Sustainable self-doping engineering: Transforming pulping black liquor into hierarchical sulfur-doped porous carbon architectures for advanced supercapacitor electrodes

The pore structure and specific surface area of electrode materials are critical to supercapacitor performance. Herein, sulfur-doped KOH-activated alkali lignin-based porous carbon (KALC) with a high specific surface area is synthesized from alkali lignin (AL), a byproduct of sulfate pulping, via pre-oxidation and KOH-activated carbonization. The pre-oxidation step enhances the carbonization efficiency and structural stability. The resulting KALC exhibits a predominantly microporous structure with a specific surface area of 1887.4 m²/g and a pore volume of 0.792 cm³/g, significantly surpassing those of directly carbonized alkali lignin-based carbon (ALC). Electrochemical tests demonstrate that KALC achieves a specific capacitance of 396 F/g at 0.8 A/g and retains 95 % capacitance over 5000 cycles in a symmetric supercapacitor. This work presents a simple and sustainable strategy for converting industrial lignin into high-performance heteroatom-doped carbon materials for energy storage.