Two‐Step Zinc Chloride and Potassium Hydroxide Activated Nitrogen and Oxygen Co‐Doped Hierarchical Porous Biocarbon for High‐Performance Supercapacitors

ABSTRACT In this study, we have prepared N and O co‐doped porous carbon from walnut shells and significantly improved its physico‐chemical properties by a two‐step chemical activation approach using ZnCl 2 and KOH. The synergistic effect of ZnCl 2 and KOH activation resulted in a well‐developed hierarchical porous architecture with a high specific surface area (901 m 2 .g −1 ) and abundant O and N functional groups, as confirmed by XPS analysis. These features facilitated efficient ion transport and enhanced charge storage capacity. Tested in a three‐electrode supercapacitor configuration using a 1 M H 2 SO 4 electrolyte, the optimized carbon electrode exhibited a high specific capacitance of 520 F·g −1 at 1 A·g −1 , and outstanding cycling stability over 5000 cycles with a capacity retention greater than 99%. This work presents a sustainable and efficient route to designing high‐performance electrode materials from biowaste for next‐generation energy storage devices.