Covalent organic frameworks (COFs) for electrochemical applications

2D Conjugated Covalent Organic Frameworks: Defined Synthesis and Tailor-Made Functions

ConspectusCovalent organic frameworks (COFs) are an emerging class of crystalline porous polymers and have received tremendous attention and research interest. COFs can be classified into two-dimensional (2D) and three-dimensional (3D) analogues. Resembling the architectures of porous graphene, 2D conjugated COFs have exhibited promising prospects in many fields, such as gas storage and separation, heterogeneous catalysis, sensing, photocatalysis, environmental remediation, drug delivery, energy storage and conversion, and so forth. However, efficient structural design for high-throughput production of crystalline 2D COFs remains challenging.In this Account, we summarize our recent contributions to the design, synthesis, and application exploration of 2D conjugated COFs. First, we raised an efficient "two-in-one" strategy for the facile synthesis of 2D imine COFs with good reproducibility and solvent adaptability. Thanks to this elaborate molecular design strategy, we could easily modulate the topology of COFs and fabricate COF films. In addition, we developed two approaches to stabilize the 2D conjugated COFs by using planar building blocks and donor-acceptor structures. We also proposed a skeleton engineering strategy to design COFs as electrode materials, through which redox-active orthoquinone moieties were stepwise-incorporated in the skeletons of isostructural 2D imine-linked COFs. This strategy enabled systematic investigations on a series of 2D conjugated COFs with analogous structures but different numbers of active sites for energy storage, which provides a good platform to unveil the underlying structure-property relationships. In addition, we recently developed a new kind of arylamine-linked 2D conjugated COFs. The electroactive diphenylamine linkages endowed these 2D conjugated COFs with extended conjugation and improved stability, which also conferred these COFs with excellent pseudocapacitive energy storage performance. Moreover, tailor-made sulfur-rich COFs were introduced that were synthesized by selective introduction of polysulfide or sulfonyl groups on the COF skeletons and were used for Li storage and proton conduction. At the end, the key challenges of 2D conjugated COFs toward practical applications and their future prospects are suggested. We hope that this Account will evoke new inspirations and innovative work in the field of 2D conjugated COFs in the near future, especially in some burgeoning and interdisciplinary research areas.

Fast-Decoding Algorithm for Electrode Processes at Electrified Interfaces by Mean-Field Kinetic Model and Bayesian Data Assimilation: An Active-Data-Mining Approach for the Efficient Search and Discovery of Electrocatalysts

The microscopic origins of the activity and selectivity of electrocatalysts has been a long-lasting enigma since the 19th century. By applying an active-data-mining approach, employing a mean-field kinetic model and a statistical approach of Bayesian data assimilation, we demonstrate here a fast decoding to extract key properties in the kinetics of complicated electrode processes from current-potential profiles in experimental and literary data. As the proof-of-concept, kinetic parameters on the four-electron oxygen reduction reaction in the 0.1 M HClO4 solution (ORR: O2 + 4e- + 4H+ → 2H2O) of various platinum-based single-crystal electrocatalysts are extracted from our own experiments and third-party literature to investigate the microscopic electrode processes. Furthermore, data assimilation of the mean-field ORR model and experimental data is performed based on Bayesian inference for the inductive estimation of kinetic parameters, which sheds light on the dynamic behavior of kinetic parameters with respect to overpotential. This work shows that a fast-decoding algorithm based on a mean-field kinetic model and Bayesian data assimilation is a promising data-driven approach to extract key microscopic features of complicated electrode processes and therefore will be an important method toward building up advanced human-machine collaborations for the efficient search and discovery of high-performance electrochemical materials.

Tying Covalent Organic Frameworks through Alkene Metathesis and Supported Platinum as Efficient Catalysts for Hydrosilylation

Recently there has been a great interest in covalent organic frameworks due to their fascinating properties. Current approaches to improve their hydrolytic stability mainly rely on the transformation of the dynamic bonds into strong and irreversible bonds, but these approaches also reduce the versatility of the frameworks. Herein, we would like to demonstrate a solution to this dilemma by forming hierarchical bonds through olefin metathesis to produce highly stable COFs. Our approach allows unprecedented opportunities for post-modification of the inner space through the dynamic imine bonds while maintaining the integrity of the framework. Specifically, we demonstrate an amorphous-to-crystalline transformation. In addition, the porosity can be enhanced by up to 70% after full removal of the amine subunits. Overall, our work provides a new direction for the generation of highly stable while still versatile COFs. Meanwhile, platinum(II) complexes can be supported on BHU-2 (Pt@BHU-2) or BHU-2-Oxidate(Pt@BHU-2-Oxidate) as efficient catalysts for hydrosilylation.

A facile and scalable synthetic method for covalent organic nanosheets: ultrasonic polycondensation and photocatalytic degradation of organic pollutants

Covalent organic framework nanosheets (COF NSs or CONs), as compared to their bulk counterparts two-dimensional (2D) covalent organic frameworks (COFs), exhibit superior performance in many aspects due to their fully accessible active sites benefiting from their ultrathin porous 2D structures. The development of a scalable synthetic methodology for CONs is crucial to further exploration of their unique properties and practical applications. Herein, we report an efficient strategy to fabricate ultrathin CONs through direct polycondensation of monomers under ultrasonic treatment and mild conditions. This method is facile and scalable, which is demonstrated by gram-scale synthesis of two ultrathin 2D CONs in several hours. Moreover, the as-prepared ultrathin CONs show excellent heterogeneous photocatalytic performance for the degradation of organic pollutants (dyes as representatives), remarkably superior to the bulk COFs prepared from the corresponding monomers under solvothermal conditions. This research provides a new roadmap for the scalable and facile synthesis of ultrathin CONs, which is of paramount importance for fully exploring the tremendous potential of this emerging type of 2D material.

We develop a strategy to efficiently fabricate ultrathin covalent organic framework nanosheets (CONs) through direct polycondensation of monomers under ultrasonic treatment. The CONs exhibit excellent photocatalytic performance for the degradation of organic pollutants.

Dual Rate-Modulation Approach for the Preparation of Crystalline Covalent Triazine Frameworks Displaying Efficient Sodium Storage

A dual rate-modulation approach was implemented for the first time to create crystalline covalent triazine frameworks. Based on a new polycondensation approach, regulating the condensation rate via the exploitation of a modulated aldehyde monomer and addition of an extrinsic inhibitor affords inherent control over the polymer growth and therefore provides tunable crystallinities and porosities for the resulting triazine frameworks. The existence of rich redox-active triazine linkages gives rise to obtaining exceptional sodium storage, where 239 mAh g^-1 at 1.0 A g^-1 is obtained after 200 cycles. We anticipate this new protocol based on the dynamic imine metathesis will facilitate new possibilities for the construction of crystalline covalent triazine frameworks and promote their energy-related applications.

Design strategies for improving the crystallinity of covalent organic frameworks and conjugated polymers: a review

Highly crystalline covalent organic frameworks (COFs) or conjugated polymers (CPs) are very important and highly desirable because these materials would display better performance in diverse devices and provide more structure-property related information. However, how to achieve highly crystalline or single-crystal COFs and CPs is very challenging. Recently, many research studies have demonstrated the possibility of enhancing the crystallinity of COFs and CPs. Thus, it is timely to offer an overview of the important progress in improving the crystallinity of COFs and CPs from the viewpoint of design strategies. These strategies include polycondensation reaction optimization, improving the planarity, fluorine substitution, side chain engineering, and so on. Furthermore, the challenges and perspectives are also discussed to promote the realization of highly crystalline or single-crystal COFs and CPs.

Robust and emissive covalent organic frameworks via intramolecular hydrogen bond interaction

Covalent organic frameworks (COFs) are a new class of crystalline porous polymers with periodic structure in the skeleton and pre-designable pore structure. COFs merge excellent crystallinity, porosity, stability, and emission,...

Light-Driven Molecular Motors Embedded in Covalent Organic Frameworks

The incorporation of molecular machines into the backbone of porous framework structures will facilitate nano actuation, enhanced molecular transport, and other out-of-equilibrium host-guest phenomena in well-defined 3D solid materials. In...

Fabrication of Hierarchical Nanoreactor based on COFs for Cascade Enzyme Catalysis

A hierarchical nanoreactor based on covalent organic frameworks (COFs) was fabricated using polystyrene spheres (PSs) as template. When applied in cascade catalysis, the nanoreactor improved significantly the catalytic activity of...

Crystallizing Sub 10 nm Covalent Organic Framework Thin Films via Interfacial-Residual Concomitance

Synthesis of covalent organic framework (COF) thin films on different supports with high crystallinity and porosity is crucial for their potential applications. We have designed a new synchronized methodology, residual crystallization (RC), to synthesize sub 10 nm COF thin films. These residual crystallized COF thin films showcase high surface area, crystallinity, and conductivity at room temperature. We have used interfacial crystallization (IC) as a rate-controlling tool for simultaneous residual crystallization. We have also diversified the methodology of residual crystallization by utilizing two different crystallization pathways: fiber-to-film (F-F) and sphere-to-film (S-F). In both cases, we could obtain continuous COF thin films with high crystallinity and porosity grown on various substrates (the highest surface area of a TpAzo COF thin film being 2093 m² g^-1). Precise control over the crystallization allows the synthesis of macroscopic defect-free sub 10 nm COF thin films with a minimum thickness of ∼1.8 nm. We have synthesized two COF thin films (TpAzo and TpDPP) using F-F and S-F pathways on different supports such as borosilicate glass, FTO, silicon, Cu, metal, and ITO. Also, we have investigated the mechanism of the growth of these thin films on various substrates with different wettability. Further, a hydrophilic support (glass) was used to grow the thin films in situ for four-probe system device fabrication. All residual crystallized COF thin films exhibit outstanding conductivity values. We could obtain a conductivity of 3.7 × 10^-2 mS cm^-1 for the TpAzo film synthesized by S-F residual crystallization.

2D Salphen-based heteropore covalent organic frameworks for highly efficient electrocatalytic water oxidation

The construction of heteroporous covalent organic frameworks (COFs) is still a challenge. Herein, a series of 2D COFs with hexagonal and quadrilateral pores were constructed via in situ salphen or metal salphen formation. Metallized salphen-based COFs can be used as electrocatalysts towards water oxidation with an overpotential of 266 mV at 10 mA cm^-2.

Controllable Synthesis of COFs-Based Multicomponent Nanocomposites from Core-Shell to Yolk-Shell and Hollow-Sphere Structure for Artificial Photosynthesis

The precise tuning and multi-dimensional processing of covalent organic frameworks (COFs)-based materials into multicomponent superstructures with appropriate diversity are essential to maximize their advantages in catalytic reactions. However, up to now, it remains an ongoing challenge for the precise design of COFs-based multicomponent nanocomposites with diverse architectures. Herein, a metal organic framework (MOF)-sacrificed in situ acid-etching (MSISAE) strategy that enables continuous synthesis of core-shell, yolk-shell, and hollow-sphere COFs-based nanocomposites through tuning of core decomposition (NH₂ -MIL-125 into TiO₂ ) rate is developed. More importantly, due to the multiple active sites, fast transfer of carriers, increased light utilization ability, et al, one of the obtained samples, NH₂ -MIL-125/TiO₂ @COF-366-Ni-OH-HAc (yolk-shell) with special three components, exhibits high photocatalytic CO₂ -to-CO conversion efficiency in the gas-solid mode. The MSISAE strategy developed in this work achieves the precise morphology design and control of multicomponent hybrid composites based on COFs, which may pave a new way in devealoping porous crystalline materials with powerful superstructures for multifunctional catalytic reactions.

Ionic Covalent Organic Frameworks for Energy Devices

Covalent organic frameworks (COFs) are a class of porous crystalline materials whose facile preparation, functionality, and modularity have led to their becoming powerful platforms for the development of molecular devices in many fields of (bio)engineering, such as energy storage, environmental remediation, drug delivery, and catalysis. In particular, ionic COFs (iCOFs) are highly useful for constructing energy devices, as their ionic functional groups can transport ions efficiently, and the nonlabile and highly ordered all-covalent pore structures of their backbones provide ideal pathways for long-term ionic transport under harsh electrochemical conditions. Here, current research progress on the use of iCOFs for energy devices, specifically lithium-based batteries and fuel cells, is reviewed in terms of iCOF backbone-design strategies, synthetic approaches, properties, engineering techniques, and applications. iCOFs are categorized as anionic COFs or cationic COFs, and how each of these types of iCOFs transport lithium ions, protons, or hydroxides is illustrated. Finally, the current challenges to and future opportunities for the utilization of iCOFs in energy devices are described. This review will therefore serve as a useful reference on state-of-the-art iCOF design and application strategies focusing on energy devices.

Arousing Electrochemiluminescence Out of Non-Electroluminescent Monomers within Covalent Organic Frameworks

Covalent organic frameworks (COFs) with stable long-range ordered arrangements are promising materials for organic optoelectronics. However, their electrochemiluminescence (ECL) from non-ECL active monomers has not been realized. Here, we report a design strategy for ECL-emitting COF family. The donors and acceptors co-crystallized and stacked into the highly aligned array of olefin-linked COFs, so that electrons can be transported freely. By this means, a tunable ECL is activated from non-ECL molecules with the maximum efficiency of 32.1% in water with the dissolved oxygen as an inner coreactant, and no additional noxious co-reactant is needed any more. Quantum chemistry calculations further demonstrate that this design reduces the COFs' band gaps and the overlap of electrons and holes in the excited state for better photoelectric properties and stronger ECL signals. This work exploits a basis to envisage the broad application potential of ECL-COFs for various biosensors and light-emitting display.

Synergistic effects of Co/CoO nanoparticles on imine-based covalent organic frameworks for enhanced OER performance

The development of non-noble metal electrocatalysts toward the oxygen evolution reaction (OER) is a key challenge in advancing electrocatalytic water splitting, which is essential for the commercialization of clean and renewable energy. A covalent organic framework (COF) has a precise and controllable structure, high π-π conjugation, large surface area, and porosity and shows great potential as an OER electrocatalyst. However, the relative conductivity and inherent instability greatly limit the further improvement of its performance. Herein, imine-based COF-supported Co/CoO nanoparticles (Co/CoO@COF) were developed for the high-performance electrocatalytic OER. For the Co/CoO@COF catalyst, Co/CoO could form a conjugation effect with the COF, which can increase the electron cloud density of the delocalized large π bond, then improve the conductivity. The combination of Co/CoO and COF effectively enhances the structural stability of the catalyst and enriches the catalytic active sites. Under alkaline conditions, the Co/CoO@COF shows a very low overpotential of 278 mV at a current density of 10 mA cm^-2, and a Tafel slope of 80.11 mV dec^-1 which is better than that of commercial RuO₂.

Structure-Performance Relationships of Covalent Organic Framework Electrode Materials in Metal-Ion Batteries

Covalent organic frameworks (COFs) have shown great potential as high-performance electrode materials for metal-ion batteries in view of their relatively high capacity, flexibly designable structure, ordered porous structure, and limited solubility in electrolyte. To develop more attractive COF electrode materials, it is necessary to understand their structure-performance relationships. This Perspective focuses on discussing the relationships between structure (molecular structure, micro structure, and electrode structure) and performance (voltage, capacity, cycling stability, and rate performance) of COF electrode materials in metal-ion batteries. Among the reported COF electrode materials, those with all linkages being redox active based on C═O and C═N groups would be the most promising cathode materials because of their high capacity (∼500 mAh g^-1), moderate working voltage (∼2 V vs Li⁺/Li), and good cycling stability. To accelerate practical application of COF electrode materials in metal-ion batteries, future work should pay more attention to function-oriented molecular structure design via theoretical simulations, as well as full-cell fabrication and evaluation. This Perspective will stimulate high-quality research that might lead to future commercialization.

Covalent Organic Frameworks and Their Derivatives for Better Metal Anodes in Rechargeable Batteries

Metal anodes based on a plating/stripping electrochemistry such as metallic Li, Na, K, Zn, Ca, Mg, Fe, and Al are recognized as promising anode materials for constructing next-generation high-energy-density rechargeable metal batteries owing to their low electrochemical potential, high theoretical specific capacity, superior electronic conductivity, etc. However, inherent issues such as high chemical reactivity, severe growth of dendrites, huge volume changes, and unstable interface largely impede their practical application. Covalent organic frameworks (COFs) and their derivatives as emerging multifunctional materials have already well addressed the inherent issues of metal anodes in the past several years due to their abundant metallophilic functional groups, special inner channels, and controllable structures. COFs and their derivatives can solve the issues of metal anodes by interfacial modification, homogenizing ion flux, acting as nucleation seeds, reducing the corrosion of metal anodes, and so on. Nevertheless, related reviews are still absent. Here we present a detailed review of multifunctional COFs and their derivatives in metal anodes for rechargeable metal batteries. Meanwhile, some outlooks and opinions are put forward. We believe the review can catch the eyes of relevant researchers and supply some inspiration for future research.

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.