N–O–S Co–Doped Hierarchical Porous Carbons Prepared by Mild KOH Activation of Ammonium Lignosulfonate for High–Performance Supercapacitors

The development of porous carbon materials that meet the demands of commercial supercapacitors is challenging, primarily due to the requirements for high energy and power density, as well as large-scale manufacturing capabilities. Herein, we present a sustainable and cost-effective method for synthesizing N–O–S co-doped hierarchical porous carbons (designated as ALK_x) from ammonium lignosulfonate (AL), an industrial by–product. This process employs a low KOH/AL mass ratio (x ≤ 0.75) and a carbonization temperature of 900 °C. The resulting materials, ALK₀.₅₀ and ALK_0.75, exhibit an exceptionally high specific surface area (>2000 m² g⁻¹), a well-balanced micro-mesoporous structure, and tunable heteroatom content, which collectively enhance their electrochemical performance in both aqueous and ionic liquid electrolytes. Notably, ALK_0.75 features a heteroatom content of 13.2 at.% and a specific surface area of 2406 m² g⁻¹, owing to its abundant small mesopores. When tested as an electrode in a two–electrode supercapacitor utilizing a 6 M KOH electrolyte, it achieves a high specific capacitance of 250 F g⁻¹ at a current density of 0.25 A g⁻¹ and retains 197 F g⁻¹ even at 50 A g⁻¹, demonstrating remarkable rate capability. In contrast, ALK_0.50, characterized by a lower heteroatom content and an optimized pore structure, exhibits superior compatibility with the ionic liquid electrolyte EMIMBF₄. A symmetric supercapacitor constructed with ALK_0.50 electrodes attains a high energy density of 90.2 Wh kg⁻¹ at a power density of 885.5 W kg⁻¹ (discharge time of 60 s). These findings provide valuable insights into heteroatom doping and the targeted regulation of pore structures in carbon materials, while also highlighting new opportunities for the high-value utilization of AL.