Effects of activation route and doping on porosity and nitrogen retention on natural fibers

In this study, the effect of urea and asparagine as potential N-doping precursors was compared in both one-step (activation) and two-step processes (hydrothermal carbonization + activation) using natural fibers (Kapok) as a precursor. The resulting activated carbons (ACs) were characterized for surface functional groups (FTIR and XPS), specific surface area (SSA, m 2 /g) and surface morphology. A comprehensive analytical approach was used to assess the surface chemistry, nitrogen speciation, and porosity development of the materials. Respect to dopant selection, urea doped materials led to a better N incorporation and higher specific surface area than those with asparagine. AC-HTC-600-U exhibited higher SSA (approximately 350 m 2 /g) and N-pyridine and N-pyrrolic species compared to AC-HTC-800-U (approximately 150 m 2 /g), while AC-HTC-800 showed higher N-graphitic species. Discrepancies between XPS and elemental analysis suggested a heterogeneous N distribution, associated to the dopant decomposition mechanism. Furthermore, SSA decay between 600 °C and 800 °C may support the microstructure collapse, highlighting the importance of a deep DFT analysis of the pore structure. These findings underscore the importance of dopant selection and temperature control in tailoring nitrogen retention and pore development in biomass-derived carbons and provide process-carbon structure pathway for designing sustainable N-doped porous carbons for supercapacitor electrodes and related energy-storage applications. • 2-step process promoted consistent N/C ratios across different activation temperatures. • Impregnation with urea and asparagine yields ACs with higher N content. • At 800 °C, micropore structure collapsed and N become graphitic. • AC-HTC-600-U has high SSA and enriched pyridinic/pyrrolic N species.