Preparation Of Nanocellulose-based Carbon Aerogels From Bubble Template

Carbon aerogel with unique porous structure,good thermal/electrical network,excellent compressibility and elasticity shows applications in wearable devices.Compared to traditional methods such as sol-gel method and hydrothermal technique,novel emulsion templated method can fabricate carbon aerogels with highly-ordered structure and multifunction.Due to the instability of bubble emulsion and mass/volume loss during carbonization,it’s still a big challenge to prepare biomass-based carbon aerogel via bubble-templated method.Recently,scientists have prepared bubble-templated carbon aerogels from graphene oxide（r GO）,however,the weak van del Waals interaction among graphene nanosheets is relatively weak,which usually results in poor mechanical performances.In this paper,we first use CNF suspension with high viscosity to obtain stable CNF bubble emulsion,pure CNF aerogel is prepared after simple freeze-drying.The addition of NH₄H₂PO₄ and pre-oxidation process significantly reduce the weight/volume loss during carbonization,giving rise to pure CNF carbon aerogel（C-CNF）with high performance.Then,we fabricate CNF/MXene composite carbon aerogel（C-CNF/MX）via the bubble template method,the addition of conducive MXene enhances the sensitivity of carbon aerogel and allow it to detect very small pressure.Finally,we used graphene oxide liquid crystal（GOLC）stabilized bubble as template to prepare r GO/CNF carbon aerogel（CAG）.The welding of CNF among r GO nanosheets can significantly enhance the interaction among r GO nanosheets and thus the compressive mechanical strength of CAG（stress almost doubles）,endowing it with super elasticity and excellent fatigue resistance.The specific contents are as follows:1.CNF with high viscosity is served as carbon precursor to prepare C-CNF with bubble and ice crystal as dual-template.The addition of NH₄H₂PO₄ and pre-oxidation process significantly reduce the wight/volume loss during carbonization.The carbon aerogel shows unique porous structure,with big spherical pores derived from bubble template and small honeycomb pores templated by ice crystal.The closely-packed small pores endow carbon aerogel with high sensitivity to pressure,while the big spherical pores enable carbon aerogel to withstand extreme compressive strain.Thus,C-CNF shows excellent compressive performances,the stress and height retention after 5000 cycles at 50%strain are high up to 87.0%and 91.7%,respectively.Besides,C-CNF exhibits rapid and sensitive current response to external stress,with a high sensitivity of 0.51 k Pa^-1 at a wide stress range of 0-30 k Pa,making it an idea material for wearable devices.2.With CNF and MXene as carbon precursors,C-CNF/MX is fabricated via bubble template method.Both CNF and MXene show good hydrophily,CNF can work both as nano-dispersant,avoiding the restacking of MXene sheets during freeze casting,and as nano-enhancer to improve the interaction between MXene and CNF,contributing to the construction of 3D network.Due to the thermal stability of MXene sheets,composite carbon aerogel shows small weight loss during carbonization.The high conductivity of MXene sheets impart C-CNF/MX with higher sensitivity(1.01 k Pa^-1)than C-CNF(0.51 k Pa^-1).C-CNF/MX can effectively detect body movement and human pulse,exhibiting potential application in wearable devices.3.CAG with ultralow density yet high mechanical performances is successfully fabricated from graphene oxide（GO）liquid crystal stabilized bubbles with the enhancement of cellulose nanofiber（CNF）.The incorporation of CNF into reduced graphene oxide（r GO）nanosheets enhances the interaction among r GO nanosheets through welding effect,which restricts the slip of r GO nanosheets and the detachment among microspheres,leading to a significant improvement on mechanical properties.The as-prepared carbon aerogel with tightly packaged cell-wall architecture displays ultrahigh compressibility（up to 99%strain）and elasticity（90.1%and 99.0%of stress and height retention after 10000 cycles at 50%strain）,which is superior to those of the present bubble-templated carbon aerogels and many other carbon materials through various methods.The structural feature leads to rapid and stable current response and high sensitivity to external strain,allowing the carbon aerogel to detect very small pressure and various human motions from finger bending to pulse.These advantages make the carbon aerogel promising for flexible electronic devices.