Monolithic nanocarbon-based CO2 solid sorbents offer fast mass transport, easy handling, minor pressure drop, and cycle operation stability because of the interconnected threedimensional network of pores that provides a unique porous structure. In this work, following a one-step water-based method, graphene-based monoliths were produced by a spontaneous reduction-induced self-assembly of graphene oxide nanoplatelets under mild conditions (45-90 degrees C). By varying the reaction temperature and amount of reducing agent (ascorbic acid, AsA), the engineering of the porous structure of the monoliths was performed and resulted in a portfolio of different monoliths with different capacities for CO2 adsorption. It was found that the monolith produced at the highest temperature and with the lowest AsA amount possessed the highest specific surface area and porosity as well as a high level of functionalization. As a result, this monolith presented an excellent CO2 capture performance of 2.1 mmol/g at T = 25 degrees C and P = 1 atm. This value is between the highest achieved in CO2 sorption in comparison to that of similar and nontreated materials. The selectivity of this monolith for CO2 capture over that for N-2 at 25 degrees C and atmospheric pressure is 53, presenting a high viability for practical applications. The monolith was shown to lose capacity in cycle operations, probably because of the collapse of the smallest pores, which was solved by the addition of a small amount of polymer particles during the one-step synthesis of the monolithic structures. This modification provides for an excellent stability over five adsorption/desorption cycles.