Preparation And Characterization Of Cellulose Fiber Flexible Substrate And Self-supporting Carbon Fiber Network Material

Cellulose is the most abundant natural polymer on earth.Cellulose fiber is the main raw material of paper.Conventional paper made of cellulose fibers is rough,opaque and hydrophilic.Herein,transparent paper,which can be used as substrate material of flexible electronics,was prepared by impregnating conventional paper with tetraethyl silicate?TEOS?hydrolysate followed by coating the paper with polydimethylsiloxane?PDMS?,which has similar refractive index to cellulose.Cellulose fiber networks with open structures were prepared by freeze-drying wood fiber suspension after the fibers were oxidized with sodium periodate.The fiber networks were further changed into carbon fiber networks through a low-temperature precarbonization and a high-temperature carbonization.The as-prepared carbon fiber networks have excellent electrical conductivity,good compressibility and flexibility,and it can be used as the electrode of a supercapacitor.Ni O/Ni Fe₂O₄and Ni O/Ni Co₂O₄were loaded on the surface of carbon fiber networks by coprecipitation-calcination method,while Ni Fe-LDH and Ni Co Al-LDH were grown in situ on the surface of carbon fiber networks by a hydrothermal method,forming various carbon fibers/hydrotalcite-based composite electrode materials.Cotton cloth woven composed of high crystallinity cotton cellulose fibers was used to prepare frees-supporting carbon fiber networks via oxidization with sodium periodate followed bylow-temperature precarbonization and high temperature carbonization.The porous self-supporting carbon fiber networks were directly used as supercapacitor electrode.The electrochemical and mechanical properties of various carbon fiber networks and carbon fiber/hydrotalcite-based composites were tested.The experimental results were shown as follows.After the paper was impregnated with tetraethyl silicate hydrolysate,silica nanoparticles produced by the hydrolysis of tetraethyl silicate adhered to the fiber surfaces,which helped PDMS penetrate into the fiber networks before curing and crosslinking,and promoted the filling of PDMS in the pores among fibers.The coating of PDMS could drive out the air in paper,and form a hydrophobic transparent coating layer on the paper surface after the curing of PDMS.This greatly improved the hydrophobicity of the paper,whose contact angle was as high as about 110°.Meanwhile,the hydrophobicity of paper was rather stable,and the paper transmittance reached 95%.The paper coated with PDMS also displayed improved thermal stability,tensile strength,deformation resistance and flexibility.Compared with the pristine coniferous wood fibers,the fibers oxidized by sodium periodate became shorter,and have wrinkled surfaces.The porous cellulose fiber networks obtained by freeze-drying well remained their frameworks after carbonization,forming a flexible and compressible carbon fiber networks.For the oxidation of cellulose fibers with sodium periodate,when the concentration of sodium periodate reached 5%,the fiber networks after carbonization had the best performance as an electrode material.In the three electrode system that employed 6mol/L KOH aqueous solution as electrolyte,the specific capacitance of carbonized fiber networks reaches 250 F/g when the voltage window is between-1?-0.2V and the current density is 0.5 A/g.After 1000 loading cycles,the specific capacitance is still as high as 100 F/g.At the pressure of 35 KPa,the deformation rate of carbonized fiber networks reaches 60%.At the compression speed of 20 mm/min,the rebound rate remains above 80%after 50 times of compression.Compared with the carbonized fiber networks without oxidation before carbonization,the fiber networks oxidized by sodium periodate before carbonization have a great improvement in electrochemical properties and mechanical properties.The electrode materials loaded with Ni O/Ni Fe₂O₄,Ni O/Ni Co₂O₄,Ni Fe-LDH or Ni Co Al-LDH pseudocapacitive materials on carbonized fibers displayed typical pseudocapacitive behavior.In three electrode system that used 6 mol/L KOH as electrolyte,the carbon fibers/Ni O/Ni Fe₂O₄and carbon fibers/Ni O/Ni Co₂O₄composites prepared by a coprecipitation-calcination method had a specific capacitance of about 380.2 F/g and 495 F/g,respectively while the carbon fibers/Ni Fe-LDH and carbon fibers/Ni Co Al-LDH composites had a specific capacitance of about 293.7 F/g and 343.8 F/g,respectively,at a voltage window of-1?0.5 V and a current density of 0.5 A/g.Among these carbon fibers/LDH-based composites,the carbon fiber/Ni O/Ni Co₂O₄composite not only showed a larger specific capacitance,but also had a better compressibility.At the pressure of 11 KPa,the deformation rate reached 60%,and the rebound rate remained above 50%after 50times of compression at the compression speed of 20 mm/min,.After being oxidized by sodium periodate followed by low-temperature precarbonization and high-temperature carbonization,cotton cloth has been changed into porous self-supporting carbon fiber network material with high specific surface area.The as-prepared self-supporting carbon fiber network material has good electrochemical performance when it was used as a bulk electrode.Among the carbonized cotton cloth oxidized by sodium periodate of different concentrations before carbonization,the carbonized cotton cloth oxidized by 5%sodium periodate before carbonization has the largest specific surface area of 686.87 m²/g,and the best electrochemical performance as an bulk electrode.In the three electrode system that employed 6 mol/L KOH as electrolyte,its specific capacitance reaches550 F/g when the voltage window is-1.6?0.5V and the current density is 0.5 A/g,and does not changed significantly after 1000 charge-discharge cycles at the current density of 4A/g,indicative of an excellent cycle stability.