Functional Kapok Fiber Cellulose Aerogel-Based Composites:Preparation,Structure And Properties

Cellulose aerogel has the excellent properties of low density,high porosity,three-dimensional network structure,good mechanical properties,biocompatibility and renewablability,which is a kind of natural polymer material with broad development prospects.The porous structure and abundant oxygen-containing functional groups make it ideal functionalized composites.In this study,based on the characteristics of tubular structure and rich cellulose content of kapok fiber,cellulose aerogels with different structures were prepared by homogeneous-filtration molding-atmospheric pressure drying and high frequency ultrasonic-freezing molding-freeze drying,respectively.Then a variety of cellulose aerogel-based composite materials were prepared through carbonization,doping,modification or other physical and chemical processes.The applications of these composite materials in thermal insulation,flame retardancy,supercapacitors,seawater desalination and atmospheric water harvesting were explored.The main research contents are as follows.Utilizing the tubular structure of kapok fiber and the method of homogenization-filtration molding-atmospheric pressure drying,cellulose aerogel with high mechanical strength and a tubular structure for thermal insulation and fire retardancy was prepared by blending with（NH₄）₂HPO₄ and montmorillonite（MMT）.The results showed that（NH₄）₂HPO₄ and MMT were uniformly mixed in the cellulose suspension.Hydrogen bonds between the hydroxyl groups of cellulose promote the formation of a tighter gel network in the filtration molding process and further increase the strength of the composite aerogel.The tubular structure effectively reduces the density of the composite aerogel and makes it retain low density while maintaining high strength.The prepared sample had an ultrahigh compressive strength of 32 MPa and a low density of 42 mg/cm³.The gaseous voids of the tubular structure break the continuity of the heat transfer path,thereby inhibiting the heat conduction,which makes the thermal conductivity of the composite as low as 54 m W/（m·K）and thermal insulation performance improve by 37.4^oC.When the composite aerogel was in contact with the flame,the dense carbon layer enhanced by MMT and（NH₄）₂HPO₄ formed and effectively isolated combustible volatile compounds,heat and oxygen during the combustion process.In addition,the tubular structure further prevents the thermal decomposition products from entering the gas phase to participate in the combustion process,which reduces the penetration pressure of the dense char layer.For the optimal sample,the ignition time was 9 s;the limiting oxygen index was 43.9%;the peak heat release rate was53.11 k W/m²;the total heat release rate was 5.51 MJ/m²,and the total smoke generation was1.27 m²/m².Based on the tubular structure of kapok fiber and the method of homogenization-filtration molding-atmospheric pressure drying,Ru O₂-doped cellulose carbon aerogel with tubular structure was prepared and used as the electrode material of supercapacitor by combining with carbonization process.The influence of the preparation conditions on the electrochemical performance was studied.The corresponding regularity between the electrochemical performance and the electrode structure was discussed.The results showed that the carbonization temperature affected the tubular structure,pore size distribution and graphitization degree of the electrode material,thus affecting the electrochemical performance.When the carbonization temperature was 800^oC,the prepared carbon aerogel had relatively complete microtube structure,moderate pore size structure and degree of graphitization.For the sample carbonized under800^oC,the doping amount of Ru was 1.91 wt%;the specific surface area was 228 m²/g and I_D/I_G=1.131.The prepared electrode had a specific capacitance of 433 F/g in the KOH electrolyte at a current density of 1 A/g.The specific capacitance retention rate was 63.3%at a current density of 20 A/g.The specific capacitance retention rate was about 91.5%after 2 000 cycle tests.This work solves the problem of low specific capacitance of pure carbon-based electrodes,and also improves the current situation of high cost and poor cycle stability of pure Ru O₂ electrodes.But the tubular structure of carbon aerogels prepared by this method cannot remain intact at high temperatures.In view of the problem that the tubular structure cannot remain intact at high temperature in Chapter 4,nitrogen-doped carbon aerogels with tubular structure for electrode materials of supercapacitors were prepared by in-situ polymerization of pyrrole and carbonization process.The results showed that when the ratio of pyrrole to cellulose suspension was 1:100（V/V）,the wall thickness of the carbonized microtube was 0.650μm;the tubular structure was complete and smooth;nitrogen content was 6.05 wt%;I_D/I_G=0.870.Furthermore,the prepared electrode material had a specific capacitance of 274 F/g at a current density of 1 A/g in 6 M KOH electrolyte.After 4 000 cycle tests,the capacitance retention rate was about 97.7%.Based on the self-assembly properties of cellulose and the method of high frequency ultrasonic-freeze casting-freeze drying,anisotropic aerogel was prepared by bidirectional freeze-casting using N,P-codoped carbon quantum dots（CDs）as dopants.Then,a laminar r GO/CDs/CMF composite carbon aerogel were further obtained by carbonization.The applications of it in elastic conductors,supercapacitors and capacitive deionization technologies were explored.The results showed that the bidirectional growth of ice crystals made the sample a laminar structure.The sample had good compressibility,which can be restored to its original state after 80%strain.For the optimal sample,the specific surface area was 601 m²/g;the content of N and P was 4.74 at%and 5.37 at%,respectively,and the conductivity was 14.2 S/cm.The addition of CDs adjusted the surface group composition,increased the ratio of pyridinic nitrogen and quaternary ammonium nitrogen,thereby increasing the active sites for electrochemical reactions and further reducing the charge transfer resistance of electrode materials.For samples in 1 M Na Cl,6M KOH and 1M H₂SO₄ electrolytes,the specific capacitances at a current density of 1 A/g were 232,330 and 305 F/g;the charge transfer resistance were 1.11,0.08 and 0.18Ω;and the electrolyte diffusion resistance were 0.42,0.51 and 0.39Ω,respectively.After 5 000cycle tests,the specific capacitance of the sample was 101%of the initial specific capacitance at a current density of 10 A/g in 6 M KOH electrolyte.Furthermore,the deionization capacity was32.59 mg/g,and the salt adsorption rate was 0.69 mg/（g·min）in an initial Na Cl solution of 504mg/L under the working voltage of 1.2 V.Anisotropic aerogels were prepared by unidirectional freezing using NH₄H₂PO₄ as a dopant and activator at the same time.After carbonization and CO₂ activation,a mesoporous carbon aerogel with a xylem structure was obtained.The application in elastic conductors,supercapacitors and capacitive deionization technologies was explored.The results showed that the prepared sample had a xylem-like structure and good compressibility.After 500 compression cycles at 50%strain,the sample retained 98%of its original height.When the ratio of cellulose to NH₄H₂PO₄ was 1:1 and the CO₂ activation time was 120 min,the conductivity was 16.4 S/cm;the doping amounts of N and P were 4.29 at%and 3.05 at%;the specific surface area was 1 553m²/g;the pore volume was 0.81 cm³/g,and the mesoporosity was 70.37%.The optimized sample had a specific capacitance of 367 F/g in 6M KOH electrolyte at a current density of 1 A/g.The capacitance retention rate was about 95%after 5 000 cycles of testing.Capacitive deionization experiments showed that the desalination capacity was 26.4 mg/g,and the salt adsorption rate was 2.24 mg/（g·min）in an initial Na Cl solution of 504 mg/L under the working voltage of 1.2V.A composite aerogel was obtained with MFC/Fe₃O₄ as a photothermal conversion layer,MFC/GBs as a thermal insulation layer and MFC/PVA as a support layer through the high frequency ultrasonic-freeze casting-freeze drying process and used as an integrated single-module evaporator for seawater desalination.The results showed that each functional layer can be effectively combined into a whole by layer-by-layer freezing while exerting their respective advantages.The photothermal conversion layer exhibited a high absorbance of 97%in a wet state.GBs can prevent water intrusion and reduce the thermal conductivity of the insulation layer to 0.1008 W/（m·K）in a wet state.The thermal insulation layer inhibited the downward transfer of heat,which increased the temperature of the evaporator by 2～3^oC.In the wet state,the temperature of the evaporator can rise from 25^oC to 47^oC within 900 s under a solar radiation of1 sun.By changing the state of water in the hydrophilic framework,increasing the number of evaporation sites,and confining water in the porous structure,the evaporation enthalpy of the evaporator reduced to 949.0 J/g,which made it have an ultra-high evaporation rate of 3.17kg/（h·m²）under a solar radiation of 1 sun.In addition,the hydrophilic interpenetrating framework of MFC/PVA ensures the stability and sufficient water supply of the evaporator,achieving an efficient,sustainable,durable and salt-resistant evaporator.An aerogel-based composite with MFC/Cl-ppy as the light-to-heat conversion layer and MFC/E-Co Cl₂ as the moisture-absorbing layer was prepared through the"high frequency ultrasonic-freeze casting-freeze drying"process and used as a humidity-responsive aerogel-based evaporator for atmospheric water harvesting.The results showed that the evaporator had a high absorbance of 97%and a high evaporation rate of 0.9550 g/（g·h）.The highly porous structure of cellulose aerogels greatly improves the absorption capacity and absorption kinetics at low relative humidity.Under the relative humidity of 25%to 85%,the aerogel-based evaporator had a water absorption capacity of 0.39～2.05 kg/kg within 3.5 h,and can quickly evaporate within 1.5 h under the solar radiation of 1 sun.By utilizing its humidity-indicating function together with fast absorption kinetics,the aerogel-based evaporator performed 3absorption-desorption cycles in one natural day outdoors,producing 2.81 kg of water per kilogram of absorbent.This aerogel-based evaporator achieves visible real-time monitoring of moisture change responding to humidity and sunlight changes flexibly in the actual environment which makes it possible to realize multiple absorption-desorption cycles on a random sunny day.