Efficient Ion Sieving in Covalent Organic Framework Membranes with Sub-2-Nanometer Channels

A hollow fiber supported ionic liquid membrane contactor for continuous extraction of lithium from high magnesium/lithium ratio brine

High-efficiency lithium (Li+) extraction from a salt-lake brine with a low Li concentration and a high Mg/Li mass ratio poses a great challenge owing to the great physical and chemical similarities between Mg2+ and Li+. In this study, a hollow fiber (HF) membrane with an inside diameter of 0.872 mm and an outside diameter of 1.228 mm was fabricated using nonsolvent induce phase separation method with 14-crown-4 ether functionalized polyimide (14C4PI) as a polymer matrix. The organic phase, a solution of tributyl phosphate and sodium bis(trifluoromethylsulfonyl)imide (NaNTf2) was filled into porous membranes as the solid phase using an impregnation method to construct a supported ionic liquid membranes (SILMs) contactor for lithium extraction from simulated salt-lake brine. The feed and stripping phases of the contactor were a mixed Mg/Li solution and 0.5 mol/L HCl, respectively. The contactor operated continuously for 120 h. The results showed an HF membrane with an average pore size of 20.1 nm, porosity of 73.6 % and a breaking strength of 5.64 MPa. When the Mg/Li mass ratio in the feed was approximately 3.5, the mass transfer rate of Li+ and the separation factor (SFLi-Mg) obtained from the contactor with a packing density of 1.5 % were 0.468 μm/s and 15.86, respectively. After further increasing the mass ratio of the Mg/Li solution to 35 in the feed and the membrane packing density to 15 %, the mass transfer rate of Li+ and the SFLi-Mg increased to 0.623 μm/s and 30.8, respectively. This implies that the SILMs contactor with a high packing density of HF membrane showed good operating stability and enhanced ion extraction efficiency. The high performance was ascribed to the crown ether groups in 14C4PI achieving specific recognition of Li+ through the size-sieving effect. Specifically, the inter-molecular interaction between crown ether and ionic liquids (ILs) improved the stability of the filled ILs. Meanwhile, ILs provided ionic transfer channels and promoted the dehydration process of Li+, leading to a high SFLi-Mg. In addition, NaNTf2 adopted to replace traditional ILs prevented cation loss and provided an efficient and continuous method for extracting lithium from salt-lake. The above advantages are expected to achieve large-scale extraction of lithium ions.

Construction of Local-Ion Trap in Phase-Reversed Mixed Matrix COF Membranes for Ultrahigh Ion Selectivity

Artificial molecular/ion traps afford grand potential in membrane-based separation processes. However, the existing trap-based architectures often confer over-strong binding forces, which severely impede the release of bound solutes during their transmembrane diffusion processes. Herein, we propose an unprecedented local-ion trap bearing moderate binding force and additional repulsion force in a type of phase-reversed mixed matrix covalent organic framework (PRCOF) membrane. By implementing COF as a continuous phase and polymer as a dispersed phase at the molecular level, the local-ion trap is formed in the COF channels equipped with free amino groups from polyethyleneimine (PEI). This unique local-ion trap built by electronegative COF nano-domains and electropositive PEI nano-domains offers appropriate interaction toward Li+, which allows the precise recognition and rapid transport of Li+ in the membrane channels. By tuning the microenvironments of local-ion trap, the optimum PRCOF-1 membrane exhibits considerably high actual selectivity of 190 along with a rapid Li+ permeation rate of 0.262 mol h-1 m-2 in dealing with a Li+/Mg2+ binary mixture. This work provides in-depth insights into the design of high-performance membranes with appropriate chemical interactions.

Membrane-Ion Interactions Creating Dual-Nanoconfined Channels for Superior Mixed Ion Separations

Ion transport in environments with mixed ions is crucial for numerous real-world applications. However, most membranes suffer from reduced selectivity in the mixed ion environment. Herein, the concept of dual-nanoconfined channels in covalent organic frameworks (COFs) is demonstrated for superior mixed mono-/multivalent ion separation. By incorporating acidic functionalities into the pore wall and coupling with a suitable pore size in the COF membranes, multivalent ions interact strongly with the acidic groups. This shields the interactions of the wall with monovalent ions, leaves sufficient space for fast monovalent ion passage, and in the meantime blocks multivalent ions, creating well-defined dual-nanoconfinement for mixed ion separation. Consequently, mixed Li+/Mg2+ ion selectivity exceeding 1,300 and mono-/trivalent ion selectivity over 9,000 is achieved. Molecular dynamics simulation and experiment with multiple ion pairs further confirm the mechanisms. The study sheds light on a fundamentally new design principle for mixed ion separation, offering the potential to reach remarkable selectivity in practical applications.

Enhancing Selective Ion Transport by Stacking Covalent Organic Framework Monolayers

Nanopore-based power generation represents an efficient way for harvesting salinity gradient energy. Due to its ultrahigh ion conductivity and moderate ion selectivity, the crystalline covalent organic framework (COF) monolayer demonstrates the record-high output power density by mixing river water and seawater. To further improve energy conversion performance, it is necessary to enhance ion selectivity while achieving high membrane permeability. Here, a layer-by-layer stacking approach is developed to notably enhance the selective ion transport of ultra-thin COF layers, offering advantageous in both conversion efficiency and scalability. Under a standard NaCl salinity gradient (0.5 M/0.1 M), the ratio of ionic mobility between Cl- and Na+ increases from 1.4 to 2.9 with stacking the anion-selective COF monolayer from one to ten layers, leading to a more than seven-fold enhancement in osmotic energy conversion efficiency. By maximizing selectivity and permeability, the output power can reach 411 pW by stacking three layers in a single device. This strategy provides an effective approach for the integration of atomically thin membranes in selective mass transport applications.

Intrinsic roles of nanosheet characteristics in two-dimensional montmorillonite membranes for efficient Li+/Mg2+ separation

Stacking two-dimensional (2D) nanosheets into lamellar membranes holds great promise in the selective separation of Li+ and Mg2+ from salt-lake brines, but revealing the intrinsic effect of nanosheet properties on the ion transport remains a great challenge. The primary reasons are inevitable emerging defects and changes in surface functional groups during nanosheet preparation. Here, we successfully demonstrated the intrinsic dependence of ion separation on the size and layer charge density of 2D building blocks using defect-free and inherently permanent charged clay nanosheets. The smaller-sized nanosheets readily assembled into lamellar membranes with narrower nanochannel dimension, which facilitated the steric hindrance effect to improve the Li+/Mg2+ selectivity. Experiments and calculations demonstrated the layer charge density-dependent ion separation as well, for which a novel mechanism of intrinsic selective separation driven from the energy barrier difference of ions transport was proposed. Based on the "internal" regulation of the intrinsic nanosheet properties, MMT membranes realized stable and efficient Li+/Mg2+ separation under extreme conditions, multi-cycle and long-term experiments, with an optimal SLi/Mg of 38.9, superior to most of the reported state-of-the-art membranes. This work reveals the intrinsic interplay of nanosheet properties tuning the ion transport and separation, which will inspire the design and development of advanced 2D lamellar membranes, particularly for sustainable and environmental energy exploitation.

Covalent Organic Framework Membranes: Synthesis Strategies and Separation Applications

Covalent organic frameworks (COFs) have emerged as highly promising materials for membrane separations due to their high porosity, tunable pore sizes, ordered crystalline structures, and exceptional chemical stability. With these features, COF membranes possess greater selectivity and permeability than conventional materials, making them the preferred choice in various fields, including membrane separations. Fascinating research endeavors have emerged encompassing fabrication strategies for COF-based membranes and their diverse separation applications. Hence, this review summarizes the latest advancements in COF synthesis, including COF powders and continuous COF-based membranes and their applications in separation membranes. Special consideration was given to regulation strategies for the performance optimization of COF membranes in separation applications, such as pore size, hydrophilicity/hydrophobicity, surface charge, crystallinity, and stability. Furthermore, applications of COF membranes in water treatment, metal ion separation, organic solvent nanofiltration, and gas separation are comprehensively reviewed. Finally, the research results and future prospects for the development of COF membranes are discussed. Future research may be focused on the following key directions: (1) single-crystal COF fabrication, (2) cost-effective membrane preparation, (3) subnanometer pore engineering, (4) advanced characterization techniques, and (5) AI-assisted development.

Multicomponent Nacre-Like Heterogeneous Nanochannels with Ion Sieving and Light-Sensitivity Properties for Improved Energy Conversion

Nanochannel membranes are currently being employed for selective ion transport and salinity gradient energy capture. However, the poor functionality of nanochannel membranes and the unfavorable influence of multivalent cations result in low energy conversion efficiency, limiting the energy conversion performance of nanochannel membranes. Herein, multicomponent nacre-like heterogeneous nanochannels composed of carboxymethyl chitosan (CMC)-intercalated composite two-dimensional (2D) GO, C3N4 nanosheets, and polyethylene terephthalate (PET) channels are developed using an interfacial assembly strategy. Benefiting from the asymmetric structure, the heterogeneous membrane achieves enhanced ion diffusion and unidirectional ion transport behaviors. Subsequently, a high power density of 8.46 W/m2 is achieved in artificial seawater and river water by the heterogeneous membrane. Notably, the introduction of CMC into 2D lamellar channels endows the heterogeneous membrane with outstanding ion sieving performance, thereby mitigating the unfavorable influence of divalent cations on energy conversion, reducing it from 32.08% to 19.85%. Furthermore, the heterogeneous membrane exhibits sensitive and stable light-responsive ion transport behavior, profiting from the light sensitivity of GO and C3N4 nanosheets, and an improvement of 11.29% in energy conversion performance can be accomplished with light irradiation. This work proposes an idea to design multifunctional nanochannel membranes to achieve highly efficient salinity gradient energy conversion.

Improving the ion sieving performance of MOF polycrystalline membranes based on interface modification

Metal-organic framework (MOF) membranes exhibit promising potential for high-precision molecule and ion sieving due to their uniform and tunable pore structures. Nevertheless, it remains a challenge to address interfacial compatibility to obtain a high-performance MOF membrane. This is because there is a weak interfacial interaction between the MOF and the substrate, which leads to non-selective areas. This work presents an interface-coating method to facilitate dense nucleation and rapid growth of MOF crystals on the substrate towards reinforcing interaction and eliminating defects. Polydopamine (PDA) and β-cyclodextrin (β-CD) were co-assembled on polyvinylidene fluoride (PVDF) substrates to modify the surface chemistry and enhance interfacial compatibility, thereby facilitating the dense growth of ZIF-8. The ZIF-8/PVDF membranes demonstrated an excellent K+ permeance of 0.33 mol m-2 h-1 and a K+/Mg2+ selectivity of 30.16. The simulation results indicate that the channel of Mg2+ through ZIF-8 must overcome a greater transport energy barrier than that of K+, resulting in a higher selectivity of K+/Mg2+.

Recent advances in the design principles of lithium selective membranes

The growing demand for lithium in energy storage applications has intensified the need for efficient lithium extraction technologies, with membrane processes emerging as a promising approach. Among various membrane technologies, nanostructured membranes with precisely engineered channels have shown exceptional potential for selective lithium extraction due to their ability to control ion transport at the molecular level. This review provides a comprehensive analysis of the fundamental design principles governing lithium-selective membranes, with a specific focus on nanochannel-based systems. We examine the critical parameters that influence lithium selectivity, including surface charge distribution, nanochannel dimensions, morphology, and wettability, while exploring how these factors interact with external driving forces to enable selective ion transport. This work extensively analyzes recent developments in nanochannel engineering and ion transport mechanisms, providing crucial insights into optimizing membrane selectivity and performance. By critically analyzing current challenges in scaling up these technologies and identifying promising research directions, this work provides a roadmap for developing next-generation lithium-selective membranes with enhanced efficiency and selectivity.

Covalent organic framework membranes for lithium extraction: facilitated ion transport strategies to enhance selectivity

The surging global demand for lithium, driven by the proliferation of electric vehicles and energy storage technologies, has exposed significant limitations in conventional lithium extraction methods, including inefficiency and environmental harm. Covalent organic frameworks (COFs) have emerged as a promising platform to address this challenge and enable more sustainable lithium extraction, owing to their unique advantages such as precisely tunable pore sizes, robust stability, and the ability to incorporate functional binding sites for selective ion transport. This review focuses on structural design and functionalization strategies in COFs to optimize lithium-ion separation, highlighting how pore confinement effects, tailored interlayer stacking arrangements, and strategic functional group modifications can dramatically enhance Li+ selectivity over competing ions present in brine solutions. A particular emphasis is placed on the fundamental energy barriers associated with lithium-ion transport. In particular, we discuss how appropriately designed pore environments and lithium-binding functional groups reduce the dehydration energy required for Li+ to enter and traverse COF nanochannels, thereby facilitating faster and more selective Li+ conduction. We also survey recent advancements in COF-based lithium separation technologies, such as high-performance COF membranes and sorbents for extracting lithium from brines and seawater, evaluating their potential, as well as remaining challenges, for sustainable industrial implementation. This review provides a comprehensive understanding of how advanced COF engineering can enable efficient and selective lithium-ion transport, offering valuable insights for the development of next-generation lithium extraction materials and technologies.

Advanced Microporous Framework Membranes for Sustainable Separation

Advancements in membrane-based separation hinge on the design of materials that transcend conventional limitations. Microporous materials, including metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), macrocycles, and porous organic cages (POCs) offer unprecedented control over pore architecture, chemical functionality, and transport properties, making them promising candidates for next-generation membrane technologies. The well-defined and tunable micropores provide a pathway to directly address the permeability-selectivity trade-off inherent in conventional polymer membranes. Here, this review explores the latest advancements in these four representative microporous membranes, emphasizing their breakthroughs in hydrocarbon separation, liquid-phase molecular sieving, and ion-selective transport, particularly focusing on their structure-performance relationships. While their tailored structures enable exceptional performance, practical adoption requires overcoming hurdles in scalability, durability, and compatibility with industrial processes. By offering insights into membrane structure optimization and innovative design strategies, this review provides a roadmap for advancing microporous membranes from laboratory innovation to real-world implementation, ultimately supporting global sustainability goals through energy-efficient separation processes.

Randomly oriented covalent organic framework membrane for selective Li+ sieving from other ions

Certain biological channels exhibit remarkable selectivity, effectively distinguishing between competing cations. If artificial membranes could achieve similar precision in differentiating competing ions from Li+, it could advance sustainable technologies in lithium extraction. In this study, we present a covalent organic framework (COF) membrane featuring a randomly oriented structure that enables selective separation of major competing ions from Li+. The random orientation results in narrow pores, which impart size-based selectivity among alkaline ions. Additionally, the COF incorporates sulfonic groups that preferentially bind to Na+ and K+, facilitating their transport while retaining Li+. These synergistic mechanisms endow the membrane with a selectivity beyond detection limit for K+ and Na+ over Li+. When driven by an electrical potential, the ion flux through the membrane is enhanced by over an order of magnitude. Notably, the membrane also permits the transport of Mg2+ and Ca2+ while still rejecting Li+, leveraging differences in their ion mobility. This work should advance the design and construction of biomimetic materials for the extraction of valuable species from seawater and other aqueous sources.

Anthraquinone substituents modulate ionic hydrogen-bonded organic frameworks to achieve high ionic conductivity for alkali metal ions

Herein, we report two charge-assisted hydrogen-bonded organic frameworks (iHOF-24 and iHOF-25) with 3D/2D hydrogen-bonding networks, which exhibit high ionic conductivity for alkali metal ions. Among them, the conductivity of Li+ is higher than that of Na+ and K+, and the ionic conductivities of Li@iHOF-24 and Li@iHOF-25 at 30 °C were 9.44 × 10-5 and 9.85 × 10-5 S cm-1. This change is attributed to the distance between neighboring carbonyl groups in iHOF-24 and iHOF-25, as well as the radius of the loaded alkali metal ions.

Selective Electrochemical Capture of Monovalent Cations Using Crown Ether-Functionalized COFs

Electrochemical adsorption offers a promising approach for the separation of monovalent cations, which is an important but challenging subject in separation science. However, progress in this area has been hampered by the lack of suitable materials with effective ion selectivity. In this work, we present the synthesis of covalent organic frameworks (COFs) functionalized with a series of crown ethers (NCx-TAB-COFs, x donate 12, 15, 18, indicating the size of crown ether) for the efficient and highly selective electrochemical capture of monovalent cations. In our design, crown ether moieties act as confinement sites, imparting high selectivity for different monovalent cations depending on the cavity dimensions of the crown ether present. COFs electrodes prepared using the novel crown-COFs exhibit superior performance for the selective sequestration of monovalent (alkali metal) cations. Notably, 18-crown-6 ether-substituted COF (NC18-TAB-COF) shows a remarkable selectivity (14.26) for K+ over Na+ and a substantial Rb+/Na+ selectivity of 22.4. Furthermore, NCx-TAB-COFs maintain their remarkable selectivity and capacity under mixed-cation conditions. Density functional theory calculations and molecular dynamics simulations suggest that the unexpectedly high selectivity for larger cations is likely due to diverse binding modes in conjunction with the porous structure of the COFs. Given their lower dehydration-free energies and smaller hydrodynamic radii, K+, Rb+, and Cs+ more readily permeate the confined channels of COFs. In contrast, Na+ and Li+, with higher dehydration-free energies and hydrodynamic radii, diffuse into the NCx-TAB-COFs structure at a much slower rate and are bound predominantly to the surfaces of the COFs.

Ultrathin zwitterionic COF membranes from colloidal 2D-COF towards precise molecular sieving

Membrane technology is an important component of resource recovery. Covalent organic frameworks (COFs) with inherent long-range ordered structure and permanent porosity are ideal materials for fabricating advanced membrane. Zwitterionic COFs have unique features beyond single ionic COFs containing anions or cations. Here, a zwitterionic colloidal 2D-COF (TpPa-Py) is synthesized via a single-phase method. ultrathin zwitterionic COF membranes are fabricated via a facile blade-coating method. Experimental and molecular dynamics simulation results showed that due to the unique amphiphilic nature of the TpPa-Py, the TpPa1-Py1 membrane exhibits high level permeance and rejection of both positively and negatively charged dyes. Moreover, the TpPa1-Py1 membrane exhibits excellent dye/dye and dye/salt separation performance. The selectivity factors were 89 for the separation of acid blue and rhodamine B, and 47.8 for the separation of methyl blue and NaCl. This work provides a promising solution for the development of high-performance membranes tailored for resource recovery of dye wastewater, addressing a critical need in water treatment.

Biomimetic ion channels with subnanometer sizes for ion sieving: a mini-review

The remarkable ion selectivity of biological systems has inspired the development of artificial ion channels with Ångström-scale precision, expanding their potential applications in ion separation, energy conversion, and water purification. This mini-review systematically examines fundamental ion-sieving mechanisms operating at the subnanoscale, highlighting advanced fabrication strategies involving synthetic ion channels on lipid bilayers and solid-state ion channels. We further explore membrane material innovations spanning zero-dimensional nanopores to three-dimensional crystalline frameworks, emphasizing structure-function relationships in channel design. The discussion concludes with critical perspectives on scalability challenges and future research directions, outlining pathways toward next-generation sustainable ion sieving technologies.

Dehydration-enhanced Ion Recognition of Triazine Covalent Organic Frameworks for High-resolution Li+/Mg2+ Separation

The precise and rapid extraction of lithium from salt-lake brines is critical to meeting the global demand for lithium resources. However, it remains a major challenge to design ion-transport membranes with accurate recognition and fast transport path for the target ion. Here, we report a triazine covalent organic framework (COF) membrane with high resolution for Li+ and Mg2+ that enables fast Li+ transport while almost completely inhibiting Mg2+ permeation. The remarkably high rejection of Mg2+ by the COF membrane is achieved via imposed ion dehydration and the construction of the energy well. The proper hydrophilic environment of the COF channel promotes the dissociation of Li+ from the negatively charged functional groups, allowing Li+ for hopping transport supported by the sulfonate side-chains to shorten the diffusion path of Li+. Under high-salinity electrodialysis conditions, the COF membrane demonstrates robust Li+/Mg2+ separation performance (No Mg2+ were detected in the collected solution), achieving efficient lithium recovery and high product purity (Li2CO3: 99.3 %). This membrane design strategy enables high energy efficiency and powerful lithium extraction in the electrodialysis lithium extraction process, and can be generalized to other energy and separation related membranes.

Molecular Design of Positively Charged 3D Covalent-Organic Framework Membranes for Li+/Mg2+ Separation

3D covalent-organic framework (3D COF) membranes have unique features such as smaller pore sizes and more interconnected networks compared with 2D COF counterparts. However, the complicated and unmanageable fabrication hinders their rapid development. Molecular simulation, which can efficiently explore the structure-performance relationship of membranes, holds great promise in accelerating the development of 3D COF membranes. In this study, a series of 3D-COF membranes (TFPM-Pa-X) is designed with different charge densities (fully charged, partially charged, and neutral) and interpenetration numbers (2-, 3-, 4-, and 5-fold), subsequently investigate their contributions to Li+/Mg2+ separation through molecular simulation. Membrane morphology and pore size are found to strongly depend on the charged density and interpenetration number. The pore size and Cl- ion density play a crucial role in governing membrane separation performance. TFPM-Pa-X membrane with a smaller interpenetration number and a higher charge density promotes Li+/Mg2+ separation. The fully charged 2-fold interpenetrated membrane has superior performance in breaking the trade-off between the flux of Li+ (JLi +) and the selectivity of Li+ over Mg2+ (SLi + /Mg 2+). This study may facilitate the rational design of new 3D COF membranes for high-performance ion separation.

Regulation of Ion Binding Sites in Covalent Organic Framework Membranes for Enhanced Selectivity under High Ionic Competition

The strategic spatial positioning of ion affinity sites within biological ion channels and their cooperative binding with the targeted ions are pivotal for enhancing ion recognition and ensuring exceptional selectivity in high ionic competition scenarios. However, the application of these principles to artificial ion channels remains largely unexplored. Herein, we present a series of covalent organic framework (COF) membranes, engineered with oxygen functional groups aligned along the rims of oriented COF pore channels of varying sizes to achieve a precise spatial arrangement of ion affinity sites. A notable COF membrane, featuring subnanometer pores decorated alternately with carbonyl and amide groups, demonstrated outstanding selectivity, achieving a Li/Mg selectivity ratio of 513 under equal mole and electrodialysis conditions. Impressively, as the Mg/Li ratio in the source solution increased to 16.6, the selectivity ratio rose to 833, significantly exceeding the reductions typically seen in conventional selective electrodialysis and nanofiltration methods. Both simulation and experimental analyses indicate that this exceptional selectivity stems from the cooperative binding between the oxygen functional groups and Li+ ions within the confined nanochannels, facilitating the preferential transport of Li+ ions. These findings provide a promising approach for designing selective ion extraction systems that function effectively in highly competitive environments.

Solution-processable polymer membranes with hydrophilic subnanometre pores for sustainable lithium extraction

Membrane-based separation processes hold great promise for sustainable extraction of lithium from brines for the rapidly expanding electric vehicle industry and renewable energy storage. However, it remains challenging to develop high-selectivity membranes that can be upscaled for industrial processes. Here we report solution-processable polymer membranes with subnanometre pores with excellent ion separation selectivity in electrodialysis processes for lithium extraction. Polymers of intrinsic microporosity incorporated with hydrophilic functional groups enable fast transport of monovalent alkali cations (Li+, Na+ and K+) while rejecting relatively larger divalent ions such as Mg2+. The polymer of intrinsic microporosity membranes surpasses the performance of most existing membrane materials. Furthermore, the membranes were scaled up and integrated into an electrodialysis stack, demonstrating excellent selectivity in simulated salt-lake brines. This work will inspire the development of selective membranes for a wide range of sustainable separation processes critical for resource recovery and a global circular economy.

Amine-Modified ZIF Composite Membranes: Regulated Nanochannel Interactions for Enhanced Cation Transport and Precise Separation

Electromembrane water treatment technologies are attracting attention for their energy efficiency and precise separation of counterions. However, ion-exchange membranes exhibit low ionic conductance and selectivity for ions with similar charges. In this study, we developed a novel ZIF-8 composite membrane with amine-modified nanochannels through an in situ PEI-assisted seeding and secondary growth method. An integral and uniform selective layer was formed, and the amine-modified nanochannels induced differential transport of Li+, Na+, K+, and Mg2+ via the dehydration-hydration process. The composite membrane possessed a lower energy barrier for Na+ transport (Ea = 13 kJ mol-1) compared to Mg2+ (Ea = 17 kJ mol-1), showing a Na+ flux of 3.7 × 10-8 mol·cm-2·s-1 and a Na+/Mg2+ permselectivity of 52 (∼60 times higher than the commercial membrane). The physicochemical and electrochemical properties of the composite membranes were systematically characterized, revealing the significant role of the Mg2+ layer in increasing Mg2+ repulsion and facilitating Na+ diffusion. Besides, DFT simulation and interaction energy calculation elucidated that a moderate binding energy and compensation effect between ions and nanochannels, which can be precisely regulated by PEI incorporation, are crucial for the favorable passage of Na+ while maintaining high Mg2+ rejection. The membrane also demonstrated performance stability during a 5-day test and maintained high selectivity across varying salinity and pH conditions. This work advances the development of efficient cation separation membranes for sustainable desalination and resource recovery.

Engineering of Covalent Organic Framework Nanosheet Membranes for Fast and Efficient Ion Sieving: Charge-Induced Cation Confined Transport

Artificial membranes with ion-selective nanochannels for high-efficiency mono/divalent ion separation are of great significance in water desalination and lithium-ion extraction, but they remain a great challenge due to the slight physicochemical property differences of various ions. Here, the successful synthesis of two-dimensional TpEBr-based covalent organic framework (COF) nanosheets, and the stacking of them as consecutive membranes for efficient mono/divalent ion separation is reported. The obtained COF nanosheet membranes with intrinsic one-dimensional pores and abundant cationic sites display high permeation rates for monovalent cations (K+, Na+, Li+) of ≈0.1-0.3 mol m-2 h-1, while the value of divalent cations (Ca2+, Mg2+) is two orders of magnitude lower, resulting in an ultrahigh mono/divalent cation separation selectivity up to 130.4, superior to the state-of-the-art ion sieving membranes. Molecular dynamics simulations further confirm that electrostatic interaction controls the confined transport of cations through the cationic COF nanopores, where multivalent cations face i) strong electrostatic repulsion and ii) steric transport hindrance since the large hydrated divalent cations are retarded due to a layer of strongly adsorbed chloride ions at the pore wall, while smaller monovalent cations can swiftly permeate through the nanopores.

Approaching infinite selectivity in membrane-based aqueous lithium extraction via solid-state ion transport

As the gap between lithium supply and demand continues to widen, the need to develop ion-selective technologies, which can efficiently extract lithium from unconventional water sources, grows increasingly crucial. In this study, we investigated the fundamentals of applying a solid-state electrolyte (SSE), typically used in battery technologies, as a membrane material for aqueous lithium extraction. We find that the anhydrous hopping of lithium ions through the ordered and confined SSE lattice is highly distinct from ion migration through the hydrated free volumes of conventional nanoporous membranes, thus culminating in unique membrane transport properties. Notably, we reveal that the SSE provides unparalleled performance with respect to ion-ion selectivity, consistently demonstrating lithium ion selectivity values that are immeasurable by even the part-per-billion detection limit of mass spectrometry. Such exceptional selectivity is shown to be the result of the characteristic size and charge exclusion mechanisms of solid-state ion transport, which may be leveraged in the design of next-generation membranes for resource recovery.

Heteropolyacid Ligands in Two-Dimensional Channels Enable Lithium Separation from Monovalent Cations

Extracting lithium from salt lakes requires ion-selective membranes with customizable nanochannels. However, it remains a major challenge to separate alkali cations due to their same valences and similar ionic radius. Inspired by the K+ channel of KcsA K+, significant progress has been made in adjusting nanochannel size to control the ion selectivity dominated by alkali cations dehydration. Besides, several works involved incorporating ligands, such as crown ether, into nanochannels based on coordination chemistry to try to promote alkali cation selectivity; nevertheless, only the separation between mono-/bivalent cations has been achieved. Herein, a series of heteropolyacid (HPA) ligands are designed to functionalize two-dimensional (2D) nanochannels, achieving superior lithium perm-selectivity over other alkali cations (16 for Li+/K+), with the Li+ permeation rate increased to four times that of the pristine 2D membrane. We discover that the switching of an ion between its hydration and ion-HPA coordination states elucidates ion-selective transport, and the relatively lower depth of energy well for the exchange from Li+ hydration to Li+-HPA coordination results in the separation of Li+ from other alkali cations. This work demonstrates a principle for exploring novel ligands to develop alkali cation-selective membranes, expanding the potential applications of ion separation membranes in lithium extraction from aquatic sources.

Constructing new-generation ion exchange membranes under confinement regime

Ion exchange membranes (IEMs) enable fast and selective ion transport and the partition of electrode reactions, playing an important role in the fields of precise ion separation, renewable energy storage and conversion, and clean energy production. Traditional IEMs form ion channels at the nanometer-scale via the assembly of flexible polymeric chains, which are trapped in the permeability/conductivity and selectivity trade-off dilemma due to a high swelling propensity. New-generation IEMs have shown great potential to break this intrinsic limitation by using microporous framework channels for ion transport under a confinement regime. In this Review, we first describe the fundamental principles of ion transport in charged channels from nanometer to sub-nanometer scale. Then, we focus on the construction of new-generation IEMs and highlight the microporous confinement effects from sub-2-nm to sub-1-nm and further to ultra-micropores. The enhanced ion transport properties brought by the intense size sieving and channel interaction are elucidated, and the corresponding applications including lithium separation, flow battery, water electrolysis, and ammonia synthesis are introduced. Finally, we prospect the future development of new-generation IEMs with respect to the intricate microstructure observation, in-situ ion transport visualization, and large-scale membrane fabrication.

Linear Enhanced 3D Nanofluid Force-Electric Conversion Device

The inherent trade-off between permeability and selectivity has constrained further improvement of passive linear force-electric conversion performance in nanofluidic pressure sensors. To overcome this limitation, a 3D nanofluidic membrane with high mechanical strength utilizing aramid nanofibers/carbon nanofiber (ANF/CNF) dual crosslinking is developed. Due to the abundant surface functional groups of CNF and the high mechanical strength of ANF, this large-scale integrated 3D nanofluidic membrane exhibits advantages of high flux, high porosity, and short ion transport path, demonstrating superior force-electric response compared to conventional 1D and 2D configurations. The enhancement mechanism of the ANF/CNF membrane is systematically investigated through experimental results and theoretical calculations. The optimized device has a sensitivity of 111 nA cm-2 kPa-1, a response/recovery time of 63/68 ms, and a stability of 45 000 cycles. This study successfully overcomes the inherent performance limitations of traditional nanofluidic membranes, offering promising potential for applications across artificial intelligence, the Internet of Things, and smart wearable devices.

Imine-Linked 3D Covalent Organic Framework Membrane Featuring Highly Charged Sub-1 nm Channels for Exceptional Lithium-Ion Sieving

Coupling ion exclusion and interaction screening within sub-nanoconfinement channels in novel porous material membranes hold great potential to realize highly efficient ion sieving, particularly for high-performance lithium-ion extraction. Diverse kinds of advanced membranes have been previously reported to realize this goal but with moderate performance and complex operations gained. Herein, these issues are circumvented by preparing the consecutive and intact imine-linked three-dimensional covalent organic framework (i.e., COF-300) membranes via a simple solvothermal approach and employing the intrinsically interconnected sub-1 nm one-dimensional channels for exceptional lithium-ion sieving. The synthesized membranes with highly charged angstrom scale channels of ≈0.78 nm achieve an excellent Li+ permeance (0.123 mol m-2 h-1) with an ultrahigh Li+/Mg2+ of 36 in the binary system. The experimental measurement and theoretical calculation reveal that a channel size right exactly between Li+ and Mg2+ enables restricted Mg2+ penetration. Meanwhile, the ion affinity interaction screening with imine groups further strengthens the fast Li+ permeability but severely suppresses the Mg2+ passage. In particular, the synthesized three-dimensional covalent organic framwork membranes also have a remarkable separation performance during a long-term operation test without sacrificing trade-off, demonstrating chemistry stability and mechanical integrity under the high-salinity aqueous environment.

Staggered-Stacking Two-Dimensional Covalent Organic Framework Membranes for Molecular and Ionic Sieving

Two-dimensional covalent organic frameworks (2D COFs), a family of crystalline materials with abundant porous structures offering nanochannels for molecular transport, have enormous potential in the applications of separation, energy storage, and catalysis. However, 2D COFs remain limited by relatively large pore sizes (>1 nm) and weak interlayer interactions between 2D nanosheets, making it difficult to achieve efficient membranes to meet the selective sieving requirements for water molecules (0.3 nm) and hydrated salt ions (>0.7 nm). Here, we report a high-performance 2D COF membrane with narrowed channels (0.7 × 0.4 nm2) and excellent mechanical performance constructed by the staggered stacking of cationic and anionic 2D COF nanosheets for selectively sieving of water molecules and hydrated salt ions. The mechanical performance has been improved by two times than that of single-phase 2D COF membranes due to the enhanced interlayer interactions between nanosheets. The stacked 2D COF membranes exhibit significantly improved monovalent salt ions rejection ratio (up to 77.9%) compared with single-phase COF membranes (∼49.2%), while maintaining comparable water permeability. The design of stacked 2D COF membranes provides a potential strategy for constructing high-performance nanoporous membranes to achieve precise molecular and ionic sieving.

Efficient Light-Driven Ion Pumping for Deep Desalination via the Vertical Gradient Protonation of Covalent Organic Framework Membranes

Traditional desalination methods face criticism due to high energy requirements and inadequate trace ion removal, whereas natural light-driven ion pumps offer superior efficiency. Current synthetic systems are constrained by short exciton lifetimes, which limit their ability to generate sufficient electric fields for effective ion pumping. We introduce an innovative approach utilizing covalent-organic framework membranes that enhance light absorption and reduce charge recombination through vertical gradient protonation of imine linkages during acid-catalyzed liquid-liquid interfacial polymerization. This technique creates intralayer and interlayer heterojunctions, facilitating interlayer hybridization and establishing a robust built-in electric field under illumination. These improvements enable the membranes to achieve remarkable ion transport across extreme concentration gradients (2000:1), with a transport rate of approximately 3.2 × 1012 ions per second per square centimeter and reduce ion concentrations to parts per billion. This performance significantly surpasses that of conventional reverse osmosis systems, representing a major advancement in solar-powered desalination technology by substantially reducing energy consumption and secondary waste.

Heterogeneous Covalent Organic Framework Membranes Mediated by Polycations for Efficient Ions Separation

Precise ions sieving at angstrom-scale is gaining tremendous attention thanks to its significant impact at the water-energy nexus. Herein, a novel polycation-modulated interfacial polymerization (IP) strategy is developed to prepare a heterogeneously charged covalent organic frameworks (COFs) membrane. Cationic poly(diallyldimethylammonium chloride) (PDDA) regulates the growth and assembly of anionic COFs nanosheets, which thus provides a negative, smooth top surface and positive, rough bottom surface, indicating the presence of heterogeneously charged angstrom-scale channels through the membrane. Experiments and simulations are conducted to understand the facilitated ions transport behavior relative to specific interactions raised by heterogeneously charged channels and angstrom-scale steric hinderance as well, rendering the membrane with robust mono-/divalent cations sieving capabilities. The selectivity (61.6) of Li+ to Mg2+ in mixed saline under the continuous cross-flow filtration mode is superior to most of the reported nanofiltration membranes. This polycation-mediated interfacial polymerization strategy offers a compelling opportunity to develop versatile heterogeneously charged COF membranes for exquisite ion sieving.

Zwitterionic Nanochannels in Covalent Organic Framework Membranes for Improved Flux and Antifouling Property

Zwitterionic membranes demonstrate excellent antifouling property in water purification. The covalent organic frameworks (COFs), due to the ordered channels and abundant organic functional groups, have distinct superiority in constructing zwitterionic surfaces.Here, the zwitterionic COF membrane is prepared with precise framework structures and uniform charge distribution. The negatively charged 4,4'-diaminobiphenyl-2,2'-sisulphonic acid sodium (SA) and positively charged ethidium bromide (EB) fragments are used to react with 1,3,5-triformylphloroglucinol (TP) at the gas-liquid interface to prepare zwitterionic COF membrane. The complementary charged fragments in the inter-layer and inner-layer facilitate the formation of continuous and tight hydration layer on the membrane surface and pore walls to resist the adsorption of pollutants. The zwitterionic COF membrane effectively resists both negatively charged bovine serum albumin and positively charged lysozyme pollutants with flux recovery ratio (FRR) of 97% and 85%, respectively. Furthermore, the regular nano-channels and balanced interactions between water and surface/pore walls of the zwitterionic membrane result in outstanding permeability of up to 146 L m-2 h-1 bar-1 and excellent dye/salt separation selectivity. The water permeation and antifouling mechanism of membranes are elucidated by experimental and molecular dynamics calculation. Zwitterionic COF membranes can find promising applications in preparing high-performance antifouling membranes.

High performance, pH-resistant membranes for efficient lithium recovery from spent batteries

Cation separation under extreme pH is crucial for lithium recovery from spent batteries, but conventional polyamide membranes suffer from pH-induced hydrolysis. Preparation of high performance nanofiltration membranes with excellent pH-resistance remains a challenge. Here we synthesize a high performance nanofiltration membrane (1,4,7,10-Tetraazacyclododecane (TAD)-1,3,5-Tris(bromomethyl)benzene (TBMB) thin film composite membranes (TFCMs)) with excellent pH-stability through interfacial quaternization reaction between TAD and TBMB. Due to the high stability of "C-N" bonds in TAD-TBMB TFCMs, its separation performance is stable even after 70 days immersion in concentrated acid (3 M H2SO4, HNO3, or HCl) and base (3 M NaOH), which is at least 15 times more stable than benchmark commercial membranes. The membrane shows an overall separation performance (11.3 L m-2 h-1 bar-1 (LMHB), RCo2+: 97% in 2 M H2SO4) due to the size sieving and the intensified charge repulsion, outperforming many of the state-of-the-art membranes. Finally, the TAD-TBMB TFCM remains stable during 30-days continuous nanofiltration of 2 M H2SO4 and leachate (2 M H2SO4, ions: 6.2 g L-1) from spent batteries.

Reverse-Selective Anion Separation Relies on Charged "Hourglass" Gate

According to the hydration size and charge property of separated ions, the transport channel can be constructed to achieve precision ion separation, but the ion geometry as a separation parameter to design the channel structure is rarely reported. Herein, a reverse-selective anion separation membrane composed of a metal-organic frameworks (MOFs) layer with a charged "hourglass" channel as an ion-selective switch to manipulate oxoanion transport is developed. The gate in "hourglass" with tetrahedral geometry similar to the oxoanion (such as SO2- 4, Cr 2O2- 7, and MnO- 4) boosts the transmission effect oxoanion much larger than Cl- through geometric matching and Coulomb interaction. Specific channel structure exhibits an abnormal selectivity for SO2- 4/Cl- of 20, Cr 2O2- 7/Cl- of 6.6, and MnO- 4/Cl- of 4.0 in a binary-ion system. The transfer behavior of SO2- 4 in the channel revealed by molecular dynamics simulation and density functional theory calculation further indicates the mechanism of the abnormal separation performance. The universality of the membrane structure is validated by the formation of different nitrogen-containing modified layers, which also achieves in situ growth of the MOFs layer, and exhibits similar reversal separation performance. The geometric configuration control of ion transport channels presents a novel effective strategy to realize the precise separation of target ions.

Modular Customized Biomimetic Nanofluidic Diode for Tunable Asymmetric Ion Transport

Artificial ion diodes, inspired by biological ion channels, have made significant contributions to the fields of physics, chemistry, and biology. However, constructing asymmetric sub-nanofluidic membranes that simultaneously meet the requirements of easy fabrication, high ion transport efficiency, and tunable ion transport remains a challenge. Here, a direct and flexible in situ staged host-guest self-assembly strategy is employed to fabricate ion diode membranes capable of achieving zonal regulation. Coupling the interfacial polymerization process with a host-guest assembly strategy, it is possible to easily manipulate the type, order, thickness, and charge density of each module by introducing two oppositely charged modules in stages. This method enables the tuning of ion transport behavior over a wide range salinity, as well as responsive to varying pH levels. To verify the potential of controllable diode membranes for application, two ion diode membranes with different ion selectivity and high charge density are coupled in a reverse electrodialysis device. This resulted in an output power density of 63.7 W m-2 at 50-fold NaCl concentration gradient, which is 12 times higher than commercial standards. This approach shows potential for expanding the variety of materials that are appropriate for microelectronic power generation devices, desalination, and biosensing.

Emerging Advances around Nanofluidic Transport and Mass Separation under Confinement in Atomically Thin Nanoporous Graphene

Membrane separation stands as an environmentally friendly, high permeance and selectivity, low energy demand process that deserves scientific investigation and industrialization. To address intensive demand, seeking appropriate membrane materials to surpass trade-off between permeability and selectivity and improve stability is on the schedule. 2D materials offer transformational opportunities and a revolutionary platform for researching membrane separation process. Especially, the atomically thin graphene with controllable porosity and structure, as well as unique properties, is widely considered as a candidate for membrane materials aiming to provide extreme stability, exponentially large selectivity combined with high permeability. Currently, it has shown promising opportunities to develop separation membranes to tackle bottlenecks of traditional membranes, and it has been of great interest for tremendously versatile applications such as separation, energy harvesting, and sensing. In this review, starting from transport mechanisms of separation, the material selection bank is narrowed down to nanoporous graphene. The study presents an enlightening overview of very recent developments in the preparation of atomically thin nanoporous graphene and correlates surface properties of such 2D nanoporous materials to their performance in critical separation applications. Finally, challenges related to modulation and manufacturing as well as potential avenues for performance improvements are also pointed out.

Boosting lithium/magnesium separation performance of selective electrodialysis membranes regulated by enamine reaction

Monovalent cation exchange membranes (MCEMs) have progressively played an important role in the field of ion separation. However, according to transition state theory (TST), synchronously tuning the enthalpy barrier (△H) and entropy barrier (△S) for cation transport to improve ion separation performance is challenging. Here, the enamine reaction between the -NH- and -CHO groups is applied to regulate the subsequent Schiff-base reaction between the -CHO and -NH2 groups, which reduces the positive charges of the selective layer but increases the steric hindrance. The increased -T△S (△S term) for cation transport plays an important role in improving Li+/Mg2+ separation performance. The optimal positively-charged glutaraldehyde@piperazine/polyethyleneimine assembled membrane (M-Glu@PIP/PEI) has a perm-selectivity (Li+/Mg2+) of 31.83 with a Li+ flux of 1.87 mol·m-2·h-1, surpassing the Li+/Mg2+ separation performance of state-of-the-art monovalent ion selective membranes (MISMs). Most importantly, the selective electrodialysis (S-ED) process with M-Glu@PIP/PEI can be directly applied to treat simulated salt-lake brines (SLBs), and its superior Li+/Mg2+ separation performance and operational stability enables 74.44 % of the lithium resources with a Li+ purity of 34.02 % to be recovered. This study presents new insights into the design of high-performance MCEMs for energy-efficient resource recovery.

Covalent organic framework membranes with vertically aligned nanorods for efficient separation of rare metal ions

Covalent organic frameworks (COFs) have emerged as promising platforms for membrane separations, while remaining challenging for separating ions in a fast and selective way. Here, we propose a concept of COF membranes with vertically aligned nanorods for efficient separation of rare metal ions. A quaternary ammonium-functionalized monomer is rationally designed to synthesize COF layers on porous substrates via interfacial synthesis. The COF layers possess an asymmetric structure, in which the upper part displays vertically aligned nanorods, while the lower part exhibits an ultrathin dense layer. The vertically aligned nanorods enlarge contact areas to harvest water and monovalent ions, and the ultrathin dense layer enables both high permeability and selectivity. The resulting membranes exhibit exceptional separation performances, for instance, a Cs+ permeation rate of 0.33 mol m-2 h-1, close to the value in porous substrates, and selectivities with Cs+/La3+ up to 75.9 and 69.8 in single and binary systems, highlighting the great potentials in the separation of rare metal ions.

Boosted Intracavity Aperture in Macrocyclic Amines Enabling Finely Regulated Microporous Membranes

Finely tuning the pore structure of traditional nanofiltration (NF) membranes is challenging but highly effective for achieving efficient separations. Herein, we propose a concept of using macrocyclic amines (1,4,7-triazacyclononane, 3A; 1,4,7,10-tetraazacyclododecane, 4A1; and 1,4,8,11-tetraazacyclotetradecane, 4A2) with different intra-annular apertures to finely modulate the pore structure of microporous membranes via interfacial polymerization (IP). The boost in the intracavity size of the building blocks results in heightened steric hindrance of these amine monomers, leading to a controlled increase in membrane pore size, as demonstrated by both film characterizations and multiscale simulations. In conjunction with the increased intracavity size, the water permeability follows an augmented trend of 3A-TMC, 4A1-TMC, and 4A2-TMC (TMC: trimesoyl chloride) while exhibiting increased molecular weight cut-offs due to larger free-volume elements and stronger pore interconnectivity. Our proposed macrocyclic amine design strategy provides a guideline for finely regulated microporous membranes with high potential in NF-related applications.

Covalent Organic Framework Membranes with Regulated Orientation for Monovalent Cation Sieving

Continuous covalent organic framework (COF) thin membranes have garnered broad concern over the past few years due to their merits of low energy requirements, operational simplicity, ecofriendliness, and high separation efficiency in the application process. This study marks the first instance of fabricating two distinct, self-supporting COF membranes from identical building blocks through solvent modulation. Notably, the precision of the COF membrane's separation capabilities is substantially enhanced by altering the pore alignment from a random to a vertical orientation. Within these confined channels, the membrane with vertically aligned pores and micron-scale stacking thickness demonstrates rapid and selective transportation of Li+ ions over Na+ and K+ ions, achieving Li+/K+ and Li+/Na+ selectivity ratios of 38.7 and 7.2, respectively. This research not only reveals regulated orientation and layer stacking in COF membranes via strategic solvent selection but also offers a potent approach for developing membranes specialized in Li+ ion separation.

Nanofluidic Membrane-Assisted Organic Electrochemical Transistors for Bioinspired Gustatory Sensation Based on Selective Cation Transport

Natural organisms have evolved precise sensing systems relying on unique ion channels, which can efficiently perceive various physical/chemical stimuli based on ionic signal transmission in biological fluid environments. However, it is still a huge challenge to achieve extensive applications of the artificial counterparts as an efficient wet sensing platform due to the fluidity of the working medium. Herein, nanofluidic membranes with selective cation transport properties and solid-state organic electrochemical transistors (OECTs) with amplified signals are integrated together to mimic human gustatory sensation, achieving ionic gustatory reagent recognition and a portable configuration. Cu-HHTP nanofluidic membranes with selective cation transport through their uniform micropores are constructed first, followed by assembly with OECTs to form the designed nanofluidic membrane-assisted OECTs (nanofluidic OECTs). As a result, they can distinguish typically ionic gustatory reagents, and even ionic liquids (ILs), demonstrating enhanced gustatory perception performance under a wide concentration range (10-7-10-1 m) compared with those of conventional OECTs. The linear correlations between the response and the reagent concentration further indicate the promising potential for practical application as a next-generation sensing platform. It is suggested that nanofluidic membranes mediated intramembrane cation transport based on the steric hindrance effect, resulting in distinguishable and improved response to multiple ions.

Giant gateable thermoelectric conversion by tuning the ion linkage interactions in covalent organic framework membranes

Efficient energy conversion using ions as carriers necessitates membranes that sustain high permselectivity in high salinity conditions, which presents a significant challenge. This study addresses the issue by manipulating the linkages in covalent-organic-framework membranes, altering the distribution of electrostatic potentials and thereby influencing the short-range interactions between ions and membranes. We show that a charge-neutral covalent-organic-framework membrane with β-ketoenamine linkages achieves record permselectivity in high salinity environments. Additionally, the membrane retains its permselectivity under temperature gradients, providing a method for converting low-grade waste heat into electrical energy. Experiments reveal that with a 3 M KCl solution and a 50 K temperature difference, the membrane generates an output power density of 5.70 W m-2. Furthermore, guided by a short-range ionic screening mechanism, the membrane exhibits adaptable permselectivity, allowing reversible and controllable operations by finely adjusting charge polarity and magnitude on the membrane's channel surfaces via ion adsorption. Notably, treatment with K3PO4 solutions significantly enhances permselectivity, resulting in a giant output power density of 20.22 W m-2, a 3.6-fold increase over the untreated membrane, setting a benchmark for converting low-grade heat into electrical energy.

Simultaneous Manipulation of Membrane Enthalpy and Entropy Barriers towards Superior Ion Separations

Sub-nanoporous membranes with ion selective transport functions are important for energy utilization, environmental remediation, and fundamental bioinspired engineering. Although mono/multivalent ions can be separated by monovalent ion selective membranes (MISMs), the current theory fails to inspire rapid advances in MISMs. Here, we apply transition state theory (TST) by regulating the enthalpy barrier (ΔH) and entropy barrier (ΔS) for designing next-generation monovalent cation exchange membranes (MCEMs) with great improvement in ion selective separation. Using a molecule-absorbed porous material as an interlayer to construct a denser selective layer can achieve a greater absolute value of ΔS for Li+ and Mg2+ transport, greater ΔH for Mg2+ transport and lower ΔH for Li+ transport. This recorded performance with a Li+/Mg2+ perm-selectivity of 25.50 and a Li+ flux of 1.86 mol ⋅ m-2 ⋅ h-1 surpasses the contemporary "upper bound" plot for Li+/Mg2+ separations. Most importantly, our synthesized MCEM also demonstrates excellent operational stability during the selective electrodialysis (S-ED) processes for realizing scalability in practical applications.

A Bioinspired Membrane with Ultrahigh Li+/Na+ and Li+/K+ Separations Enables Direct Lithium Extraction from Brine

Membranes with precise Li+/Na+ and Li+/K+ separations are imperative for lithium extraction from brine to address the lithium supply shortage. However, achieving this goal remains a daunting challenge due to the similar valence, chemical properties, and subtle atomic-scale distinctions among these monovalent cations. Herein, inspired by the strict size-sieving effect of biological ion channels, a membrane is presented based on nonporous crystalline materials featuring structurally rigid, dimensionally confined, and long-range ordered ion channels that exclusively permeate naked Li+ but block Na+ and K+. This naked-Li+-sieving behavior not only enables unprecedented Li+/Na+ and Li+/K+ selectivities up to 2707.4 and 5109.8, respectively, even surpassing the state-of-the-art membranes by at least two orders of magnitude, but also demonstrates impressive Li+/Mg2+ and Li+/Ca2+ separation capabilities. Moreover, this bioinspired membrane has to be utilized for creating a one-step lithium extraction strategy from natural brines rich in Na+, K+, and Mg2+ without utilizing chemicals or creating solid waste, and it simultaneously produces hydrogen. This research has proposed a new type of ion-sieving membrane and also provides an envisioning of the design paradigm and development of advanced membranes, ion separation, and lithium extraction.

Ion transport mechanisms in covalent organic frameworks: implications for technology

Covalent organic frameworks (COFs) have emerged as promising materials for ion conduction due to their highly tunable structures and excellent electrochemical stability. This review paper explores the mechanisms of ion conduction in COFs, focusing on how these materials facilitate ion transport across their ordered structures, which is crucial for applications such as solid electrolytes in batteries and fuel cells. We discuss the design strategies employed to enhance ion conductivity, including pore size optimization, functionalization with ionic groups, and the incorporation of solvent molecules and salts. Additionally, we examine the various applications of ion-conductive COFs, particularly in energy storage and conversion technologies, highlighting recent advancements and future directions in this field. This review paper aims to provide a comprehensive overview of the current state of research on ion-conductive COFs, offering insights into their potential to design highly ion-conductive COFs considering not only fundamental studies but also practical perspectives for advanced electrochemical devices.

Covalent organic framework membrane reactor for boosting catalytic performance

Membrane reactors are known for their efficiency and superior operability compared to traditional batch processes, but their limited diversity poses challenges in meeting various reaction requirements. Herein, we leverage the molecular tunability of covalent organic frameworks (COFs) to broaden their applicability in membrane reactors. Our COF membrane demonstrates an exceptional ability to achieve complete conversion in just 0.63 s at room temperature-a benchmark in efficiency for Knoevenagel condensation. This performance significantly surpasses that of the corresponding homogeneous catalyst and COF powder by factors of 176 and 375 in turnover frequency, respectively. The enhanced concentration of reactants and the rapid removal of generated water within the membrane greatly accelerate the reaction, reducing the apparent activation energy. Consequently, this membrane reactor enables reactions that are unattainable using both COF powders and homogeneous catalysts. Considering the versatility, our findings highlight the substantial promise of COF-based membrane reactors in organic transformations.

Microporous Materials in Polymer Electrolytes: The Merit of Order

Solid-state batteries (SSBs) have garnered significant attention in the critical field of sustainable energy storage due to their potential benefits in safety, energy density, and cycle life. The large-scale, cost-effective production of SSBs necessitates the development of high-performance solid-state electrolytes. However, the manufacturing of SSBs relies heavily on the advancement of suitable solid-state electrolytes. Composite polymer electrolytes (CPEs), which combine the advantages of ordered microporous materials (OMMs) and polymer electrolytes, meet the requirements for high ionic conductivity/transference number, stability with respect to electrodes, compatibility with established manufacturing processes, and cost-effectiveness, making them particularly well-suited for mass production of SSBs. This review delineates how structural ordering dictates the fundamental physicochemical properties of OMMs, including ion transport, thermal transfer, and mechanical stability. The applications of prominent OMMs are critically examined, such as metal-organic frameworks, covalent organic frameworks, and zeolites, in CPEs, highlighting how structural ordering facilitates the fulfillment of property requirements. Finally, an outlook on the field is provided, exploring how the properties of CPEs can be enhanced through the dimensional design of OMMs, and the importance of uncovering the underlying "feature-function" mechanisms of various CPE types is underscored.

Ion-Selective Transport Promotion Enabled by Angstrom-Scale Nanochannels in Dendrimer-Assembled Polyamide Nanofilm for Efficient Electrodialysis

The ion permeability and selectivity of membranes are crucial in nanofluidic behavior, impacting industries ranging from traditional to advanced manufacturing. Herein, we demonstrate the engineering of ion-conductive membranes featuring angstrom-scale ion-transport channels by introducing ionic polyamidoamine (PAMAM) dendrimers for ion separation. The exterior quaternary ammonium-rich structure contributes to significant electrostatic charge exclusion due to enhanced local charge density; the interior protoplasmic channels of PAMAM dendrimer are assembled to provide additional degrees of free volume. This facilitates the monovalent ion transfer while maintaining continuity and efficient ion screening. The dendrimer-assembled hybrid membrane achieves high monovalent ion permeance of 2.81 mol m-2 h-1 (K+), reaching excellent mono/multivalent selectivity up to 20.1 (K+/Mg2+) and surpassing the permselectivities of state-of-the-art membranes. Both experimental results and simulating calculations suggest that the impressive ion selectivity arises from the significant disparity in transport energy barrier between mono/multivalent ions, induced by the "exterior-interior" synergistic effects of bifunctional membrane channels.

Advancing Ion Separation: Covalent-Organic-Framework Membranes for Sustainable Energy and Water Applications

ConspectusMembranes are pivotal in a myriad of energy production processes and modern separation techniques. They are essential in devices for energy generation, facilities for extracting energy elements, and plants for wastewater treatment, each of which hinges on effective ion separation. While biological ion channels show exceptional permeability and selectivity, designing synthetic membranes with defined pore architecture and chemistry on the (sub)nanometer scale has been challenging. Consequently, a typical trade-off emerges: highly permeable membranes often sacrifice selectivity and vice versa. To tackle this dilemma, a comprehensive understanding and modeling of synthetic membranes across various scales is imperative. This lays the foundation for establishing design criteria for advanced membrane materials. Key attributes for such materials encompass appropriately sized pores, a narrow pore size distribution, and finely tuned interactions between desired permeants and the membrane. The advent of covalent-organic-framework (COF) membranes offers promising solutions to the challenges faced by conventional membranes in selective ion separation within the water-energy nexus. COFs are molecular Legos, facilitating the precise integration of small organic structs into extended, porous, crystalline architectures through covalent linkage. This unique molecular architecture allows for precise control over pore sizes, shapes, and distributions within the membrane. Additionally, COFs offer the flexibility to modify their pore spaces with distinct functionalities. This adaptability not only enhances their permeability but also facilitates tailored interactions with specific ions. As a result, COF membranes are positioned as prime candidates to achieve both superior permeability and selectivity in ion separation processes.In this Account, we delineate our endeavors aimed at leveraging the distinctive attributes of COFs to augment ion separation processes, tackling fundamental inquiries while identifying avenues for further exploration. Our strategies for fabricating COF membranes with enhanced ion selectivity encompass the following: (1) crafting (sub)nanoscale ion channels to enhance permselectivity, thereby amplifying energy production; (2) implementing a multivariate (MTV) synthesis method to control charge density within nanochannels, optimizing ion transport efficiency; (3) modifying the pore environment within confined mass transfer channels to establish distinct pathways for ion transport. For each strategy, we expound on its chemical foundations and offer illustrative examples that underscore fundamental principles. Our efforts have culminated in the creation of groundbreaking membrane materials that surpass traditional counterparts, propelling advancements in sustainable energy conversion, waste heat utilization, energy element extraction, and pollutant removal. These innovations are poised to redefine energy systems and industrial wastewater management practices. In conclusion, we outline future research directions and highlight key challenges that need addressing to enhance the ion/molecular recognition capabilities and practical applications of COF membranes. Looking forward, we anticipate ongoing advancements in functionalization and fabrication techniques, leading to enhanced selectivity and permeability, ultimately rivaling the capabilities of biological membranes.

Elucidating the transport of water and ions in the nanochannel of covalent organic frameworks by molecular dynamics

Inspired by biological channels, achieving precise separation of ion/water and ion/ion requires finely tuned pore sizes at molecular dimensions and deliberate exposure of charged groups. Covalent organic frameworks (COFs), a class of porous crystalline materials, offer well-defined nanoscale pores and diverse structures, making them excellent candidates for nanofluidic channels that facilitate ion and water transport. In this study, we perform molecular simulations to investigate the structure and kinetics of water and ions confined within the typical COFs with varied exposure of charged groups. The COFs exhibit vertically arrayed nanochannels, enabling diffusion coefficients of water molecules within COFs to remain within the same order of magnitude as in the bulk. The motion of water molecules manifests in two distinct modes, creating a mobile hydration layer around acid groups. The ion diffusion within COFs displays a notable disparity between monovalent (M+) and divalent (M2+) cations. As a result, the selectivity of M+/M2+ can exceed 100, while differentiation among M+ is less pronounced. In addition, our simulations indicate a high rejection (R > 98%) in COFs, indicating their potential as ideal materials for desalination. The chemical flexibility of COFs indicates that would hold significant promise as candidates for advanced artificial ion channels and separation membranes.