Hydrophilicity gradient in covalent organic frameworks for membrane distillation

Rapid Synthesis of Single-Crystal Covalent Organic Framework with Controllable Crystal Habits

Covalent organic frameworks (COFs) linked by poorly reversible covalent bonds lack dynamic formation and cleavage, so the synthesis of their single-crystal structures necessitates slow crystallization rates to mitigate defect formation. This, however, inherently restricts opportunities for facet-selective engineering beyond traditional compositional and topological controls. To address this fundamental limitation, we developed an acetal/CH3COOH protocol that paradoxically accelerated crystallization while enhancing structural perfection, reducing the synthesis time for 60 μm-sized single-crystal COF-300 to 1 h, while achieving crystal sizes of up to 120 μm within 48 h, and 300 μm after 30 days. Capitalizing on this accelerated synthesis platform, we systematically interrogated crystallization landscapes through multiparameter exploration─modulator chemoselectivity, catalyst dosages, temporal evolution, and reactive temperature─to decode possible growth mechanisms of single-crystal COFs. Based on these, the relationship between reaction conditions and the crystal size, size distribution, shape, and growth dynamics of single-crystal COFs was trained and predicted by a machine learning (ML) model.

Mass Transport Based on Covalent Organic Frameworks

ConspectusMass transport is fundamental to biological systems and industrial processes, governing chemical reactions, substance exchange, and energy conversion across various material scales. In biological systems, ion transport, such as proton migration through voltage-gated proton channels, regulates cellular potential, signaling, and metabolic balance. In industrial processes, transporting molecules through solid, liquid, or gas phases dictates reactant contact and diffusion rates, directly impacting reaction efficiency and conversion. Optimizing these processes necessitates the design of efficient interfaces or channels to enhance mass transport.Crystalline porous materials, particularly covalent organic frameworks (COFs), offer an excellent platform for investigating and optimizing mass transport. With ordered, pre-engineered nano- or subnanometer pores, COFs enable confined substance transport and garnered significant attention for energy conversion, catalysis, drug delivery, adsorption, and separation applications. Deeper investigations into the mass transport mechanism in COFs at the molecular level are crucial for advancing materials science, chemistry, and chemical engineering.Our group focuses on COFs to explore multisubstance cooperative transport mechanisms and structure-activity relationships for ions, water, and gases. We have expanded the linker chemistry of COFs by developing irreversible α-aminoketone-linked COFs and introducing the irreversible Suzuki coupling reaction into COF preparation. We proposed strategies such as side-chain-induced dipole-facilitated stacking and prenucleation and slow growth to achieve record large pore sizes and highly oriented nanochannels. We implemented exfoliation and an interwoven strategy to accelerate ion transport at complex interfaces, refined gas permeability in molecular sieve-based membranes through precise pore size engineering, and elucidated the effects of pore size and hydrophobicity/hydrophilicity on water phase transition and diffusion. Building on these insights, we designed novel open framework ionomers to tailor the microenvironment of electrocatalytic interfaces and uncovered multiple substance transport mechanisms. The synergistically enhanced transport of ions, water, and gas across three-phase interfaces effectively modulates the electrochemical CO2 reduction reaction pathway and significantly boosts the power density of proton-exchange membrane fuel cells (PEMFCs).In this Account, we summarize recent advances in COF-based ion and molecular transport, emphasizing nanochannel construction strategies, including linkage, pore size, orientation, and function gradient modulations. We discuss the functional design of COFs, correlations between pore structure and transport properties, and their applications in gas separation, energy storage, and catalysis. Finally, we outline current challenges and future opportunities in synthetic chemistry, mass transport mechanisms, and applications. By understanding mass transport phenomena from microscopic particles to macroscopic scales, this Account aims to provide molecular design strategies for optimizing multisubstance transport across three-phase interfaces, aligning mass transport with reaction processes and offering insights to enhance catalytic efficiency and energy conversion performance.

Valence Engineering via Polyoxometalate-Induced on Vanadium Centers for Efficient Aqueous Zinc-Ion Batteries

Layered vanadium-based compounds have attracted attention as cathode materials for aqueous zinc-ion batteries (AZIBs) because of their low cost, high theoretical specific capacity, and abundant vanadium valence states. However, the slow migration of Zn2+ ions and their poor cycling stability hinder their practical application in AZIBs. Herein, using a one-pot solvothermal method, the polyoxometalates (POMs) were inserted into the aluminum vanadate interlayer spacing, and a series of novel 3D nanoflower cathode materials (HAVO-MMo6-X) were successfully fabricated. The unique electron-rich structure of the POMs accelerated the migration of Zn2+ on the cathode to obtain a high specific capacity. Owing to the synergistic pillar effect of POMs and HAVO, the interlayer spacing of HAVO-FeMo6-50 increased to approximately 14.33 Å. X-ray absorption fine structure spectroscopy was used to analyze the coordination environments of the cathode materials. A combination of in situ and ex situ characterization techniques demonstrated the storage mechanism of Zn2+ during the charge/discharge process. Furthermore, the experimental results and DFT calculations indicated that the introduction of POMs had the dual function of improving conductivity and reducing the Zn2+ migration barrier. Thus, this work provides a new perspective on the synergistic interaction between POMs and metal compounds and offers insights into the design of functionally rich nanomaterials.

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.

Engineering Bipolar Covalent Organic Framework Membranes for Selective Acid Extraction

Nitric acid (HNO3) is a vital industrial chemical, and its recovery from complex waste streams is essential for sustainability and resource optimization. This study demonstrates the effectiveness of bipolar covalent organic framework (COF) membranes with tunable ionic site distributions as a solution for this challenge. The membranes are fabricated by layering anionic COF nanosheets on cationic COF layers, supported by a porous substrate. The resulting membranes exhibit significant rectifying behavior, driven by the asymmetric charge polarity and the intrinsic electric field, which enhances HNO3 transport. The transmembrane diffusion coefficient of 2.74 × 10-5 cm2 s-1 exceeds the self-diffusion rate of NO3 -, leading to increased HNO3 flux and selectivity compared to the individual anionic and cationic COF membranes. The optimized bipolar membrane configuration achieves remarkable separation factors, ranging from 22 to 242,000 for HNO₃, in comparison to other solutes such as HCl, H2SO4, H3PO4, and various metal salts in an eight-component mixed waste stream. This results in a substantial increase in HNO₃ purity, from 12.5% to 94.1% after a single membrane separation. With the broad range of COF materials and the versatility of the proposed membrane design, this work represents a significant advancement in chemical separation technologies.

Reverse filling approach to mixed matrix covalent organic framework membranes for gas separation

Mixed-matrix membranes that combine the merits of polymer and filler materials offer high potential for molecular separations, but precisely engineering the filler phase structure to give full play to the role of filler materials remains challenging. Herein, we explore a reverse-filling approach to fabricate mixed-matrix membranes with continuous and vertically penetrating covalent organic framework channels for CO2 separation. Covalent organic framework nanosheets as building blocks are pre-assembled into a robust and vertically oriented covalent organic framework scaffold via ice templating method, with the subsequent polyimide filling into the scaffold. The scaffold inherits the intrinsic CO2-philic pore structure of nanosheets, which serves as fast and selective CO2 transport channels in the membrane. The resulting membrane exhibits high CO2 permeability of 972 Barrer and CO2/CH4 selectivity of 58, along with long-term stability and scale-up capability. This approach may stimulate the thinking about how to design advanced mixed-matrix membranes.

Enzyme-Mimic Photoinitiated Flow-Polymerization with High Stereoselectivity under Mild Conditions

Enzymatic reactions can achieve efficient flow-polymerization with specificity and high stereoselectivity. However, current enzyme-mimic polymerization systems cannot achieve high stereoregularity in flow reactions under mild conditions. This inefficient chain control may be due to the absence of a specific catalyst structure for the target monomer. This study reports a model of enzyme-mimic catalytic material for the polymerization of a specific monomer. In particular, the specific enzyme-mimic photoinitiated flow-polymerization of benzyl acrylate was realized at 22 °C using zinc porphyrin metal-organic framework (Zn-PMOF) membranes with one-dimensional nanochannels, achieving the efficient synthesis of highly heterotactic polymers. Under visible light irradiation, the zinc porphyrin core on the membrane surface could initiate polymerization, while copper porphyrin MOF with similar structures could not. The specific channel structure of the Zn-PMOF membrane provided space for stereochemical control. Control experiments, density functional theory simulations, and spectroscopic characterizations show that the combination of size effect and channel-monomer interactions realized higher monomer conversion and polymer stereoregularity in the flow reaction. Furthermore, the crystallinity, shear stress, and ionic conductivity of enzyme-mimic polymers were considerably better than those of bulk polymerization products. Thus, this study provides a method for enzyme-mimic polymerization with high stereoselectivity under mild conditions.

Creating Sodium Ion Channels via De Novo Encapsulation of Ionophores for Enhanced Water Energy Harvesting

Biological ion channels achieve remarkable permselectivity and cation discrimination through the synergy of their intricate architectures and specialized ionophores within confined nanospaces, enabling efficient energy conversion. Emulating such selectivity in synthetic nanochannels, however, remains a persistent challenge. To address this, a novel host-guest assembly membrane is developed by incorporating sodium-selective ionophores into a β-ketoenamine-linked covalent organic framework (COF). This design confers exceptional permselectivity and Na+ selectivity, achieving Na+/K+ and Na+/Li+ selectivity ratios of 3.6 and 103, respectively, along with near-perfect Na+/Cl- selectivity under a 0.5 M || 0.01 M salinity gradient. Notably, the membrane dynamically switches its permselectivity to favor anion transport in the presence of high-valent cations (e.g., Ca2+), overcoming limitations such as uphill cation diffusion and back currents observed in conventional cation-selective membranes. This adaptive behavior yields a 4.6-fold increase in output power density in Ca2+-rich environments. These findings advance the design of biomimetic nanochannels with unparalleled ion selectivity and enhanced energy conversion efficiency.

Bioinspired Photo-Thermal Catalytic System Using Covalent Organic Framework-Based Aerogel for Synchronous Seawater Desalination and H2O2 Production

Efficient utilization of solar energy is widely regarded as a crucial solution to addressing the energy crisis and reducing reliance on fossil fuels. Coupling photothermal and photochemical conversion can effectively improve solar energy utilization yet remains challenging. Here, inspired by the photosynthesis system in green plants, we report herein an artificial solar energy converter (ASEC) composed of light-harvesting units as solar collector and oriented ionic hydrophilic channels as reactors and transporters. Based on such architecture, the obtained ASEC (namely ASEC-NJFU-1) can efficiently realize parallel production of freshwater and H2O2 from natural seawater under natural light. The total solar energy conversion (SEC) of ASEC-NJFU-1 reaches up to 8047 kJ m-2 h-1, corresponding to production rates of freshwater and H2O2 are 3.56 kg m-2-1 h-1 and 19 mM m-2 h-1, respectively, which is a record-high value among all photothermal-photocatalytic systems reported to date. Mechanism investigation of combining spectrum and experimental studies indicated that the high SEC performance for ASEC-NJFU-1 was attributed to the presence of plant bioinspired architecture with carbon nanotubes as solar-harvestor and COF-based oriented aerogel as reactors and transporters. Our work thus establishes a novel artificial photosynthesis system for highly efficient solar energy utilization.

Biomass-Derived Carbon and Carbon Nanofiber-Integrated Electrospun Janus Membranes: A New Frontier in Membrane Distillation

Membrane distillation (MD) is an emerging desalination technique that uses low-grade energy to extract water vapor from saline solutions. In a thermally driven MD system, achieving a lower heat transfer and a higher mass transportation rate is desirable. To balance the trade-off between heat transfer and mass transportation, we developed novel dual-layered electrospun Janus nanofibrous membranes in this study, showing asymmetric wettability on each layer. The developed Janus membrane was constructed with a bottom hydrophilic layer composed of PVDF-co-HFP/biomass-derived jute carbon (JC) particles, and the top hydrophobic layer was formed using PH/carbon nanofibers (PH/CNF). The effect of distinct carbon nanoparticles on the prepared membranes was investigated by analyzing their chemical structure, morphology, water contact angle (WCA), pore size, porosity, thickness, liquid entry pressure, and mechanical and thermal stability. The hydrophobic layer of the optimized Janus membrane exhibited a WCA of 138 ± 1°, and the hydrophilic surface showed 72 ± 4°. Additionally, the optimized Janus membrane composed of a hydrophobic PH/0.5 wt % CNF layer and PH/0.5 wt % JC hydrophilic layer experienced an outstanding improvement in water flux (with 70 g L-1 of NaCl content), reaching to a value of 71.72 kg m-2 h-1 (∼162% improvement compared to the pristine PH membrane), while maintaining a salt rejection of >99.99% for 24 h of water gap membrane distillation. Notably, the optimum Janus PH-0.5CNF/PH-0.5JC membrane demonstrated an astonishing long-term stability with real seawater, exhibiting a remarkable flux of 78.42 kg m-2 h-1, which is ∼547% higher than commercially available PVDF membranes, while maintaining a salt rejection of 99.98% after 50 h. The proposed strategies provide a novel approach to fabricate an electrospun Janus membrane, and their performance highlights a strong potential candidate to be used in commercial water desalination plants.

Stabilizing Li-Metal Electrode via Anion-Induced Desolvation in a Covalent Organic Framework Separator

Although Li-metal batteries have been widely used as high-capacity batteries, they are highly susceptible to electrolytes that lead to dendritic or dead Li growth, which significantly reduces the stability of Li-metal electrodes. Herein, we report an anionic covalent organic framework (sulfonate COF: Bd-COF) as a Li+-solvate dissociator that strips solvent molecules from encapsulated Li+ to stabilize Li-metal electrodes. The homogeneous and dense ionic COF separator was prepared using a template-assisted interface in-suit polymerization engineering. Notably, the well-developed anionic groups within the COF channels could as counter-charge ligands to Li+, that adsorb Li+-solvates and induce their partial desolvation. Meanwhile, the ordered anionic groups on the surface of COF pores provide continuous ion channels for Li+ migration, facilitating the removal of solvent molecules from Li+-solvated species. Combined with the dense nanoporous feature, the COF membrane was found to be effective in suppressing Li-dendrites and parasitic reactions. The Bd-COF/Celgard membrane realizes uniform Li deposition on Li-metal electrodes, exhibiting excellent cycling performance in Li-symmetric batteries and high-voltage Li-metal batteries with LiNi0.6Mn0.2Co0.2O2 cathodes, showcasing the application prospects of ion-conductive covalent organic frameworks in lithium battery separators.

In/Outside Catalytic Sites of the Pore Walls in One-Dimensional Covalent Organic Frameworks for Oxygen Reduction Reaction

Pore channels play a decisive role in mass transport in catalytic systems. However, the influences of the location of catalytic sites inside or outside of the pore walls on the performance were still under-explored due, because it is difficult to construct sites anchored in or outside of pore walls. Herein, one-dimensional covalent organic frameworks with precisely anchored active sites were used to explore the effects of channels on a typical oxygen reduction reaction (ORR) catalysis. Electrocatalytic evaluations showed that single Pt sites located inside of the channels exhibited higher kinetic activity compared to those anchored outside. The in situ spectroscopic analysis revealed that local reconstruction of Pt-Cl breaking and potential-induced anion transport occurred more effectively inside the channels. The superior anion transportability and kinetic activity of the inside-channel active sites also facilitated *OH desorption during the ORR process outperforming their outside-channel counterparts. The results of this study provide strategies for designing active sites in porous catalysts for heterogeneous catalysis.

Precise Separation of Complex Ultrafine Molecules through Solvating Two-Dimensional Covalent Organic Framework Membranes

Isoporous nanomaterials, with their proven potential for accurate molecular sieving, are of substantial interest in propelling sustainable membrane techniques. Covalent organic frameworks (COFs) are prominent due to their customizable isopores and chemistry. Still, the discrepancy in experimental and theoretical structures poses a challenge to developing COF membranes for molecular separations. Here, we report high-selectivity sieving of complex ultrafine molecules through solvating pore-to-pore-aligned two-dimensional COF membranes. Our structurally oriented membrane shows reversible interlayer expansion with intralayer shift in response to solvent exposure. This dynamic deformation induced by solvents leads to a reduction in the aperture of the membrane's sieving pores, which correlates with the number of COF layers. The resultant membranes yield largely improved molecular selectivity to discriminate binary and trinary complex mixtures, exceeding the conventional COF membranes. The membrane's robustness against solvents and physical aging permits extremely stable microporosity and reliable operation for over 3000 h. This exceptional performance positions our membrane as an alternative to enriching and purifying value-added chemicals, such as active pharmaceutical ingredients.

Engineered Surface Wettability of Nanomaterials for Efficient Uranium Extraction from Seawater

The extraction of uranium from seawater offers a sustainable pathway to secure nuclear fuel supplies, crucial for the transition to low-carbon energy systems. However, the low concentration of uranium and interference from competing ions pose significant challenges to extraction efficiency. Surface wettability engineering has become a key factor in enhancing the performance of nanomaterials. In this Perspective, we explore how surface wettability influences the performance of the nanomaterials in three uranium extraction scenarios: chemical adsorption, electro-assisted enhanced adsorption, and photo/electrocatalytic reduction. Strategies for optimizing this property are discussed, alongside recommendations and future directions in material design and characterization methods, aiming to accelerate the practical application of nanomaterials in seawater uranium extraction.

Industrialization of Covalent Organic Frameworks

Covalent organic frameworks (COFs) have attracted broad interest because of their well-defined, customizable, highly stable, and porous structures. COFs have shown significant potential for various practical applications, such as gas storage/purification, drug purification, water treatment, catalysis, and battery applications. Scaling up COFs is highly desirable to meet industrial application demands but is hindered by the limitations of synthesis methods and the high cost of reactants. Recently, emerging green synthesis methods, such as mechanochemical synthesis and flux synthesis, have offered promising solutions to these challenges (e.g., ton-scale production of COFs has been achieved by companies recently). This Perspective provides an overview of the state of the art with respect to the industrial production of COFs and discusses factors influencing the large-scale production of COFs. Directions and opportunities for improving the performance and sustainability of COFs toward industrial applications are also emphasized.

Tillandsia-Inspired Asymmetric Covalent Organic Framework Membranes for Unidirectional Low-Friction Water Collection

Friction plays a pivotal role in many phenomena of physical chemistry and has long been in the focus of research thereof. As a crucial parameter, friction in membranes' inner and/or outer surface can be minimized to reduce solvent inlet resistance and enlarge inner pore fluid flux, ideally reaching near frictionless transport of water at nanoscale. Inspired by the leaf structure of Tillandsia, a porous membrane with a rough surface and a hydrophilic inlet together with hydrophobic pore channels was designed and fabricated, based on covalent organic frameworks (COFs). Combined with COFs' inherent highly oriented pore structures, the as-made asymmetric membranes through chemical etching can minimize the solvent critical intrusion pressure and enable inner pore low friction water transport. Ultimately, obtained COF membranes succeeded in trapping fog from air and achieved a water harvesting rate (WHR) of 1570 mg cm-2 h-1, together with small molecular pollutants filtrated off in the meantime. Intriguingly, the synthesized asymmetric COF membranes illustrated unidirectional low friction water collecting and transporting features, the successful imitation of T. macdougallii. This work presents a practical strategy to construct functional porous membranes for low friction water collection and transport, and created a model paradigm to design fluid transporting pore channels.

Three-Dimensional Covalent Organic Framework with Dense Lithiophilic Sites as Protective Layer to Enable High-Performance Lithium Metal Battery

Lithium (Li) metal batteries with remarkable energy densities are restrained by short lifetime and low Coulombic efficiency (CE), resulting from the accumulative Li dendrites and dead Li during cycling. Here, we prepared a new three-dimensional (3D) covalent organic framework (COF) with dense lithiophilic sites (heteoatom weight contents of 32.32 wt %) as an anodic protective layer of Li metal batteries. The 3D COF was synthesized using a [6+4] synthesis strategy by inducing flexible 6-connected cyclotriphosphazene derivative aldehyde and 4-connected porphyrin-based tetraphenylamines. Both phosphazene and porphyrin rings in the COF served as electron-rich and lithiophilic sites, enhancing a homogeneous Li+ flux via 3D direction towards highly smooth and compact Li deposition. The Li/Por-PN-COF-Cu cells achieved a record average CE of 99.1 % for 320 cycles with smooth Li deposition. Meanwhile, the abundant lithiophilic sites can promote fast Li+ transport with Li+ transference number of 0.87, enabling LiFePO4 full cell with stable stripping/plating processes even at a harsh rate of 5 C. Theoretical calculations revealed that the strong interaction force between Li+ and the COF facilitated the dissolution of Li+ from the electrolyte, and the low migration barrier of 1.08 eV indicated a favorable interaction between the Li+ ions and the π-electron system.

3D Covalent Organic Framework Membrane with Interactive Ion Nanochannels for Hydroxide Conduction

Crystalline porous materials, known for their ordered structures, hold promise for efficient hydroxide conductivity in alkaline fuel cells with limited ionic densities. However, the rigid cross-linking of porous materials precludes their processing into membranes, while composite membranes diminish materials' conductivity advantage due to the interrupted phases. Here, we report a self-standing three-dimensional covalent organic framework (3D COF) membrane with efficient OH-transport through its interconnected 3D ionic nanochannels. The large-area, homogeneously connected COF membrane, with an 8 cm diameter and 20 μm thickness, was prepared using an interface polymerization strategy assisted by sacrificial templates of a polyacrylonitrile membrane. At the microscopic level, the introduction of imidazolium salt-building units resulted in a noninterpenetrated structure of 3D COF, creating a 3D interactive continuous hydrophilic channel for OH--conduction. The 3D COF membrane demonstrated high conductivity (169 mS/cm at 80 °C, 100% humidity) and achieved a peak power density of 160 mW/cm2 in H2/O2 single-cell tests. This COF interface polymerization strategy brings new possibilities to address the challenges of porous material membrane formation and is expected to advance their practical applications in the field of ion transport.

Determining Covalent Organic Framework Structures Using Electron Crystallography and Computational Intelligence

The structural characterization of new materials often poses immense challenges, especially when obtaining single-crystal structures is difficult, which is a common difficulty with covalent organic frameworks (COFs). Despite this, understanding the atomic structure is crucial as it provides insights into the arrangement and connectivity of organic building blocks, offering the opportunity to establish the correlation of structure-function relationships and unravel material properties. In this study, we present an approach for determining the structures of COFs, an integration of electron crystallography and computational intelligence (COF+). By applying established chemistry knowledge and employing particle swarm optimization (PSO) for trial structure generation, we overcome existing limitations, thus paving the way for advancements in COF structural determination. We have successfully implemented this technique on four representative COFs, each with unique characteristics. These examples underline the accuracy and efficacy of our approach in addressing the challenges tied to COF structural determination. Furthermore, our approach has revealed new structure candidates with different topologies or interpenetrations that are chemically feasible. This discovery demonstrates the capability of our algorithm in constructing trial COF structures without being influenced by topological factors. Our new approach to COF structure determination represents a significant advancement in the field and opens new avenues for exploring the properties and applications of COF materials.

Morphological Tuning of Covalent Organic Framework Single Crystals

The synthesis of high-quality single-crystal covalent organic frameworks (COFs) presents significant challenges, particularly in achieving precise control over their morphologies. Herein, we present a straightforward strategy to fine-tune the morphology of COF single crystals. Using rigid triptycene derivatives as the core building blocks and varying the amounts of aniline modulators, we successfully synthesized a series of high-quality COF single crystals with different aspect ratios and well-defined facets, JUC-663-X (X = 30 to 135, the equivalent of aniline). Their structures were characterized using PXRD, TEM, and N2 adsorption analyses to confirm the structural consistency. The study of the growth mechanism and DFT calculations elucidated the crucial role of aniline as a modulator in facilitating anisotropic competitive binding throughout the crystal growth process. Furthermore, our findings demonstrate that the aspect ratio of these single crystals significantly influences the adsorption properties of Rh B. This research not only paves new paths in the synthesis and morphological control of COF single-crystal materials but also provides profound insights into the relationship between COF morphology and functional performance.

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.

Turing covalent organic framework membranes via heterogeneous nucleation synthesis for organic solvent nanofiltration

Although covalent organic frameworks (COFs) demonstrate notable potential for developing advanced separation membranes, contemporary COF membranes still lack controlled stacking and highly efficient sieving. Here, Turing-architecture COF membranes were constructed by engineering a reaction-diffusion assembly process via heterogeneous nucleation synthesis with tannic acid (TA). TA covalently binds with amine monomers to form a composite precursor with increased reactivity and decreased diffusivity. This altered the pathway of Schiff base reactions with aldehyde monomers, fulfilling suitable reaction-diffusion conditions, and ultimately formed the labyrinthine stripe or spot-patterned Turing COF film with controlled stacking and uniform pore structure. This endows our COF membrane with highly efficient molecule sieving ability for organic solvent nanofiltration while exhibiting a flux that is 621% greater than that of commercial membranes. Thus, this study provides a paradigm for the in situ synthesis of highly efficient COF membranes for diversely sustainable separations.

Azobenzene-Bridged Covalent Organic Frameworks Boosting Photocatalytic Hydrogen Peroxide Production from Alkaline Water: One Atom Makes a Significant Improvement

Covalent organic frameworks (COFs) have been demonstrated as promising photocatalysts for hydrogen peroxide (H2O2) production. However, the construction of COFs with new active sites, high photoactivity, and wide-range light absorption for efficient H2O2 production remains challenging. Herein, we present the synthesis of a novel azobenzene-bridged 2D COF (COF-TPT-Azo) with excellent performance on photocatalytic H2O2 production under alkaline conditions. Notably, although COF-TPT-Azo differs by only one atom (-N=N- vs. -C=N-) from its corresponding imine-linked counterpart (COF-TPT-TPA), COF-TPT-Azo exhibits a significantly narrower band gap, enhanced charge transport, and prompted photoactivity. Remarkably, when employed as a metal-free photocatalyst, COF-TPT-Azo achieves a high photocatalytic H2O2 production rate up to 1498 μmol g-1 h-1 at pH = 11, which is 7.9 times higher than that of COF-TPT-TPA. Further density functional theory (DFT) calculations reveal that the -N=N- linkages are the active sites for photocatalysis. This work provides new prospects for developing high-performance COF-based photocatalysts.

A comprehensive review of covalent organic frameworks (COFs) and their derivatives in environmental pollution control

Covalent organic frameworks (COFs) have gained considerable attention due to their design possibilities as the molecular organic building blocks that can stack in an atomically precise spatial arrangement. Since the inception of COFs in 2005, there has been a continuous expansion in the product range of COFs and their derivatives. This expansion has led to the evolution of three-dimensional structures and various synthetic routes, propelling the field towards large-scale preparation of COFs and their derivatives. This review will offer a holistic analysis and comparison of the spatial structure and synthesis techniques of COFs and their derivatives. The conventional methods of COF synthesis (i.e., ultrasonic chemical, microwave, and solvothermal) are discussed alongside the synthesis strategies of new COFs and their derivatives. Furthermore, the applications of COFs and their derived materials are demonstrated in air, water, and soil pollution management such as gas capture, catalytic conversion, adsorption, and pollutant removal. Finally, this review highlights the current challenges and prospects for large-scale preparation and application of new COFs and the derived materials. In line with the United Nations Sustainable Development Goals (SDGs) and the needs of digital-enabled technologies (AI and machine learning), this review will encompass the future technical trends for COFs in environmental pollution control.

Dual-Asymmetric Janus Membranes Based on Two-Dimensional Nanowebs with Superspreading Surface for High-Performance Desalination

Distillation membranes with hydrophobic surfaces and defined pores are considered a promising solution for seawater desalination. Most existing distillation membranes exhibit three-dimensional (3D) stacking bulk structures, where the zigzag water-repellent channels often lead to limited permeability and high energy costs. Here, we created two-dimensional nanowebs directly from the polymer/sol solution to construct dual-asymmetric Janus membranes. By tailoring the phase separation rate, the polymer phase evolved into continuous hydrophilic webs in situ weld on the microporous hydrophobic layer. The webs possess true-nanoscale architectures (internal fiber diameter of ∼20 nm, pore size of ∼140 nm) with enhanced roughness, serving as a superspreading surface to reach a water contact angle of 0° in 1.7 s. Benefiting from the architecture and wettability dual asymmetries, the obtained Janus membrane shows high-efficiency desalination performance (salt rejection >99%, flux of 11 kg m-2 h-1, and energy efficiency of 79%) with a thickness of 6.7 μm. Such a fascinating nanofibrous web-based Janus membrane may inspire the design of advanced liquid separation materials.

Optimizing selectivity via membrane molecular packing manipulation for simultaneous cation and anion screening

Advancing membranes with enhanced solute-solute selectivity is essential for expanding membrane technology applications, yet it presents a notable challenge. Drawing inspiration from the unparalleled selectivity of biological systems, which benefit from the sophisticated spatial organization of functionalities, we posit that manipulating the arrangement of the membrane's building blocks, an aspect previously given limited attention, can address this challenge. We demonstrate that optimizing the face-to-face orientation of building blocks during the assembly of covalent-organic-framework (COF) membranes improves ion-π interactions with multivalent ions. This optimization leads to extraordinary selectivity in differentiating between monovalent cations and anions from their multivalent counterparts, achieving selectivity factors of 214 for K+/Al3+ and 451 for NO3-/PO43-. Leveraging this attribute, the COF membrane facilitates the direct extraction of NaCl from seawater with a purity of 99.57%. These findings offer an alternative approach for designing highly selective membrane materials, offering promising prospects for advancing membrane-based technologies.

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.

Covalent organic frameworks in tribology - A perspective

Two-dimensional covalent organic frameworks (2D COFs) are an emerging class of crystalline porous materials formed through covalent bonds between organic building blocks. COFs uniquely combine a large surface area, an excellent stability, numerous abundant active sites, and tunable functionalities, thus making them highly attractive for numerous applications. Especially, their abundant active sites and weak interlayer interaction make these materials promising candidates for tribological research. Recently, notable attention has been paid to COFs as lubricant additives due to their excellent tribological performance. Our review aims at critically summarizing the state-of-art developments of 2D COFs in tribology. We discuss their structural and functional design principles, as well as synthetic strategies with a special focus on tribology. The generation of COF thin films is also assessed in detail, which can alleviate their most challenging drawbacks for this application. Subsequently, we analyze the existing state-of-the-art regarding the usage of COFs as lubricant additives, self-lubrication composite coatings, and solid lubricants at the nanoscale. Finally, critical challenges and future trends of 2D COFs in tribology are outlined to initiate and boost new research activities in this exciting field.

Tunable Wettability of a Dual-Faced Covalent Organic Framework Membrane for Enhanced Water Filtration

Membrane technology plays a central role in advancing separation processes, particularly in water treatment. Covalent organic frameworks (COFs) have transformative potential in this field due to their adjustable structures and robustness. However, conventional COF membrane synthesis methods are often associated with challenges, such as time-consuming processes and limited control over surface properties. Our study demonstrates a rapid, microwave-assisted method to synthesize self-standing COF membranes within minutes. This approach allows control over the wettability of the surface and achieves superhydrophilic and near-hydrophobic properties. A thorough characterization of the membrane allows a detailed analysis of the membrane properties and the difference in wettability between its two faces. Microwave activation accelerates the self-assembly of the COF nanosheets, whereby the thickness of the membrane can be controlled by adjusting the time of the reaction. The superhydrophilic vapor side of the membrane results from -NH2 reactions with acetic acid, while the nearly hydrophobic dioxane side has terminal aldehyde groups. Leveraging the superhydrophilic face, water filtration at high water flux, complete oil removal, increased rejection with anionic dye size, and resistance to organic fouling were achieved. The TTA-DFP-COF membrane opens new avenues for research to address the urgent need for water purification, distinguished by its synthesis speed, simplicity, and superior separation capabilities.

Preparation of sp2 carbon-bonded π-conjugated COF aerogels by ultrasound-assisted mild solvothermal reaction for multi-functional applications

Molding COFs into aerogels from monomers can establish interpenetrating spatial network structures on the centimeter scale that increase the accessibility of dominant pore channels and the convenience of real application, which radically gets rid of the difficult reprocessing problems of insoluble and non-fusible powder COFs. However, the construction of bulk COF structures and achieving crystallinity are often incompatible, especially with sp2 carbon-based COFs, whose powder synthesis has been quite demanding. Herein, for the first time, we report an efficient method to prepare sp2 carbon-linked π-conjugated DFB-TMTA-COF (DT-COF) aerogels by an ultrasound-assisted mild solvothermal technique and freeze-drying. Particularly, unlike the typical synthesis methods of vacuum deoxygenation, high temperature and long reaction time, crystalline DT-COF aerogels can be obtained by reacting at 90 °C for 48 h without vacuum sealing. The fluffy, hierarchical porous flower-shaped microsphere clustering of DT-COF aerogels contributes to excellent mechanical properties and better host-guest interactions, which are favorable to utilize the benefits of the highly conjugated structure of channels. As a proof of concept, DT-COF aerogels have been used in absorption, batteries, and sensors, demonstrating enhanced functionality and effectiveness.

Accelerating Anhydrous Proton Transport in Covalent Organic Frameworks: Pore Chemistry and its Impacts

Proton conduction is important in both fundamental research and technological development. Here we report designed synthesis of crystalline porous covalent organic frameworks as a new platform for high-rate anhydrous proton conduction. By developing nanochannels with different topologies as proton pathways and loading neat phosphoric acid to construct robust proton carrier networks in the pores, we found that pore topology is crucial for proton conduction. Its effect on increasing proton conductivity is in an exponential mode other than linear fashion, endowing the materials with exceptional proton conductivities exceeding 10-2 S cm-1 over a broad range of temperature and a low activation energy barrier down to 0.24 eV. Remarkably, the pore size controls conduction mechanism, where mesopores promote proton conduction via a fast-hopping mechanism, while micropores follow a sluggish vehicle process. Notably, decreasing phosphoric acid loading content drastically reduces proton conductivity and greatly increases activation energy barrier, emphasizing the pivotal role of well-developed proton carrier network in proton transport. These findings and insights unveil a new general and transformative guidance for designing porous framework materials and systems for high-rate ion conduction, energy storage, and energy conversion.

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.

Covalent Organic Framework Membranes with Patterned High-Density Through-Pores for Ultrafast Molecular Sieving

The creation of uniformly molecular-sized through-pores within polymeric membranes and the direct evidence of these pores are essential for fundamentally understanding the transport mechanism and improving separation efficiency. Herein, we report an electric-field-assisted interface synthesis approach to fabricating large-area covalent organic framework (COF) membranes that consist of preferentially oriented single-crystalline COF domains. These single-crystalline frameworks were translated into high-density, vertically aligned through-pores across the entire membrane, enabling the direct visualization of membrane pores with an ultrahigh resolution of 2 Å using the low-dose high-resolution transmission electron microscopy technique (HRTEM). The density of directly visualized through-pores was quantified to be 1.2 × 1017 m-2, approaching theoretical predictions. These COF membranes demonstrate ultrahigh solvent permeability, which is 10 times higher than that of state-of-the-art organic solvent nanofiltration membranes. When applied to high-value pharmaceutical separations, their COF membranes exhibit 2 orders of magnitude higher methanol permeance and 20-fold greater enrichment efficiency than their commercial counterparts.

Dynamic Cleavage-Remodeling of Covalent Organic Networks into Multidimensional Superstructures

Superstructures with complex hierarchical spatial configurations exhibit broader structural depth than single hierarchical structures and the associated broader application prospects. However, current preparation methods are greatly constrained by cumbersome steps and harsh conditions. Here, for the first time, a concise and efficient thermally responsive dynamic synthesis strategy for the preparation of multidimensional complex superstructures within soluble covalent organic networks (SCONs) with tunable morphology from 0D hollow supraparticles to 2D films is presented. Mechanism study reveals the thermally responsive dynamic "cleavage-remodeling" characteristics of SCONs, synthesized based on the unique bilayer structure of (2.2)paracyclophane, and the temperature control facilitates the process from reversible solubility to reorganization and construction of superstructures. Specifically, during the process, the oil-water-emulsion two-phase interface can be generated through droplet jetting, leading to the preparation of 0D hollow supraparticles and other bowl-like complex superstructures with high yield. Additionally, by modulating the volatility and solubility of exogenous solvents, defect-free 2D films are prepared relying on an air-liquid interface. Expanded experiments further confirm the generalizability and scalability of the proposed dynamic "cleavage-remodeling" strategy. Research on the enrichment mechanism of guest iodine highlights the superior kinetic mass transfer performance of superstructural products compared to single-hierarchical materials.

Constructing Photocatalytic Covalent Organic Frameworks with Aliphatic Linkers

Photocatalytic covalent organic frameworks (COFs) are typically constructed with rigid aromatic linkers for crystallinity and extended π-conjugation. However, the essential hydrophobicity of the aromatic backbone can limit their performances in water-based photocatalytic reactions. Here, we for the first time report the synthesis of hydrophilic COFs with aliphatic linkers [tartaric acid dihydrazide (TAH) and butanedioic acid dihydrazide] that can function as efficient photocatalysts for H2O2 and H2 evolution. In these hydrophilic aliphatic linkers, the specific multiple hydrogen bonding networks not only enhance crystallization but also ensure an ideal compatibility of crystallinity, hydrophilicity, and light harvesting. The resulting aliphatic linker COFs adopt an unusual ABC stacking, giving rise to approximately 0.6 nm nanopores with an improved interaction with water guests. Remarkably, both aliphatic linker-based COFs show strong visible light absorption, along with a narrow optical band gap of ∼1.9 eV. The H2O2 evolution rate for TAH-COF reaches up to 6003 μmol h-1 g-1, in the absence of sacrificial agents, surpassing the performance of all previously reported COF-based photocatalysts. Theoretical calculations reveal that the TAH linker can enhance the indirect two-electron oxygen reduction reaction for H2O2 production by improving the O2 adsorption and stabilizing the *OOH intermediate. This study opens a new avenue for constructing semiconducting COFs using nonaromatic linkers.

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.

Chemistries and materials for atmospheric water harvesting

Atmospheric water harvesting (AWH) is recognized as a crucial strategy to address the global challenge of water scarcity by tapping into the vast reserves of atmospheric moisture for potable water supply. Within this domain, sorbents lie in the core of AWH technologies as they possess broad adaptability across a wide spectrum of humidity levels, underpinned by the cyclic sorption and desorption processes of sorbents, necessitating a multi-scale viewpoint regarding the rational material and chemical selection and design. This Invited Review delves into the essential sorption mechanisms observed across various classes of sorbent systems, emphasizing the water-sorbent interactions and the progression of water networks. A special focus is placed on the insights derived from isotherm profiles, which elucidate sorbent structures and sorption dynamics. From these foundational principles, we derive material and chemical design guidelines and identify key tuning factors from a structural-functional perspective across multiple material systems, addressing their fundamental chemistries and unique attributes. The review further navigates through system-level design considerations to optimize water production efficiency. This review aims to equip researchers in the field of AWH with a thorough understanding of the water-sorbent interactions, material design principles, and system-level considerations essential for advancing this technology.

Covalent Organic Frameworks for Water Harvesting from Air

Despite approximately 70 % of the earth being covered by water, water shortage has emerged as an urgent social challenge. Sorbent-based atmospheric water harvesting stands out as a potent approach to alleviate the situation, particularly in arid regions. This method requires adsorbents with ample working capacity, rapid kinetics, low energy costs, and long-term stability under operating conditions. Covalent organic frameworks (COFs) are a novel class of crystalline porous materials and offer distinct advantages due to their high specific surface area, structural diversity, and robustness. These properties enable the rational design and customization of their water-harvesting capabilities. Herein, the basic concepts about the water sorption process within COFs, including the parameters that qualitatively or quantitatively describe their water isotherms and the mechanism are summarized. Then, the recent methods used to prepare COFs-based water harvesters are reviewed, emphasizing the structural diversity of COFs and presenting the common empirical understandings of these endeavors. Finally, challenges and research concepts are proposed to help develop next-generation COFs-based water harvesters.

Solvent-Responsive Nonporous Adaptive Crystals Derived from Pyridinium Hydrochloride and the Application in Iodine Adsorption

Nonporous adaptive crystals (NACs) are crystalline nonporous materials that can undergo a structural adaptive phase transformation to accommodate specific guest via porous cavity or lattice voids. Most of the NACs are based on pillararenes because of their flexible backbone and intrinsic porous structure. Here a readily prepared organic hydrochloride of 4-(4-(diphenylamino)phenyl)pyridin-1-ium chloride (TPAPyH), exhibiting the solvent dimension-dependent adaptive crystallinity is reported. Wherein it forms a nonporous α crystal in a solvent with larger dimensions, while forming two porous β and γ crystals capable of accommodating solvent molecules in solvent with small size. Furthermore, the thermal-induced single-crystal-to-single-crystal (SCSC) transition from the β to α phase can be initiated. Upon exposure to iodine vapor or immersion in aqueous solution, the nonporous α phase transforms to porous β phase by adsorbing iodine molecules. Owing to the formation of trihalide anion I2Cl- within the crystal cavity, TPAPyH exhibits remarkable performance in iodine storage, with a high uptaking capacity of 1.27 g g-1 and elevated iodine desorption temperature of up to 110 and 82 °C following the first and second adsorption stage. The unexpected adaptivity of TPAPyH inspires the design of NACs for selective adsorption and separation of volatile compound from organic small molecules.

Harnessing Solar-Salinity Synergy with Porphyrin-Based Ionic Covalent-Organic-Framework Membranes for Enhanced Ionic Power Generation

Nature seamlessly integrates multiple functions for energy conversion, utilizing solar energy and salinity gradients as the primary drivers for ionic power generation. The creation of artificial membranes capable of finely controlling ion diffusion within nanoscale channels, driven by diverse forces, remains a challenging endeavor. In this study, we present an innovative approach: an ionic covalent-organic framework (COF) membrane constructed using chromophoric porphyrin units. The incorporation of ionic groups within these nanoconfined channels imparts the membrane with exceptional charge screening capabilities. Moreover, the membrane exhibits photoelectric responsivity, enhancing the ion conductivity upon exposure to light. As a result, this leads to a substantial increase in the output power density. In practical terms, when subjected to a salinity gradient of 0.5/0.01 M NaCl and exposed to light, the device achieved an outstanding peak power density of 18.0 ± 0.9 W m-2, surpassing the commercial benchmark by 3.6-fold. This innovative membrane design not only represents a significant leap forward in materials science but also opens promising avenues for advancing sustainable energy technologies.

Redox Molecular Junction Metal-Covalent Organic Frameworks for Light-assisted CO2 Energy Storage

Visible-light sensitive and bi-functionally favored CO2 reduction (CRR)/evolution (CER) photocathode catalysts that can get rid of the utilization of ultraviolet light and improve sluggish kinetics is demanded to conquer the current technique-barrier of traditional Li-CO2 battery. Here, a kind of redox molecular junction sp2c metal-covalent organic framework (i.e. Cu3-BTDE-COF) has been prepared through the connection between Cu3 and BTDE and can serve as efficient photocathode catalyst in light-assisted Li-CO2 battery. Cu3-BTDE-COF with redox-ability, visible-light-adsorption region, electron-hole separation ability and endows the photocathode with excellent round-trip efficiency (95.2 %) and an ultralow voltage hysteresis (0.18 V), outperforming the Schiff base COFs (i.e. Cu3-BTDA-COF and Cu3-DT-COF) and majority of the reported photocathode catalysts. Combined theoretical calculations with characterizations, Cu3-BTDE-COF with the integration of Cu3 centers, thiazole and cyano groups possess strong CO2 adsorption/activation and Li+ interaction/diffusion ability to boost the CRR/CER kinetics and related battery property.

Enhancing Sustainable Energy Conversion Efficiency by Incorporating Photoelectric Responsiveness into Multiporous Ionic Membrane

The evolution of porous membranes has revitalized their potential application in sustainable osmotic-energy conversion. However, the performance of multiporous membranes deviates significantly from the linear extrapolation of single-pore membranes, primarily due to the occurrence of ion-concentration polarization (ICP). This study proposes a robust strategy to overcome this challenge by incorporating photoelectric responsiveness into permselective membranes. By introducing light-induced electric fields within the membrane, the transport of ions is accelerated, leading to a reduction in the diffusion boundary layer and effectively mitigating the detrimental effects of ICP. The developed photoelectric-responsive covalent-organic-framework membranes exhibit an impressive output power density of 69.6 W m-2 under illumination, surpassing the commercial viability threshold by ≈14-fold. This research uncovers a previously unexplored benefit of integrating optical electric conversion with reverse electrodialysis, thereby enhancing energy conversion efficiency.

Photo-Induced Geometry and Polarity Gradients in Covalent Organic Frameworks Enabling Fast and Durable Molecular Separations

Azobenzene, which activates its geometric and chemical structure under light stimulation enables noninvasive control of mass transport in many processes including membrane separations. However, producing azobenzene-decorated channels that have precise size tunability and favorable pore wall chemistry allowing fast and durable permeation to solvent molecules, remains a great challenge. Herein, an advanced membrane that comprises geometry and polarity gradients within covalent organic framework (COF) nanochannels utilizing photoisomerization of azobenzene groups is reported. Such functional variations afford reduced interfacial transfer resistance and enhanced solvent-philic pore channels, thus creating a fast solvent transport pathway without compromising selectivity. Moreover, the membrane sets up a densely covered defense layer to prevent foulant adhesion and the accumulation of cake layer, contributing to enhanced antifouling resistance to organic foulants, and a high recovery rate of solvent permeance. More importantly, the solvent permeance displays a negligible decline throughout the long-term filtration for over 40 days. This work reports the geometry and polarity gradients in COF channels induced by the conformation change of branched azobenzene groups and demonstrates the strong capability of this conformation change in realizing fast and durable molecular separations.

Constructing spherical-beads-on-string structure of electrospun membrane to achieve high vapor flux in membrane distillation

Hydrophobic membranes with a reentrant-like structure have shown high hydrophobicity and high anti-wetting properties in membrane distillation (MD). Here, PVDF spherical-beads-on-string (SBS) fibers were electrospun on nonwoven fabric and used in the MD process. Such a reentrant-like structure was featured with fine fibers, a low ratio of bead length to bead diameter, and high bead frequency. It was revealed that the SBS-structured membranes exhibited an exceptional capability for vapor flux, due to the formation of a network of more interconnected macropores than that of fibers and fusiform-beads-on-string structures, ensuring unimpeded vapor diffusion. In the desalination of formulated seawater (3.5 wt.% NaCl solution), a vapor flux of 61 ± 3 kg m-2 h-1 with a salt rejection of >99.98 % was achieved at a feed temperature of 60 °C. Furthermore, this SBS structured membrane showed satisfactory seawater desalination performance with a stable flux of 40 kg m-2 h-1 over a 27 h MD process. These findings suggest a viable approach for fabricating SBS-structured membranes that significantly enhance vapor flux in MD for desalination applications. Besides, the hydrophobic membranes with SBS structure can be prepared by single-step electrospinning, and it is facile to scale-up manufacture. This strategy holds promise for advancing the development of high-performance MD membranes tailored for efficient seawater desalination processes.

Crosslinking and fluorination reinforced PTFE nanofibrous membrane with excellent amphiphobic performance for low-scaling membrane distillations

Membrane distillation (MD) has emerged as a promising technology for desalination and concentration of hypersaline brine. However, the efficient preparation of a structurally stable and salinity-resistant membrane remains a significant challenge. In this study, an amphiphobic polytetrafluoroethylene nanofibrous membrane (PTFE NFM) with exceptional resistance to scaling has been developed, using an energy-efficient method. This innovative approach avoids the high-temperature sintering treatment, only involving electrospinning with PTFE/PVA emulsion and subsequent low-temperature crosslinking and fluorination. The impact of the PVA and PTFE contents, as well as the crosslinking and subsequent fluorination on the morphology and MD performance of the NFM, were systematically investigated. The optimized PTFE NFM displayed robust amphiphobicity, boasting a water contact angle of 155.2º and an oil contact angle of 132.7º. Moreover, the PTFE NFM exhibited stable steam flux of 52.1 L·m-2·h-1 and 26.7 L·m-2·h-1 when fed with 3.5 wt % and 25.0 wt % NaCl solutions, respectively, and an excellent salt rejection performance (99.99 %, ΔT = 60 °C) in a continuous operation for 24 h, showing exceptional anti-scaling performance. It also exhibited stable anti-wetting and anti-fouling properties against surfactants (sodium dodecyl sulfate) and hydrophobic contaminants (diesel oil). These results underscore the significant potential of the PTFE nanofibrous membrane for practical applications in desalination, especially in hypersaline or polluted aqueous environments.

Solar-driven abnormal evaporation of nanoconfined water

Intrinsic water evaporation demands a high energy input, which limits the efficacy of conventional interfacial solar evaporators. Here, we propose a nanoconfinement strategy altering inherent properties of water for solar-driven water evaporation using a highly uniform composite of vertically aligned Janus carbon nanotubes (CNTs). The water evaporation from the CNT shows the unexpected diameter-dependent evaporation rate, increasing abnormally with decreasing nanochannel diameter. The evaporation rate of CNT10@AAO evaporator thermodynamically exceeds the theoretical limit (1.47 kg m-2 hour-1 under one sun). A hybrid experimental, theoretical, and molecular simulation approach provided fundamental evidence of different nanoconfined water properties. The decreased number of H-bonds and lower interaction energy barrier of water molecules within CNT and formed water clusters may be one of the reasons for the less evaporative energy activating rapid nanoconfined water vaporization.

Large-Area Ultrathin Metal-Organic Framework Membranes Fabricated on Flexible Polymer Supports for Gas Separations

Ultrathin continuous metal-organic framework (MOF) membranes have the potential to achieve high gas permeance and selectivity simultaneously for otherwise difficult gas separations, but with few exceptions for zeolitic-imidazolate frameworks (ZIF) membranes, current methods cannot conveniently realize practical large-area fabrication. Here, we propose a ligand back diffusion-assisted bipolymer-directed metal ion distribution strategy for preparing large-area ultrathin MOF membranes on flexible polymeric support layers. The bipolymer directs metal ions to form a cross-linked two-dimensional (2D) network with a uniform distribution of metal ions on support layers. Ligand back diffusion controls the feed of ligand molecules available for nuclei formation, resulting in the continuous growth of large-area ultrathin MOF membranes. We report the practical fabrication of three representative defect-free MOF membranes with areas larger than 2,400 cm2 and ultrathin selective layers (50-130 nm), including ZIFs and carboxylate-linker MOFs. Among these, the ZIF-8 membrane displays high gas permeance of 3,979 GPU for C3H6, with good mixed gas selectivity (43.88 for C3H6/C3H8). To illustrate its scale-up practicality, MOF membranes were prepared and incorporated into spiral-wound membrane modules with an active area of 4,800 cm2. The ZIF-8 membrane module presents high gas permeance (3,930 GPU for C3H6) with acceptable ideal gas selectivity (37.45 for C3H6/C3H8).

Construction of Layer-Blocked Covalent Organic Framework Heterogenous Films via Surface-Initiated Polycondensations with Strongly Enhanced Photocatalytic Properties

Imine-linked covalent organic frameworks (COFs) usually show high crystallinity and porosity, while vinyl-linked COFs have excellent semiconducting properties and stability. Therefore, achieving the advantages of imine- and vinyl-linkages in a single COF material is highly interesting but remains challenging. Herein, we demonstrate the fabrication of a layer-blocked COF (LB-COF) heterogeneous film that is composed of imine- and vinyl-linkages through two successive surface-initiated polycondensations. In brief, the bottom layer of imine-linked COF film was constructed on an amino-functionalized substrate via Schiff-base polycondensation, in which the unreacted aldehyde edges could be utilized for initiating aldol polycondensation to prepare the second layer of vinyl-linked COF film. The resultant LB-COF film displays excellent ordering due to the crystalline templating effect from the bottom imine-linked COF layer; meanwhile, the upper vinyl-linked COF layer could strongly enhance its stability and photocatalytic properties. The LB COF also presents superior performance in photocatalytic uranium extraction (320 mg g-1), which is higher than the imine-linked (35 mg g-1) and the vinyl-linked (295 mg g-1) counterpart. This study provides a novel surface-initiated strategy to synthesize layer-blocked COF heterogeneous films that combine the advantages of each building block.

High-Strength Polycrystalline Covalent Organic Framework with Abnormal Thermal Transport Insensitive to Grain Boundary

Grain boundaries (GBs) in two-dimensional (2D) covalent organic frameworks (COFs) unavoidably form during the fabrication process, playing pivotal roles in the physical characteristics of COFs. Herein, molecular dynamics simulations were employed to elucidate the fracture failure and thermal transport mechanisms of polycrystalline COFs (p-COFs). The results revealed that the tilt angle of GBs significantly influences out-of-plane wrinkles and residual stress in monolayer p-COFs. The tensile strength of p-COFs can be enhanced and weakened with the tilt angle, which exhibits an inverse relationship with the defect density. The crack always originates from weaker heptagon rings during uniaxial tension. Notably, the thermal transport in p-COFs is insensitive to the GBs due to the variation of minor polymer chain length at defects, which is abnormal for other 2D crystalline materials. This study contributes insights into the impact of GBs in p-COFs and offers theoretical guidance for structural design and practical applications of advanced COFs.

Desirable Strong and Tough Adhesive Inspired by Dragonfly Wings and Plant Cell Walls

The production of wood-based panels has a significant demand for mechanically strong and flexible biomass adhesives, serving as alternatives to nonrenewable and toxic formaldehyde-based adhesives. Nonetheless, plywood usually exhibits brittle fracture due to the inherent trade-off between rigidity and toughness, and it is susceptible to damage and deformation defects in production applications. Herein, inspired by the microstructure of dragonfly wings and the cross-linking structure of plant cell walls, a soybean meal (SM) adhesive with great strength and toughness was developed. The strategy was combined with a multiple assembly system based on the tannic acid (TA) stripping/modification of molybdenum disulfide (MoS2@TA) hybrids, phenylboronic acid/quaternary ammonium doubly functionalized chitosan (QCP), and SM. Motivated by the microstructure of dragonfly wings, MoS2@TA was tightly bonded with the SM framework through Schiff base and strong hydrogen bonding to dissipate stress energy through crack deflection, bridging, and immobilization. QCP imitated borate chemistry in plant cell walls to optimize interfacial interactions within the adhesive by borate ester bonds, boron-nitrogen coordination bonds, and electrostatic interactions and dissipate energy through sacrificial bonding. The shear strength and fracture toughness of the SM/QCP/MoS2@TA adhesive were 1.58 MPa and 0.87 J, respectively, which were 409.7% and 866.7% higher than those of the pure SM adhesive. In addition, MoS2@TA and QCP gave the adhesive good mildew resistance, durability, weatherability, and fire resistance. This bioinspired design strategy offers a viable and sustainable approach for creating multifunctional strong and tough biobased materials.