Synthesis And Electrochemical Properties Of Cellulose-based Carbon Nanofibril/Manganese Oxide Aerogels

With the aggravation of environmental pollution,energy shortage and other problems,the task of developing new clean energy is imminent.Cellulose has been widely concerned because of its abundant,renewable and pollution-free advantages.Supercapacitors also play a significant role in energy storage due to their high power density and long service life.In this paper,2,2,6,6-tetramethylpiperidine-1-hydroxyl radical（TEMPO）oxidation was used to fabricate cellulose nanofibril（TOCN）,while freeze drying and high temperature carbonization were used to prepare cellulose-based carbon nanofibril（CTOCN）aerogel materials with abundant pore structure and high specific surface area.On this basis,manganese oxide was introduced into the carbon framework by hydrothermal method to improve the specific capacitance of the composite.Meanwhile,the electrochemical performance of the composite material was improved by nitrogen doping,sulfonated carbon nanotubes modification and increasing the reaction temperature.（1）CTOCN/MnO₂ aerogel composites were prepared by hydrothermal reaction.It showed that when the amount of MnO₂ was 20.8 wt%,a well-distributed composite could be formrd and maintained a stable three-dimensional pore structure.The specific surface area of 3-CTOCN/MnO₂sample with the mass ratio of CTOCN to KMnO₄ in the process of hydrothermal synthesis of 1:3 was 152.1 m² g^-1,and the pore of the sample was mainly mesoporous.Among the five groups of samples,3-CTOCN/MnO₂delivered the best performance,with a specific capacitance of 171.1 F g^-1 at 0.5 A g^-1.After 5000 cycles under 3 A g^-1,the specific capacitance retained 98.4%.The asymmetric supercapacitor was prepared with 3-CTOCN/MnO₂ as positive electrode and activated carbon（AC）as negative electrode.The device possessed a power density of 619.2 W kg^-1 and an energy density of 8.6 W h kg^-1.In addition,the device possessed a good cycle life,with a specific capacitance retention rate of 99.4%after 4500 cycles at 3 A g^-1.（2）Nitrogen doped carbon nanofibril/Mn₃O₄ aerogel composites were prepared by hydrothermal reaction,which contained a certain amount of MnOOH,and appropriate amount of nitrogen doping could form the composite with uniform distribution of each component.The specific surface area of the 40-NCMn sample with the mass ratio of CTOCN to urea of 1:40 was 475.3 m² g^-1,and the main pores of it was mesoporous.The 40-NCMn sample delivered the best performance,and the specific capacitance was 275.5 F g^-1 at 1 A g^-1.After 3000 cycles at 10 A g^-1,the specific capacitance retained 80.2%.The asymmetric supercapacitor possessed a power density of 600 W kg^-1 and an energy density of 23.3 W h kg^-1.In addition,the device maintained a good cycle life,with a specific capacitance retention rate of 99.2%after 3000 cycles of 5 A g^-1.The results indicated that adding N atoms into carbon skeleton could change the surface functional groups of carbon materials and improved their electrochemical properties.And because Mn₃O₄ and MnOOH are traditional pseudocapacitive materials,the introduction of Mn₃O₄ and MnOOH by hydrothermal method can promote the performance of electrode materials.（3）Sulfonated carbon nanotubes（SCNT）/carbon nanofibril/Mn₃O₄ aerogel composites were prepared by hydrothermal reaction.The specific surface area of 40-SCMN sample with a SCNT mass of 40 wt%was 409.4 m² g^-1,and the main pores of it was mesoporous.The 40-SCMn sample delivered the best performance,and the specific capacitance was 306.9 F g^-1 at 1 A g^-1.After 5000 cycles at 10 A g^-1,the specific capacitance retained 87.5%.The asymmetric supercapacitor possessed a power density of 600 W kg^-1 and an energy density of 28.9 W h kg^-1.The specific capacitance retention rate of the device was 98.8%after 3000 cycles at 3 A g^-1.The introduction of SCNT endowed the material with excellent electrical conductivity,which was conducive to the rapid diffusion of electrons.Moreover,the grafted sulfonic acid groups on the surface of SCNT posssessed electrostatic repulsion,and carbon nanofibrils could block SCNT agglomeration,which was beneficial to the uniform distribution of SCNT in carbon aerogel.