Here we analyze how changes in the charge density of activating alkali cations (lithium, sodium, and potassium) alters the synthesis and resulting physicochemical properties of N-rich activated carbons. In general, the synthesis reagents had significant influence on the total nitrogen content (2–24 at%), the chemical environment of the nitrogen species, the specific surface area (∼600–4300 m2 g−1), and the types of pores that formed in the activated materials. Each sample was screened for carbon dioxide (CO2) and nitrogen (N2) gas adsorption. From application of the ideal adsorbed solution theory, the predicted CO2/N2 selectivity spanned a large range from ∼8 to ∼150 at 15% CO2 and was dependent on d-spacing, surface N content, and porosity. Finally, the materials were analyzed with a simplified temperature swing adsorption model to estimate the optimal working capacity and regeneration energy of the materials in a cyclic process. Overall, this study demonstrates that while the precursor nitrogen content drives significant changes in the isotherm shape, a careful choice of activating cation during synthesis of advanced porous carbons can strongly influence physicochemical properties and the resulting thermodynamics and selectivity of CO2 adsorption.