Highly efficient quasi-static water desalination using monolayer graphene oxide/titania hybrid laminates

There has been a growing interest in water purification by graphene oxide (GO) laminate membranes due to their exceptional hydrophilicity, high throughput, and extraordinary separation performance originating from their two-dimensional and well-defined nanostructure. However, the swelling and stability in an aqueous environment are areas of concern for the GO laminate membranes. Here, a novel methylimidazolium ionic liquid-reduced GO (mimG)-assembled GO laminate membrane (mimG-GO) with remarkable stability was fabricated by a vacuum-assisted strategy for water purification. Methylimidazolium-based ionic liquid-reduced graphene oxide (mimG) was prepared by a facile nucleophilic ring-opening mechanism. Fabricated membranes were thoroughly characterized for stability, structural, permeance, and rejection properties in an aqueous environment. A combination of cationic mimG and GO nanosheets improves membrane stability in the aqueous environment via cation-π interactions and creates nanofluidic channels for facile water transport while yielding significant enhancement in the salt and dye separation performance. The pore size and the number of nanofluidic channels were precisely controlled via material deposition and laminate thickness to remove salts from water. The mimG-GO laminate membrane containing 72.2 mg m-2 deposition showed a permeance of 14.9 LMH bar-1, 50% higher than 9.7 LMH bar-1 of the neat GO laminate membrane, in addition to an increase in Na2SO4 salt rejection from 46.6 to 77.4%, overcoming the flux-rejection trade-off. The mimG-GO laminate membrane also rejected various anionic dyes (i.e., 99.9% for direct red 80 (DR 80), 96.8% for reactive black 5 (RB 5), and 91.4% for methyl orange (MO)). The mimG-GO laminate membrane containing 361.0 mg m-2 deposition showed the highest rejection for Na2SO4 (92.1%) and 99.9% rejection for DR 80, 99.0% rejection for RB 5, and 98.1% rejection for MO dyes keeping a flux of 2.6 LMH bar-1. Partial reduction and covalent grafting of ionic liquid moieties on GO helped to enhance the cation-π interaction between GO laminates, which showed enhanced stability, frictionless water transport, with high salt and dye rejection. Moreover, a simultaneous improvement in water permeance and solute rejection reveals the great potential of ionic liquid-functionalized GO laminate membranes for water-based applications.

Graphene oxide-cation interaction: Inter-layer spacing and zeta potential changes in response to various salt solutions

Graphene oxide (GO) laminates possess unprecedented fast water‐transport channels. However, how to fully utilize these unique channels in order to maximize the separation properties of GO laminates remains a challenge. Here, a bio‐inspired membrane that couples an ultrathin surface water‐capturing polymeric layer (<10 nm) and GO laminates is designed. The proposed synergistic effect of highly enhanced water sorption from the polymeric layer and molecular channels from the GO laminates realizes fast and selective water transport through the integrated membrane. The prepared membrane exhibits highly selective water permeation with an excellent water flux of over 10 000 g m−2 h−1, which exceeds the performance upper bound of state‐of‐the‐art membranes for butanol dehydration. This bio‐inspired strategy demonstrated here opens the door to explore fast and selective channels derived from 2D or 3D materials for highly efficient molecular separation.

Enhanced desalination performance of forward osmosis membranes based on reduced graphene oxide laminates coated with hydrophilic polydopamine

The nanocapillarity phenomenon involves ultralow frictional flow of water molecules through nanoscale channels, and here we study this using exceptionally large number of nanochannels within graphene oxide (GO) laminates. The nanoconfined water molecules in GO nanochannels form square lattice (as in the ice bilayer), which melts and jumps across the channels, similar to slip flow, with mean speed of the order of 1 m/s. This ease of liquid spreading in GO laminate is used to delay the critical heat flux (CHF) phenomenon in water pool boiling, by preventing formation/growth of dry spots. The water nanocapillarity speed is derived based on the measured water penetration flux, and the CHF enhancement (up to 140%) is demonstrated on a 1-μm-thick GO laminate. The GO laminate offers efficient surface modifications for increased transport efficiency (and safety margin) of pool boiling heat transfer systems.

Low-pressure loose GO composite membrane intercalated by CNT for effective dye/salt separation

The influence of cations intercalated in graphene oxide membranes in tuning H2/CO2 separation performance

Subnanometer interlayer space in graphene oxide (GO) laminates is desirable for use as permselective membrane nanochannels. Although the facile modification of the local structure of GO enables various nanochannel functionalizations, precisely controlling nanochannel space is still a challenge, and the roles of confined nanochannel chemistry in selective water/ion separation have not been clearly defined. In this study, macrocyclic molecules with consistent basal plane but varying side groups were used to conjunct with GO for modified nanochannels in laminates. We demonstrated the side-group dependence of both the angstrom-precision tunability for channel free space and the energy barrier setting for ion transport, which challenges the permeability-selectivity trade-off with a slightly decreased permeance from 1.1 to 0.9 L m-2 h-1 bar-1 but an increased salt rejection from 85% to 95%. This study provides insights into the functional-group-dependent intercalation modifications of GO laminates for understanding laminate structural control and nanochannel design.

Bio-inspired molecular bridge anchoring GO laminates onto PAN substrate for molecular separation

Development of a PEM with insignificant methanol permeability at highly concentrated methanol supply is considered a major advancement in the direct methanol fuel cell (DMFC) technology. GO, a popular precursor for graphene synthesis, is decorated with surface oxidative groups (-COOH, -OH, C=O, etc.), which render proton conductivity to GO under hydrated conditions [1]. Its proton conductivity, along with a highly impermeable graphene backbone makes GO a suitable material for a highly selective PEM. We prepared a GO membrane, which exhibited two orders of magnitude higher proton to methanol selectivity, as compared to Nafion [2]. The membrane didn’t suffer any open cell voltage drop, even at 5 and 10 M methanol solution.GO is prepared by chemically exfoliating graphite flakes via oxidation. Different recipes have been reported in literature for oxidation of graphite, which result in GO with different oxidation levels. Oxidation level of GO is known to impact its physiochemical properties (flake size, surface defects), which should affect mass transport of species across it. Herein, we present effect of oxidation level of GO on its proton and methanol transport characteristics. At first, we studied the impact of mean flake size on these transport processes, by preparing GO using Hummer’s method, and varying its mean flake size via sonication, while keeping a constant oxidation level. A 10 µm thick GO laminate is prepared by vacuum filtration of its aqueous dispersion. Methanol permeability of the GO laminate is observed to be varying linearly with flake size (Fig.1). This is because, being similar in size to water, methanol molecules also follow a tortuous transport path around a GO flake [3]. Hence, changing the flake size changes the diffusion path length for methanol. On the contrary, proton conductivity changed rather insignificantly with flake size (Fig.1). This could be due to the presence of proton selective atomic formations on the GO surface, which allows protons to be transported through the GO flake (Fig.3). We conducted a TEM study on the GO flakes, which revealed presence of surface defects of different sizes. Based on the size of these defects, possible proton transport paths across the GO flakes is discussed in Fig.4.GO with three different oxidation levels is prepared using process developed by Marcano et al. [4] by varying the weight ratio of graphite flakes to KMnO4. GO 1 to 3 (refers to products with an increasing oxidation level) is prepared by using weight ratios (Graphite: KMnO4) of 1:2, 1:4 and 1:6 respectively. Mean flake size of GO is reduced with increase in the oxidation level; mean flake sizes for GO-1, GO-2 and GO-3 measuring 77, 56 and 34 µm. Reduction in flake size is due to unzipping of GO flakes under oxidative environments. Therefore, methanol permeability across the GO laminate changes with oxidation level (Fig.2). However, unlike in Fig.1, methanol permeability doesn’t change linearly with flake size; rather it varies polynomially (order 3). This is because, in addition to breaking down of flakes, surface defects on GO surface also enlarge under the oxidative environment. This further adds to the methanol permeability across the GO laminate. Proton conductivity also increases with increase in the oxidation level. As mean flake size doesn’t have a significant effect on proton conductivity of GO flakes, this increase is primarily due to increase in the number of surface oxidative groups, which are responsible for the in-plane ionic conductivity of the GO flakes.[1] Karim et al. J. Am. Chem. Soc. 135 (2013) 8097−8100.[2] Paneri et al. J. Power Sources (2013)(Under review).[3] Boukhvalov et al.Nano Lett. 13 (2013) 3930.[4] Marcano et al. ACS Nano 4 (2010) 4806.

With excellent mass transport properties, graphene oxide (GO)-based lamellar membranes are believed to have great potential in water desalination. In order to quantify whether GO-based membranes are indeed suitable for reverse osmosis (RO) desalination, three sub-micrometer thick GO-based lamellar membranes: GO-only, reduced GO (RGO)/titania (TO) nanosheets and RGO/TO/chitosan (CTS) are prepared, and their RO desalination performances are evaluated in a home-made RO test apparatus. The photoreduction of GO by TO improves the salt rejection, which increases slowly with the membrane thickness. The RGO/TO/CTS hybrid membranes exhibit higher rejection rates of only about 30% (greater than threefold improvement compared with a GO-only membrane) which is still inferior compared to other commercial RO membranes. The low rejection rates mainly arise from the pressure-induced weakening of the ion–GO interlayer interactions. Despite the advantages of simple, low-cost preparation, high permeability and selectivity of GO-based lamellar membranes, as the current desalination performances are not high enough to afford practical application, there still remains a great challenge to realize high performance separation membranes for water desalination applications.

Confined intercalating sulfonated graphene quantum dots into GO laminates for fast alkali recovery

Controlling assembly behaviors of laminar GO membranes in organic solvents by altering GO-solvent interactions

High-efficiency water-transport channels using the synergistic effect of superhydrophilic PA layer and robust BU cross-linked GO laminates

Impact of synthesis conditions on physicochemical and transport characteristics of graphene oxide laminates