Ion-Gated Gas Separation through Porous Graphene

Porous graphene holds great promise as a one-atom-thin, high-permeance membrane for gas separation, but to precisely control the pore size down to 3–5 Å proves challenging. Here we propose an ion-gated graphene membrane comprising a monolayer of ionic liquid-coated porous graphene to dynamically modulate the pore size to achieve selective gas separation. This approach enables the otherwise nonselective large pores on the order of 1 nm in size to be selective for gases whose diameters range from 3 to 4 Å. We show from molecular dynamics simulations that CO 2, N 2, and CH 4 all can permeate through a 6 Å nanopore in graphene without any selectivity. But when a monolayer of [emim][BF 4 ] ionic liquid (IL) is deposited on the porous graphene, CO 2 has much higher permeance than the other two gases. We find that the anion dynamically modulates the pore size by hovering above the pore and provides affinity for CO 2, while the larger cation (which cannot go through the pore) holds the anion in place via electrostatic attraction. This composite membrane is especially promising for CO 2 /CH 4 separation, yielding a CO 2 /CH 4 selectivity of about 42 and CO 2 permeance of ∼10 5 GPU (gas permeation unit). We further demonstrate that selectivity and permeance can be tuned by the anion size, pore size, and IL thickness. The present work points toward a promising direction of using the atom-thin ionic liquid/porous graphene hybrid membrane for high-permeance, selective gas separation that allows a greater flexibility in substrate pore size control.