Weakly Humidity-Dependent Proton-Conducting COF Membranes

Conjugated molecularly imprinted polymers based on covalent organic frameworks: Fluorescent sensing platform for specific capture of urea and elimination of ethyl carbamate

This study described the preparation of an azide covalent organic framework-embedded molecularly imprinted polymers (COFs(azide)@MIPs) platform for urea adsorption and indirect ethyl carbamate (EC) removal from Chinese yellow rice wine (Huangjiu). By modifying the pore surface of COFs using the copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction, COFs(azide) with a high fluorescence quantum yield and particular recognition ability were inventively produced. In order to selectively trap urea, the COFs(azide) were encased in an imprinted shell layer via imprinting technology. With a detection limit (LOD) of 0.016 μg L-1 (R2 = 0.9874), the COFs(azides)@MIPs demonstrated a good linear relationship with urea in the linear range of 0-5 μg L-1. Using real Huangjiu samples, the spiking recovery trials showed the viability of this sensing platform with recoveries ranging from 88.44 % to 109.26 % and an RSD of less than 3.40 %. The Huangjiu processing model system achieved 38.93 % EC reduction by COFs(azides)@MIPs. This research will open up new avenues for the treatment of health problems associated with fermented alcoholic beverages, particularly Huangjiu, while also capturing and removing hazards coming from food.

Tailor-Made Heterocharged Covalent Organic Framework Membrane for Efficient Ion Separation

Pore size sieving, Donnan exclusion, and their combined effects seriously affect ion separation of membrane processes. However, traditional polymer-based membranes face some challenges in precisely controlling both charge distribution and pore size on the membrane surface, which hinders the ion separation performance, such as heavy metal ion removal. Herein, the heterocharged covalent organic framework (COF) membrane is reported by assembling two kinds of ionic COF nanosheets with opposite charges and different pore sizes. By manipulating the stacking quantity and sequence of two kinds of nanosheets, the impact of membrane surface charge and pore size on the separation performance of monovalent and multivalent ions is investigated. For the separation of anions, the effect of pore size sieving is dominant, while for the separation of cations, the effect of Donnan exclusion is dominant. The heterocharged TpEBr/TpPa-SO3H membrane with a positively charged upper layer and a negatively charged bottom layer exhibits excellent rejection of multivalent anions and cations (Ni2+, Cd2+, Cr2+, CrO4 2-, SeO3 2-, etc). The strategy provides not only high-performance COF membranes for ion separation but also an inspiration for the engineering of heterocharged membranes.

Multichannel Nanogenerator-Driven Collaborative Metabolic Fingerprint Diagnostic Strategy for Early Screening and Risk Evaluation of Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD), along with its progressive forms nonalcoholic steatohepatitis (NASH) and NASH fibrosis, has emerged as a global health crisis. However, the absence of robust screening and risk evaluation tools contributes to the underdiagnosis of NAFLD. Herein, we reported a multichannel nanogenerator-assisted laser desorption/ionization mass spectrometry (LDI-MS) platform for early screening and risk evaluation of NAFLD. Specifically, titanium oxide nanosheets (TiNS) and covalent-organic framework nanosheets (COFNS) were employed as nanogenerators with excellent optical properties and exhibited efficient desorption/ionization during the LDI-MS process. Only ∼0.025 μL of serum without pretreatments and separation, serum metabolic fingerprints (SMFs) can be extracted within seconds. Notably, integrated SMFs from TiNS and COFNS significantly improved diagnostic performance and achieved the area under the curve (AUC) values of 1.000 with 100% sensitivity and 100% specificity for the validation sets of global diagnosis, early diagnosis, high-risk NASH, and NASH fibrosis evaluation. Additionally, four biomarker panels were identified, and their diagnostic AUC values were more than 0.944. Ultimately, key metabolic pathways indicating the change from simple NAFLD to high-risk NASH and NASH fibrosis were uncovered. This work provided a noninvasive and high-throughput screening and risk evaluation strategy for NAFLD healthcare management, thus contributing to the precise treatment of the NALFD.

Biomineralization-Inspired Synthesis of Hybrid COF Nanosheets toward Efficient Desalination Membranes

Synthesis of covalent organic framework nanosheets (CONs) with high aspect ratio is crucial to their assembly into advanced membranes. Nonetheless, the π-π stacking between covalent organic framework (COF) layers often leads to thick CONs. Herein, inspired by biomineralization process, a series of aspect ratio CONs >15 000 is synthesized by multifunctional polyelectrolytes which not only provide the nucleation sites for pre-assembly with COF monomer, but also suppress π-π interaction for anisotropic growth through protonation. The membrane assembled from CONs exhibited water permeance of 341 kg m-2 h-1 and salt rejection of 99.5% in desalination, outperforming ever-reported membranes. This method establishes a platform for the synthesis of crystalline nanosheets.

Room-Temperature Superprotonic Conductivity beyond 10-1 S cm-1 in a Co(II) Coordination Polymer

An efficient design of crystalline solid-state proton conductors (SSPCs) is crucial for the progress of clean energy applications. Developing such materials to make them work at room temperature with a conductivity of ≥10-1 S cm-1 is of significant interest in terms of technical and commercial aspects. Utilizing the recently highlighted "coordinated-water-driven proton conduction" approach, herein, we have rationally synthesized two highly stable and scalable 1D Co(II) coordination polymers (CPs) as SSPCs, PCM-2 {[Co(bpy)(H2O)2(NO3)2]·H2O}n and PCM-3 {[Co2(bpy)2(SO4)2(H2O)6].4H2O}n, with distinct alignments in coordinated water and coordinated oxo-anions (nitrate and sulfate, respectively). The acidity of the metal-bound water molecules in PCM-2 is further enhanced through cooperative long-range continuous H bonds with coordinated Brønsted basic nitrates (proton acceptors), leading to ultrahigh superprotonic conductivities even at 25 °C (1.03 × 10-1 S cm-1 under 95% RH), and reached a maximum of 2.99 × 10-1 S cm-1 at 85 °C (95% RH). The conductivity at 25 °C is even higher than that of commercial Nafion 117 (6.74 × 10-2 S cm-1 at 100% RH). The absence of such an H-bonding interaction in PCM-3 (closed loops) resulted in a lesser conductivity of 5.87 × 10-5 S cm-1 (95% RH, 85 °C). PCM-2 represents the first example of SSPC exhibiting conductivity in the order 10-1 S cm-1 at ambient temperature (25 °C) with excellent recyclability.

Fabrication of -SO3H-functionalized polyphosphazene-reinforced proton conductive matrix-mixed membranes

Proton exchange membranes (PEMs) have emerged as very promising membranes for automotive applications because of their notable proton conductivity at low temperatures. These membranes find extensive utilization in fuel cells. Several polymeric materials have been used, but their application is constrained by their expense and intricate synthetic processes. Affordable and efficient synthetic methods for polymeric materials are necessary for the widespread commercial use of PEM technology. The polymeric combination of hexachlorocyclotriphosphazene (HCCP) and 4,4-diamino-2,2-biphenyldisulfonic acid facilitated the synthesis of PP-(PhSO3H)2, a polyphosphazene with built-in -SO3H moieties. Characterization revealed that it was a porous organic polymer with high stability. PP-(PhSO3H)2 exhibited a proton conductivity of up to 8.24 × 10-2 S cm-1 (SD = ±0.031) at 353 K under 98% relative humidity (RH), which was more than two orders of magnitude higher than that of its -SO3H-free analogue, PP-(Ph)2 (2.32 × 10-4 S cm-1) (SD = ±0.019) under identical conditions. Therefore, for application in a PEM fuel cell, PP-(PhSO3H)2-based matrix-mixed membranes (PP-(PhSO3H)2-MMMs) were fabricated by mixing them with polyacrylonitrile (PAN) in various ratios. The proton conductivity could reach up to 6.11 × 10-2 S cm-1 (SD = ±0.0048) at 353 K and 98%RH, when the weight ratio of PP-(PhSO3H)2 : PAN was 3 : 1, the value of which was comparable with those of commercially available electrolytes used in PEM fuel cells. PP-(PhSO3H)2-MMM (3 : 1) had an extended lifetime of reusability. Using phosphazene and bisulfonated multiple-amine modules as precursors, we demonstrated that a porous organic polymer with a highly effective proton-conductive matrix-mixed membrane for PEM fuel cells could be produced readily by an intuitive polymeric reaction.

Ultra-Strong Janus Covalent Organic Framework Membrane with Smart Response to Organic Vapor

Vapor-driven smart Janus materials have made significant advancements in intelligent monitoring, control, and interaction, etc. Nevertheless, the development of ultrafast response single-layer Janus membrane, along with a deep exploration of the smart response mechanisms, remains a long-term endeavor. Here, the successful synthesis of a high-crystallinity single-layer Covalent organic framework (COF) Janus membrane is reported by morphology control. This kind of membrane displays superior mechanical properties and specific surface area, along with excellent responsiveness to CH2Cl2 vapor. The analysis of the underlying mechanisms reveals that the vapor-induced breathing effect of the COF and the stress mismatch of the Janus structure play a crucial role in its smart deformation performance. It is believed that this COF Janus membrane holds promise for complex tasks in various fields.

"All in One" Strategy for Achieving Superprotonic Conductivity by Incorporating Strong Acids into a Robust Imidazole-Linked Covalent Organic Framework

The fabrication of solid-state proton-conducting electrolytes possessing both high performance and long-life reusability is significant but challenging. An "all-in-one" composite, H3PO4@PyTFB-1-SO3H, including imidazole, sulfonic acid, and phosphoric acid, which are essential for proton conduction, was successfully prepared by chemical post-modification and physical loading in the rationally pre-synthesized imidazole-based nanoporous covalent organic framework (COF), PyTFB-1. The resultant H3PO4@PyTFB-1-SO3H exhibits superhigh proton conductivity with its value even highly up to 1.15 × 10-1 S cm-1 at 353 K and 98% relative humidity (RH), making it one of the highest COF-based composites reported so far under the same conditions. Experimental studies and theoretical calculations further confirmed that the imidazole and sulfonic acid groups have strong interactions with the H3PO4 molecules and the synergistic effect of these three groups dramatically improves the proton conductivity properties of H3PO4@PyTFB-1-SO3H. This work demonstrated that by aggregating multiple proton carriers into one composite, effective proton-conducting electrolyte can be feasibly achieved.

Continuous Ultrathin Zwitterionic Covalent Organic Framework Membrane Via Surface-Initiated Polymerization Toward Superior Water Retention

Efficient construction of proton transport channels in proton exchange membranes maintaining conductivity under varied humidity is critical for the development of fuel cells. Covalent organic frameworks (COFs) hold great potential in providing precise and fast ion transport channels. However, the preparation of continuous free-standing COF membranes retaining their inherent structural advantages to realize excellent proton conduction performance is a major challenge. Herein, a zwitterionic COF material bearing positive ammonium ions and negative sulphonic acid ions is developed. Free-standing COF membrane with adjustable thickness is constructed via surface-initiated polymerization of COF monomers. The porosity, continuity, and stability of the membranes are demonstrated via the transmission electron microscopy (TEM), atomic force microscopy (AFM), and scanning electron microscopy (SEM) characterization. The rigidity of the COF structure avoids swelling in aqueous solution, which improves the chemical stability of the proton exchange membranes and improves the performance stability. In the higher humidity range (50-90%), the prepared zwitterionic COF membrane exhibits superior capability in retaining the conductivity compared to COF membrane merely bearing sulphonic acid group. The established strategy shows the potential for the application of zwitterionic COF in the proton exchange membrane fuel cells.

Recent advances in two-dimensional polymers: synthesis, assembly and energy-related applications

Two-dimensional polymers (2DPs) are a class of 2D crystalline polymer materials with definite structures, which have outstanding physical-chemical and electronic properties. They cleverly link organic building units through strong covalent bonds and can construct functional 2DPs through reasonable design and selection of different monomer units to meet various application requirements. As promising energy materials, 2DPs have developed rapidly in recent years. This review first introduces the basic overview of 2DPs, such as their historical development, inherent 2D characteristics and diversified topological advantages, followed by the summary of the typical 2DP synthesis methods recently (including "top-down" and "bottom-up" methods). The latest research progress in assembly and processing of 2DPs and the energy-related applications in energy storage and conversion are also discussed. Finally, we summarize and prospect the current research status, existing challenges, and future research directions of 2DPs.

Recent bio-applications of covalent organic framework-based nanomaterials

Appearing as a new class of functional organic materials, covalent organic frameworks (COFs) have aroused a huge wave of interest in versatile fields ever since they were first proposed in 2005. Thanks to but not limited to their ultralight weights, high surface areas, ordered channels, variable functional groups and well-defined crystal structures, the applications of COF-based biomaterials in the fields of drug loading and delivery, photodynamic therapy, photothermal therapy, bioimaging, etc. are comprehensively summarized and introduced. The existing challenges and future prospects for this emerging but hot research direction are also discussed. It is hoped that this review will serve as a guidance for future research on COFs as multifunctional bioplatforms.

Recent advances of pure/independent covalent organic framework membrane materials: preparation, properties and separation applications

Covalent organic frameworks (COF) are porous crystalline polymers connected by covalent bonds. Due to their inherent high specific surface area, tunable pore size, and good stability, they have attracted extensive attention from researchers. In recent years, COF membrane materials developed rapidly, and a large amount of research work has been presented on the preparation methods, properties, and applications of COF membranes. This review focuses on the research on independent/pure continuous COF membranes. First, based on the membrane formation mechanism, COF membrane preparation methods are categorized into two main groups: bottom-up and top-down. Four methods are presented, namely, solvothermal, interfacial polymerization, steam-assisted conversion, and layer by layer. Then, the aperture, hydrophilicity/hydrophobicity and surface charge properties of COF membranes are summarized and outlined. According to the application directions of gas separation, water treatment, organic solvent nanofiltration, pervaporation and energy, the latest research results of COF membranes are presented. Finally, the challenges and future directions of COF membranes are summarized and an outlook provided. It is hoped that this work will inspire and motivate researchers in related fields.

Covalent Organic Framework Ionomer Steering the CO2 Electroreduction Pathway on Cu at Industrial-Grade Current Density

CO2 electroreduction holds great promise for addressing global energy and sustainability challenges. Copper (Cu) shows great potential for effective conversion of CO2 toward specific value-added and/or high-energy-density products. However, its limitation lies in relatively low product selectivity. Herein, we present that the CO2 reduction reaction (CO2RR) pathway on commercially available Cu can be rationally steered by modulating the microenvironment in the vicinity of the Cu surface with two-dimensional sulfonated covalent organic framework nanosheet (COF-NS)-based ionomers. Specifically, the selectivity toward methane (CH4) can be enhanced to more than 60% with the total current density up to 500 mA cm-2 in flow cells in both acidic (pH = 2) and alkaline (pH = 14) electrolytes. The COF-NS, characterized by abundant apertures, can promote the accumulation of CO2 and K+ near the catalyst surface, alter the adsorption energy and surface coverage of *CO, facilitate the dissociation of H2O, and finally modulate the reaction pathway for the CO2RR. Our approach demonstrates the rational modulation of reaction interfaces for the CO2RR utilizing porous open framework ionomers, showcasing their potential practical applications.

Ambient Aqueous Synthesis of Imine-Linked Covalent Organic Frameworks (COFs) and Fabrication of Freestanding Cellulose Nanofiber@COF Nanopapers

Covalent organic frameworks (COFs) are usually synthesized under solvothermal conditions that require the use of toxic organic solvents, high reaction temperatures, and complicated procedures. Additionally, their insolubility and infusibility present substantial challenges in the processing of COFs. Herein, we report a facile, green approach for the synthesis of imine-linked COFs in an aqueous solution at room temperature. The key behind the synthesis is the regulation of the reaction rate. The preactivation of aldehyde monomers using acetic acid significantly enhances their reactivity in aqueous solutions. Meanwhile, the still somewhat lower imine formation rate and higher imine breaking rates in aqueous solution, in contrast to conventional solvothermal synthesis, allow for the modulation of the reaction equilibrium and the crystallization of the products. As a result, highly crystalline COFs with large surface areas can be formed in relatively high yields in a few minutes. In total, 16 COFs are successfully synthesized from monomers with different molecular sizes, geometries, pendant groups, and core structures, demonstrating the versatility of this approach. Notably, this method works well on the gram scale synthesis of COFs. Furthermore, the aqueous synthesis facilitates the interfacial growth of COF nanolayers on the surface of cellulose nanofibers (CNFs). The resulting CNF@COF hybrid nanofibers can be easily processed into freestanding nanopapers, demonstrating high efficiency in removing trace amounts of antibiotics from wastewater. This study provides a route to the green synthesis and processing of various COFs, paving the way for practical applications in diverse fields.

Bioinspired Heterogeneous Construction of Lignocellulose-Reinforced COF Membranes for Efficient Proton Conduction

The exponential interest in covalent organic frameworks (COFs) arises from the direct correlation between their diverse and intriguing properties and the modular design principle. However, the insufficient interlamellar interaction among COF nanosheets greatly hinders the formation of defect-free membranes. Therefore, developing a methodology for the facile fabrication of these materials remains an enticing and highly desirable objective. Herein, ultrahigh proton conductivity and superior stability are achieved by taking advantage of COF composite membranes where 2D TB-COF nanosheets are linked by 1D lignocellulosic nanofibrils (LCNFs) through π-π and electrostatic interactions to form a robust and ordered structure. Notably, the high concentration of -SO3 H groups within the COF pores and the abundant proton transport paths at COFs-LCNFs interfaces impart composite membranes ultrahigh proton conductivity (0.348 S cm-1 at 80 °C and 100% RH). Moreover, the directional migration of protons along the stacked nanochannels of COFs is facilitated by oxygen atoms on the keto groups, as demonstrated by density functional theory (DFT) calculations. The simple design concept and reliable operation of the demonstrated mixed-dimensional composite membrane are expected to provide an ideal platform for next-generation conductive materials.

Manipulation of Cationic Group Density in Covalent Organic Framework Membranes for Efficient Anion Transport

Anion exchange membranes with high anion conductivity are highly desired for electrochemical applications. Increasing ion exchange capacity is a straightforward approach to enhancing anion conductivity but faces a challenge in dimensional stability. Herein, we report the design and preparation of three kinds of isoreticular covalent organic framework (COF) membranes bearing tunable quaternary ammonium group densities as anion conductors. Therein, the cationic groups are integrated into the backbones by flexible ether-bonded alkyl side chains. The highly quaternary ammonium-group-functionalized building units endow COF membranes with abundant cationic groups homogeneously distributed in the ordered channels. The flexible side chains alleviate electrostatic repulsion and steric hindrance caused by large cationic groups, ensuring a tight interlayer stacking and multiple interactions. As a result, our COF membranes achieve a high ion exchange capacity and exceptional dimensional stability simultaneously. Furthermore, the effect of the ionic group density on the ion conductivity in rigid COF channels is systematically explored. Experiments and simulations reveal that the ionic group concentration and side chain mobility jointly determine the ion transport behavior, resulting in the abnormal phenomenon that the anion conductivity is not positively correlated to the ionic group density. The optimal COF membrane achieves the ever-reported highest hydroxide ion conductivity over 300 mS cm-1 at 80 °C and 100% RH. This study offers insightful guidelines on the rational design and preparation of high-performance anion conductors.

Crystallizing Self-Standing Covalent Organic Framework Membranes for Ultrafast Proton Transport in Flow Batteries

Covalent organic frameworks (COFs) display great potential to be assembled into proton conductive membranes for their uniform and controllable pore structure, yet constructing self-standing COF membrane with high crystallinity to fully exploit their ordered crystalline channels for efficient ionic conduction remains a great challenge. Here, a macromolecular-mediated crystallization strategy is designed to manipulate the crystallization of self-standing COF membrane, where the -SO3 H groups in introduced sulfonated macromolecule chains function as the sites to interact with the precursors of COF and thus offer long-range ordered template for membrane crystallization. The optimized self-standing COF membrane composed of highly-ordered nanopores exhibits high proton conductivity (75 mS cm-1 at 100 % relative humidity and 20 °C) and excellent flow battery performance, outperforming Nafion 212 and reported membranes. Meanwhile, the long-term run of membrane is achieved with the help of the anchoring effect of flexible macromolecule chains. Our work provides inspiration to design self-standing COF membranes with ordered channels for permselective application.

Photo-Driven Quasi-Topological Transformation Exposing Highly Active Nitrogen Cation Sites for Enhanced Photocatalytic H2 O2 Production

Herein, the exposure of highly-active nitrogen cation sites has been accomplished by photo-driven quasi-topological transformation of a 1,10-phenanthroline-5,6-dione-based covalent organic framework (COF), which contributes to hydrogen peroxide (H2 O2 ) synthesis during the 2-electron O2 photoreduction. The exposed nitrogen cation sites with photo-enhanced Lewis acidity not only act as the electron-transfer motor to adjust the inherent charge distribution, powering continuous and stable charge separation, and broadening visible-light adsorption, but also providing a large number of active sites for O2 adsorption. The optimal catalyst shows a high H2 O2 production rate of 11965 μmol g-1  h-1 under visible light irradiation and a remarkable apparent quantum yield of 12.9 % at 400 nm, better than most of the previously reported COF photocatalysts. This work provides new insights for designing photo-switchable nitrogen cation sites as catalytic centers toward efficient solar to chemical energy conversion.

Synergistically Boosting the Circularly Polarized Luminescence of Functionalized Pillar[5]arenes by Polymerization and Aggregation

Supramolecular polymers based on chiral macrocycles have attracted increasing attention in the field of circularly polarized luminescence (CPL) owing to their unique properties. However, the construction of macrocyclic supramolecular polymers with highly efficient CPL properties in aggregate states still remains challenging. Herein, w e constructed a class of macrocycle-based coordination polymers by combining the planar chiral properties of pillar[5]arene with the excellent fluorescence properties of aggregation-induced emission luminogens. The formation of polymers enhances both the fluorescence and chiral properties, resulting in chiral supramolecular polymers with remarkable CPL properties. Increasing the aggregation degree of the polymers can further improve their CPL properties, as evidenced by a 21-fold increase in the dissymmetry factor and an over 25-fold increase in the fluorescence quantum yield in the aggregate state compared to the solution state. Such a synergistic effect of polymerization- and aggregation-enhanced CPL can be explained by the restriction of intramolecular motions and aggregation-induced conformation confinement. This work provides a promising method for developing highly efficient CPL supramolecular polymers.

Hydrothermal Synthesis of Highly Crystalline Zwitterionic Vinylene-Linked Covalent Organic Frameworks with Exceptional Photocatalytic Properties

Ionic covalent organic frameworks (COFs) featuring both crystallinity and ionic characteristics have attracted tremendous attention in recent years. Compared with single anion- or cation-containing ionic COFs, zwitterionic COFs possess unique functionalities beyond single ionic COFs such as tunable charge density and superhydrophilic and highly ion-conductive characteristics, endowing them with huge potential in various applications. However, it remains a considerable challenge to directly synthesize robust, highly crystalline zwitterionic COFs from the original building blocks. Herein, we report a green hydrothermal synthesis strategy to prepare highly crystalline zwitterionic vinylene-linked COFs (ZVCOFs) from the predesigned zwitterionic building block by utilizing 4-dimethylaminopyridine (DMAP) as the high-efficiency catalyst for the first time. Detailed theoretical calculations and experiments revealed that both the high catalytic activity of DMAP and the unique role of water contributed to the formation of highly crystalline ZVCOFs. It was found that the participation of water could not only remarkably reduce the activation energy barrier and thus enhance the reaction reversibility but also enable the hydration of zwitterionic sites and facilitate ordered layered arrangement, which are favorable for the ZVCOF crystallization. Benefiting from the highly π-conjugated structure and hydrophilic characteristic, the obtained ZVCOFs achieved an ultrahigh sacrificial photocatalytic hydrogen evolution rate of 2052 μmol h-1 under visible light irradiation with an apparent quantum yield up to 47.1% at 420 nm, superior to nearly all COF-based photocatalysts ever reported. Moreover, the ZVCOFs could be deposited on a support as a photocatalytic film device, which demonstrated a remarkable photocatalytic performance of 402.1 mmol h-1 m-2 for hydrogen evolution.

Colloidal 2D Covalent Organic Framework-Tailored Nanofiltration Membranes for Precise Molecular Sieving

Covalent organic frameworks (COFs) with tunable pore sizes and ordered structures are ideal materials for engineering nanofiltration (NF) membranes. However, most of the COFs prepared by solvothermal synthesis are unprocessable powders and fail to form well-structured membranes, which seriously hinders the development of COF NF membranes. Herein, colloidal 2D-COFs with processable membrane formation ability were synthesized by oil-in-water emulsion interfacial polymerization technology. COF NF membranes with tailored thickness and surface charge were fabricated via a layer-by-layer (LBL) assembly strategy. The prepared COF NF membrane achieved precise sieving of dye molecules with high permeance (85 L·m-2·h-1·bar-1). In this work, the strategy of prepared COF NF membranes based on colloid 2D-COF LBL assembly is proposed for the first time, which provides a new idea for the on-demand design and preparation of COF membranes for precise molecular sieving.

A 3D Covalent Organic Framework with In-situ Formed Pd Nanoparticles for Efficient Electrochemical Oxygen Reduction

Non-platinum noble metals are highly desirable for the development of highly active, stable oxygen reduction reaction (ORR) electrocatalysts for fuel cells and metal-air batteries. However, how to improve the utilization of non-platinum noble metals is an urgent issue. Herein, a highly efficient catalyst for ORR was prepared through homogeneous loading of Pd precursors by a domain-limited method in a three-dimensional covalent organic framework (COF) followed by pyrolysis. The morphology of the Pd nanoparticles (Pd NPs) was well maintained after carbonization, which was attributed to the rigid structure of the 3D COF. Thanks to the uniform distribution of Pd NPs in the carbon, the catalyst exhibited a remarkable half-wave potential of 0.906 V and a Tafel slope of 70 mV dec-1 in 0.1 M KOH, surpassing the commercial Pt/C catalyst (0.863 V and 75 mV dec-1 ). Furthermore, a maximum power density of 144.0 mW cm-2 was achieved at 252 mA cm-2 , which was significantly higher than the control battery (105.1 mW cm-2 ). This work not only provides a simple strategy for in-situ preparation of highly dispersible metal catalysts in COFs, but also offers new insights into the ORR electrocatalysis.

Continuous Covalent Organic Frameworks Membranes: From Preparation Strategies to Applications

Covalent organic frameworks (COFs) are porous crystalline polymeric materials formed by the covalent bonding of organic units. The abundant organic units library gives the COFs species diversity, easily tuned pore channels, and pore sizes. In addition, the periodic arrangement of organic units endows COFs regular and highly connected pore channels, which has led to the rapid development of COFs in membrane separations. Continuous defect-free and high crystallinity of COF membranes is the key to their application in separations, which is the most important issue to be addressed in the research. This review article describes the linkage types of covalent bonds, synthesis methods, and pore size regulation strategies of COFs materials. Further, the preparation strategies of continuous COFs membranes are highlighted, including layer-by-layer (LBL) stacking, in situ growth, interfacial polymerization (IP), and solvent casting. The applications in separation fields of continuous COFs membranes are also discussed, including gas separation, water treatment, organic solvent nanofiltration, ion conduction, and energy battery membranes. Finally, the research results are summarized and the future prospect for the development of COFs membranes are outlined. More attention may be paid to the large-scale preparation of COFs membranes and the development of conductive COFs membranes in future research.

Water/Vapor Assisted Fabrication of Large-Area Superprotonic Conductive Covalent Organic Framework Membranes

Fabrication of large-area ionic covalent organic framework membranes (iCOMs) remains a grand challenge. Herein, the authors report the liquid water and water vapor-assisted fabrication of large-area superprotonic conductive iCOMs. A mixed monomer solution containing 1,3,5-triformylphloroglucinol (TFP) in 1,4-dioxane and p-diaminobenzenesulfonic acid (DABA) in water is first polymerized to obtain a pristine membrane which subsequently underwent crystallization process in mixed vapors containing water vapor. During the polymerization stage, water played a role of a diluting agent, weakening the Coulombic repulsion between sulfonic acid groups. During the crystallization stage, water vapor played a role of a structure-directing agent to facilitate the formation of highly crystalline, large-area iCOMs. The resulting membranes achieved a proton conductivity value of 0.76 S cm-1 at 90 °C under 100% relative humidity, which is among the highest ever reported. Using liquid water and water vapor as versatile additives open a novel avenue to the fabrication of large-area membranes from covalent organic frameworks and other kinds of crystalline organic framework materials.

3D covalent organic framework membrane with fast and selective ion transport

3D ionic covalent organic framework (COF) membranes, which are envisioned to be able to break the trade-off between ion conductivity and ion selectivity, are waiting for exploitation. Herein, we report the fabrication of a 3D sulfonic acid-functionalized COF membrane (3D SCOF) for efficient and selective ion transport, using dual acid-mediated interfacial polymerization strategy. The 3D SCOF membranes possess highly interconnected ion transport channels, ultramicroporous pore sizes (0.97 nm), and abundant sulfonate groups (with a high ion exchange capacity of 4.1 mmol g-1), leading to high proton conductivity of 843 mS cm-1 at 90 °C. When utilized in osmotic energy conversion, a high power density of 21.2 W m-2, and a remarkable selectivity of 0.976 and thus an exceptional energy conversion efficiency of 45.3% are simultaneously achieved. This work provides an alternative approach to 3D ionic COF membranes and promotes the applications of 3D COFs in ion transport and separation.

Liquid-Liquid Phase Separation of Aqueous Ionic Liquids in Covalent Organic Frameworks for Thermal Switchable Proton Conductivity

Covalent organic frameworks (COFs) have regular channels that can accommodate guest molecules to provide highly conductive solid electrolytes. However, designing smart, conductive COFs remains a great challenge. Herein, we report the first example of PEG-functionalized ionic liquids (ILs) anchored on the COF walls by strong hydrogen bonding to fabricate thermally responsive COFs (ILm@COF). We found that similar to the traditional IL/water mixture, the ILs undergo lower critical solution temperature (LCST)-type phase behavior within COF nanopores under high moisture levels. However, the phase separation temperature of aqueous IL decreases in COF channels due to the strong interaction between the IL and COF. Thus, the proton conductivity of ILm@COF can be reversibly switched by phase miscibility and separation in COF nanopores, and there is no obvious decrease even after 20 switching cycles. Our work provides important clues for understanding liquid-liquid phase separation in a confined nanospace and opens a new pathway to switchable proton conductivity.

Recent progress of covalent organic frameworks membranes: Design, synthesis, and application in water treatment

To date, significant efforts have been devoted to eliminating hazardous components to purify wastewater through the development of various nanomaterials. Covalent organic frameworks (COFs), an important branch of the porous crystalline family, possess the peculiarity of ultrahigh surface area, adjustable pore size, and facile functionality. Exciting studies from design fabrication to potential applications in water treatment by COF-based membranes (COMs) have emerged. This review summarizes various preparation strategies and synthesis mechanisms for COMs, including layer-by-layer stacking, in situ growth, interfacial polymerization, and electrochemical synthesis, and briefly describes the advanced characterization techniques for COMs. Moreover, the application of COMs in heavy metal removal, dye separation, purification of radionuclides, pollutant detection, sea water desalination, and so on, is described and discussed. Finally, the perspectives on future opportunities for designing COMs in water purification have been proposed.

Locating Hydrogen Positions for COF-300 by Cryo-3D Electron Diffraction

Covalent organic frameworks (COFs) have wide-ranging applications, and their host-guest interactions play an essential role in the achievement of COF functions. To investigate these host-guest interactions, it is necessary to locate all atoms, especially hydrogen atoms. However, it is difficult to determine the hydrogen atomic positions in COFs because of the complexities in synthesizing high-quality large single crystals. Three-dimensional electron diffraction (3D ED) has unique advantages for the structural determination of nanocrystals and identification of light atoms. In this study, it was demonstrated for the first time that the hydrogen atoms of a COF, not only on the framework but also on the guest molecule, can be located by 3D ED using continuous precession electron diffraction tomography (cPEDT) under cryogenic conditions. The host-guest interactions were clarified with the location of the hydrogen atoms. These findings provide novel insights into the investigation of COFs.

Recent Progress of Crystalline Porous Frameworks for Intermediate-Temperature Proton Conduction

Proton exchange membranes (PEMs), especially for work under intermediate temperatures (100-200 °C), have attracted great interest because of the high CO toleration and facial water management of the corresponding proton exchange membrane fuel cells (PEMFCs). Traditional polymer PEMs faced challenges of low stability and proton carrier leaking. Crystalline porous materials, such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), are promising to overcome these issues contributed by nanometer-sized channels. Herein we summarized the recent development of MOF/COF-based intermediate-temperature proton conductors. The strategies of framework engineering and pore impregnation were introduced in detail for raising proton conductivity. The proton-conducting mechanism was described as well. This spotlight will provide new insight into the fabrication of MOF/COF proton conductors under intermediate-temperature and anhydrous conditions.

Chitosan-Linked Dual-Sulfonate COF Nanosheet Proton Exchange Membrane with High Robustness and Conductivity

2D materials that can provide long-range ordered channels in thin-film form are highly desirable for proton exchange membranes (PEMs). Covalent organic framework nanosheets (CONs) are promising 2D materials possessing intrinsic porosity and high processability. However, the potential of CONs in PEMs is limited by loose sheet stacking and interfacial grain boundary, which lead to unsatisfied mechanical property and discontinuous conduction pathway. Herein, chitosan (CS), a natural polymer with rich NH2 groups, is designed as the linker of dual-sulfonate CONs (CON-2(SO3 H)) to obtain CON-2(SO3 H)-based membrane. Ultrathin CON-2(SO3 H) with high crystallinity and large lateral size is synthesized at water-octanoic acid interface. The high flexibility of CS chains and their electrostatic interactions with SO3 H groups of CON-2(SO3 H) enable effective connection of CON-2(SO3 H), thus endowing membrane dense structure and exceptional stability. The stacked CON-2(SO3 H) constructs regular hydrophilic nanochannels containing high-density SO3 H groups, and the electrostatic interactions between CON-2(SO3 H) and CS form interfacial acid-base pairs transfer channels. Consequently, CON-2(SO3 H)@CS membrane simultaneously achieves superior proton conductivity of 353 mS cm-1 (under 80 °C hydrated condition) and tensile strength of 95 MPa. This work highlights the advantages of proton-conducting porous CON-2(SO3 H) in advanced PEMs and paves a way in fabricating robust CON-based membranes for various applications.

Enhanced Proton Transfer in Proton-Exchange Membranes with Interconnected and Zwitterion-Functionalized Covalent Porous Material Structures

Excellent proton-conductive accelerators are indispensable for efficient proton-exchange membranes (PEMs). Covalent porous materials (CPMs), with adjustable functionalities and well-ordered porosities, show much promise as effective proton-conductive accelerators. In this study, an interconnected and zwitterion-functionalized CPM structure based on carbon nanotubes and a Schiff-base network (CNT@ZSNW-1) is constructed as a highly efficient proton-conducting accelerator by in situ growth of SNW-1 onto carbon nanotubes (CNTs) and subsequent zwitterion functionalization. A composite PEM with enhanced proton conduction is acquired by integrating CNT@ZSNW-1 with Nafion. Zwitterion functionalization offers additional proton-conducting sites and promotes the water retention capacity. Moreover, the interconnected structure of CNT@ZSNW-1 induces a more consecutive arrangement of ionic clusters, which significantly relieves the proton transfer barrier of the composite PEM and increases its proton conductivity to 0.287 S cm^-1 under 95 % RH at 90 °C (about 2.2 times that of the recast Nafion, 0.131 S cm^-1 ). Furthermore, the composite PEM displays a peak power density of 39.6 mW cm^-2 in a direct methanol fuel cell, which is significantly higher than that of the recast Nafion (19.9 mW cm^-2 ). This study affords a potential reference for devising and preparing functionalized CPMs with optimized structures to expedite proton transfer in PEMs.

Functional Separator Enabled by Covalent Organic Frameworks for High-Performance Li Metal Batteries

Uncontrolled ion transport and susceptible SEI films are the key factors that induce lithium dendrite growth, which hinders the development of lithium metal batteries (LMBs). Herein, a TpPa-2SO₃ H covalent organic framework (COF) nanosheet adhered cellulose nanofibers (CNF) on the polypropylene separator (COF@PP) is successfully designed as a battery separator to respond to the aforementioned issues. The COF@PP displays dual-functional characteristics with the aligned nanochannels and abundant functional groups of COFs, which can simultaneously modulate ion transport and SEI film components to build robust lithium metal anodes. The Li//COF@PP//Li symmetric cell exhibits stable cycling over 800 h with low ion diffusion activation energy and fast lithium ion transport kinetics, which effectively suppresses the dendrite growth and improves the stability of Li+ plating/stripping. Moreover, The LiFePO4//Li cells with COF@PP separator deliver a high discharge capacity of 109.6 mAh g^-1 even at a high current density of 3 C. And it exhibits excellent cycle stability and high capacity retention due to the robust LiF-rich SEI film induced by COFs. This COFs-based dual-functional separator promotes the practical application of lithium metal batteries.

Phosphonated Ionomers of Intrinsic Microporosity with Partially Ordered Structure for High-Temperature Proton Exchange Membrane Fuel Cells

High mass transport resistance within the catalyst layer is one of the major factors restricting the performance and low Pt loadings of proton exchange membrane fuel cells (PEMFCs). To resolve the issue, a novel partially ordered phosphonated ionomer (PIM-P) with both an intrinsic microporous structure and proton-conductive functionality was designed as the catalyst binder to improve the mass transport of electrodes. The rigid and contorted structure of PIM-P limits the free movement of the conformation and the efficient packing of polymer chains, resulting in the formation of a robust gas transmission channel. The phosphonated groups provide sites for stable proton conduction. In particular, by incorporating fluorinated and phosphonated groups strategically on the local side chains, an orderly stacking of molecular chains based on group assembly contributes to the construction of efficient mass transport pathways. The peak power density of the membrane electrode assembly with the PIM-P ionomer is 18-379% greater than that of those with commercial or porous catalyst binders at 160 °C under an H₂/O₂ condition. This study emphasizes the crucial role of ordered structure in the rapid conduction of polymers with intrinsic microporosity and provides a new idea for increasing mass transport at electrodes from the perspective of structural design instead of complex processes.

Super-hydrophilic sulfonate-modified covalent organic framework nanosheets for efficient separation and enrichment of glycopeptides

Highly efficient extraction of glycopeptides prior to mass spectrometry detection is extremely crucial for glycoproteomic research, especially in disease biomarker research. Reported here is the first time by applying two-dimensional (2D) covalent organic framework (COFs) nanosheets for highly efficient enrichment of glycopeptides. Particularly, by incorporating hydrophilic monomers through a bottom-up strategy, the 2D COF nanosheets (denoted as NUS-9) displayed an ultra-high graft density of sulfonic groups and super-hydrophilicity. In addition, because of the large surface area, low steric hindrance, high chemical stability, and abundant accessibility sites of 2D COF nanosheets, NUS-9 exhibited remarkable efficiency for glycopeptide enrichment, involving excellent detection sensitivity (0.01 fmol μL^-1), outstanding enrichment capability, and good enrichment selectivity (1:1500, horseradish peroxidase (HRP) tryptic digest to bovine serum albumin (BSA) tryptic digest), and recovery (92.2 ± 2.0%). Moreover, the NUS-9 was able to unambiguously detect 631 endogenous glycopeptides from human saliva, demonstrating an unparalleled high efficiency in glycopeptide enrichment. Gene ontology analyses of proteins from human saliva enriched by NUS-9 demonstrated its potential for comprehensive glycoproteome analysis.

Spacer-Engineered Ionic Channels in Covalent Organic Framework Membranes toward Ultrafast Proton Transport

Side-chain engineering of covalent organic frameworks as advanced ion conductors is a critical issue to be explored. Herein, ionic covalent organic framework membranes (iCOFMs) with spacer-engineered ionic channel are de novo designed and prepared. The ionic channels are decorated with side chains comprising spacers having different carbon chain lengths and the -SO₃ H groups at the end. Attributed to the synergistic contribution from the spacers and the -SO₃ H groups, the iCOFM with moderate-length spacer exhibit the highest through-plane proton conductivity of 889 mS cm^-1 at 90 °C.

Monolayer-Assisted Surface-Initiated Schiff-Base-Mediated Aldol Polycondensation for the Synthesis of Crystalline sp2 Carbon-Conjugated Covalent Organic Framework Thin Films

sp² carbon-conjugated covalent organic frameworks (sp²c-COFs) with superb in-plane π-conjugations, high chemical stability, and robust framework structure are expected to be ideal films/membranes for a wide range of applications including energy-related devices and optoelectronics. However, so far, sp²c-COFs have been mainly limited to microcrystalline powders, and this consequently hampered their performances in devices. Herein, we report a simple and robust methodology to fabricate large-area, free-standing, and crystalline sp²c-COF films (TFPT-TMT and TB-TMT) on various solid substrates (e.g., fluorine-doped tin oxide, aluminum sheet, polyacrylonitrile membrane) by self-assembly monolayer-assisted surface-initiated Schiff-base-mediated aldol polycondensation (namely, SI-SBMAP). The resultant sp²c-COF films show lateral sizes up to 120 cm² and tunable thickness from tens of nanometers to a few micrometers. Owing to the robust framework and highly ordered quasi-1D channels, the sp²c-COF membrane-based osmotic power generator presents an output power density of 14.1 W m^-2 under harsh conditions, outperforming most reported COF membranes as well as commercialized benchmark devices (5 W m^-2). This work demonstrates a simple and robust interfacial methodology for the fabrication of sp²c-COF films/membranes for green energy applications and potential optoelectronics.

Ultramicroporous Covalent Organic Framework Nanosheets with Functionality Pair for Membrane C2 H2 /C2 H4 Separation

Gas separation efficiency of covalent organic framework (COF) membrane can be greatly elevated through precise functionalization. A pair-functionalized COF membrane of 1,3,5-triformylphloroglucinol (TP) and isoquinoline-5,8-diamine (IQD) monomers in two and three nodes is designed and synthesized. TP-IQD is crystallized in a two-dimensional structure with a pore size of 6.5 Å and a surface area of 289 m²  g^-1 . This COF possesses N-O paired groups which cooperatively interact with C₂ H₂ instead of C₂ H₄ . TP-IQD nanosheets of ≈10 μm in width and ≈4 nm in thickness are prepared by mechanical exfoliation; they are further processed with 6FDA-ODA polymer into a hybrid membrane. High porosity and functionality pair of TP-IQD offer the membrane with significantly increased C₂ H₂ permeability and C₂ H₂ /C₂ H₄ selectivity which are 160 % and 430 % higher of pure 6FDA-ODA. The boosted performance demonstrates high efficiency of the pair-functionality strategy for the synthesis of separation-led COFs.

Bio-Inspired Synthetic Hydrogen-Bonded Organic Frameworks for Efficient Proton Conduction

Hydrogen-bonded organic frameworks (HOFs) are a rising class of promising proton-conducting materials. However, they always suffer from the inherent contradiction between chemical stability and proton conduction. Herein, inspired by the self-assembly of lipid bilayer membranes, a series of aminomethylphosphonic acid-derived single-component HOFs are successfully developed with different substituents attached to the phosphonate oxygen group. They remain highly stable in strong acid or alkaline water solutions for one month owing to the presence of charge-assisted hydrogen bonds. Interestingly, in the absence of external proton carriers, the methyl-substituted phosphonate-based HOF exhibits a very high proton conductivity of up to 4.2 × 10^-3  S cm^-1 under 80 °C and 98% relative humidity. This value is not only comparable to that of HOFs consisting of mixed ligands but also is the highest reported in single-component HOFs. A combination of single-crystal structure analysis and density functional theory calculations reveals that the high conductivity is attributed to the strengthened H-bonding interactions between positively charged amines and negatively charged phosphonate groups in the channel of bio-inspired HOFs. This finding demonstrates that the well-defined molecular structure of proton conductors is of great importance in the precise understanding of the relationship between structure and property.

Surfactant-Assisted Sulfonated Covalent Organic Nanosheets: Extrinsic Charge for Improved Ion Transport and Salinity-Gradient Energy Harvesting

Charge-governed ion transport is the vital property of nanofluidic channels for salinity-gradient energy harvesting and other electrochemical energy conversion technologies. 2D nanofluidic channels constructed by nanosheets exhibit great superiority in ion selectivity, but a high ion transport rate remains challenging due to the insufficiency of intrinsic surface charge density in nanoconfinement. Herein, extrinsic surface charge into nanofluidic channels composed of surfactant-assisted sulfonated covalent organic nanosheets (SCONs), which enable tunable ion transport behaviors, is demonstrated. The polar moiety of surfactant is embedded in SCONs to adjust in-plane surface charges, and the aggregation of nonpolar moiety results in the sol-to-gel transformation of SCON solution for membrane fabrication. The combination endows SCON/surfactant membranes with considerable water-resistance, and the designable extrinsic charges promise fast ion transport and high ion selectivity. Additionally, the SCON/surfactant membrane, serving as a power generator, exhibits huge potential in harvesting salinity-gradient energy where corresponding output power density can reach up to 9.08 W m^-2 under a 50-fold salinity gradient (0.5 m NaCl|0.01 m NaCl). The approach to extrinsic surface charge provides new and promising insight into regulating ion transport behaviors.

Single Solution-Phase Synthesis of Charged Covalent Organic Framework Nanosheets with High Volume Yield

Covalent organic framework nanosheets (COF-NSs) are emerging building blocks for functional materials, and their scalable fabrication is highly desirable. Current synthetic methods suffer from low volume yields resulting from confined on-surface/at-interface growth space and complex multiple-phase synthesis systems. Herein, we report the synthesis of charged COF-NSs in open space using a single-phase organic solution system, achieving magnitudes higher volume yields of up to 18.7 mg mL^-1 . Charge-induced electrostatic repulsion forces enable in-plane anisotropic secondary growth from initial discrete and disordered polymers into large and crystalline COF-NSs. The charged COF-NS colloidal suspensions are cast into thin and compact proton exchange membranes (PEMs) with lamellar morphology and oriented crystallinity, displaying outstanding proton conductivity, negligible dimensional swelling, and good H₂ /O₂ fuel cell performance.

Large-area Free-standing Metalloporphyrin-based Covalent Organic Framework Films by Liquid-air Interfacial Polymerization for Oxygen Electrocatalysis

Synthesizing large-area free-standing covalent organic framework (COF) films is of vital importance for their applications but is still a big challenge. Herein, we reported the synthesis of large metalloporphyrin-based COF films and their applications for oxygen electrocatalysis. The reaction of meso-benzohydrazide-substituted metal porphyrins with tris-aldehyde linkers afforded free-standing COF films at the liquid-air interface. These films can be scaled up to 3000 cm² area and display great mechanical stability and structural integrity. Importantly, the Co-porphyrin-based films are efficient for electrocatalytic O₂ reduction and evolution reactions. A flexible, all-solid-state Zn-air battery was assembled using the films and showed high performance with a charge-discharge voltage gap of 0.88 V at 1 mA cm^-2 and high stability under bent conditions (0° to 180°). This work thus presents a strategy to synthesize functionalized COF films with high quality for uses in flexible electronics.

Hydrogen Bond Activation by Pyridinic Nitrogen for the High Proton Conductivity of Covalent Triazine Framework Loaded with H3 PO4

Under high temperature anhydrous conditions, it is still a formidable challenge to improve the performance of proton-conducting materials based on H₃ PO₄ and elucidate its proton conduction mechanism. Herein, a highly stable covalent triazine frameworks (CTFs) based on H₃ PO₄ is reported. The more pyridinic nitrogen CTFs contain, the higher proton conductivity is. Compared with H₃ PO₄ @CTF-L with less pyridinic nitrogen, H₃ PO₄ @CTF-H has a higher proton conductivity of 1.6×10^-1  S cm^-1 at 150 °C under anhydrous conditions, which does not decay after about 18 months exposure in air. The high proton conductivity is associated with the formation and breaking of the activated N_triazine ⋯H⁺ ⋯H₂ PO₄ ^- pairs by pyridinic nitrogen of CTFs. The outstanding long-term stability is mainly attributed to the ultra-strong triazine skeleton structure of CTFs.