Hydrogen-sieving single-layer graphene membranes obtained by crystallographic and morphological optimization of catalytic copper foil

Gas separation membranes based on single-layer-graphene are highly attractive because the size of graphene nanopores can be tuned to separate gases by the size-sieving mechanism. A prerequisite for this, the synthesis of high-quality polycrystalline single-layer graphene film by chemical vapor deposition (CVD), is extremely crucial. The quality of graphene in the context of membranes is reflected by the size and the density of the intrinsic vacancy defects, and is affected by the catalytic metal substrate and the CVD environment. Generally, expensive high-purity Cu foil is used to obtain gas-sieving performance from single-layer graphene. For the eventual scale-up of graphene membranes, it is highly attractive to use low-cost Cu foils, however, as we show here, these Cu foils are rough and graphene membranes derived from these foils do not yield gas-sieving performance. Herein, we conduct a systematic high-temperature annealing study on two separate, commercial, low-cost Cu foils leading to their transformation to Cu(111). The annealing process smoothened the Cu surface, decreasing the root mean square (RMS) surface roughness from over 200 nm to close to 100 nm. The RMS roughness on the individual Cu step, measured using the scanning tunneling microscopy (STM), was only 0.23 nm. The smooth, oriented Cu grains yielded single-layer graphene with a significantly lower defect density with ID/IG ratio decreasing from 0.18 ± 0.02 to 0.04 ± 0.01. Finally, single-layer graphene films, synthesized on the annealed low-purity Cu foil, yielded H2-selective membranes with H2 permeance reaching 1000 gas permeation units (GPU) in combination with attractive H2/CH4 and H2/C3H8 selectivities of 13 and 26, respectively.

Crystallization of Size-Selective Nanopores in Graphene for Gas Separation

Mojtaba Rezaei

Nanoporous single-layer graphene (N-SLG) membranes, owing to their single-atom thinness, have the potential to exceed the permeance and selectivity limits of gas separation membranes. However, two key issues in the top-down N-SLG synthesis need to be addressed to achieve scalable, high-performance membranes: a) reproducible synthesis of high-quality SLG film by chemical vapor deposition (CVD) on a low-cost Cu substrate, and b) introducing high-density of pores with a narrow pore-size-distribution (PSD). This dissertation addresses these issues by developing a method to prepare smooth and oriented Cu foil by a facile approach to obtain high-quality SLG membranes. On the fundamental science front, it explores two novel methods of tuning the PSD in graphene for gas separation. Low-cost Cu foils are rough, and result in membranes with large nonselective intrinsic vacancy defects which hampers their application in gas separation. Herein, we conduct a systematic high-temperature annealing study on two separate, commercial, low-cost Cu foils leading to their transformation to smooth Cu(111), decreasing their surface roughness by ~ 2-fold. The smooth, oriented Cu foils yielded SLG with a significantly lower defect density with ID/IG ratio decreasing from 0.18 Â± 0.02 to 0.04 Â± 0.01. The intrinsic defects in these SLG films were H2 selective with H2 permeance reaching 1000 gas permeation units (GPU; flux normalized with transmembrane pressure difference) and attractive H2/CH4 and H2/C3H8 selectivities of 13 and 26, respectively. To decouple the pore nucleation and expansion in SLG, we utilize CO2 as a mild etchant to expand the existing pores at temperatures ranging 750 â 1000 Â°C. CO2 could uniformly expand the intrinsic pores in SLG in a controlled manner, down to a few Ã/min, without nucleating new defects in the SLG basal plane. Furthermore, we revealed two distinct kinetic zones for the reaction of CO2 with graphene edges, with the transition happening around the pore diameter of ~ 2 nm. Etching rate of the larger expanded-pores was constant and independent of the pore size. The expansion was thermally activated with an activation energy of 2.71 eV, consistent with the literature based on ab-initio calculation for CO2 dissociative chemisorption on zigzag edges. In comparison, the etching rate was an order of magnitude slower in smaller pores indicating that geometrical confinement in smaller pores play an important role. An exponential relation between the density of expanded pores and etching temperature, with an activation energy of 3.58 eV, was observed. Next, we develop a novel method to fabricate N-SLG with a high pore-density while maintaining a narrow PSD. We exposed the highly porous SLG (treated by O2 plasma) with a broad PSD to graphene CVD condition in presence of both CH4 and CO2. The pore expansion (as a result of etching) and shrinkage (as a result of growth) reached a comparable rate. Moreover, CO2 suppressed the graphene grain nucleation, leading to high-quality graphene synthesis. Membranes with H2 permeance reaching 10000 GPU and H2/C3H8 selectivity of 26 were fabricated by optimizing the CH4/CO2 ratio at 800Â°C.In summary, we address the current obstacles in SLG membrane development by utilizing CO2 in graphene CVD environment to tune the PSD of SLG membranes. Moreover, a simple annealing method to optimize the morphological and crystallographic properties of Cu foils for high-quality SLG synthesis is proposed.

EPFL2022

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

Kumar Varoon Agrawal, Deepu Joseph Babu, Kuang-Jung Hsu, Cédric Karel J Van Goethem, Luis Francisco Villalobos Vazquez de la Parra

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 highperformance 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 the year 2018 and 2021, respectively. We conclude with insights and perspectives highlighting the key scientific and technological gaps that must be addressed in the future research. Superscript/Subscript Available

AMER CHEMICAL SOC2022

High-permeance polymer-functionalized single-layer graphene membranes that surpass the postcombustion carbon capture target

Kumar Varoon Agrawal, Guangwei He, Shiqi Huang, Mounir Driss Mensi, Emadeddin Oveisi, Mojtaba Rezaei, Luis Francisco Villalobos Vazquez de la Parra, Jing Zhao

Membrane-based postcombustion carbon capture can reduce the capture penalty in comparison to absorbent-based separation. To realize this, high-performance membranes are urgently needed with a CO2 permeance exceeding 1000 gas permeation units or GPU, and a CO2/N2 mixture separation factor exceeding 20. Here, we report a new class of organic–inorganic hybrid membranes based on single-layer graphene with a selective layer thinner than 20 nm. For this, the impermeable graphene lattice is exposed to oxygen plasma leading to a high percentage of vacancy defects (porosity up to 18.5%) and is then functionalized with CO2-philic polymeric chains. Treating a gas stream mimicking flue gas, the hybrid membranes yield a six-fold higher CO2 permeance (6180 GPU with a CO2/N2 separation factor of 22.5) than the performance target. Membranes prepared with a combination of optimized graphene porosity, pore size, and functional groups yield a CO2 permeance up to 11 790 GPU. Other membranes yield a CO2/N2 selectivity up to 57.2.