Effect of Covalent Organic Frameworks Containing Different Groups on Properties of Sulfonated Poly(ether ether ketone) Matrix Proton Exchange Membranes

The rich -SO₃H groups enable sulfonated poly (ether ether ketone) (SPEEK) to possess excellent proton conductivities in proton exchange membrane (PEM), but cause excessive water absorption, resulting in the decline of dimensional stability. It is a challenge to resolve the conflict between conductivity and stability. Owing to its unique structural designability, covalent organic frameworks (COFs) have been used to regulate the performances of PEMs. The authors propose the use of COFs with acidic and basic groups for meeting the requirements of proton conductivity and dimensional stability. Herein, COFs containing different groups (sulfoacid, pyridine, and both) were uniformly dispersed into the SPEEK matrix by in situ synthesis, and the effects on the properties of SPEEK matrix PEMs were revealed. The sulfoacid group significantly improves proton conductivities. At 60 °C, under 95% RH, the conductivity of the SPEEK/TpPa-SO₃H-20 composite membrane was 443.6 mS·cm^-1, which was 3.3 times that of the pristine SPEEK membrane. The pyridine group reduced the swelling ratio at 50 °C from 220.7% to 2.4%, indicating an enhancement in dimensional stability. Combining the benefits of sulfoacid and pyridine groups, SPEEK/TpPa-(SO₃H-Py) composite membrane has a conductivity of 360.3 mS·cm^-1 at 60 °C and 95% RH, which is 1.86 times that of SPEEK, and its swelling ratio is 11.8%, about 1/20 of that of SPEEK membrane. The method of in situ combination and regulation of groups open up a way for the development of SPEEK/COFs composite PEMs.

Atomically Thin, Ionic-Covalent Organic Nanosheets for Stable, High-Performance Carbon Dioxide Electroreduction

The incorporation of charged functional groups is effective to modulate the activity of molecular complexes for the CO₂ reduction reaction (CO₂ RR), yet long-term heterogeneous electrolysis is often hampered by catalyst leaching. Herein, an electrocatalyst of atomically thin, cobalt-porphyrin-based, ionic-covalent organic nanosheets (CoTAP-iCONs) is synthesized via a post-synthetic modification strategy for high-performance CO₂ -to-CO conversion. The cationic quaternary ammonium groups not only enable the formation of monolayer nanosheets due to steric hindrance and electrostatic repulsion, but also facilitate the formation of a *COOH intermediate, as suggested by theoretical calculations. Consequently, CoTAP-iCONs exhibit higher CO₂ RR activity than other cobalt-porphyrin-based structures: an 870% and 480% improvement of CO current densities compared to the monomer and neutral nanosheets, respectively. Additionally, the iCONs structure can accommodate the cationic moieties. In a flow cell, CoTAP-iCONs attain a very small onset overpotential of 40 mV and a stable total current density of 212 mA cm^-2 with CO Faradaic efficiency of >95% at -0.6 V for 11 h. Further coupling the flow electrolyzer with commercial solar cells yields a solar-to-CO conversion efficiency of 13.89%. This work indicates that atom-thin, ionic nanosheets represent a promising structure for achieving both tailored activity and high stability.

Fast Hydroxide Conduction via Hydrogen-Bonding Network Confined in Benzimidazolium-Functionalized Covalent Organic Frameworks

In both hydrophobic and hydrophilic nanochannels, confined water clusters spontaneously form dense internal hydrogen bond networks and hence exhibit fast mass-transfer kinetics. Covalent organic frameworks (COFs), a porous polymer, enables one-dimensional open channels to achieve ordered assembly guided by synthetic techniques and allows the accommodation of a large number of water molecules within the nanochannels. In the field of alkaline anion exchange membrane fuel cells, it has been a long-term pursuit of scientists to build abundant hydrogen bonds around hydrogen oxides (OH^-) to improve the conduction of OH^- by increasing the water content. Here, we designed and synthesized a OH^- conductor by assembling benzimidazolium into COFs, and a significantly high conductivity of 10^-1 S cm^-1 was achieved at 353 K. Theoretical calculations showed that the water clusters confined in the pores of COFs and the regularly arranged hydroxides cooperatively formed a dense hydrogen bond network and OH^- conducted diffusive conduction through the Grotthuss hopping of protons in this hydrogen bond network.

Hydrophilic Covalent Organic Frameworks Coated Steel Sheet As a Mass Spectrometric Ionization Source for the Direct Determination of Zearalenone and Its Derivatives

Zearalenone has attracted worldwide attention due to its toxic properties and threat to public health. A rapid determination method for zearalenone and its derivatives by hydrophilic covalent organic frameworks coated steel sheet (HCOFCS) combined with ambient mass spectrometry (AMS) was developed. The HCOFCS behaved as both a tip for solid-phase microextraction and a solid substrate for electrospray ionization mass spectrometry (ESI-MS). To evaluate the HCOFCS-ESI-MS method, five zearalenone and its derivatives in milk samples were determined, including zearalenone (ZEA), α-zearalenol (α-ZEL), β-zearalenol (β-ZEL), α-zearalanol (α-ZAL), and β-zearalanol (β-ZAL). After the extraction procedure, the HCOFCS was directly added with a high voltage for ESI-MS, and the analysis could be completed within 1 min. The developed method showed good linearity in the range 0.1-100 μg/L with a coefficient of determination (R²) > 0.9991. The limits of detection (LODs) and limits of quantitation (LOQs) ranged from 0.05 to 0.1 and 0.2 to 0.3 μg/L, respectively. The results demonstrated that the HCOFCS combined with ESI-MS can be used for the rapid and sensitive determination of trace ZEA and its derivatives in milk samples with satisfactory recoveries from 80.58% to 109.98% and reproducibility with relative standard deviations (RSDs) no more than 11.18%. Furthermore, HCOFCS showed good reusability, which could reuse at least 10 extraction cycles with satisfactory adsorption performance.

Humidity-Independent Artificial Olfactory Array Enabled by Hydrophobic Core-Shell Dye/MOFs@COFs Composites for Plant Disease Diagnosis

As a class of important artificial olfactory system, the colorimetric sensor array possesses great potential for commercialization due to its cost-effectiveness and portability. However, when applied to practical applications, the humidity interference from ambient environment and dissatisfactory sensitivity for trace target VOCs are largely unsolved problems. To overcome the problems, we developed a series of dye/MOFs@COFs gas-sensing materials with core-shell structure using a hydrophobization strategy by encapsulation of dye/metal-organic frameworks (MOFs) into hydrophobic covalent organic frameworks (COFs). Benefiting from the hydrophobic property of the COF shell, the dye/MOFs@COFs composites were endowed with excellent humidity-resistance even under 100% relative humidity (RH). Moreover, due to the uniform distribution of dyes on the porous MOFs, the dye/MOFs@COFs sensors also exhibited improved sensitivity at the sub-ppm level, compared with conventional dye sensors. On basis of the excellent humidity-resistance and improved sensitivity, an artificial olfactory array based on dye/MOFs@COFs composites was proven to be a successful practical application in early and accurate detection of wheat scab (1 day after inoculation) by monitoring its released VOC markers. The synthetic strategy for core-shell dye/MOFs@COFs is applicable to a wide range of colorimetric sensor arrays, endowing them with excellent humidity-resistance and sensitivity for the feasibility of practical applications.

Effect of Nitrogen Atom Introduction on the Photocatalytic Hydrogen Evolution Activity of Covalent Triazine Frameworks: Experimental and Theoretical Study

The construction of high-performance photocatalyst has always been explored. Covalent organic frameworks (COFs), especially keto-amine-linked COFs, have many advantages, such as adjustable bandgaps, π-π stacking structure, excellent response ability to visible light, high specific surface area, high mobility of carrier carriers, good physical and chemical stability, and so on, showing strong potential applications in photocatalytic solar energy conversion and hydrogen production. Two analogous covalent triazine frameworks (CTFs), T3H-CTF and T3N-CTF, have been synthesized via Schiff-base condensation reactions between 2,4,6-trihydroxybenzene-1,3,5-tricarbalehyde (MOP) and the corresponding triazine-based aromatic amines under solvothermal condition. For T3N-CTF, the peripheral aromatic linker to the central triazine unit was the pyridine unit, instead of the benzene unit in the T3H-CTF unit. T3N-CTF had a hydrogen production rate (HPR) of 6485.05 μmol g^-1  h^-1 , much higher than that of T3H-CTF (2028.06 μmol g^-1  h^-1 ). Accordingly, T3N-CTF had a much higher apparent quantum yield (AQY) of 12.2 % than that of T3H-CTF (4.12 %) at 405 nm. The experimental and theoretical results showed that the extended light absorption range, enlarged surface area, and enhanced separation and transportation efficiencies of charge carriers of T3N-CTF compared with T3H-CTF were uniformly induced by the introduction of peripheral nitrogen atoms into the skeleton of former CTF, which eventually boosted the visible-light induced hydrogen evolution reaction (HER). The work suggests a new method for enhancing the intrinsic HER activity by modulating the electronic features of the conjugated COFs by the introduction of pyridinic N atoms.

Two-Dimensional Porphyrin-Based Covalent Organic Framework with Enlarged Inter-layer Spacing for Tunable Photocatalytic CO2 Reduction

Two-dimensional (2D) porphyrin-based covalent organic frameworks (COFs) are one of the most promising candidates for photocatalytic carbon dioxide reduction reaction (CO₂RR), which however still suffer from the hindered mass transfer during the catalysis procedure associated with the close packing of 2D COF layers due to the strong axial π-π stacking. Herein, condensation between the porphyrinic aldehydes p-MPor-CHO (M = H₂, Co, and Ni) and 3,8-diamino-6-phenyl-phenanthridine (DPP) affords new porphyrin-based 2D COF architecture MPor-DPP-COFs (M = H₂, Co, and Ni). The bulky phenyl substituent at the phenanthridine periphery of the linking unit reduces the axial π-π stacking, providing an enlarged inter-layer spacing of 6.0 Å according to high-resolution transmission electron microscopy. This, in combination with the large surface area (1021 m² g^-1) revealed by nitrogen sorption measurements at 77 K for CoPor-DPP-COF possessing electroactive Co ions, endows it with excellent photocatalytic activity for CO₂RR with a CO generation rate of 10 200 μmol g^-1 h^-1 and a CO selectivity up to 82%. This work affords new ideas for achieving efficient photocatalytic CO₂RR upon fine-tuning the inter-layer spacing of 2D COFs.

Impact of Pore Flexibility in Imine-Linked Covalent Organic Frameworks on Benzene and Cyclohexane Adsorption

This work focuses on the impact of covalent organic frameworks' (COFs) pore flexibility in the adsorption and separation of benzene and cyclohexane. With this aim, we have selected the imine-linked 3D COFs COF-300 and LZU-111 as examples of flexible and rigid frameworks, respectively. Optimized syntheses at room temperature or in solvothermal conditions enabled us to selectively isolate the narrow-pore form of COF-300 (COF-300-rt) or a mixture of the narrow-pore and a larger-pore form (COF-300-st), respectively, with different textural properties (BET specific surface area = 39 or 1270 m²/g, respectively, from N₂ adsorption at 77 K). In the case of LZU-111, only the room temperature route was successful, leading to the known microporous framework. COF-300-rt, COF-300-st, and LZU-111 were studied for benzene and cyclohexane adsorption and separation in static and dynamic conditions. At 298 K and 1 bar, these COFs adsorb more benzene (251, 221, and 214 cm³/g STP, respectively) than cyclohexane (175, 133, and 164 cm³/g STP, respectively). Moreover, the benzene and cyclohexane isotherms of COF-300-rt and COF-300-st are characterized by steps, as expected with a flexible material. Indeed, in situ powder X-ray diffraction experiments on benzene- and cyclohexane-impregnated batches enabled us to trap, for the first time, a sequence of forms of COF-300 with different pore aperture, rationalizing the stepped hysteretic isotherms. Finally, benzene/cyclohexane separation was evaluated using a benzene/cyclohexane 50:50 v/v flow at different temperatures (T = 298, 323, and 348 K): LZU-111 does not selectively retain any of the two components, while COF-300 exhibits stronger benzene-COF interactions also in dynamic conditions.

Facile, Direct, De Novo Synthesis of an Alkyl Phosphoric Acid-Decorated Covalent Organic Framework

The development and understanding of proton conductors based on phosphoric acid are critical for the field of chemistry, biology, and energy. Covalent organic frameworks (COFs), featuring highly crystalline structures and controllable pore sizes, are suitable for constructing phosphoric acid-based proton conductors. However, because of tedious and intricate synthesis, how to develop COFs based on phosphoric acid remains a substantial challenge. Herein, we contributed a side-chain decorated strategy to construct a phosphoric acid-functionalized, imine-linked COF by de novo synthesis. The phosphoric acid side chains with vigorous motion integrating with 1D nanochannels endow the resulting COF with intrinsic proton conductivity. This work expectantly provides a competitive alternative for producing phosphoric acid-functionalized COFs with high intrinsic proton conductivity. This article is protected by copyright. All rights reserved.

In Situ Growth of Covalent Organic Framework Nanosheets on Graphene as the Cathode for Long-Life High-Capacity Lithium-Ion Batteries

The poor electronic and ionic conductivities of covalent organic frameworks (COFs) severely restrict the development of COF-based electrodes for practical rechargeable batteries, therefore inspiring more research interest from the direction of both material synthesis and technology. Herein, a dual-porous COF, USTB-6, with good crystallinity and rich redox-active sites is conceived and fabricated by the polymerization of 2,3,8,9,14,15-hexa(4-formylphenyl)diquinoxalino [2,3-a:2',3'-c]phenazine and 2,7-diaminopyrene-4,5,9,10-tetraone. In particular, the heterogeneous polymerization of the same starting materials in the presence of graphene affords uniformly dispersed COF nanosheets with a thickness of 8.3 nm on a conductive carbon substrate, effectively enhancing the electronic conductivity of the COF-based electrode. Such a graphene-supported USTB-6 nanosheets cathode when used in a lithium-ion battery exhibits a specific capacity of 285 mA h g^-1 at a current density of 0.2 C and excellent rate performance with a prominent capacity of 188 mA h g^-1 at 10 C. More importantly, a capacity of 170 mA h g^-1 is retained by using the USTB-6 nanosheets cathode after 6000 cycles charge and discharge measurement at 5 C.

Progress in Hybridization of Covalent Organic Frameworks and Metal-Organic Frameworks

Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) hybrid materials are a class of porous crystalline materials that integrate MOFs and COFs with hierarchical pore structures. As an emerging porous frame material platform, MOF/COF hybrid materials have attracted tremendous attention, and the field is advancing rapidly and extending into more diverse fields. Extensive studies have shown that a broad variety of MOF/COF hybrid materials with different structures and specific properties can be synthesized from diverse building blocks via different chemical reactions, driving the rapid growth of the field. The allowed complementary utilization of π-conjugated skeletons and nanopores for functional exploration has endowed these hybrid materials with great potential in challenging energy and environmental issues. It is necessary to prepare a "family tree" to accurately trace the developments in the study of MOF/COF hybrid materials. This review comprehensively summarizes the latest achievements and advancements in the design and synthesis of MOF/COF hybrid materials, including COFs covalently bonded to the surface functional groups of MOFs (MOF@COF), MOFs grown on the surface of COFs (COF@MOF), bridge reaction between COF and MOF (MOF+COF), and their various applications in catalysis, energy storage, pollutant adsorption, gas separation, chemical sensing, and biomedicine. It concludes with remarks concerning the trend from the structural design to functional exploration and potential applications of MOF/COF hybrid materials.

Covalent Organic Framework Nanocarriers of Singlet Oxygen for Oxygen-Independent Concurrent Photothermal/Photodynamic Therapy to Ablate Hypoxic Tumors

Photodynamic therapy (PDT) of cancers is seriously restricted by tumor hypoxia. In addition to the intrinsic hypoxic microenvironment, continuous photoirradiation further aggravates intratumoral hypoxia, thereby reducing the PDT effect significantly. Oxygen-independent PDT is recognized as an efficient approach to overcome this issue. Herein, singlet oxygen (¹ O₂ )-stored covalent organic framework (COF) nanoparticles loading the near-infrared (NIR) dye cypate, which realize oxygen-independent ¹ O₂ production for concurrent photothermal therapy (PTT) and PDT under NIR irradiation, are presented. The cypate-loading COF nanoparticles are prepared by using the photosensitizers and ¹ O₂ -stored molecules via formation of Schiff base bonds, followed by coverage of poly(vinyl pyrrolidone). The COF nanoparticles significantly improve the photostability and photothermal conversion efficiency of cypate by protecting them from photodegradation under NIR irradiation. Upon 660 nm laser irradiation, ¹ O₂ is produced by the photosensitizer motifs and is successfully stored by the ¹ O₂ -stored moieties. After intravenous injection and tumor accumulation, the COF nanoparticles can generate heat quickly upon 808 nm irradiation which induces the efficient release of the stored ¹ O₂ to ablate tumors via O₂ -independent concurrent PTT/PDT. Accordingly, the COF nanocarriers of ¹ O₂ provide a paradigm to develop O₂ -independent concurrent PTT/PDT for precise cancer treatment upon NIR irradiation.

Radical Covalent Organic Frameworks Associated with Liquid Na-K toward Dendrite-Free Alkali Metal Anodes

Liquid sodium-potassium (Na-K) alloy has the characteristics of high abundance, low redox potential, high capacity, and no dendrites, which has become an ideal alternative material for potassium/sodium metal anodes. However, the high surface tension of liquid sodium potassium alloy at room temperature makes it inconvenient in practical use. Here, the Na-K as reducing agent treats with hydrazone linkages of covalent organic frameworks (COFs) and obtain the carbon-oxygen radical COFs (COR-Tf-DHzDM-COFs). The preparation method solves the problems that the preparation process of the traditional Na-K composite anode is complex and has high cost. The structures of the COR-Tf-DHzDM-COFs are characterized by X-ray diffraction （XRD), fourier transform infrared (FT-IR), electron paramagnetic resonance (EPR), and solid-state NMR measurements. It is the first time that carbon-oxygen radical COFs from bulk COFs are constructed by one-step method and the operation is flexible, convenient, and high rate of quality, which is suitable for big production and widely used. The cycle stability of the composite Na-K anode is improved, which provides a new idea for the design of high-performance liquid metal anode.

Bottom-Up Interfacial Design of Covalent Organic Frameworks for Highly Efficient and Selective Electrocatalysis of CO2

Assembling molecular catalytic centers into crosslinked networks is widely used to fabricate heterogeneous catalysts but they often suffer loss in activity and selectivity accompanied by unclear causes. Here, a strategy for the construction of heterogeneous catalysts to induce activity and selectivity by bottom-up introduction of segregated electron-conduction and mass-transport interfaces into the catalytic materials is reported. The catalytic skeletons are designed to possess different π orderings for electron motion and the open channels are tailored to install finely engineered walls for mass transport, so that origins of activity and selectivity are correlated. The resultant covalent organic framework catalysts with ordered π skeletons and solvophobic pores increase activity by two orders of magnitude, enhance selectivity and energy efficiency by 70-fold, and broaden the voltage range, to promote CO₂ transformation under ambient conditions. The results open a way to precise interfacial design of actionable heterogeneous catalysts for producing feedstocks from CO₂ .

Covalent Organic Framework Based on Azacalix[4]arene for the Efficient Capture of Dialysis Waste Products

Azacalix[n]arenes (ACAs) are lesser-known cousins of calix[n]arenes that contain amine bridges instead of methylene bridges, so they generally have higher flexibility due to enlarged cavities. Herein, we report a highly substituted cationic azacalix[4]arene-based covalent organic framework (ACA-COF) synthesized by the Zincke reaction under microwave irradiation. The current work is a rare example of a synthetic strategy that utilizes the chemical functionalization of an organic macrocycle to constrain its conformational flexibility and, thereby, produce an ordered material. Considering the ACA cavity dimensions, and the density and diversity of the polar groups in ACA-COF, we used it for adsorption of uric acid and creatinine, two major waste products generated during hemodialysis treatment in patients with renal failure. This type of application, which has the potential to save ∼400 L of water per patient per week, has only been recognized in the last decade, but could effectively address the problem of water scarcity in arid areas of the world. Rapid adsorption rates (up to k = 2191 g mg^-1 min^-1) were observed in our COF, exceeding reported values by several orders of magnitude.

An Anti-Aromatic Covalent Organic Framework Cathode with Dual-Redox Centers for Rechargeable Aqueous Zinc Batteries

Covalent organic frameworks (COFs) are promising cathode candidates with high structural stability. However, they contain redox inactive linkages and experience low redox potential. Herein, a full anti-aromatic microporous COF cathode material of TAQ-BQ is designed for aqueous zinc batteries. The anti-aromatic conjugation effectively lowers the energy level of the lowest unoccupied molecular orbital as revealed by theoretical calculations, which corresponds to an elevated redox potential. Besides, the structure contains imino active sites at the linkages, in addition to carbonyl at the active parts. As a result, the TAQ-BQ cathode exhibits a voltage of 1.53 V/1.54 V and between 1.35 and 0.45 V in zinc cells. It delivers 208 mAh g^-1 capacity at 0.1 A g^-1 and maintains 136 mAh g^-1 at 2 A g^-1. Stable cycling is realized for 1000 cycles with 87% capacity retention. The co-de/insertion of Zn²⁺ and protons is identified for energy storage. Our work reveals the promises of COF cathode materials for aqueous zinc batteries.

Covalent Organic Frameworks-TpPa-1 as an Emerging Platform for Electrochemical Sensing

Covalent organic frameworks (COFs) are a new type of metal-free porous architecture with a well-designed pore structure and high stability. Here an efficient electrochemical sensing platform was demonstrated based on COFs TpPa-1 constructed by 1,3,5-triformylphloroglucinol (Tp) with p-phenylenediamine (Pa-1), which possesses abundant nitrogen and oxo-functionalities. COFs TpPa-1 exhibited good water dispersibility and strong adsorption affinities for Pd²⁺ and thus was used as loading support to modify Pd²⁺. The Pd²⁺-modified COFs TpPa-1 electrode (Pd²⁺/COFs) showed high electrocatalytic activity for both hydrazine oxidation reaction and nitrophenol reduction reaction. In addition, TpPa-1-derived nitrogen-doped carbon presented high activity for the electro-oxidation of reduced glutathione (GSH), and sensitive electrochemical detection of GSH was achieved. The presented COFs TpPa-1 can be utilized as a precursor as well as support for anchoring electro-active molecules and nanoparticles, which will be useful for electrochemical sensing and electrocatalysis.

Suppressing the Excitonic Effect in Covalent Organic Frameworks for Metal-Free Hydrogen Generation

Photocatalytic hydrogen generation is a promising solution for renewable energy production and plays a role in achieving carbon neutrality. Covalent organic frameworks (COFs) with highly designable backbones and inherent pores have emerged as novel photocatalysts, yet the strong excitonic effect in COFs can impede the promotion of energy conversion efficiency. Here, we propose a facile approach to suppress the excitonic effect in COFs, which is by narrowing the band gap and increasing the dielectric screening via a rational backbone design and chemical modifications. Based on the GW-BSE method, we uncover a linear relationship between the electronic dielectric constant and the inverse square of the optical band gap of COFs of the Lieb lattice. We further demonstrate that both reduced exciton binding energy and enhanced sunlight absorption can be simultaneously realized in COFs with a narrow band gap. Specifically, we show that one of our designed COFs whose exciton binding energy is nearly half that of g-C₃N₄ is capable of metal-free hydrogen production under near-infrared light irradiation. Our results showcase an effective method to suppress the excitonic effect in COFs and also pave the way for their applications in photocatalytic, photovoltaic, and other related solar energy conversions.

Hydroxyl-Functionalized Covalent Organic Frameworks as High-Performance Supercapacitors

Covalent organic frameworks (COFs) have attracted significant interest because of their heteroatom-containing architectures, high porous networks, large surface areas, and capacity to include redox-active units, which can provide good electrochemical efficiency in energy applications. In this research, we synthesized two novel hydroxy-functionalized COFs-TAPT-2,3-NA(OH)_2, TAPT-2,6-NA(OH)₂ COFs-through Schiff-base [3 + 2] polycondensations of 1,3,5-tris-(4-aminophenyl)triazine (TAPT-3NH₂) with 2,3-dihydroxynaphthalene-1,4-dicarbaldehyde (2,3-NADC) and 2,6-dihydroxynaphthalene-1,5-dicarbaldehyde (2,6-NADC), respectively. The resultant hydroxy-functionalized COFs featured high BET-specific surface areas up to 1089 m² g^-1, excellent crystallinity, and superior thermal stability up to 60.44% char yield. When used as supercapacitor electrodes, the hydroxy-functionalized COFs exhibited electrochemical redox activity due to the presence of redox-active 2,3-dihydroxynaphthalene and 2,6-dihydroxynaphthalene in their COF skeletons. The hydroxy-functionalized COFs showed specific capacitance of 271 F g^-1 at a current density of 0.5 A g^-1 with excellent stability after 2000 cycles of 86.5% capacitance retention. Well-known pore features and high surface areas of such COFs, together with their superior supercapacitor performance, make them suitable electrode materials for use in practical applications.

Synthesis and Visualization of Entangled 3D Covalent Organic Frameworks with High-Valency Stereoscopic Molecular Nodes for Gas Separation

The structural diversity of three-dimensional (3D) covalent organic frameworks (COFs) are limited as there are only a few choices of building units with multiple symmetrically distributed connection sites. To date, 4 and 6-connected stereoscopic nodes with T_d , D_3h , D_3d and C₃ symmetries have been mostly reported, delivering limited 3D topologies. We propose an efficient approach to expand the 3D COF repertoire by introducing a high-valency quadrangular prism (D_4h ) stereoscopic node with a connectivity of eight, based on which two isoreticular 3D imine-linked COFs can be created. Low-dose electron microscopy allows the direct visualization of their 2-fold interpenetrated bcu networks. These 3D COFs are endowed with unique pore architectures and strong molecular binding sites, and exhibit excellent performance in separating C₂ H₂ /CO₂ and C₂ H₂ /CH₄ gas pairs. The introduction of high-valency stereoscopic nodes would lead to an outburst of new topologies for 3D COFs.

2D Covalent Organic Frameworks Toward Efficient Photocatalytic Hydrogen Evolution

Efficiently producing clean energy is of great importance for sustainable development of the environment. Solar-driven water splitting for H₂ evolution has an important role among the renewable energy technologies. Developing high-performance and cost-effective photocatalysts is still a critical task before practical application. 2D Covalent organic frameworks (COFs) as photocatalysts have recently attracted widespread interest thanks to their tunable optical bandgaps, tailor-made functionality, excellent crystallinity, high specific surface area, and good photo- and chemical stability. This Review focuses on the representative progress and remaining challenges in 2D COF-based photocatalysts for hydrogen evolution.

CoN5 Sites Constructed by Anchoring Co Porphyrins on Vinylene-Linked Covalent Organic Frameworks for Electroreduction of Carbon Dioxide

Developing effective electrocatalysts for CO₂ reduction (CO₂ RR) is of critical importance for producing carbon-neutral fuels. Covalent organic frameworks (COFs) are an ideal platform for constructing catalysts toward CO₂ RR, because of their controllable skeletons and ordered pores. However, most of these COFs are synthesized from Co-porphyrins or phthalocyanines-based monomers, and the available building units and resulting catalytic centers in COFs are still limited. Herein, Co-N₅ sites are first developed through anchoring Co porphyrins on an olefin-linked COF, where the Co active sites are uniformly distributed in the hexagonal networks. The strong electronic coupling between Co porphyrins and COF is disclosed by various characterizations such as X-ray absorption spectroscopy (XAS) and density functional theory calculation (DFT). Thanks to the CoN₅ sites, the catalytic COF shows remarkable catalytic activity with Faraday efficiencies (FE_CO ) of 84.2-94.3% at applied potentials between -0.50 and -0.80 V (vs RHE), and achieves a turnover frequency of 4578 h^-1 at -1.0 V. Moreover, the theoretical calculation further reveals that the CoN₅ sites enable a decrease in the overpotential for the formation COOH*. This work provides a design strategy to employ COFs as scaffold for fabricating efficient CO₂ electrocatalysts.

Core-Shell Carbon-Based Bifunctional Electrocatalysts Derived from COF@MOF Hybrid for Advanced Rechargeable Zn-Air Batteries

The development of highly active carbon-based bifunctional electrocatalysts for both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is highly desired, but still full of challenges in rechargeable Zn-air batteries. Metal organic frameworks (MOFs) and covalent organic frameworks (COFs) have gained great attention for various applications due to their attractive features of structural tunability, high surface area and high porosity. Herein, a core-shell structured carbon-based hybrid electrocatalyst (H-NSC@Co/NSC), which contains high density active sites of MOF-derived shell (Co/NSC) and COF-derived hollow core (H-NSC), is successfully fabricated by direct pyrolysis of covalently-connected COF@ZIF-67 hybrid. The core-shell H-NSC@Co/NSC hybrid manifests excellent catalytic properties toward both OER and ORR with a small potential gap (∆E = 0.75 V). The H-NSC@Co/NSC assembled Zn-air battery exhibits a high power-density of 204.3 mW cm^-2 and stable rechargeability, outperforming that of Pt/C+RuO₂ assembled Zn-air battery. Density functional theory calculations reveal that the electronic structure of the carbon frameworks on the Co/NSC shell can be effectively modulated by the embedded Co nanoparticles (NPs), facilitating the adsorption of oxygen intermediates and leading to enhanced catalytic activity. This work will provide a strategy to design highly-efficient electrocatalysts for application in energy conversion and storage.

Boosting the Capacity of Aqueous Li-Ion Capacitors via Pinpoint Surgery in Nanocoral-Like Covalent Organic Frameworks

Aqueous lithium storage devices are promising candidates for next-generation energy storage applications, featuring low-cost, safety, environmental benignness, and grid-scale merits. Developing reliable anode materials with fast Li⁺ diffusion is paramount to stimulate their development. Herein, the electrochemical performance and mechanism of a redox-active β-ketoenamine-linked covalent organic framework (COF) (2,6-diaminoanthraquinone and 2,4,6-triformylphloroglucinol COF, DAAQ-TFP-COF) for lithium storage in aqueous electrolyte are explored for the first time. Systematic studies demonstrate that, by the conversion of neutral COF into anionic COF via a pinpoint surgery on the β-ketoenamine linkage, the resultative COF shows doubled Li⁺ storage capacity (132 mAh g^-1 at 0.5 A g^-1 , 87% of theoretical specific capacity), good rate capability (108 mAh g^-1 at 10 A g^-1 ), and excellent cyclability in 1000 cycles. This pinpoint surgery can be promising in extending the electrochemical applications of β-ketoenamine-linked COFs. The Li⁺ storage mechanism is investigated by ex situ electron paramagnetic resonance, in situ/ex situ Fourier transform infrared investigations, and density functional theory calculations. As a proof of new concept, a novel aqueous lithium-ion capacitor assembled with DAAQ-TFP-COF anode delivers high specific capacitance of 224 F g^-1 (0.1 A g^-1 ), supercapacitor-level power density (≈4000 W kg^-1 ), and long cyclability.

Construction of a Few-Layered COF@CNT Composite as an Ultrahigh Rate Cathode for Low-Cost K-Ion Batteries

Potassium-ion batteries (PIBs) are attracting great interest for large-scale energy storage owing to the abundant resources and low redox potential of K⁺/K. However, the large volume changes and slow kinetics caused by the larger ionic radius of K⁺ for cathode materials remain a critical challenge for PIBs. Herein, we construct few-layered covalent organic frameworks integrated with carboxylated carbon nanotubes (DAAQ-COF@CNT) as cathode materials for PIBs. The synthesized DAAQ-COF@CNT features numerous active sites, a stable conductive framework, and an appropriate surface area with nanopores, which can render high electrical conductivity, shorten the ion/electron diffusion distance, and accelerate K⁺ diffusion. In consequence, the DAAQ-COF@CNT delivers a high reversible capacity of 157.7 mAh g^-1 at 0.1 A g^-1, an excellent rate capability of 111.2 mAh g^-1 at 1 A g^-1, and a long cycling stability of 77.6% capacity retention after 500 cycles at 0.5 A g^-1. The integrated characterization of ex situ X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and theoretical simulation discloses that the storage mechanism of DAAQ-COF@CNT is based on the reversible reaction between electroactive C═O groups and K⁺ during two successive steps. This work provides a promising high-performance cathode material for PIBs and encourages the development of new types of covalent organic frameworks for PIBs.