Centimeter-scale gas-sieving nanoporous single-layer graphene membrane

Abstract

High-permeance, molecular-sieving, nanoporous single-layer graphene (NSLG) membranes are highly promising for gas separation. However, the formation of cracks during the transfer of NSLG to a low-cost porous support is difficult to avoid. These cracks are detrimental to gas selectivity, and therefore, make the scale-up of the gas-sieving NSLG membranes challenging. To mitigate the crack formation on low-cost macroporous supports, herein, we demonstrate mechanical reinforcement of the graphene film with a two-layer composite carbon film. The bottom layer of the composite film is a 100-nm-thick block-copolymer film derived nanoporous carbon (NPC) film with a pore size of 20–30 nm. This layer makes an intimate contact with NSLG and prevents generation of crack. However, the NPC film by itself is not robust enough to cover the rough surface of low-cost macroporous supports and tends to generate occasional cracks. This is prevented by spin-coating a 500-nm-thick multi-walled carbon nanotube (MWNT) film, hosting pore size of 200–300 nm, on top of the NPC film. This imparts enough mechanical strength to NSLG/NPC film to be successfully suspended on a low-cost, macroporous, nonwoven metal wire mesh on a centimeter-scale while completely avoiding cracks. As a result, H2/CH4 and H2/CO2 selectivities of 11–23 and 5–8, respectively, higher than the corresponding Knudsen selectivities of 2.8 and 4.7, respectively, are obtained from the centimeter-scale NSLG membranes. The reinforced membranes are mechanically robust and can successfully withstand transmembrane pressure difference of 4 bar. When the MWNT film is directly coated on NSLG without using the intermediate NPC layer, the gas sieving behavior is not observed, likely due to the development of nanoscale cracks. This underlines the crucial role of the hierarchical pore structure in the composite carbon film in realizing the gas-sieving graphene membranes.