De Novo Design of Covalent Organic Framework Membranes toward Ultrafast Anion Transport

Microporous poly(aryl piperidinium) hydroxide exchange membranes with multi-directional branched structure for high performance fuel cells

Hydroxide exchange membranes (HEMs) are important materials for energy conversion devices in hydroxide exchange membrane fuel cells (HEMFCs). This study details a series of multi-directional branched HEMs containing octaphenylcyclotetrasiloxane (OCSi). The OCSi structure allows for the establishment of continuous OH- conducting channels within the membrane while addressing the prevailing trade-off between ionic conductivity and size/mechanical stability. Thanks to the formation of fine microphase-separated morphologies, the quaternized poly(octaphenylcyclotetrasiloxane-terphenyl-piperidinium) (QPOCSi-TP-2) membrane has high conductivity (152.9 mS cm-1 at 80 °C), excellent mechanical stability (tensile strength of 76.5 MPa) and outstanding chemical stability (1500 h in 5 M NaOH at 80 °C). In H2/O2 cell tests at 80 °C, the peak power density of the QPOCSi-TP-2 membrane reaches 1.26 W cm-2. During 120 h of operation at 100 m A cm-2, the voltage degradation rate of the cell is 1.02 mV h-1.

Thiazolium-Linked Crystalline Porous Covalent Organic Frameworks for Mixed Electronic-Ionic Transport

Developing efficiently mixed electronic-ionic (MEI) conductive microcosmic pathways within a single functional material is essential yet challenging for electronic devices. Covalent organic frameworks (COFs) feature pre-designed functionalities and uniform pores, making them highly desirable platforms for transporting electrons and ions. However, for MEI conductive COFs, achieving high crystallinity when incorporating high-density ionic groups within the extensively π-electron delocalized skeletons remain a challenge due to intermolecular interactions. Herein, we reported a "pre-polymerization followed by self-ionization" approach to synthesize new thiazolium-linked COFs (MEICOFs, M═Cu, Co, Fe), where the ionic groups synthesized following the connection of building blocks. MEICOFs demonstrated broad ultraviolet-visible-near-infrared absorption bands and narrow bandgaps. As a proof of concept, the mixed electronic and hydroxide ionic conductivity of CuEICOF was determined to be 55.2 and 0.01 S m-1, respectively. Moreover, MEICOFs film could directly catalyze the oxygen reduction reaction without additional conductive agent and the rotation of the electrode.

Effects of Hydration Level and Hydrogen Bonds on Hydroxide Transport Mechanisms in Anion Exchange Membranes

The transport of hydroxide in anion exchange membranes (AEMs) is generally determined by multiple factors, including hydration levels, pore morphologies, and the hydration shells of cationic groups and hydroxides. Thus, clarifying the working mechanisms benefits the proposal of strategies for enhancing the hydroxide transport, thereby enabling a rational design of high-performance AEMs. Herein, by using ReaxFF molecular dynamics (MD) simulations and RDAnalyzer, this study explores the straightforward but effective correlations for steric hindrance versus hydration shell, hydration level versus free/associated diffusion, and strong (short) hydrogen bond (SHB) versus vehicular/Grotthuss diffusion. The theoretical investigations indicate that higher steric hindrance of cationic groups results in less water in the first hydration shell of cationic groups in AEMs. Meanwhile, a higher hydration level facilitates wider hydrophilic pores of AEMs and increases the ratio of the free diffusion mechanism of hydroxides. Interestingly, this study finds a strong correlation between the number of SHBs and the Grotthuss diffusion, thereby enhancing the understanding of the high conductivity of covalent organic framework (COF)-based AEMs that contain obvious SHBs. This work provides a theoretical view for fine-tuning the free/associated and vehicular/Grotthuss transport of hydroxide in AEMs.

Noncovalent Complex Modulated Fabrication of COF Membrane for Organic Solvent Nanofiltration

Covalent organic framework (COF) has been recognized as a disruptive material for fabricating organic molecular sieve membranes. Acquiring crystalline and defect-free COF membranes directly on polymeric substrates is important for practical applications yet is highly challenging. In this study, a noncovalent complex (NCX) modulated fabrication of COF membrane on hydrolyzed polyacrylonitrile (HPAN) substrate via counter diffusion is proposed. The triaminoguanidine chloride-phytic acid noncovalent complex (Tg-PA NCX) is uniformly introduced onto the HPAN substrate, transferring the substrate into an ideal seeding layer and catalytic platform for COF nucleation and growth. Enhanced Tg monomer concentration derived from NCX enhance its concentration near the surface is observed, facilitating the heterogeneous nucleation for COF fabrication. In addition, the PA in the NCX acted as a catalyst, promoting the growth of the highly crystalline COF membrane afterward. The resulting TgTb/NCX/HPAN membrane shows a defect-free surface with high crystallinity. Also, it displays over 90% rejection to various dyes rejection rates (e.g., Evans blue, Methyl blue, Congo red, and Amido Black) and high solvent permeation for organic solvents (e.g., ethanol: 57.9 ± 2.1 L m-2h-1 bar-1, n-hexane: 215.0 ± 2.7 L m-2h-1 bar-1). This work holds great potential as a platform technology for fabricating high-performance organic molecular sieve membranes.

Advanced Anion Exchange Membranes: Structural Insights and Property Optimization

With the increasing demand for clean energy, driven by advancements in science and technology, anion exchange membrane fuel cells (AEMFCs) have emerged as a promising solution for efficient and clean energy conversion. As the core component of AEMFCs, anion exchange membranes (AEMs) are crucial for ion transport and the separation of the cathode and anode. The performance of AEMs primarily depends on two key factors: ionic conductivity and stability, which often require a delicate balance. The ion conduction process is closely linked to the membrane's microscopic structure. This concept article reviews the development of various AEM types, including homogeneous polymer membranes, hybrid membranes, and nanoporous framework membranes, with a focus on their structural characteristics. Additionally, it explores the design and optimization of AEMs in relation to key properties such as ionic conductivity, dimensional stability, and alkali resistance, providing a reference for future innovations in ion-exchange membranes for AEMFCs.

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.

Computational Screening Guiding the Development of a Covalent-Organic Framework-Based Gas Sensor for Early Detection of Lithium-Ion Battery Electrolyte Leakage

This study presents a computationally guided approach for selecting covalent organic frameworks (COFs) for the selective detection of the trace ethylene carbonate (EC) vapor, a key indicator of electrolyte leakage from lithium-ion batteries (LIBs). High-throughput screening, employing grand canonical Monte Carlo (GCMC) simulation complemented by density functional theory (DFT) calculations, was used to identify potential COF candidates from the CURATED COF database. Among the screened materials, an imine COF functionalized with quaternary ammonium (QA) groups, named COF-QA-4, exhibited a high adsorption capacity (5.88 mmol/g) and selectivity of EC vapor. DFT analysis revealed strong molecular interactions driven by a partial charge transfer mechanism between EC and the COF-QA-4 framework, underpinning its superior adsorption properties. Experimental validation through chemiresistive gas sensors fabricated with COF-QA-4 demonstrated excellent sensitivity and reversibility to 1.15 ppmv of EC vapor, maintaining consistent performance over three response-recovery cycles. This work highlights the potential of computationally guided material discovery for advancing sensor technologies in LIB safety monitoring.

Freestanding Phosphonium Covalent Organic Frameworks with Efficient Hydroxide Conduction for Zinc-Air Batteries

Owing to their well-defined crystalline pore structures and ordered functional ionic groups along the skeleton, ionic covalent organic frameworks (iCOFs) exhibit excellent performance and have significant potential for use in energy storage and conversion devices. Herein, we for the first time developed cationic phosphonium COFs with high hydroxide conduction even with low ion exchange capacity (IEC). Specifically, we synthesized COFs containing quaternary phosphonium groups as excellent ion transport moieties. Then, we fabricated freestanding phosphonium membranes through a vapor-assisted method, which exhibited high hydroxide conductivity of 126 mS cm-1 at 80 °C from a minimal IEC of 1.17 mmol g-1. The resulting film was successfully applied to zinc-air batteries, demonstrating energy density of 96.1 mW cm-2, specific capacity of 95.0 mAh cm-2, and stable operation over 2,300 min. Overall, in addition to investigating a novel cationic functional group, we demonstrated a freestanding film formation method of COF-based materials. The findings can provide a solid foundation for advancing the field of iCOFs to ion transport and promoting electrochemical applications.

Facilitating Rapid OH-/H2O Transport in Anion Exchange Membranes via Ultra-Stable Heteroatom-Free Micropores

Efficient OH- conduction in anion exchange membranes (AEMs) is pivotal for the advancement and industrialization of sustainable electrochemical technologies in alkaline environments, including water electrolysis, fuel cells, and CO2 electroreduction. We here designed AEMs with a novel class of rigid heteroatom-free micropores (HFMs), engineered at the molecular level to facilitate rapid ionic transport in an ultra-stable manner. By manipulating monomers, our design strategically controls the torsional angles and energy barriers within the polymeric backbones, creating sub-nanometer ionic channels that precisely regulate porosity. These hydrophilic micropores significantly enhance the mobility of OH-/H2O, achieving over a 150 % increase in self-diffusion coefficient compared to commercial AEMs and elevating OH- conductivity to a leading 215 mS cm-1 at 80 °C. Moreover, the robust carbon-carbon bond construction in HFMs offers the stability that is four orders of magnitude higher under severe alkaline conditions compared to existing wisdoms, with a demonstrated operational lifespan of over 4000 hours. The integration of HFM-AEMs into water electrolyzers not only supports the use of platinum group metal-free catalysts but also exhibits exceptional energy efficiency and extended durability, highlighting their substantial potential for wide-ranging applications in emerging electrochemical technologies.

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.

De Novo Design of Aminopropyl Quaternary Ammonium-Functionalized Covalent Organic Frameworks for Enhanced Polybenzimidazole Anion Exchange Membranes

Quaternary ammonium functionalized covalent organic frameworks (COFs) have great potential to enhance hydroxide transport owing to crystalline ordered 1D nanochannels, however, suffer from limited quaternary ammonium functional monomers and poor membrane-forming ability. In this work, a novel aminopropyl quaternary ammonium-functionalized COF (DCOF) is designed and synthesized via a bottom-up strategy. The self-supporting DCOF membrane exhibits high crystallinity with a dense and orderly arrangement of quaternary ammonium groups (IEC, 2.07 mmol g-1), achieving a high hydroxide conductivity of 172.5 mS cm-1 and an extremely low water swelling of 5.3% at 80 °C. The exfoliated DCOF colloidal suspension is further incorporated into quaternary ammonium di-cation grafted polybenzimidazoles (DPBI) matrix. Molecular simulations reveal strong electrostatic and van der Waals interfacial interactions between DCOF and DPBI, which enable a high doping content of 20 wt.% and interconnected ionic channels through the surface and nanochannels of the DCOF. The DCOF/DPBI-20% membrane exhibits a tensile strength of 29.7 MPa, a hydroxide conductivity of 135.3 mS cm-1, and a low swelling ratio of 37.2% at 80 °C. A H2/O2 single cell assembled with the membrane reaches a peak power density of 323 mW cm- 2, surpassing most recently reported COF-based membranes.

Diffusion limited synthesis of wafer-scale covalent organic framework films for adaptative visual device

Synthesizing high-crystalline covalent organic framework films is highly desired to advance their applications in two-dimensional optoelectronics, but it remains a great challenge. Here, we report a diffusion-limited synthesis strategy for wafer-scale uniform covalent organic framework films, in which pre-deposited 4,4',4″,4‴-(1,3,6,8-Tetrakis(4-aminophenyl) pyrene is encapsulated on substrate surface with a layer of covalent organic framework prepolymer. The polymer not only prevents the dissolution of precursor, but limits the reaction with terephthalaldehyde dissolved in solution, thereby regulating the polymerization process. The size depends on growth substrates, and 4-inch films have been synthesized on silicon chips. Their structure, thickness, patterning and crystallization degree can be controlled by adjusting building blocks and polymerization chemistries, and molybdenum disulfide have been used as substrates to construct vertical heterostructure. The measurements reveal that using covalent organic framework as a photosensitive layer, the heterojunction displays enhanced photoelectric performance, which can be used to simulate the adaptative function of visual system.

Phosphorylated Covalent Organic Framework Membranes Toward Ultrafast Single Lithium-Ion Transport

Developing all-solid-state electrolytes, the chips for lithium-metal batteries, with superior electrochemical and mechanical properties awaits the disruptive materials. Herein, ionic covalent organic framework membranes are explored as solid-state electrolytes for single Li+ conduction. In the membrane, the anion groups act as Li+ transporter, determining Li+ binding capacity and releasing ability, whereas the oxygen-containing groups act as Li+ co-transporter, creating relay sites between adjacent Li+ transporters for rapid hopping. The membrane exhibits an unprecedented Li+ conductivity of 1.7 mS cm-1 with a Li+-transference number close to unity at room temperature. Additionally, the membrane possesses high flexibility, low interfacial resistance, and excellent cycling performance at room temperature. This work paves an unprecedented path for the advancement of next-generation Li+ conductors in solid-state electrolytes.

Regulating the Porosity and Bipolarity of Polyimide-Based Covalent Organic Framework for Advanced Aqueous Dual-Ion Symmetric Batteries

The all-organic aqueous dual-ion batteries (ADIBs) have attracted increasing attention due to the low cost and high safety. However, the solubility and unstable activity of organic electrodes restrict the synergistic storage of anions and cations in the symmetric ADIBs. Herein, a novel polyimide-based covalent organic framework (labeled as NTPI-COF) is constructed, featured with the boosted structure stability and electronic conductivity. Through regulating the porosity and bipolarity integrally, the NTPI-COF possesses hierarchical porous structure (mesopore and micropore) and abundant bipolar active centers (C═O and C─N), which exhibits rapid dual-ion transport and storage effects. As a result, the NTPI-COF as the electrodes for ADIBs deliver a high reversible capacity of 109.7 mA h g-1 for Na+ storage and that of 74.8 mA h g-1 for Cl- storage at 1 A g-1, respectively, and with a capacity retention of 93.2% over 10 000 cycles at 10 A g-1. Additionally, the all-organic ADIBs with symmetric NTPI-COF electrodes achieve an impressive energy density of up to 148 W h kg-1 and a high power density of 2600 W kg-1. Coupling the bipolarity and porosity of the all-organic electrodes applied in ADIBs will further advance the development of low-cost and large-scale energy storage.

Electrostatic Repulsion Facilitated Ion Transport in Covalent-Organic Framework Membranes

Covalent-organic framework (COF) membranes are increasingly used for many potential applications including ion separation, fuel cells, and ion batteries. It is of central importance to fundamentally and quantitatively understand ion transport in COF membranes. In this study, a series of COF membranes is designed with different densities and arrangements of functional groups and subsequently utilize molecular simulation to provide microscopic insights into ion transport in these membranes. The membrane with a single-sided layer exhibits the highest chloride ion (Cl-) conductivity of 77.2 mS cm-1 at 30 °C. Replacing the single-sided layer with a double-sided layer or changing layer arrangement leads to a decrease in Cl- conductivity up to 33% or 53%, respectively. It is revealed that the electrostatic repulsion between ions serves as a driving force to facilitate ion transport and the positions of functional groups determine the direction of electrostatic repulsion. Furthermore, the ordered pores generate concentrated ions and allow rapid ion transport. This study offers bottom-up inspiration on the design of new COF membranes with moderate density and proper arrangement of functional groups to achieve high ion conductivity.

Self-standing membranes for separation: Achievements and opportunities

Supported membranes and mixed matrix membranes have a limitation of harming the mass transfer due to the incompatibility between the support layer or the matrix and the active components of the membrane. Self-standing membranes, which could structurally abandon the support layer, altogether avoid the adverse effect, thus greatly facilitating the transmembrane mass transfer process. However, the abandonment of the support layer also reduces the membrane's mechanical properties and formability. In this review, our emphasis will be on self-standing membranes within the realm of materials and separation engineering. We will explore the materials employed in the fabrication of self-standing membranes, highlighting their ability to simultaneously enhance membrane performance and promote self-standing characteristics. Additionally, we will delve into the diverse techniques utilized for crafting self-standing membranes, encompassing interfacial polymerization, filtration, solvent casting, Langmuir-Blodgett & layer-by-layer assembly, electrospinning, compression, etc. Throughout the discussion, the merits and drawbacks associated with each of these preparation methods were elucidated. We also provide a brief overview of the applications of self-standing membranes, including water purification, gas separation, organic solvent nanofiltration, electrochemistry, and membrane reactor, as well as a brief description of the general strategies for performance enhancement of self-standing membranes. Finally, the current status of self-standing membranes and the challenges they may encounter were discussed.

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.

Structural Modulation of Covalent Organic Frameworks for Efficient Hydrogen Peroxide Electrocatalysis

The electrochemical production of hydrogen peroxide (H2O2) using metal-free catalysts has emerged as a viable and sustainable alternative to the conventional anthraquinone process. However, the precise architectural design of these electrocatalysts poses a significant challenge, requiring intricate structural engineering to optimize electron transfer during the oxygen reduction reaction (ORR). Herein, we introduce a novel design of covalent organic frameworks (COFs) that effectively shift the ORR from a four-electron to a more advantageous two-electron pathway. Notably, the JUC-660 COF, with strategically charge-modified benzyl moieties, achieved a continuous high H2O2 yield of over 1200 mmol g-1 h-1 for an impressive duration of over 85 hours in a flow cell setting, marking it as one of the most efficient metal-free and non-pyrolyzed H2O2 electrocatalysts reported to date. Theoretical computations alongside in situ infrared spectroscopy indicate that JUC-660 markedly diminishes the adsorption of the OOH* intermediate, thereby steering the ORR towards the desired pathway. Furthermore, the versatility of JUC-660 was demonstrated through its application in the electro-Fenton reaction, where it efficiently and rapidly removed aqueous contaminants. This work delineates a pioneering approach to altering the ORR pathway, ultimately paving the way for the development of highly effective metal-free H2O2 electrocatalysts.

Ion transport mechanisms in covalent organic frameworks: implications for technology

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

Ultrafast Interfacial Self-Assembly toward Bioderived Polyester COF Membranes with Microstructure Optimization

The precise manipulation of the microstructure (pore size, free volume distribution, and connectivity of the free-volume elements), thickness, and mechanical characteristics of membranes holds paramount significance in facilitating the effective utilization of self-standing membranes. In this contribution, the synthesis of two innovative ester-linked covalent-organic framework (COF) membranes is first reported, which are generated through the selection of plant-derived ellagic acid and quercetin phenolic monomers in conjunction with terephthaloyl chloride as a building block. The optimization of the microstructure of these two COF membranes is systematically achieved through the application of three different interfacial electric field systems: electric neutrality, positive electricity, and negative electricity. It is observed that the positively charged system facilitates a record increase in the rate of membrane formation, resulting in a denser membrane with a uniform pore size and enhanced flexibility. In addition, a correlation is identified wherein an increase in the alkyl chain length of the surfactants leads to a more uniform pore size and a decrease in the molecular weight cutoff of the COF membrane. The resulting COF membrane exhibits an unprecedented combination of high water permeance, superior sieving capability, robust mechanical strength, chemical robustness for promising membrane-based separation science and technology.

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.

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.

Ultrathin Free-Standing Porous Aromatic Framework Membranes for Efficient Anion Transport

Porous aromatic frameworks (PAFs) show promising potential in anionic conduction due to their high stability and customizable functionality. However, the insolubility of most PAFs presents a significant challenge in their processing into membranes and subsequent applications. In this study, continuous PAF membranes with adjustable thickness were successfully created using liquid-solid interfacial polymerization. The rigid backbone and the stable C-C coupling endow PAF membrane with superior chemical and dimensional stabilities over most conventional polymer membranes. Different quaternary ammonium functionalities were anchored to the backbone through flexible alkyl chains with tunable length. The optimal PAF membrane exhibited an OH- conductivity of 356.6 mS ⋅ cm-1 at 80 °C and 98 % relative humidity. Additionally, the PAF membrane exhibited outstanding alkaline stability, retaining 95 % of its OH- conductivity after 1000 hours in 1 M NaOH. To the best of our knowledge, this is the first application of PAF materials in anion exchange membranes, achieving the highest OH- conductivity and exceptional chemical/dimensional stability. This work provides the possibility for the potential of PAF materials in anionic conductive membranes.

Fabrication of Nitrogen Based Magnetic Conjugated Microporous Polymer for Efficient Extraction of Neonicotinoids in Water Samples

Facile and sensitive methods for detecting neonicotinoids (NEOs) in aquatic environments are crucial because they are found in extremely low concentrations in complex matrices. Herein, nitrogen-based magnetic conjugated microporous polymers (Fe3O4@N-CMP) with quaternary ammonium groups were synthesized for efficient magnetic solid-phase extraction (MSPE) of NEOs from tap water, rainwater, and lake water. Fe3O4@N-CMP possessed a suitable specific surface area, extended π-conjugated system, and numerous cationic groups. These properties endow Fe3O4@N-CMP with superior extraction efficiency toward NEOs. The excellent adsorption capacity of Fe3O4@N-CMP toward NEOs was attributed to its π-π stacking, Lewis acid-base, and electrostatic interactions. The proposed MSPE-HPLC-DAD approach based on Fe3O4@N-CMP exhibited a wide linear range (0.1-200 µg/L), low detection limits (0.3-0.5 µg/L), satisfactory precision, and acceptable reproducibility under optimal conditions. In addition, the established method was effectively utilized for the analysis of NEOs in tap water, rainwater, and lake water. Excellent recoveries of NEOs at three spiked levels were in the range of 70.4 to 122.7%, with RSDs less than 10%. This study provides a reliable pretreatment method for monitoring NEOs in environmental water samples.

Stable Anion Exchange Membrane Bearing Quinuclidinium for High-performance Water Electrolysis

Anion exchange membranes (AEMs) are core components in anion exchange membrane water electrolyzers (AEM-WEs). However, the stability of functional quaternary ammonium cations, especially under high temperatures and harsh alkaline conditions, seriously affects their performance and durability. Herein, we synthesized a 1-methyl-3,3-diphenylquinuclidinium molecular building unit. Density functional theory (DFT) calculations and accelerated aging analysis indicated that the quinine ring structure was exceedingly stable, and the SN2 degradation mechanism dominated. Through acid-catalyzed Friedel-Crafts polymerization, a series of branched poly(aryl-quinuclidinium) (PAQ-x) AEMs with controllable molecular weight and adjustable ion exchange capacity (IEC) were prepared. The stable quinine structure in PAQ-x was verified and retained in the ex situ alkaline stability. Furthermore, the branched polymer structure reduces the swelling rate and water uptake to achieve a tradeoff between dimensional stability and ionic conductivity, significantly improving the membrane's overall performance. Importantly, PAQ-5 was used in non-noble metal-based AEM-WE, achieving a high current density of 8 A cm-2 at 2 V and excellent stability over 2446 h in a gradient constant current test. Based on the excellent alkaline stability of this diaryl-quinuclidinium group, it can be further considered as a multifunctional building unit to create multi-topological polymers for energy conversion devices used in alkaline environments.

Hydroxide Ion Conduction through Viologen-Based Covalent Organic Frameworks (vCOFs): An Approach toward the Advancement

Alkaline fuel cells rely on the movement of hydroxide anions (OH-) for their operation, yet these anions face challenges in efficient conduction due to their limited diffusion coefficient and substantial mass compared to proton (H+) transport. Within the covalent organic framework structure, ordered channels offer a promising solution for the OH- ion transport. Herein, we synthesized a cationic covalent organic framework (vTAPA) via the solvothermal-assisted Zincke reaction. vTAPA showcases excellent stability in harsh basic solution (12 M) and a wide range of pH. This framework facilitates OH- conduction through its one-dimensional network through the anion exchange process. We employed various tertiary ammonium salts (tetramethyl, tetraethyl, and tetrabutyl ammonium hydroxide) to exchange trapped anionic chloride ions inside the vTAPA structure with OH- ions. The density functional theory (DFT) study exhibited that the anion exchange process is very favorable, as the vTAPA framework offers preferable interaction sites for OH- ions. The impact of steric hindrance from these tertiary ammonium salts on the OH- conduction performance was extensively investigated. Butyl@vTAPA exhibited a high OH- ion conductivity of 1.05 × 10-4 S cm-1 at 90 °C under 98% relative humidity (RH). Our uniquely designed cationic covalent organic frameworks (COF) created a platform for a preferential transport network of hydroxide ions, and this is the first report of directly used COFs for hydroxide ion conduction without any vigorous postsynthetic modification.

Bioinspired Gradient Covalent Organic Framework Membranes for Ultrafast and Asymmetric Solvent Transport

Gradients play a pivotal role in membrane technologies, e.g., osmotic energy conversion, desalination, biomimetic actuation, selective separation, and more. In these applications, the compositional gradients are of great relevance for successful function implementation, ranging from solvent separation to smart devices; However, the construction of functional gradient in membranes is still challenging both in scale and directions. Inspired by the specific function-related, graded porous structures in glomerular filtration membranes, a general approach for constructing gradient covalent organic framework membranes (GCOMx) applying poly (ionic liquid)s (PILs) as template is reported here. With graded distribution of highly porous covalent organic framework (COF) crystals along the membrane, GCOMx exhibts an unprecedented asymmetric solvent transport when applying different membrane sides as the solvent feed surface during filtration, leading to a much-enhanced flux (10-18 times) of the "large-to-small" pore flow comparing to the reverse direction, verified by hydromechanical theoretical calculations. Upon systematic experiments, GCOMx achieves superior permeance in nonpolar (hexane ≈260.45 LMH bar-1) and polar (methanol ≈175.93 LMH bar-1) solvents, together with narrow molecular weight cut-off (MWCO, 472 g mol-1) and molecular weight retention onset (MWRO, <182 g mol-1). Interestingly, GCOMx shows significant filtration performance in simulated kidney dialysis, revealing great potential of GCOMx in bionic applications.

Alkyl modified cationic COFs for preferential trapping of charge dispersed perrhenate: Synergistic hydrophobicity and anion-recognition effects

Charge dispersed oxoanionic pollutants (such as TcO4- and ReO4-) with low hydrophilicity are typically difficult to be preferentially extracted. Recently, cationic covalent organic frameworks (COFs) have received considerable attention for anions trapping. Two cationic COFs, denoted as Tp-S and Tp-D, were synthesized by incorporating ethyl and cyclic alkylated diquats into 2,2'-bipyridine-based COF. A synergistic effect of hydrophobic channel and anion-recognition sites were achieved by branched chains, which effectively surmounted the Hofmeister bias. Both Tp-S and Tp-D exhibited raising removal performance for surrogate ReO4- at high acidity with adsorption capacities of 435.6 and 291.4 mg g-1, respectively. Obvious variations caused by side chains were displayed in microstructures and adsorption performance. Specially, compared with Tp-D, Tp-S demonstrated desirable priority in uptake capacity and selectivity. In a real-scenario experiment, Tp-S could remove 72.8 % of ReO4- in a simulated Hanford LAW stream, which was attributed to the spatial effects and charge distribution arising from the open and flexible side chains of Tp-S. Otherwise, the rigid cyclic chains endowed pyridine-base Tp-D material an unprecedented alkaline stability. Spectra and theoretical calculations revealed a mechanism of preferential capture based on electrostatic interaction and hydrogen bonding between charge dispersed ReO4-/TcO4- and Tp-S/Tp-D. This work provides an innovative perspective to tailored materials for the treatment of oxoanionic contaminants.

Covalent Organic Framework Membranes and Water Treatment

Covalent organic frameworks (COFs) are an emerging class of highly porous crystalline organic polymers comprised entirely of organic linkers connected by strong covalent bonds. Due to their excellent physicochemical properties (e.g., ordered structure, porosity, and stability), COFs are considered ideal materials for developing state-of-the-art separation membranes. In fact, significant advances have been made in the last six years regarding the fabrication and functionalization of COF membranes. In particular, COFs have been utilized to obtain thin-film, composite, and mixed matrix membranes that could achieve effective rejection (mostly above 80%) of organic dyes and model organic foulants (e.g., humic acid). COF-based membranes, especially those prepared by embedding into polyamide thin-films, obtained adequate rejection of salts in desalination applications. However, the claims of ordered structure and separation mechanisms remain unclear and debatable. In this perspective, we analyze critically the design and exploitation of COFs for membrane fabrication and their performance in water treatment applications. In addition, technological challenges associated with COF properties, fabrication methods, and treatment efficacy are highlighted to redirect future research efforts in realizing highly selective separation membranes for scale-up and industrial applications.

N-Methylquinuclidinium-Based Anion Exchange Membrane with Ultrahigh Alkaline Stability

Anion-exchange-membrane (AEM) water electrolysis is a promising technology for hydrogen production from renewable energy sources. However, the bottleneck of its development is the poor comprehensive performance of AEM, especially the stability at highly concentrated alkaline condition and temperature. Herein, a new cationic group N-methylquinuclidinium with enhanced alkaline stability is proposed and hereby a full-carbon chain poly(aryl quinuclidinium) AEM is prepared. Compared with reported AEMs, it shows ultrahigh comprehensive alkaline stability (no chemical decomposition, no decay of conductivity) in 10 m NaOH aqueous solution at 80 °C for more than 1800 h, excellent dimensional stability (swelling ratio: <10% in pure water, <2% in 10 m NaOH) in OH- form at 80 °C, high OH- conductivity (≈139.1 mS cm-1 at 80 °C), and high mechanical properties (tensile strength: 41.5 MPa, elongation at break: 50%). The water electrolyzer using the AEM exhibits a high current density (1.94 A cm-2 at 2.0 V) when assembled with nickel-alloy foam electrodes, and high durability when assembled with nickel foam electrodes.

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.

Solutions Are the Problem: Ordered Two-Dimensional Covalent Organic Framework Films by Chemical Vapor Deposition

Covalent organic frameworks (COFs) are a promising class of crystalline polymer networks that are useful due to their high porosity, versatile functionality, and tunable architecture. Conventional solution-based methods of producing COFs are marred by slow reactions that produce powders that are difficult to process into adaptable form factors for functional applications, and there is a need for facile and fast synthesis techniques for making crystalline and ordered covalent organic framework (COF) thin films. In this work, we report a chemical vapor deposition (CVD) approach utilizing co-evaporation of two monomers onto a heated substrate to produce highly crystalline, defect-free COF films and coatings with hydrazone, imine, and ketoenamine COF linkages. This all-in-one synthesis technique produces highly crystalline, 40 nm-1 μm-thick COF films on Si/SiO2 substrates in less than 30 min. Crystallinity and alignment were proven by using a combination of grazing-incidence wide-angle X-ray scattering (GIWAXS) and transmission electron microscopy (TEM), and successful conversion of the monomers to produce the target COF was supported by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and UV-vis measurements. Additionally, we used atomic force microscopy (AFM) to investigate the growth mechanisms of these films, showing the coalescence of triangular crystallites into a smooth film. To show the wide applicability and scope of the CVD process, we also prepared crystalline ordered COF films with imine and ketoenamine linkages. These films show potential as high-quality size exclusion membranes, catalytic platforms, and organic transistors.

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.

Photoelectric responsive ionic channel for sustainable energy harvesting

Access to sustainable energy is paramount in today's world, with a significant emphasis on solar and water-based energy sources. Herein, we develop photo-responsive ionic dye-sensitized covalent organic framework membranes. These innovative membranes are designed to significantly enhance selective ion transport by exploiting the intricate interplay between photons, electrons, and ions. The nanofluidic devices engineered in our study showcase exceptional cation conductivity. Additionally, they can adeptly convert light into electrical signals due to photoexcitation-triggered ion movement. Combining the effects of salinity gradients with photo-induced ion movement, the efficiency of these devices is notably amplified. Specifically, under a salinity differential of 0.5/0.01 M NaCl and light exposure, the device reaches a peak power density of 129 W m-2, outperforming the current market standard by approximately 26-fold. Beyond introducing the idea of photoelectric activity in ionic membranes, our research highlights a potential pathway to cater to the escalating global energy needs.

Chiral Ionic Covalent Organic Framework as an Enantioselective Fluorescent Sensor for Phenylalaninol Determination

Phenylalaninol (PAL) is a significant chemical intermediate widely utilized in drug development and chiral synthesis, for instance, as a reactant for bicyclic lactams and oxazoloisoindolinones. Since the absolute stereochemical configuration significantly impacts biological action, it is crucial to evaluate the concentration and enantiomeric content of PAL in a quick and convenient manner. Herein, an effective PAL enantiomer recognition method was reported based on a chiral ionic covalent organic framework (COF) fluorescent sensor, which was fabricated via one-step postquaternization modification of an achiral COF by (1R, 2S, 5R)-2-isopropyl-5-methylcyclohexyl-carbonochloridate (L-MTE). The formed chiral L-TB-COF can be applied as a chiral fluorescent sensor to recognize the stereochemical configuration of PAL, which displayed a turn-on fluorescent response for R-PAL over that of S-PAL with an enantioselectivity factor of 16.96. Nonetheless, the single L-MTE molecule had no chiral recognition ability for PAL. Moreover, the ee value of PAL can be identified by L-TB-COF. Furthermore, density functional theory (DFT) calculations demonstrated that the chiral selectivity came from the stronger binding affinity between L-TB-COF and R-PAL in comparison to that with S-PAL. L-TB-COF is the first chiral ionic COF employed to identify chiral isomers by fluorescence. The current work expands the range of applications for ionic COFs and offers fresh suggestions for creating novel chiral fluorescent sensors.

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.

Advanced Covalent Organic Framework-Based Membranes for Recovery of Ionic Resources

Membrane technology has shown a viable potential in conversion of liquid-waste or high-salt streams to fresh waters and resources. However, the non-adjustability pore size of traditional membranes limits the application of ion capture due to their low selectivity for target ions. Recently, covalent organic frameworks (COFs) have become a promising candidate for construction of advanced ion separation membranes for ion resource recovery due to their low density, large surface area, tunable channel structure, and tailored functionality. This tutorial review aims to analyze and summarize the progress in understanding ion capture mechanisms, preparation processes, and applications of COF-based membranes. First, the design principles for target ion selectivity are illustrated in terms of theoretical simulation of ions transport in COFs, and key properties for ion selectivity of COFs and COF-based membranes. Next, the fabrication methods of diverse COF-based membranes are classified into pure COF membranes, COF continuous membranes, and COF mixed matrix membranes. Finally, current applications of COF-based membranes are highlighted: desalination, extraction, removal of toxic metal ions, radionuclides and lithium, and acid recovery. This review presents promising approaches for design, preparation, and application of COF-based membranes in ion selectivity for recovery of ionic resources.

Covalent Organic Frameworks with Ionic Liquid-Moieties (ILCOFs): Structures, Synthesis, and CO2 Conversion

CO₂, an acidic gas, is usually emitted from the combustion of fossil fuels and leads to the formation of acid rain and greenhouse effects. CO₂ can be used to produce kinds of value-added chemicals from a viewpoint based on carbon capture, utilization, and storage (CCUS). With the combination of unique structures and properties of ionic liquids (ILs) and covalent organic frameworks (COFs), covalent organic frameworks with ionic liquid-moieties (ILCOFs) have been developed as a kind of novel and efficient sorbent, catalyst, and electrolyte since 2016. In this critical review, we first focus on the structures and synthesis of different kinds of ILCOFs materials, including ILCOFs with IL moieties located on the main linkers, on the nodes, and on the side chains. We then discuss the ILCOFs for CO₂ capture and conversion, including the reduction and cycloaddition of CO₂. Finally, future directions and prospects for ILCOFs are outlined. This review is beneficial for academic researchers in obtaining an overall understanding of ILCOFs and their application of CO₂ conversion. This work will open a door to develop novel ILCOFs materials for the capture, separation, and utilization of other typical acid, basic, or neutral gases such as SO₂, H₂S, NOx, NH₃, and so on.

Fast Hydroxide Conduction via Hydrogen-Bonding Network Confined in Benzimidazolium-Functionalized Covalent Organic Frameworks

In both hydrophobic and hydrophilic nanochannels, confined water clusters spontaneously form dense internal hydrogen bond networks and hence exhibit fast mass-transfer kinetics. Covalent organic frameworks (COFs), a porous polymer, enables one-dimensional open channels to achieve ordered assembly guided by synthetic techniques and allows the accommodation of a large number of water molecules within the nanochannels. In the field of alkaline anion exchange membrane fuel cells, it has been a long-term pursuit of scientists to build abundant hydrogen bonds around hydrogen oxides (OH^-) to improve the conduction of OH^- by increasing the water content. Here, we designed and synthesized a OH^- conductor by assembling benzimidazolium into COFs, and a significantly high conductivity of 10^-1 S cm^-1 was achieved at 353 K. Theoretical calculations showed that the water clusters confined in the pores of COFs and the regularly arranged hydroxides cooperatively formed a dense hydrogen bond network and OH^- conducted diffusive conduction through the Grotthuss hopping of protons in this hydrogen bond network.

Azo-Branched Covalent Organic Framework Thin Films as Active Separators for Superior Sodium-Sulfur Batteries

Sodium-Sulfur (Na-S) batteries are outstanding for their ultrahigh capacity, energy density, and low cost, but they suffer from rapid cell capacity decay and short lifespan because of serious polysulfide shuttle and sluggish redox kinetics. Herein, we synthesize thin films of covalent organic frameworks (COFs) with azobenzene side groups branched to the pore walls. The azobenzene branches deliver dual functions: (1) narrow the pore size to the sub-nanometer scale thus inhibiting the polysulfide shuttle effect and (2) act as ion-hopping sites thus promoting the Na⁺ migration. Consequently, the Na-S battery using the COF thin film as the separator exhibits a high capacity of 1295 mA h g^-1 at 0.2 C and an extremely low attenuation rate of 0.036% per cycle over 1000 cycles at 1 C. This work highlights the importance of separator design in upgrading Na-S batteries and demonstrates the possibility of functionalizable framework materials in developing high-performance energy storage systems.

Controllable Construction of Amino-Functionalized Dynamic Covalent Porous Polymers for High-Efficiency CO2 Capture from Flue Gas

The design of high-efficiency CO₂ adsorbents with low cost, high capacity, and easy desorption is of high significance for reducing carbon emissions, which yet remains a great challenge. This work proposes a facile construction strategy of amino-functional dynamic covalent materials for effective CO₂ capture from flue gas. Upon the dynamic imine assembly of N-site rich motif and aldehyde-based spacers, nanospheres and hollow nanotubes with spongy pores were constructed spontaneously at room temperature. A commercial amino-functional molecule tetraethylenepentamine could be facilely introduced into the dynamic covalent materials by virtue of the dynamic nature of imine assembly, thus inducing a high CO₂ capacity (1.27 mmol·g^-1) from simulated flue gas at 75 °C. This dynamic imine assembly strategy endowed the dynamic covalent materials with facile preparation, low cost, excellent CO₂ capacity, and outstanding cyclic stability, providing a mild and controllable approach for the development of competitive CO₂ adsorbents.

Polyamide Covalent Organic Framework Membranes for Molecular Sieving

Polyamide is an important class of membrane materials for separation technology. The polyamide membranes currently used are amorphous, and thus, their pore structures are disordered, which inevitably decreases their performance in separation. Herein, we report a new type of polyamide membranes which are fabricated from amide-linked covalent organic frameworks (COFs), a class of crystalline porous polymers with well-ordered pore structures. Thanks to the structural advantages of amide-linked COFs, the polyamide COF membranes not only exhibit high permeability (482.3 L m^-2 h^-1 bar^-1 to water) and high rejection rate to organic dyes (>99% for methylene blue) but also display excellent stability under a harsh environment. The vantage of the polyamide COF membranes is also manifested by the comparison of their mechanical property, stability, and separation performance with that of the membranes fabricated from the COFs having the same building blocks but linked with imine and amine linkages. This work demonstrates that amide-linked COFs, which combine the structural features of COFs and polyamide, could be a new type of advanced materials for the fabrication of high-performance separation membranes.

Role of water environment in chemical degradation of a covalent organic framework tethered with quaternary ammonium for anion exchange membranes

The anion exchange membrane (AEM) is a main component for AEM fuel cells. Recently, a series of electrolytes based on covalent organic frameworks (COFs) functionalized with quaternary ammonium (QA) of showed extraordinary ionic conductivities thanks to the intrinsic porosity of the COF structures, which also provide a robust backbone for good mechanical strength. However, the chemical stability of the COF-based AEMs in alkaline conditions is yet to be understood. Here we systematically investigate the chemical degradation of the COF-based structures tethered with alkyl spacers by combining molecular dynamics (MD) simulations and density functional theory (DFT) calculations. We find that the water environment protects the cationic groups from chemical degradation in terms of both physical and chemical effects, which play a synergistic role. Moreover, we introduce the effective density of water as an order parameter to quantitatively characterize the level of degradation of the COF-based systems with similar design of architecture. The results provide guidance for estimation of the chemical stability of COF-based AEMs.

Transformation of Porous Organic Cages and Covalent Organic Frameworks with Efficient Iodine Vapor Capture Performance

The reaction of 5,5'-([2,2'-bipyridine]-5,5'-diyl)diisophthalaldehyde (BPDDP) with cyclohexanediamine and [benzidine (BZ)/[2,2'-bipyridine]-5,5'-diamine (BPDA)], respectively, affords a nitrogen-rich porous organic cage BPPOC and two two-dimensional (2D) covalent organic frameworks (COFs), USTB-1 and USTB-2 (USTB = University of Science and Technology Beijing), under suitable conditions. Interestingly, BPPOC with a single-crystal X-ray diffraction structure is able to successfully transform into USTB-1 and USTB-2 (newly converted COFs denoted as USTB-1c and USTB-2c, respectively) upon exchange of the imine unit of cyclohexanediamine in the cage by BZ and BPDA. Such a transformation also enables the isolation of analogous COFs (USTB-3c and USTB-4c) on the basis of an isostructural organic cage, BTPOC, which is derived from 5,5'-([2,2'-bithiophene]-4,4'-diyl)diisophthalaldehyde (BTDDP) and cyclohexanediamine. However, the conventional solvothermal reaction between BTDDP and BPDA leads to an impure phase of USTB-4 containing incompletely converted aldehyde groups due to the limited solubility of the building block. The newly prepared COFs have been characterized by nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy. In particular, BPPOC is able to absorb the iodine vapor with an uptake of 5.64 g g^-1, breaking the porous organic cage's (POC's) record value of 3.78 g g^-1. Nevertheless, the cage-derived COFs exhibit improved iodine vapor adsorption capability in comparison with the directly synthesized counterparts, with the highest uptake of 5.80 g g^-1 for USTB-1c. The mechanism investigation unveils the superiority of nitrogen atoms to sulfur atoms for POCs in iodine vapor capture with the assistance of definite crystal structures. This, in combination with porosity, synergistically influences the iodine vapor capture capacity of COFs.