Boosting the electrochemical performance of lignin-based carbon fibers for supercapacitors via synergistic Ni/P co-doping

Lignin-based carbon fibers (LCFs) offer a promising route for both the high-value utilization of underused lignin and the development of green, low-cost electrode materials for supercapacitors. However, achieving high energy density while maintaining power density and long-term cycling stability remains a major challenge for LCF-based supercapacitors. In this study, a series of Ni/P co-doped LCFs were prepared via electrospinning, thermostabilization, and carbonization, without additional activation steps. Nickel acetylacetonate (Ni(acac) 2 ) was incorporated to enhance the thermal stability of lignin fibers during thermostabilization and to facilitate catalytic graphitization during carbonization. Phosphoric acid (H 3 PO 4 ) was introduced at various stages to tailor pore architecture, enhance surface activity, and further promote graphitic structure development. Furthermore, co-doping with Ni and P at 1,400 °C enabled the in-situ formation of Ni 2 P, which contributed additional pseudocapacitance. Benefiting from the integrated structural and compositional design, the resultant supercapacitor based on P-LSF-1400 delivered an excellent electrochemical performance, achieving a specific capacitance of 156.4 F g −1 , an energy density of 48.9 Wh kg −1 , and a power density of 802 W kg −1 , with 95 % capacitance retention after 5,000 cycles. This synergistic strategy places P-LSF-1400 well above most carbon-based electrodes derived from biomass, highlighting its strong potential for practical energy storage applications.