From Biomass to High‐Performance Electrodes: Hierarchical Porous Oxy‐Carbon for Redox‐Mediated Supercapacitors

ABSTRACT The development of high‐performance, sustainable energy storage systems is essential to address the growing global energy demand. In this study, hierarchical porous oxy‐carbon (HPOC) was synthesized from Zelkova serrata leaf biowaste via a one‐step pyrolysis method and evaluated as an electrode material for symmetric supercapacitors (SSCs). The electrochemical performance of the HPOC electrode was examined with and without a redox‐active electrolyte. Among various concentrations tested, 10 mM potassium ferricyanide (K ₃ [Fe(CN) ₆ ]) was identified as the optimal redox additive, offering substantial performance enhancement. The introduction of the reversible Fe ³⁺ /Fe ²⁺ redox couple nearly doubled the specific capacitance compared to the bare electrolyte. The as‐synthesized HPOC exhibited a high specific surface area of 997 m ² g ⁻¹ and a hierarchical pore structure, enriched with electrochemically active carboxylic functional groups that contributed to enhanced charge storage. The synergistic combination of electric double‐layer capacitance and faradaic redox reactions enabled the redox‐mediated SSC to deliver an energy density of 14.45 Wh kg ⁻¹ and a maximum power density of 5000 W kg ⁻¹ . This work presents an eco‐friendly and scalable approach to converting biomass waste into functional electrode materials and emphasizes the potential of redox‐active electrolytes to significantly boost supercapacitor performance without compromising electrode integrity.