Tailoring porosity and surface nitrogen sites in n‑self‑doped biochar for CO2 mitigation applications

Excessive carbon dioxide emissions have triggered severe environmental degradation. This urgent scenario necessitates the transition of global energy systems toward net-zero carbon emissions. N‑self‑doped biochars are promising multifunctional materials achieves low-cost carbon dioxide (CO 2 ) adsorption from flue gas and green electrochemical energy storage, which are both regarded as viable net-zero-carbon-emission technology. However, the trade-off between porosity and surface nitrogen sites with varying calcination temperatures poses great challenges for performance optimization of the biochar. Thus, in this work, spirulina was used as a precursor to synthesize N‑self‑doped biochar in different calcination temperature (450, 550, 650, and 750 °C). The as-prepared biochar delivers an outstanding CO 2 uptake of 5.09mmol g −1 at 25 °C and a high specific capacitance of 321.77F g −1 at 1 A g −1 . Multimodal characterizations and density functional theory (DFT) calculations reveal that how porosity and surface nitrogen sites affect the performance. These results demonstrate that the core of modulating the performance of N-self-doped biochar lies in achieving an optimal balance between porosity and surface nitrogen active sites. Specifically, retained nitrogen functional groups (especially N-6) guarantee high CO 2 uptake without excessive high-temperature activation for micropores. While specific surface area (S BET ) and micropore volume govern capacitance, N doping remedies poor conductivity of low-graphitized biochar. Based on the above findings, this work provides an important theoretical basis for developing high-performance multifunctional nitrogen-doped biochar to advance low-cost clean energy applications.