Ion-Gated Gas Separation through Porous Graphene
ZQ Tian and SM Mahurin and S Dai and DE Jiang, NANO LETTERS, 17, 1802-1807 (2017).
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 angstrom 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 angstrom. We show from molecular dynamics simulations that CO2, N-2 and CH4 all can permeate through a 6 angstrom nanopore in graphene without any selectivity. But when a monolayer of emim BF4 ionic liquid (IL) is deposited on the porous graphene, CO2 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 CO2, 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 CO2/CH4 separation, yielding a CO2/CH4 selectivity of about 42 and CO2, permeance of similar to 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.
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