Preparation And Properties Of Nanocellulose-Based Conductive Energy Storage Materials With A Sandwich Structure

Cellulose nanofibers(CNF)is a novel biomass polymer material.It has many excellent properties with compared to traditional polymers,such as:high flexibility,heat resistance,hydrophilicity,biocompatibility and degradability.As a result,CNF have broad application prospects in the fields of fine chemicals,nano-materials,and biological medicine.Also,high specific surface area,high Young's modulus,and high crystallinity of CNF make it suitable candidates as a substrate material for developing electrical energy storage devices with light flexibility.With the fine structure and abundant oxygen-containing active groups of CNF,it is of great research significance to expand its further application in the field of conductive energy storage.In this thesis,CNF was first produced using mechanical treatment combined with pre-oxidation process.This whole fabrication process was not only simple and easy to operate,but also low energy consumption and high yield;Secondly,a simple layer-by-layer assembly method was used to prepare a kind of sandwich structured conductive paper based on cellulose nanofibers.Thirdly,a flexible All-in-One supercapacitor film was successfully prepared.The electrochemical properties of CNF film as separator and electrolyte material were studied.Furthermore,utilizing the advantages of hydrophilicity and mechanical reinforcement of CNF,a flexible sandwich-like film electrode was developed based on polypyrrole,graphene and CNF,which show good flexibility and electrochemical properties.The main results can be summarized as follows:(1)CNF was successfully isolated from wood cellulose using high intensity ultrasonication combined with TEMPO/NaBr/NaClO oxidation pretreatment.The amount of carboxylate groups on the surface of CNF could be adjusted by altering the amount of NaClO added.This pretreatment could greatly reduce the energy consumption during the preparation process and increase the yield of CNF.When the carboxylate content increased to 1.87 mmol g-1,the yield of CNF could reach 86%,correspondingly the size of the individual nanofiber became thinner.The structure and properties of CNF were characterized.(2)A sandwich structured Graphene/CNF/Graphene conductive paper was successfully prepared through a three-step vacuum filtration followed by chemical reduction process.The electrical conductivity and mechanical properties of the sandwich conductive paper could be adjusted by changing the amount of graphene oxide(GO)and CNF dispersions.Hydroiodic acid was selected as the chemical reducing agent for the reduction of GO.The structure and properties of reduced graphene oxide(RGO)were characterized by X-ray diffraction,Raman and X-ray photoelectron spectroscopy.As a result,a highly conductive network on the top and bottom surface of sandwich structured conductive paper could be obtained without destroy its structural integrity.The as-fabricated paper exhibited an intrinsic electrical conductivity anisotropy,and the in-plane electrical conductivity were significantly enhanced,while it was insulating along the cross-plane direction.The electrical conductive anisotropy was characterized by a multimeter and Four-probe system,its optimized in-plane electrical conductivity could reach up to 4382 S m-1 with 4 wt%RGO.In addition,the hydrophobicity,char-forming and electromagnetic shielding effectiveness of the sandwich structured conductive paper were also studied.(3)Based on the electrical conductive anisotropy of the sandwich structured paper,the polypyrrole coated graphene oxide(GO@PPy)nanocomposites were electrodeposited on its two sides,which was consistent with the integrated electrode/separator/electrode sandwich structure for supercapacitor.The capacitance performance of GO@PPy materials could be adjusted by altering the amount of GO added and electrodeposition time.The microstructure,porosity and electrolyte uptake capability of pure CNF film was investigated by Scanning electron microscopy and Nitrogen adsorption/desorption method.For comparison,two kinds of supercapacitors were prepared by using PVA/H3PO4 and CNF/H2SO4 as separator/electrolyte,respectively.As a result,the integrated supercapacitor film exhibited a high areal and volume capacitance of 64.8 m1 cm-2 and 31.0 F·cm3.This integrated supercapacitor film was mechanically flexible,and the capacitance value were not be effected under different bending angles.It also possessed good mechanical properties and its tensile strength could reach up to 75 MPa.(4)Based on the active materials PPy,RGO,and CNF with high flexibility,high specific surface area and high mechanical strength,a novel sandwich-like structured electrode film was fabricated through a simple vacuum filtration followed by chemical reduction process.In a three-electrode system,this electrode film exhibited excellent capacitance performance,a capacitance of 304 F g-1 was obtained.The effects of CNF on the capacitance properties of RGO and PPy materials were also studied,including the energy storage mechanism with the ion and electron transportion.A solid state symmetric supercapacitor was assembled by two pieces of the resultant films as electrodes and CNF/H2SO4 membrane as the polymer electrolyte.It possessed a high specific capacitance of 625.6 F g-1 at 0.22 A g-1,a maximum volumetric energy density of 21.7 Wh kg-1 at the power density of 0.11 kW kg-1 and a good capacitance retention of 75.4%after 5000 cycles.The results shown that the assembled supercapacitor had excellent electrochemical behavior.