Abstract <p>Owing to the high specific surface area and adjustable pore size distribution, biomass-derived carbon nano-onions (CNOs) have lately aroused the interest of researchers in supercapacitor applications. Herein, we employ ajwain oil-derived, porous CNOs for supercapacitor applications. In this study, carbon nano-onions (CNOs) are synthesized through a simple, scalable, one-step “wick-and-oil” flame method followed by KOH activation. Microstructural analysis reveals monodisperse carbon nanoparticles with high porosity, attributed to the long-chain hydrocarbons and active components such as phenolic carvacrol and thymol present in ajwain oil, which contribute to high carbon yield and improved ion diffusion pathways. Electrochemical studies are performed using a three-electrode setup in 6 M KOH electrolyte. The CNO electrodes exhibits excellent charge transfer properties, achieving a specific capacitance of ~ 276.2 F g^–1 at 1 A g^–1. The charge storage mechanism has analyzed through surface- and diffusion-controlled contribution analysis. At lower scan rates, diffusion charge contribution is more dominant, whereas at higher scan rates, a significant surface charge contribution of 88.4% at 100 mV s^–1 demonstrated rapid electrochemical kinetics and superior rate capability. The surface-based charge contribution transformed into diffusion-based charge contribution when transiting from high scan rate to low scan rates. Thus, these findings highlight that analyzing surface and diffusion charge contributions provides valuable insights for optimizing and enhancing supercapacitor performance.</p>