Selective Ion Transport Research Articles

Electro-osmotic Flow Generation via a Sticky Ion Action.

Selective transport of ions through nanometer-sized pores is fundamental to cell biology and central to many technological processes such as water desalination and electrical energy storage. Conventional methods for generating ion selectivity include placement of fixed electrical charges at the inner surface of a nanopore through either point mutations in a protein pore or chemical treatment of a solid-state nanopore surface, with each nanopore type requiring a custom approach. Here, we describe a general method for transforming a nanoscale pore into a highly selective, anion-conducting channel capable of generating a giant electro-osmotic effect. Our molecular dynamics simulations and reverse potential measurements show that exposure of a biological nanopore to high concentrations of guanidinium chloride renders the nanopore surface positively charged due to transient binding of guanidinium cations to the protein surface. A comparison of four biological nanopores reveals the relationship between ion selectivity, nanopore shape, composition of the nanopore surface, and electro-osmotic flow. Guanidinium ions are also found to produce anion selectivity and a giant electro-osmotic flow in solid-state nanopores via the same mechanism. Our sticky-ion approach to generate electro-osmotic flow can have numerous applications in controlling molecular transport at the nanoscale and for detection, identification, and sequencing of individual proteins.

Determining obstructive or promoting effects of anions and DOM on phosphate transport combining with Donnan dialysis and selective electrodialysis

In recent, nutrient recovery from wastewater by selective electrodialysis has been getting more attention, especially for non-renewable elements like phosphorus. However, phosphates usually possess a slow transfer flux in the electrodialysis process. This study aimed to decipher the influence of competing anions and dissolved organic matters (DOM) on phosphate migration with and without electric field. Anions that do and do not participate in protonation-deprotonation reactions showed discrepant effects on competitive phosphate transport. Carbonates enhance the sorption of phosphates in the anion exchange membrane and their diffusion through the membrane. However, this advantage gradually reduced with the increasing voltage during the selective electrodialysis, caused by the development of water splitting and an increase of the carbonate charge in the membrane. DOM substances, especially humic and fulvic acids, delay the phosphate transfer by the formation of fouling and biofouling and substantially intensify water splitting. With the increase of electric field, fouling of humic and fulvic acids were enhanced, while the presence of DOM has little effect on phosphate in Donnan dialysis. These findings help the understanding of phosphorus transfer in the electrodialysis process, and providing insights into the underlying mechanisms of membrane selectivity in competitive ion transport.

Distinct ion transport behavior between graphene oxide and UV-irradiated reduced graphene oxide membranes

Graphene oxide (GO) membranes are promising materials for achieving selective ion permeation in aqueous solutions. However, the swelling of GO laminates, caused by the affinity of water molecules for oxygen-containing groups (OCGs) on GO, significantly limits the practical prospects of GO membranes for selective ion transport and separation applications. Selectively removing OCGs from GO has been shown to decrease the interlayer distance between individual GO sheets, thereby improving the ability of laminate membranes to discriminate between different ions based on a size-exclusion mechanism. In this work, GO reduced through exposure to ultraviolet light (UV-rGO) is shown to display ion transport behavior that raises questions about size-exclusion as the dominant mechanism determining selective ion transport in GO membranes. Notably, smaller monovalent Li+ cations are found to permeate more slowly through UV-rGO membranes than larger divalent metal cations such as Ca2+ and Mg2+. This unexpected behavior results in a 3 – 4 fold improvement in the separation selectivity between these representative cations with UV-rGO compared to pristine GO membranes. Through a joint experimental and theoretical study, we identified the factors influencing selective ion transport through the nanochannels in UV-rGO membranes. We demonstrate that the selective removal of hydroxyl (–OH) groups from the basal planes of GO enhances the interactions of metal cations with functional groups located at the edges of GO, thereby resulting in a lower ratio of free Li+ in solution, rather than stationary Li+ adsorbed onto the surface GO, compared to Ca2+ cations. Based on these results, we propose a separation mechanism analogous to chromatography, which emphasizes the crucial role of different OCGs on GO surfaces in controlling selective ion transport through GO membranes.

Toward Selective Transport of Monovalent Metal Ions with High Permeability Based on Crown Ether-Encapsulated Metal-Organic Framework Sub-Nanochannels.

Achieving selective transport of monovalent metal ions with high precision and permeability analogues to biological protein ion channels has long been explored for fundamental research and various applications, such as ion sieving, mineral extraction, and energy harvesting and conversion. However, it still remains a significant challenge to construct artificial nanofluidic devices to realize the trade-off effects between selective ion transportation and high ion permeability. In this work, we report a bioinspired functional micropipet with in situ growth of crown ether-encapsulated metal-organic frameworks (MOFs) inside the tip and realize selective transport of monovalent metal ions. The functional ion-selective micropipet with sub-nanochannels was constructed by the interfacial growth method with the formation of composite MOFs consisting of ZIF-8 and 15-crown-5. The resulting micropipet device exhibited obvious monovalent ion selectivity and high flux of Li+ due to the synergistic effects of size sieving in subnanoconfined space and specific coordination of 15-crown-5 toward Na+. The selectivity of Li+/Na+, Li+/K+, Li+/Ca2+, and Li+/Mg2+ with 15-crown-5@ZIF-8-functionalized micropipet reached 3.9, 5.2, 105.8, and 122.4, respectively, which had an obvious enhancement compared to that with ZIF-8. Notably, the ion flux of Li+ can reach up to 93.8 ± 3.6 mol h-1·m-2 that is much higher than previously reported values. Furthermore, the functional micropipet with 15-crown-5@ZIF-8 sub-nanochannels exhibited stable Li+ selectivity under various conditions, such as different ion concentrations, pH values, and mixed ion solutions. This work not only provides new opportunities for the development of MOF-based nanofluidic devices for selective ion transport but also facilitates the promising practical applications in lithium extraction from salt-like brines, sewage treatment, and other related aspects.

Effect of Organic Cation Adsorption on Ion-Transport Selectivity in a Cation-Permselective Nanopore Membrane.

A key knowledge gap in the emerging field of nanofluidics concerns how the ionic composition and ion-transport properties of a nanoconfined solution differ from those of a contacting bulk solution. We and others have been using potentiometric concentration cells, where a nanopore or nanotube membrane separates salt solutions of differing concentrations to explore this issue. The membranes studied contained a fixed pore/tube wall anionic charge, which ideally would prohibit anions and salt from entering the pore/tube-confined solution. We have been investigating experimental conditions that allow for this ideally permselective cation state to be achieved. Results of potentiometric investigations of a polymeric nanopore membrane (10 ± 2 nm-diameter pores) with anionic charge due to carbonate are presented here. While studies of this type have been reported using alkaline metal and alkaline earth cations, there have been no analogous studies using organic cations. This paper uses a homologous series of tetraalkylammonium ions to address this knowledge gap. The key result is that, in contrast to the inorganic cations, the ideal cation-permselective state could not be obtained under any experimental conditions for the organic cations. We propose that this is because these hydrophobic cations adsorb onto the polymeric pore walls. This makes ideality impossible because each adsorbed alkylammonium must bring a charge-balancing anion, Cl-, with it into the nanopore solution. The alkylammonium adsorption that occurred was confirmed and quantified by using surface contact angle measurements.

Use of Function Diagrams in Physiology Education: Nessie the Nephron and Navigating the Lochs

Function diagrams focus on physiological concepts rather than associated structures and can serve as elaboration tools and mnemonic aids. A function diagram prototype of the gastrointestinal system was recently described (Wilson & Barrett, Adv Physiol Educ, 45:264–268, 2021). Currently, a functional diagram of the glomerulus, nephron, urinary system, and bladder is proposed. Colloquially named “Nessie the Nephron” to loosely capture the looping structure and illusive understanding of renal blood flow, glomerular filtration, and epithelial transport for the student, not to mention the nearly mythical countercurrent multiplier. “Navigating the Lochs” references Nessie’s possible habitat and more importantly the movement and storage of the modified ultrafiltrate from the nephron. The primary analogies that form the structure of this function diagram are: Rain Barrel, Soaker Hose, & Hose Nozzle (afferent and efferent blood flow and filtration pressure); Water Purification System (glomerular filtration barrier forming multiple step filtration process involving the capillary, basement membrane, and podocytes); Mixed Recycling Machine (proximal tubule individual, co-, and bulk transport); Desiccator & Briner (thin descending limb removal of water and thin/thick ascending limb movement of salt); Conveyor Belt Picker (distal nephron selective ion transport); and Concentrator (collecting duct water and urea transport). The primary analogies that elaborate “Navigating the Lochs” are Aqueduct & Cistern System (fluid movement and collection); and Pressure Gauge, Syringe Bulb, and Two-valve Plumbing (bladder storage and micturition). Complimenting these analogies is the rich potential for inclusion of clinical and comparative applications and examples to link previous knowledge and strengthen memories for future retrieval. University of Kentucky College of Medicine. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Eliminating lithium dendrites via dependable ion regulation of charged nanochannels

Reviving lithium metal anode is of great significance to developing high-energy-density batteries after addressing the lithium dendrite. Constructing functional separator with charged nanochannel configuration has been proven to be an effective way to inhibit nucleation of lithium dendrites due to the achievement of a fast and selective lithium ion transport. While the ion regulation capability of existing functional separators cannot be maximized because their nanochannels are much larger than the electric double layer (EDL) regions that dominate ion transport. Considering the tiny EDL thickness (∼1 nm) formed in the electrolyte, the sulfonate-rich covalent organic framework (COF) is developed as functional separator to achieve a dependable ion regulation based on the nano-confined channel (∼1.55 nm) and surface negative charges. Consequently, a high Li+ transference number tLi+ (0.85) and sufficient ionic conductivity (0.53 mS cm−1) are obtained simultaneously, which effectively alleviates dendrite nucleation and growth of lithium metal anode for over 1000 h. Moreover, the principle for fast and selective lithium-ion transport is revealed from a new perspective of EDL region embedded in the nanochannel. Overall, this work demonstrates a strategy to eliminate lithium dendrites via ion regulation of charged nanochannels, which is anticipated to promote the practical application of lithium metal batteries.

Enhancing selective ammonium transport in membrane electrochemical systems

Recovering ammonia nitrogen from wastewater is a sustainable strategy that simultaneously addresses both nitrogen removal and fertilizer production. Membrane electrochemical system (MES), which utilizes electrochemical redox reactions to transport ammonium ions through cation exchange membranes, has been considered as an effective technology for ammonia recovery from wastewater. In this study, we develop a mathematical model to systematically investigate the impact of co-existing ions on the transport of ammonium (NH4+) ions in MES. Our analysis elucidates the importance of pH values on both the NH4+ transport and inert ion (Na+) transport. We further comprehensively assess the system performance by varying the concentration of Na+ in the system. We find that while the inert cation in the initial anode compartment competes with NH4+ transport, NH4+ dominates the cation transport in most cases. The transport number of Na+ surpasses NH4+ only if the fraction of Na+ to total cation is extremely high (>88.5%). Importantly, introducing Na+ ions into the cathode compartment significantly enhances the ammonia transport due to the Donnan dialysis. The analysis of selective ion transport provides valuable insights into optimizing both selectivity and efficiency in ammonia recovery from wastewater.

Chitosan Composite Membrane with Efficient Hydroxide Ion Transport via Nano-Confined Hydrogen Bonding Network for Alkaline Zinc-Based Flow Batteries.

The development of membranes with rapid and selective ionic transport is imperative for diverse electrochemical energy conversion and storage systems, including fuel cells and flow batteries. However, the practical application of membranes is significantly hindered by their limited conductivity and stability under strong alkaline conditions. Herein, a unique composite membrane decorated with functional Cu2+ cross-linked chitosan (Cts-Cu-M) is reported and their high hydroxide ion conductivity and stability in alkaline flow batteries are demonstrated. The underlying hydroxide ions transport of the membrane through Cu2+ coordinated nano-confined channels with abundant hydrogen bonding network via Grotthuss (proton hopping) mechanism is proposed. Consequently, the Cts-Cu-M membrane achieves high hydroxide ion conductivity with an area resistance of 0.17Ωcm2 and enables an alkaline zinc-based flow battery to operate at 320mAcm-2, along with an energy efficiency of ≈80%. Furthermore, the membrane enables the battery for 200cycles of long-cycle stability at a current density of 200mAcm-2. This study offers an in-depth understanding of ion transport for the design and preparation of high-performance membranes for energy storage devices and beyond.

Anion exchange membrane based on poly(arylene ether sulfone)s functionalized with quinuclidinium-piperidinium dual cations for vanadium redox flow battery applications

Vanadium redox flow batteries (VRFBs) have gained significant interest as a prospective energy storage solution due to their scalability and extended cycle durability. The efficient functioning of VRFBs relies significantly on anion exchange membranes (AEMs), which facilitate the selective transport of ions while preventing crossover. In this study, we present a novel AEM based on poly(arylene ether sulfone)s (PAES) functionalized with dual cations (quinuclidinium-piperidinium) (DC-PAES) bearing a butyl spacer, explicitly designed for VRFB applications. The DC-PAES membrane containing dual cations separated by a butyl spacer results in a balanced combination of hydrophilicity and hydrophobicity, which enables efficient ion transport across the membrane. The DC-PAES membrane showed hydroxide conductivity of 0.085 S/cm at 80 °C, having an ion exchange capacity (IEC) of 1.42 mmol/g. The resulting DC-PAES membrane displayed low permeability (2.1 × 10−7 cm2 min−1) and high selectivity (12.3 × 105 S min cm−3) to vanadium. The performance of the AEMs fabricated for VRFB application showed better Coulombic (97.1 %), energy (90.0 %), and voltage efficiency (92.63 %) at 40 mAcm−2 higher than Nafion 117. The reported AEMs revealed excellent stability due to the steric and ring constraints of quinuclidinium and piperidinium cation, hindering the attack of oxidative species. Furthermore, the DC-PAES membrane demonstrated improved battery cycling performance lasting up to 150 cycles under in-situ conditions, making it a promising separator material for VRFB applications.

Ion-Concentration-Hopping Heterolayer Gel for Ultrahigh Gradient Energy Conversion.

Conventional solid ion channel systems relying on single one- or two-dimensional confined nanochannels enabled selective and ultrafast convective ion transport. However, due to intrinsic solid channel stacking, these systems often face pore-pore polarization and ion concentration blockage, thereby restricting their efficiency in macroscale ion transport. Here, we constructed a soft heterolayer-gel system that integrated an ion-selective hydrogel layer with a water-barrier organogel layer, achieving ultrahigh cation selectivity and flux and effectively providing high-efficiency gradient energy conversion on a macroscale order of magnitude. Specifically, the hydrogel layer featured an unconfined 3D network, where the fluctuations of highly hydrated polyelectrolyte chains driven by thermal dynamics enhanced cation selectivity and mitigated transfer energy barriers. Such chain fluctuation mechanisms facilitated ion-cluster internal transmission, thereby enhancing ion concentration hopping for more efficient ion-selective transport. Compared to the existing rigid nanochannel-based gradient energy conversion systems, such a heterogel-based power generator exhibited a record power density of 192.90 and 1.07 W/m2 at the square micrometer scale and square centimeter scale, respectively (under a 500-fold artificial solution). We anticipate that such heterolayer gels would be a promising candidate for energy separation and storage applications.

Electricity generation from carbon dioxide adsorption by spatially nanoconfined ion separation

Selective ion transport underpins fundamental biological processes for efficient energy conversion and signal propagation. Mimicking these ‘ionics’ in synthetic nanofluidic channels has been increasingly promising for realizing self-sustained systems by harvesting clean energy from diverse environments, such as light, moisture, salinity gradient, etc. Here, we report a spatially nanoconfined ion separation strategy that enables harvesting electricity from CO2 adsorption. This breakthrough relies on the development of Nanosheet-Agarose Hydrogel (NAH) composite-based generators, wherein the oppositely charged ions are released in water-filled hydrogel channels upon adsorbing CO2. By tuning the ion size and ion-channel interactions, the released cations at the hundred-nanometer scale are spatially confined within the hydrogel network, while ångström-scale anions pass through unhindered. This leads to near-perfect anion/cation separation across the generator with a selectivity (D-/D+) of up to 1.8 × 106, allowing conversion into external electricity. With amplification by connecting multiple as-designed generators, the ion separation-induced electricity reaching 5 V is used to power electronic devices. This study introduces an effective spatial nanoconfinement strategy for widely demanded high-precision ion separation, encouraging a carbon-negative technique with simultaneous CO2 adsorption and energy generation.

Highly selective ion transport by freestanding Zn-Imidazole complex intercalated graphene oxide membrane for enhanced blue energy harvesting

Generating electricity from seawater by utilizing nanofluidic structures is an appealing strategy for achieving zero-emission energy goals. A nanofluidic channel that is atomically small, tunable, and mechanically stable is the key to solving this problem. In this paper, for the first time, we report the construction of centimeter-sized freestanding zinc-imidazole complex (ZnIm) intercalated graphene oxide (GO) membrane (GO-ZnIm) having uniform 2D nanofluidic channels with interlayer spacing of 9 Å. The pristine GO membrane rapidly disintegrates in an aqueous medium due to membrane swelling; however, treatment with an aqueous zeolitic imidazole framework (ZIF-8) suspension, rendering ZnIm complex, provides long-term stability by restricting the excessive interlayer separation of GO sheets. From ion transport, diffusion, and variable load studies, it is observed that, besides stabilizing the GO membrane, intercalation of ZnIm significantly improves average cation selectivity (from 0.46 for GO to 0.74 for GO-ZnIm, measured up to 1000 concentration gradient) and experimental power density (from 1.7 Wm−2 for GO to 10.8 Wm−2 for GO-ZnIm, at 1000 concentration gradient). Moreover, the practical applicability of the GO-ZnIm membrane has also been investigated under artificial estuary conditions at ambient temperature, rendering an osmotic power density of 4.5 Wm−2, which is very close to the commercial benchmark of 5 Wm−2. A mechanically robust and flexible membrane made up of cost-effective components (GO and ZnIm), able to deliver an osmotic power density close to commercial benchmark at ambient temperature is the main highlight of the present GO-ZnIm membrane, which can be a promising candidate for applications such as separation, desalination, and blue energy harvesting.

Development of Structure-Property Relationships for Ammonium Transport through Charged Organogels.

Ammonia is a promising carbon-free fuel, but current methods to produce ammonia are energy intensive. New methods are thereby needed, with one promising method being electrochemical nitrogen reduction cells. Efficient cell operation requires robust catalysts but also efficient membrane separators that permit the selective transport of ions while minimizing the transport of the products across the cell. Commercial membranes have an unknown morphology which makes designing improved cells challenging. To address this problem, we synthesized a series of membranes with controlled crosslinking density and chemical composition to understand their impact on ammonium transport. Higher crosslinking density led to lower ammonium permeability. At the highest crosslinking density, similar ammonium permeability was observed independent of the water volume fraction and hydrophobicity of the monomers. These results suggest new directions to develop membranes with reduced ammonium crossover to improve the efficiency of these electrochemical cells.

Highly ion-selective graphene-oxide-based membranes for nanofluidic osmotic energy conversion

Graphene oxide (GO) membranes have shown great potential for water treatment and osmotic energy conversion. However, the inherent challenge with GO membranes lies in their instability in water. While reduction techniques can improve stability, they simultaneously compromise selective ion transport properties due to the loss of functional groups. Here, we report preassembly modified GO (GOTA) membranes by using tannic acid (TA) as modification reagent. It is found that the TA molecules not only act as reducing agent, enhancing the aqueous stability of the membrane by generating stacked graphitic sp2 domains but also serve as intercalating agent, improving the selective ion transport properties by creating localized areas with high charge density and enlarged interlayer spacing. The cation selective number of GOTA membranes is ∼0.94–0.89 for different salinity gradients, much better than the thermally reduced GO membranes. In energy conversion system, the output power density approaches ∼0.32 W m−2 for the artificial seawater and river water with energy conversion efficiency of 41.7%. Meanwhile, the GOTA membranes exhibit excellent structural stability in various solution environments. This work demonstrates the importance of nanofluidic structure in improving the performance of ion-selective membranes and provides insights into the construction of advanced osmotic energy conversion system.

Ion Transport Behavior in van der Waals Gaps of 2D Materials.

2D materials, with advantages of atomic thickness and novel physical/chemical characteristics, have emerged as the vital building blocks for advanced lamellar membranes which possess promising potential in energy storage, ion separation, and catalysis. When 2D materials are stacked together, the van der Waals (vdW) force generated between adjacent layered nanosheets induces the construction of an ordered lamellar membrane. By regulating the interlayer spacing down to the nanometer or even sub-nanometer scale, rapid and selective ion transport can be achieved through such vdW gaps. The further improvement and application of qualified 2D materials-based lamellar membranes (2DLMs) can be fulfilled by the rational design of nanochannels and the intelligent micro-environment regulation under different stimuli. Focusing on the newly emerging advances of 2DLMs, in this review, the common top-down and bottom-up synthesis approaches of 2D nanosheets and the design strategy of functional 2DLMs are briefly introduced. Two essential ion transport mechanisms within vdW gaps are also involved. Subsequently, the responsive 2DLMs based on different types of external stimuli and their unique applications in nanofluid transport, membrane-based filters, and energy storage are presented. Based on the above analysis, the existing challenges and future developing prospects of 2DLMs are further proposed.

Naphthalene Diimide-Based Metallacage as an Artificial Ion Channel for Chloride Ion Transport.

Developing synthetic molecular devices for controlling ion transmembrane transport is a promising research field in supramolecular chemistry. These artificial ion channels provide models to study ion channel diseases and have huge potential for therapeutic applications. Compared with self-assembled ion channels constructed by intermolecular weak interactions between smaller molecules or cyclic compounds, metallacage-based ion channels have well-defined structures and can exist as single components in the phospholipid bilayer. A naphthalene diimide-based artificial chloride ion channel is constructed through efficient subcomponent self-assembly and its selective ion transport activity in large unilamellar vesicles and the planar lipid bilayer membrane by fluorescence and ion-current measurements is investigated. Molecular dynamics simulations and density functional theory calculations show that the metallacage spans the entire phospholipid bilayer as an unimolecular ion transport channel. This channel transports chloride ions across the cell membrane, which disturbs the ion balance of cancer cells and inhibits the growth of cancer cells at low concentrations.

Ultra-Thin Ion Exchange Membranes by Low Ionomer Blending for Energy Harvesting.

Exploring the utilization of ion exchange membranes (IEMs) in salinity gradient energy harvesting, a technique that capitalizes on the salinity difference between seawater and freshwater to generate electricity, this study focuses on optimizing PVDF to Nafion ratios to create ultra-thin membranes. Specifically, our investigation aligns with applications such as reverse electrodialysis (RED), where IEMs facilitate selective ion transport across salinity gradients. We demonstrate that membranes with reduced Nafion content, particularly the 50:50 PVDF:Nafion blend, retain high permselectivity comparable to those with higher Nafion content. This challenges traditional understandings of membrane design, highlighting a balance between thinness and durability for energy efficiency. Voltage-current analyses reveal that, despite lower conductivity, the 50:50 blend shows superior short-circuit current density under salinity gradient conditions. This is attributed to effective ion diffusion facilitated by the blend's unique microstructure. These findings suggest that blended membranes are not only cost-effective but also exhibit enhanced performance for energy harvesting, making them promising candidates for sustainable energy solutions. Furthermore, these findings will pave the way for advances in membrane technology, offering new insights into the design and application of ion exchange membranes in renewable energy.

Construction of Selective Ion Transport Polymer at Anode-Electrolyte Interface for Stable Aqueous Zinc-Ion Batteries.

Rampant dendrite formation and serious adverse parasitic reactions induced by migration of dissolved V/Mn cathode ions on Zn anode have hampered the high performance of aqueous zinc-ion batteries (AZIBs). Inspired by the coordination chemistry between functional groups of polymer and electrolyte ions, a freestanding layer consisting of dopamine-functionalized polypyrrole (DA-PPy) nanowires served as a selective ion transport layer at the anode-electrolyte interface to address these two issues, which could simultaneously avoid polarization caused by the introduction of an additional interface. On the one hand, the DA-PPy layer displays excellent zinc ion and charge transfer ability, as well as provides chemical homochanneling for zinc ions at the interface, which endow the DA-PPy layer with properties as a chemical guider and physical barrier for dendrite inhibition. On the other hand, the DA-PPy layer can trap excess transition metal ions fleeing from the cathodes, thus serving as a chemical barrier, preventing the formation of Vx+/Mnx+-passivation on the surface of the zinc anode. Consequently, the AZIBs based on V2O5 and MnO2 cathodes involving the DA-PPy functional layer show a great improvement in the capacity retention.

Ultrahigh Lithium Selective Transport in Two-Dimensional Confined Ice.

Inspired by selective ion transport in biological membrane proteins, researchers developed artificial ion channels that sieve monovalent cations, catering to the increasing lithium demand. In this work, we engineered an ion transport channel based on the confined ice within two-dimensional (2D) capillaries and found that the permselectivity of monovalent cations depends on the anisotropy of the confined ice. Particularly, the 2D confined ice showed an anomalous lithium selective transport along the (002) direction in the vermiculite capillary, with the Li+/Na+ and Li+/K+ permselectivity reaching up to 556 ± 86 and 901 ± 172, respectively, superior to most ion-selective channels. However, the 2D confined ice along the (100) direction showed less Li+ permselectivity. Additionally, the anisotropy of 2D confined ice can be tuned by adjusting the interlayer spacing. By providing insights into the ion transport in the 2D confined ice, our work may inspire more design of monovalent ion-selective channels for efficient lithium separation.