Carbon Nanotube Network Structures For High-efficiency Photothermal Water Evaporation

With the rapid increase of population and industrial pollution,the shortage of fresh water resources has gradually become one of the most serious problems in the world.In recent years,the solar-thermal steam generation has been widely studied because of its advantages of high photothermal conversion efficiency,environmental friendliness,and no pollution.The photothermal materials and the efficient water evaporation structures are the core of this field.In addition,how to improve the salt resistance and achieve multifunctional integration are also the recent research focus.In this paper,based on the control of the microstructure and surface modification of carbon nanotube(CNT)network,a strategy of temperature gradient enhancement constructed by hydrophilic and hydrophobic CNT network is proposed to realize efficient solar steam generation.In addition,inspired by the principle of water transport in plants,the bionic design and large-area braided manufacturing technology of flexible solar thermal water evaporation structure are also proposed.Through the integration with the thermoelectric(TE)module,the functional integration of solar thermal steam generation and TE power generation is realized.The main research results of this paper are as follows.1.A simple and rapid deposition amorphous carbon chemical vapor deposition method was developed to prepare elastic CNT network.This method can be used to controllable carbon modification inside the CNT network.By optimizing the parameters of deposition process,the microstructure,density,mechanics and pore size of the elastic CNT network could be effectively controlled.The oleophobic and hydrophobic CNT networks were obtained by chemical fluorination.The hydrophilic CNT network was obtained by modification of polyvinyl alcohol.2.Based on the different hydrophilic and hydrophobic properties of CNT networks,a gradient heating effect with additional energy was developed to improve the water evaporation rate.A heat collecting zone was constructed using a hydrophobic CNT film around the hydrophilic carbon nanotube network.Under solar irradiation,due to differences in wettability,the formation of a temperature gradient between the outer heater and the intermediate water evaporator can input additional heat energy to the central water evaporation zone.Thus,the evaporation rate of water is greatly increased.The results show that the evaporation rate of water in the CNT network enhanced by temperature gradient heating can reach as high as 4.2 kgm^-2h^-1 under 1 sun.Additionally,the outdoor testing confirms the feasibility of the structure in large-area integration and fresh water collection.3.Inspired by the principle of water transport in plants,a bionic photothermal water evaporator with high environmental adaptability was designed.A flexible bionic tree-like solar-thermal water evaporator was constructed by weaving a porous CNT film and a hydrophilic cotton fabric together,in which the hydrophilic cotton fabric was used as the root structure for water transport,and the CNT film was used for solar absorption and water evaporation.The unique water-transport root design also enables it to work in various complex environments such as wet sand and soil.After the evaporation of high-concentration brine for a long time,salt particles are precipitated on the surface of the evaporator.After being placed in the dark for a period of time,the precipitated salt dissolves again,featuring excellent self-cleaning performance.The braided method not only makes it easy to achieve large-scale industrial manufacturing,but also makes its structure with excellent flexibility,causing that it can effectively remove pollutants by washing.This kind of flexible solar thermal water evaporation structure can be achieved by low-cost manufacturing with long service life,thus displaying a broad application prospect.4.Besides seawater desalination,the photothermal interface water evaporation also contains a lot of other abundant energy such as temperature-difference heat energy,which can be utilized.Herein,TE as power generation module and paraffin as thermal storage module are introduced into the structure of solar thermal water evaporation with enhanced temperature gradient heating.This design increases the water evaporation rate from 4.2 to 5.0 kgm^-2h^-1,and most importantly,gives the system a temperature-difference power generation function.The power density generation reaches 0.4 Wm^-2,realizing the integration of water evaporation and power generation.Notably,the phase-changing induce heat storage function of paraffin enables the system to maintain a certain amount of water evaporation and power generation function for some time after the light source is turned off.