Morphology Regulation And Electrochemical Properties Of Bio-based Carbon Nanocomposite Fibers

Compared with traditional capacitors,supercapacitors have a higher energy density,higher power density,better charge-discharge performance,and longer cycle life than lithium-ion batteries.Therefore,supercapacitors have attracted more attention in the research of energy storage devices.Self-supported carbon nanofibers(CNF)with high SSA and rich pore structure are used as electrode materials for supercapacitors.Their excellent electrochemical performance meets people's urgent needs for fast charging and long battery life of energy storage equipment.Lignin is the main agricultural and forestry waste biomass resource in China.Phenol depolymerized lignin phenolic resin(PDLF)synthesized from lignin can be used as a high-quality carbon source for the preparation of CNF.This greatly improves the high-value utilization of biomass resources and realizes the concept of low cost and green environmental protection of carbon materials.In this paper,PDLF resin was used as a carbon source,CNF and their composites were prepared by electrospinning and atomic layer deposition(ALD).The microstructure,thermal stability,and pore structure of the samples were investigated by scanning electron microscopy,thermogravimetric analyzer,and N₂ adsorption and desorption.The electrochemical properties of the materials were tested and analyzed in the three-electrode and two-electrode system respectively.The main research contents are as follows:(1)The alkali lignin was depolymerized under high temperature and pressure,and then the active site of phenolic hydroxyl group was improved by phenolic modification.It was used to replace 50%phenol by condensation reaction with hemicellulose-derived furfural in an alkaline environment to form PDLF resin.The molecular weight,and reaction mechanism were characterized by FTIR,NMR,and GPC.The curing kinetics and thermal stability were studied by thermal analysis.(2)PDLF resin was used as the carbon source for electrospinning,and CNF was obtained by gradient temperature curing and high-temperature carbonization.The effects of spinning conditions,such as concentration of spinning solution,voltage,and receiving distance,on the diameter and morphology of CNF,and finally on the electrochemical performance were discussed.The results showed that CNF-50%-14KV-15cm has a uniform diameter,smooth surface and the best microstructure under the conditions of 14 k V,15 cm,and spinning liquid concentration of 50%.The specific surface area(SSA)and pore volume of CNF-50%-14KV-15cm were 376 m²·g^-1 and0.20 cm³·g^-1,respectively.The specific capacitance of CNF-50%-14KV-15cm is 241F·g^-1 at the current density of 0.1 A·g^-1,and the capacity retention is 68%at 20 A·g^-1.The capacity retention rate is 98%after 1000 charge-discharge cycles at 1 A·g^-1.At the power density of 100 W·kg^-1,the energy density is 20.83 Wh·kg^-1.(3)CNF was activated by soft template F127,and the SSA and pore volume of ACNF were increased to 1134 m²·g^-1 and 0.63 cm³·g^-1,respectively.The specific capacitance of ACNF-F127 is 293 F·g^-1 at the current density of 0.1 A·g^-1,and 160 F·g^-1 at 20 A·g^-1.The capacity retention rate increased by 11%after 1000 charge-discharge cycles at 1 A·g^-1.At the power density of 100 W·kg^-1,the energy density is 26.25Wh·kg^-1.The specific capacitance and energy density increased by 47%and 26%than CNF sample,respectively.(4)After different ALD cycles(300,600,900),a layer of uniform nickel oxide nanoparticles was deposited on the surface of ACNF.Nickel oxide nanoparticles can not only increase the mechanical stability of ACNF,but also improve the specific capacitance through oxidation-reduction reaction.Good mechanical stability enables supercapacitors to have longer cycle life and better rate performance.The SSA and pore volume of NiO/ACNF-600 were 792 m²·g^-1 and 0.45 cm³·g^-1,respectively.The electrochemical performance test results show that the specific capacitance of NiO/ACNF-600 is 992 F·g^-1 at the current density of 1 A·g^-1,and 320 F·g^-1 at 20 A·g^-1.After 1000 charge-discharge cycles at 1 A·g^-1,the capacity retention rate increased by18%.The energy density of NIO/ACNF-600 is 96.11 Wh·kg^-1 at the power density of100 W·kg^-1.The specific capacitance is increased by 6 times and the energy density is increased by 4.8 times than CNF sample.(5)The specific capacitance of TiO₂/ACNF composites prepared by ALD is 778F·g^-1 at 0.1 A·g^-1.At the current density of 1 A·g^-1,the specific capacitance is 372 F·g^-1,and the capacity retention rate is 83%after 1000 charge-discharge cycles.At the power density of 100 W·kg^-1,the energy density of TiO₂/ACNF is 36.67 Wh·kg^-1.Compared with the CNF sample,the specific capacitance is increased by 57%and the energy density is increased by 1.8 times.