Nanoporous Graphene Membranes Research Articles

A Reverse Nonequilibrium Molecular Dynamics Algorithm for Coupled Mass and Heat Transport in Mixtures.

We present a new method for introducing stable nonequilibrium concentration gradients in molecular dynamics simulations of mixtures. This method extends earlier reverse nonequilibrium molecular dynamics (RNEMD) methods, which use kinetic energy scaling moves to create temperature or velocity gradients. In the new scaled particle flux (SPF-RNEMD) algorithm, energies and forces are computed simultaneously for a molecule existing in two nonadjacent regions of a simulation box, and the system evolves under a linear combination of these interactions. A continuously increasing particle scaling variable is responsible for the migration of the molecule between the regions as the simulation progresses, allowing for simulations under an applied particle flux. To test the method, we investigate diffusivity in mixtures of identical but distinguishable particles and in a simple mixture of multiple Lennard-Jones particles. The resulting concentration gradients provide Fick diffusion constants for mixtures. We also discuss using the new method to obtain coupled transport properties using simultaneous particle and thermal fluxes to compute the temperature dependence of the diffusion coefficient and activation energies for diffusion from a single simulation. Lastly, we demonstrate the use of this new method in interfacial systems by computing the diffusive permeability of a molecular fluid moving through a nanoporous graphene membrane.

Dense fluid transport through nanoporous graphene membranes in the limit of steric exclusion

Dense fluid transport through nanoporous graphene membranes in the limit of steric exclusion

COF-anchored design of nanoporous graphene membranes for ultrafast and selective organic separation

Nanoporous graphene has attracted extensive attention as a type of disruptive membrane materials for organic separation. However, the fabrication of an ideal graphene membrane with high permeability and selectivity remains challenging due to the inevitable non-selective defects. Here, a strategy based on synergetic effect of graphene membranes and covalent organic framework (COF) nanounits is developed. Atomically thin nanoporous graphene provides high solvent flux and stability. To prevent the leakage from inevitable non-selective pores in graphene without sacrificing flux, porous COF nanounits are directly anchored at the defective sites. The fabricated membranes exhibit record-high solvent flux of 49.81 L m−2 h−1 (1–2 orders of magnitude higher than that of state-of-the-art membranes), and relatively low reverse solute flux of 1.69 g m−2 h−1 in organic solvent forward osmosis (OSFO) process. The strategy we proposed gives dramatic impetus to the development of OSFO and other membrane separation processes.

3D hierarchical aquaporin-like nanoporous graphene membrane with engineered tripartite nanochannels for efficient oil/water separation

The aqueous system is being polluted by the untreated direct discharge of industrial oily wastewater into the ecosystem. Due to its low cost, energy economy, and sustainability, the advanced membrane filtration method is regarded as one of the best methods for treating oily wastewater. Its exceptional atomic thickness and superior amphiphilic properties of graphene oxide (GO) nanosheet make it one of the finest 2D constituents for creating membranes with high permeability. Nevertheless, the interlayer d-spacing of multi-stacked GO membranes is crucial since it is responsible for the permeability/selectivity trade-off. To efficiently separate oil-in-water emulsion using an in-situ polymerization technique, we developed an aquaporin-like 3D hierarchical multi-functionalized nanoporous graphene (NPG) membrane with tripartite nanochannels. The extraordinary, prepared membrane displayed both ultra-water-permeability of 2490 L m−2 h−1.bar along with superior selectivity. Consequently, the permeance of the aquaporin-like 3D hierarchical multi-functionalized NPG membrane achieves a higher flux than the GO membrane, while the oil rejection reaches ~97%.

Beyond the single-layer limit for desalination based on conical channels of multilayer nanoporous graphene membranes

Nanoporous graphene monolayer has been proved to be a candidate for desalination with high permeability due to its ultrathin nature, however, it is still challenging to produce at large scales, which hampers its practical applications. Here, we propose a multilayer membrane strategy for high-efficiency desalination beyond the single-layer limit using conical channels formed by graphene multilayer of nanopores. By varying the number of layers and the cone angle, molecular dynamics simulations demonstrate that the water flux for the channels made of four-layer graphene at the cone angles of 8.6°, 19.1° and 35° is much larger than that the monolayer graphene with the pore size identical to that of the smallest pore of multilayer. Based on the channel with cone angle of 19.1°, we further find that N-functionalized conical channels exhibit promising desalination performance in terms of both water permeability and salt rejection. This work provides a useful guideline for the application of multilayer graphene membranes in the field of desalination.

Preanchoring Enabled Directional Modification of Atomically Thin Membrane for High-Performance Osmotic Energy Generation.

Salinity gradient energy is an environmentally friendly energy source that possesses potential to meet the growing global energy demand. Although covalently modified nanoporous graphene membranes are prospective candidates to break the trade-off between ion selectivity and permeability, the random reaction sites and inevitable defects during modification reduce the reaction efficiency and energy conversion performance. Here, we developed a preanchoring method to achieve directional modification near the graphene nanopores periphery. Numerical simulation revealed that the improved surface charge density around nanopores results in exceptional K+/Cl- selectivity and osmotic energy conversion performance, which agreed well with experimental results. Ionic transport measurements showed that the directionally modified graphene membranes achieved an outstanding power density of 81.6 W m-2 with an energy conversion efficiency of 35.4% under a 100-fold salinity gradient, outperforming state-of-the-art graphene-based nanoporous membranes. This work provided a facile approach for precise modification of nanoporous graphene membranes and opened up new ways for osmotic power harvesting.

Rational design of reverse osmosis membranes for boron removal: A counter-intuitive relationship between boron rejection and pore size

Owing to the toxicity of excess boron, developing reverse osmosis membranes that are capable of effectively removing boric acid, a boron specie commonly exists in seawater, is critical to advance water purification technology. To date, design principles of reverse osmosis membranes for boron removal have not yet been systematically explored. Using nanoporous graphene as a model system and by employing state-of-the-art molecular simulation techniques, this study discovers the critical role of the adsorptive competition of boric acid against water in the nanopores of membranes in their design. Specifically, the preferential adsorption of water onto the pore rims can be exploited to impede the permeation of boric acid, facilitating the size-exclusion effect for much improved boron rejection. Moreover, a well-balanced adsorptive competition is desired; water adsorption should not be too strong so that efficient water permeation can still be preserved. This results in membranes that are not only highly permeable but also capable of effectively removing boric acid. Detailed investigations on energetic, dynamics, and spatial distribution of water and boric acid as well as their free energy landscapes are also conducted for better understandings. Moreover, bilayer nanoporous graphene membranes with a heterogeneous design are identified to effectively block both boric acid and salt with a decent water permeation. Overall, the outcomes achieved herein can fundamentally guide the rational design of reverse osmosis membranes for more efficient and effective boron removal.

Bioinspired Large‐Area Atomically‐Thin Graphene Membranes

AbstractNanoporous graphene membranes are attractive for molecular separations, but it remains challenging to maintain sufficient mechanical strength during scalable fabrication and module development. Inspired by the composite structure of cell membranes and cell walls, a large‐area atomically thin nanoporous graphene membrane supported by a fiber‐reinforced structure with strong interlamellar adhesion is designed. Compared with other graphene‐based membranes of large scale, the fracture stress, fracture strength, and tensile stiffness of the composite membranes can be enhanced by a factor of 17, 67, and 94, respectively. This fiber‐reinforced structure also confers stability of the composite membrane to different curvature states and repeated bending processes after 10 000 times, which provides an opportunity for modularization. The breathable function of such membrane with an ultrahigh gas permeance (≈8.6–23 L m−2 d−1 Pa−1) and an ultralow water vapor transportation rate (WVTR) (≈23–129 g L m−2 d−1) is observed, superior to most commercial materials. This work provides a facile method to fabricate large‐area graphene membranes and paves the road to practical application in the membrane separation field for other 2D films.

Oscillating electric field assisted nano-incised graphene kirigami for ultrahigh-performance water desalination membrane

Benefiting from the single-atom thickness, dense honeycomb structure, nano-size pores, and adjustable selection capability, graphene kirigami (GK) is expected to be a promising candidate for the membrane of water desalination. However, due to the low bending stiffness of the GK structure, the fluctuation in incision size and the accumulation of salt ions around the incisions always limit the GK membrane from higher water desalination efficiency. In this study, we adopted an oscillating electric field to improve the water desalination efficiency of nano-incised GK via molecular dynamic (MD) simulation. The results reveal that with the oscillating electric field's assistance, the water permeability of the GK membrane achieves 1351.6 L/m2/h/bar theoretically under the condition of 100 % salt rejection, 5.9–7.1 times higher than the reported nano-porous graphene membrane and 2–5 orders of magnitude higher than the existing reverse osmosis membranes. The oscillating electric field can enhance the water desalination efficiency of the proposed GK membranes mainly by alleviating the structural fluctuation of the GK membrane and dredging the permeation pathway for water molecules. The considerable potential of the oscillating electric field-assisted GK membrane reported by this study will attract great attention in the field of water separation and purification.

From molecular sieving to gas effusion through nanoporous 2D graphenes: Comparison between analytical predictions and molecular simulations.

In this paper, we study the permeation of polyatomic gas molecules through 2D graphene membranes. Using equilibrium molecular dynamics simulations, we investigate the permeation of pure gas compounds (CH4, CO2, O2, N2, and H2) through nanoporous graphene membranes with varying pore sizes and geometries. Our simulations consider the recrossing mechanism, often neglected in previous studies, which has a significant effect on permeation for intermediate pore size to molecular diameter ratios. We find that the permeation process can be decoupled into two steps: the crossing process of gas molecules through the pore plane and the escaping process from the pore region to a neighboring adsorption site, which prevents recrossing. To account for these mechanisms, we use a permeance model expressed as the product of the permeance for the crossing process and the probability of molecule escape. This phenomenological model is extended to account for small polyatomic gas molecules and to describe permeation regimes ranging from molecular sieving to effusion. The proposed model captures the temperature dependence and provides insights into the key parameters of the gas/membrane interaction controlling the permeance of the system. This work lays the foundation for predicting gas permeance and exploring membrane separation factors in 2D materials such as graphene.

Evaluation of water desalination performances of functionalized nanoporous graphene membranes by molecular dynamics simulation

Nanoporous graphene membrane (NPGM) is a promising candidate for water desalination due to its unique features, such as chemical stability, high resistance to chlorine and fouling, excellent mechanical strength, and tunable permeability. In this work, the water desalination performances of small- and large-pore, pristine and functionalized NPGMs, with the latter containing amide, sulfonic acid, thiourea, and carbamate functional groups, were evaluated by molecular dynamics (MD) simulation. Our results indicate that pore functionalization leads to a decrease in water flow rate through the membrane. Among functionalized NPGMs, large-pore, amide-functionalized NPGM exhibited the highest water flow rate. In terms of ion rejection performance, pore functionalization resulted in partial Na+ and Cl− ion rejection by both small- and large-pore NPGMs. For small-pore, functionalized NPGMs, the rejection of Na+ and Cl− ions were higher than 98%. Comparing between the ion rejection performances of large-pore, functionalized NPGMs, the following order of Na+ ion rejection rates were observed: carbamate > sulfonic acid > thiourea > amide. The carbamate functionalization of large-pore NPGM led to about 21% and 18% Na+ and Cl− rejection, respectively. This observation is in good correlation with the smallest effective pore diameter in the carbamate-functionalized NPGM and the largest ion-carbamate interaction energies.

Designing a biomimetic graphene nanopore with valence selectivity between cations

Biological ion channels exhibit many remarkable mass-transport properties, including high selectivity and permeability, which are also highly desired in practical applications. Herein, using molecular dynamics simulations, we design a biomimetic graphene nanopore with valence selectivity for monovalent cations, by tailoring pore size and modifying the pore rim with four negatively charged carboxylate groups to mimic the selectivity filter of bacterial Na+ channels such as NavAb. The results indicate that under electric fields, the biomimetic nanopore preferentially conducts Na+ over Ca2+, as the carboxylate groups in the nanopore selectively bind Ca2+ and slow down its passage through the nanopore. In contrast, Na+ interacts with the carboxylate groups less strongly and passes across the nanopore quickly. Under strong fields, a Cl− concentration polarization layer quickly forms on one side of the nanopore and promotes the formation of stable clusters of Cl− and Ca2+ pairs bound at the carboxylate groups, which severely hinders the transport of Cl− and occasionally suspends the transport of cations across the nanopore. These molecular insights provide some design principles and implications for fabricating nanoporous graphene membranes for separation applications, such as heavy metal removal from wastewater.

Nanoporous multilayer graphene oxide membrane for forward osmosis metal ion recovery from spent Li-ion batteries

The recovery of Li and rare metals from the spent electrodes of Li-ion batteries (LIBs) is becoming increasingly important. Herein, nanoporous multilayer graphene oxide (NMG) membranes were fabricated via the confined thermal annealing method to achieve Li-ion selectivity. A graphene oxide (GO) membrane was prepared by coating the membrane with an aqueous layer of GO liquid crystal via the scalable bar coating method, followed by hot-pressing. The influence of the GO interlayer spacing and nanopore presence on the ion separation performance was investigated. The NMG membrane shows different ion permeance phenomena depending on ion concentration. The permeation rate of divalent ions (Co, Ni, Mn) through the NMG membrane was faster than that of Li ions in solution with low ionic strength, whereas the membrane was Li-ion selective in mixed ionic solutions or at high ionic strength. This result indicates that electrostatic interaction by oxygen groups is important at low ionic strength, but size exclusion by interlayer spacing and adsorption by nanopore is critical at high ionic strength. The ion permeation mechanisms were further examined based on molecular calculations, which revealed that the binding energies between the divalent ions and nanopores or basal plane of graphene were high, resulting in slow ion permeation. When the NMG membrane was combined in a forward osmosis system, Li-ion can be separated continuously from the mixture solution simulated from the spent Li-battery electrode. This approach shows the high potential of nanoporous multilayer graphene membrane for Li extraction in particular with dense pore structure and around 7 Å of d-spacing.

Microwave-assisted design of nanoporous graphene membrane for ultrafast and switchable organic solvent nanofiltration

Layered two-dimensional materials can potentially be utilized for organic solvent nanofiltration (OSN) membrane fabrication owing to their precise molecular sieving by the interlayer structure and excellent stability in harsh conditions. Nevertheless, the extensive tortuosity of nanochannels and bulky solvent molecules impede rapid permeability. Herein, nanoporous graphene (NG) with a high density of sp2 carbon domain was synthesized via sequential thermal pore activation of graphene oxide (GO) and microwave-assisted reduction. Due to the smooth sp2 carbon domain surfaces and dense nanopores, the microwave-treated nanoporous graphene membrane exhibited ultrafast organic solvent permeance (e.g., IPA: 2278 LMH/bar) with excellent stability under practical cross-flow conditions. Furthermore, the membrane molecular weight cut-off (MWCO) is switchable from 500 Da size of molecule to sub-nanometer-size molecules depending on the solvent type, and this switching occurs spontaneously with solvent change. These properties indicate feasibility of multiple (both binary and ternary) organic mixture separation using a single membrane. The nanochannel structure effect on solvent transport is also investigated using computation calculations.

Theoretical investigation of graphene kirigami membrane for natural gas processing

Membrane separation technology plays a vital role in natural gas processing due to its cost-effectiveness, excellent efficiency and low energy consumption. In this study, we demonstrate the superior permeability and selection capability of the graphene kirigami (GK) membrane for natural gas purification via molecular dynamics insights. The results indicate that the separation rate of the GK membrane can reach up to 4.1 × 106 GPU (gas permeance unit) with nearly 100% selectivity of the permeating gases mixture (H2S, CO2 and N2) over the non-permeating gas (CH4). Furthermore, the Monte Carlo simulation reveals that the unique geometry of the GK structure introduces a funnel-shaped adsorption channel to guide specific molecules’ separation and enhance the efficiency of the membrane. Compared with the nano-porous graphene (NPG) membrane, the gas separation rate of the GK membrane can be increased by more than four times. Moreover, the GK membrane does not require the modification of functional groups in natural gas purification, which provides great convenience for future applications in practical projects. The considerable potential of the GK membrane underpinned by the findings of this study will attract broad interest in the field of separation and purification.

Atomic boundary position and steric effects on ion transport and separation through nanoporous graphene membrane

Atomic boundary position and steric effects on ion transport and separation through nanoporous graphene membrane

The validity of the continuum modeling limit in a single pore flows to the molecular scale.

The discrete characteristics of molecules become dominant in the molecular regime when the surface-to-volume ratio becomes very high. Using the well-established continuum approach is questionable due to this dominant behavior. Due to the lack of perfect modeling of such a small-scale system, the experimentalist must rely on the trial and error method. Here we analyze the water transport mechanism through a nanoporous graphene membrane at the molecular level by adopting the classical molecular dynamics (MD) simulation. The results for SPC/E water molecules were compared with those obtained for liquid argon atoms and continuum Sampson's equation predictions. We find that the effect of local variants such as density layering, interatomic forces, slip velocity, and geometric boundary conditions become exponentially dominant with decreasing nanopore size. Consequently, the continuum assumptions break down at 1.5 nm pore diameter due to neglecting the dominant local properties.

Removal of methanol from water by capacitive deionization system combined with functional nanoporous graphene membrane

In this article, molecular dynamics simulations were used to examine the feasibility of capacitive deionization (CDI) system combined with a functionalized nanoporous graphene (NPG) membrane for removing methanol from water. The radial distribution function of electrode-methanol and methanol-water, the self-diffusion coefficient of methanol and water, the water density near the membrane, the interaction energy between methanol and membrane, the hydrogen bond structure between methanol and water, and the 2D density map of methanol molecules near the membrane under different electric field (EF) (to simulate the effect of capacitance) were examined to evaluate the separation performance of NPG membranes with hydrogen-passivated pores for methanol. The findings show that an EF with appropriate strength can decrease the amount of water molecules near methanol, increase the self-diffusion coefficient of methanol and water, increase hydrophobicity of hydrogenated pores, decrease the interaction between the NPG membrane and methanol, and weaken hydrogen bond interaction between water and methanol molecules. All these findings suggest that an appropriate EF can improve the NPG membrane's permeability to methanol, and verify the feasibility of CDI system combined with hydrogenated NPG membrane to remove methanol from water. This study is expected to propose a potential CDI application technology, and also give a novel idea for the removal of small organic molecules in water by functionalized NPG membrane.

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

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 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.

Polyethylene oxide-intercalated nanoporous graphene membranes for ultrafast H2/CO2 separation: Role of graphene confinement effect on gas molecule binding

H2/CO2 separation is essential for various industrial applications, such as hydrocarbon steam reforming for H2 production. Herein, a layered composite membrane composed of nanoporous graphene (NG) and polyethylene oxide (PEO) is developed. The membrane demonstrates an ultrafast H2 permeability of ca. 32 240 barrer with a moderate H2/CO2 selectivity of 25, which far surpasses the upper bound of the separation performance demonstrated by existing membrane materials. The regularly layered structure is fabricated by facilitating the intercalation of PEO via an aqueous graphene-oxide liquid crystal (GOLC) scaffold. After preparing the layered GO/PEO membrane, hot pressing is conducted to activate dense nanopores on the basal plane of graphene, resulting in an NG/PEO membrane. The permeation of gas molecules is significantly enhanced owing to the presence of nanopores and expansion of the interlayer spacing by PEO. In contrast, CO2 permeation is lower than that of H2 owing to its strong binding interaction with PEO, and molecular simulations demonstrate that this CO2–PEO interaction is significantly enhanced owing to the confinement of PEO in the graphene interlayer spacing, particularly at d-spacing of approximately 7–8 Å.