Pore Characteristics for Efficient CO₂ Storage in Hydrated Carbons

Development of new approaches for carbon dioxide (CO2) capture is important in both scientific and technological aspects. One of the emerging methods in CO2 capture research is based on the use of gas-hydrate crystallization in confined porous media. Pore dimensions and surface functionality of the pores play important roles in the efficiency of CO2 capture. In this report, we summarize work on several porous carbons (PCs) that differ in pore dimensions that range from supermicropores to mesopores, as well as surfaces ranging from hydrophilic to hydrophobic. Water was imbibed into the PCs, and the CO2 uptake performance, in dry and hydrated forms, was determined at pressures of up to 54 bar to reveal the influence of pore characteristics on the efficiency of CO2 capture and storage. The final hydrated carbon materials had H2O-to-carbon weight ratios of 1.5:1. Upon CO2 capture, the H2O/CO2 molar ratio was found to be as low as 1.8, which indicates a far greater CO2 capture capacity in hydrated PCs than ordinarily seen in CO2-hydrate formations, wherein the H2O/CO2 ratio is 5.72. Our mechanistic proposal for attainment of such a low H2O/CO2 ratio within the PCs is based on the finding that most of the CO2 is captured in gaseous form within micropores of diameter <2 nm, wherein it is blocked by external CO2-hydrate formations generated in the larger mesopores. Therefore, to have efficient high-pressure CO2 capture by this mechanism, it is necessary to have PCs with a wide pore size distribution consisting of both micropores and mesopores. Furthermore, we found that hydrated microporous or supermicroporous PCs do not show any hysteretic CO2 uptake behavior, which indicates that CO2 hydrates cannot be formed within micropores of diameter 1–2 nm. Alternatively, mesoporous and macroporous carbons can accommodate higher yields of CO2 hydrates, which potentially limits the CO2 uptake capacity in those larger pores to a H2O/CO2 ratio of 5.72. We found that high nitrogen content prevents the formation of CO2 hydrates presumably due to their destabilization and associated increase in system entropy via stronger noncovalent interactions between the nitrogen functional groups and H2O or CO2.