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 (K3[Fe(CN)6]) was identified as the optimal redox additive, offering substantial performance enhancement. The introduction of the reversible Fe3+/Fe2+ redox couple nearly doubled the specific capacitance compared to the bare electrolyte. The as-synthesized HPOC exhibited a high specific surface area of 997 m2 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.