Modular Solvent-Free Carbon Removal: A Solid-Sorbent Reactor for Sequential Direct Air Capture and Flue Gas Decarbonization

Scaling carbon removal to gigatonne-per-year levels requires capture technologies that are simultaneously efficient, environmentally clean, and deployable across heterogeneous emission sources, from dilute atmospheric CO₂ (∼420 ppm) to concentrated industrial flue gas (1-33 vol %). Established amine- and hydroxide-based systems typically employ circulating liquid capture media and cyclic regeneration, making solvent handling, regeneration energy, and capture-medium degradation central to process complexity and economics. Here, an alternative solvent-free carbon capture process is demonstrated that addresses both source types within a single sequential platform. A composite sodium hydroxide-sodium percarbonate sorbent captures ambient CO₂ via humidity-activated sodium peroxide (Na₂O₂) formation, fully converting NaOH to Na₂CO₃ with a capacity of 0.545 g_CO2 g_NaOH^-1 . The resulting carbonate is subsequently converted to bicarbonate under 15 vol % CO₂ through water-mediated trona-phase growth, and is stacked in a modular reactor to achieve complete sequential capture at 0.612 g_CO2 g_sorbent ^-1. The end product, sodium bicarbonate, may be stored or used as feedstock for downstream electrochemical and chemical syntheses, offering a complementary solvent-free pathway for sequential carbon capture.