Preparation And Properties Of Functionalized Nanocellulose-based Carbon Aerogel

Cellulose is the most abundant renewable natural polymer on the earth,which has the advantages of green and low price.It is of great significance to realize sustainable development to utilize cellulose resources.In this paper,functional nanocellulose-based carbon aerogel was prepared by reasonable design and improved the adsorption performance,mechanical properties and electrical properties.The forming mechanism and properties of the carbon aerogel were studied.The applications of nanocellulose-based carbon aerogel in environmental protection,energy storage materials and sensors were discussed.Nanocellulose with high aspect ratio was obtained by TEMPO oxidation method.Nanocellulose dispersion was frozen at different freezing temperatures,and nanocellulose carbon aerogel was prepared by freeze-drying and pyrolysis at high temperature.It was found that with the decrease of freezing temperature,the formation rate of ice crystal was fast and the volume was small,and the specific surface area of carbon aerogel increased gradually.Freezing temperature had no effect on hydrophobic property of carbon aerogel.In the process of inert gas atmosphere pyrolysis,nanocellulose was dehydrated,decarboxylated and decarbonylated.The removal of hydrophilic groups(C=O,C-O and-OH)causes the carbon aerogel to produce high hydrophobicity(a contact angle of 139.30),and the cellulose is converted to amorphous carbon structure after pyrolysis.The obtained carbon aerogel has low density(5.8mg/cm3),high porosity(99.67%)and high specific surface area(161.16m2/gl).The adsorption capacity of the organic solution is as high as 110-260 g/g.After repeated cyclic adsorption and desorption,it still can maintain a high adsorption capacity.By studying the adsorption process of gasoline and diesel.It was found that the lower density of carbon aerogel,the higher the oil absorption capacity.The rate of oil absorption is related to the viscosity of oil.The lower the viscosity,the faster the adsorption rate.The adsorption process is dominated by chemical adsorption.During the pyrolysis process,the volume of nanocellulose carbon aerogel shrinks dramatically(60%-80%),which greatly reduces the available effective volume and greatly limits its application in the field of adsorption.Therefore,the strategy of compound graphene oxide with high carbon content was adopted to prepare cellulose/graphene gel under low temperature hydrothermal conditions,and carbon aerogel was prepared through freeze drying and high temperature pyrolysis.During the hydrothermal process,the cellulose and graphene oxide could form hydrogen bonds directly.In the pyrolysis process,the cellulose was removed of oxygen functional groups,the graphene oxide was removed the residual oxygen containing functional groups and repaired the conjugate structure of graphene,which endow the high hydrophobic of carbon aerogel(contact Angle is 131.5°).Graphene oxide with high carbon content kept the volume of cellulose carbon aerogel unchanged,and the pore structure of the aerogel remained intact in the carbon aerogel.The obtained carbon aerogel has high compressibility of 80%,stable structures,low density of 4.8 cm/mg3,high mechanical stability,and high adsorption capacity.The adsorption capacity of oil and organic solution was reached to 160-281 g/g,and the volume adsorption capacity was up to 80.5-95.0%.By means of different desorption methods such as evaporation,combustion and compression,the adsorption capacity of carbon aerogels still maintained highly adsorption capacity after repeated cycles of adsorption.In order to solve the problems of low strength and poor fatigue resistance of nanocellulose based carbon aerogel,nanocellulose/graphene carbon aerogel with regular layered structure was successfully prepared by bidirectional freezing process.The carbon aeogel has low density(3.26 mg/cm3),high conductivity(0.32 S/m),high compressibility(90%)and high fatigue resistance.Cyclic compression was performed for 1 × 104 times under 20%and 50%strain,and the plastic deformation was only 0.1%and 8.2%,respectively.The energy dissipation of each cyclic compression under 80%strain was only 0.32mJ/cm3.The elastic enhancement mechanism of carbon aerogel is mainly due to the regular arrangement and bending of lamellar structural units in the compression process,rather than the shedding,sliding and tearing between the structural units.In addition,graphene enhanced the mechanical stability of carbon aerogel,and carbon nanocellulose enhanced the elasticity of carbon aerogel.The regular layered porous structure of carbon aerogel and the synergistic effect of graphene/carbon nanocellulose endow carbon aerogel with excellent mechanical properties.The obtained carbon aerogels had high sensitivity of 0.26 kpa-1,which has potential applications in wearable flexible sensors.The preparation process of nanocellulose carbon aerogel is complex,and freeze-drying or supercritical drying is inevitable in the drying process.Therefore,a strategy of using melamine foam as rigid framework was proposed,and nanocellulose was coated on the framework of melamine foam by means of dip coating,so as to prepare flexible carbon aerogel under normal pressure drying.It was found that the rigid skeleton of melamine foams prevented the structure collapse of nanocellulose aerogels during the atmospheric drying.The melamine foams provided the nitrogen contend and increased wettability of carbon aerogels.The obtained carbon aerogels possessed a low density of 11.23 mg/cm3,high compressity of 60%,high nitrogen content of 6.53%,high conductivity of 0.378 S/cm and sensitivity of 1.841 kPa-1,which has potential applications in wearable sensors.In addition,a series of nitrogenous groups(-NH2,pyridine N,C=N,graphite N)are formed during the pyrolysis process,and these nitrogen-containing groups work together to provide a high mass ratio capacitor(92.2 F/g)and an area ratio capacitor(461 mF/cm2)(the current density is 0.1 mA/cm2),which has potential applications for compressible supercapacitor electrodes.