Stable Lignin-Rich Nanofibers for Binder-Free Carbon Electrodes in Supercapacitors

In this work, 1,2,4,5-benzenetetracarboxylic acid (BT) is used as a cross-linker to improve the oxidative thermostabilization of lignin-rich nanofibers so that the activated carbon nanofiber (ACNF) sheets can be prepared from lignins. As sources, we used organosolv lignins from sugarcane bagasse (LB), para rubber wood chip (LR), and palm kernel shell (LP). The addition of BT imparts hydrogen bond interactions between BT and lignin and increases the oxidation degree of lignin during the stabilization, enabling retention of nanofiber morphology after heat treatments. The preparation of activated carbon in the form of nanofibers substantially enhances the specific surface area of the materials when compared with the activated carbon powders from pure lignins. The properties of the resulting ACNFs are influenced by the structure and composition of lignins. With the synergistic effects of a small fiber diameter (688 ± 80 nm), a high content of heteroatom (O atomic content ∼5.1%), a high graphitic carbon structure (ID/IG ratio ∼0.98), a high conductivity (NI>0.1nA/A ∼3479 ± 404 pixel μm–2 at 0.5 V), and a high specific surface area (∼2195 m2 g–1), the ACNFs from LB show the highest specific capacitance of 182 F g–1 (at a current density of 0.1 A g–1 in 1.0 M H2SO4 electrolyte) with good cycling stability (95.7% capacitance retention after 6000 cycles). A maximum energy density of 31 W h kg–1 at a power density of 240 W kg–1 can be achieved from the LB/PEO/BT ACNFs. Their performance is almost comparable to that of polyacrylonitrile ACNFs. These findings demonstrate a simple and effective approach to produce lignin-based ACNFs from diverse biomass feedstock for sustainable development of renewable materials for supercapacitor electrodes.