Fabrication Of Lignin-based Carbon Nanofibers For Electrode Material Of Electrochemical Capacitors

Lignin, the second most abundant natural polymer after cellulose, is considered to be the largest biomass material of aromatic functionality with high carbon content above 60%, making it the most attractive sustainable precursor for carbon materials. It is significant to enable the conversion of lignin into high-value chemicals or materials. One of the newly explored applications for lignin is to be utilized in producing carbon fibers for the electrode materials of electrochemical capacitors.In this study, IO-LCNFs were fabricated by electrospinning of carbon precursor solutions consisted of polymers（polyethylene oxide and bamboo acetic acid lignin（AAL））, additive（ferric acetylacetonate（Fe（acac）3）） and the solvent（N,N-dimethylformamide（DMF））, followed by carbonization after thermalstabilization. The morphology and thermomechanical behavior of IO-LCNFs was observed using field emission scanning electron microscope（SEM）, a transmission electron microscope（TEM） and thermal gravity analysis（TGA）. The crystal structural characterization of the samples was carried out by using X-ray diffractometer（XRD）, Raman spectrometer and X-ray photoelectron spectroscopy（XPS）. The as-prepared IO-LCNFs used as electrode materials and measured by cyclic voltammetry tests, galvanostatic charge/discharge tests and electrochemical impedance spectroscopy tests. While the IO-LCNFs were converted to activated carbon fibers（A-IO-LCNFs） by carbonization in a stream of nitrogen and its electrochemical properties were also tested. The results were as below:Low proportions of Fe（acac）3 when AAL/ Fe（acac）3 ratio was 99/1 resulted in fusion among the adjacent carbon fibers; While when high proportions of Fe（acac）3 was added, smooth and separated carbon fibers were fabricated with diameters of 300⁵00nm; Iron oxide particles with diameters of 30⁶0nm were both exposed outside and well-distributed in IO-LCNFs. The iron oxide particels were a mixture of hematite, magnetite and also iron metal.The as-prepared IO-LCNFs were used as electrode material. When the electrolyte was 1M Na₂SO₄, the IO-LCNFs electrode materials acted as the electrical double-layer capacitor, and the specific capacitance was 9²0 F/g. While chosen 1M Na₂SO₃ as electrolyte, the electrode had redox reaction. Among these, the most obvious redox peaks are observed for IO-LCNFs with Fe（acac）3 ratio of 20%, and the specific capacitance of IO-LCNFs-20 was 72.1F/g.IO-LCNFs were converted to A-IO-LCNFs by activating by water steam to decorate its surface, and the electrochemical properties were also tested. After activation, more holes and defects were observed on the surface of A-IO-LCNFs-10 and A-IO-LCNFs-15. While the micropore volume and mesopore volume of A-IO-LCNFs-10 were enlarged. And its BET surface area was raised from344.63m²/g to 566.45m²/g. In the electrochemical tests, A-IOLCNFs-10 and A-IO-LCNFs-15’s specific capacitance were raised from 26.5F/g, 35.5F/g to 51.4F/g, 42.4F/g, respectively. While crystalline particulates were exposed from the surface of A-IO-LCNFs-20 and A-IO-LCNFs-25. The fiber morphology was hardly recognized from AIO-LCNFs-25. A-IO-LCNFs-20’s BET surface area was down from 335.44m²/g to 273.80m²/g. A-IO-LCNFs-20 and A-IO-LCNFs-25’s specific capacitance were decreased from 72.1F/g, 55.7F/g to 38.5F/g, 3.4F/g, respectively. In summary, when low proportions of Fe（acac）3 were added, it maintained the fiber morphology of IO-LCNFs, when this kind IO-LCNFs were activated, the specific surface area and the porosity were raised, thus the electrochemical properties were also improved. While when high proportions of Fe（acac）3 were added, more crystalline iron oxide particles were synthesized during activation process and the carbon fiber morphology were destroyed. Thus there were more resistance for electrolyte to the electrode materials. And their electrochemical properties declined.