growth of population and environmental pollution,especially water pollution,global fresh water resources are becoming one of the severe challenges facing mankind.In recent years,the water evaporation technology driven by solar thermal conversion can not only relieve the pressure on energy and the environment but also greatly contribute to the urgent problem of shortage of freshwater resources.With the development of nanotechnology,more and more functional materials such as metal nanomaterials,semiconductor materials,and carbon materials have been found to have excellent light absorption and photothermal conversion properties.Among them,carbon-based materials are regarded as one of the most promising photothermal materials due to their advantages of high light absorption rate and high stability.In addition,the structural design of the photothermal converter and the use of interface evaporation,especially the utilize of some porous structures of the material can significantly increase the light absorption area,can greatly improve the efficiency of the photothermal conversion.Therefore,in order to obtain high-efficiency solar thermal water evaporation materials,this paper prepared graphene-based three-dimensional photothermal aerogels and Janus structure carbon nanotubes/polyvinyl alcohol（CNT/PVA）porous composites by freeze-drying.And the relevant photothermal conversion properties were studied.The main research contents and results are as follows:（1）Carboxylated multi-walled carbon nanotubes（MWCNT-COOH）were used as photothermal materials and polyvinyl alcohol as the framework.After mixing,porous composites with wide wavelength absorption,high conversion efficiency,and Janus structure were obtained through gel sol and freeze-drying methods.In the evaporator,the anisotropic wettability of the hydrophobic PDMS and hydrophilic PVA porous composite makes it a good evaporator,which can maintain a stable water evaporation rate in the seawater desalination experiment,and in the longterm test process there is no salt precipitation on the surface shows that the salt rejection rate after desalination can reach 99.86%.In the test,the evaporator floats on the water-air interface,which greatly reduces the heat loss to the water body and improves the photothermal conversion efficiency.Under one sunlight intensity（1k W m-2）,the evaporation rate can reach 1.34 kg m-2 h-1,and the photothermal conversion efficiency can reach 85.71%.In addition,the organic content and heavy metal ion concentration in the lake water and wastewater after purification treatment are greatly reduced,with the removal rate reaching 93.14% and 96.81%,respectively.The cycle stability test and outdoor evaporation experiment have also verified its photothermal performance in practical applications.In conclusion,the porous solar evaporator with Janus structure and good water channels have high efficiency of water evaporation and salt tolerance.（2）Considering that the interface evaporation method floating on the water still has a large contact area with the water body,which is likely to cause more heat loss,based on the above work,the evaporation system structure design that isolates the evaporation layer from the bottom water body can ensure Stabilize the evaporation of water while reducing the heat transferred to the water body.Using graphene oxide nanosheets and sodium alginate as raw materials,a flexible graphene oxide framework（GOF）is formed through covalent cross-linking and hydrogen bonding under mild conditions.After simple reduction,a new type of flexible r GO-SA photothermal aerogel with a three-dimensional interconnected porous network structure is obtained.The aerogel has good mechanical properties,excellent broadband light absorption（95.87%）,low thermal conductivity（0.049 W m-2 K-1）,and a high salt removal rate（99.28%）.Combining with the structure optimal design,the water evaporation rate of the aerogel can reach 1.86 kg m-2h-1 under one solar irradiation,which has great potential in application fields such as seawater desalination.