Preparation of nitrogen-doped biochar with different potassium sources and its CO2 adsorption performance

Nitrogen-doped biochars are prepared from corn stalk using different potassium salts (KOH, KHCO 3 , K 2 CO 3 , and potassium citrate) as activators, followed by KCl-assisted modification. Among all samples, PB2 (K 2 CO 3 /KCl co-activated) shows the highest CO 2 uptake of 4.28 mmol/g at 25 °C and 1 bar. This improved performance is attributed to the combined effects of abundant ultramicropores and narrow micropores, particularly pores below 0.6 nm, together with abundant surface functional groups, including graphitic nitrogen and C-N/C-O species. The micropores provide a high density of adsorption sites, which promotes efficient pore filling. Meanwhile, the nitrogen and oxygen functionalities enhance CO 2 affinity by modulating the electronic properties and increasing surface polarity. Cyclic stability is evaluated by thermogravimetric analysis (40 °C, atmospheric pressure). PB2 exhibits an initial CO 2 adsorption capacity of 2.05 mmol/g, which decreases to 1.69 mmol/g after five adsorption-desorption cycles, corresponding to a capacity retention of 82.44%. Although this cyclic performance is lower than those of PB, PB1, and PB3, PB2 combines the highest single-point CO 2 uptake with acceptable cyclic stability. This result indicates a trade-off between adsorption capacity and cycling stability. Overall, the results demonstrate that both pore structure and surface chemistry are critical to CO 2 adsorption performance. Among all samples, PB2 exhibits the most favorable combination of these characteristics.