Covalent Organic Frameworks for Batteries

Covalent organic frameworks (COFs) have emerged as an exciting new class of porous materials constructed by organic building blocks via dynamic covalent bonds. They have been extensively explored as potentially superior candidates for electrode materials, electrolytes, and separators, due to their tunable chemistry, tailorable structures, and well-defined pores. These features enable rational design of targeted functionalities, facilitate the penetration of electrolytes, and enhance ion transport. This review provides an in-depth summary of the recent progress in the development of COFs for diverse battery applications, including lithium-ion, lithium–sulfur, sodium-ion, potassium-ion, lithium–CO₂, zinc-ion, zinc–air batteries, etc. This comprehensive synopsis pays particular attention to the structure and chemistry of COFs and novel strategies that have been implemented to improve battery performance. Additionally, current challenges, possible solutions, and potential future research directions on COFs for batteries are discussed, laying the groundwork for future advances for this exciting class of material.     

Covalent Organic Frameworks: Structures,Synthesis, and Applications

Covalent organic frameworks (COFs) are crystalline porous polymers formed by a bottom‐up approach from molecular building units having a predesigned geometry that are connected through covalent bonds. They offer positional control over their building blocks in two and three dimensions. This control enables the synthesis of rigid porous structures with a high regularity and the ability to fine‐tune the chemical and physical properties of the network. This Feature Article provides a comprehensive overview over the structures realized to date in the fast growing field of covalent organic framework development. Different synthesis strategies to meet diverse demands, such as high crystallinity, straightforward processability, or the formation of thin films are discussed. Furthermore, insights into the growing fields of COF applications, including gas storage and separations, sensing, electrochemical energy storage, and optoelectronics are provided.     

Covalent Organic Frameworks Based Heterostructure in Solar-To-Fuel Conversion

Photocatalytic reactions for fuel generation are crucial for the world's energy needs. Covalent-Organic-Frameworks (COFs) have been extensively studied as promising designable photocatalysts for these reactions due to their efficient visible-light absorption, suitable energy-band structure, facilitated intramolecular charge separation, and fast mass transfer. However, the activities of pristine COFs remain unsatisfactory, due to intermolecular charge recombination. Recently, COF-based heterostructures, which combine COFs with metal-sulfides, metal-oxides, carbon materials, or MOFs, have attracted increasing attention for enhancing solar-to-fuel conversion efficiency by facilitating interfacial photo-generated carrier separation, sensitizing wide-gap semiconductors, and promoting surface redox reactions. Thus, a review of the state-of-the-art progress of COF-based heterostructure photocatalysts in reactions such as H₂ evolution, CO₂ reduction, O₂ evolution, H₂O splitting and CO₂ splitting is crucial for the design of new photocatalysts to promote solar-to-fuel conversion. In this review, the COF-based heterostructures photocatalysts are highlighted based on their synthesis, properties, and reasons for enhanced activities. Moreover, design principles are raised for such photocatalysts for each fuel generation reaction, based on insights into related research. Finally, this review is concluded by proposing future trends for COF-based heterostructures photocatalysts, with attention to the design of COFs and supports, analyzing the photocatalytic reaction dynamics, together with considering practical applications.     

Defective 2D Covalent Organic Frameworks for Postfunctionalization

Defects are deliberately introduced into covalent organic frameworks (COFs) via a three‐component condensation strategy. The defective COFs (dCOF‐NH₂‐Xs, X = 20, 40, and 60) possess favorable crystallinity and porosity, as well as have active amine functional groups as anchoring sites for further postfunctionalization. By introducing imidazolium functional groups onto the pore walls of COFs via the Schiff‐base reaction, dCOF‐ImBr‐Xs‐ and dCOF‐ImTFSI‐Xs‐based materials are employed as all‐solid‐state electrolytes for lithium‐ion conduction with a wide range of working temperatures (from 303 to 423 K), and the ion conductivity of dCOF‐ImTFSI‐60‐based electrolyte reaches 7.05 × 10^?3 S cm^?1 at 423 K. As far as it is known, it is the highest value for all polymeric crystalline porous material based all‐solid‐state electrolytes. Furthermore, Li/dCOF‐ImTFSI‐60@Li/LiFePO₄ all‐solid Li‐ion battery displays satisfactory battery performance under 353 K. This work not only provides a new methodology to construct COFs with precisely controlled defects for postfunctionalization, but also makes them promising candidate materials as all‐solid‐state electrolytes for lithium‐ion batteries operate at high temperatures.     

2D Covalent Organic Frameworks for Biomedical Applications

Covalent organic frameworks (COFs) are an emerging class of organic crystalline polymers with well‐defined molecular geometry and tunable porosity. COFs are formed via reversible condensation of lightweight molecular building blocks, which dictate its geometry in two or three dimensions. Among COFs, 2D COFs have garnered special attention due to their unique structure composed of two‐dimensionally extended organic sheets stacked in layers generating periodic columnar π‐arrays, functional pore space, and their ease of synthesis. These unique features in combination with their low density, high crystallinity, large surface area, and biodegradability have made them an excellent candidate for a plethora of applications ranging from energy to biomedical sciences. In this article, the evolution of 2D COFs is briefly discussed in terms of different types of chemical linkages, synthetic strategies of bulk and nanoscale 2D COFs, and their tunability from a biomedical perspective. Next, the biomedical applications of 2D COFs specifically for drug delivery, phototherapy, biosensing, bioimaging, biocatalysis, and antibacterial activity are summarized. In addition, current challenges and emerging approaches in designing 2D COFs for advanced biomedical applications are discussed.     

Azobenzene Functionalized Organic Covalent Frameworks: Controlled Morphologies and Photo-Regulated Adsorption

Covalent organic frameworks (COFs) containing azobenzene building blocks carry great potential for use in intelligent storage, separation, chemical sensing, and catalysis due to their intriguing photo-responsiveness. However, azobenzene units are often exploited as the linkers to form the framework of COFs, thereby restricting their molecular motion and photoisomerization. Herein, a simple yet robust template-free solvothermal strategy is reported to yield azobenzene-dangled COFs (Azo-COFs) with their azobenzene moieties suspending within the pores. The crystallinity, specific surface area, and morphology of Azo-COFs can be conveniently tailored by changing the ratio of amine to aldehyde monomers. Notably, the Azo-COFs provide sufficient free space for the reversible trans-to-cis isomerization of the dangled azobenzene units inside the pores, thus reversibly regulating surface wettability of Azo-COFs. The adsorption capacity of Azo-COFs toward organic dye molecules is increased by 3.7-fold when irradiated with ultraviolet light, which can be ascribed to the intelligent closing/opening of molecular gates rendered by photoisomerization of azobenzene moieties. As such, the ability to photoregulate the adsorption of Azo-COFs highlights their significance in functioning as smart porous nanomaterials for applications in cargo release, molecular sieves, ion transport, energy conversion systems, and environmental remediation.     

Two-Photon Absorption Induced Cancer Immunotherapy Using Covalent Organic Frameworks

Immune checkpoint blockade therapy is revolutionizing the traditional treatment model of multiple tumor types, but remains ineffective for a large subset of patients. Photodynamic therapy (PDT) has been shown to induce cancer cell death and provoke an immune response, and may represent a potential strategy to synergize with immune checkpoint blockade therapy. However, the limited tissue penetration of exciting light for conventional PDT largely hinders its application in the clinic and its further combination with immunotherapy. Here, a serrated packing covalent organic framework (COF), COF-606, with excellent two-photon absorption (2PA) property and photostability, largely avoids aggregation-caused quenching, therefore offering high reactive oxygen species (ROS) generation efficiency; it is used as a 2PA photosensitizer for PDT in deep tumor tissue. COF-606 induced PDT is shown to be efficient in inducing immunogenic cell death, provoking an immune response and normalizing the immunosuppressive status for the first time. This makes it possible to combine 2PA induced PDT using COF with programmed cell death protein 1 immune checkpoint blockade therapy. Such combination leads to strong abscopal tumor-inhibiting efficiency and long-lasting immune memory effects, standing as a promising combinatorial therapeutic strategy for cancer treatment.     

Defects and Disorder in Covalent Organic Frameworks for Advanced Applications

Crystalline porous organic polymers (CPPs) or covalent organic frameworks (COFs), are composed by light elements linked by covalent bonds. Despite the remarkable progress attained, there are still bottlenecks limiting further development, some of them related to the presence of defects during their synthesis as well as in-depth understanding of structure of active centers and/or details of the reaction mechanism. Indeed, very often the proposed structures are far from reality because defects and disorders have not been considered. The present review provides an illustrative overview of “defects and disorder in COFs”. These defects include those not only generated during the synthesis and manipulation of COFs, but also lack of crystallinity, stacking disorder and network vacancies. The review starts giving general remarks on organic COFs and their synthetic methods, followed by different methods to play and manage defects, how to minimize them or how to take advantage of them to gain new properties and applications. Selected characterization techniques to quantify defective structures and active sites in COFs are also presented. Finally, the challenges and future opportunities in the field have been summarized in the last section.     

Imaging Solvent-Triggered Gaseous Iodine Uptake on Single Covalent Organic Frameworks

Covalent organic frameworks (COFs) are promising solid absorbents for the treatment of gaseous iodine. However, extensive efforts are still focused on empirical optimizations of specific binding sites and pore structures in COFs, and the chemical control of gaseous iodine uptake on COFs remains challenging. In this study, the chemically triggered sorption properties of COF-300 for I₂ vapors at the single-particle level with the dark-field microscope (DFM) are explored. The present operando single-particle DFM imaging method enables the direct visualization of an adsorption activity transformation from inactive COF-300 to active solvated COF-300 toward gaseous I₂ vapors. Exploiting the useful reaction information from time-lapsed DFM images, the tunable adsorption performance of solvated COF-300 is quantitatively compared by various solvents. The results illustrate that the isopropanol (IPA)-solvated COF-300 achieves the optimum adsorption capacity for I₂ among the absorbents. The reaction mechanism is elucidated to be the channel size enlargement and modification of internal surface chemistry in the IPA-solvated COF-300, producing a stable I₂/IPA-solvated COF-300 complex after the sorption reaction. The present chemical control of the sorption behavior of COF-300 revealed by DFM opens up a new fundamental paradigm for rationally developing high-performance COF-based absorbents for removing I₂ vapors.     

All-in-One Hollow Flower-Like Covalent Organic Frameworks for Flexible Transparent Devices

Covalent organic frameworks (COFs) exhibit great potential in the application of functional electronic devices. However, there has been no report of the precise fabrication of 3D all-in-one hollow micro/nanostructures based on COFs. Here, for the first time, all-in-one hollow dioxin-based COF-316 microflowers are synthesized measuring 5–7 µm and with interconnected hollow petals through a self-template strategy. The growth mechanism involves the collaborative process of self-assembly of nanoparticles, inside-out Ostwald ripening, and epitaxial growth. Due to the intrinsic porosity and interconnected hollow structure, COF-316 can uniformly composite with polypyrrole (PPy) through the “interior” and “exterior” functionalization, in which the hydrogen bond interaction enhances the charge transfer efficiency and structural stability in the charge/discharge process. The COF-316@PPy flexible transparent supercapacitors exhibit an areal specific capacitance (C_A) of 783.6 µF cm^–2 at 3 µA cm^–2 and long-term cycling stability. This work will boost research on the valuable design concepts of 3D hollow COF materials for energy storage devices.     

Ionic Dye Based Covalent Organic Frameworks for Photothermal Water Evaporation

Conversion of solar energy into heat for water evaporation is of great significance to provide clean and sustainable technology for water purification by using inexhaustible sunlight. In this field, one of the challenges comes from the design of high-performance photothermal materials powerful in light harvesting, light-to-heat conversion, and water activation. Herein, it is demonstrated that rationalization of the ionic covalent organic framework (iCOF) can simultaneously satisfy these multiple requirements and a new iCOF STTP is constructed through the Schiff base chemistry in a rapid microwave-assisted solvothermal route by using a hydrophilic dye molecule safranineT as the ionic building block. The integrated dye-related ionic moieties greatly strengthen the light absorbance (>97%) throughout the entire solar spectrum from UV–vis to the infrared region. The framework ionic moieties provide strong polarization to reduce the exciton dissociation energy for enhanced photothermal effect, and in addition, promote sufficient water activation to decrease the water evaporation enthalpy. As an outcome, the STTP driven solar water evaporator affords a fast water evaporation rate of 3.55 kg h⁻¹ m⁻² and high solar-to-vapor efficiency of 95.8%. This study highlights the potential of designing iCOF materials for photothermal applications.     

Hierarchically Macro–Microporous Covalent Organic Frameworks for Efficient Proton Conduction

Assembling molecular proton carriers into crosslinked networks is widely used to fabricate proton conductors, but they often suffer losses in conduction efficiency and stability accompanied by unclear causes. Covalent organic frameworks (COFs), with well-defined crystal frameworks and excellent stability, offer a platform for exploring the proton transfer process. Herein, a strategy to construct proton conductors that induce conductivity and stability by introducing bottom-up hierarchical structure, mass transport interfaces, and host–guest interactions into the COFs is proposed. The proton-transport platforms are designed to possess hierarchically macro–microporous structure for proton storage and mass transport. The protic ionic liquids, with low proton dissociation energies investigated by DFT calculation, are installed at open channel walls for faster proton motion. As expected, the resultant proton conductors based on a covalent organic framework (PIL_0.5@m-TpPa-SO₃H) with hierarchical pores increase conductivity by approximately three orders of magnitude, achieving the value of 1.02 × 10⁻¹ S cm⁻¹ (90 °C, 100% RH), and maintain excellent stability. In addition, molecular dynamics simulations reveal the mechanism of “hydrogen-bond network” for proton conduction. This work offers a fresh perspective on COF-based material manufacturing for high-performance proton conductors via a protocol of macro-micropores.     

Electrolyte Interphase Built from Anionic Covalent Organic Frameworks for Lithium Dendrite Suppression

Lithium (Li) metal batteries hold considerable promise for numerous energy-dense applications. However, the dendritic Li anode produced during Li⁺/Li deposition-stripping endangers battery safety and shortens cycle lifespan. Herein, an electrolyte interphase built from 2D anionic covalent organic frameworks (ACOF) is coated on Li for dendrite suppression. The ACOF with Li⁺-affinity facilitates rapid and exclusive passage of Li-ions from the electrolyte, yielding near-unity Li⁺ transference number (0.82) and ionic conductivity beyond 3.7 mS cm^-1 at the interphase. Such high transport efficiency of Li-ions can fundamentally circumvent the Li⁺ deficiency that results in dendrite formation. Pairing the ACOF-coated Li against a high-voltage LiCoO₂ cathode (4.5 V) achieves exceptional cycle stability, mitigated polarization, as well as improved rate capability. Accordingly, this strategy vastly expands the pool of electrolyte interphases that can be used for coating and protecting Li anode.     

2D Hexagonal Covalent Organic Radical Frameworks as Tunable Correlated Electron Systems

Quantum materials hold huge technological promise but challenge the fundamental understanding of complex electronic interactions in solids. The Mott metal–insulator transition on half-filled lattices is an archetypal demonstration of how quantum states can be driven by electronic correlation. Twisted bilayers of 2D materials provide an experimentally accessible means to probe such transitions, but these seemingly simple systems belie high complexity due to the myriad of possible interactions. Herein, it is shown that electron correlation can be simply tuned in experimentally viable 2D hexagonally ordered covalent organic radical frameworks (2D hex-CORFs) based on single layers of half-filled stable radical nodes. The presented carefully procured theoretical analysis predicts that 2D hex-CORFs can be varied between a correlated antiferromagnetic Mott insulator state and a semimetallic state by modest out-of-plane compressive pressure. This work establishes 2D hex-CORFs as a class of versatile single-layer quantum materials to advance the understanding of low dimensional correlated electronic systems.     

Flexible,Mechanically Stable,Porous Self-Standing Microfiber Network Membranes of Covalent Organic Frameworks: Preparation Method and Characterization

Covalent organic frameworks (COFs) show advantageous characteristics, such as an ordered pore structure and a large surface area for gas storage and separation, energy storage, catalysis, and molecular separation. However, COFs usually exist as difficult-to-process powders, and preparing continuous, robust, flexible, foldable, and rollable COF membranes is still a challenge. Herein, such COF membranes with fiber morphology for the first time prepared via a newly introduced template-assisted framework process are reported. This method uses electrospun porous polymer membranes as a sacrificial large dimension template for making self-standing COF membranes. The porous COF fiber membranes, besides having high crystallinity, also show a large surface area (1153 m² g⁻¹), good mechanical stability, excellent thermal stability, and flexibility. This study opens up the possibility of preparation of large dimension COF membranes and their derivatives in a simple way and hence shows promise in technical applications in separation, catalysis, and energy in the future.     

Thiol-Functionalized Covalent Organic Frameworks as Thermal History Indictor for Temperature and Time History Monitoring

Various products, including foods and pharmaceuticals, are sensitive to temperature fluctuations. Thus, temperature monitoring during production, transportation, and storage is critical. Facile indicators are required to monitor temperature conditions via color changes in real time. This study aimed to prepare and apply thiol-functionalized covalent organic frameworks (COFs) as a novel indicator for monitoring thermal history and temperature abuse. The COFs underwent obvious color changes from bright yellow to purple after exposure to different temperatures for varying durations. The reaction kinetics are analyzed under isothermal conditions, which reveal that the order of reaction rates is k_−20°C < k_4°C < k_20°C < k_35°C < k_55°C. The activation energy (E_a) of the COFs is calculated using the Arrhenius equation as 50.71 kJ moL⁻¹. The COFs are capable of sensitive color changes and offer a broad temperature tracking range, thereby demonstrating their application potential for the monitoring of temperature and time exposure history during production, transportation, and storage. This excellent performance thermal history indicator also shows promise for expanding the application field of COFs.     

Multibranched Octupolar Module Embedded Covalent Organic Frameworks Enable Efficient Two‐Photon Fluorescence

Covalent organic frameworks (COFs) have emerged as potential light emitting polymers for optoelectronic and optical devices, but their nonlinear optical properties, particularly two‐photon absorption and fluorescence (TPA/TPF), have seldom been explored. Herein, to construct octupolar three‐branched modules (e.g., acceptor 3‐(donor‐core), triphenylbenzene core) within a 2D cyano‐sp²c‐conjugated framework is proposed that results in two‐photon luminescent COFs, combining a large TPA cross section and high quantum yield (QY). Such octupolar module‐embedded sp²c‐conjugated COFs emit not only intense one‐photon fluorescence with QY of 27.2% in the solid state and 38.1% in tetrahydrofuran—superior to almost all reported COFs, but also efficient two‐photon fluorescence with large TPA cross section of 1225 GM—remarkably surpassing the corresponding cyano‐sp²c‐linked model compounds (104 GM). The finding highlights the synergy between sp²c‐conjugated framework and octupolar modules that leads to markedly improved TPA response owing to extended conjugated length, enhanced planarity and multidimensional intramolecular interaction. In view of the versatility of the branched chromophore, the proposed design idea is expected to be used to exploit more two‐photon active COF materials for a range of applications. Multiple uses of the COF in information encryption and warm white light‐emitting diodes are also exemplified.     

Dual-Active-Center of Polyimide and Triazine Modified Atomic-Layer Covalent Organic Frameworks for High-Performance Li Storage

Covalent organic frameworks (COFs) have received great attention as electrode materials in the lithium-ion batteries due to their exceptional crystallinity, easily chemical modification, and adjustable porous distribution. However, their practical application remains hindered by the insufficient Li⁺ active sites and long ion diffusion in the bulk materials. To tackle those issues, combining the virtues of high stable skeleton structure of large molecular, atomic-layer thickness feature, and multi-active sites, a novel atomic-layer COF cathode (denoted as E-TP-COF) with a dual-active-center of CO and CN group is developed. The atomic-layer thick structure improves the capturing and diffusion of Li-ion. Both active sites of CN and CO groups generate more capacity. The large molecular structure avoids the dissolubility challenge in electrolytes. As a result, the lithium-ion batteries assembled with E-TP-COF delivers a high initial capacity of 110 mAh g⁻¹ with a high capacity retention of 87.3% after 500 cycles. Furthermore, the Li⁺ diffusion mechanism is also confirmed through in(ex) situ technology and density functional theory calculation in detailing. This new strategy may exploit an important application of COFs in electrochemical energy storage and conversion.