Single-layer graphene oxide (SLGO) is emerging as a new-generation membrane material for high-flux, high-selectivity water purification, owing to its favorable two-dimensional morphology that allows facile fabrication of ultrathin membranes with subnanometer interlayer channels. However, reliable and precise molecular sieving performance still necessarily depends on thick graphene oxide (GO) deposition that usually leads to low water flux. This trade-off between selectivity and flux significantly impedes the development of ultrathin GO membranes. In this work, we demonstrate that the selectivity/flux trade-off can be broken by self-assembly of SLGO via simple deposition rate control. We find GO membranes, prepared by slow deposition of SLGO flakes, exhibit considerably improved salt rejection, while counterintuitively having 2.5-4 times higher water flux than that of membranes prepared by fast deposition. This finding has extensive implications of designing/tuning interlayer nanostructure of ultrathin GO membranes by simply controlling SLGO deposition rate and thus may greatly facilitate their development for high flux, high selectivity water purification.

Among the current CO2 capture technologies, membrane gas separation has many inherent advantages over other conventional techniques. However, fabricating gas separation membranes with both high CO2 permeance and high CO2/N2 selectivity, especially under wet conditions, is a challenge. In this study, sub-20-nm thick, layered graphene oxide (GO)-based hollow fiber membranes with grafted, brush-like CO2-philic agent alternating between GO layers are prepared by a facile coating process for highly efficient CO2/N2 separation under wet conditions. Piperazine, as an effective CO2-philic agent, is introduced as a carrier-brush into the GO nanochannels with chemical bonding. The membrane exhibits excellent separation performance under simulated flue gas conditions with CO2 permeance of 1,020 GPU and CO2/N2 selectivity as high as 680, demonstrating its potential for CO2 capture from flue gas. We expect this GO-based membrane structure combined with the facile coating process to facilitate the development of ultrathin GO-based membranes for CO2 capture.

Robust, gas-impeding water-conduction nanochannels that can sieve water from small gas molecules such as hydrogen (H2), particularly at high temperature and pressure, are desirable for boosting many important reactions severely restricted by water (the major by-product) both thermodynamically and kinetically. Identifying and constructing such nanochannels into large-area separation membranes without introducing extra defects is challenging. We found that sodium ion (Na+)–gated water-conduction nanochannels could be created by assembling NaA zeolite crystals into a continuous, defect-free separation membrane through a rationally designed method. Highly efficient in situ water removal through water-conduction nanochannels led to a substantial increase in carbon dioxide (CO2) conversion and methanol yield in CO2 hydrogenation for methanol production.

a significant advantage, greatly facilitates GO deposition from solution using water as a low cost and environment-friendly solvent. [3] Recently, GO has attracted great attention as a novel 2-D membrane material in water purification application because of its excellent mechanical property, atomically thin thickness, excellent dispersion in water, and ease to form compact membrane structure or to be added into polymer matrix. [3,4] Concept demonstration/preliminary studies on using graphene-based membranes for water purification were focused on simulations for single layer graphene/ GO/reduced GO (rGO) with structural defects. Cohen-Tanugi et al., [5] using molecular dynamics (MD) simulation, found that hydrogenated and hydroxylated defects with appropriate sizes on graphene could have 2-3 orders of magnitude higher water permeability than commercial reverse osmosis (RO) membranes but similarly high salt rejection, suggesting great potential of single-layered graphene membranes for desalination. Lin et al. [6] showed by MD simulations that thermally reducing GO with different initial epoxy to hydroxyl ratios and different oxygen concentrations may generate selective defects on rGO for high water permeability and high salt rejection desalination. Figure 2a shows representative structures of rGO after reduction at 2,500 K, when GO flakes with different starting oxygen concentrations and epoxy concentrations or epoxy/hydroxyl ratios are used. With the increase of oxygen concentration and epoxy concentration, rGO becomes more defective and has bigger nanopores because of more carbon removal from the GO matrix. This suggests pores on rGO may be controlled by controlling starting GO composition and reduction conditions. Further, they studied desalination performance of defects on rGO after reduction at different temperatures and using GO with different oxygen concentrations and epoxy concentrations (Figure 2b). Too low oxygen concentration (17%) leads to complete water blocking irrespective of reduction temperature and initial epoxy concentration or epoxy/hydroxyl ratio. At higher initial oxygen concentration (25% and 33%), high water flux and 99% salt rejection can be obtained depending on reduction temperature epoxy/hydroxyl ratio. These promising simulation results, therefore, suggest appropriately reducing GO with desired starting composition As a newly emerging 2-dimensional (2-D) material with sub-nanometer thickness, graphene oxide (GO) has been widely studied either as a pure/skeleton membrane material or as an additive in and a functional coating on matrix membranes for water purification because of its unique physico-chemicomechanical properties. Manipulating or incorporating this novel 2-D material effectively into a membrane structure has been shown to significantly improve membrane performance, including increased water permeability, alleviated fouling, improved antibacterial properties, etc., which will eventually lead to lower energy consumption, longer lifetime, and lower maintenance cost. As the pure/...

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