Unveiling the role of pentagonal topological defects in lignocellulose-derived self-assembled N/O co-doped micro-mesoporous biochar for enhanced CO₂ adsorption

The physicochemical properties of biochar are critical for CO2 adsorption; however, the synergistic effect of doped nitrogen and topological defect in biochar on CO2 adsorption capacity remains uncertain. Here, N/O coupled topological defect co-doped biochars (NWBCs-T) were successfully synthesized via self-assembly temperature-controlled carbonization/annealing within 500–900 °C from red bayberry pits. The influence mechanism of temperature on the formation, transformation, and interaction of N/O and pentagonal topological defect functionalities, as well as their impact on CO2 adsorption, was systematically investigated. The results revealed that NWBC-900 exhibited the highest CO2 adsorption capacity of 60 mg/g, primarily attributed to the prominent synergistic effect between N/O active sites and topological defect, rather than the physical adsorption force from micro-mesoporous pores, as evidenced by relevant analyses and Pearson heatmaps. Notably, the presence of pentagonal topological defects exerts a profound enhancing effect on CO2 adsorption of edge graphitic-N and C-O-C sites, yet exerts weaker or opposite effects on other N/O configurations. Further insights from XPS and NMR analyses indicated a notable surge in pentagonal topological defects at elevated annealing temperatures, along with a reduction in total and pyrrolic nitrogen content. DFT calculation findings confirmed that pentagonal topological defects introduced at elevated temperatures adjust electron distribution, thereby facilitating improved electron transfer and boosting adsorption binding of NWBC-900 for CO2. This work provides new insights into the conversion of waste biomass into green-efficient biochar for carbon capture.