Enhanced Proton Conductivity of Imidazole-Doped Thiophene-Based Covalent Organic Frameworks via Subtle Hydrogen Bonding Modulation

A critical review of COFs-based photocatalysis for environmental remediation

Covalent organic frameworks (COFs) are highly porous crystalline polymers formed through covalent bonding of molecular building blocks. Numerous fabrication strategies have been developed, including solvothermal, ionothermal, microwave, mechanochemical, and sonochemical methods, alongside ligand substitution and post-modification techniques, which allow for precise control over the structures and properties of COFs. The exceptional physicochemical stability, large specific surface area, broad visible light absorption, and extended π-conjugated systems have sparked significant interest in photocatalytic applications. Recently, COFs have shown remarkable efficacy in environmental remediation, demonstrating the ability to degrade a wide range of organic pollutants, including dyes, antibiotics, and drugs, as well as to reduce/oxidize heavy metals such as Cr(VI), U(VI), and As(III), in addition to targeting biological pollutants. This review comprehensively explores recent advancements in COFs-based photocatalysis, covering synthetic methods, COF types, modification method, theoretical calculations, environmental applications, and underlying mechanisms. Additionally, the challenges and opportunities for COFs as a robust, cost-effective technology in practical applications was discussed, and offering valuable insights for researchers in environmental remediation, materials science, and photocatalysis.

Catalytic Reinforcement in Hybrid Imidazole Functionalized Cobalt Phthalocyanine and Ketjenblack for Boosted Hydrogen Evolution

Hydrogen energy is widely regarded as one of the cleanest forms of green energy due to its bio-friendly nature. However, its commercialization is restricted by many issues. One of the major issues is related to high production cost, which can be overcome by designing of effective catalysts through simple, innovative and cost-effective approach for efficient green hydrogen production. In this study, we report the synthesis of an eco-friendly, affordable, and highly redox-active tetra-imidazole functionalized cobalt phthalocyanine (TImCoPc) through a straightforward method. To ensure its purity and effectiveness as an electrocatalyst for the hydrogen evolution reaction (HER) in 0.5 M H₂SO₄, various analytical techniques are employed for its thorough characterization. The electrocatalytic performance and conductivity of TImCoPc is further enhanced by forming hybrid with highly conductive Ketjenblack (KB) in various ratios. Among the different ratios, the 3.5 : 1.5 (TImCoPc/KB) composite exhibited excellent HER performance, with an onset potential closer to that of Pt/C and an overpotential (η₁₀) of -108 mV, closely approaching the η₁₀ of Pt/C (-36 mV). Additionally, the optimized hybrid showed a lower Tafel slope of 44 mV/dec, higher turnover frequency of 3.66x10-10 mol s-1 cm-2 and mass activity of 4.121 A mg-1, with an excellent catalytic stability, thereby proving to be one of the superior electrocatalysts for HER in terms of cost, methodology, activity and efficiency. The observed enhancement in HER activity of the hybrid can be accounted to the synergistic effects that reduce the metal ion's d-band center and improve π-π interactions between the hybrid components. The composite also demonstrated an increased electrochemical active surface area. This work highlights the potential of the TImCoPc/KB (3.5 : 1.5) composite as a cost-effective and efficient catalyst for sustainable hydrogen production.

Skeleton Regulation of Covalent-Organic Frameworks From 2D to 3D Networks for High Anhydrous Proton Conduction

Developing new materials for anhydrous proton conduction under high-temperature conditions is very challenging but significant for proton exchange membrane fuel cells. Herein, a series of highly crystalline and robust covalent-organic frameworks (COFs) with different skeletons (2D and 3D) is designed and synthesized using steric hindrance engineering of the monomer. Moreover, a [4 + 2] construction approach is used to construct 3D COFs with entangled networks, which can be further post-modified with phosphite acid groups to improve intrinsic proton conduction. After loading with imidazole, COFs can realize a proton conductivity of 1.06 × 10-2 S cm-1 under anhydrous conditions, among the best proton-conducting COF materials loading imidazole. These materials show high stability at loading and testing conditions and maintain high proton conductivity over a wide temperature range (100-160 °C). This work provides a skeleton regulation approach to design materials for anhydrous proton conduction, showing great potential as high-temperature proton exchange membranes.

Simultaneous Achievement of Enhanced Nonlinear Optical Absorption and Nonlinear Refraction in Highly Crystalline 2D Covalent Organic Frameworks Ultrathin Films

Large enhancement of nonlinear absorption and nonlinear refraction are achieved simultaneously in highly ordered two dimensional (2D) covalent organic framework (COF) films prepared by solidliquid interface one-step method to overcome the weakness of COF powders in solubility. In the intrinsic nonlinear optical response obtained at 532 nm with 5 ns pulse, the nonlinear absorption coefficients (β) of two COF films are -4.87 × 10-5 and -1.29 × 10-5 m W-1, respectively. Simultaneously, the fitted closed-aperture curves also show large nonlinear refractive indexes (n2), -5.62 × 10-12 m2 W-1 and -0.76 × 10-12 m2 W-1. The 4f coherent imaging performed at the same condition with a single-shot pulse further verifies the outstanding nonlinear optical response without any damage probably experienced in the Z-scan technique. Moreover, the differences in framework electronic structure and photoexcited states between two COF films are compared to explain the difference in nonlinear optical response. All the results indicate that two COF crystalline films with intrinsic giant nonlinear optical response can be capable of modulating both amplitude and phase of light, providing huge potential in all-optical manipulating and switching at the nanoscale as outstanding nonlinear optical materials.

Anhydrous Solid-State Proton Conduction in Crystalline MOFs, COFs, HOFs, and POMs

Strategic design of solid-state proton-conducting electrolytes for application in anhydrous proton-exchange membrane fuel cells (PEMFCs) has gained burgeoning interest due to a spectrum of advantageous features, including higher CO tolerance and ease in the water management systems. Toward this direction, crystalline materials like metal-organic frameworks (MOFs), covalent organic frameworks (COFs), hydrogen-bonded organic frameworks (HOFs), and polyoxometalates (POMs) are emerging PEM materials, offering strategic structural engineering through crystallography, thus enabling ultrahigh anhydrous proton conductivity up to 10-2-10-1 S/cm. This Perspective highlights significant progress achieved thus far with such crystalline platforms in the domain of anhydrous proton conduction across a wide temperature window (sub-zero to above 100 °C). Based on their structural backgrounds, these platforms are categorized into four classes (viz. MOFs, COFs, HOFs, and POMs) with a detailed evolutionary timeline since their emergence early in 2009. Insightful discussions with a key focus on the strategies undertaken to attain anhydrous proton conductivity along with implementation in fuel cell technology through membrane electrode assembly are presented. A section on "Critical Analysis and Future Prospects" provides decisive key viewpoints on those overlooked issues with future endorsement (e.g., performance assessment with CO tolerance analysis and fuel cell test stand) for further development while comparing them with other anhydrous platforms from both academic and industrial perspectives.

Tuning the Crowding Effect of Water and Imidazole in Covalent Organic Frameworks for Proton Conduction

The proton conduction of imidazole under confined conditions has attracted widespread attention from researchers. Under anhydrous conditions, the proton transfer behavior is primarily governed by the molecular dynamics of imidazole. However, within a water-mediated system, the crowding effect of water and imidazole in a confined space may outweigh the intrinsic properties of imidazole itself. In this study, we have meticulously adjusted the structural fragments within the covalent organic frameworks (COFs), fine-tuning the saturation level of imidazole loading and adjusting the crowding degree of imidazole and water molecules. As a result, the two COF composites exhibit distinctly different proton conduction mechanisms from 32 to 100% relative humidity (RH), of which one possesses proton conduction progressively shifting from the Grotthuss mechanism to the vehicle mechanism, while the other has proton conduction undergoing a transition from the vehicle mechanism at 32% RH through the Grotthuss mechanism at 75% RH and finally back to the vehicle mechanism at 100% RH. These results highlight the critical role of the crowding effect of water and imidazole within confined spaces in proton conduction.

High Proton Conductivity of Acid Impregnated COFs Stabilized by Post-Oxidation

The investigation of proton conduction processes within artificial nanopores using phosphoric acid (H3PO4) and sulfuric acid (H2SO4) not only sheds light on the mechanisms of proton conduction for these strong acids in confined environments, while also provides critical insights into the proper understanding of biological transmembrane proton transport. However, the synthesis of stable and acid-resistant host frameworks is yet a major challenges. By following that, the present study is conducted with the aim of improving the chemical stability of an imine-based COF (CPOF-10) by converting it into an amide-linked COF (CPOF-11) via a post-oxidative approach. In which, the integration of an appropriate amount of imidazole groups into the framework facilitates the efficient impregnation of liquid proton-conducting acids. The obtained results indicate the ten times greater proton conductivity of H3PO4@CPOF-11 than that of H3PO4@CPOF-10, thereby, successfully achieving 8.63 × 10-2 S cm-1 at 160 °C, under nitrogen (N2) atmosphere. Moreover, the highly stable CPOF-11 tolerated H2SO4 doping, delivering a high proton conductivity of up to 1.70 × 10-1 S cm-1 at 140 °C, with a significantly low activation energy of 0.05 eV. To the best of the knowledge, this activation energy (0.05 eV) of H2SO4@CPOF-11 is found to be one of the lowest value among all the reported proton-conducting materials. Thus, this study will provide new understanding for the fabrication of advanced porous organic materials in fuel cells application.

Anisotropic Superprotonic Conduction in a Layered Single-Component Hydrogen-Bonded Organic Framework with Multiple In-Plane Open Channels

Hydrogen-bonded organic frameworks (HOFs) are promising proton conductive materials because of their inherent and abundant hydrogen-bonding sites. However, most superprotonic-conductive HOFs are constructed from multiple components to enable favorable framework architectures and structural integrity. In this contribution, layered HOF-TPB-A3 with a single component is synthesized and exfoliated. The exfoliated nanoplates exhibited anisotropic superprotonic conduction, with in-plane proton conductivities reaching 1.34 × 10-2 S cm-1 at 296 K and 98% relative humidity (RH). This outperforms the previously reported single-component HOFs and is comparable with the state-of-the-art multiple-component HOFs. The high and anisotropic proton conductive properties can be attributed to the efficient proton transport along multiple open channels parallel to their basal planes. Moreover, an all-solid-state (ASS) proton rectifier device is demonstrated by combining HOF-TPB-A3 and a hydroxide ion-conducting layered double hydroxide (LDH). This work suggests that single-component HOFs with multiple open channels offer more opportunities as versatile platforms for proton conductors, making them promising candidates for conducting media in protonic devices.

Photothermal Conversion Porous Organic Polymers: Design, Synthesis, and Applications

Solar energy is a primary form of renewable energy, and photothermal conversion is a direct conversion process with tunable conversion efficiency. Among various kinds of photothermal conversion materials, porous organic polymers (POP) are widely investigated owing to their controllable molecular design, tailored porous structures, good absorption of solar light, and low thermal conductivity. A variety of POP, such as conjugated microporous polymers (CMP), covalent organic frameworks (COF), hyper-crosslinked porous polymers (HCP), polymers of intrinsic microporosity (PIM), porous ionic polymers (PIP), are developed and applied in photothermal conversion applications of seawater desalination, latent energy storage, and biomedical fields. In this review, a comprehensive overview of the recent advances in POP for photothermal conversion is provided. The micro molecular structure characteristics and macro morphology of POP are designed for applications such as seawater desalination, latent heat energy storage, phototherapy and photodynamic therapy, and drug delivery. Besides, a probe into the underlying mechanism of structural design for constructing POP with excellent photothermal conversion performance is methodicalized. Finally, the remaining challenges and prospective opportunities for the future development of POP for solar energy-driven photothermal conversion applications are elucidated.

Ultrafine Spatial Modulation of Diazapyrene-Based Two-Dimensional Conjugated Covalent Organic Frameworks

Tailormade bottom-up synthesis of covalent organic frameworks (COFs) from various functional building blocks offer not only tunable topology and pore size but also multidimensional properties. High crystallinity is one of the prerequisites for their structures and associated physicochemical properties. Among different π-conjugated motifs for constructing COFs, pyrene-based tetragonal structures are effective in achieving highly ordered and crystalline states. In the present research, we demonstrated that the substitution of pyrene with 2,7-diazapyrene produces nearly "flat" structures of two-dimensional (2D) COF layers by controlling the torsional angle of linker molecules. Featuring finite pore diameters and excellent thermodynamic stability of ∼500 °C, ordered face-to-face (slipped AA) stacking arrangements were produced. Extended electrical conjugation spanning 2D frames with modest optical bandgaps (Eg) of ∼2.1 eV shows the planar character of diazapyrene-based COFs. The stacking of the conjugated 2D frames with small Eg values is also beneficial for the formation of highly stable conducting pathways in the crystalline state, which was confirmed by the results of the microwave conductivity measurements. Nitrogen centers in diazapyrene units also play a key role as the active sites for proton transfer, and the maximum proton conductivity of σ = 10-2 S cm-1 was achieved along the cocontinuous nanopore structures surrounded by the active sites. Results show that tetragonal COFs based on diazapyrene can be used as a highly crystalline two-dimensional material with special electrical and proton-conducting capabilities.

Accelerating Anhydrous Proton Transport in Covalent Organic Frameworks: Pore Chemistry and its Impacts

Proton conduction is important in both fundamental research and technological development. Here we report designed synthesis of crystalline porous covalent organic frameworks as a new platform for high-rate anhydrous proton conduction. By developing nanochannels with different topologies as proton pathways and loading neat phosphoric acid to construct robust proton carrier networks in the pores, we found that pore topology is crucial for proton conduction. Its effect on increasing proton conductivity is in an exponential mode other than linear fashion, endowing the materials with exceptional proton conductivities exceeding 10-2 S cm-1 over a broad range of temperature and a low activation energy barrier down to 0.24 eV. Remarkably, the pore size controls conduction mechanism, where mesopores promote proton conduction via a fast-hopping mechanism, while micropores follow a sluggish vehicle process. Notably, decreasing phosphoric acid loading content drastically reduces proton conductivity and greatly increases activation energy barrier, emphasizing the pivotal role of well-developed proton carrier network in proton transport. These findings and insights unveil a new general and transformative guidance for designing porous framework materials and systems for high-rate ion conduction, energy storage, and energy conversion.

Superprotonic conductivity of ketoenamine covalent-organic frameworks grafted by imidazole-based units

The achievement of covalent organic frameworks (COFs) with high stability and exceptional proton conductivity is of tremendous practical importance and challenge. Given this, we hope to prepare the highly stable COFs carrying CN connectors and enhance their proton conductivity via a post-modification approach. Herein, one COF, TpTta, was successfully synthesized by employing 1,3,5-triformylphloroglucinol (Tp) and 4,4',4″-(1,3,5-triazine-2,4,6-triyl)-trianiline (Tta) as starting materials, which has a β-ketoenamine structure bearing a large amount of -NH groups and intramolecular H-bonds. TpTta was then post-modified by inserting imidazole (Im) and histamine (His) molecules, yielding the corresponding COFs, Im@TpTta and His@TpTta, respectively. As a result, their proton conductivities were surveyed under changeable temperatures (30-100 °C) and relative humidities (68-98 %), revealing a degree of temperature and humidity dependence. Impressively, under identical conditions, the optimum proton conductivities of the two post-modified COFs are 1.14 × 10-2 (Im@TpTta) and 3.45 × 10-3 S/cm (His@TpTta), which are significantly greater than that of the pristine COF, TpTta (2.57 × 10-5 S/cm). Finally, their proton conduction mechanisms were hypothesized based on the computed activation energy values, water vapor adsorption values, and structural properties of these COFs. Additionally, the excellent electrochemical stability of the produced COFs was expressed, as well as the prospective application value.

Biomimetic Helical Hydrogen Bonded Organic Framework Membranes for Efficient Uranium Recovery from Seawater

Inspired by the uranyl-imidazole interactions via nitrogen's (N's) of histidine residues in single helical protein assemblies with open framework geometry that allows through migration/coordination of metal ions. Here, preliminary components of a stable hydrogen-bonded organic framework (HOF) are designed to mimic the stable single helical open framework with imidazole residues available for Uranium (U) binding. The imidazolate-HOF (CSMCRI HOF2-S) is synthesized with solvent-directed H-bonding in 1D array and tuned hydrophobic CH-π interactions leading to single helix pattern having enhanced hydrolytic stability. De-solvation led CSMCRI HOF2-P with porous helical 1D channels are transformed in a freestanding thin film that showcased improved mass transfer and adsorption of uranyl carbonate. CSMCRI HOF2-P thin film can effectively extract ≈14.8 mg g-1 in 4 weeks period from natural seawater, with > 1.7 U/V (Uranium to Vanadium ratio) selectivity. This strategy can be extended for rational designing of hydrolytically stable, U selective HOFs to realize the massive potential of the blue economy toward sustainable energy.

Comparative Study on Proton Conductivity and Mechanism Analysis of Two Imidazole Modified Imine-Based Covalent Organic Frameworks

This work elucidates the potential impact of intramolecular H-bonds within the pore walls of covalent organic frameworks (COFs) on proton conductivity. Employing DaTta and TaTta as representative hosts, it was observed that their innate proton conductivities (σ) are both unsatisfactory and σ(DaTta)<σ(TaTta). Intriguingly, the performance of both imidazole-loaded products, Im@DaTta and Im@TaTta is greatly improved, and the σ of Im@DaTta (0.91×10-2  S cm-1 ) even surpasses that of Im@TaTta (3.73×10-3  S cm-1 ) under 100 °C and 98 % relative humidity. The structural analysis, gas adsorption tests, and activation energy calculations forecast the influence of imidazole on the H-bonded system within the framework, leading to observed changes in proton conductivity. It is hypothesized that intramolecular H-bonds within the COF framework impede efficient proton transmission. Nevertheless, the inclusion of an imidazole group disrupts these intramolecular bonds, leading to the formation of an abundance of intermolecular H-bonds within the pore channels, thus contributing to a dramatic increase in proton conductivity. The related calculation of Density Functional Theory (DFT) provides further evidence for this inference.

Recent advancements of covalent organic frameworks (COFs) as proton conductors under anhydrous conditions for fuel cell applications

Recent electrochemical energy conversion devices require more advanced proton conductors for their broad applications, especially, proton exchange membrane fuel cell (PEMFC) construction. Covalent organic frameworks (COFs) are an emerging class of organic porous crystalline materials that are composed of organic linkers and connected by strong covalent bonds. The unique characteristics including well-ordered and tailorable pore channels, permanent porosity, high degree of crystallinity, excellent chemical and thermal stability, enable COFs to be the potential proton conductors in fuel cell devices. Generally, proton conduction of COFs is dependent on the amount of water (extent of humidity). So, the constructed fuel cells accompanied complex water management system which requires large radiators and airflow for their operation at around 80 °C to avoid overheating and efficiency roll-off. To overcome such limitations, heavy-duty fuel cells require robust proton exchange membranes with stable proton conduction at elevated temperatures. Thus, proton conducting COFs under anhydrous conditions are in high demand. This review summarizes the recent progress in emerging COFs that exhibit proton conduction under anhydrous conditions, which may be prospective candidates for solid electrolytes in fuel cells.

Water/Vapor Assisted Fabrication of Large-Area Superprotonic Conductive Covalent Organic Framework Membranes

Fabrication of large-area ionic covalent organic framework membranes (iCOMs) remains a grand challenge. Herein, the authors report the liquid water and water vapor-assisted fabrication of large-area superprotonic conductive iCOMs. A mixed monomer solution containing 1,3,5-triformylphloroglucinol (TFP) in 1,4-dioxane and p-diaminobenzenesulfonic acid (DABA) in water is first polymerized to obtain a pristine membrane which subsequently underwent crystallization process in mixed vapors containing water vapor. During the polymerization stage, water played a role of a diluting agent, weakening the Coulombic repulsion between sulfonic acid groups. During the crystallization stage, water vapor played a role of a structure-directing agent to facilitate the formation of highly crystalline, large-area iCOMs. The resulting membranes achieved a proton conductivity value of 0.76 S cm-1 at 90 °C under 100% relative humidity, which is among the highest ever reported. Using liquid water and water vapor as versatile additives open a novel avenue to the fabrication of large-area membranes from covalent organic frameworks and other kinds of crystalline organic framework materials.

Liquid-Liquid Phase Separation of Aqueous Ionic Liquids in Covalent Organic Frameworks for Thermal Switchable Proton Conductivity

Covalent organic frameworks (COFs) have regular channels that can accommodate guest molecules to provide highly conductive solid electrolytes. However, designing smart, conductive COFs remains a great challenge. Herein, we report the first example of PEG-functionalized ionic liquids (ILs) anchored on the COF walls by strong hydrogen bonding to fabricate thermally responsive COFs (ILm@COF). We found that similar to the traditional IL/water mixture, the ILs undergo lower critical solution temperature (LCST)-type phase behavior within COF nanopores under high moisture levels. However, the phase separation temperature of aqueous IL decreases in COF channels due to the strong interaction between the IL and COF. Thus, the proton conductivity of ILm@COF can be reversibly switched by phase miscibility and separation in COF nanopores, and there is no obvious decrease even after 20 switching cycles. Our work provides important clues for understanding liquid-liquid phase separation in a confined nanospace and opens a new pathway to switchable proton conductivity.

Recent Progress of Crystalline Porous Frameworks for Intermediate-Temperature Proton Conduction

Proton exchange membranes (PEMs), especially for work under intermediate temperatures (100-200 °C), have attracted great interest because of the high CO toleration and facial water management of the corresponding proton exchange membrane fuel cells (PEMFCs). Traditional polymer PEMs faced challenges of low stability and proton carrier leaking. Crystalline porous materials, such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), are promising to overcome these issues contributed by nanometer-sized channels. Herein we summarized the recent development of MOF/COF-based intermediate-temperature proton conductors. The strategies of framework engineering and pore impregnation were introduced in detail for raising proton conductivity. The proton-conducting mechanism was described as well. This spotlight will provide new insight into the fabrication of MOF/COF proton conductors under intermediate-temperature and anhydrous conditions.

Design, Preparation, and High Intrinsic Proton Conductivity of Two Highly Stable Hydrazone-Linked Covalent Organic Frameworks

Assembling crystalline materials with high stability and high proton conductivity as a potential alternative to the Nafion membrane is a challenging topic in the field of energy materials. Herein, we concentrated on the creation and preparation of hydrazone-linked COFs with super-high stability to explore their proton conduction. Fortunately, two hydrazone-linked COFs, TpBth and TaBth, were solvothermally prepared by using benzene-1,3,5-tricarbohydrazide (Bth), 2,4,6-trihydroxy-benzene-1,3,5-tricarbaldehyde (Tp), and 2,4,6-tris(4-formylphenyl)-1,3,5-triazine (Ta) as monomers. Their structures were simulated by Material Studio 8.0 software and confirmed by the PXRD pattern, demonstrating a two-dimensional framework with AA packing. The presence of a large number of carbonyl groups as well as -NH-NH₂- groups on the backbone is responsible for their super-high water stability as well as high water absorption capacity. AC impedance tests demonstrated a positive correlation between the water-assisted proton conductivity (σ) of the two COFs and the temperature and humidity. Under 100 °C/98% RH, the highest σ values of TpBth and TaBth can reach 2.11 × 10^-4 and 0.62 × 10^-5 S·cm^-1, which are among the high σ values of the reported COFs. Their proton-conductive mechanisms were highlighted by structural analyses as well as N₂ and H₂O vapor adsorption data and activation energy values. Our systematic research affords ideas for the synthesis of proton-conducting COFs with high σ values.

In Situ Rapid Electrochemical Fabrication of Porphyrin-Based Covalent Organic Frameworks: Novel Fibers for Electro-Enhanced Solid-Phase Microextraction

Electro-enhanced solid-phase microextraction (EE-SPME) is a bright separation and enrichment technique that integrates solid-phase microextraction with the electric field. It retains the excellent extraction performance of SPME technology while having the advantages of efficient driving of electric field and special interaction between the electric field and electrons in the molecules of material structure. Replacing conventional SPME fibers with highly efficient and highly conductive original EE-SPME fibers is critical for the practical applications of these technologies. Here, a novel fiber preparation strategy was proposed to obtain a highly conductive porphyrin-based covalent organic framework (POR-COF) by one-step electropolymerization. Benefiting from the excellent semiconducting properties of porphyrin groups, the POR-COF can be spontaneously polymerized on the fiber surface under an appropriate voltage within a few hours. Its performance was evaluated by the EE-SPME of phthalate esters (PAEs) from food and environmental samples, followed by gas chromatography-tandem triple quadrupole mass spectrometry (GC-MS/MS) technology. The results showed that the POR-COF fiber has been successfully used for the detection of trace PAEs in beverages, industrial wastewater, lake water, and oyster samples with high adsorption selectivity and satisfactory sensitivity. The remarkable extraction properties are mainly attributed to the synergistic effect from material characteristics and electrical parameters' control in the extraction process. The presented strategy for the controlled design and synthesis of highly conductive porphyrin-based covalent organic framework fibers offers prospects in developing EE-SPME technologies.

Enhancement of Visible-Light-Driven Hydrogen Evolution Activity of 2D π-Conjugated Bipyridine-Based Covalent Organic Frameworks via Post-Protonation

Photocatalytic hydrogen (H₂ ) evolution represents a promising and sustainable technology. Covalent organic frameworks (COFs)-based photocatalysts have received growing attention. A 2D fully conjugated ethylene-linked COF (BTT-BPy-COF) was fabricated with a dedicated designed active site. The introduced bipyridine sites enable a facile post-protonation strategy to fine-tune the actives sites, which results in a largely improved charge-separation efficiency and increased hydrophilicity in the pore channels synergically. After modulating the degree of protonation, the optimal BTT-BPy-PCOF exhibits a remarkable H₂ evolution rate of 15.8 mmol g^-1  h^-1 under visible light, which surpasses the biphenyl-based COF 6 times. By using different types of acids, the post-protonation is proved to be a potential universal strategy for promoting photocatalytic H₂ evolution. This strategy would provide important guidance for the design of highly efficient organic semiconductor photocatalysts.

Metal-Organic Framework-Derived N-Doped Porous Carbon for a Superprotonic Conductor at above 100 °C

The development of proton conductors capable of working at above 100 °C is of great significance for proton exchange membrane electrolysis cells (PEMECs) and proton exchange membrane fuel cells (PEMFCs) but remains to be an enormous challenge to date. In this work, we demonstrate for the first time that the N-doped porous carbon derived from metal-organic frameworks (MOFs) with great superiority can be exploited for high-performing proton conductors at above 100 °C. Through the pyrolysis of ZIF-8, the N-doped porous carbon (ZIF-8-C) featuring high chemical resistance to Fenton's reagent was readily prepared and then served as a robust host to accommodate H₃PO₄ molecules for proton transport. Upon impregnation with H₃PO₄, the resulting PA@ZIF-8-C exhibits low water swelling and high proton conduction of over 10^-2 S cm^-1 at a temperature above 100 °C, which is superior to many reported proton conductors. This work provides a new approach for the design of high-performing proton conductors at above 100 °C.

Asymmetric Hydrophosphonylation of Imines to Construct Highly Stable Covalent Organic Frameworks with Efficient Intrinsic Proton Conductivity

Imine-linked covalent organic frameworks (COFs) have received widespread attention because of their structure features such as high crystallinity and tunable pores. However, the intrinsic reversibility of the imine bond leads to the poor stability of imine-linked COFs under strong acid conditions. Also, their thermal stability is significantly lower than that of many other COFs. Herein, we report for the first time that the reversible imine bonds in the skeleton of COFs can be locked through the asymmetric hydrophosphonylation reaction of phosphite. The functionalized COFs not only retain the crystallinity and porous structure but also exhibit evidently improved chemical and thermal stabilities. In addition, the phosphorous acid groups generated by acidic hydrolysis attached to the skeleton endow the COFs with good intrinsic proton conductivity. Due to the diversity of phosphite derivatives and imine-linked COFs, this work may provide an avenue for extending the COF structures and functions through the asymmetric hydrophosphonylation reaction.

Developing a Unique Hydrogen-Bond Network in a Uranyl Coordination Framework for Fuel Cell Applications

Crystalline materials with persistent high anhydrous proton conductivity that can be directly used as a practical electrolyte of the intermediate-temperature proton exchange membrane fuel cells for durable power generation remain a substantial challenge. The present work proposes a unique way of the axial uranyl oxo atoms as hydrogen-bond acceptors to form a dense hydrogen-bonded network within a stable uranyl-based coordination polymer, UO₂(H₂PO₃)₂(C₃N₂H₄)₂ (HUP-3). It exhibits stable and efficient anhydrous proton conductivity over a super-wide temperature range (-40-170 °C). It was also assembled into a H₂/O₂ fuel cell as the electrolyte and shows a high electrical power density of 11.8 mW·cm^-2 at 170 °C, which is among one of the highest values reported from crystalline solid electrolytes. The cell was tested for over 12 h without notable power loss.

A Visible Light/Heat Responsive Covalent Organic Framework for Highly Efficient and Switchable Proton Conductivity

In recent years, covalent organic frameworks (COFs) have attracted enormous interest as a new generation of proton-exchange membranes, chemical sensors and electronic devices. However, to design high proton conductivity COFs,...

Thiophene-Based Covalent Organic Frameworks: Synthesis, Photophysics and Light-Driven Applications

Porous crystalline materials, such as covalent organic frameworks (COFs), have emerged as some of the most important materials over the last two decades due to their excellent physicochemical properties such as their large surface area and permanent, accessible porosity. On the other hand, thiophene derivatives are common versatile scaffolds in organic chemistry. Their outstanding electrical properties have boosted their use in different light-driven applications (photocatalysis, organic thin film transistors, photoelectrodes, organic photovoltaics, etc.), attracting much attention in the research community. Despite the great potential of both systems, porous COF materials based on thiophene monomers are scarce due to the inappropriate angle provided by the latter, which hinders its use as the building block of the former. To circumvent this drawback, researchers have engineered a number of thiophene derivatives that can form part of the COFs structure, while keeping their intrinsic properties. Hence, in the present minireview, we will disclose some of the most relevant thiophene-based COFs, highlighting their basic components (building units), spectroscopic properties and potential light-driven applications.

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

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

Controllable Synthesis and Performance Modulation of 2D Covalent-Organic Frameworks

Covalent-organic frameworks (COFs) are especially interesting and unique as their highly ordered topological structures entirely built from plentiful π-conjugated units through covalent bonds. Arranging tailorable organic building blocks into periodically reticular skeleton bestows predictable lattices and various properties upon COFs in respect of topology diagrams, pore size, properties of channel wall interfaces, etc. Indeed, these peculiar features in terms of crystallinity, conjugation degree, and topology diagrams fundamentally decide the applications of COFs including heterogeneous catalysis, energy conversion, proton conduction, light emission, and optoelectronic devices. Additionally, this research field has attracted widespread attention and is of importance with a major breakthrough in recent year. However, this research field is running with the lack of summaries about tailorable construction of 2D COFs for targeted functionalities. This review first covers some crucial polymeric strategies of preparing COFs, containing boron ester condensation, amine-aldehyde condensation, Knoevenagel condensation, trimerization reaction, Suzuki CC coupling reaction, and hybrid polycondensation. Subsequently, a summary is made of some representative building blocks, and then underlines how the electronic and molecular structures of building blocks can strongly influence the functional performance of COFs. Finally, conclusion and perspectives on 2D COFs for further study are proposed.

Pyrimidine-Functionalized Covalent Organic Framework and its Cobalt Complex as an Efficient Electrocatalyst for Oxygen Evolution Reaction

A pyrimidine-modified covalent organic framework (COF-Pyr) was designed to be synthesized via the Povarov reaction. The nitrogen atom on the pyrimidine showed excellent coordination ability to metal ions. Their stable metal composite material (Co@COF-Pyr) exhibited remarkable performance for electrocatalytic oxygen evolution reaction (OER) in 1.0 m KOH aqueous solution. The overpotential was 450 mV at 10 mA cm^-2 . The Co@COF-Pyr with large specific surface area (392 m²  g^-1 ) and regular crystal structure provided free passage for H₂ O to move and make them fully contact with the uniformly dispersed cobalt ions on the surface. Thus, the turnover frequency of Co@COF-Pyr was 0.1 s^-1 at the overpotential of 370 mV, which was higher than most reported OER catalysts. This work provided a new way to design and prepare nitrogen-containing heterocyclic functionalized COFs. They can be combined with metal ions to expand the application of COFs in the field of electrocatalysis.

Accumulation of Sulfonic Acid Groups Anchored in Covalent Organic Frameworks as an Intrinsic Proton-Conducting Electrolyte

Covalent organic frameworks (COFs) are a novel class of crystalline porous polymers, which possess high porosity, excellent stability, and regular nanochannels. 2D COFs provide a 1D nanochannel to form the proton transport channels. The abovementioned features afford a powerful potential platform for designing materials as proton transportation carriers. Herein, the authors incorporate sulfonic acid groups on the pore walls as proton sources for enhancing proton transport conductivity in the 1D channel. Interestingly, the sulfonic acid COFs (S-COFs) electrolytes being binder free exhibit excellent proton conductivity of ≈1.5 × 10^-2 S cm^-1 at 25 ℃ and 95% relative humidity (RH), which rank the excellent performance in standard proton-conducting electrolytes. The S-COFs electrolytes keep the high proton conduction over the 24 h. The activation energy is estimated to be as low as 0.17 eV, which is much lower than most reported COFs. This research opens a new window to evolve great potential of structural design for COFs as the high proton-conducting electrolytes.

Anhydrous Proton Conduction in Crystalline Porous Materials with a Wide Working Temperature Range

Crystalline porous materials (CPMs), exhibiting high surface areas, versatile structural topologies, and tunable functionality, have attracted much attention in the field of proton exchange membrane fuel cells (PEMFC) for their great potential in solid electrolytes. However, most hydrated CPM proton conductors suffer from the narrow working temperature and the high water/humidity dependence. Considering the practical application in different working environments, CPMs with high anhydrous conductivity from subzero to moderate temperature (>100 °C) are desirable, but it is still a huge challenge. Herein we summarized our recent research work in the anhydrous CPM proton conductors, including to rationally tune the structures of CPMs by using the strategies of pore engineering and protonic species control to achieve wide working temperature conduction, as well as to clarify the conducting mechanism. This spotlight will provide clues to flexibly design and fabricate wide-working-temperature CPM conductors with high protonic conductivity.

Cationic Covalent Organic Framework as an Ion Exchange Material for Efficient Adsorptive Separation of Biomolecules

Although covalent organic frameworks (COFs) have earned significant interest in separation applications, the use of COFs in biomolecule separation remains unexplored. We examined the ionic COF Py-BPy²⁺-COF as an ion exchange material for biomolecule separation. After characterizing the properties of the synthesized COF with a variety of techniques, binding experiments with both large and small biomolecules were performed. High adsorption capacities of amino acids with different hydrophobicity and charge, as well as proteins of different isoelectric points and molecular weights, were determined in batch equilibrium experiments. Desorption experiments with mixtures of model proteins demonstrated an ability to successfully separate one protein from another with the selectivity hypothesized to be a combination of the isoelectric point, hydrophobicity, and ability to penetrate the crystalline material. Overall, the results demonstrated that Py-BPy²⁺-COF can be exploited as a robust crystalline anion exchange biomolecule separation material.

Proton Conduction Mechanism for Anhydrous Imidazolium Hydrogen Succinate Based on Local Structures and Molecular Dynamics

Anhydrous organic crystalline materials incorporating imidazolium hydrogen succinate (Im-Suc), which exhibit high proton conduction even at temperatures above 100 °C, are attractive for elucidating proton conduction mechanisms toward the development of solid electrolytes for fuel cells. Herein, quantum chemical calculations were used to investigate the proton conduction mechanism in terms of hydrogen-bonding (H-bonding) changes and restricted molecular rotation in Im-Suc. The local H-bond structures for proton conduction were characterized by vibrational frequency analysis and compared with corresponding experimental data. The calculated potential energy surface involving proton transfer (PT) and imidazole (Im) rotational motion showed that PT between Im and succinic acid was a rate-limiting step for proton transport in Im-Suc and that proton conduction proceeded via the successive coupling of PT and Im rotational motion based on a Grotthuss-type mechanism. These findings provide molecular-level insights into proton conduction mechanisms for Im-based (or -incorporated) H-bonding organic proton conductors.

Self-Standing combined covalent-organic-framework membranes for subzero conductivity assisted by ionic liquids

The development of proton-conducting materials in cold regions is still at the initial stage due to the challenge in breaking the subzero temperature limit, especially in covalent organic frameworks (COFs). Herein, we fabricated a series of proton-conductive COFs as self-standing, highly flexible combined membranes (ssc-COFMs) composed of a processable TpBD-Me₂ and a conductive Tp-TG_Cl, in-situ encapsulated proton-conducting ionic liquids (PCILs) as additional proton sources into backbones. Compositions and microstructures of ssc-COFMs are monitored by XRD, FTIR, nitrogen adsorption and elemental analysis. Comparison to other porous organic conductors, a great advance propelled renders the combined COF membranes to have a high protonic conductivities at medium and subzero temperatures (243 to 353 K), owing to the resultant multifaceted synergistic effect of multiple proton units. Specifically, the proton conductivities of the ssc-COFMs loaded with -SO₄H functionalized PCILs reaches 2.87 × 10^-4 S cm^-1 (~58% RH) and 9.93 × 10^-4 S cm^-1 (~98% RH) at 243 K, together with 6.84 × 10^-2 S·cm^-1 under 353 K and ~ 98% RH.

A novel 2D mesoporous phosphazene-anthraquinone-based covalent organic polymer: synthesis, characterization and supercapacitor applications

A novel phosphazene anthraquinone-based covalent organic polymer (HD-1) was successfully designed and synthesized through a simple polymerization reaction. The as-prepared material was used as an electrode active material for a supercapacitor.

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

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

Design and application of covalent organic frameworks for ionic conduction

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

Solving the COF trilemma: towards crystalline, stable and functional covalent organic frameworks

Strategies in covalent organic frameworks and adjacent fields are highlighted for designing stable, ordered and functional materials.