Template-free approach to fabricate uniformly N-doped hierarchical porous carbons from waste oil

The rational pore structure and abundant surface functional groups of hierarchical porous carbons (HPCs) are important for their practical application in supercapacitors. The liquid linoleic acid and solid melamine were thoroughly mixed and subjected to carbonization under autogenic pressure at elevated temperatures (CAPET), followed by KOH activation, to produce uniformly N-doped HPCs. The structure and surface chemical properties are controlled by adjusting the N-doped ratio. This adjustment results in high conductivity, abundant ion-accessible surfaces, hierarchical porosity with appropriate micro-mesoporous channels, as well as the presence of N and O heteroatoms. The addition of melamine markedly increased the surface area to 3474.1 m² g⁻¹ and the mesopore volume proportion to 72.9%‒77.3% in the HPCs. The crystal structure and functional groups of the HPCs were revealed by X-ray diffractometer, Raman spectrometry, and X-ray photoelectron spectrometry, indicating that LA-HPCs-N0.5 is a promising electrode material for supercapacitors. This material presented excellent capacitance storage performance and cycling stability, with a specific capacitance of 430.2 F g⁻¹ at 1 A g⁻¹ in a 6 mol L⁻¹ KOH electrolyte system, and the capacitance retention rate was 86.5% after 2000 cycles of charging and discharging at 10 A g⁻¹. This study has successfully demonstrated that the template-free preparation of N-doped HPCs from waste oil is feasible, economical, and sustainable.