Covalent organic frameworks in tribology - A perspective

Two-dimensional covalent organic frameworks (2D COFs) are an emerging class of crystalline porous materials formed through covalent bonds between organic building blocks. COFs uniquely combine a large surface area, an excellent stability, numerous abundant active sites, and tunable functionalities, thus making them highly attractive for numerous applications. Especially, their abundant active sites and weak interlayer interaction make these materials promising candidates for tribological research. Recently, notable attention has been paid to COFs as lubricant additives due to their excellent tribological performance. Our review aims at critically summarizing the state-of-art developments of 2D COFs in tribology. We discuss their structural and functional design principles, as well as synthetic strategies with a special focus on tribology. The generation of COF thin films is also assessed in detail, which can alleviate their most challenging drawbacks for this application. Subsequently, we analyze the existing state-of-the-art regarding the usage of COFs as lubricant additives, self-lubrication composite coatings, and solid lubricants at the nanoscale. Finally, critical challenges and future trends of 2D COFs in tribology are outlined to initiate and boost new research activities in this exciting field.

Heterointerface regulation of covalent organic framework-anchored graphene via a solvent-free strategy for high-performance supercapacitor and hybrid capacitive deionization electrodes

Covalent organic frameworks (COFs) with customizable geometry and redox centers are an ideal candidate for supercapacitors and hybrid capacitive deionization (HCDI). However, their poor intrinsic conductivity and micropore-dominated pore structures severely impair their electrochemical performance, and the synthesis process using organic solvents brings serious environmental and cost issues. Herein, a 2D redox-active pyrazine-based COF (BAHC-COF) was anchored on the surface of graphene in a solvent-free strategy for heterointerface regulation. The as-prepared BAHC-COF/graphene (BAHCGO) nanohybrid materials possess high-speed charge transport offered by the graphene carrier and accelerated electrolyte ion migration within the BAHC-COF, allowing ions to effectively occupy ion storage sites inside BAHC. As a result, the BAHCGO//activated carbon asymmetric supercapacitor achieves a high energy output of 61.2 W h kg-1 and a satisfactory long-term cycling life. More importantly, BAHCGO-based HCDI possesses a high salt adsorption capacity (SAC) of 67.5 mg g-1 and excellent long-term desalination/regeneration stability. This work accelerates the application of COF-based materials in the fields of energy storage and water treatment.

Simultaneous electrochemical sensing of heavy metal ions (Zn2+, Cd2+, Pb2+, and Hg2+) in food samples using a covalent organic framework/carbon black modified glassy carbon electrode

In this study, for the first time, a selective electrochemical sensor by glassy carbon electrode (GCE) modified with the covalent organic framework (COF) and carbon black (CB) was introduced and applied to simultaneous sensing of Zn2+, Cd2+, Pb2+, and Hg2+ via differential pulse anodic stripping voltammetry (DPASV). The COF is supplied through a condensation reaction between melamine and trimesic acid. The COF and CB, which are used to modify the GCE surface, increase electrochemical activity. The linearity to determine ions was achieved as Zn2+: 0.009-1100 nM, Cd2+: 0.005-1100 nM, Pb2+: 0.003-1100 nM, and Hg2+: 0.001-1100 nM. Besides, the detection limits for Zn2+, Cd2+, Pb2+, and Hg2+ have obtained 0.003, 0.002, 0.001 and 0.0003 nM, respectively. The CB-COF/GCE was applied to simultaneously measure the ions in food samples. For validation, atomic absorption spectrometry (AAS) was applied to measure the amount of target metal ions as a standard method in real samples.

Covalent Organic Framework/Graphene Hybrids: Synthesis, Properties, and Applications

To address current energy crises and environmental concerns, it is imperative to develop and design versatile porous materials ideal for water purification and energy storage. The advent of covalent organic frameworks (COFs), a revolutionary terrain of porous materials, is underscored by their superlative features such as divinable structure, adjustable aperture, and high specific surface area. However, issues like inferior electric conductivity, inaccessible active sites impede mass transfer and poor processability of bulky COFs restrict their wider application. As a herculean stride forward, COF/graphene hybrids amalgamate the strengths of their constituent components and have in consequence, enticed significant scientific intrigue. Herein, the current progress on the structure and properties of graphene-based materials and COFs are systematically outlined. Then, synthetic strategies for preparing COF/graphene hybrids, including one-pot synthesis, ex situ synthesis, and in situ growth, are comprehensively reviewed. Afterward, the pivotal attributes of COF/graphene hybrids are dissected in conjunction with their multifaceted applications spanning adsorption, separation, catalysis, sensing, and energy storage. Finally, this review is concluded by elucidating prevailing challenges and gesturing toward prospective strides within the realm of COF/graphene hybrids research.

Understanding the Behavior of Supercapacitor Materials via Electrochemical Impedance Spectroscopy: A Review

Energy harvesting and energy storage are two critical aspects of supporting the energy transition and sustainability. Many studies have been conducted to achieve excellent performance devices for these two purposes. As energy-storing devices, supercapacitors (SCs) have tremendous potential to be applied in several sectors. Some electrochemical characterizations define the performance of SCs. Electrochemical impedance spectroscopy (EIS) is one of the most powerful analyses to determine the performance of SCs. Some parameters obtained from this analysis include bulk resistance, charge-transfer resistance, total resistance, specific capacitance, response frequency, and response time. This work provides a holistic and comprehensive review of utilizing EIS for SC characterization. Overall, researchers can benefit from this review by gaining a comprehensive understanding of the utilization of electrochemical impedance spectroscopy (EIS) for characterizing supercapacitors (SCs), enabling them to enhance SC performance and contribute to the advancement of energy harvesting and storage technologies.

Rapid, simple, and simultaneous electrochemical determination of cadmium, copper, and lead in Baijiu using a novel covalent organic framework based nanocomposite

It is of great significance to develop a simple and rapid electrochemical sensor for simultaneous determination of heavy metal ions (HMIs) in Baijiu by using new nanomaterials. Here, graphene (GR) was utilized to combine with covalent organic frameworks (COFs) that was synthesized via the aldehyde-amine condensation between 2, 5-dimethoxyterephthalaldehyde (DMTP) and 1, 3, 5-tris(4-aminophenyl) benzene (TAPB) to prepare a new GR/COFDPTB/GCE sensor for electrochemical sensing multiple HMIs. Compared with the glass carbon electrode (GCE), GR/GCE and COFDPTB/GCE, the developed sensor exhibited excellent electrochemical analysis ability for the simultaneous detection of Cd2+, Pb2+, and Cu2+ owing to the synergistically increased the specific surface area, the periodic porous network and plenty of effective binding sites, as well as the enhanced conductivity. Under the optimized experimental parameters, the proposed sensor showed good linearity range of 0.1-25 μM for Cd2+, and both 0.1-11 μM for Pb2+ and Cu2+ with the detection limits of Cd2+, Pb2+, and Cu2+ being 0.011 μM, 8.747 nM, and 6.373 nM, respectively. Besides, the designed sensor was successfully applied to the simultaneous detection of the three HMIs in Baijiu samples, suggesting its good practical application performance and a new method for the rapid detection of HMIs being expended.

Covalent organic frameworks: spotlight on applications in the pharmaceutical arena

Covalent organic frameworks (COFs) have much potential in the field of analytical separation research due to their distinctive characteristics, including easy modification, low densities, large specific surface areas and permanent porosity. This article provides a historical overview of the synthesis and broad perspectives on the applications of COFs. The use of COF-based membranes in gas separation, water treatment (desalination, heavy metals and dye removal), membrane filtration, photoconduction, sensing and fuel cells is also covered. However, these COFs also demonstrate great promise as solid-phase extraction sorbents and solid-phase microextraction coatings. In addition to various separation applications, this work aims to highlight important advancements in the synthesis of COFs for chiral and isomeric compounds.

Redox-Active Phenanthrenequinone Molecules and Nitrogen-Doped Reduced Graphene Oxide as Active Material Composites for Supercapacitor Applications

Carbon-based supercapacitor electrodes are generally restricted in energy density, as they rely exclusively on electric double-layer capacitance (EDLC). The introduction of redox-active organic molecules to obtain pseudocapacitance is a promising route to develop electrode materials with improved energy densities. In this work, we develop a porous nitrogen-doped reduced graphene oxide and 9,10-phenanthrenequinone composite (N-HtrGO/PQ) via a facile one-step physical adsorption method. The electrochemical evaluation of N-HtrGO/PQ using cyclic voltammetry showed a high capacitance of 605 F g-1 in 1 M H2SO4 when the composite consisted of 30% 9,10-phenanthrenequinone and 70% N-HtrGO. The measured capacitance significantly exceeded pure N-HtrGO without the addition of redox-active molecules (257 F g-1). In addition to promising capacitance, the N-HtrGO/30PQ composite showed a capacitance retention of 94.9% following 20,000 charge/discharge cycles. Based on Fourier transform infrared spectroscopy, we postulate that the strong π-π interaction between PQ molecules and the N-HtrGO substrate enhances the specific capacitance of the composite by shortening pathways for electron transfer while improving structural stability.

Solvent-Free Synthesis of Covalent Organic Framework/Graphene Nanohybrids: High-Performance Faradaic Cathodes for Supercapacitors and Hybrid Capacitive Deionization

Covalent organic frameworks (COFs) with flexible periodic skeletons and ordered nanoporous structures have attracted much attention as potential candidate electrode materials for green energy storage and efficient seawater desalination. Further improving the intrinsic electronic conductivity and releasing porosity of COF-based materials is a necessary strategy to improve their electrochemical performance. Herein, the employed graphene as the conductive substrate to in situ grow 2D redox-active COF (TFPDQ-COF) with redox activity under solvent-free conditions to prepare TFPDQ-COF/graphene (TFPDQGO) nanohybrids and explores their application in both supercapacitor and hybrid capacitive deionization (HCDI). By optimizing the hybridization ratio, TFPDQGO exhibits a large specific capacitance of 429.0 F g-1 due to the synergistic effect of the charge transport highway provided by the graphene layers and the abundant redox-active centers contained in the COF skeleton, and the assembled TFPDQGO//activated carbon (AC) asymmetric supercapacitor possesses a high energy output of 59.4 Wh kg-1 at a power density of 950 W kg-1 and good cycling life. Furthermore, the maximum salt adsorption capacity (SAC) of 58.4 mg g-1 and stable regeneration performance is attained for TFPDQGO-based HCDI. This study highlights the new opportunities of COF-based hybrid materials acting as high-performance supercapacitor and HCDI electrode materials.

Progress and Perspectives on Promising Covalent-Organic Frameworks (COFs) Materials for Energy Storage Capacity

In recent years, a new class of highly crystalline advanced permeable materials covalent-organic frameworks (COFs) have garnered a great deal of attention thanks to their remarkable properties, such as their large surface area, highly ordered pores and channels, and controllable crystalline structures. The lower physical stability and electrical conductivity, however, prevent them from being widely used in applications like photocatalytic activities and innovative energy storage and conversion devices. For this reason, many studies have focused on finding ways to improve upon these interesting materials while also minimizing their drawbacks. This review article begins with a brief introduction to the history and major milestones of COFs development before moving on to a comprehensive exploration of the various synthesis methods and recent successes and signposts of their potential applications in carbon dioxide (CO2 ) sequestration, supercapacitors (SCs), lithium-ion batteries (LIBs), and hydrogen production (H2 -energy). In conclusion, the difficulties and potential of future developing with highly efficient COFs ideas for photocatalytic as well as electrochemical energy storage applications are highlighted.

Recent advances in the utilization of covalent organic frameworks (COFs) as electrode materials for supercapacitors

Due to their excellent stability, ease of modification, high specific surface area, and tunable redox potentials, covalent organic frameworks (COFs) as potential electrodes in supercapacitors (SCs) have raised much research interest because these materials can enable the achievement of high electric double-layer supercapacitance and high pseudocapacitance. Here, the design strategies and SC applications of COF-based electrode materials are summarized. The detailed principles are introduced first, followed by discussions on strategies with diverse examples. The updated advances in design and applications are also discussed. Finally, in the outlook section, we provide some guidelines on the rational design of COF-based electrode materials for high-performance SCs, which we hope will inspire novel concepts for COF-based supercapacitors.

Integrated carbon nanotube and triazine-based covalent organic framework composites for high capacitance performance

As a rising class of crystallographic organic polymers, covalent-organic frameworks (COFs) have high specific surface areas, ordered pore structures, and designability, which exhibit broad application prospects in the energy storage sector. However, their low electrical conductivity hinders their potential use in supercapacitors. To improve the electrical conductivity, we introduced carboxylated multi-walled carbon nanotubes to obtain a series of carbon nanotube@COF composites by a facile one-pot method, in which 2D TFA-COFs are in situ grown on the surface of carboxylated multi-walled carbon nanotubes. Among them, the CNT@TFA-COF-3 composite exhibits good crystallinity, regular pores, excellent stability and a specific surface area of 1034 m² g^-1. As expected, as a capacitive electrode material, the CNT@TFA-COF composite shows improved electrochemical performance. Notably, the value of specific capacitance of the CNT@TFA-COF-3 composite (338 F g^-1) is about 8.5, 4.9, and 7.5 times higher than those of TFA-COFs, CNTs, and the CNT/TFA-COF physically mixed complex at a current density of 1.0 A g^-1, respectively. Furthermore, the CNT@TFA-COF-3 supercapacitor exhibits long-term cycle chemical stability and splendid rate capability even after 7000 charge-discharge cycles. The successful preparation of the CNT@TFA-COF-3 composite can provide new ideas for the construction of new COF-based composites and the development of new materials for energy storage.

Synthesis of Covalent Organic Frameworks (COFs)-Nanocellulose Composite and Its Thermal Degradation Studied by TGA/FTIR

At present, the synthesis methods of crystalline porous materials often involve powder products, which not only affects the practical application but also has complex synthesis operations and limited scale. Based on the mechanochemical method, we choose COF-TpPa-1, preparing TpPa-1-DANC composites. Covalent organic frameworks (COFs) are a kind of crystalline material formed by covalent bonds of light elements. COFs possess well pore structure and high thermal stability. However, the state of synthesized powders limits their application. Cellulose nanocrystals (CNCs) are promising renewable micron materials with abundant hydroxyl groups on their surface. It is possible to prepare high-strength materials such as film, water, and aerogel. Firstly, the nanocellulose was oxidized by the sodium periodate method to obtain aldehyde cellulose nanocrystals (DANC). TpPa-1-DANC not only had the crystal characteristic peak of COFs at 2θ ≈ 5° but also had a BET surface area of 247 m²/g. The chemical bonds between COFs and DANC formed by Schiff base reaction appeared in FTIR and XPS. The pyrolysis behavior of the composite was characterized by TG-IR, which showed that the composite had good thermal stability. With the advantages of nanocellulose as a material in every dimension, we believe that this method can be conducive to the large-scale synthesis of COFs composites, and has the possibility of multi-form synthesis of COFs.

COF-43 based voltammetric sensor for Ag(I) determination: optimization of experimental conditions by Box-Behnken design

Hydrazone-linked covalent organic framework-43 (COF-43) was synthesized and the carbon paste electrode (CPE) modified with this COF was used as a voltammetric sensor to measure silver(I). Various characterization tests (XRD, FTIR, BET, SEM/EDX, electrochemical impedance (EIS), and cyclic voltammetry (CV)) were performed on the synthesized COF-43 and the prepared COF-43/CPE. Box-Behnken design was used to optimize the preparation and operation conditions of the sensor. EIS and CV investigations reveal the diffusive characteristics of silver transport in the electrode matrix. An appropriate mechanism for the sensor procedure has been suggested and ratified by electrochemical and SEM/EDX techniques. The COF-43 used has several recognition elements for the selective binding of silver ion and due to its high porosity provides a large space for the deposition and reduction of large amounts of silver. Therefore, due to the correct selection of COF used in the construction of the sensor, high selectivity and sensitivity for the prepared sensor has been achieved. The obtained data disclosed that the modification of the carbon paste electrode by COF-43 significantly improves the analytical characteristics of the sensor, which specifies the performance of COF-43 as a sensory material for determining silver(I). The obtained calibration curve is linear in the concentration range from 0.001 μM to 10.0 μM and the detection limit is 1.5 × 10^-10 M. Various statistical tests have been employed to evaluate the sensor performance. The appropriate accuracy and precision of the proposed method were confirmed using the analysis of variance (ANOVA) approach. Potential interferences were investigated and it was found that the other species did not have a significant effect on the sensor performance. The prepared sensor has been successfully used to measure silver in two samples of photographic effluents, bleaching, and fixing agents. The results from the analysis of real samples demonstrate the reliable applicability of the fabricated sensor.

Nitrogen-enriched micro-mesoporous carbon derived from polymers organic frameworks for high-performance capacitive deionization

Nitrogenization is an effective method for improving the capacitive deionization (CDI) performance of porous carbon materials. In particular, polymer organic frameworks with heteroatom doping, containing an ordered pore structure and excellent electrochemical stability, are ideal precursors for carbon materials for high-performance CDI. In this study, a nitrogen-enriched micro-mesoporous carbon (NMC) electrode was fabricated by carbonizing a Schiff base network-1 at 500, 600, and 700 °C. Scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, N₂ adsorption-desorption, the contact angle of water, cyclic voltammetry, and electrochemical impedance spectroscopy were used to characterize the morphological structure, wettability, Brunauer-Emmett-Teller surface areas, and electrochemical performance of the NMCs. The results showed that the NMC carbonized at 600°C achieved the best specific capacitance (152.33 F/g), as well as a high electrosorption capacity (25.53 mg/g) because of its chemical composition (15.57% N) and surface area (312 m²/g). These findings prove that NMC is viable as an electrode material for desalination by high-performance CDI applications.

Covalent organic frameworks as multifunctional materials for chemical detection

Sensitive and selective detection of chemical and biological analytes is critical in various scientific and technological fields. As an emerging class of multifunctional materials, covalent organic frameworks (COFs) with their unique properties of chemical modularity, large surface area, high stability, low density, and tunable pore sizes and functionalities, which together define their programmable properties, show promise in advancing chemical detection. This review demonstrates the recent progress in chemical detection where COFs constitute an integral component of the achieved function. This review highlights how the unique properties of COFs can be harnessed to develop different types of chemical detection systems based on the principles of chromism, luminescence, electrical transduction, chromatography, spectrometry, and others to achieve highly sensitive and selective detection of various analytes, ranging from gases, volatiles, ions, to biomolecules. The key parameters of detection performance for target analytes are summarized, compared, and analyzed from the perspective of the detection mechanism and structure-property-performance correlations of COFs. Conclusions summarize the current accomplishments and analyze the challenges and limitations that exist for chemical detection under different mechanisms. Perspectives on how future directions of research can advance the COF-based chemical detection through innovation in novel COF design and synthesis, progress in device fabrication, and exploration of novel modes of detection are also discussed.

Novel melamine-based porous organic polymers: synthesis, characterizations, morphology modifications, and their applications in lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries have been considered to be one of the most promising energy storage devices in the next generation. However, the insulating properties of sulfur and the shuttle effect of soluble lithium polysulfides (LiPSs) seriously hinder the practical application of Li-S batteries. In this paper, a novel porous organic polymer (HUT3) was prepared based on the polycondensation between melamine and 1,4-phenylene diisocyanate. The micro morphology of HUT3 was improved byin situgrowth on different mass fractions of rGO (5%, 10%, 15%), and the obtained HUT3-rGO composites were employed as sulfur carriers in Li-S batteries with promoted the sulfur loading ratio and lithium-ion mobility. Attributed to the synergistic effect of the chemisorption of polar groups and the physical constraints of HUT3 structure, HUT3-rGO/S electrodes exhibits excellent capacity and cyclability performance. For instance, HUT3-10rGO/S electrode exhibits a high initial specific capacity of 950 mAh g^-1at 0.2 C and retains a high capacity of 707 mAh g^-1after 500 cycles at 1 C. This work emphasizes the importance of the rational design of the chemical structure and opens up a simple way for the development of cathode materials suitable for high-performance Li-S batteries.

Two-Dimensional Polymers and Polymerizations

Synthetic chemists have developed robust methods to synthesize discrete molecules, linear and branched polymers, and disordered cross-linked networks. However, two-dimensional polymers (2DPs) prepared from designed monomers have been long missing from these capabilities, both as objects of chemical synthesis and in nature. Recently, new polymerization strategies and characterization methods have enabled the unambiguous realization of covalently linked macromolecular sheets. Here we review 2DPs and 2D polymerization methods. Three predominant 2D polymerization strategies have emerged to date, which produce 2DPs either as monolayers or multilayer assemblies. We discuss the fundamental understanding and scope of each of these approaches, including: the bond-forming reactions used, the synthetic diversity of 2DPs prepared, their multilayer stacking behaviors, nanoscale and mesoscale structures, and macroscale morphologies. Additionally, we describe the analytical tools currently available to characterize 2DPs in their various isolated forms. Finally, we review emergent 2DP properties and the potential applications of planar macromolecules. Throughout, we highlight achievements in 2D polymerization and identify opportunities for continued study.

Progress and Perspectives on Covalent-organic Frameworks (COFs) and Composites for Various Energy Applications

Covalent-organic frameworks (COFs), being a new member of the crystalline porous materials family, have emerged as important materials for energy storage/conversion/generation devices. They possess high surface areas, ordered micro/mesopores, designable structures and an ability to precisely control electro-active groups in their pores, which broaden their application window. Thanks to their low weight density, long range crystallinity, reticular nature and tunable synthesis approach towards two and three dimensional (2D and 3D) networks, they have been found suitable for a range of challenging electrochemical applications. Our review focuses on the progress made on the design, synthesis and structure of COFs and their composites for various energy applications, such as metal-ion batteries, supercapacitors, water-splitting and solar cells. Additionally, attempts have been made to correlate the structural and mechanistic characteristics of COFs with their applications.

Triazine-based porous organic polymers with enhanced electronegativity as multifunctional separator coatings in lithium-sulfur batteries

The commercialization of lithium-sulfur batteries is seriously affected by the shuttle behavior and slow conversion kinetics of polysulfides. Herein, a new porous organic polymer (POP) is synthesized and grown on reduced graphene oxide (rGO) in situ to improve battery performance, which serves as an efficient polysulfide adsorber and catalytic promoter for polysulfide conversion. The polar POP shows strong chemisorption to polysulfides, which is confirmed by a series of calculations and experimental results. As a popular conductive substrate, rGO offers an electron transport channel for sulfur and polysulfide conversion. Due to the synergistic functions of composite materials, the batteries with POP@rGO modified separators retain a high specific capacity of 697.3 mA h g-1 and a minimum capacity fading rate of 0.04% per cycle at 1C over 500 cycles. Besides, even at a high sulfur loading of 5 mg cm-2, a high area capacity of 4.27 mA h cm-2 can also be achieved, which shows that it has great potential in promoting the commercialization of lithium-sulfur batteries.

Mixed-metal hybrid ultramicroporous material (HUM) precursor to graphene-supported tetrataenite as a highly active and durable NPG catalyst for the OER

Current interest in investigating non-precious group (NPG) metals for catalyzing the oxygen evolution reaction (OER) has revealed that doping of Ni hydroxides with Fe results in the dramatic enhancement of catalytic activity. Herein, a facile pathway to construct tetrataenite, an NiFe alloy of extraterrestrial origin and to address the limited electrical conductivity of metal oxides/hydroxides by directly constructing them atop graphene sheets is described. In this approach, a one-pot, bottom-up assembly of hybrid ultramicroporous materials (HUMs) was carried out, in the presence of suspended graphene (G), to homogeneously deposit the HUMs on unmodified graphene sheets, affording HUMs@G. Single metal (SIFSIX-3-Ni@G) and mixed metal (SIFSIX-3-NiFe@G) HUMs can be readily synthesized from their respective metal salts to afford a well-designed catalyst for the OER. The pyrolysis of SIFSIX-3-NiFe@G resulted in the deposition of the nanoalloy tetrataenite on G, demonstrating an exceptionally low OER onset potential of 1.44 V vs. RHE and reduced overpotential at 10 mA cm-2 (η10 = 266 mV). The synergy between the composition of the active catalyst and the electronically conductive support was attained by designing a reaction system encoding the self-assembly of a crystalline pre-catalyst on G sheets.

Covalent organic frameworks (COFs) for electrochemical applications

Covalent organic frameworks are a class of extended crystalline organic materials that possess unique architectures with high surface areas and tuneable pore sizes. Since the first discovery of the topological frameworks in 2005, COFs have been applied as promising materials in diverse areas such as separation and purification, sensing or catalysis. Considering the need for renewable and clean energy production, many research efforts have recently focused on the application of porous materials for electrochemical energy storage and conversion. In this respect, considerable efforts have been devoted to the design and synthesis of COF-based materials for electrochemical applications, including electrodes and membranes for fuel cells, supercapacitors and batteries. This review article highlights the design principles and strategies for the synthesis of COFs with a special focus on their potential for electrochemical applications. Recently suggested hybrid COF materials or COFs with hierarchical porosity will be discussed, which can alleviate the most challenging drawback of COFs for these applications. Finally, the major challenges and future trends of COF materials in electrochemical applications are outlined.

Covalent Organic Frameworks: Design, Synthesis, and Functions

Covalent organic frameworks (COFs) are a class of crystalline porous organic polymers with permanent porosity and highly ordered structures. Unlike other polymers, a significant feature of COFs is that they are structurally predesignable, synthetically controllable, and functionally manageable. In principle, the topological design diagram offers geometric guidance for the structural tiling of extended porous polygons, and the polycondensation reactions provide synthetic ways to construct the predesigned primary and high-order structures. Progress over the past decade in the chemistry of these two aspects undoubtedly established the base of the COF field. By virtue of the availability of organic units and the diversity of topologies and linkages, COFs have emerged as a new field of organic materials that offer a powerful molecular platform for complex structural design and tailor-made functional development. Here we target a comprehensive review of the COF field, provide a historic overview of the chemistry of the COF field, survey the advances in the topology design and synthetic reactions, illustrate the structural features and diversities, scrutinize the development and potential of various functions through elucidating structure-function correlations based on interactions with photons, electrons, holes, spins, ions, and molecules, discuss the key fundamental and challenging issues that need to be addressed, and predict the future directions from chemistry, physics, and materials perspectives.

Multifunctional carbon nanotubes covalently coated with imine-based covalent organic frameworks: exploring structure-property relationships through nanomechanics

The assembly of 3-dimensional covalent organic frameworks on the surface of carbon nanotubes is designed and successfully developed for the first time via the hybridization of imine-based covalent organic frameworks (COF-300) and oxidized MWCNTs by one-pot chemical synthesis. The resulting hybrid material ox-MWCNTs@COF exhibits a conformal structure that consists of a uniform amorphous COF layer covering the ox-MWCNT surface. The measurements of individual hybrid nanotube mechanical strength performed with atomic force microscopy provide insights into their stability and resistance. The results evidence a very robust hybrid tubular nanostructure that preserves the benefits obtained from COF, such as CO2 adsorption. Further digestion of the organic structure with aniline enables the study of the interplay between the hybrid interface and its nanomechanics. This new hybrid nanomaterial presents exceptional mechanical and electrical properties, merging the properties of the CNT template and COF-300.

Facile synthesis of a covalent organic framework (COF) based on the reaction of melamine and trimesic acid incorporated electrospun nanofiber and its application as an electrochemical tyrosinamide aptasensor

Schematic presentation of the COFCNF platform for the highly sensitive detection of tyrosinamide by using a solution containing [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ as a redox probe.

Bulk COFs and COF nanosheets for electrochemical energy storage and conversion

The current advances, structure-property relationship and future perspectives in covalent organic frameworks (COFs) and their nanosheets for electrochemical energy storage (EES) and conversion (EEC) are summarized.

Ultrastable Triazine-Based Covalent Organic Framework with an Interlayer Hydrogen Bonding for Supercapacitor Applications

Covalent organic frameworks (COFs) with redox-active units are a class of ideal materials for electrochemical-energy-storage devices. A novel two-dimensional (2D) PDC-MA-COF with redox-active triazine units was prepared via aldehyde-amine condensation reaction by using 1,4-piperazinedicarboxaldehyde (PDC) and melamine (MA) as structural units, which possessed high specific surface area (S_BET = 748.2 m² g^-1), narrow pore width (1.9 nm), large pore volume (1.21 cm³ g^-1), and high nitrogen content (47.87%), for pseudocapacitance application. The interlayer C-H···N hydrogen bonding can "lock" the relative distance between two adjacent layers to avoid an interlayer slip, which is more conducive to maintaining the ordered pore structure of the COF and improving a fast charge transfer between the electrode interface and triazine units. The PDC-MA-COF exhibited an excellent electrochemical performance with the highest specific capacitance of 335 F g^-1 along with 19.71% accessibility of the redox-active triazine units in a three-electrode system and 94 F g^-1 in a two-electrode system at 1.0 A g^-1 current density. Asymmetric supercapacitor of PDC-MA-COF//AC assembled using PDC-MA-COF and activated carbon (AC) as positive and negative electrode materials, respectively, exhibited a high energy density of 29.2 W h kg^-1 with a power density of 750 W kg^-1. At the same time, it also showed an excellent cyclic stability and could retain 88% of the initial capacitance after 20 000 charge-discharge cycles, which was better than those of the most of the analogous materials reported previously. This study provided a new strategy for designing redox-active COFs for pseudocapacitive storage.

Functional π-Conjugated Two-Dimensional Covalent Organic Frameworks

Fingerprints of π-conjugated compounds are ubiquitous in nature and play a crucial part in human existence. For instance, cis-retinal, an endogenous π-conjugated molecule present in the eye, performs a vital role in the function of visual perception. π-Conjugated molecules have also received a great deal of attention owing to their intriguing optical properties and created a surge in optoelectronics. Varieties of π-conjugated molecules/oligomers have been developed and explored for a number of applications such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), organic photovoltaics (OPVs), and sensors, among others. While the extended π-delocalization in one-dimensional (1D) polymers versus oligomers produce superior optical and electronic properties, further extension of π-delocalization to the second dimension (2D) is expected to give rise even more intriguing properties as revealed by theoretical studies. As a matter of fact, graphene is the best example of 2D-conjugated polymers, but its zero-band-gap behavior is a major impediment for semiconducting applications. In contrast, it was challenging to prepare 2D crystalline polymers until the discovery of boroxine/boronate ester linked covalent organic frameworks (COFs) by Yaghi and co-workers. COFs are a new class of porous crystalline polymers in which organic building blocks are held together by covalent bonds. These polymers exhibit potential applications in gas storage, energy storage, photocatalyst, heterogeneous catalysis, sensors, etc. However, the first π-conjugated COF was realized in 2009 via the introduction of imine linker (-C═N-) between the building blocks. Since then, wide varieties of COFs with various π-delocalization promoting spacers have been developed and explored their electronic and optical properties and pertinent applications. In this review, we will highlight the importance of 2D π-conjugated COFs and their achievements in developing novel functionalities.

Porous Polymers as Multifunctional Material Platforms toward Task-Specific Applications

Exploring advanced porous materials is of critical importance in the development of science and technology. Porous polymers, being famous for their all-organic components, tailored pore structures, and adjustable chemical components, have attracted an increasing level of research interest in a large number of applications, including gas adsorption/storage, separation, catalysis, environmental remediation, energy, optoelectronics, and health. Recent years have witnessed tremendous research breakthroughs in these fields thanks to the unique pore structures and versatile skeletons of porous polymers. Here, recent milestones in the diverse applications of porous polymers are presented, with an emphasis on the structural requirements or parameters that dominate their properties and functionalities. The Review covers the following applications: i) gas adsorption, ii) water treatment, iii) separation, iv) heterogeneous catalysis, v) electrochemical energy storage, vi) precursors for porous carbons, and vii) other applications (e.g., intelligent temperature control textiles, sensing, proton conduction, biomedicine, optoelectronics, and actuators). The key requirements for each application are discussed and an in-depth understanding of the structure-property relationships of these advanced materials is provided. Finally, a perspective on the future research directions and challenges in this field is presented for further studies.

pH-Responsive Hybrid Hydrogels as Antibacterial and Drug Delivery Systems

This study describes the design and synthesis of organic⁻inorganic hybrid hydrogels based on an interpenetrating polymer network (IPN) composed of polyaspartic acid crosslinked by graphene nanosheets as the primary network and poly(acrylamide-co-acrylic acid) as the secondary network. Silver, copper oxide, and zinc oxide nanoparticles were formed within the gel matrix, and the obtained hydrogel was applied to a load and controlled release of curcumin. The loading of curcumin and the release of this drug from the gels depended on the nanoparticle's (NP's) content of hydrogels as well as the pH of the medium. The synthesized hydrogels showed antibacterial activity against E. coli and S. aureus bacteria. The ability of the synthesized hydrogels to incapacitate bacteria and their loading capacity and controlled release of curcumin qualify them for future therapies such as wound-dressing applications.

Fabrication of porous covalent organic frameworks as selective and advanced adsorbents for the on-line preconcentration of trace elements against the complex sample matrix

Herein, for the first time, the typical porous Covalent Organic Frameworks (COFs) CTpBD with superior chemical stability and large surface area were applied as sorbents for solid phase extraction of trace ions via flow injection followed by inductively coupled plasma mass spectrometry (ICP-MS) detection. The well-prepared and fully-characterized CTpBD COFs were filled in solid phase extraction cartridge as novel and robust adsorbents for element analysis. Separation and enrichment of Cr (III), Mn (II), Co (II), Ni (II), Cd (II), V (V), Cu (II), As (III), Se (IV), and Mo (VI) was then carried out, and the contents were measured by ICP-MS. Owing to the large surface area and instinctive porous structure of CTpBD, preconcentration of the target trace elements via COF-filled on-line SPE column has achieved low detection limits of 2.1-21.6ngL^-1 along with a wide linearity range at 0.05-25μgL^-1 for all target ions. The relative standard deviations (RSD) of 1.2%-4.3% obtained via 11 parallel determinations at the sample concentration of 100ngL^-1 revealed excellent repeatability of the developed methods Our proposed methods have been successfully utilized for trace element analysis in environmental and food samples.

Electrochemical Stimuli-Driven Facile Metal-Free Hydrogen Evolution from Pyrene-Porphyrin-Based Crystalline Covalent Organic Framework

A [2 + 2] Schiff base type condensation between 5,10,15,20-tetrakis(4-aminophenyl)porphyrin (TAP) and 1,3,6,8-tetrakis (4-formylphenyl) pyrene (TFFPy) under solvothermal condition yields a crystalline, quasi-two-dimensional covalent organic framework (SB-PORPy-COF). The porphyrin and pyrene units are alternatively occupied in the vertex of 3D triclinic crystal having permanent microporosity with moderately high surface area (∼869 m² g^-1) and promising chemical stability. The AA stacking of the monolayers give a pyrene bridged conducting channel. SB-PORPy-COF has been exploited for metal free hydrogen production to understand the electrochemical behavior using the imine based docking site in acidic media. SB-PORPy-COF has shown the onset potential of 50 mV and the Tafel slope of 116 mV dec^-1. We expect that the addendum of the imine based COF would not only enrich the structural variety but also help to understand the electrochemical behavior of these class of materials.

A New Triazine-Based Covalent Organic Framework for High-Performance Capacitive Energy Storage

The new covalent organic framework material TDFP-1 was prepared through a solvothermal Schiff base condensation reaction of the monomers 1,3,5-tris-(4-aminophenyl)triazine and 2,6-diformyl-4-methylphenol. Owing to its high specific surface area of 651 m²  g^-1 , extended π conjugation, and inherent microporosity, TDFP-1 exhibited an excellent energy-storage capacity with a maximum specific capacitance of 354 F g^-1 at a scan rate of 2 mV s^-1 and good cyclic stability with 95 % retention of its initial specific capacitance after 1000 cycles at 10 A g^-1 . The π-conjugated polymeric framework as well as ionic conductivity owing to the possibility of ion conduction inside the micropores of approximately 1.5 nm make polymeric TDFP-1 a favorable candidate as a supercapacitor electrode material. The electrochemical properties of this electrode material were measured through cyclic voltammetry, galvanic charge-discharge, and electrochemical impedance spectroscopy, and the results indicate its potential for application in energy-storage devices.

Spray drying for making covalent chemistry II: synthesis of covalent–organic framework superstructures and related composites

Here we report a method that combines the spray-drying technique with a dynamic covalent chemistry process to synthesize zero-dimensional, spherical and microscale superstructures made from the assembly of imine-based COF nanocrystals.

Interfacial synthesis of ordered and stable covalent organic frameworks on amino-functionalized carbon nanotubes with enhanced electrochemical performance

Highly ordered 2D COF_TTA–DHTAwas synthesized on amino-functionalized MWCNTs with high crystallinity, regular pore structures and improved electrochemical performance.

Covalent organic frameworks based on Schiff-base chemistry: synthesis, properties and potential applications

Covalent organic-frameworks (COFs) are an emerging class of porous and ordered materials formed by condensation reactions of organic molecules.

The potential of a graphene-supported porous-organic polymer (POP) for CO2 electrocatalytic reduction

A composite of porous-organic polymer and graphene demonstrates electrocatalytic activity toward CO₂ reduction in aqueous medium.