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A versatile chemical activation approach for the fabrication of sustainable porous carbons with a pore network tunable from micro- to hierarchical micro-/mesoporous is hereby presented. It is based on the use of a less corrosive and less toxic chemical, i.e., potassium oxalate, rather than the widely used KOH. The fabrication procedure is exemplified for glucose as precursor, although it can be extended to other biomass derivatives (saccharides) with similar results. When potassium oxalate alone is used as activating agent, highly microporous carbons are obtained (S BET ≈ 1300-1700 m 2 g -1 ). When a melamine-mediated activation process is used, hierarchical micro-/mesoporous carbons with surface areas as large as 3500 m 2 g -1 are obtained. The microporous carbons are excellent adsorbents for CO 2 capture at low pressure and room temperature, able to adsorb 4.2-4.5 mmol CO 2 g -1 at 1 bar and 1.1-1.4 mmol CO 2 g -1 at 0.15 bar. However, the micro-/mesoporous carbons provide record-high room temperature CO 2 uptakes at 30 bar of 32-33 mmol g -1 CO 2 and 44-49 mmol g -1 CO 2 at 50 bar. The findings demonstrate the key relevance of pore size in CO 2 capture, with narrow micropores having the leading role at pressures <1 bar and supermicropores/small mesopores at high pressures. In this regard, the fabrication strategy presented here allows fine-tuning of the pore network to maximize both the overall CO 2 uptake and the working capacity at any target pressure.