Controlling nitrogen doping and micropore development in biomass-derived carbons via in situ and ex situ hydrothermal co-carbonisation and phosphoric acid activation

This work elucidates how the timing of phosphoric acid incorporation governs the interplay between nitrogen doping and micropore development in biomass-derived carbons obtained via hydrothermal co-carbonisation of olive stone and sheep wool. By systematically comparing in situ and ex situ activation routes, the study demonstrates that the stage of acid incorporation determines the reaction pathways available during hydrothermal co-carbonisation: in the in situ route, H₃PO₄ catalyses condensation reactions between amine and carbonyl groups — including Schiff base formation and Maillard- and Mannich-type pathways — thereby governing the extent of nitrogen–carbon bond formation and the subsequent development of micropore architecture during thermal activation. This route yields carbons with a high specific surface area (952 m² g⁻¹) and enhanced nitrogen incorporation (up to 2.25 wt% in bulk and 3.4 at% on the surface), while preserving a well-developed microporous structure. In contrast, increasing nitrogen content leads to a progressive loss of microporosity, revealing a trade-off between heteroatom incorporation and pore development. Pore size distribution analysis shows that ultramicropores (<0.7 nm) account for ∼41% of the total pore volume in the in situ sample, compared to ∼22% for the ex situ counterpart, indicating a more efficient development of narrow micropores. XPS analysis further reveals a redistribution of nitrogen functionalities, with partial transformation of pyrrolic species into more stable pyridinic and graphitic configurations. Overall, the in situ introduction of phosphoric acid enables a more effective balance between nitrogen doping and micropore development, providing insights into the rational design of biomass-derived functional carbons.