Eco‐Conversion of Waste Engine Oil Into Defect‐Rich Soot‐Derived Activated Carbons for Aqueous Zinc‐Ion Capacitors and Supercapacitors

ABSTRACT In this study, onion‐like soot nanoparticles (ST) are synthesized from waste engine oil through a rapid and low‐cost flame synthesis method and subsequently activated with potassium hydroxide (KOH) to obtain activated carbon (SAC). Two modified variants were produced by introducing melamine and urea during the activation process, yielding SAC‐M and SAC‐U, respectively. Structural analyses confirmed that the incorporation of melamine and urea significantly influences the pore structure, surface chemistry, and electronic properties. The electrochemical behavior of these activated carbons is examined in symmetric supercapacitors and aqueous zinc (Zn)‐ion capacitors (ZICs). All devices exhibited typical electric double‐layer capacitor (EDLC) behavior with minor faradaic contributions. Among all samples, SAC‐U ZIC delivered the highest specific capacitance (451.1 F g ⁻¹ at 0.1 A g ⁻¹ ), which can be attributed to its more developed porous structure and higher defect density, while SAC‐M demonstrated enhanced rate capability and cycling stability in ZIC configurations. The SAC‐U‐based ZIC achieved a remarkable energy density of 203.0 Wh kg ⁻¹ at 90.0 W kg ⁻¹ , outperforming many reported aqueous ZIC systems. Overall, the findings indicate that structural and surface changes induced by the incorporation of melamine or urea during synthesis significantly influence electrochemical performance, providing guidance for the design of efficient carbon‐based electrodes.