Gas Separation Membranes with Atom-Thick Nanopores: The Potential of Nanoporous Single-Layer Graphene

Abstract

Conspectus Gas separation is one of the most important industrial processes and is poised to take a larger role in the transition to renewable energy, e.g., carbon capture and hydrogen purification. Conventional gas separation processes involving cryogenic distillation, solvents, and sorbents are energy intensive, and as a result, the energy footprint of gas separations in the chemical industry is extraordinarily high. This has motivated fundamental research toward the development of novel materials for high-performance membranes to improve the energy efficiency of gas separation. These novel materials are expected to overcome the intrinsic limitations of the conventional membrane material, i.e., polymers, where a longstanding trade-off between the separation selectivity and the permeance has motivated research into nanoporous materials as the selective layer for the membranes. In this context, atom-thick materials such as nanoporous single-layer graphene constitute the ultimate limit for the selective layer. Gas transport from atom-thick nanopores is extremely fast, dependent primarily on the energy barrier that the gas molecule experiences in translocating the nanopore. Consequently, the difference in the energy barriers for two gas molecules determines the gas pair selectivity. In this Account, we summarize the development in the field of nanoporous single-layer graphene membranes for gas separation. We start by discussing the mechanism for gas transport across atom-thick nanopores, which then yields the crucial design elements needed to achieve high-performance membranes: (i) nanopores with an adequate electron-density gap to sieve the desired gas component (e.g., smaller than 0.289, 0.33, 0.346, 0.362, and 0.38 nm for H2, CO2, O2, N2, and CH4, respectively), (ii) narrow pore size distribution to limit the nonselective effusive transport from the tail end of the distribution, and (iii) high density of selective pores. We discuss and compare the state-of-the-art bottom-up and top-down routes for the synthesis of nanoporous graphene films. Mechanistic insights and parameters controlling the size, distribution, and density of nanopores are discussed. Fundamental insights are provided into the reaction of ozone with graphene, which has been successfully used by our group to develop membranes with record-high carbon capture performance. Postsynthetic modifications, which allow the tuning of the transport by (i) tailoring the relative contributions of adsorbed-phase and gas-phase transport, (ii) competitive adsorption, and (iii) molecular cutoff adjustment, are discussed. Finally, we discuss practical aspects that are crucial in successfully preparing practical membranes using atom-thick materials as the selective layer, allowing the eventual scale-up of these membranes. Crack- and tear-free preparation of membranes is discussed using the approach of mechanical reinforcement of graphene with nanoporous carbon and polymers, which led to the first reports of millimeter- and centimeter-scale gas-sieving membranes in t...

Selective Photonic Gasification of Strained Oxygen Clusters on Graphene for Tuning Pore Size in the Å Regime

“…Generally, one always observes a trade-off between gas permeance and gas pair selectivity because the conventional routes for pore incorporation have concomitant pore formation and expansion, which leads to increased pore size at a higher pore density. , However, in the approach we employed, we were able to successfully increase both gas permeance and gas pair selectivity in a systematic way. For the smallest gas that we probed (H 2 ), the increase in gas permeance was significant (a 12-fold increase between 43 and 80 °C).…”

“…35 The following fundamental understandings are crucial: identification of precursors which evolve into vacancy defects or pores; understanding how precursor properties (e.g., chemical composition, structure, and size) are related to that of the pores; and understanding the energetics of the underlying processes (i.e., precursor nucleation and growth and the conversion of precursor to pore). 35,36 Progress has been achieved in some of these aspects. For example, it is now well understood that epoxy groups are generated on the graphitic lattice upon oxidation.…”

Grand Canonical Monte Carlo Simulation Experiences of Methane and Ethane Adsorption Behaviors on Simplified Organic Shale Formed by Graphene Layering

“…This contrast in total adsorption might cause relatively larger deviations in calculations such as reservoir capacity. Moreover, we examined the solid−fluid interaction potential based on the Lennard-Jones potential (6)(7)(8)(9)(10)(11)(12), which was used in the simulations. The results show that the solid−fluid interaction energy between the graphene sheets in the walls and fluid particles (pure methane and ethane) increased as the pore size decreased.…”

“…Graphene nanopores have potential applications for gas separation such as the separation of methane and ethane from natural gas. The applications of graphene nanopores for gas separation are studied, and researchers used theoretical calculations to show that graphene nanopores could be used to separate methane and ethane with high efficiency . Similarly, the adsorption of methane and ethane on graphene nanopores is investigated, and researchers used molecular dynamics simulations to study the adsorption of methane and ethane on graphene nanopores.…”

Mechanistic Insights on Functionalization of Graphene with Ozone

“…Controlled incorporation of defects in graphene by surface functionalization is attractive for modulating its electronic, , magnetic, sorption, , diffusion, − and catalytic properties. , Among various functional groups, the epoxy group, which is essentially an O atom bonded to two neighboring C atoms without breaking the C–C bond, is of fundamental interest. Epoxy groups alter the local density of states of graphene, which can be used for stepwise tuning of band gap of graphene .…”