Surface Modification Of Carbon Fiber Membrane And Its Application In Solar-driven Seawater Desalination

In recent years,the global freshwater shortage has become an increasingly serious problem.To alleviate the freshwater crisis,it is urgent to develop seawater desalination technologies.Among them,solar-driven seawater desalination has attracted great attention due to its high-efficient,low-cost and environmental-friendly features.The key for solar evaporation is to construct efficient seawater evaporation devices and to develop solar absorbers that have excellent light absorption ability,high photothermal conversion efficiency.At present,researchers have developed a variety of photothermal materials（including metal,semiconductor,polymer and carbon-based photothermal conversion materials）,and developed three main evaporation modes（bulk evaporation mode,floating evaporation mode and hanging evaporation mode）.Among them,the bulk and floating models are limited by low evaporation rate and salt precipitation.To solve these problems,the photothermal membrane is suspended with its one or two edges immersed in the seawater tanks,and the seawater flows downward along the surface of the membrane.At the same time,sunlight irradiates on the surface of the photothermal membrane to promote the seawater evaporation,and the concentrated seawater drops at the bottom of the membrane.This hanging-model evaporator has the advantages of double-surface evaporation,low heat dissipation,high evaporation speed and no salt precipitation.The key for the evaporator is to prepare hydrophilic photothermal membranes with wide-band responses.Carbon fiber membrane（CFM）has good light absorption and photothermal properties,and is expected to be an efficient solar absorber.However,CFM hydrophobic and lacks micro geometry,resulting in low surface area.In order to optimize CFM,this paper developed super-hydrophilic CFM by surface modification,and constructed a hanging-model seawater evaporator for solar-driven seawater desalination.The main research contents are as follows:（1）Modification of CFM with polydopamine and construction of hanging-model seawater evaporatorCFM usually lacks of hydrophilic active groups on its surface,and cannot be soaked by water,leading to difficulties to directly use it to construct hanging-model evaporator.Polydopamine（PDA）is a kind of material with excellent hydrophilicity and adhesiveness,and also has good light absorption and photothermal properties.In this chapter,a layer of PDA film was in-situ grown on the surface of CFM by simple self-polymerization method with dopamine as precursor under weak alkaline condition（pH=8.5）,obtaining PDA coated carbon fiber membrane（CFM@PDA）.After loading PDA,the light absorption efficiency of CFM at 250～2500 nm increased from 94.4%to 96.7%.Importantly,the hydrophilic groups（-OH and-NH₂）in the PDA polymer chain endow CFM with super hydrophilicity.Beneficial from the fibrous structure,CFM@PDA performs efficiently water transport ability.After construction of hanging-model evaporator,the CFM@PDA（2×16 cm²）exhibited a water transmission speed of 26.35 g/h,which is 3.6 times higher than that（7.28 g/h）of the same size commercial cotton cloth.Under the illumination of simulated sunlight(1.0 k W m^-2),the evaporation rate of CFM@PDA reached 1.79kg m^-2 h^-1.After long-time evaporation process,the CFM@PDA showed no salt crystal on their surface,performing obviously superior evaporation ability to a traditional floating device.In addition,the good combination ability of PDA facilitates CFM@PDA to have good stability in the cycling evaporation experiments.（2）Fabrication of MOF coated CFM and the construction of hanging-model seawater evaporatorIn order to modify CFM with hydrophilicity and introduce multi-level micro-structures,copper doped zinc-based MOF nanosheets（CFM/MOF）are loaded on the surface of CFM.Put CFM in the reaction solution,without heating and stirring,and stand at room temperature for 6 h,MOF nanosheets can be self-assembled on the surface of CFM.The MOF nanosheets with uniform thickness（3～5 nm）is stacked on the CFM surface to form a nano pore structure,which increases the CFM fiber diameter from～6.6μm to～13.3μm.The contact angle of CFM is 113°before the loading of MOF nanosheets,and becomes 0°after that,indicating that CFM/MOF has excellent water infiltration effect.The surface of CFM/MOF has different light absorption under dry and wet conditions.In the wavelength range of 380～2500 nm,the light absorption of dry CFM/MOF is 78.0～85.6%,and that of wet CFM/MOF is 85.4～98.6%.This is because the light incident first refracts at the air/water interface and then arrives at the surface of the MOF nanosheets.After multiple refractions and reflections on the muti-stage structure by these nanosheets,a light-trapping effect is generated,which reduces the reflectivity and improves the light absorption.On the hanging-model evaporator,CFM/MOF shows excellent water transmission capacity.When the initial height h of the left tank solution is 1 cm,the water transmission capacity reaches 5.82 g/h.The evaporation rate of CFM/MOF is as high as 1.88 kg m^-2 h^-1 under 1 times sunlight.In addition,CFM/MOF has excellent evaporation stability,structural stability and efficient desalination capacity,and has good application prospects in solar desalination.In this paper,two kinds of superhydrophilic photothermal CFM are fabricated by simple preparation methods and equipped in hanging-model evaporators.The hanging CFM-based membranes show high light absorption effect in broad spectrum,high evaporation rate,and they have the ability to prevent salt accumulation during long-time evaporation.Therefore,the present CFM-based membranes provide the possibility to promote the large-scale practical application of photothermal membranes.