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.     

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.     

Non-Interpenetrated 3D Covalent Organic Framework with Dia Topology for Au Ions Capture

The 3D covalent organic frameworks (COFs) have attracted considerable attention owing to their unique structural characteristics. However, most of 3D COFs have interpenetration phenomena, which will result in decreased surface area and porosities, and thus limited their applications in molecular/gas capture. Developing 3D COFs with non-fold interpenetration is challenging but significant because of the existence of non-covalent interactions between the adjacent nets. Herein, a new 3D COF (BMTA-TFPM-COF) with dia topology and non-fold interpenetration for Au ion capture is first demonstrated. The constructed COF exhibits a high Brunauer–Emmett–Teller surface area of 1924 m² g⁻¹, with the pore volume of 1.85 cm³ g⁻¹. The high surface area and abundant cavities as well as the abundant exposed CN linkages due to the non-interpenetration enable to absorb Au³⁺ with high capacity (570.18 mg g⁻¹), selectivity (99.5%), and efficiency (68.3% adsorption of maximum capacity in 5 min). This work provides a new strategy to design 3D COFs for ion capture.     

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.     

Use in Photoredox Catalysis of Stable Donor–Acceptor Covalent Organic Frameworks and Membrane Strategy

Optoelectronic attributes notwithstanding donor–acceptor covalent organic frameworks (D–A COFs) are not durable photocatalysts in many cases. Herein, a stabilization strategy of D–A COFs by intramolecular hydrogen (H)-bonds and a membrane-based mass transfer strategy for photocatalytic modulation are reported. The crystalline stability design of COF is cored at the strong π–π interactions and the H-bonds of adjacent tetrakis(4-formylphenyl)pyrene and naphthalenediimide units and the D–A charge transfer is designed for efficiency optimization. The well-defined, stable structure and charge dynamics of D–A COF, and the structure-controlled reactive oxygen species yields are confirmed. In two photoredox models, the COF presents both robust activity and stability and is further integrated with the mass transfer optimization of the COFs/polyvinylidene fluoride membrane. The membrane is recycled at least 15 times, and the turnover frequency value of g-scale amine coupling is as high as 62.4 h⁻¹. This work offers a facile approach to the stabilization design of D–A COFs and explores a general membrane-based mass transfer strategy for photocatalysis.     

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.     

Rapid Photochemical Synthesis of Sea‐Urchin‐Shaped Hierarchical Porous COF‐5 and Its Lithography‐Free Patterned Growth

Despite potential advantages of covalent organic frameworks (COFs) in wide area applications, several limitations in conventional solvothermal synthesis, such as long reaction time and high reaction temperature, reduce reaction efficiency and prohibit technical processes for practical applications. Therefore, the development of a novel synthesis method that provides better reaction efficiency and spatial controllability has become a critical challenge. Herein, a photochemical synthesis of C₉H₄BO₂ (COF‐5) is demonstrated for the first time, by which “sea urchin‐shaped” COF‐5 (UV‐COF‐5) with uniform size is synthesized with a highly enhanced growth rate, ≈48 times faster than that of the solvothermal method for 75% yield. In addition, an enlarged surface area is measured from UV‐COF‐5, which originates from its hierarchical morphology. The selectively increased growth rate of UV‐COF‐5 in the [001] direction observed by microscopic analysis results in the local 1D morphology of the hierarchical structure. Density functional theory calculations determine that the enhanced growth rate along the [001] direction can be understood by the characteristic of the interlayer orbital coupling at the frontier energy region. In addition, this study successfully demonstrates the preparation of COF‐5 patterns without any complicated postsynthesis lithography process, but simply by utilizing optical masks during the photochemical method.     

Covalent Assembly of Two-Dimensional COF-on-MXene Heterostructures Enables Fast Charging Lithium Hosts

2D heterostructured materials combining ultrathin nanosheet morphology, defined pore configuration, and stable hybrid compositions, have attracted increasing attention for fast mass transport and charge transfer, which are highly desirable features for efficient energy storage. Here, the chemical space of 2D–2D heterostructures is extended by covalently assembling covalent organic frameworks (COFs) on MXene nanosheets. Unlike most COFs, which are generally produced as solid powders, ultrathin 2D COF-LZU1 grows in situ on aminated Ti₃C₂T_x nanosheets with covalent bonding, producing a robust MXene@COF heterostructure with high crystallinity, hierarchical porosity, and conductive frameworks. When used as lithium hosts in Li metal batteries, lithium storage and charge transport are significantly improved. Both spectroelectrochemical and theoretical analyses demonstrate that lithiated COF channels are important as fast Li⁺ transport layers, by which Li ions can be precisely nucleated. This affords dendrite-free and fast-charging anodes, which would be difficult to achieve using individual components.     

Unzipped Carbon Nanotube/Graphene Hybrid Fiber with Less “Dead Volume” for Ultrahigh Volumetric Energy Density Supercapacitors

The development of 1D fiber-shaped supercapacitors (SCs) with high volumetric energy density is of great significance for miniature wearable electronics, where limiting the device's volume is critical. In this study, a partially unzipped carbon nanotube/reduction graphene oxide (PUCNT/RGO) hybrid fiber with less “dead volume” and a well-ordered porous structure is fabricated via wet spinning of a mixed partially unzipped oxidized carbon nanotube (PUOCNT)/GO solution and chemical reduction. The spinning solution is of low viscosity and high concentration, which can ensure smooth spinning while reducing the mass transfer during phase separation, thus lessen the “dead volume” derived from isolated pores. Moreover, PUOCNT with 1D and 2D hybrid nanoarchitecture, large specific surface area, and good water solubility can be a more effective spacer to inhibit the restacking of graphene oxide sheets while reducing the spacer itself and the large spacious voids formed “dead volume”. The all-solid-state SC assembled from the PUCNT/RGO hybrid fiber exhibits an excellent volumetric energy density of 8.63 mWh cm⁻³, exceeding the values of previously reported carbon-based fibers. The findings may open a door for finely controlling the density and pore structure of graphene-based fiber for applications in high volumetric energy storage via a scalable and efficient process.     

Bottom‐Up Preparation of Fully sp2‐Bonded Porous Carbons with High Photoactivities

Porous carbons, possessing exceptional stability, high surface area, and electric conductivity, are broadly used as superior adsorbent, supporter, or electrode material for environmental protection, industrial catalysis, and energy storage and conversion. The construction of such kinds of materials with designable structures and properties will extremely extend their potential applications, but remains a huge synthetic challenge. Herein, a bottom‐up approach is presented to synthesize one type of fully sp² carbon–bonded frameworks by transition metal–catalyzed cross‐coupling of different polyphenylenes with electron‐withdrawing 9,9′‐bifluorenylidene (9,9′‐BF) through its 2,7‐position. The resulting porous polymeric carbons exhibit substantial semiconducting properties, such as strong light‐harvesting capabilities in the visible light region, likely due to their π‐extended backbones with donor–acceptor characters. Their electronic and porous structures can be finely tuned via the polyphenylene spacers. The intriguing properties allow these porous carbons to efficiently catalyze dye degradation under visible light or even natural sunlight with high reusability. Meanwhile, associated with their intrinsic structures, these porous carbons also exhibit highly selective degradation activities toward different dyes. In particular, the photodegradation mechanism involving oxygen and electron is elucidated for the first time for such kinds of materials, related to the presence of specific 9,9′‐BF units in their π‐conjugated skeletons.     

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.     

Nickel@Nickel Oxide Core–Shell Electrode with Significantly Boosted Reactivity for Ultrahigh‐Energy and Stable Aqueous Ni–Zn Battery

The main bottlenecks of aqueous rechargeable Ni–Zn batteries are their relatively low energy density and poor cycling stability, mainly arising from the low capacity and inferior reversibility of the current Ni‐based cathodes. Additionally, the complicated and difficult‐to‐scale preparation procedures of these cathodes are not promising for large‐scale energy storage. Here, a facile and cost‐effective ultrasonic‐assisted strategy is developed to efficiently activate commercial Ni foam as a robust cathode for a high‐energy and stable aqueous rechargeable Ni–Zn battery. 3D Ni@NiO core–shell electrode with remarkably boosted reactivity and an area of 300 cm² is readily obtained by this ultrasonic‐assisted activation method (denoted as SANF). Benefiting from the in situ formation of electrochemically active NiO and porous 3D structure with a large surface area, the as‐fabricated SANF//Zn battery presents ultrahigh capacity (0.422 mA h cm^?2) and excellent cycling durability (92.5% after 1800 cycles). Moreover, this aqueous rechargeable SANF//Zn battery achieves an impressive energy density of 15.1 mW h cm^?3 (0.754 mW h cm^?2) and a peak power density of 1392 mW cm^?3, outperforming most reported aqueous rechargeable energy‐storage devices. These findings may provide valuable insights into designing large‐scale and high‐performance 3D electrodes for aqueous rechargeable batteries.     

Pyrolysis of Enzymolysis-Treated Wood: Hierarchically Assembled Porous Carbon Electrode for Advanced Energy Storage Devices

Designing energy storage devices from thick carbon electrodes with high areal/volumetric energy density via a simple and green way is very attractive but still challenging. Cellulose, as an excellent precursor for thick carbon electrodes with abundant sources and low cost, is usually activated by a chemical activator and pyrolysis route to achieve high electrochemical performance. However, there are still some problems to be addressed, such as the harsh activation conditions, easy collapse of porous structures, and the high cost. Herein, a 3D self-supporting thick carbon electrode derived from wood-based cellulose is proposed for high areal and volumetric energy density of supercapacitor from a mild, simple, and green enzymolysis treatment. Benefiting from the high specific surface area (1418 m² g⁻¹) and abundant active sites on the surface of wood-derived hierarchically porous structures and enzymolysis-induced micropores and mesopores, the assembled symmetry supercapacitor from the thick carbon electrode can realize the high areal/volumetric energy density of 0.21 mWh cm⁻²/0.99 mWh cm⁻³ with excellent stability of 86.58% after 15 000 long-term cycles at 20 mA cm⁻². Significantly, the simple and universal strategy to design material with high specific surface area, provides a new research idea for realizing multi-functional application.     

Fast and Large Lithium Storage in 3D Porous VN Nanowires–Graphene Composite as a Superior Anode Toward High‐Performance Hybrid Supercapacitors

Li‐ion hybrid capacitors (LIHCs), consisting of an energy‐type redox anode and a power‐type double‐layer cathode, are attracting significant attention due to the good combination with the advantages of conventional Li‐ion batteries and supercapacitors. However, most anodes are battery‐like materials with the sluggish kinetics of Li‐ion storage, which seriously restrict the energy storage of LIHCs at the high charge/discharge rates. Herein, vanadium nitride (VN) nanowire is demonstated to have obvious pseudocapacitive characteristic of Li‐ion storage and can get further gains in energy storage through a 3D porous architecture with the assistance of conductive reduced graphene oxide (RGO). The as‐prepared 3D VN–RGO composite exhibits the large Li‐ion storage capacity and fast charge/discharge rate within a wide working widow from 0.01–3 V (vs Li/Li⁺), which could potentially boost the operating potential and the energy and power densities of LIHCs. By employing such 3D VN–RGO composite and porous carbon nanorods with a high surface area of 3343 m² g^?1 as the anode and cathode, respectively, a novel LIHCs is fabricated with an ultrahigh energy density of 162 Wh kg^?1 at 200 W kg^?1, which also remains 64 Wh kg^?1 even at a high power density of 10 kW kg^?1.     

A Solar Responsive Battery Based on Charge Separation and Redox Coupled Covalent Organic Framework

Solar-responsive battery holds great promise in solar-to-electrochemical energy storage, but is impeded by the lack of efficient photoelectrochemical-cathodes. Herein, a crystalline mesoporous (≈4.0 nm) covalent organic framework (TA-PT COF) with repeating units consisting of covalently linked triphenylamine (TPA) and perylenetetracarboxylic diimide (PTCDI) is presented. The repeating unit functions as both a donor–acceptor pair and a dual-redox site to realize a molecule-level coupling of intramolecular charge separation (τCS = 136.2 ps, τCR = 949 ps) and reversible redox chemistry (C=O/C O−, TPA/TPA+). Equipped with this photoelectrochemical cathode, a reversible aqueous solar-responsive battery delivered a reliable voltage-response of 376 mV, an extra round-trip efficiency of 35% and a good light durability (500 cycles). A photo-coupled electron/mass transfer mechanism of photoelectrons for Zn²⁺ storage and holes for OTf− storage is further revealed, shedding light on a new photoelectrochemical cathode design based on charge separation and redox-coupled COF for efficient solar-responsive batteries.     

In Situ Growth of Imine-Bridged Anion-Selective COF/AAO Membrane for Ion Current Rectification and Nanofluidic Osmotic Energy Conversion

Facing the energy crisis, using the salinity gradient between seawater and freshwater for osmotic energy conversion is a direct way to obtain energy. So far, most nanofluidic membranes utilized for osmotic energy generation are cation-selective. Given that both anion- and cation-selective membranes have the identical importance for energy conversion devices, it is of great significance to develop anion-selective membranes. Herein, an anion-selective membrane is synthesized by in situ growth of imine-bridged covalent organic framework (COF) on ordered anodic aluminum oxide (AAO) at room temperature. The imine groups and residual amino groups of COF can combine with protons in neutral solution, enabling the COF positively charged and efficiently transport of anions. Particularly, due to the asymmetry in the charge and structure of COF/AAO, the as-prepared membrane exhibits excellent ionic current rectification property, which can inhibit ion concentration polarization effectively and possess high ion selectivity and permeability. Using the present COF/AAO membrane, salinity gradient energy can be successfully harvested from solutions with high salt content, and the output power density reached 17.95W m⁻² under a 500-fold salinity gradient. The study provides a new avenue for construction and application of anion-selective membranes in the smart ion transport and efficient energy conversion.     

Biomass Organs Control the Porosity of Their Pyrolyzed Carbon

In most electrochemical energy storage and conversion devices, nanostructured carbon materials play essential roles. One‐step carbonization of some biomass materials has recently been demonstrated as a promising route to produce high surface area carbon without introducing extra activation agents. Here, this study shows the importance of physiologic function of plant organs in the microstructure and porosity of formed carbon nanomaterials. The lotus stem pyrolyzed carbon at 800 °C presents a specific surface area of 1610 m² g^?1, about 55% higher than the porous carbon from the leaves. A similar organ‐dependent effect in the porosity of the pyrolyzed carbon is also observed in other plants with wide disparity in the stems and leaves, such as celery and asparagus lettuce, largely due to the higher metal ion content in the stems, which plays the role of ion transportation for plants. Furthermore, optimizing the celery stem pyrolyzing condition can produce carbon with specific surface area as high as 2194 m² g^?1 without any extra activation process. As a supercapacitor electrode, the porous carbon pyrolyzed from lotus stems exhibits a specific capacitance of 174 F g^?1 at a scan rate of 5 mV s^?1 in 6 M KOH aqueous electrolyte, with 72% capacitance retention at a high scan rate of 500 mV s^?1 and good stability over 10 000 cycles.     

Covalent–Organic Framework (COF)-Core–Shell Composites: Classification,Synthesis, Properties,and Applications

Core–shell structures, where the “guest” material is encapsulated within a protective shell, integrate the advantages of different materials to enhance the overall properties of the composite. Covalent–organic frameworks (COFs) are favorable candidates for composing core–shell structures due to their inherent porosity, good activity, excellent stability, and other advantages. In particular, COFs as shells to encapsulate other functional materials are becoming increasingly popular in the fields of environmental remediation and energy conversion. However, there is a lack of reviews on COF-based core–shell materials. In this context, this review provides a systematic summary of the current research on COF-based core–shell composites. First, a simple classification is made for COF-based core–shell composites. The second part of the review describes the main synthesis methods. The changes brought about by the COF shell and core–shell structures on the properties of the composites and their applications in photocatalysis, electrocatalysis, adsorption, sensing, and supercapacitors are then emphasized. Finally, new perspectives on the future development and challenges of composites are presented. The purpose of this study is to provide future insights into the design and application of COF-based core–shell composites.     

Highly Efficient Electrocatalysts for Oxygen Reduction Reaction Based on 1D Ternary Doped Porous Carbons Derived from Carbon Nanotube Directed Conjugated Microporous Polymers

One‐dimensional (1D) porous materials have shown great potential for gas storage and separation, sensing, energy storage, and conversion. However, the controlled approach for preparation of 1D porous materials, especially porous organic materials, still remains a great challenge due to the poor dispersibility and solution processability of the porous materials. Here, carbon nanotube (CNT) templated 1D conjugated microporous polymers (CMPs) are prepared using a layer‐by‐layer method. As‐prepared CMPs possess high specific surface areas of up to 623 m² g^?1 and exhibit strong electronic interactions between p‐type CMPs and n‐type CNTs. The CMPs are used as precursors to produce heteroatom‐doped 1D porous carbons through direct pyrolysis. As‐produced ternary heteroatom‐doped (B/N/S) 1D porous carbons possess high specific surface areas of up to 750 m² g^?1, hierarchical porous structures, and excellent electrochemical‐catalytic performance for oxygen reduction reaction. Both of the diffusion‐limited current density (4.4 mA cm^?2) and electron transfer number (n = 3.8) for three‐layered 1D porous carbons are superior to those for random 1D porous carbon. These results demonstrate that layered and core–shell type 1D CMPs and related heteroatom‐doped 1D porous carbons can be rationally designed and controlled prepared for high performance energy‐related applications.     

Collectively Exhaustive Electrodes Based on Covalent Organic Framework and Antagonistic Co‐Doping for Electroactive Ionic Artificial Muscles

In this study, high‐performance ionic soft actuators are developed for the first time using collectively exhaustive boron and sulfur co‐doped porous carbon electrodes (BS‐COF‐Cs), derived from thiophene‐based boronate‐linked covalent organic framework (T‐COF) as a template. The one‐electron deficiency of boron compared to carbon leads to the generation of hole charge carriers, while sulfur, owing to its high electron density, creates electron carriers in BS‐COF‐C electrodes. This antagonistic functionality of BS‐COF‐C electrodes assists the charge‐transfer rate, leading to fast charge separation in the developed ionic soft actuator under alternating current input signals. Furthermore, the hierarchical porosity, high surface area, and synergistic effect of co‐doping of the BS‐COF‐Cs play crucial roles in offering effective interaction of BS‐COF‐Cs with poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), leading to the generation of high electro‐chemo‐mechanical performance of the corresponding composite electrodes. Finally, the developed ionic soft actuator based on the BS‐COF‐C electrode exhibits large bending strain (0.62%), excellent durability (90% retention for 6 hours under operation), and 2.7 times higher bending displacement than PEDOT:PSS under extremely low harmonic input of 0.5 V. This study reveals that the antagonistic functionality of heteroatom co‐doped electrodes plays a crucial role in accelerating the actuation performance of ionic artificial muscles.