Adsorption of CO2 gas on graphene-polymer composites

Three-dimensional graphene-polymer porous materials have been proposed recently as potential adsorbents for carbon dioxide capture. We report results from molecular dynamics simulations on the adsorption of CO2 gas by composite systems formed by six different types of polymers. All composites are characterized by a constant polymer-to-graphene mass-ratio of 1.02 and their capture capacity as a function of the gas pressure is analyzed relative to the adsorption of nitrogen and methane. The results were then compared to the performance obtained from a bare-graphene sheet. More specifically, we examined the abilities of hydrogen-bond donor groups, amines and amides, as well as aromatic rings to promote and discriminate CO2 capture. We find that bare-graphene displays the highest capacity to adsorb CO2. Nevertheless, increasing the number of amine/amide protic groups of the polymer augments the adsorption. In fact, the best performing polymer in our study, which contains three protic groups per monomer, exhibits capture of CO2 almost as good as bare-graphene. Furthermore, these protic polymers form substantial intra-polymer hydrogen bonds and consequently exhibit cohesive behavior. In these cases, the aggregation of the polymer resulted in partial exposure of the graphene sheet on which the gas can adsorb as well. In contrast aprotic polymers, such as poly(styrene) or poly(methyl-methacrylate), spread extensively on graphene and are characterized by small capacities to adsorb CO2. For all systems studied, CO2 is adsorbed preferentially relative to N-2 and CH4. This preferential adsorption is almost constant as a function of the gas pressure with values ranging from 2 to 12.