How water adsorbed on porous graphene affects CO₂ capture and separation

Abstract

When capturing CO2 for combating global warming, water molecules are omnipresent and can play a detrimental role in many carbon capture approaches. Particularly, water not only can compete with CO2 for adsorption in various porous sorbents, but also may permanently damage their porous structures. Membrane filtration of CO2 by porous graphene is a promising approach thanks to its high-selectivity and energy-efficiency, as demonstrated in both experimental and theoretical studies. Recent studies have shown that a graphene surface is typically covered by 2–3 layers of water molecules, which is particularly relevant in separation applications due to the presence of water vapor in both flue gas and in air. However, the presence of water was rarely considered in previous theoretical studies for the CO2 filtration. Here, using all-atom molecular dynamics simulations, we investigate how a thin film of water molecules adsorbed on porous graphene can affect the efficiency of CO2 capture and its separation from N2. Surprisingly, we find that the design of pore termination (hydrophobic vs. hydrophilic) can affect the thickness of water's solvation shell around the pore edge, which consequently can help improve the selectivity for CO2 over N2 even for pores (e.g. 5–15 Å in radius) much larger than CO2/N2 sizes due to an unveiled pore-edge-dominated sieving mechanism. It is worth noting that in all previous theoretical studies only graphene pores comparable in size to gas molecules have exhibited a sieving effect for separating CO2 from N2. Surprisingly, the transport rate of gas is proportional to R1 (where R is the effective pore-radius reduced by the thickness of water's solvation shell around the pore edge), instead of R2 as is common for macroscopic pores.