Preparation Of Lignin-based Carbon Nanofibers And Its Applications In Supercapacitors

Supercapacitors are considered as one of the most promising new energy storage devices due to its advantages such as high-power density,good cycling performance and fast charge/discharge rate.The electrode material of supercapacitor is a key factor to determine its performance.Electrospun carbon nanofibers are ideal electrode materials for supercapacitors because of its advantages such as large aspect ratio and shorter electron transport path.However,the used precursor materials for electrospinning are usually derived from petrochemical products,such as polyacrylonitrile(PAN)and polyvinyl alcohol(PVA),which are expensive and nonrenewable.Finding cheap and renewable precursors is of great concern.Lignin is considered as an excellent carbon precursor material due to its low price,high carbon content and high crosslinking of aromatic heteropolymers.In this thesis,lignin-based carbon nanofibers were prepared by electrospinning and applied to supercapacitors.The electrochemical properties of the prepared nanofibers were studied.The specific work is as follows:(1)Lignin and PAN were used as precursors,and polymethyl methacrylate(PMMA)was used as pore-forming agent and cross-linking agent for spinning.The effects of PMMA’s content in the spinning solution on the morphology,structure and specific surface area of the nanofibers were investigated by scanning electron microscopy(SEM),transmission electron microscopy(TEM)and physical adsorption apparatus.The electrochemical properties of the nanofibers were also tested.The results showed that the prepared carbon nanofibers with cross-linked structure and large specific surface area have excellent electrochemical properties.The electrode specific capacitance can reach 233 F g-1 when the content of PMMA is 50%.When current density increases from 0.5 A g-1 to 100 A g-1,the specific capacity retention is 54.5%.Meanwhile,the assembled symmetric supercapacitor has a good cycle stability with a capacitance retention of 96.8%after 50,000 cycles at the current density of 2 A g-1.(2)Polycondensation of lignin can produce lignin-based phenolic resin,then used as precursor for spinning,which greatly reduces the amount of polymer,and get carbon membrane derived form lignin(LCM).LCM is used as matrix to prepare α-Fe2O3/LCM by template method and LCM@ppy-10 by electrodeposition.The morphology and structure of the materials were characterized by scanning electron microscopy(SEM),transmission electron microscopy(TEM)and X-ray diffraction(XRD),and the electrochemical properties of the materials were also tested.The results showed that the prepared α-Fe2O3/LCM and LCM@ppy-10 electrodes have excellent electrochemical performance,whose specific capacitance can reach 487 mF cm-2 and 440 mF cm-2 respectively.(3)The LCM@ppy-10 and α-Fe2O3/LCM electrode materials prepared in the previous chapter are assembled into asymmetric capacitors and solid asymmetric supercapacitors.It is showed that LCM@ppy-10//α-Fe2O3/LCM asymmetric supercapacitors exhibit excellent capacity performance(101 mF cm-2 at a current density of 1 mA cm-2)and cycling performance(95%capacity retention after 10,000 cycles)at a working window of 1.6 V.The assembled LCM@ppy-10//α-Fe2O3/LCM solid-state asymmetric supercapacitor can reach 89 mF cm-2 and 1.74 F cm-3,respectively,and the maximum energy density and power density can reach 0.62 mWh cm-3 and 77.2 mW cm-3,respectively.