Cellulose Based Highly Conductive Film And Its Application In Supercapacitors

With the rapid development of wearable electronic technology,energy storage devices with high flexibility,light weight and good electrochemical performance have attracted people's attention.The problems of unstable physical connection,poor mechanical properties and high interface resistance in traditional laminated supercapacitors need to be further solved.Nano-cellulose has the advantages of high mechanical strength,biodegradability and strong compatibility with electrically active materials such as graphene and carbon nanotubes,so it is an excellent raw material for the preparation of flexible supercapacitors.In this paper,through layer-by-layer self-assembly,cellulose nanoparticles were used as diaphragm and electrode materials to prepare an integrated flexible supercapacitor with high conductivity and good tensile strength.In this thesis,cellulose nanofiber(NFC)was prepared by mechanical grinding method using pulpboard as raw material in the oxidation system of 2,2,6,6-tetramethylpiperidine nitrogen oxide(TEMPO)/sodium bromide/sodium hypochlorite.The hydroxymethyl group of cellulose was oxidized to carboxyl group.NFC containing carboxyl group on the surface as a dispersant for CNT,so that CNT can exist stably in water without agglomeration.The ultraviolet spectrum analysis of CNT dispersed by NFC showed that the absorption peak position of CNT dispersed by NFC was the same as that of sodium cetylbenzene sulfonate.And there was no precipitation in the dispersion for one month.The conductive film of carbon nanotube/cellulose nanofiber/carbon nanotube(CNT/NFC/CNT)sandwich structure was prepared by alternating filtration of the CNT dispersion and the aqueous dispersion of NFC.When the mass fraction of CNT in the film was 19.2 wt%,the tensile strength of the sandwich conductive film was 60.8 MPa,and the Young's modulus was 3.8 GPa,which was similar to that of the pure NFC film,indicating that the film had good mechanical properties.The surface resistance of CNT/NFC/CNT thin films by four-probe technique showed that the conductivities of the upper and lower surfaces of the films were 44.4 and 43.7 S·cm-1,respectively.By using the digital multimeter to test the conductivity of the thin film,it was found that the CNT/NFC/CNT film has the distinct characteristic of conductive anisotropy.That was,the direction perpendicular to the film showed conductive insulation,and the conductivity in the horizontal direction was as high as 90.8 S·cm-1.Based on the anisotropic conductivity of CNT/NFC/CNT thin film,PPy-CNT composite active material was electrodeposited on the surface of the film,and an integrated "electrode-diaphragm-electrode" supercapacitor with NFC as electrolyte diaphragm was prepared.The test results showed that the specific surface area of the integrated capacitor was as high as 27.6 m2·g-1,the surface pore diameter was large,which was beneficial to the transfer of electrolyte ions and electrons.When the electrodeposition time was 900 s,the electrochemical performance of the material was the best.The electrochemical performance of the capacitor in the two-electrode test system showed that when the current density was 1 mA-cm-2,the area specific capacitance of the capacitor was 56.25 mF·cm-2,and the volume specific capacitance was 11.25 F·cm-3,while the area and volume specific capacitance of the capacitor prepared by pure CNT were only 10.55 mF·cm-2 and 2.93 F·cm-3.When the PPy-CNT composite active material was electrodeposited,CNT and PPy wrapped the PPy outside the CNT through the action of ?-? thus reducing the contact between the CNT.As a result,the resistance of electrolyte ions passing through the conductive layer was reduced.Under the power density of 0.08W·cm-3,the volumetric energy density of the capacitor was as high as 1 mWh·cm-3.At the same time,the electrochemical performance of the prepared integrated supercapacitor remained unchanged at any bending angle,indicating that the prepared supercapacitor had good bending performance,and the specific capacity still maintained 90.48%of the initial value after 1000 cycles.The integrated supercapacitor prepared in this paper provided a new development way for the field of portable wearable electronic devices and flexible sustainable power supply.