Preparation And Application Of Nanocellulose Based Conductive Composites

The increasingly serious climate change and fossil fuel depletion problem urgently requires green and renewable energy to meet the needs of development,which makes the research on efficient energy storage technology extremely important.Supercapacitors have drawn great attention due to their high-power density,fast charge-discharge rate,and excellent cycling stability.However,the low energy density limits the practical application of supercapacitors.The key to increasing energy density without sacrificing the original advantages depends on the design and construction of high-performance electrode materials.Nanocellulose has numerous advantages including renewability,high aspect ratio,large specific surface area,high porosity,and rich surface groups,making it an ideal candidate for green substrates and the preparation of conductive composites.Meanwhile,nanocellulose is also an ideal carbon source,which can be converted into conductive carbon materials by simple carbonization.Benefiting from its unique structure and physicochemical properties,nanocellulose is highly competitive as a substrate for supercapacitors.Nevertheless,the inherent insulating property of nanocellulose will block the electron transport in the composites,and poor interface stability between the conductive materials and nanocellulose also has a great impact on the electrochemical performance of conductive composites.In addition,nanocellulose-derived carbon materials usually have problems such as low carbon yield and limited specific capacitance.Therefore,improving the capacitance and cycling stability is the key point of current research on nanocellulose-based electrode materials.Based on this,this paper focuses on the modification of nanocellulose and the structural design of conductive composites,aiming to prepare high-performance nanocellulose-based conductive composites that can be used as supercapacitor electrodes.The main research contents are described as follows:1.Preparation of graphitized cellulose nanocrystals:the surface of nanocellulose was dehydrated and carbonized by the high heat released when H₂SO₄ generated a high concentration of protons to prepare the cellulose nanocrystals with highly oriented graphitized carbon layers,which solved the problem of non-conductivity.The morphology and surface graphitic carbon layer structure of GCNC can be effectively controlled by adjusting the reaction time.GCNC had typical rod-like nanostructures and well-maintained cellulose properties,and the retained oxygen-containing functional groups endowed GCNC with excellent wettability and good solution dispersibility.Benefiting from the robust and high mobility conductive graphitic carbon layer structure,GCNC had a specific capacitance of 139.4 F g^-1 and excellent cycling stability（100%capacitance retention after 3000 cycles）.2.Preparation and performance of GCNC/manganese dioxide heterostructure composites:the GCNC/manganese dioxide（Mn O₂）heterostructure composites were prepared through an in-situ growth strategy using GCNC as the conductive support framework.The uniform loading of Mn O₂ on the GCNC surface improved the agglomeration problem of Mn O₂ and increased the effective specific surface area in contact with electrolyte ions.Meanwhile,GCNC and Mn O₂ are stably connected by covalent bonds,constructing a reliable heterostructure with good interfacial bonding,which solved the problem of interface firmness of composites and effectively reduced the contact resistance at the GCNC/Mn O₂ interface.The GCNC/Mn O₂ heterostructure composites exhibited excellent electrochemical performance,which achieved a high specific capacitance of 528.2 F g^-1at the current density of 0.5 A g^-1.In addition,the assembled symmetric supercapacitor based on GCNC/Mn O₂-15m M showed high rate capability and outstanding cycling stability（100%capacitance retention after 3000cycles）.3.Construction and performance of 3D porous carbon aerogel composites:carbon aerogel composites（CNFAs）with 3D interconnected hierarchical porous nanostructures were prepared through engineering the pyrolysis chemistry of nanocellulose using inorganic salts,which solved the problems of low carbon yield and mechanical fragility of nanocellulose-derived carbon materials.CNFAs with excellent mechanical properties,large specific surface area,and hierarchical porous network can offer abundant channels for efficient diffusion and transport of the electrolyte ions,while more oxygen-containing functional groups are retained on the surface of CNFAs contributing to generating pseudocapacitance.CNFAs exhibited an exceptionally high capacitance of 440.3 F g^-1at the current density of 1.0 A g^-1,which was significantly superior to the recently reported carbon materials derived from cellulose or other biomass.The further assembled symmetric supercapacitor based on CNFAs-8%showed excellent rate capability,high energy density(0.081 m Wh cm^-2),and impressive cycling stability（100%capacitance retention after 7000 cycles）.4.Construction and performance of fiber-shaped asymmetric supercapacitor:the CNFA@ESF composite electrodes were prepared by ultrasound-assisted cycle drying processes using silk fiber（SF）as a flexible substrate.The calcium chloride/deionized water/ethanol ternary solvent system was employed to slightly dissolve the surface of SF,combined with the ultrasound-assisted cycle drying method,which solved the problems of low loading of CNFA and weak bonding fastness between CNFA and SF.The CNFA@ESF exhibited excellent tensile strength（251.9 MPa）and elongation at break（35.3%）,as well as high flexibility.Meanwhile,the uniform loading of CNFA on the SF surface formed a highly interconnected conductive network structure,endowing the CNFA@ESF with good electrochemical performance,which showed an area specific capacitance of 44.44 m F cm^-2,excellent rate performance,and cycle stability（91%capacitance retention after 5000 cycles）.In addition,the fiber-shaped asymmetric supercapacitor was assembled with CNFA₃@ESF as the negative electrode and the GCNC/Mn O₂@ESF as the positive electrode,which had a volumetric capacitance of66.88 m F cm^-3 and the maximum power density and energy density are 522.3μW cm^-3and 0.60μWh cm^-3,respectively.