Covalent Organic Frameworks for Batteries

Covalent organic frameworks (COFs) have emerged as an exciting new class of porous materials constructed by organic building blocks via dynamic covalent bonds. They have been extensively explored as potentially superior candidates for electrode materials, electrolytes, and separators, due to their tunable chemistry, tailorable structures, and well-defined pores. These features enable rational design of targeted functionalities, facilitate the penetration of electrolytes, and enhance ion transport. This review provides an in-depth summary of the recent progress in the development of COFs for diverse battery applications, including lithium-ion, lithium–sulfur, sodium-ion, potassium-ion, lithium–CO₂, zinc-ion, zinc–air batteries, etc. This comprehensive synopsis pays particular attention to the structure and chemistry of COFs and novel strategies that have been implemented to improve battery performance. Additionally, current challenges, possible solutions, and potential future research directions on COFs for batteries are discussed, laying the groundwork for future advances for this exciting class of material.     

Covalent Organic Frameworks Based Heterostructure in Solar-To-Fuel Conversion

Photocatalytic reactions for fuel generation are crucial for the world's energy needs. Covalent-Organic-Frameworks (COFs) have been extensively studied as promising designable photocatalysts for these reactions due to their efficient visible-light absorption, suitable energy-band structure, facilitated intramolecular charge separation, and fast mass transfer. However, the activities of pristine COFs remain unsatisfactory, due to intermolecular charge recombination. Recently, COF-based heterostructures, which combine COFs with metal-sulfides, metal-oxides, carbon materials, or MOFs, have attracted increasing attention for enhancing solar-to-fuel conversion efficiency by facilitating interfacial photo-generated carrier separation, sensitizing wide-gap semiconductors, and promoting surface redox reactions. Thus, a review of the state-of-the-art progress of COF-based heterostructure photocatalysts in reactions such as H₂ evolution, CO₂ reduction, O₂ evolution, H₂O splitting and CO₂ splitting is crucial for the design of new photocatalysts to promote solar-to-fuel conversion. In this review, the COF-based heterostructures photocatalysts are highlighted based on their synthesis, properties, and reasons for enhanced activities. Moreover, design principles are raised for such photocatalysts for each fuel generation reaction, based on insights into related research. Finally, this review is concluded by proposing future trends for COF-based heterostructures photocatalysts, with attention to the design of COFs and supports, analyzing the photocatalytic reaction dynamics, together with considering practical applications.     

Defective 2D Covalent Organic Frameworks for Postfunctionalization

Defects are deliberately introduced into covalent organic frameworks (COFs) via a three‐component condensation strategy. The defective COFs (dCOF‐NH₂‐Xs, X = 20, 40, and 60) possess favorable crystallinity and porosity, as well as have active amine functional groups as anchoring sites for further postfunctionalization. By introducing imidazolium functional groups onto the pore walls of COFs via the Schiff‐base reaction, dCOF‐ImBr‐Xs‐ and dCOF‐ImTFSI‐Xs‐based materials are employed as all‐solid‐state electrolytes for lithium‐ion conduction with a wide range of working temperatures (from 303 to 423 K), and the ion conductivity of dCOF‐ImTFSI‐60‐based electrolyte reaches 7.05 × 10^?3 S cm^?1 at 423 K. As far as it is known, it is the highest value for all polymeric crystalline porous material based all‐solid‐state electrolytes. Furthermore, Li/dCOF‐ImTFSI‐60@Li/LiFePO₄ all‐solid Li‐ion battery displays satisfactory battery performance under 353 K. This work not only provides a new methodology to construct COFs with precisely controlled defects for postfunctionalization, but also makes them promising candidate materials as all‐solid‐state electrolytes for lithium‐ion batteries operate at high temperatures.     

First-Principles Study of Electronic Structure and Thermoelectric Properties of Ge-Doped Tin Clathrates

We calculated the electronic structure and thermoelectric properties of the Ge-doped quaternary clathrate Ba-Ga-Sn-Ge. The electronic structure was calculated by using the WIEN2k code, which is based on the full-potential augmented plane-wave method. Using this method, we calculated the total energies for several Ge configurations to determine the positions of Ge atoms in the unit cell. The calculated Ge positions were in good agreement with the experimental results. Based on the resulting Ge positions, the band structure and thermoelectric properties of the Ba-Ga-Sn-Ge clathrates were calculated.     

First-Principles Study of the Electronic Structure and Thermoelectric Properties of Al-Doped ZnO

ZnO materials doped with elements such as Al, Ga, etc. are of great interest for high-temperature thermoelectric applications. In this work, the effects of Al doping on the electronic structure and thermoelectric properties of the ZnO system are presented. The energy band structure and density of states of Al-doped ZnO were investigated using the projector-augmented plane wave pseudopotential method within the local density approximation. The calculated energy band structure was then used in combination with the Boltzmann transport equation to calculate the thermoelectric parameters of Al-doped ZnO. The electronic structure calculation showed that the position of the Fermi level of the doped sample was shifted to a higher energy level compared with the undoped material. The conduction band near the Fermi energy was a combination of hybridized Zn sp-orbitals and Al s-orbital. The calculated thermoelectric properties were compared with the experimental results, showing some agreement. For the Al-doped ZnO system, the Seebeck coefficient was shown to be negative and its absolute value increased with temperature. The electrical conductivity and electronic thermal conductivity followed the trend of the experimental results.     

2D Covalent Organic Frameworks for Biomedical Applications

Covalent organic frameworks (COFs) are an emerging class of organic crystalline polymers with well‐defined molecular geometry and tunable porosity. COFs are formed via reversible condensation of lightweight molecular building blocks, which dictate its geometry in two or three dimensions. Among COFs, 2D COFs have garnered special attention due to their unique structure composed of two‐dimensionally extended organic sheets stacked in layers generating periodic columnar π‐arrays, functional pore space, and their ease of synthesis. These unique features in combination with their low density, high crystallinity, large surface area, and biodegradability have made them an excellent candidate for a plethora of applications ranging from energy to biomedical sciences. In this article, the evolution of 2D COFs is briefly discussed in terms of different types of chemical linkages, synthetic strategies of bulk and nanoscale 2D COFs, and their tunability from a biomedical perspective. Next, the biomedical applications of 2D COFs specifically for drug delivery, phototherapy, biosensing, bioimaging, biocatalysis, and antibacterial activity are summarized. In addition, current challenges and emerging approaches in designing 2D COFs for advanced biomedical applications are discussed.     

共价有机框架/多壁碳纳米管复合材料及其双电层电容器性能

共价有机框架(COF)材料是一种特殊的结晶性有机多孔材料,具有多种有机官能团结构,同时有着非常低的骨架密度以及较高的比表面积。通过熔融热法制备TpPa-COF材料并与导电性能优异的多壁碳纳米管(MWCNT)复合制得TpPa-COF@MWCNT纳米复合材料,复合材料的微观形貌通过扫描电子显微镜(SEM)和透射电子显微镜(TEM)进行表征,通过循环伏安法对用于超级电容器的TpPa-COF@MWCNT纳米复合材料的电化学性能进行研究。实验验证了该复合材料在不同扫描速度下的循环伏安曲线均呈现优异的双电层电容特性。当电流密度高达1 A·g-1时,该复合材料的比电容仍达到25 F·g-1,在2 A·g-1的电流密度下测得5 000次循环后比电容的保持率略高于100%,表现出良好的大电流充放电性能和应用前景。     

Covalent Organic Frameworks Enable Sustainable Solar to Hydrogen Peroxide

As a chemical product with rapidly expanding demand in the field of modern energy and environmental applications, hydrogen peroxide (H₂O₂) has garnered widespread attention. However, the existing industrial production of H₂O₂ is plagued by high energy consumption, harmful waste emission, and severe safety issues, making it difficult to satisfy the environmental/economic production concept. Artificial photosynthesis offers a viable strategy for green and sustainable H₂O₂ production since it uses sunlight as an energy source to initiate the reaction of oxygen and water to produce H₂O₂. Among various photocatalysts, covalent organic frameworks (COFs), featuring highly ordered skeletons and well-defined active sites, have emerged as promising photocatalysts for H₂O₂ production. This review presents the nascent and burgeoning area of photocatalytic H₂O₂ production based on COFs. First, a brief overview of photocatalytic technology is provided, followed by a detailed introduction to the principles and evaluation of the photocatalytic H₂O₂ generation. Subsequently, the latest research progress on the judicious design of COFs for H₂O₂ photosynthesis is expounded, with a particular emphasis on manipulating the electronic structures and redox active sites. Finally, an outlook on the challenges and future opportunities is proposed, in the hope of stimulating further explorations of novel molecular-designed COFs for sustainable photosynthesis.     

Two-Photon Absorption Induced Cancer Immunotherapy Using Covalent Organic Frameworks

Immune checkpoint blockade therapy is revolutionizing the traditional treatment model of multiple tumor types, but remains ineffective for a large subset of patients. Photodynamic therapy (PDT) has been shown to induce cancer cell death and provoke an immune response, and may represent a potential strategy to synergize with immune checkpoint blockade therapy. However, the limited tissue penetration of exciting light for conventional PDT largely hinders its application in the clinic and its further combination with immunotherapy. Here, a serrated packing covalent organic framework (COF), COF-606, with excellent two-photon absorption (2PA) property and photostability, largely avoids aggregation-caused quenching, therefore offering high reactive oxygen species (ROS) generation efficiency; it is used as a 2PA photosensitizer for PDT in deep tumor tissue. COF-606 induced PDT is shown to be efficient in inducing immunogenic cell death, provoking an immune response and normalizing the immunosuppressive status for the first time. This makes it possible to combine 2PA induced PDT using COF with programmed cell death protein 1 immune checkpoint blockade therapy. Such combination leads to strong abscopal tumor-inhibiting efficiency and long-lasting immune memory effects, standing as a promising combinatorial therapeutic strategy for cancer treatment.     

包覆在MWCNT表面的共价有机框架复合材料及其超级电容器性能

共价有机框架(COF)是一类由C、O、N和B等元素通过共价键连接的特殊有机多孔结晶材料,是金属有机框架(MOF)材料后又一重要的三维有序材料,其具有多种有机官能团结构,兼具很低的骨架密度和较高的比表面积。通过微波加热和阴离子取代制备了SJTU-COFAcO材料,并与具有良好导电性能的多壁碳纳米管(MWCNT)复合制得SJTU-COF-AcO@MWCNT纳米复合材料。材料的微观形貌通过扫描电子显微镜(SEM)等进行表征。通过组建三电极的超级电容器系统对复合材料的电化学性能进行了研究,在电流密度1 A·g~(-1)下,该材料的质量比电容能够达到29.2 F·g~(-1),循环5 000次后比电容仍能保持90.9%,表明其作为电容器电极材料具有良好的性能和广阔的应用前景。     

Covalent Organic Frameworks: Structures,Synthesis, and Applications

Covalent organic frameworks (COFs) are crystalline porous polymers formed by a bottom‐up approach from molecular building units having a predesigned geometry that are connected through covalent bonds. They offer positional control over their building blocks in two and three dimensions. This control enables the synthesis of rigid porous structures with a high regularity and the ability to fine‐tune the chemical and physical properties of the network. This Feature Article provides a comprehensive overview over the structures realized to date in the fast growing field of covalent organic framework development. Different synthesis strategies to meet diverse demands, such as high crystallinity, straightforward processability, or the formation of thin films are discussed. Furthermore, insights into the growing fields of COF applications, including gas storage and separations, sensing, electrochemical energy storage, and optoelectronics are provided.     

Defects and Disorder in Covalent Organic Frameworks for Advanced Applications

Crystalline porous organic polymers (CPPs) or covalent organic frameworks (COFs), are composed by light elements linked by covalent bonds. Despite the remarkable progress attained, there are still bottlenecks limiting further development, some of them related to the presence of defects during their synthesis as well as in-depth understanding of structure of active centers and/or details of the reaction mechanism. Indeed, very often the proposed structures are far from reality because defects and disorders have not been considered. The present review provides an illustrative overview of “defects and disorder in COFs”. These defects include those not only generated during the synthesis and manipulation of COFs, but also lack of crystallinity, stacking disorder and network vacancies. The review starts giving general remarks on organic COFs and their synthetic methods, followed by different methods to play and manage defects, how to minimize them or how to take advantage of them to gain new properties and applications. Selected characterization techniques to quantify defective structures and active sites in COFs are also presented. Finally, the challenges and future opportunities in the field have been summarized in the last section.     

All-in-One Hollow Flower-Like Covalent Organic Frameworks for Flexible Transparent Devices

Covalent organic frameworks (COFs) exhibit great potential in the application of functional electronic devices. However, there has been no report of the precise fabrication of 3D all-in-one hollow micro/nanostructures based on COFs. Here, for the first time, all-in-one hollow dioxin-based COF-316 microflowers are synthesized measuring 5–7 µm and with interconnected hollow petals through a self-template strategy. The growth mechanism involves the collaborative process of self-assembly of nanoparticles, inside-out Ostwald ripening, and epitaxial growth. Due to the intrinsic porosity and interconnected hollow structure, COF-316 can uniformly composite with polypyrrole (PPy) through the “interior” and “exterior” functionalization, in which the hydrogen bond interaction enhances the charge transfer efficiency and structural stability in the charge/discharge process. The COF-316@PPy flexible transparent supercapacitors exhibit an areal specific capacitance (C_A) of 783.6 µF cm^–2 at 3 µA cm^–2 and long-term cycling stability. This work will boost research on the valuable design concepts of 3D hollow COF materials for energy storage devices.     

Imaging Solvent-Triggered Gaseous Iodine Uptake on Single Covalent Organic Frameworks

Covalent organic frameworks (COFs) are promising solid absorbents for the treatment of gaseous iodine. However, extensive efforts are still focused on empirical optimizations of specific binding sites and pore structures in COFs, and the chemical control of gaseous iodine uptake on COFs remains challenging. In this study, the chemically triggered sorption properties of COF-300 for I₂ vapors at the single-particle level with the dark-field microscope (DFM) are explored. The present operando single-particle DFM imaging method enables the direct visualization of an adsorption activity transformation from inactive COF-300 to active solvated COF-300 toward gaseous I₂ vapors. Exploiting the useful reaction information from time-lapsed DFM images, the tunable adsorption performance of solvated COF-300 is quantitatively compared by various solvents. The results illustrate that the isopropanol (IPA)-solvated COF-300 achieves the optimum adsorption capacity for I₂ among the absorbents. The reaction mechanism is elucidated to be the channel size enlargement and modification of internal surface chemistry in the IPA-solvated COF-300, producing a stable I₂/IPA-solvated COF-300 complex after the sorption reaction. The present chemical control of the sorption behavior of COF-300 revealed by DFM opens up a new fundamental paradigm for rationally developing high-performance COF-based absorbents for removing I₂ vapors.     

Ionic Dye Based Covalent Organic Frameworks for Photothermal Water Evaporation

Conversion of solar energy into heat for water evaporation is of great significance to provide clean and sustainable technology for water purification by using inexhaustible sunlight. In this field, one of the challenges comes from the design of high-performance photothermal materials powerful in light harvesting, light-to-heat conversion, and water activation. Herein, it is demonstrated that rationalization of the ionic covalent organic framework (iCOF) can simultaneously satisfy these multiple requirements and a new iCOF STTP is constructed through the Schiff base chemistry in a rapid microwave-assisted solvothermal route by using a hydrophilic dye molecule safranineT as the ionic building block. The integrated dye-related ionic moieties greatly strengthen the light absorbance (>97%) throughout the entire solar spectrum from UV–vis to the infrared region. The framework ionic moieties provide strong polarization to reduce the exciton dissociation energy for enhanced photothermal effect, and in addition, promote sufficient water activation to decrease the water evaporation enthalpy. As an outcome, the STTP driven solar water evaporator affords a fast water evaporation rate of 3.55 kg h⁻¹ m⁻² and high solar-to-vapor efficiency of 95.8%. This study highlights the potential of designing iCOF materials for photothermal applications.     

Thermoelectric Properties of Organic Charge-Transfer Compounds

We measured the thermoelectric (TE) properties of compressed pellets of various organic charge-transfer (CT) complexes, such as (TTF)(TCNQ), (BO)(TCNQ) and (ET)₂(HCNAL), where TTF, TCNQ, BO, ET, and HCNAL represent tetrathiafulvalene, tetracyanoquinodimethane, bis(ethylenedioxy)-tetrathiafulvalene, bis(ethylenedithio)tetrathiafulvalene, and 2,5-dicyano- 3,6-dihydroxy-p-benzoquinone, respectively. The metallic (TTF)(TCNQ) and semiconducting (BO)(TCNQ) complexes showed Seebeck coefficients (S) of −18 μV/K and −30 μV/K at 300 K, respectively. On the contrary, the Mott insulator (ET)₂(HCNAL) was found to show a rather high absolute S (−116 μV/K at 300 K), the magnitude of which is comparable to those of the conventional inorganic TE materials. With increasing temperature (170 K to 300 K), the electrical conductivity was increased about two orders of magnitude while the S value was nearly constant. These results suggest that S values could be determined mainly by spin entropy (configurations) of carriers in the Mott insulator (ET)₂(HCNAL). The magnitude of the observed S value was compared with that derived from a theoretical model (generalized Heikes formula).     

Azobenzene Functionalized Organic Covalent Frameworks: Controlled Morphologies and Photo-Regulated Adsorption

Covalent organic frameworks (COFs) containing azobenzene building blocks carry great potential for use in intelligent storage, separation, chemical sensing, and catalysis due to their intriguing photo-responsiveness. However, azobenzene units are often exploited as the linkers to form the framework of COFs, thereby restricting their molecular motion and photoisomerization. Herein, a simple yet robust template-free solvothermal strategy is reported to yield azobenzene-dangled COFs (Azo-COFs) with their azobenzene moieties suspending within the pores. The crystallinity, specific surface area, and morphology of Azo-COFs can be conveniently tailored by changing the ratio of amine to aldehyde monomers. Notably, the Azo-COFs provide sufficient free space for the reversible trans-to-cis isomerization of the dangled azobenzene units inside the pores, thus reversibly regulating surface wettability of Azo-COFs. The adsorption capacity of Azo-COFs toward organic dye molecules is increased by 3.7-fold when irradiated with ultraviolet light, which can be ascribed to the intelligent closing/opening of molecular gates rendered by photoisomerization of azobenzene moieties. As such, the ability to photoregulate the adsorption of Azo-COFs highlights their significance in functioning as smart porous nanomaterials for applications in cargo release, molecular sieves, ion transport, energy conversion systems, and environmental remediation.     

Electrolyte Interphase Built from Anionic Covalent Organic Frameworks for Lithium Dendrite Suppression

Lithium (Li) metal batteries hold considerable promise for numerous energy-dense applications. However, the dendritic Li anode produced during Li⁺/Li deposition-stripping endangers battery safety and shortens cycle lifespan. Herein, an electrolyte interphase built from 2D anionic covalent organic frameworks (ACOF) is coated on Li for dendrite suppression. The ACOF with Li⁺-affinity facilitates rapid and exclusive passage of Li-ions from the electrolyte, yielding near-unity Li⁺ transference number (0.82) and ionic conductivity beyond 3.7 mS cm^-1 at the interphase. Such high transport efficiency of Li-ions can fundamentally circumvent the Li⁺ deficiency that results in dendrite formation. Pairing the ACOF-coated Li against a high-voltage LiCoO₂ cathode (4.5 V) achieves exceptional cycle stability, mitigated polarization, as well as improved rate capability. Accordingly, this strategy vastly expands the pool of electrolyte interphases that can be used for coating and protecting Li anode.     

2D Hexagonal Covalent Organic Radical Frameworks as Tunable Correlated Electron Systems

Quantum materials hold huge technological promise but challenge the fundamental understanding of complex electronic interactions in solids. The Mott metal–insulator transition on half-filled lattices is an archetypal demonstration of how quantum states can be driven by electronic correlation. Twisted bilayers of 2D materials provide an experimentally accessible means to probe such transitions, but these seemingly simple systems belie high complexity due to the myriad of possible interactions. Herein, it is shown that electron correlation can be simply tuned in experimentally viable 2D hexagonally ordered covalent organic radical frameworks (2D hex-CORFs) based on single layers of half-filled stable radical nodes. The presented carefully procured theoretical analysis predicts that 2D hex-CORFs can be varied between a correlated antiferromagnetic Mott insulator state and a semimetallic state by modest out-of-plane compressive pressure. This work establishes 2D hex-CORFs as a class of versatile single-layer quantum materials to advance the understanding of low dimensional correlated electronic systems.