Highly porous N-doped carbon monoliths have been successfully prepared by using binary H₃PO₄–HNO₃ mixed acid as a co-activating agent for the first time and sodium alginate (SA), a natural biopolymer, as a carbon precursor. The resultant N-doped carbon monoliths have a narrow size distribution and high content of pyrrolic N. Particularly, the sample SA-2N-P with the highest surface area (1740 m² g⁻¹) exhibits the highest CO₂ adsorption capacity of 8.99 mmol g⁻¹ at 273 K and 4.57 mmol g⁻¹ at 298 K, along with an initial CO₂ adsorption energy of 43 kJ mol⁻¹ at lower CO₂ coverage and 32 kJ mol⁻¹ at higher CO₂ coverage. Remarkably, this sample also shows the highest CO₂ capacity [66.44 mg (CO₂) g⁻¹ (adsorbent) at 25 °C and 0.15 atm] under low CO₂ pressures, which is of more relevance for flue gas applications. Furthermore, the selectivity of CO₂ over N₂ is also calculated for the binary gas mixture [V (N₂) : V (CO₂) = 85 : 15] according to the ideal adsorbed solution theory (IAST). Combined with its simple preparation, high adsorption capacity, and high selectivity for CO₂, the sample SA-2N-P is one of the promising solid-state absorbents reported so far for CO₂ capture and storage.