Facile and sustainable synthesis of sodium lignosulfonate derived hierarchical porous carbons for supercapacitors with high volumetric energy densities

Interconnected hierarchical porous carbon was successfully prepared by direct carbonization of industrial waste sodium lignosulfonate without additional templating and activation agents. The as-prepared carbon sample shows a moderate specific surface area of 903 m² g⁻¹ and high contents of 8.11 at% oxygen and 1.76 at% nitrogen, which could improve the electrolyte-affinitive surface area in an aqueous electrolyte. When used as electrode materials for symmetric supercapacitors in 7 M KOH electrolytes, the as-synthesized carbon sample exhibits a significantly high gravimetric capacitance of 247 F g⁻¹, a volumetric capacitance of 240 F cm⁻³, and an areal capacitance of 27.4 μF cm⁻² at a current density of 0.05 A g⁻¹. Moreover, a superior energy density of 8.4 W h L⁻¹ (at 13.9 W L⁻¹) and a power density of 5573.1 W L⁻¹ (at 3.5 W h L⁻¹), as well as a remarkable cycling stability after 20 000 cycles at two different current densities were achieved for the assembled supercapacitors.