Two-dimensional (2D) nanomaterials are used for membrane separation because of their unique structures and properties. In this respect, 2D graphitic carbon nitride (g-C3N4) nanosheets are considered promising for constructing membranes because of their integral in-plane nanopores.
Study: Fast hydrogen purification through graphitic carbon nitride nanosheet membranes. Image Credit: Shawn Hempel/Shutterstock.com
Nevertheless, the non-selective defects induced during conventional top-down preparation and undesirable restacking limit the application of g-C3N4 nanosheets in gas separation. An article published in Nature Communications presented lamellar g-C3N4 nanosheets prepared via the bottom-up method and utilized as membranes for gas separation.
The fabricated membranes provided exceptional selectivity for various gas mixtures and showed a hydrogen (H2) permeability of 1.3 × 10−6 moles per square meter per second per pascal due to interlayer paths and high-density sieve channels.
Interestingly, membranes with high H2 permeability were stable even under harsh environments, such as wet atmospheres, temperature swings, and long-term operation (>200 hours). Thus, lamellar membranes fabricated from g-C3N4 nanosheets served as efficient gas separators.
g-C3N4 Nanosheet-Based Molecular Sieving Membrane
Hydrogen is a carbon-free gas that is a next-generation sustainable energy source. However, during production, hydrogen is associated with large molecules such as carbon dioxide (CO2), nitrogen (N2), and methane (CH4), which must be separated before practical application.
Conventional separation methods, such as pressure swing adsorption and distillation, are highly energy-consuming. However, the recently established membrane separation technology is an energy-conserving alternative for purifying H2 gas. However, the trade-off between selectivity and permeability limits the application of conventional polymer membranes in separation processes.
2D nanosheet membranes are excellent options for high-performance separation owing to the tunable pore size of 2D nanosheets and the interlayer channels of neighboring nanosheets. Graphene and its variants are among the most extensively researched 2D materials.
In addition to graphene and its derivatives, other 2D nanosheet materials, including zeolites, metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), MXenes, and g-C3N4, have also exhibited unprecedented potential for different filtration and separation processes.
g-C3N4 nanosheets are the building blocks of molecular sieving membranes. These nanosheets have molecular-sized pores constituting units of tri-s-triazine across the 2D plane with nanometer-thickness. Hence, g-C3N4 nanosheet membranes are excellent candidates for H2 separation.
However, owing to the difficulties in obtaining high-quality g-C3N4 nanosheets, only a few papers have discussed gas separation using g-C3N4 nanosheet membranes. Like other 2D materials, g-C3N4 nanosheets were prepared from bulk materials via top-down exfoliation. This process involves material exposure to harsh environments to eliminate bulk g-C3N4’s interlayer interactions, which also induces structural deterioration and causes defects in the resulting g-C3N4 nanosheets.
Graphitic Carbon Nitride Nanosheet Membranes for H2 Purification
Using strong hydrochloric acid, researchers previously constructed g-C3N4 nanosheet membranes with 1.5–3 nanometer-sized artificial nanopores. Although these membranes demonstrated remarkable performance in nanofiltration, they had large nanopores that restricted H2 purification. Previous reports have mentioned that strong π–π interactions among the nanosheets result in tight films, blocking the integral in-plane nanopores.
In this study, high-quality g-C3N4 nanosheets were prepared using a bottom-up approach. The prepared g-C3N4 was delaminated into nanosheets via thermal polycondensation. Here, adopting the bottom-up method instead of the top-down method to prepare g-C3N4 nanosheets skipped the exfoliation step and consequently prevented structural damage to the g-C3N4 nanosheets.
Furthermore, using isopropanol as a dispersant weakened the π–π interactions between the g-C3N4 nanosheets, resulting in g-C3N4 membranes with disoriented stacking structures assembled from high-quality g-C3N4 nanosheets. The prepared g-C3N4 nanosheet membranes showed excellent H2 permeability of 1.3 × 10−6 moles per square meter per second per pascal with high selectivity for the separation of various gaseous mixtures.
Density functional theory (DFT) and molecular dynamics (MD) simulations showed that the process of gas separation across the g-C3N4 membrane was due to the synergistic effects of size restriction and interactions of gas molecules with g-C3N4 nanosheets. Thus, the present study helped to develop a g-C3N4 nanosheet membrane and offered a method for creating alternative 2D nanosheet membranes for gas separation.
In summary, the fabricated lamellar g-C3N4 nanosheet membranes exhibited outstanding gas separation ability. Adopting the bottom-up method to prepare the membranes prevented the structural deterioration associated with top-down exfoliation.
The introduction of guest molecules during the membrane preparation prevented the g-C3N4 nanosheets from restacking. Moreover, the synergistic effect of the disordered stacking structure and high-density sieving channels increased the H2 permeance of the prepared nanosheet membranes.
These membranes are stable, even under harsh fabrication conditions. The long-term stability of the g-C3N4 membrane makes it a promising candidate for H2 purification, providing an opportunity to maintain carbon neutrality.
Zhou, Y et al. (2022). Fast hydrogen purification through graphitic carbon nitride nanosheet membranes. Nature Communications. https://www.nature.com/articles/s41467-022-33654-6