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In this study, the facile and sustainable synthesis of highly microporous carbons is explored to reduce the extensive use of harsh activating agents and solvents. The role of potassium citrate (PC) as a greener activating agent in addition to the conventional ZnCl₂ is investigated in the mechanochemical solvent-free preparation of highly microporous carbon materials from chestnut tannin (CT), a biomass-type carbon precursor. A small amount of potassium citrate as a chemical activator coupled with CO₂ activation at 700 °C afforded carbons with higher specific surface area (1256 m² g⁻¹) and larger micropore volume (0.54 cm³ g⁻¹) as compared to the carbons activated with both PC and ZnCl₂. The high microporosity of the PC-activated carbon materials, significantly enlarged after CO₂ activation from micropore volume of 0.16 to 0.54 cm³ g⁻¹, makes them favorable for CO₂ adsorption, as evidenced by high adsorption capacity of 3.55 mmol g⁻¹ at ambient conditions (25 °C, 1 bar). This study shows that the solvent-free mechanochemical processing of tannin in the presence of PC is a promising method for obtaining highly microporous carbon materials.