No abstract

Covalent organic frameworks (COFs) are penetrated with uniform and ordered nanopores, implying their great potential in molecular/ion separations. As an imine-linked, stable COF, TpPa-1 is receiving tremendous interest for molecular sieving membranes. Theoretically, atomically thin TpPa-1 monolayers exhibit extremely high water permeance but unfortunately no rejection to ions because of its large pore size (∼1.58 nm). The COF monolayers tend to stack to form laminated multilayers, but how this stacking influences water transport and ion rejections remains unknown. Herein, we investigate the transport behavior of water and salt ions through multilayered TpPa-1 COFs by nonequilibrium molecular dynamics simulations. By analyzing both the interfacial and interior resistance for water transport, we reveal that with rising stacking number of COF multilayers exhibit increasing ion rejections at the expense of water permeance. More importantly, stacking in the offset eclipsed fashion significantly reduces the equivalent pore size of COF multilayers to 0.89 nm, and ion rejection is correspondingly increased. Remarkably, 25 COF monolayers stacked in this fashion give 100% MgCl2 rejection, whereas water permeance remains 1 to 2 orders of magnitude higher than that of commercial nanofiltration membranes. This work demonstrates the rational design of fast membranes for desalination by tailoring stacking number and fashion of the COF monolayers.

We present an exfoliation-free and scalable strategy to prepare few-layered CONs by the interface-confined synthesis. The resultant CONs are assembled into selective layers for molecular separations.

Water flow inside polyamide (PA) reverse osmosis (RO) membranes is studied by steady state nonequilibrium molecular dynamics (NEMD) simulations in this work. The PA RO membrane is constructed with the all-atom model, and the density and average pore size obtained thereby are consistent with the latest experimental results. To obtain the time-independent water flux, a steady state NEMD method is used under various pressure drops. The water flux in our simulations, which is calculated under higher pressure drops, is in a linear relation with the pressure drops. Hence, the water flux in lower pressure drops can be reliably estimated, which could be compared with the experimental results. The molecular details of water flowing inside the membrane are considered. The radial distribution function and residence time of water around various groups of polyamide are introduced to analyze the water velocities around these groups, and we find that water molecules flow faster around benzene rings than around carboxyl or amino groups in the membrane, which implies that the main resistance of mass transport of water molecules comes from the carboxyl or amino groups inside the membranes. This finding is in good consistency with experimental results and suggests that less free carboxyl or amino groups should be generated inside RO membranes to enhance water permeance.

No abstract

It is generally considered that ion rejection of a desalination membrane is independent of the operation pressure drops (ΔPs), which is typically not higher than 10 MPa. However, this may not be true for pressures as high as hundreds of megapascals usually used in simulations. Therefore, simulation results of high ΔPs cannot be directly used to predict real-world ion rejections, which is often overlooked. Herein, we investigate the ion rejection of carbon nanotube membranes in a large scale of ΔPs via nonequilibrium molecular dynamics simulations. With effective pressure drops (ΔP e 's) increased from 2.85 to 996 MPa, the ion rejection drops from 100% to nearly zero. Rather than directly investigating the rejection, the relationships of ion and water fluxes with ΔPs are separately investigated. With rising ΔP e s, the water flux increases linearly, while the ion flux undergoes a two-stage increase: first, an exponential increase at ΔP e ≤ 53.4 MPa and then a linear increase. An equation describing the ΔP e -dependent ion rejection is then developed based on these observations. Moreover, the rejection mechanism is also discovered, which indicates that the enhanced input energy makes ions easier to overcome the energy barrier rather than the molecular-configurational reasons. These findings are expected to fill the big gaps between simulations and experiments and may also be helpful for the rational design of the next-generation desalination membranes.

Membrane separation is playing increasingly important role in providing clean water. Simulations predict that membrane pores with strong hydrophobicity produce ultrahigh water permeability as a result of low friction. However, experiments demonstrate that hydrophilic pores favor higher permeability. Herein we simulate water molecules transporting through interlayers of two-dimensional nanosheets with various hydrophilicities using nonequilibrium molecular dynamics. We reveal that there is a threshold pressure drop (ΔPT), exceeding which stable water permeability appears. Strongly hydrophobic pores exhibit extremely high ΔPT, prohibiting the achievement of ultrahigh water permeability under the experimentally accessible pressures. Under pressures < ΔPT, water flows in hydrophobic pores in a running-stop mode because of alternative wetting and nonwetting, thus leading to significantly reduced permeability. We discover that hydrophilic modification to one surface of the nanosheet can remarkably reduce ΔPT by > 99%, indicating a promising strategy to experimentally realize ultrafast membranes.

Water electrolysis, which is a promising high‐purity H2 production method, lacks pH‐universality; moreover, highly efficient electrocatalysts that accelerate the sluggish anodic oxygen evolution reaction (OER) are scarce. Geometric structure engineering and electronic structure modulation can be efficiently used to improve catalyst activity. Herein, a facile Ar plasma treatment method to fabricate a composite of uniformly dispersed iridium‐copper oxide nanoclusters supported on defective graphene (DG) to form IrCuOx@DG, is described. Acid leaching can be used to remove Cu atoms and generate porous IrOx nanoclusters supported on DG (P–IrOx@DG), which can serve as efficient and robust pH‐universal OER electrocatalysts. Moreover, when paired with commercial 20 wt% Pt/C, P–IrOx@DG can deliver current densities of 350.0, 317.6, and 47.1 mA cm−2 at a cell voltage of 2.2 V for overall water splitting in 0.5 m sulfuric acid, 1.0 m potassium hydroxide, and 1.0 m phosphate buffer solution, respectively, outperforming commercial IrO2 and nonporous IrOx nanoclusters supported on DG (O–IrOx@DG). Probing experiment, X‐ray absorption spectroscopy, and theoretical calculation results demonstrate that Cu removal can successfully create P–IrOx nanoclusters and introduce unsaturated Ir atoms. The optimum binding energies of oxygenated intermediate species on unsaturated Ir sites and ultrafine IrOx nanoclusters contribute to the high intrinsic OER catalytic activity of P–IrOx@DG.