Effect of lignin structure in different biomass resources on the performance of lignin-based carbon nanofibers as supercapacitor electrode

Lignin is proposed to an attractive stuff for fabricating functional composite nano-carbon materials owing to its high carbon yield and low-cost. In this work, in order to unlocking the mechanism of the plant species of lignin on electrochemical energy storage capacity of lignin-based carbon nanofibers (LCNFs), three typical lignin from representative hardwood (poplar, PR), softwood (pine, PE) and grass (corn stalk, CS) materials, and containing different structural units (syringyl units (S), guaiacyl units (G), and/or p-hydroxyphenyl unit (H)) were chosen to prepare LCNFs with polyacrylonitrile (PAN) as blending agent. As compared with other LCNFs, poplar lignin (PRL) based LCNFs-PRL (5:5) with independent filamentous morphology networks exhibited higher tensile strength of 35.32 MPa, greater specific surface area of 1062.5 m2/g and larger specific capacitance of 349.2 F/g. Moreover, in a two electrode system, the assembled LCNFs-PRL (5:5)//LCNFs-PRL (5:5) symmetric supercapacitor also displayed an excellent energy density of 39.6 Wh/kg at the power density of 5000 W/kg and outstanding cycling stability of 90.52 % after 5000 cycles in Na2SO4 aqueous electrolyte. The results proved that hardwood lignin was a good green raw material for LCNFs. The mechanism of lignin chemical structure on the properties was demonstrated in detail by various characterization analysis, such as electrochemical test, nuclear magnetic resonance spectroscopy (NMR) and gel permeation chromatography (GPC), which indicated that the structural units, molecular weight, homogeneity, hydrophilic and hydrophobic groups of lignin were more important for fabricating high quality LCNFs.