【作者】 李伟；

【导师】 于中振；

【作者基本信息】 北京化工大学 ， 材料科学与工程， 2022， 博士

【摘要】 淡水资源短缺是全球面临的非常严峻的问题,为了解决这一难题,人们陆续开发了众多的水清洁方法,然而这些方法往往需要电能和化学能的参与,不经济也不环保。太阳能是地球表面最丰富的可再生能源,利用丰富的太阳能进行海水淡化和废水处理对于净化水来说是非常有前景的。传统的太阳能海水淡化方法存在太阳能利用率低、水蒸发速率慢、光热转换效率低等问题。目前人们正在努力探究光热系统背后所隐藏的科学问题,也开发出许多具有高水蒸发速率的有机复合光热材料。对于碳纳米管（CNTs）和石墨烯等纯碳材料而言,它们具有优异的化学和热稳定性,是一种良好的光热转换材料,但由于碳材料自身对水蒸发焓的影响较小,使它们的水蒸发速率依然处于较低水平。因此,针对碳基材料水蒸发速率慢和光热转换效率低的问题,我们直接利用或构筑垂直排列孔道来加快水从底部向蒸发表面的传输,利用含氧官能团来降低水的蒸发焓,并通过动态压缩的方式实现对水能量态的进一步调控,从而使所开发的太阳能驱动水蒸发器呈现出优异的太阳能蒸汽生成性能,可用于海水淡化和废水处理。主要研究内容和结果如下:1、具有垂直排列孔道的木材/氧化铁/碳纳米管复合蒸发器的制备及其水蒸发性能研究:通过电弧放电、化学气相沉积和激光烧蚀等方法制备CNTs通常需要高温、高能量密度或者惰性气体保护等,使CNTs的制备工艺相对复杂。在本章中,我们开发了一种在大气环境中通过一步加热燃烧法快速制备大量CNTs的新型方法。以乙酰丙酮铁为前驱体,将其快速均匀地撒在预热的高温热台上,乙酰丙酮铁会受热分解成铁纳米颗粒和碳原子,分别作为CNTs生长的催化剂和碳源,并按照“尖端生长”机理快速生成大量CNTs。最后,将生成的CNTs负载到具有垂直排列孔道的木材表面来制备具有双层结构的木材/氧化铁/碳纳米管复合蒸发器。CNTs作为光吸收层,可以高效捕获太阳能并将太阳能转换为热能来驱动水蒸发。木材层作为隔热层,由于其具有极低的热导率,可以防止热量向水体的散失,并且木材中的垂直孔道可以减少水的传输路径,从而加快水向蒸发表面的传输。因此,该复合蒸发器可以实现1.42 kg m^-2 h^-1的水蒸发速率以及高达87.2%的光热转换效率。2、具有垂直排列孔道的还原氧化石墨烯/MXene复合水凝胶的制备及其水蒸发性能研究:石墨烯等碳基光热转换材料具有优异的化学和热稳定性,但由于碳材料自身对蒸发焓的影响较小,导致其水蒸发速率远远低于有机光热转换材料的速率。相比于二维石墨烯薄膜,具有三维结构的还原氧化石墨烯（RGO）水凝胶可以促进光的吸收,并利用毛细效应加快水从底部向蒸发表面的传输。在本章中,我们刻意保留RGO网络中的大量含氧官能团,通过使其与水分子形成氢键作用来降低水的蒸发焓,从而提高水蒸发效率。为了进一步加快水在材料内部的传输,我们利用定向冷冻技术在RGO水凝胶中构筑垂直排列孔道来缩短水的传输路径。此外,由于过渡金属碳化物/碳氮化物（MXene）纳米片具有近乎100%的面内光热转换效率,我们通过马兰戈尼效应将其渗透到RGO骨架中来进一步改善光热转化性能,从而在一个太阳光强度下实现高达2.09 kg m^-2 h^-1的水蒸发速率和93.5%的光热转换效率,其性能优于多数无机光热转换材料的性能。3、具有可控压缩性的还原氧化石墨烯泡沫的制备及其压缩过程中水能量态变化研究:克服无机体系中水蒸发速率低的难题依然是一个挑战。目前,大多数研究者主要从水传输速度、光吸收能力和加热方式等方面进行改进,却极少探究光热转换材料自身对水能量态的影响。在本章中,我们首次通过动态压缩的方式对具有垂直排列孔道的还原氧化石墨烯（V-RGO）泡沫中水的能量态进行调节。随着压缩程度的增加,自由水、中间水、结合水的比例含量发生了明显的变化,因此可以利用这一变化实现对水蒸发焓的调控。当压缩到47.1%时,对应的中间水含量最高为42.09%,水的蒸发焓最低为1306 J g^-1,此时最有利于水蒸发。另外,我们还充分利用环境能量来提高水蒸发效率,由于V-RGO泡沫的侧面温度低于环境温度,所以除了太阳光直接辐照输入的能量外,蒸发器的侧面还会通过热对流和热辐射的形式从周围环境中额外吸收能量来促进水蒸发。在降低水的蒸发焓和利用环境能量的协同作用下,V-RGO泡沫实现高达3.34 kg m^-2h^-1的水蒸发速率和104.1%的太阳能蒸汽生成效率,高于大多数碳基光热转换材料的速率。更多还原

【Abstract】 The shortage of fresh water resource is a very serious problem in the world and many water cleaning methods have been developed to solve this dilemma.However,these methods often require the involvement of electric energy and chemical energy,which is neither economical nor environmentally friendly.Solar energy is the most abundant renewable energy on the earth and the use of abundant solar energy for seawater desalination and wastewater treatments is highly promising for purifying water.However,traditional solar-driven seawater desalination approaches have the problems of low utilization of solar energy,slow evaporation rate of water,and low efficiency of solar-thermal energy conversion.At present,people are trying to explore the scientific problems behind the solar-thermal energy conversion system,and have developed many organic composite photothermal materials with high water evaporation rates.Pure carbon materials,such as carbon nanotubes（CNTs）and graphene,have high chemical and thermal stabilities and are promising solar-thermal energy conversion materials,but their water evaporation rates are still at a low level due to the less significant effect of the carbon materials on the water vaporization enthalpy.Therefore,in order to solve the problems of small water evaporation rates and low solar-thermal energy conversion efficiencies of carbon-based materials,we directly utilize or construct vertically aligned channels to accelerate the water transport from the bottom to the top evaporation surface,and take advantage of oxygen-containing functional groups to reduce the water vaporization enthalpy,and regulate the water energy state by a dynamic compression technique.The developed solar-driven water evaporators exhibit high solar steam generation performances and are competent for the desalination of seawater and putification of wastewater.The the main research contents and results are given as follows:1.Preparation of wood/iron oxide/carbon nanotube composite evaporators with vertically aligned channels and studies on their water evaporation performances:The preparations of CNTs by arc-discharge,chemical vapor deposition and laser ablation usually require high temperature,high energy density and inert gas protection,which are relatively complex.In this chapter,we develop a new method to rapidly prepare a large number of CNTs by one-step combustion in an atmospheric environment.Taking iron acetylacetonate as a precursor,we quickly and evenly sprinkle it on the preheated high-temperature hot plate.The iron acetylacetonate is heated and rapidly decomposed into iron nanoparticles and carbon atoms,which can be the catalyst and the carbon source for the growth of CNTs,respectively.A large amount of CNTs are generated followed by the mechanism of“tip growth”.Finally,the generated CNTs are loaded on the surface of the wood with vertically aligned channels to prepare wood/iron oxide/carbon nanotube composite evaporators with a double-layer structure.As a light absorption layer,CNTs can effectively absorb solar energy and convert solar energy into thermal energy to drive the water evaporation.As a heat insulation layer,the wood can prevent the loss of heat to the bulk water due to its extremely low thermal conductivity.Moreover,the vertical channels in the wood reduce the water transport paths and accelerate the water transport to the evaporation surface.Therefore,this evaporator can achieve a water evaporation rate of 1.42 kg m^-2h^-1 and a solar-thermal energy conversion efficiency of 87.2%.2.Preparation of reduced graphene oxide/MXene composite hydrogels with vertically aligned channels and studies on their water evaporation performances:Althogh graphene and other carbon-based solar-thermal energy conversion materials have excellent chemical and thermal stability,their water evaporation rates are far lower than those of organic photothermal materials due to the little effect of the carbon matrix on water vaporization enthalpy.Compared with two-dimensional graphene films,three-dimensional reduced graphene oxide（RGO）hydrogels can promote the the light absorption and accelerate the water transport from the bottom to the evaporating surface by the capillary effect.In this chapter,a large number of oxygen-containing functional groups are retained deliberately in the RGO network for forming numerous hydrogen bonds with water molecules,thereby reducing the vaporization enthalpy of water and promoting the water evaporation efficiency.To further accelerate the water transport,we construct vertically aligned channels in the RGO hydrogel by directional freezing for shortening the water transport paths.In addition,because transition metal carbide/carbonitride（MXene）nanosheets possess nearly 100%internal light-to-heat conversion efficiency,they are infiltrated into the RGO skeleton on the basis of the Marangoni effect to further improve the solar-thermal energy conversion performances,achieving a water evaporation rate of 2.09 kg m^-2 h^-1 and a solar-thermal energy conversion efficiency of 93.5%under 1 sun illumination,higher than those of most inorganic photothermal conversion materials.3.Preparation of reduced graphene oxide foams with controllable compression and the changes of water energy state in compression process:It is still a challenge to solve the problem of low evaporation rates in inorganic solar steam generation systems.Curently,most researches focus on improving water transport rate,light absorption capacity,and heating modes.The influence of the photothmeral material itself on water energy states is rarely explored.In this chapter,we adjust the energy state of water in the porous channels of the vertically aligned reduced graphene oxide（V-RGO）foams for the first time by dynamic compression.The contents of free water,intermediate water,and bound water can be varied obviously with varying the mechanical compression degree,which can be used to control the vaporization enthalpy of water.When the compressive strain is 47.1%,the corresponding content of intermediate water is up to 42.09%,and the minimum vaporization enthalpy of water is 1306J g^-1,which is very conducive to the evaporation of water.In addition,we also make full use of environment energy to enhance water evaporation efficiency.Besides the direct solar energy input,the sidewall of the evaporator can absorb extra energy from the adjacent environment in the forms of thermal convection and thermal radiation because the sidewall temperature of the V-RGO foam is lower than the environment temperature,which can further promote the water evaporation behavior.Under the synergistic effect of the reduced vaporization enthalpy of water and the use of environmental energy,the V-RGO foam achieves a water evaporation rate of 3.34 kg m^-2 h^-1and a solar steam generation efficiency of 104.1%,higher than those of other carbon-based solar-thermal energy conversion materials.更多还原