Nitrogen Self-Doping Hierarchical Pore Biochar for Enhanced CO₂ Capture: Modulation of Pore Structure and Surface Properties

Negative emission technology aims to remove greenhouse gases such as CO₂ from the atmosphere, which is an important way to achieve efficient carbon reduction. The use of biochar materials as adsorbents can significantly reduce the cost while effectively capturing CO₂. In this work, carp fish scales were used as biomass raw materials, a small amount of KOH was used as an activator to provide ionic active sites, and the activation ion K⁺ was uniformly introduced into the material through a premixed hydrothermal reaction, followed by a single-step carbonization and activation process at a moderate temperature, along with an acid wash, to produce a N self-doping, layered-structured biochar with high porosity and uniform pore channels. A series of hierarchical biochars with dissimilar physicochemical properties were prepared by varying the carbonization temperature. Among them, the sample prepared at 700 °C (AF-700) has an ultrahigh specific surface area of 1371 m² g^-1, a pore volume of 0.85 cm³ g^-1, and showed the highest adsorption performance of 3.34 mmol g^-1 (0 °C, 1 bar). We also fitted the adsorption curves of the biochar using the Langmuir-Freundlich isotherm model and calculated the adsorption selectivity of the material for CO₂ (N₂ and Ar) with ideal adsorption solution theory. The results indicate that N self-doped hierarchical structure biochar exhibits high CO₂ adsorption efficiency and is composited in an economical and simple way, thus holding great potential for the large-scale production of efficient CO₂ adsorbents.