The Preparation Of Solar-driven Water Evaporators And Studies On Their Solar-thermal Energy Conversion Performances

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.