Designing Covalent Organic Frameworks with a Tailored Ionic Interface for Ion Transport across One‐Dimensional Channels

The research progress on COF solid-state electrolytes for lithium batteries

Lithium metal batteries have garnered significant attention due to their high energy density and broad application prospects. However, the practical use of traditional liquid electrolytes is constrained by safety and stability challenges. In the exploration of novel electrolytes, solid-state electrolyte materials have emerged as a focal point. Covalent organic frameworks (COFs), with their large conjugated structures and unique electronic properties, are gradually gaining attention as an emerging class of solid-state electrolyte materials. In recent years, outstanding electrochemical performance has been achieved through the design and synthesis of various types of COF-based solid-state electrolytes, along with successful integration with other functional materials. This review will provide an overview of the research progress on COFs as solid-state electrolyte materials for lithium metal batteries and offer insights into their future potential in battery technology.

An integrated "rigid-flexible" strategy by side chain engineering towards high ion-conduction cationic covalent organic framework electrolytes

In recent years, solid-state lithium metal batteries (SSLMBs) have become a new development trend, and it has become a top priority to design solid-state electrolytes (SSEs) that can rapidly and stably transport lithium ions in a variety of climatic environments. In this work, an integrated "rigid-flexible" dual-functional strategy is proposed to develop a cationic covalent organic framework (EO-BIm-iCOF) with well-defined flexible oligo(ethylene oxide) (EO) chains as an SSE for SSLMBs. As expected, the synergistic effects of the rigid cationic framework and flexible EO chains not only promote the dissociation of LiTFSI salts, but also greatly improve the transport of lithium ions, which endows LITFSI@EO-BIm-iCOF SSEs with a high Li+ conductivity of 1.08 × 10-4 S cm-1 and ionic transference number of 0.69 at room temperature. Besides, the molecular dynamics (MD) simulations have also elucidated the diffusion and transport mechanism of lithium ions in LITFSI@EO-BIm-iCOF SSEs. Interestingly, the assembled SSLMBs wherein LiFePO4 is paired with LITFSI@EO-BIm-iCOF SSEs display decent electrochemical properties at higher and lower temperatures. This work provides a great development prospect for the application of cationic COFs in solid-state batteries.

Porous organic crystals crosslinked by free-radical reactions

Two hydrogen-bonded crosslinked organic frameworks (HCOFs) were synthesized via free radical reactions utilizing butadiene and isoprene as crosslinkers. These HCOFs exhibit high crystallinity, enabling detailed structural characterization via single-crystal X-ray diffraction analysis. Subsequently, one of the olefin-rich HCOFs was converted to a hydroxylated framework through hydroboration-oxidation while maintaining the high crystallinity.

Exceptional Anhydrous Proton Conduction in Covalent Organic Frameworks

Covalent organic frameworks (COFs) offer an irreplaceable platform for mass transport, as they provide aligned one-dimensional channels as pathways. Especially, proton conduction is of great scientific interest and technological importance. However, unlike proton conduction under humidity, anhydrous proton conduction remains a challenge, as it requires robust materials and proceeds under harsh conditions. Here, we report exceptional anhydrous proton conduction in stable crystalline porous COFs by integrating neat phosphoric acid into the channels to form extended hydrogen-bonding networks. The phosphoric acid networks in the pores are stabilized by hierarchical multipoint and multichain hydrogen-bonding interactions with the 3D channel walls. We synthesized five hexagonal COFs that possess different pore sizes, which are gradually tuned from micropores to mesopores. Remarkably, mesoporous COFs with a high pore volume exhibit an exceptional anhydrous proton conductivity of 0.31 S cm-1, which marks the highest conductivity among all examples reported for COFs. We observed that the proton conductivity is dependent on the pore volume, pore size, and content of phosphoric acid. Increasing the pore volume improves the proton conductivity in an exponential fashion. Remarkably, changing the pore volume from 0.41 to 1.60 cm3 g-1 increases the proton conductivity by 1150-fold. Interestingly, as the pore size increases, the activation energy barrier of proton conduction decreases in linear mode. The mesopores enable fast proton hopping across the channels, while the micropores follow sluggish vehicle conduction. Experiments on tuning phosphoric acid loading contents revealed that a well-developed hydrogen-bonding phosphoric acid network in the pores is critical for proton conduction.

Multipolar Conjugated Polymer Framework Derived Ionic Sieves via Electronic Modulation for Long-Life All-Solid-State Li Batteries

Here, we build a tunable multipolar conjugated polymer framework platform via pore wall chemistry to probe the role of electronic structure engineering in improving the Li+ conduction by theoretical studies. Guided by theoretical prediction, we develop a new cyano-vinylene-linked multipolar polymer framework namely CNF-COF, which can act as efficient ion sieves to modify solid polymer electrolytes to simultaneously tune Li+ migration and stable Li anodes for long-lifespan all-solid-state (ASS) Li metal batteries at high rate. The dual-decoration of cyano and fluorine groups in CNF-COF favorably regulates electronic structure via multipolar donor-acceptor electronic effects to afford proper energy band structure and abundant electron-rich sites for enhanced oxidative stability, facilitated ion-pair dissociation and suppressed anion movements. Thus, the CNF-COF incorporation into poly (ethylene oxide) (PEO) electrolytes not only renders fast selective Li+ transport but also facilitates the Li dendrite suppression. Specifically, the constructed PEO composite electrolyte with an ultra-low CNF-COF content of only 0.5 wt % is endowed with a wide electrochemical window, a high ionic conductivity of 0.634 mS cm-1 at 60 °C and a large Li+ transference number of 0.81-remarkably outperforming CNF-COF-free counterparts (0.183 mS cm-1 and 0.22). As such, the Li symmetric cell delivers stable Li plating/stripping over 1400 h at 0.1 mA cm-2. Impressively, by coupling with LiFePO4 (LFP) cathodes, the assembled ASS Li battery under 60 °C allows for stable cycling over 2000 cycles at 1 C and over 1000 cycles even at 2 C with a large capacity retention of ~75 %, surpassing most reported ASS Li batteries using PEO-based electrolytes.

Light-Gating Crystalline Porous Covalent Organic Frameworks

We report light gating in synthetic one-dimensional nanochannels of stable crystalline porous covalent organic frameworks. The frameworks consist of 2D hexagonal skeletons that are extended over the x-y plane and stacked along the z-direction to create dense yet aligned 1D mesoporous channels. The pores are designed to be photoadaptable by covalently integrating tetrafluoro-substituted azobenzene units onto edges, which protrude from walls and offer light-gating machinery confined in the channels. The implanted tetrafluoroazobenzene units are thermally stable yet highly sensitive to visible light to induce photoisomerization between the E and Z forms. Remarkably, photoisomerization induces drastic changes in intrapore polarity as well as pore shape and size, which exert profound effects on the molecular adsorption of a broad spectrum of compounds, ranging from inorganic iodine to organic dyes, drugs, and enzymes. Unexpectedly, the systems respond rapidly to visible lights to gate the molecular release of drugs and enzymes. Photoadaptable covalent organic frameworks with reversibly convertible pores offer a platform for constructing light-gating porous materials and tailorable delivery systems, remotely controlled by visible lights.

Quantitative Construction of Boronic-Ester Linkages in Covalent Organic Frameworks for the Carbon Dioxide Reduction

Covalent organic frameworks (COFs) have been utilized for catalyzing the reduction of carbon dioxide (CO2RR) due to their atomic metal centers and controllable pore channels, which are facilitated by different covalent bonds. However, the exploration of boron-based linkages in these catalytic COFs has been limited owing to potential instability. Herein, we present the construction of boronic ester-linked COFs through nucleophilic substitution reactions in order to catalyze the CO2 RR. The inclusion of abundant fluorine atoms within the frameworks enhances their hydrophobicity and subsequently improves water tolerance and chemical stability of COFs. The content of boron atoms in the COF linkages was carefully controlled, with COFs featuring a higher density of boron atoms exhibiting increased electronic conductivity, enhanced reductive ability, and stronger binding affinity towards CO2 . Consequently, these COFs demonstrate improved activity and selectivity. The optimized COFs achieve the highest activity, achieving a turnover frequency of 1695.3 h-1 and a CO selectivity of 95.0 % at -0.9 V. Operando synchrotron radiation measurements confirm the stability of Co (II) atoms as catalytically active sites. By successfully constructing boronic ester-linked COFs, we not only address potential instability concerns but also achieve exceptional catalytic performance for CO2 RR.

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.

Drug delivery using biocompatible covalent organic frameworks (COFs) towards a therapeutic approach

Covalent organic frameworks (COFs) are constructed exclusively with lightweight organic scaffolds, which can have a 2D or 3D architecture. The ease of synthesis, robust skeleton and tunable properties of COFs make them superior candidates among their counterparts for a wide range of uses including biomedical applications. In the biomedical field, drug delivery or photodynamic-photothermal (PDT-PTT) therapy can be individually considered a potential parameter to be investigated. Therefore, this comprehensive review is focused on drug delivery using COFs, highlighting the encapsulation and decapsulation of drugs by COF scaffolds and their delivery in biological media including live cells. Versatile COF scaffolds together with the delivery of several drug molecules are considered. We attempted to incorporate the status of drug encapsulation and decapsulation considering a wide range of recent publications.

Tuning Crystallinity and Stacking of Two-Dimensional Covalent Organic Frameworks through Side-Chain Interactions

Two-dimensional covalent organic frameworks (2D COFs) form as layered 2D polymers whose sheets stack through high-surface-area, noncovalent interactions that can give rise to different interlayer arrangements. Manipulating the stacking of 2D COFs is crucial since it dictates the effective size and shape of the pores as well as the specific interactions between functional aromatic systems in adjacent layers, both of which will strongly influence the emergent properties of 2D COFs. However, principles for tuning layer stacking are not yet well understood, and many 2D COFs are disordered in the stacking direction. Here, we investigate effects of pendant chain length through a series of 2D imine-linked COFs functionalized with n-alkyloxy chains varying in length from one carbon (C1 COF) to 11 carbons (C11 COF). This series reveals previously unrecognized and unanticipated trends in both the stacking geometry and crystallinity. C1 COF adopts an averaged eclipsed geometry with no apparent offset between layers. In contrast, all subsequent chain lengths lead to some degree of unidirectional slip stacking. As pendant chain length is increased, trends show average layer offset increasing to a maximum of 2.07 Å in C5 COF and then decreasing as chain length is extended through C11 COF. Counterintuitively, shorter chains (C2-C4) give rise to lower yields of weakly crystalline materials, while longer chains (C6-C9) produce greater yields of highly crystalline materials, as confirmed by powder X-ray diffraction and scanning electron microscopy. Molecular dynamics simulations corroborate these observations, suggesting that long alkyl chains can interact favorably to promote the self-assembly of sheets. In situ proton NMR spectroscopy provides insights into the reaction equilibrium as well as the relationship between low COF yields and low crystallinity. These results provide fundamental insights into principles of supramolecular assembly in 2D COFs, demonstrating an opportunity for harnessing favorable side-chain interactions to produce highly crystalline materials.

Gradient Channel Segmentation in Covalent Organic Framework Membranes with Highly Oriented Nanochannels

Covalent organic frameworks (COFs) offer an exceptional platform for constructing membrane nanochannels with tunable pore sizes and tailored functionalities, making them promising candidates for separation, catalysis, and sensing applications. However, the synthesis of COF membranes with highly oriented nanochannels remains challenging, and there is a lack of systematic studies on the influence of postsynthetic modification reactions on functionality distribution along the nanochannels. Herein, we introduced a "prenucleation and slow growth" approach to synthesize a COF membrane featuring highly oriented mesoporous channels and a high Brunauer-Emmett-Teller surface area of 2230 m2 g-1. Functional moieties were anchored to the pore walls via "click" reactions and coordinated with Cu ions to serve as segmentation functions. This led to a remarkable H2/CO2 separation performance that surpassed the Robeson upper bound. Moreover, we found that the functionalities distributed along the nanochannels could be influenced by functionality flexibility and postsynthetic reaction rate. This strategy paved the way for the accurate design and construction of COF-based artificial solid-state nanochannels with high orientation and precisely controlled channel environments.

Multicomponent Covalent Organic Framework Solid Electrolyte Allowing Effective Li-Ion Dissociation and Diffusion for All-Solid-State Batteries

Organic solid electrolytes compatible with all-solid-state Li metal batteries (LMBs) are essential to ensuring battery safety, high energy density, and long-term cycling performance. However, it remains a challenge to develop an approach to provide organic solid electrolytes with capabilities for the facile dissociation of strong Li-ion pairs and fast transport of ionic components. Herein, a diethylene glycol-modified pyridinium covalent organic framework (DEG-PMCOF) with a well-defined periodic structure is prepared as a multicomponent solid electrolyte with a cationic moiety of high polarity, an additional flexible ion-transporter, and an ordered ionic channel for all-solid-state LMBs. The DEG-containing pyridinium groups of DEG-PMCOF allow a lower dissociation energy of Li salts and a smaller energy barrier of Li-ion transport, leading to high ion conductivity (1.71 × 10-4 S cm-1) and a large Li-ion transfer number (0.61) at room temperature in the solid electrolyte. The DEG-PMCOF solid electrolyte exhibits a wide electrochemical stability window and effectively suppresses the formation of Li dendrites and dead Li in all-solid-state LMBs. Molecular dynamics and density functional theory simulations provide insights into the mechanisms for the enhanced Li-ion transport driven by the integrated diffusion process based on hopping motion, vehicle motion, and free diffusion of DEG-PMCOF. The all-solid-state LMB assembled with a DEG-PMCOF solid electrolyte displays a high specific capacity with a retention of 99% and an outstanding Coulombic efficiency of 99% at various C-rates during long-term cycling. This DEG-PMCOF approach can offer an effective route to design various solid-state Li batteries.

Covalent Organic Frameworks for Neuromorphic Devices

Neuromorphic computing could enable the potential to break the inherent limitations of conventional von Neumann architectures, which has led to widespread research interest in developing novel neuromorphic memory devices, such as memristors and bioinspired artificial synaptic devices. Covalent organic frameworks (COFs), as crystalline porous polymers, have tailorable skeletons and pores, providing unique platforms for the interplay with photons, excitons, electrons, holes, ions, spins, and molecules. Such features encourage the rising research interest in COF materials in neuromorphic electronics. To develop high-performance COF-based neuromorphic memory devices, it is necessary to comprehensively understand materials, devices, and applications. Therefore, this Perspective focuses on discussing the use of COF materials for neuromorphic memory devices in terms of molecular design, thin-film processing, and neuromorphic applications. Finally, we provide an outlook for future directions and potential applications of COF-based neuromorphic electronics.

Ammonia-Induced Anomalous Ion Transport in Covalent Organic Framework Nanochannels

More anomalous transport behaviors have been observed with the rapid progress in nanofabrication technology and characterization tools. The ions/molecules inside nanochannels can act dramatically different from those in the bulk systems and exhibit novel mechanisms. Here, we have reported the fabrication of a nanodevice, covalent organic frameworks covered theta pipette (CTP), that combine the advantages of theta pipette (TP), nanochannels framework, and field-effect transistors (FETs) for controlling and modulating the anomalous transport. Our results show that ammonia, a weak base, causes a continuous supply of ions inside covalent organic framework (COF) nanochannels, leading to an abnormally high current depending on the ionic/molecular size and the pore size of the nanochannel. Furthermore, CTP can distinguish different concentrations of ammonia and have all of the qualities of a nanosensor.

Rational Fabrication of Ionic Covalent Organic Frameworks for Chemical Analysis Applications

The rapid development of advanced material science boosts novel chemical analytical technologies for effective pretreatment and sensitive sensing applications in the fields of environmental monitoring, food security, biomedicines, and human health. Ionic covalent organic frameworks (iCOFs) emerge as a class of covalent organic frameworks (COFs) with electrically charged frames or pores as well as predesigned molecular and topological structures, large specific surface area, high crystallinity, and good stability. Benefiting from the pore size interception effect, electrostatic interaction, ion exchange, and recognizing group load, iCOFs exhibit the promising ability to extract specific analytes and enrich trace substances from samples for accurate analysis. On the other hand, the stimuli response of iCOFs and their composites to electrochemical, electric, or photo-irradiating sources endows them as potential transducers for biosensing, environmental analysis, surroundings monitoring, etc. In this review, we summarized the typical construction of iCOFs and focused on their rational structure design for analytical extraction/enrichment and sensing applications in recent years. The important role of iCOFs in the chemical analysis was fully highlighted. Finally, the opportunities and challenges of iCOF-based analytical technologies were also discussed, which may be beneficial to provide a solid foundation for further design and application of iCOFs.

Facile Synthesis of Quinolinecarboxylic Acid-Linked Covalent Organic Framework via One-Pot Reaction for Highly Efficient Removal of Water-Soluble Pollutants

To efficiently eliminate highly polar organic pollutants from water has always been a difficult issue, especially in the case of ultralow concentrations. Herein, we present the facile synthesis of quinolinecarboxylic acid-linked COF (QCA-COF) via the Doebner multicomponent reaction, possessing multifunction, high specific surface area, robust physicochemical stability, and excellent crystallinity. The marked feature lies in the quinolinyl and carboxyl functions incorporated simultaneously to QCA-COF in one step. The major cis-orientation of carboxyl arms in QCA-COF was speculated by powder X-ray diffraction and total energy analysis. QCA-COF demonstrates excellent adsorption capacity for water-soluble organic pollutants such as rhodamine B (255.7 mg/g), methylene blue (306.1 mg/g), gentamycin (338.1 mg/g), and 2,4-dichlorophenoxyacetic acid (294.1 mg/g) in water. The kinetic adsorptions fit the pseudo-second order model and their adsorption isotherms are Langmuir model. Remarkably, QCA-COF can capture the above four water-soluble organic pollutants from real water samples at ppb level with higher than 95% removal efficiencies and excellent recycling performance.

A Fluorinated Covalent Organic Framework with Accelerated Oxygen Transfer Nanochannels for High-Performance Zinc-Air Batteries

The establishment of abundant three-phase interfaces with accelerated mass transfer in air cathodes is highly desirable for the development of high-rate and long-cycling rechargeable zinc-air batteries (ZABs). Covalent organic frameworks (COFs) exhibit tailored nanopore structures, facilitating the rational tuning of their specific properties. Here, by finely tuning the fluorinated nanopores of a COF, a novel air cathode for rechargeable ZABs is unprecedentedly designed and synthesized. COF nanosheets are decorated with fluorinated alkyl chains, which shows high affinity to oxygen (O₂ ), in its nanopores (fluorinated COF). The fluorinated COF nanosheets are stacked into well-defined O₂ -transport channels, which are then assembled into aerophilic "nano-islands" on the hydrophilic FeNi layered-double-hydroxide (FeNi LDH) electrocatalyst surface. Therefore, the mass-transport "highway" for O₂ and water is segregated on the nanoscale, which significantly enlarges the area of three-phase boundaries and greatly promotes the mass-transfer therein. ZABs based on the COF-modified air cathode deliver a small charge/discharge voltage gap (0.64 V at 5 mA cm^-2 ), a peak power density (118 mW cm^-2 ), and a stable cyclability. This work provides a feasible approach for the design of the air cathodes for high-performance ZABs, and will expand the new application of COFs.

One-Dimensional Covalent Organic Frameworks for the 2e- Oxygen Reduction Reaction

Two-dimensional covalent organic frameworks (2D COFs) are often employed for electrocatalytic systems because of their structural diversity. However, the efficiency of atom utilization is still in need of improvement, because the catalytic centers are located in the basal layers and it is difficult for the electrolytes to access them. Herein, we demonstrate the use of 1D COFs for the 2e^- oxygen reduction reaction (ORR). The use of different four-connectivity blocks resulted in the prepared 1D COFs displaying good crystallinity, high surface areas, and excellent chemical stability. The more exposed catalytic sites resulted in the 1D COFs showing large electrochemically active surface areas, 4.8-fold of that of a control 2D COF, and thus enabled catalysis of the ORR with a higher H₂ O₂ selectivity of 85.8 % and activity, with a TOF value of 0.051 s^-1 at 0.2 V, than a 2D COF (72.9 % and 0.032 s^-1 ). This work paves the way for the development of COFs with low dimensions for electrocatalysis.

Application and Research Progress of Covalent Organic Frameworks for Solid-State Electrolytes in Lithium Metal Batteries

Covalent organic frameworks (COFs) are a class of crystalline porous organic polymers with periodic networks that are constructed from small molecular units via covalent bonds, which have low densities, high porosity, large specific surface area, and ease of functionalization. The one-dimension nanochannels in COFs offer an effective means of transporting lithium ions while maintaining a stable structure over a wide range of temperatures. As a new category of ionic conductors, COFs exhibit unparalleled application potential in solid-state electrolytes. Here, we provide a comprehensive summary of recent applications and research progress for COFs in solid-state electrolytes of lithium metal batteries and discuss the possible development directions in the future. This review is expected to provide theoretical guidance for the design of high-performance solid-state electrolytes.

Sustained-release nanocapsule based on a 3D COF for long-term enzyme prodrug therapy of cancer

A well-designed three-dimensional (3D) covalent organic framework (COF) was constructed as a nanocapsule for the encapsulation of horseradish peroxidase (HRP), which could realize sustained release of HRP to prolong the duration of the therapeutic agents and promote long-term enzyme prodrug therapy.

Porous polyelectrolyte frameworks: synthesis, post-ionization and advanced applications

Porous organic polymers (POPs), which feature high surface areas, robust skeletons, tunable pores, adjustable functionality and versatile applicability, have constituted a designable platform to develop advanced organic materials. Endowing polyelectrolytes with the distinct characteristics of POPs will attract mounting interest as the structural diversity of polyelectrolytes will bring the new hope of intriguing applications and potential benefits. In this review, the striking progress in ionized POPs (i-POPs) has been systematically summarized with regard to their synthetic strategies and applications. In the synthesis of i-POPs, we illustrate the representative ionic building blocks and charged functional groups capable of constructing the polyelectrolyte frameworks. The synthetic methods, including direct synthesis and post-modification, are detailed for the i-POPs with amorphous or crystalline structures, respectively. Subsequently, we outline the distinctive performances of i-POPs in adsorption, separation, catalysis, sensing, ion conduction and biomedical applications. The survey concerns the interplay between the surface chemistry, ionic interaction and pore confinement that cooperatively promote the performance of i-POPs. Finally, we conclude with the remaining challenges and promising opportunities for the on-going development of i-POPs.

Ionic Covalent Organic Frameworks for Energy Devices

Covalent organic frameworks (COFs) are a class of porous crystalline materials whose facile preparation, functionality, and modularity have led to their becoming powerful platforms for the development of molecular devices in many fields of (bio)engineering, such as energy storage, environmental remediation, drug delivery, and catalysis. In particular, ionic COFs (iCOFs) are highly useful for constructing energy devices, as their ionic functional groups can transport ions efficiently, and the nonlabile and highly ordered all-covalent pore structures of their backbones provide ideal pathways for long-term ionic transport under harsh electrochemical conditions. Here, current research progress on the use of iCOFs for energy devices, specifically lithium-based batteries and fuel cells, is reviewed in terms of iCOF backbone-design strategies, synthetic approaches, properties, engineering techniques, and applications. iCOFs are categorized as anionic COFs or cationic COFs, and how each of these types of iCOFs transport lithium ions, protons, or hydroxides is illustrated. Finally, the current challenges to and future opportunities for the utilization of iCOFs in energy devices are described. This review will therefore serve as a useful reference on state-of-the-art iCOF design and application strategies focusing on energy devices.

Covalent Organic Framework (COF)-Based Hybrids for Electrocatalysis: Recent Advances and Perspectives

Developing highly efficient electrocatalysts for renewable energy conversion and environment purification has long been a research priority in the past 15 years. Covalent organic frameworks (COFs) have emerged as a burgeoning family of organic materials internally connected by covalent bonds and have been explored as promising candidates in electrocatalysis. The reticular geometry of COFs can provide an excellent platform for precise incorporation of the active sites in the framework, and the fine-tuning hierarchical porous architectures can enable efficient accessibility of the active sites and mass transportation. Considerable advances are made in rational design and controllable fabrication of COF-based organic-inorganic hybrids, that containing organic frameworks and inorganic electroactive species to induce novel physicochemical properties, and take advantage of the synergistic effect for targeted electrocatalysis with the hybrid system. Branches of COF-based hybrids containing a diversity form of metals, metal compounds, as well as metal-free carbons have come to the fore as highly promising electrocatalysts. This review aims to provide a systematic and profound understanding of the design principles behind the COF-based hybrids for electrocatalysis applications. Particularly, the structure-activity relationship and the synergistic effects in the COF-based hybrid systems are discussed to shed some light on the future design of next-generation electrocatalysts.

Emerging polymer electrodes for aqueous energy storage

New generation energy storage devices call for electrodes with high capacity, high cycling performance and environmental benignity. Polymer electrode materials (PEMs) are attractive for their abundant structural diversity and tunability as well as engineered conductivity, desirable processability and electrochemical properties for aqueous batteries. We herein overview the state-of-the-art development of PEMs for aqueous batteries, including conventional doped, redox-backbone, redox-pendant and hydrophilic conducting polymers. The merits and demerits of PEMs, and their structural modification and energy storage performance are discussed in detail. To provide a comprehensive understanding of polymer-based aqueous batteries, we correlate the molecular structures of PEMs with their conductivity, morphology and electrochemical behaviors. The review offers an insight into the rational design of conducting polymer electrodes for safe and cost-effective aqueous batteries.

An enzyme nanopocket based on covalent organic frameworks for long-term starvation therapy and enhanced photodynamic therapy of cancer

An enzyme nanopocket constructed from a porphyrin-based covalent organic framework (COF) was developed to co-load glucose oxidase (GOx) and catalase (CAT) for long-term starvation therapy and enhanced photodynamic therapy (PDT).

Hydroxide Anion Transport in Covalent Organic Frameworks

Hydroxide anion transport is essential for alkaline fuel cells, but hydroxide anion has an inherently low conductivity owing to its small diffusion coefficient and high mass. Ordered open channels found in covalent organic frameworks are promising as pathways to enable hydroxide anion transport, but this remains to be explored. Here we report designed synthesis of anionic covalent organic frameworks that promote hydroxide anion transport across the one-dimensional channels. Engineering cationic chains with imidazolium termini onto the pore walls self-assembles a supramolecular interface of single-file hydroxide anion chains in the channels. The frameworks facilitate hydroxide anion transport to achieve an exceptional conductivity of 1.53 × 10^-2 S cm^-1 at 80 °C, which is 2-6 orders of magnitude higher than those of linear polymers and other porous frameworks. Impedance spectroscopy at different temperatures and studies on deuterated samples reveal that hydroxide anions transport via a proton-exchange hopping mechanism. These results open a way to design framework materials for energy conversions via engineering an anionic interface.

Covalent Organic Frameworks Enabling Site Isolation of Viologen-Derived Electron-Transfer Mediators for Stable Photocatalytic Hydrogen Evolution

Electron transfer is the rate-limiting step in photocatalytic water splitting. Viologen and its derivatives are able to act as electron-transfer mediators (ETMs) to facilitate the rapid electron transfer from photosensitizers to active sites. Nevertheless, the electron-transfer ability often suffers from the formation of a stable dipole structure through the coupling between cationic-radical-containing viologen-derived ETMs, by which the electron-transfer process becomes restricted. Herein, cyclic diquats, a kind of viologen-derived ETM, are integrated into a 2,2'-bipyridine-based covalent organic framework (COF) through a post-quaternization reaction. The content and distribution of embedded diquat-ETMs are elaborately controlled, leading to the favorable site-isolated arrangement. The resulting materials integrate the photosensitizing units and ETMs into one system, exhibiting the enhanced hydrogen evolution rate (34600 μmol h^-1  g^-1 ) and sustained performances when compared to a single-module COF and a COF/ETM mixture. The integration strategy applied in a 2D COF platform promotes the consecutive electron transfer in photochemical processes through the multi-component cooperation.

Liquid-Like Phase of N,N-Dimethylpyrrolidinium Iodide Impregnated into COFs Endows Fast Lithium Ion Conduction in the Solid State

A novel kind of solid-state lithium electrolyte was fabricated by impregnating organic ionic plastic crystals (OIPCs) into the pores of covalent organic frameworks (COFs). The liquid-like phase of confined N,N-dimethylpyrrolidinium iodide (P_1,1 I) and the ordered nanochannels of COFs simultaneously stimulated the lithium ion conduction.

Precise Design of Covalent Organic Frameworks for Electrocatalytic Hydrogen Peroxide Production

Electrochemical synthesis of H₂ O₂ with high productivity is a significant challenge in electrocatalysis. Herein, we develop Mg-ion contained covalent organic frameworks (MgP-DHTA-COF), comprising stacked 2D layers, well-defined skeletons, and well-ordered monodispersed active sites, for the electrocatalytic production of H₂ O₂ directly from O₂ and H₂ O. The precise-designed MgP-DHTA-COF achieves H₂ O₂ selectivity up to 96%, high Faradaic efficiency of 91% and reliable stability for H₂ O₂ synthesis in 0.10 mol L^-1 KOH aqueous solution. Both experiments and simulations demonstrate that the pyrrolic-N fixed Mg ions in the knots promote the reactivity of COF and enhance the adsorption ability of OOH*. This work provides a valuable example for the design of an efficient electrocatalyst based on COFs for H₂ O₂ production.

Cationic covalent-organic framework for sulfur storage with high-performance in lithium-sulfur batteries

Covalent organic frameworks (COFs) with pre-designed structure and customized properties have been employed as sulfur storage materials for lithium-sulfur (Li-S) batteries. In this work, a cationic mesoporous COF (COF-NI) was synthesized by grafting a quaternary ammonium salt group onto the pore channel of COFs via a one-pot three components tandem reaction strategy. The post-functionalized COFs were utilized as the matrix framework to successfully construct the Li-S battery with high-speed capacity and long-term stability. The experimental results showed that, after loading active material sulfur, cationic COF-NI effectively suppressed the shuttle effect of the intermediate lithium polysulfide species in Li-S batteries, and exhibited better cycle stability than the as-obtained neutral COF (COF-Bu). For example, compared with COF-Bu based sulfur cathode (521 mA h g^-1), the cationic COF-NI based sulfur cathode maintained a discharge capacity of 758 mA h g^-1 after 100 cycles. These results clearly showed that appropriate pore environment of COFs can be prepared by rational design, which can reduce the shuttle effect of lithium polysulfide species and improve the performance of Li-S battery.

Covalent organic frameworks: an ideal platform for designing ordered materials and advanced applications

Covalent organic frameworks offer a molecular platform for integrating organic units into periodically ordered yet extended 2D and 3D polymers to create topologically well-defined polygonal lattices and built-in discrete micropores and/or mesopores.

Flexible thiourea linked covalent organic frameworks

Exploring covalent organic frameworks with new linkages is regarded as an efficient method to modulate the corresponding properties, and a new linkage is essential to the progress of this field.

Design and application of covalent organic frameworks for ionic conduction

This review article comprehensively summarized recent progress in the development of covalent organic framework materials for ionic conduction.

Simultaneous determination of Acetaminophen and dopamine based on a water-soluble pillar[6]arene and ultrafine Pd nanoparticle-modified covalent organic framework nanocomposite

A novel electrochemical sensing platform for the simultaneous determination of AP and DA based on a water-soluble pillar[6]arene and ultrafine Pd nanoparticle-modified covalent organic framework nanocomposite.

Electroactive Covalent Organic Frameworks: Design, Synthesis, and Applications

Covalent organic frameworks (COFs) are an emerging class of crystalline porous polymers with tailorable compositions, porosities, functionalities, and intrinsic chemical stability. The incorporation of electroactive moieties in the structure transforms COFs into electroactive materials with great potential for energy-related applications. Herein, the recent advances in the design and use of electroactive COFs as capacitors, batteries, conductors, fuel cells, water-splitting, and electrocatalysis are addressed. Their remarkable performance is discussed and compared with other porous materials; hence, perspectives in the development of electroactive COFs are presented.

Nanoscale covalent organic frameworks as theranostic platforms for oncotherapy: synthesis, functionalization, and applications

Cancer nanomedicine is one of the most promising domains that has emerged in the continuing search for cancer diagnosis and treatment. The rapid development of nanomaterials and nanotechnology provide a vast array of materials for use in cancer nanomedicine. Among the various nanomaterials, covalent organic frameworks (COFs) are becoming an attractive class of upstarts owing to their high crystallinity, structural regularity, inherent porosity, extensive functionality, design flexibility, and good biocompatibility. In this comprehensive review, recent developments and key achievements of COFs are provided, including their structural design, synthesis methods, nanocrystallization, and functionalization strategies. Subsequently, a systematic overview of the potential oncotherapy applications achieved till date in the fast-growing field of COFs is provided with the aim to inspire further contributions and developments to this nascent but promising field. Finally, development opportunities, critical challenges, and some personal perspectives for COF-based cancer therapeutics are presented.

Covalent Organic Frameworks: Pore Design and Interface Engineering

Nature evolves fascinating molecular pores to achieve unique biological functions based on a single pore or channel as observed for aquaporins and ion channels. An artificial system, on the other hand, explores porous structures to construct dense pores in materials. Progress in chemistry over the past century has greatly improved our capability to synthesize porous materials. This is evident by the advancement from inorganic to organic units, from trial-and-error tests to module fabrication and further to fully predesignable pores, and from harsh preparation protocols to ambient synthetic methods. Over the past 15 years, a molecular platform based on organic and polymer chemistry has been explored to enable the design of artificial pores to achieve different pore size, shape, wall, and interface. This becomes possible with a class of emerging polymer-covalent organic frameworks (COFs). COFs are a class of crystalline porous polymers that integrate organic units into extended molecular frameworks with periodically ordered skeletons and well-defined pores. We have focused on exploring COFs over the past 15 years to design and synthesize porous structures with the aim of developing chemistry that leads to the creation of tailor-made pore interfaces (Nagai, A. et al. Nat. Commun., 2011, 2, 536). In this Account, we summarize the general concept of our approaches to various pore interfaces by emphasizing design principle, synthetic strategy, and distinct porous features and their impacts. We illustrate pore interface design by highlighting general strategies based on direct polymerization and pore surface engineering to construct different pore walls with a diversity of functional units. One distinct feature is that these functional groups are predesigned and synthetically controlled to achieve a predetermined component, position, and density, leading to a general way to install various specific pore wall interfaces to each pore. We showcase hierarchical pore interface architectures by elucidating the nature of interplays between interfaces and molecules and ions, ranging broadly from hydrogen bond to dipole-dipole/quadrupole interactions, electrostatic interaction, acid-base interaction, coordination, and electronic interactions. We scrutinize the unique properties and functions of adsorption and separation, catalysis, energy transformation and storage, and proton and metal ion transport by disclosing functional design schemes and interface-function correlations. We predict the fundamental key issues to be addressed and show future directions in designing artificial pores to target at ultimate functions. This chemistry on pore interface engineering opens a way to porous materials that have remained challenging in the predesign of both structure and function.

Applications of Dynamic Covalent Chemistry Concept towards Tailored Covalent Organic Framework Nanomaterials: A Review

Covalent organic frameworks (COFs) are a rapidly developing class of materials that has been of immense research interest during the last ten years. Numerous reviews have been devoted to summarizing the synthesis and applications of COFs. However, the underlying dynamic covalent chemistry (DCC), which is the foundation of COFs synthesis, has never been systematically reviewed in this context. Dynamic covalent chemistry is the practice of using thermodynamic equilibriums to molecular assemblies. This Critical Review will cover the state-of-the-art use of DCC to both synthesize COFs and expand the applications of COFs. Five synthetic strategies for COF synthesis are rationalized, namely: modulation, mixed linker/linkage, sub-stoichiometric reaction, framework isomerism, and linker exchange, which highlight the dynamic covalent chemistry to regulate the growth and to modify the properties of COFs. Furthermore, the challenges in these approaches and potential future perspectives in the field of COF chemistry are also provided.

Ultralow Surface Tension Solvents Enable Facile COF Activation with Reduced Pore Collapse

Covalent organic frameworks (COFs) are organic, crystalline, highly porous materials attractive for applications such as gas storage, gas separations, catalysis, contaminant adsorption, and membrane filtration. Activation of COFs removes adsorbed solvents and impurities, but common methods for COF activation can result in the collapse of porous structure and loss of accessible surface areas. Here, we present a study of the impact of solvent surface tension on the activation process and demonstrate that activation using the ultralow surface tension solvent perfluorohexane (PFH) is simple and effective for a range of COF materials. We synthesized six different imine-based COFs through imine condensation reactions between tris(4-aminophenyl)benzene (TAPB) or 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT) and multifunctional di- and tri-benzaldehydes with different aromatic substituents. For each COF, we performed a solvent wash followed by vacuum drying using six solvents varying in surface tension from 11.9 to 72.8 mN m^-1. Through powder X-ray diffraction (PXRD) measurements combined with nitrogen adsorption and desorption analysis, we found that some COF chemistries readily lost their porosity during activation with higher surface tension solvents while others were more robust. However, all COFs could be effectively activated using PFH to produce materials with excellent crystallinity and high surface areas, comparable to those for samples activated using supercritical CO₂. This work demonstrates that the solvent surface tension used during activation has a strong impact on the potential pore collapse, and activation using PFH provides a simple and effective activation method to produce COFs with excellent crystallinities and pore structures.