Gated Transport of CO2 through Electrowetted Solid-State Nanopores

Abstract

Membrane sieving is one of the most effective and efficient approaches for capturing and separating CO2 from air or flue gas, and is promising for alleviating the greenhouse effect worldwide. Various investigations have demonstrated that porous solid-state membranes have great potential for CO2 separation, and remarkable progress has been achieved. To date, researchers have usually focused on solid-state nanopores (e.g., C6) with sizes comparable to those of the transported gases due to their specific gas–pore interactions and therefore desirable performance, while overlooking large nanopores that are inevitably present in a fabricated porous membrane. It is well-known that a large nanopore (e.g., C24) can lose its selectivity because of the effortless transport of both CO2 and its mixtures (attributed to the trade-off between permeation and selectivity). Therefore, maintaining a high selectivity for CO2 through large solid-state nanopores remains a great challenge. Here, we present the design of a CO2/N2 separation system based on water-covered graphene nanochannels (C24) in an electric field. Using molecular dynamics simulations, we found that upon introducing low electric field strengths, the system exhibited an enhanced separation selectivity of CO2/N2 (resulting from the enhanced solubility of CO2 in polarized water). At stronger electric fields, however, the graphene nanopore is gradually electrowetted, which consequently blocks the passage of both CO2 and N2. These results reveal an optimal field strength that simultaneously maximizes the transport rate and enhances the selectivity. This novel phenomenon of electric-field-controlled transport provides a practical approach for the efficient and selective separation of CO2 and N2 through large nanopores.