Traditional chemical activation methods to prepare porous carbons adsorbents consume large amounts of activating agent through physical mixing. Herein, we developed a new strategy to prepare microporous carbons with uniform ultramicropores (similar to 0.57 nm) through ionic activation method by using potassium carboxylate as an activating agent, which was uniformly incorporated onto the surface of hydrothermally prepared hydrochar. The ultramicroporous carbon adsorbent developed by this new method showed high CO2 uptakes of 5.87 and 3.82 mmol g(-1) at 0 and 25 degrees C, respectively. We demonstrated that small amounts of uniformly distributed potassium carboxylate contributed to the development of ultramicroporosity. This research paves the way for a new route to prepare ultramicroporous carbons as efficient CO2 adsorbents by using significantly lower amounts of activating agents and hence the process may be not only cost-effective but also avoids negative impacts of traditional chemical activation.