Synthesis Of Lignin-based Carbon Nanofibers For Flexible Supercapacitor Electrodes

Lignin is one of the most abundant biopolymers in nature,and is regarded as a potential raw material for many high value-added products.Therefore,the high-value utilization of lignin has been paid more and more attention.As the only non-petroleum-based biomass source that can provide a large amount of renewable aryl compounds,lignin is an ideal precursor for the production of carbon materials.Converting low-cost lignin into high value-added carbon nanofibers for flexible electrode materials and applying it to supercapacitors,which can not only make full use of the lignin,but also effectively alleviate the problem of energy shortage.In this paper,lignin is used as the carbon source,and Hemin as the source both of iron and nitrogen,the super-flexible carbon nanofibers with outstanding folding resistance were fabricated by an electrospinning technique and high-temperature carbonization,demonstrating huge application potential in wearable and portable electronic products.In this paper,three types of iron-nitrogen co-doped lignin-based porous carbon nanofibers with different structures were prepared.The microscopic morphology,composition,microcrystalline structure,specific surface area and pore size distribution of carbon nanofibers were analyzed through a variety of characterization methods.Cyclic voltammetry,galvanostatic charge-discharge,and AC impedance spectroscopy were conducted on the samples in a three-electrode system and a two-electrode system respectively to explore their electrochemical performance.The results shown as below:1.The fibrous network structure of lignin-based carbon nanofibers and the rich microporous structure on the fibers provide a high specific surface area and a large number of ion channels,shortening the diffusion distance of electrolyte ions,which create good conditions for rapid charge and discharge.The Fe O_xand Fe₃C particles embedded in the carbon fibers generated pseudocapacitance through redox reaction with electrolyte ions,which enhances the capacitance of the supercapacitors.Nitrogen doping improves the surface wettability and conductivity of the material,which further promotes the electrochemical properties.2.Iron-N co-doped lignin-based carbon nanofibers with Hemin content of 20%(CNF@Fe-N-20)obtained a specific capacitance of 148.1 F g^-1 at a current density of 0.5 A g^-1.The energy density of the symmetrical supercapacitor based on CNF@Fe-N-20 reached14.27 W h kg^-1 at the power density of 177.14 W kg^-1,and maintained 92.8%of initial capacitance after 5000 charge/discharge cycles,indicating an excellent long-term cycle stability.3.The core-shell structure of lignin-based carbon nanofibers(SCNF@Fe-N)can effectively improve the contact efficiency between electrolyte ions and redox active sites by exposing Fe O_x particles to the surface of carbon fibers as much as possible.SCNF@Fe-N-5obtains a specific capacitance of 138.1 F g^-1 at a current density of 0.5 A g^-1.Compared with CNF@Fe-N-20,SCNF@Fe-N-5 reached the equivalent capacitance by a reduced Hemin content,which effectively saves the preparation cost.SCNF@Fe-N-5 showed an energy density of 13.2 W h kg^-1 at the power density of 168 W kg^-1,and holded a capacitance retention of 94.2%after 5000 charge/discharge cycles,providoing the possibility for a long working life.4.The construction of hollow structure effectively reduces the diffusion resistance of electrolyte ions in the electrode,leading to an improvement of migration speed of ions.Compared with solid carbon fiber,the overall electrochemical performance of hollow carbon nanofiber(HCNF@Fe-N)has been significantly improved.HCNF@Fe-N-15 achieves a high specific capacitance of 444 F g^-1 at a current density of 1 A g^-1,and maintains a specific capacitance of 73.6%at a high current density of 20 A g^-1,showing an excellent rate capacity.HCNF@Fe-N-15 exhibited a high energy density of 43.6 W h kg^-1 at the power density of799.2 W kg^-1,as well as an excellent cycling stability with 92.6%capacitance retention after5000 charge/discharge cycles at a current density of 5 A g^-1.