Textural and surface chemical properties of ACs are key to determine their performance for CO
2 capture. In this regard, continuous efforts have been made to tune the pore structure and surface functionality of ACs to improve their CO
2 adsorption performance. It is generally agreed that CO
2 adsorption on ACs is based on a micropore filling mechanism, and the CO
2 adsorption capability of ACs is mostly related to narrow micropore structure [
20]. Previous studies have demonstrated micropores with sizes smaller than 1 nm, i.e., ultra-micropores, are mainly responsible for CO
2 uptake at ambient pressure [
21,
22]. Additionally, chemical environment of ACs is also an important factor of affecting CO
2 adsorption, which can induce chemisorption, hydrogen bonding, and/or electrostatic interactions between the CO
2 molecule and surface functional groups [
23,
24]. The porosity of ACs can be effectively adjusted by carbon precursors [
25] and activation methods [
26], as well as the conditions of the method [
27]. Based on pore adjustment and surface modification, CO
2 adsorption capabilities of the reported ACs have been significantly increased, and some of the reported ACs with the appropriate pores and active sites could achieve the CO
2 adsorption capacity of > 7 mmol∙g
–1 at 0 °C and 1 bar [
28]. However, it is worth mentioning that the majority of previous studies focused mainly on improving the CO
2 gravimetric capacitive performance of porous carbons with the low packing density, which could cause the insufficient use of pores in ACs. Conversely, in comparison with the gravimetric performance, CO
2 adsorption capability per volume of porous carbons may provide a relatively realistic indication of the performance. Besides, the volumetric CO
2 adsorption capability of porous carbons is also an important indicator to determine the scale and efficiency of adsorption systems [
29–
31], which, however, was commonly overlooked in most of the previous studies [
20,
32]. To achieve a high volumetric CO
2 adsorption capacity, porosity regulation of carbon materials is the prerequisite since pore configuration and size greatly correspond to the packing density of porous carbons [
33]. Regardless gravimetric and volumetric adsorption, it is very necessary to explore the critical factors, which determine volumetric and/or gravimetric capability of porous carbons, and hence devise controllable methods for porosity regulation towards high-performance CO
2 adsorbents.