Lignin Selection Improves the Performance of Porous Carbon Nanofiber Electrodes in Freestanding Supercapacitors

Lignin is an attractive precursor for renewable carbon materials due to its low cost and comparatively high carbon yield. The resulting carbons may be used in energy storage devices (supercapacitors and batteries) and structural composites. Since lignin has substantial variability in its chemical structure, influenced by its botanical origin and the method of extraction, this work explores the effect of lignin type on the structure and performance of porous carbon materials, using electrospun carbon nanofibers applied as freestanding supercapacitor electrodes as an example. Precursor nanofiber mats were electrospun from aqueous NaOH solutions of six technical lignins, which originated from commercially relevant biomass types, i.e., hardwood (eucalyptus and beech), softwood (pine/spruce mix), and grass (Miscanthus) and three extraction methods (ethanol organosolv, Kraft, and ionosolv), with poly-(ethylene oxide) (PEO) as the spinning aid. After stabilization at 250 °C and carbonization/activation at 1000 °C, the supercapacitor performance was evaluated in symmetric two-electrode cells using an aqueous electrolyte (6 M KOH). Correlation of a large number of microstructural characteristics and lignin chemical properties showed that a high micropore (1-2 nm) volume increased gravimetric capacitance of the lignin-derived carbon nanofiber (LCNF) mats (up to 192 F/g at 0.25 A/g), while a high packing density maximized volumetric capacitance (up to 19.9 F/cm³ at 0.25 A/g). The packing density of the LCNF electrodes was strongly correlated to the viscosity of the lignin-PEO solution, showing potential for engineering material performance through spinning solution composition. Technical lignins with higher contents of phenylpropanoid linkages and hydroxyl groups, especially phenolic hydroxyl groups, exhibited increased microporosity and hence gravimetric capacity but also decreasing packing density and hence volumetric capacitance, indicating a trade-off situation. Finally, by changing the aqueous electrolyte to a water-in-salt electrolyte (12 mol/kg NaNO₃), the energy density of the best-performing supercapacitor cell was increased from 8.9 to 15.5 Wh/kg, due to the higher stable operating voltage (1.8 vs 1.2 V), delivering competitive performance with typical activated carbon powder-based supercapacitors (5-15 Wh/kg) and highlighting the importance of the whole device design for boosting performance.