Most carbon electrode materials for supercapacitors (SCs) are prepared by the conventional KOH activation method that entails several steps at higher temperatures resulting in low-energy density. Herein, we report one-step facile carbonization and low-concentration-activation aided synthesis of activated carbon (AC) from Prunus dulcis (almond) peel (AP) by using three activating agents, namely (a) KOH (AC-K1); (b) KOH with K3[Fe(CN)6] (AC-K2); (c) KOH with K3[Fe(CN)6] and K4[Fe(CN)6] (AC-K3) for high-performance symmetric supercapacitor (SSCs) applications in 6 M KOH and 1 M TEABF4 electrolytes. Detailed structural and chemical characterization studies were performed on the as-prepared AC-K1/K2/K3 samples and optimization studies revealed that AC-K3 electrode delivers the highest performance due to the presence of trace amount of iron salts in the carbon network that acts as a catalyst and induces fast charge-transfer for electrochemical reactions. The BET surface area of AC-K1, AC-K2, and AC-K3 are found to be 408.89 m2 g−1, 819.79 m2 g−1, and 1061.6 m2 g−1, respectively. AC-K3 electrode delivers an outstanding half-cell specific capacitance of 320 F g−1 at 1 A g−1 in 6 M KOH. The SC device with AC-K3 electrode delivers full-cell specific capacitance of 290 F g−1 and 370.37 F g−1 in 6 M KOH and 1 M TEABF4, respectively. The highest energy density of 41.24 Wh kg−1 at 1349 W kg−1 power with the extended potential of ~2.7 V is delivered by the AC-K3 electrode in 1 M TEABF4, which is considerably higher and unprecedented compared to other reported carbon materials. After 10 000 cycles, there is a ~100% capacitance retention. As a result of this study, the abundant bio-derived AC-K3 is found to be a potential long-term electrode material for high-energy SC applications. © 2022 John Wiley & Sons Ltd.