Preparation Of Two-Dimensional COFs-Based Composites And Their Photothermal And Electrocatalytic Properties

Benefiting from their unique structural and performance characteristics,two-dimensional covalent organic frameworks（2D-COFs）have shown promising applications in several fields.However,the practical application of COFs materials is limited due to their inherent defects including high cost,low stability,poor processability,etc.To overcome this problem,this thesis develops a variety of 2D COFs-based composites through incorporation with different functional materials.And then,their applications in solar-driven photothermal seawater desalination,air water collection and electrocatalytic oxygen evolution reaction were studied.The main content of this dissertation is as follows:（1）In order to achieve broad-band light absorption and high photothermal conversion efficiency,composite photothermal materials（MoCOF1 and MoCOF2）with core-shell structure were prepared by encapsulation of the narrow-bandgap MoS₂micron-flowers with twoβ-ketoenamine-based COFs（TpPa-COF and TpBd-COF）via in situ growth strategy.On the one hand,the introduction of narrow bandgap MoS₂ can broaden the light absorption range,enabling the light absorbption ability.In addition,the unique core-shell structure can realize controllable charge migration from COF to MoS₂,which contributes to increasing photothermal conversion efficiency via decreasing undesired radiative decay.Therefore,under 1 sun irradiation,the surface temperatures of the photothermal composites MoCOF1 and MoCOF2 can reach～78.5℃ and～81.3℃,respectively.（2）To address the challenge that the powdered MoCOF photothermal composite is difficult to machine for solar-driven photothermal evaporation,MoCOF@Gel evaporator is fabricated by introducing MoCOF composites into the PVA（polyvinyl alcohol）polymer network via in situ gelation method.This evaporator exhibits interconnected microscale pore structures,and MoCOF composites are loaded onto the pore walls of the PVA gels via hydrogen bonding interactions.Furthermore,based on the solar radiation spectrum（AM 1.5）,the MoCOF@Gel evaporator demonstrates a high light absorption rate of 86.7%,resulting in a surface temperature of approximately51.3℃under one solar irradiation.Thanks to the synergistic effect of the high photothermal conversion capability and the rapid water transfer bevavious,the MoCOF@Gel demonstrates the high evaporation rate(2.31 kg m^-2 h^-1),outstanding conversion efficiency（91.8%）,and excellent anti-biofouling capability.（3）To realize low-cost and large-scale preparation of COF-based solar-driven photothermal evaporators,COF@carbon nanotube（CNT）photothermal composites with core-shell structure were first prepared by in-situ growth method.Subsequently,using polyacrylonitrile（PAN）as a substrate,the PAN/COF@CNT composite fiber membrane with a large-size（14×25 cm²）was prepared by solution blow spinning technology.The resultant PAN/COF@CNT exhibits excellent absorption（90.6%）in the range of 300-2500 nm and excellent light-to-heat conversion ability（72.6℃）.In addition,the PAN/COF@CNT evaporator also shows smooth evaporation rate(1.42 kg m^-2 h^-1),and outstanding solar to thermal energy conversion efficiency（96.7%）under1 sun solar.Furthermore,in the continuous evaporation experiment,the water evaporation rate of PAN/COF@CNT fiber film is more than 1.40 kg m^-2 g^-1,indicating an impressive durability.（4）To enhance the water transport and water vapor diffusion capability in the process of air water collection,a hygroscopic fiber membrane containing the PPy-COF photothermal composite layer was constructed.On the one hand,the introduction of PPy（polypyrrole）reduces the band gap of COF,and thus improves the light absorption performance of the composite（～95.5%）.On the other hand,the charge transfer between the two components could suppress the radiative leap and enhance the non-radiative leap,thus improving the photothermal conversion abilicity（75.3℃,under 1 sun irradiation）.As a result,the water releasing rate was enhanced(1.43 kg m^-2 h^-1).More importantly,the inherent nanoscale through-hole structure of PPy-COF not only optimizes the water transport kinetics,but also ensures the consistency of the entire adsorption process without loss of steam diffusion channels.（5）In order to expand the use of produced water by desalination and air collection,imidazole-functionalized COF-IM materials were prepared by modifying imidazole functional groups into COF pores through the Poivarov cycloaddition reaction.In addition,the COF-IM@Co,COF-IM@Ni,and COF-IM@Fe composite catalysts were also prepared by coordination reactions with cobalt acetate,nickel chloride,and iron chloride,respectively.Among them,the COF-IM@Co exhibits an excellent oxygen evolution reaction（OER）activity with an overpotential of only 404 mV at a current density of 10 m A/cm²,which was lower than that of the other COF catalysts.Moreover,there was also almost no significant degradation of the electrocatalytic performance after 1000 CV（cyclic voltammetry）cycles.