Preparation Of Cellulose Nanofibers/MXene Based Photothermal Conversion Aerogel And Its Water Purification Study

Since the first industrial revolution,global freshwater resources have become increasingly stressed due to uneven geographical distribution and increasing pollution.As the most abundant water resource,seawater cannot be used directly due to its high salt content,so it is especially important to develop seawater desalination technology.The more mature desalination technologies include ion exchange,electrodialysis,distillation,etc.,but these methods are often accompanied by more serious energy consumption.Therefore,the solar-driven water evaporation technology with the core of photothermal conversion materials is gaining more and more attention because it does not require additional energy consumption.As an extremely abundant biomass resource in nature,cellulose nanofiber（CNF）is expected to be an excellent substrate for solar-driven photothermal water evaporators because of its wide source,low thermal conductivity,abundant surface functional groups and strong hydrophilicity.However,the limited absorption capacity of CNF itself to sunlight is the main factor limiting its application in seawater desalination.Therefore,in this paper,we designed and prepared a three-dimensional porous high-efficiency solar-driven interfacial water evaporator using biomass CNF as the main three-dimensional structural substrate by introducing highly efficient light-absorbing materials.The main study contents and findings are as follows:（1）The monolayer/few-layer two-dimensional titanium carbide（MXene）was obtained by etching MAX through in situ generation of hydrogen fluoride method.CNF was used as the three-dimensional porous substrate and MXene as the main photothermal conversion material.Both can be cross-linked by hydrogen bonding to obtain CNF/MXene（CM）aerogel due to the extremely abundant hydroxyl groups on the surface.The experimental results demonstrate that the water evaporation performance of the CM aerogel obtained by introducing MXene into CNF aerogel is excellent,and the water vapor evaporation rate reaches 1.40 kg m^-2 h^-1 under 1 sun sunlight.（2）In order to improve the performance of CM aerogels,typical narrow bandgap semiconductor iron tetroxide（Fe₃O₄）was used as the light absorption enhancer,and the surface modification of Fe₃O₄ with polydiallyldimethylammonium chloride（PDDA）to make it positively charged,and the CNF/MXene/Fe₃O₄（CMF）aerogel was prepared by electrostatic self-assembly with CNF and MXene.The experimental results demonstrated that the addition of Fe₃O₄ did not reduce the hydrophilicity of the composite aerogel,but also significantly increased the water vapor evaporation rate of the CMF aerogel to 1.64 kg m^-2 h^-1.When applied to seawater desalination,the composite aerogel was able to achieve a high water vapor evaporation rate of1.65 kg m^-2 h^-1 and maintained a high performance after ten cycles.（3）In order to further improve the water vapor evaporation performance of the composite aerogel,nickel nanochains were prepared by a combination of soft-template and magnetic field-assisted methods,and then the CNF/MXene/Ni-chain（CMN）aerogel was prepared by electrostatic self-assembly with CNF and MXene after modifying the Ni chains with PDDA.The experimental results demonstrated that the water vapor evaporation rate of CMN aerogel reached1.85 kg m^-2 h^-1 at 1 sun.When CMN aerogel was applied to seawater desalination,CMN aerogel not only maintained the efficient water vapor evaporation rate of 1.81 kg m^-2 h^-1,but also could effectively remove the major metal cations from seawater.In addition,CMN aerogel can remove heavy metals and organic pollutants from seawater up to 99%.