Template-Free Fabrication of Nitrogen-Doped Hierarchical Porous Carbons from Waste Oil. The schematic illustrates the transformation process from linoleic acid and melamine into nitrogen-doped HPCs using CAPET carbonization and subsequent KOH activation. SEM images highlight the structural evolution, while elemental mapping confirms uniform nitrogen and oxygen distribution. The table shows the increase in specific surface area and mesopore volume after activation. The graph demonstrates the enhanced specific capacitance of N-doped HPCs, with LA-HPCs-N0.5 achieving a peak capacitance of 430.2 F g−1 at A g−1.