Controlled Synthesis of Biochar with Flower-like Morphology for CO₂ Adsorption: Enrichment and Efficient Accessibility of N-Containing Sites

The N-doped biochar is recognized as a promising, cost-effective, and efficient material for CO₂ adsorption. However, achieving efficient enrichment of N-containing adsorption sites and improving their accessibility remains a bottleneck problem that restricts the adsorption performance of N-doped biochar. Herein, a synthesis strategy for nitrogen-doped biochar by one-pot ionothermal treatment of biomass and zeolitic imidazolate framework (ZIF) precursors accompanied by pyrolysis is demonstrated. Through ion thermal ZIF modification, biochar exhibits a controllable flower-like morphology with effective enrichment of nitrogen elements (nitrogen retention rates ranging from 62% to 88%). After pyrolysis, this regular morphology is retained, and a developed hierarchical pore structure is formed. Compared with pristine biochar and ZIF-derived carbon, ZIF-modified biochar has superior CO₂ adsorption capacity (up to 3.5 mmol/g) and excellent CO₂/N₂ adsorption selectivity (up to 38.6). The CO₂ adsorption capacities of ZIF-modified biochars have a good linear relationship with both bulk and surface N content, with correlative coefficients of around 0.998 and 0.950, respectively. This positional indifference reflects the effective accessibility of N-containing sites, which can be attributed to the ordered flower-like morphology and hierarchical pore structure of ZIF-modified biochar. The DFT results confirmed the importance of the number and accessibility of such N-containing sites for CO₂ adsorption.