Microporous and Mesoporous Carbon Nanofibers from Lignin by In Situ Sodium Templating for Supercapacitor Electrodes

Lignin-derived carbon nanofibers (LCF) with hierarchical micro-mesoporous architecture featuring unusually high mesopore preservation were fabricated by electrospinning. The process used a lignin/poly(ethylene oxide) precursor in an alkaline medium (sodium hydroxide, NaOH), followed by carbonization and selective HCl washing. During electrospinning, sodium species were incorporated from the alkaline spinning solution and, upon subsequent thermal treatment, converted in situ into sodium-containing domains that acted as transient porogens, enabling the development of a highly accessible pore network without conventional activation. The resulting free-standing carbon nanofiber mats exhibited a hierarchical micro-mesoporous architecture, in which 84% of the specific surface area arose from micropores accompanied by a well-defined 8-12 nm mesopore population that is expected to facilitate ion transport, maximizing the utilization of the microporous surface for charge storage. Raman analysis further revealed a defect-rich, edge-abundant sp² carbon network (I_D/I_G = 1.05). As self-supported supercapacitor electrodes in 3 M potassium hydroxide (KOH), the materials delivered a specific capacitance of ≈275 F g^-1 at 0.25 A g^-1, retained ≈87% of this value at 2.5 A g^-1, and exhibited outstanding cycling stability, with 117% capacitance retention and 99.9% coulombic efficiency after 10,000 charge-discharge cycles. This simple, low-cost, and low-waste strategy demonstrates that in situ sodium templating effectively engineers hierarchical pore architectures in lignin-derived carbon nanofibers without aggressive chemical activation, providing a sustainable platform for high-performance energy-storage electrodes.