Customizable Structural Design Of Functional Cellulose Aerogels

Cellulose aerogel,as one of the aerogel materials,has the advantages of being biodegradable and biocompatible in addition to the characteristics of traditional aerogel such as low density,high porosity,and large specific surface area,and has the potential to replace petroleum-based compounds as new green materials,which has become a research hotspot in recent years.The rich nanopores structure makes cellulose aerogel have an excellent effect on adsorption,transport,and insulation.At the same time,it is an effective way to improve the performance of cellulose aerogel and expand its application range by combining cellulose with functional nanomaterials and making them have an excellent synergistic effect through a customizable microstructure design strategy.In this work,cellulose as the main raw material,design and construct functional cellulose aerogel material,At the same time,field emission scanning electron microscopy（FESEM）,atomic force microscopy（AFM）,three-dimensional confocal microscopic Raman spectroscopy（3D Raman）,attenuated total reflection Fourier transform infrared spectroscopy（ATR-FTIR）,synchrotron X-ray small-angle scattering（SAXS）and other characterization methods were used to study the microstructure and properties of cellulose aerogels.Subsequently,a series of experiments were designed to explore the application prospects of aerogels in dye adsorption,seawater desalination,transpiration power generation,osmotic energy conversion and other fields.The main innovation points of this thesis include:（1）3D Raman imaging technology was used to explore the relationship between MOFs and cellulose nanofibers,design and synthesis of cellulose nanofibers and MOFs entanglement structure of composite aerogels,which demonstrate excellent adsorption effect on organic dyes;（2）The aligned cellulose/MXene composite aerogel films were fabricated through a directional freezing-assisted physical densification strategy for osmotic energy conversion to power generation;（3）Using the alkali/urea solvent system for dissolving cellulose and chitosan,the aligned regenerated cellulose/chitosan@GO Janus aerogels were constructed by molecular chain interface self-tangling and directional freezing method.The aerogels could realize simultaneous application in desalination and transpiration power generation.The main research content and conclusion of this thesis includes the following parts.TEMPO was used to oxidize cellulose nanofibers for controlled chelation with metal ions,and ZIF-67 was further grown on the controllable site of cellulose nanofibers to construct composite aerogel.Due to the high length-diameter ratio and specific ion site of cellulose nanofibers,ZIF-67 crystals were physically entangled by cellulose nanofibers,thus enhancing the structural stability.We used Raman imaging technology to visually monitor the distribution of metal ions and ZIF-67 nanocrystals in the gel.The cellulose nanofiber@ZIF-67 composite aerogel has a low mass density（6.18 mg/cm³）,high specific surface area（734.7 m²/g）and good underwater stability,and excellent adsorption performance for organic dyes.This study provides a method to design a microcosmic fixed-point to solve the MOF dispersion problem in nanofiber aerogel by using Raman imaging technology.Cellulose nanofibers（NCNF）with high negative surface charge were extracted from seaweed,and further modified by amination to obtain positively charged cellulose nanofiber（PCNF）.Besides,the exfoliation of MXene（NMXene）was realized,and the positively charged MXene（PMXene）was obtained by quaternization modification.Subsequently,we prepared MXene/CNF composite aerogels with oriented micro-channels by freeze casting technology,followed by the mechanical compression to construct highly oriented and structural densified MXene/CNF aerogel films.Furthermore,the ionic conductivities of positively charged PMXene/PCNF and negatively charged NMXene/NCNF were increased to 2.8×10^-3 S cm^-1 and2.9×10^-3 S cm^-1 at low salt concentrations,respectively,by adjusting the spatial geometry and charge density distribution of the orientation nano-channel.In addition,the output power density(reaching 1.79 W m^-2)of the oriented,structure-densified composite aerogel film is significantly higher than that of the oriented,structure-undensified aerogel(freeze casting,0.37W m^-2)and the non-oriented,structure densified film(vacuum filtration,0.62 W m^-2).Furthermore,the composite aerogel films can be assembled in series in P-N type to achieve a maximum output power density of up to 8.86 W m^-2.In particular,15 sets of P-N units in series can achieve an output voltage of 1.85 V,which can power a calculator.Cellulose and chitosan@GO gels were respectively dissolved and regenerated in an alkali/urea aqueous solvent.The two kinds of gels were self-entangled and combined through the molecular chain interface to prepare double-layer gels.Based on this,the ice crystal orientation growth of the cellulose hydrogel layer was induced by a temperature gradient,and the composite aerogel with a Janus structure was prepared by freeze-drying.The cellulose aerogel with an oriented micro-channel structure is used as the water transport layer of the solar evaporator,which reduces the resistance of water transmission and realizes the rapid transmission of water to the top of the aerogel.The photothermal layer is a composite aerogel constructed by chitosan@GO,which has good photothermal conversion performance.The molecular chain of chitosan is easy to form a low-density hydrogen bond with water,reducing the enthalpy of evaporation of this part of water,giving the composite aerogel excellent evaporation rate of seawater(2.5 kg m^-2 h^-1)and efficiency（90.1%）.In addition,based on the good transmission performance of the ordered cellulose water conduction layer,the ions are transported in the nano-channel of the cellulose pore wall,which enables the evaporator to generate a voltage（420 m V）and output power of 680 n W cm^-2 while the water evaporates.Therefore,the whole biomass aerogel combines the characteristics of good surface salt resistance,high evaporation rate,high power generation efficiency,biodegradability,etc.,and has great application potential in the fields of desalination,and steam energy conversion.This dissertation mainly focuses on cellulose and chitosan and uses different strategies to customize a series of cellulose/nano functional materials composite aerogels with unique structures and functions.The microstructure and properties of the composite aerogel were studied in depth to realize the application of dye adsorption,seawater desalination,transpiration power generation,and osmotic power generation.This dissertation using cellulose and other biomass resources with high added value has academic value and application prospects and conforms to a sustainable development strategy.