Adsorption using solid adsorbents is a promising technique for capturing carbon dioxide (CO2) to reduce greenhouse gas emission. In the present work, palm shell-based activated carbon was functionalized with eco-friendly activating agents including potassium carbonate, potassium acetate, binary deep eutectic solvent (DES) composed of choline chloride and ethylene glycol, and ternary DES composed of choline chloride, urea and ethylene glycol by impregnation method. Post-combustion CO2 adsorption performance of the functionalized adsorbents was evaluated in a fixed-bed adsorption column under varying adsorption temperature (25–55°C) and inlet CO2 concentration (15–20%), followed by a cyclic CO2 adsorption study to determine the regeneration ability of the adsorbents. The results revealed that activated carbon modified with potassium acetate (ACPA) exhibited remarkably high CO2 adsorption capacity of 116.5 mg/g and breakthrough time of 54 min at 25°C and 15% inlet CO2 concentration. Furthermore, ACPA demonstrated good regeneration ability even after seven adsorption-desorption cycles. Interestingly, it was found that activated carbon modified with ternary DES (AC-DES 4) exhibited significantly higher adsorption capacity than activated carbon modified with binary DES (AC-DES 2). It is worth mentioning that the present work is the first study that uses ternary DES as activating agent for such purpose.