Targeted preparation of ultramicroporous carbon via stepwise microwave activation for efficient CO₂ capture

Constructing ultramicropores is considered an effective method for enhancing the CO₂ uptake under ambient conditions. Although microwave heating shows advantages in constructing ultramicropores, its high heating rate and rapid volatile release make the pore expansion caused by H₂O and CO₂ gasification unavoidable, thereby limiting further directional regulation of ultramicropores. Here, we propose a novel stepwise microwave activation strategy that addresses the mismatch between microwave heating, volatile release, and carbon structure evolution, achieving more efficient directional regulation of ultramicropores. An initial low-power pre-activation step selectively releases volatiles, so that fewer reactive gases participate in the subsequent high-power activation stage and causing pore expansion, thereby promoting ultramicropore formation. Meanwhile, the transiently loosened carbon structure formed during the pre-activation stage further facilitates ultramicropore development. The sample obtained under optimized conditions (labeled 10-MW200) possesses a specific surface area (S_BET) of 1258 m² g⁻¹ and a remarkable ultramicroporosity (V_ultra/V_t) of 73.21%. Benefiting from this, 10-MW200 exhibits the highest CO₂ uptake of 4.21 mmol g⁻¹ among all samples at 298 K and 1 bar, while demonstrating high CO₂/N₂ adsorption selectivity (S_ads = 68) and moderate isosteric heat of adsorption (Q_st = 34.63 kJ mol⁻¹). Furthermore, the sample exhibits excellent kinetic performance (reaching equilibrium within 5 min) and outstanding regenerability (97% capacity retention after 10 cycles). This work provides new insights into achieving more efficient directional regulation of ultramicropores via microwave heating.