Ion transport in nanoconfinement differs from that in bulk and has been extensively researched across scientific and engineering disciplines
. For many energy and water applications of nanoporous ...materials, concentration-driven ion diffusion is simultaneously subjected to a local electric field arising from surface charge or an externally applied potential. Due to the uniquely crowded intermolecular forces under severe nanoconfinement (<2 nm), the transport behaviours of ions can be influenced by the interfacial electrical double layer (EDL) induced by a surface potential, with complex implications, engendering unusual ion dynamics
. However, it remains an experimental challenge to investigate how such a surface potential and its coupling with nanoconfinement manipulate ion diffusion. Here, we exploit the tunable nanoconfinement in layered graphene-based nanoporous membranes to show that sub-2 nm confined ion diffusion can be strongly modulated by the surface potential-induced EDL. Depending on the potential sign, the combination and concentration of ion pairs, diffusion rates can be reversibly modulated and anomalously enhanced by 4~7 times within 0.5 volts, across a salt concentration gradient up to seawater salinity. Modelling suggests that this anomalously enhanced diffusion is related to the strong ion-ion correlations under severe nanoconfinement, and cannot be explained by conventional theoretical predictions.
Upon shearing a microscale lithographically defined graphite mesa, the sheared section retracts spontaneously to minimize interface energy. Here, we demonstrate a sixfold symmetry of the ...self-retraction and provide a first experimental estimate of the frictional force involved, as direct evidence that the self-retraction is due to superlubricity, where ultralow friction occurs between incommensurate surfaces. The effect is remarkable because it occurs reproducibly under ambient conditions and over a contact area of up to 10×10 μm2, more than 7 orders of magnitude larger than previous scanning-probe-based studies of superlubricity in graphite. By analyzing the sheared interface, we show how the grain structure of highly oriented pyrolitic graphite determines the probability of self-retraction. Our results demonstrate that such self-retraction provides a novel probe of superlubricity, and the robustness of the phenomenon opens the way for practical applications of superlubricity in micromechanical systems.
Nanoporous laminar membranes composed of multilayered 2D nanomaterials (2D‐NLMs) are increasingly being exploited as a unique material platform for understanding solvated ion transport under ...nanoconfinement and exploring novel nanoionics‐related applications, such as ion sieving, energy storage and harvesting, and in other new ionic devices. Here, the fundamentals of solvation‐involved nanoionics in terms of ionic interactions and their effect on ionic transport behaviors are discussed. This is followed by a summary of key requirements for materials that are being used for solvation‐involved nanoionics research, culminating in a demonstration of unique features of 2D‐NLMs. Selected examples of using 2D‐NLMs to address the key scientific problems related to nanoconfined ion transport and storage are then presented to demonstrate their enormous potential and capabilities for nanoionics research and applications. To conclude, a personal perspective on the challenges and opportunities in this emerging field is presented.
Solvation‐involved nanoionics is the study of ion transport and storage in nanoporous structures immersed in liquids involving various short‐range atomic interactions. The fundamentals are introduced and the progress and opportunities enabled by 2D nanomaterial laminar membranes are discussed, which is of great interest for energy storage, desalination, ionic devices, and others.
Electrical double layer （EDL） capacitors based on recently emergent graphene materials have shown several folds performance improvement compared to conventional porous carbon materials, driving a ...wave of technology breakthrough in portable and renewable energy storage. Accordingly, much interest has been generated to pursue a comprehensive understanding of the fundamental yet elusive double layer structure at file electrode~electrolyte interface. In this paper, we carried out comprehensive molecular dynamics simulations to obtain a com- prehensive picture of how ion type, solvent properties, and charging conditions affect the EDL structure at the graphene electrode surface, and thereby its contribution to capacitance. We show that different symmetrical monovalent aqueous electrolytes M~X- （M~ = Na~, K~, Rb＋, and Cs＋; X- = F-, CI-, and I ） indeed have distinctive EDL structures. Larger ions, such as, Rb＊, Cs＊, C1, and I, undergo partial dehydration and penetrate through the first water layer next to the graphene electrode surfaces under charging. As such, the electrical potential distribution through the EDL strongly depends on the ion type. Interestingly, we further reveal that the water can play a critical role in determining the capacitance value. The change of dielectric constant of water in different electrolytes largely cancels out the variance in electric potential drop across the EDL of different ion type. Our simulation sheds new lights on how the interplay between solvent molecules and EDL structure cooperatively contributes to capacitance, which agrees with our experimental results well.
Biological ion channels have remarkable ion selectivity, permeability and rectification properties, but it is challenging to develop artificial analogues. Here, we report a metal-organic ...framework-based subnanochannel (MOFSNC) with heterogeneous structure and surface chemistry to achieve these properties. The asymmetrically structured MOFSNC can rapidly conduct K
, Na
and Li
in the subnanometre-to-nanometre channel direction, with conductivities up to three orders of magnitude higher than those of Ca
and Mg
, equivalent to a mono/divalent ion selectivity of 10
. Moreover, by varying the pH from 3 to 8 the ion selectivity can be tuned further by a factor of 10
to 10
. Theoretical simulations indicate that ion-carboxyl interactions substantially reduce the energy barrier for monovalent cations to pass through the MOFSNC, and thus lead to ultrahigh ion selectivity. These findings suggest ways to develop ion selective devices for efficient ion separation, energy reservation and power generation.
The emergence of the field of nanofluidics in the last decade has led to the development of important applications including water desalination, ultrafiltration and osmotic energy conversion. Most ...applications make use of carbon nanotubes, boron nitride nanotubes, graphene and graphene oxide. In particular, understanding water transport in carbon nanotubes is key for designing ultrafiltration devices and energy-efficient water filters. However, although theoretical studies based on molecular dynamics simulations have revealed many mechanistic features of water transport at the molecular level, further advances in this direction are limited by the fact that the lowest flow velocities accessible by simulations are orders of magnitude higher than those measured experimentally. Here, we extend molecular dynamics studies of water transport through carbon nanotubes to flow velocities comparable with experimental ones using massive crowd-sourced computing power. We observe previously undetected oscillations in the friction force between water and carbon nanotubes and show that these oscillations result from the coupling between confined water molecules and the longitudinal phonon modes of the nanotube. This coupling can enhance the diffusion of confined water by more than 300%. Our results may serve as a theoretical framework for the design of new devices for more efficient water filtration and osmotic energy conversion devices.
Biological fluoride ion channels are sub-1-nanometer protein pores with ultrahigh F
conductivity and selectivity over other halogen ions. Developing synthetic F
channels with biological-level ...selectivity is highly desirable for ion separations such as water defluoridation, but it remains a great challenge. Here we report synthetic F
channels fabricated from zirconium-based metal-organic frameworks (MOFs), UiO-66-X (X = H, NH
, and N
(CH
)
). These MOFs are comprised of nanometer-sized cavities connected by sub-1-nanometer-sized windows and have specific F
binding sites along the channels, sharing some features of biological F
channels. UiO-66-X channels consistently show ultrahigh F
conductivity up to ~10 S m
, and ultrahigh F
/Cl
selectivity, from ~13 to ~240. Molecular dynamics simulations reveal that the ultrahigh F
conductivity and selectivity can be ascribed mainly to the high F
concentration in the UiO-66 channels, arising from specific interactions between F
ions and F
binding sites in the MOF channels.
To enhance the proton permeability of the anion exchange membranes used in diffusion dialysis, porous ultrafiltration membrane with a thin skin layer rather than the dense membrane was utilized for ...the preparation of diffusion dialysis membrane. In particular, brominated poly (phenylene oxide) (BPPO) ultrafiltration membrane was fabricated via a phase inversion method and then modified by crosslinking with polyethyleneimine (PEI) and subsequent quaternization with trimethylamine (TMA) via nucleophilic substitution reaction. Diffusion dialysis related properties of the membranes such as water uptake, ion exchange capacity, and thermal and swelling stabilities were investigated. Due to the special structure of the membranes (i.e., high porosity and low thickness of the top layer (sub-1μm), as confirmed by scanning electron microscopy) the optimal membrane exhibited 6.4 times higher acid dialysis coefficient for HCl recovery from HCl/FeCl2 solution than the commercial DF-120 (thickness=320μm) membrane at the similar separation factor. The calculated acid recovery capacity can increase from 11.3 up to 83.7Lm−2d−1 by replacing DF-120 with our optimal membrane. Our findings show the porous membranes developed here have great potential for high-efficiency acid recovery through diffusion dialysis process.
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•Porous diffusion dialysis membranes were prepared.•Greatly improved proton permeability was achieved.•The asymmetric membranes showed great potential for rapid acid recovery.
Layered crystalline materials, such as graphene, boron nitride, tungsten sulfate, phosphorene, etc., have attracted enormous attentions, due to their unique crystal structures and superior ...mechanical, thermal, and physical properties. Making use of mechanical buckling is a promising route to control their structural morphology and thus tune their physical properties, giving rise to many novel applications. In this paper, we employ molecular dynamics (MD) simulations and theoretical modeling to study the compressive buckling of a column made of layered crystalline materials with the crystal layers parallel to the compressive direction. We find that the mechanical buckling of the layered crystalline materials exhibits two anomalous and counter-intuitive features as approaching the zero slenderness ratio. First, the critical buckling strain εcr has a finite value that is much lower than the material's elastic limit strain. A continuum mechanics model (by homogenizing the layered materials) is proposed for the εcr, which agrees well with the results of MD simulations. We find that the εcr solely depends on elastic constants without any structural dimension, which appears to be an intrinsic material property and thus is defined as intrinsic buckling strain (IBS), εcrIBS, in this paper. Second, below a certain nanoscale length, l0, in the compressive direction (e.g., about 20nm for graphite), the critical buckling strain εcr shows a size effect, i.e., increasing as the column length L decreases. To account for the size effect, inspired by our recently developed multi-beam shear model (Liu et al., 2011), a bending energy term of individual crystal layer is introduced in our continuum model. The theoretical model of εcr agrees well with the size effects observed in MD simulations. This study could lay a ground for engineering layered crystalline materials in various nano-materials and nano-devices via mechanical buckling.
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