Mesoporous Polyimide-Linked Covalent Organic Framework with Multiple Redox-Active Sites for High-Performance Cathodic Li Storage

Integrating Multiple Redox-Active Units into Conductive Covalent Organic Frameworks for High-Performance Sodium-Ion Batteries

The rational design of porous covalent organic frameworks (COFs) with high conductivity and reversible redox activity is the key to improving their performance in sodium-ion batteries (SIBs). Herein, we report a series of COFs (FPDC-TPA-COF, FPDC-TPB-COF, and FPDC-TPT-COF) based on an organosulfur linker, (trioxocyclohexane-triylidene)tris(dithiole-diylylidene))hexabenzaldehyde (FPDC). These COFs feature two-dimensional crystalline structures, high porosity, good conductivity, and densely packed redox-active sites, making them suitable for energy storage devices. Among them, FPDC-TPT-COF demonstrates a remarkably high specific capacity of 420 mAh g-1 (0.2 A g-1), excellent cycling stability (~87 % capacity retention after 3000 cycles, 1.0 A g-1) and high rate performance (339 mAh g-1 at 2.0 A g-1) as an anode for SIBs, surpassing most reported COF-based electrodes. The superior performance is attributed to the dithiole moieties enhancing the conductivity and the presence of redox-active carbonyl, imine, and triazine sites facilitating Na storage. Furthermore, the sodiation mechanism was elucidated through in situ experiments and density functional theory (DFT) calculations. This work highlights the advantages of integrating multiple functional groups into redox-active COFs for the rational design of efficient and stable SIBs.

Regulating the Spin-State of Cobalt in Three-Dimensional Covalent Organic Frameworks for High-Performance Sodium-Iodine Rechargeable Batteries

Although the catalytic activity is heavily reliant on the electronic structure of the catalyst, understanding the impact of electron spin regulation on electrocatalytic performance is still rarely investigated. This work presents a novel approach involving the single-atom coordination of cobalt (Co) within metalloporphyrin-based three-dimensional covalent organic frameworks (3D-COFs) to facilitate the catalytic conversion for sodium-iodine batteries. The spin state of Co is modulated by altering the oxidation state of the porphyrin-centered Co, achieving optimal catalysis for iodine reduction. Experimental results demonstrate that CoII and CoIII are incorporated into the 3D-COFs, exhibiting spin ground states of S=1/2 and S=0, respectively. The low spin state of CoIII is favorable to hybridize with the sp 3d orbitals of I3 -, thus facilitating the conversion of I3 - to I-. Density-functional theory (DFT) calculations further reveal that the presence of CoIII enhances iodide adsorption and accelerates the formation of NaI in 3D-COFs-CoIII, thereby promoting its rapid kinetic behaviors. Notably, the I2@3D-COFs-CoIII cathode achieves a high reversible capacity of 227.7 mAh g-1 after 200 cycles at 0.5 C and demonstrates exceptional cyclic stability, exceeding 2000 cycles at 10 C with a minor capacity fading rate of less than one 0.01 % per cycle.

Triazine-Based Large-Sized Single-Crystalline Two-Dimensional Covalent Organic Framework for High-Performance Lithium-Ion Batteries

A large-sized single crystalline 2D COFs with excellent crystallinity and stability was prepared through the traditional thermal solvent method. The electrochemical performance can be significantly enhanced using a straightforward hybrid approach that involves in situ growth of the 2D COFs on multi-walled carbon nanotubes (MWCNTs). Both the advantages of COFs and CNTs are mutually enhanced. The single-crystalline feature of the obtained COFs improves the structural integrity and brings excellent chemical and electrochemical stabilities for lithium-ion battery applications. The resultant COF-CNT core-shell hybrids greatly improved the conductivity and demonstrated excellent lithium-ion storage performance with a high capacity of 228 mAh g-1 (0.2 A g-1).

Conjugated Phthalocyanine-Based Mesoporous Covalent Organic Frameworks for Efficient Anodic Lithium Storage

Organic anode materials have been recognized as promising candidates for low-cost and sustainable lithium-ion batteries (LIBs), which however suffer from the inferior cycling stability and low conductivity with unsatisfactory LIBs performance. Herein, two conjugated phthalocyanine-based covalent organic frameworks (COFs), namely CoPc-Ph-COF and CoPc-3Ph-COF, are synthesized by the nucleophilic substitution reaction of hexafluorophthalocyanine cobalt (II) (CoPcF16) with 1,2,4,5-tetrahydroxybenzene and 9,10-dimethyl-2,3,6,7-tetrahydroxyanthracene, respectively. Powder X-ray diffraction and electron microscopy analysis reveal the crystalline porous structure of both COFs with a pore size of 1.6-2.4 nm, enabling facile ion transportation. Immersion experiments demonstrate the excellent stability of both COFs. I-V curve measurement discloses the superb conductivity of both COFs due to their fully π-conjugated frameworks. These merits, in combination with their N-rich skeleton, endow the two COFs with excellent anodic Li+ storage performance in terms of high specific capacities, superb rate performance, and good cycling stability. In particular, CoPc-3Ph-COF suggests a large reversible capacity of 1086 mA h g-1 at 100 mA g-1, superior to most reported organic LIBs anodes, exhibiting its promising application in high-performance LIBs.

A Covalent Triazine Framework for Photocatalytic Anti-Markovnikov Hydrofunctionalizations

Porous materials-based heterogeneous photocatalysts, performing selective organic transformations, are increasing the applicability of photocatalytic reactions due to their ability to merge traditional photocatalysis with structured pores densely decorated with catalytic moiety for efficient mass and charge transfer, as well as added recyclability. We herein disclose a porous crystalline covalent triazine framework (CTF)-based heterogeneous photocatalyst that exhibits excellent photoredox properties for different hydrofunctionalization reactions such as hydrocarboxylations, hydroamination and hydroazidations. The high oxidizing property of this CTF enables the activation of styrenes, followed by regioselective C-N and C-O bond formation at ambient conditions. A change in the physicochemical and optoelectronic properties of the CTF, upon protonation during catalysis, lies at the basis of its photocatalytic properties. This allows us to obtain hydrocarboxylations, hydroamination, and hydroazidations from a myriad of electron-donating and -withdrawing aromatic and aliphatic substrates. This catalytic approach is further extended to late-stage functionalization of bio-active molecules. Finally, detailed characterizations of the CTF and further mechanistic investigations provide mechanistic insights into these reactions.

High-Performance Polyimide Covalent Organic Frameworks for Lithium-Ion Batteries: Exceptional Stability and Capacity Retention at High Current Densities

Organic polymers are considered promising candidates for next-generation green electrode materials in lithium-ion batteries (LIBs). However, achieving long cycling stability and capacity retention at high current densities remains a significant challenge due to weak structural stability and low conductivity. In this study, we report the synthesis of two novel polyimide covalent organic frameworks (PI-COFs), COF-JLU85 and COF-JLU86, by combining truxenone-based triamine and linear acid anhydride through polymerization. These PI-COFs feature layers with pore channels embedded with 18 carbonyl groups, facilitating rapid lithium-ion diffusion and enhancing structural stability under high current densities. Compared to previously reported organic polymer materials, COF-JLU86 demonstrates the excellent performance at high current densities, with an impressive specific capacity of 1161.1 mA h g-1 at 0.1 A g-1, and outstanding cycling stability, retaining 1289.8 mA h g-1 at 2 A g-1 after 1500 cycles and 401.1 mA h g-1 at 15 A g-1 after 10000 cycles. Additionally, in situ infrared spectroscopy and density functional theory (DFT) calculations provide mechanistic insights, revealing that the high concentration of carbonyl redox-active sites and the optimized electronic structure contribute to the excellent electrochemical performance. These results highlight the potential of PI-COFs as high-performance organic electrode materials for LIBs, offering a promising solution to the challenges of long-term stability and capacity retention at high current densities.

Eco-Friendly High-Performance Symmetric All-COF/Graphene Aqueous Zinc-Ion Batteries

Developing high-performance aqueous symmetric all-organic batteries (SAOBs) by replacing metal-based batteries or batteries with organic electrolytes is highly attractive to achieve a greener rechargeable world. However, such a new energy storage system still exhibits unsatisfactory rate capability and cycling stability due to the limitations in electrode materials screening. Here, a novel covalent organic framework (COF) containing abundant CN and CO for the electrode material is designed, which is combined with graphene and assembled into all-COF/graphene batteries for the first time. Moreover, the co-storage of Zn2+ and H+ in COF can be achieved in a mild aqueous electrolyte. Impressively, benefiting from the extended porous structure of COF, plentiful active reaction sites, more extensive electron delocalization from CO modification at molecular level, as well as enhanced fast H+ storage capacity of graphene and CO in COF, this kind of SAOBs show excellent cycle life and high rate performance (over 15000 cycles with a capacity of 80 mAh g-1 at a high current density of 5 A g-1 in pouch cell). This work will open a new window for the design of high-performance aqueous organic batteries, further moving toward a more eco-friendly electrochemical world.

Fully sp2 Carbon-Conjugated Covalent Organic Frameworks with Multiple Active Sites for Advanced Lithium-Ion Battery Cathodes

Covalent organic frameworks (COFs) hold great promise for rechargeable batteries. However, the synthesis of COFs with abundant active sites, excellent stability, and increased conductivity remains a challenge. Here, chemically stable fully sp2 carbon-conjugated COFs (sp2c-COFs) with multiple active sites are designed by the polymerization of benzo[1,2-b:3,4-b':5,6-b'']trithiophene-2,5,8-tricarbaldehyde) (BTT) and s-indacene-1,3,5,7(2H,6H)-tetrone (ICTO) (denoted as BTT-ICTO). The morphology and structure of the COF are precisely regulated from "butterfly-shaped" to "cable-like" through an in situ controllable growth strategy, significantly promoting the exposure and utilization of active sites. When the unique "cable-like" BTT-ICTO@CNT is employed as lithium-ion batteries (LIBs) cathode, it exhibits exceptional capacity (396 mAh g-1 at 0.1 A g-1 with 97.9 % active sites utilization rate), superb rate capacity (227 mAh g-1 at 5.0 A g-1), and excellent cycling performance (184 mAh g-1 over 8000 cycles at 2.0 A g-1 with 0.00365 % decay rate per cycle). The lithium storage mechanism of BTT-ICTO is exhaustively revealed by in situ Fourier transform infrared, in situ Raman, and density functional theory calculations. This work provides in-depth insights into fully sp2c-COFs with multiple active sites for high-performance LIBs.

Single organic electrode for multi-system dual-ion symmetric batteries

The large void space of organic electrodes endows themselves with the capability to store different counter ions without size concern. In this work, a small-molecule organic bipolar electrode called diquinoxalino[2,3-a:2',3'-c]phenazine-2,6,10-tris(phenoxazine) (DQPZ-3PXZ) is designed. Based on its robust solid structure by the π conjugation of diquinoxalino[2,3-a:2',3'-c]phenazine (DQPZ) and phenoxazine (PXZ), DQPZ-3PXZ can indiscriminately and stably host 5 counter ions with different charge and size (Li+, Na+, K+, PF6- and FSI-). In Li/Na/K-based half cells, DQPZ-3PXZ can deliver the peak discharge capacities of 257/243/253 mAh g-1cathode and peak energy densities of 609/530/572 Wh kg-1cathode, respectively. The Li/Na/K-based dual-ion symmetric batteries can be constructed, which can be activated through the 1st charge process and show the stable discharge capacities of 85/66/72 mAh g-1cathode and energy densities of 59/50/52 Wh kg-1total mass, all running for more than 15000 cycles with nearly 100% capacity retention.

Wiring Covalent Organic Frameworks with Conducting Polymers

Covalent organic frameworks are a class of crystalline porous polymers formed by linking organic units into periodically aligned skeletons and pores. Here we report a strategy for wiring these frameworks with conducting polymers via wall engineering and polymerization. We anchored each edge site with one pyrrole unit, which is densely packed along the z direction yet protruded from pore walls. This assembly enables the polymerization of pyrrole units to form polypyrrole and creates a new polypyrrole chain conformation. The resultant framework constitutes six single file polypyrrole chains in each pore and develop spatially segregated yet built-in single molecular wires with exceptional stable polarons. Hall effect measurements revealed that the materials are p-type semiconductors, increase conductivity by eight orders of magnitude compared to the pristine frameworks, and achieve a carrier mobility as large as 13.2 cm2 V-1 s-1. Our results open an avenue to π electronic frameworks by interlayer molecular wiring with conducting polymers.

Single-Crystalline 3D Covalent Organic Frameworks with Exceptionally High Specific Surface Areas and Gas Storage Capacities

Single-crystalline covalent organic frameworks (COFs) are highly desirable toward understanding their pore chemistry and functions. Herein, two 50-100 μm single-crystalline three-dimensional (3D) COFs, TAM-TFPB-COF and TAPB-TFS-COF, were prepared from the condensation of 4,4',4″,4‴-methanetetrayltetraaniline (TAM) with 3,3',5,5'-tetrakis(4-formylphenyl)bimesityl (TFPB) and 3,3',5,5'-tetrakis(4-aminophenyl)bimesityl (TAPB) with 4,4',4″,4‴-silanetetrayltetrabenzaldehyde (TFS), respectively, in 1,4-dioxane under the catalysis of acetic acid. Single-crystal 3D electron diffraction reveals the triply interpenetrated dia-b networks of TAM-TFPB-COF with atom resolution, while the isostructure of TAPB-TFS-COF was disclosed by synchrotron single-crystal X-ray diffraction and synchrotron powder X-ray diffraction with Le Bail refinements. The nitrogen sorption measurements at 77 K disclose the microporosity nature of both activated COFs with their exceptionally high Brunauer-Emmett-Teller surface areas of 3533 and 4107 m2 g-1, representing the thus far record high specific surface area among imine-bonded COFs. This enables the activated COFs to exhibit also the record high methane uptake capacities up to 28.9 wt % (570 cm3 g-1) at 25 °C and 200 bar among all COFs reported thus far. This work not only presents the structures of two single-crystalline COFs with exceptional microporosity but also provides an example of atom engineering to adjust permanent microporous structures for methane storage.

Nanofibrous Covalent Organic Frameworks as the Cathode, Separator, and Anode for Batteries with High Energy Density and Ultrafast-Charging Performance

To meet the demand for longer driving ranges and shorter charging times of power equipment in electric vehicles, engineering fast-charging batteries with exceptional capacity and extended lifespan is highly desired. In this work, we have developed a stable ultrafast-charging and high-energy-density all-nanofibrous covalent organic framework (COF) battery (ANCB) by designing a series of imine-based nanofibrous COFs for the cathode, separator, and anode by Schiff-base reactions. Hierarchical porous structures enabled by nanofibrous COFs were constructed for enhanced kinetics. Rational chemical structures have been designed for the cathode, separator, and anode materials, respectively. A nanofibrous COF (AA-COF) with bipolarization active sites and a wider layer spacing has been designed using a triphenylamine group for the cathode to achieve high voltage limits with fast mass transport. For the anode, a nanofibrous COF (TT-COF) with abundant polar groups, active sites, and homogenized Li+ flux based on imine, triazine, and benzene has been synthesized to ensure stable fast-charging performance. As for the separator, a COF-based electrospun polyacrylonitrile (PAN) composite nanofibrous separator (BB-COF/PAN) with hierarchical pores and high-temperature stability has been prepared to take up more electrolyte, promote mass transport, and enable as high-temperature operation as possible. The as-assembled ANCB delivers a high energy density of 517 Wh kg-1, a high power density of 9771 W kg-1 with only 56 s of ultrafast-charging time, and high-temperature operational potential, accompanied by a 0.56% capacity fading rate per cycle at 5 A g-1 and 100 °C. This ANCB features an ultralong lifespan and distinguished ultrafast-charging performance, making it a promising candidate for powering equipment in electric vehicles.

Evaluation of the Polypyrrole Coupling Mode for High-Performance Dual-Ion Batteries

Due to the advantages of large interstitial sites, antisolubility, and reversible insertion and extraction of anions, polypyrrole (PPy) has become an excellent P-type electrode material for dual-ion batteries. Unfortunately, PPy electrodes inevitably suffer from low specific capacity and poor cycle stability because of structural disintegration during repeated cycling as well as poor doping ability brought on by aggregation or cross-linking within the PPy chain. In this work, PPy with different proportions of coupling mode (α-α, α-β, or β-β coupling) was derived from different preparation methods. Among them, PPy derived from the interfacial frozen polymerization method (I-PPy) is dominated by the α-α coupling mode and possesses the best anion doping ability and the highest specific capacity of 119 mAh g-1 at 100 mA g-1 compared with PPy derived from electrochemical deposition (E-PPy) and chemical oxidation method (C-PPy) (both less than 40 mAh g-1). This work verifies that increasing the proportion of the α-α coupling mode in the PPy electrode is a useful strategy to enhance the anion doping ability and capacity of dual-ion batteries.

Fully Conjugated Covalent Triazine Framework Integrating Hexaazatrinaphthylene Unit as Anode Material for High-Performance Hybrid Lithium-Ion Capacitors

As a high-performance energy storage device consisting of a battery-type anode and a capacitor-type cathode, hybrid lithium-ion capacitors (HLICs) combine the advantages of high energy density of batteries and high power density of capacitors. However, the imbalance in electrochemical kinetics between the battery-type anode and the capacitor-type cathode hinders the further development of HLICs. Fully conjugated covalent organic frameworks have great potential as electrode materials for HLICs due to the designability of their structure. Herein, a fully conjugated covalent triazine framework (PT-CTF) integrating the hexaazatrinaphthylene unit was constructed, which provides abundant active sites (C═N and C═C groups) as the pseudocapacitive anode material for HLICs. And the connection of the triazine unit of PT-CTF improves the molecular conjugate degree, facilitating the transport of electrons. The fabricated PT-CTF||AC HLICs exhibit a high energy density (164.9 Wh kg-1 at 100 mA g-1), large power density (13.1 kW kg-1 at 4 A g-1), and excellent cycling capability (72% after 10 000 cycles at 2 A g-1).

Covalent Organic Frameworks for Photocatalytic Reduction of Carbon Dioxide: A Review

Covalent organic frameworks (COFs) are crystalline networks with extended backbones cross-linked by covalent bonds. Due to the semiconductive properties and variable metal coordinating sites, along with the rapid development in linkage chemistry, the utilization of COFs in photocatalytic CO2RR has attracted many scientists' interests. In this Review, we summarize the latest research progress on variable COFs for photocatalytic CO2 reduction. In the first part, we present the development of COF linkages that have been used in CO2RR, and we discuss four mechanisms including COFs as intrinsic photocatalysts, COFs with photosensitive motifs as photocatalysts, metalated COF photocatalysts, and COFs with semiconductors as heterojunction photocatalysts. Then, we summarize the principles of structural designs including functional building units and stacking mode exchange. Finally, the outlook and challenges have been provided. This Review is intended to give some guidance on the design and synthesis of diverse COFs with different linkages, various structures, and divergent stacking modes for the efficient photoreduction of CO2.

Lithiophilic Dibenzamide Linkages to Impart Lithium Storage Capacity in Porous Polybenzamides

Polymer-based organic cathode materials have shown immense promise for lithium storage, owing to their structural diversity and functional group tunability. However, designing appropriate high-performance cathode materials with a high-rate capability and long cycle life remains a significant challenge. It is quintessential to design polymer-based electrodes with lithiophilic linkages. Herein, we design a bifurcated dibenzamide (DBA) linkage having lithiophilic functionalities. 1H NMR has been used as an experimental tool to understand the lithiophilic nature of the DBAs. Considering the strong Li+ affinity of DBAs, a series of polybenzamides have been designed as lithium storage systems. The design of porous polybenzamides consists of amides as only redox-active functionalities, and the rest are inactive phenyl units. Porous polybenzamides, when tested as cathodes against a Li-metal anode, displayed high capacity and rate performance, demonstrating their redox activity. The most efficient polybenzamide (TAm-TA) delivered a specific capacity of 248 mA h g-1 at 1C. TAm-TA retained 63% of its specific capacity at a very high rate of 10C (157 mA h g-1). Notably, polybenzamides displayed a capacity enhancement during long cycling, tending to achieve their theoretical capacity. Long cycling stability tests over 3000 cycles at a rate of 1.3C and over 6000 cycles at elevated rates (5C to 40C) demonstrate the electrochemical robustness of dibenzamide linkages. Finally, two full-cell experiments using TAm-TA as both cathode and anode were conducted, which delivered high capacity, demonstrating that TAm-TA is a promising candidate for Li+-ion batteries (LIBs). Furthermore, the ex situ Fourier transform infrared (FT-IR), X-ray photoemission spectroscopy (XPS), and density functional theory (DFT) studies revealed the stepwise lithiation/delithiation mechanism for polybenzamides.

A photocatalytic redox cycle over a polyimide catalyst drives efficient solar-to-H2O2 conversion

Circumventing the conventional two-electron oxygen reduction pathway remains a great problem in enhancing the efficiency of H2O2 photosynthesis. A promising approach to achieve outstanding photocatalytic activity involves the utilization of redox intermediates. Here, we engineer a polyimide aerogel photocatalyst with photoreductive carbonyl groups for non-sacrificial H2O2 production. Under photoexcitation, carbonyl groups on the photocatalyst surface are reduced, forming an anion radical intermediate. The produced intermediate is oxidized by O2 to produce H2O2 and subsequently restores the carbonyl group. The high catalytic efficiency is ascribed to a photocatalytic redox cycle mediated by the radical anion, which not only promotes oxygen adsorption but also lowers the energy barrier of O2 reduction reaction for H2O2 generation. An apparent quantum yield of 14.28% at 420 ± 10 nm with a solar-to-chemical conversion efficiency of 0.92% is achieved. Moreover, we demonstrate that a mere 0.5 m2 self-supported polyimide aerogel exposed to natural sunlight for 6 h yields significant H2O2 production of 34.3 mmol m-2.

Covalent organic frameworks and their composites for rechargeable batteries

Covalent organic frameworks (COFs), characterized by well-ordered pores, large specific surface area, good stability, high precision, and flexible design, are a promising material for batteries and have received extensive attention from researchers in recent years. Compared with inorganic materials, COFs can construct elastic frameworks with better structural stability, and their chemical compositions and structures can be precisely adjusted and functionalized at the molecular level, providing an open pathway for the convenient transfer of ions. In this review, the energy storage mechanism and unique superiority of COFs and COF composites as electrodes, separators and electrolytes for rechargeable batteries are discussed in detail. Special emphasis is placed on the relationship between the establishment of COF structures and their electrochemical performance in different batteries. Finally, this review summarizes the challenges and prospects of COFs and COF composites in battery equipment.

Covalent Bonding of MXene/COF Heterojunction for Ultralong Cycling Li-Ion Battery Electrodes

Covalent organic frameworks (COFs) have emerged as promising renewable electrode materials for LIBs and gained significant attention, but their capacity has been limited by the densely packed 2D layer structures, low active site availability, and poor electronic conductivity. Combining COFs with high-conductivity MXenes is an effective strategy to enhance their electrochemical performance. Nevertheless, simply gluing them without conformal growth and covalent linkage restricts the number of redox-active sites and the structural stability of the composite. Therefore, in this study, a covalently assembled 3D COF on Ti3C2 MXenes (Ti3C2@COF) is synthesized and serves as an ultralong cycling electrode material for LIBs. Due to the covalent bonding between the COF and Ti3C2, the Ti3C2@COF composite exhibits excellent stability, good conductivity, and a unique 3D cavity structure that enables stable Li+ storage and rapid ion transport. As a result, the Ti3C2-supported 3D COF nanosheets deliver a high specific capacity of 490 mAh g-1 at 0.1 A g-1, along with an ultralong cyclability of 10,000 cycles at 1 A g-1. This work may inspire a wide range of 3D COF designs for high-performance electrode materials.

Zigzag Hopping Site Embedded Covalent Organic Frameworks Coating for Zn Anode

Precise design and tuning of Zn hopping/transfer sites with deeper understanding of the dendrite-formation mechanism is vital in artificial anode protective coating for aqueous Zn-ion batteries (AZIBs). Here, we probe into the role of anode-coating interfaces by designing a series of anhydride-based covalent organic frameworks (i.e., PI-DP-COF and PI-DT-COF) with specifically designed zigzag hopping sites and zincophilic anhydride groups that can serve as desired platforms to investigate the related Zn2+ hopping/transfer behaviours as well as the interfacial interaction. Combining theoretical calculations with experiments, the ABC stacking models of these COFs endow the structures with specific zigzag sites along the 1D channel that can accelerate Zn2+ transfer kinetics, lower surface-energy, homogenize ion-distribution or electric-filed. Attributed to these superiorities, thus-obtained optimal PI-DT-COF cells offer excellent cycling lifespan in both symmetric-cell (2000 cycles at 60 mA cm-2) and full-cell (1600 cycles at 2 A g-1), outperforming almost all the reported porous crystalline materials.

Crystalline nanosheets of three-dimensional supramolecular frameworks with uniform thickness and high stability

Fabricating three dimensional (3D) supramolecular frameworks (SMFs) into stable crystalline nanosheets remains a great challenge due to the homogeneous and weak inter-building block interactions along 3D directions. Herein, crystalline nanosheets of a 3D SMF with a uniform thickness of 4.8 ± 0.1 nm immobilized with Pt nanocrystals on the surface (Q[8]/Pt NSs) were fabricated via the solid-liquid reaction between cucurbit[8]uril/H2PtCl6 single crystals and hydrazine hydrate with the help of gas and heat yielded during the reaction process. A series of experiments and theoretical calculations reveal the ultrahigh stability of Q[8]/Pt NSs due to the high density hydrogen bonding interaction among neighboring Q[8] molecules. This in turn endows Q[8]/Pt NSs with excellent photocatalytic and continuous thermocatalytic CO oxidation performance, representing the thus-far reported best Pt nano-material-based catalysts.

Molecule and Microstructure Modulations of Cyano-Containing Electrodes for High-Performance Fully Organic Batteries

Cyano-containing electrodes usually promise high theoretical potentials while suffering from uncontrollable self-dissolution and sluggish reaction kinetics. Herein, to remedy their limitations, an unprecedented core-shell heterostructured electrode of carbon nanotubes encapsulated in poly(1,4-dicyanoperfluorobenzene sulfide) (CNT@PFDCB) is rationally crafted via molecule and microstructure modulations. Specifically, the linkage of sulfide bridges of PFDCB prevents the active cyano groups from dissolving, resulting in a robust structure. The fluorinations modulate the electronic configurations in frontier orbitals, allowing higher electrical conductivity and elevated output voltage. Combined with the core-shell architecture to unlock the sluggish diffusion kinetics for both electrons and guest ions, the CNT@PFDCB exhibits an impressive capacity (203.5 mAh g-1), remarkable rate ability (127.6 mAh g-1 at 3.0 A g-1), and exceptional cycling stability (retaining 81.1 % capacity after 3000 cycles at 1.0 A g-1). Additionally, the Li-storage mechanisms regarding PFDCB are thoroughly revealed by in situ attenuated total reflection infrared spectroscopy, in situ Raman spectroscopy, and theoretical simulations, which involve the coordination interaction between Li ions and cyano groups and the electron delocalization along the conjugated skeleton. More importantly, a practical fully organic cell based on the CNT@PFDCB is well-validated that demonstrates a tremendous potential of cyanopolymer as the cathode to replace its inorganic counterparts.

3D Crown Ether Covalent Organic Framework as Interphase Layer toward High-Performance Lithium Metal Batteries

The practical application of lithium (Li) metal batteries is inhibited by accumulative Li dendrites and continuous active Li consumption during cycling, which results in a low Coulombic efficiency and short lifetime. Constructing artificial solid-electrolyte interphase (SEI) layer in Li anode, such as 2D covalent organic frameworks (COFs), is an effective strategy to restrain the formation of Li dendrites and improve cycling performance. However, the exploration of 3D COFs as protecting layers is rarely reported, because of the preconception that the interconnect pores in 3D COFs eventually cause Li dendrites in disordered direction. 3D crown ether-based COF with ffc topology as interphase layer, in which the crown ether units are arranged in parallel and vertical orientation along the electrode, is demonstrated. The strong coupling effect between the crown ether and Li+ accelerates Li+ diffusion kinetics and enables homogeneous Li+ flux, resulting in a high Li+ transference number of 0.85 and smooth Li deposition in 3D direction. Li/COF-Cu cells display a lower Li-nucleation overpotential (17.4 mV) and high average Coulombic efficiency of ≈98.6% during 340 cycles with COF incorporation. This work gives a new insight into designing COFs for energy storage systems.

3D Covalent Organic Framework with "the" Topology

Discovery of new topology covalent organic frameworks (COFs) is a mainstay in reticular chemistry and materials research because it not only serves as a stepwise guide to the designed construction of covalent-organic architectures but also helps to comprehend function from structural design point-of-view. Proceeding on this track, the first 3D COF, TUS-38, with the topology is constructed by reticulating a planar triangular 3-c node of D3h symmetry with a tetragonal prism 8-c node of D2h symmetry via [3 + 8] reversible imine condensation reaction. TUS-38 represents a twofold interpenetrated multidirectional pore network with a high degree of crystallinity and structural integrity. Interestingly, stemming from the nitrogen-rich s-triazine rings with electron-deficient character and ─C ═ N─ linkages composing the TUS-38 framework that benefit to the charge-transfer and hence dipole-dipole electrostatic interactions between the framework and iodine in addition to exclusive topological characteristics of the exotic the net, TUS-38 achieves an exemplary capacity for iodine vapor uptake reaching 6.3 g g-1. Recyclability studies evidence that TUS-38 can be reused at least five times retaining 95% of its initial adsorption capacity; while density functional theory (DFT) calculations have heightened the understanding of the interactions between iodine molecules and the framework.

12 Connecting Sites Linked Three-dimensional Covalent Organic Frameworks with Intrinsic Non-interpenetrated shp Topology for Photocatalytic H2O2 Synthesis

Developing high connectivity (>8) three-dimensional (3D) covalent organic frameworks (COFs) towards new topologies and functions remains a great challenge owing to the difficulty in getting high connectivity organic building blocks. This however represents the most important step towards promoting the diversity of COFs due to the still limited dynamic covalent bonds available for constructing COFs at this stage. Herein, highly connected phthalocyanine-based (Pc-based) 3D COFs MPc-THHI-COFs (M=H2, Ni) were afforded from the reaction between 2,3,9,10,16,17,23,24-octacarboxyphthalocyanine M(TAPc) (M=H2, Ni) and 5,5',5'',5''',5'''',5'''''-(triphenylene-2,3,6,7,10,11-hexayl)hexa(isophthalohydrazide) (THHI) with 12 connecting sites. Powder X-ray diffraction analysis together with theoretical simulations and transmission electron microscopy reveals their crystalline nature with an unprecedented non-interpenetrated shp topology. Experimental and theoretical investigations disclose the broadened visible light absorption range and narrow optical band gap of MPc-THHI-COFs. This in combination with their 3D nanochannels endows them with efficient photocatalysis performance for H2O2 generation from O2 and H2O via 2e- oxygen reduction reaction and 2e- water oxidation reaction under visible-light irradiation (λ >400 nm). This work provides valuable result for the development of high connectivity functional COFs towards diverse application potentials.

Nanostructurally Engineering Covalent Organic Frameworks for Boosting CO2 Photoreduction

Herein, a series of imine-linked covalent organic frameworks (COFs) are developed with advanced ordered mesoporous hollow spherical nanomorphology and ultra-large mesopores (4.6 nm in size), named OMHS-COF-M (M = H, Co, and Ni). The ordered mesoporous hollow spherical nanomorphology is revealed to be formed via an Ostwald ripening mechanism based on a one-step self-templated strategy. Encouraged by its unique structural features and outstanding photoelectrical property, the OMHS-COF-Co material is applied as the photocatalyst for CO2-to-CO reduction. Remarkably, it delivers an impressive CO production rate as high as 15 874 µmol g-1 h-1, a large selectivity of 92.4%, and a preeminent cycling stability. From in/ex situ experiments and density functional theory (DFT) calculations, the excellent CO2 photoreduction performance is ascribed to the desirable cooperation of unique ordered mesoporous hollow spherical host and abundant isolated Co active sites, enhancing CO2 activation, and improving electron transfer kinetics as well as reducing the energy barriers for intermediates *COOH generation and CO desorption.

Practical organic batteries: Concepts to realize high mass loading with high performance

Organic electrode materials (OEMs) have been well developed in recent years. However, the practical applications of OEMs have not been paid sufficient attention. The concept here focused on one of the essential aspects for practical applications, i. e., high mass loading of active materials. This paper summarizes the challenges posed by high-mass loading of active materials in organic batteries and discusses the possible solutions in terms of organic electrode materials, conductive additives, electrode structures, and electrolytes or battery systems. We hope this concept can stimulate more attention to practical applications of organic batteries towards industry from lab.

Recent Progress in Covalent Organic Frameworks for Cathode Materials

Covalent organic frameworks (COFs) are constructed from small organic molecules through reversible covalent bonds, and are therefore considered a special type of polymer. Small organic molecules are divided into nodes and connectors based on their roles in the COF's structure. The connector generally forms reversible covalent bonds with the node through two reactive end groups. The adjustment of the length of the connector facilitates the adjustment of pore size. Due to the diversity of organic small molecules and reversible covalent bonds, COFs have formed a large family since their synthesis in 2005. Among them, a type of COF containing redox active groups such as -C=O-, -C=N-, and -N=N- has received widespread attention in the field of energy storage. The ordered crystal structure of COFs ensures the ordered arrangement and consistent size of pores, which is conducive to the formation of unobstructed ion channels, giving these COFs a high-rate performance and a long cycle life. The voltage and specific capacity jointly determine the energy density of cathode materials. For the COFs' cathode materials, the voltage plateau of their active sites' VS metallic lithium is mostly between 2 and 3 V, which has great room for improvement. However, there is currently no feasible strategy for this. Therefore, previous studies mainly improved the theoretical specific capacity of the COFs' cathode materials by increasing the number of active sites. We have summarized the progress in the research on these types of COFs in recent years and found that the redox active functional groups of these COFs can be divided into six subcategories. According to the different active functional groups, these COFs are also divided into six subcategories. Here, we summarize the structure, synthesis unit, specific surface area, specific capacity, and voltage range of these cathode COFs.

Ultra-Thin Hydrogen-Organic-Framework (HOF) Nanosheets for Ultra-Stable Alkali Ions Battery Storage

Organic frameworks-based batteries with excellent physicochemical stability and long-term high capacity will definitely reduce the cost, carbon emissions, and metal consumption and contamination. Here, an ultra-stable and ultra-thin perylene-dicyandiamide-based hydrogen organic framework (HOF) nanosheet (P-DCD) of ≈3.5 nm in thickness is developed. When applied in the cathode, the P-DCD exhibits exceptional long-term capacity retention for alkali-ion batteries (AIBs). Strikingly, for lithium-ion batteries (LIBs), at current of 2 A g-1, the large reversible capacity of 108 mA h g-1 shows no attenuation within 5 000 cycles. For sodium-ion batteries (SIBs), the related capacity retains 91.7% within 10 000 cycles compared to the initial state, significantly much more stable than conventional organic materials reported previously. Mechanism studies through ex situ and in situ experiments and theoretical density functional theory (DFT) calculations reveal that the impressive long-term performance retention originates from the large electron delocalization, fast ion diffusion, and physicochemical stability within the ultra-thin 2D P-DCD, featuring π-π and hydrogen bonding stacking, nitrogen-rich units, and low impedance. The advantageous features demonstrate that rationally designed stable and effective organic frameworks pave the way to utilizing complete organic materials for developing next-generation low-cost and highly stable energy storage batteries.

A nitrogen-rich graphdiyne containing hexaazatrinaphthylene for high-performance lithium-ion batteries

A nitrogen-rich graphdiyne (HATN-GDY) material containing electrochemical active hexaazatrinaphthylene units was successfully prepared. HATN-GDY exhibits a superior specific capacity of 2139 mA h g-1 and firm long-term stability due to the unique 2D π-conjugated structure and the large in-plane N-cavities.

Polyimide covalent organic frameworks bearing star-shaped electron-deficient polycyclic aromatic hydrocarbon building blocks: molecular innovations for energy conversion and storage

Polyimide covalent organic frameworks (PI-COFs) are outstanding functional materials for electrochemical energy conversion and storage owing to their integrated advantages of the high electroactive feature of polyimides and the periodic porous structure of COFs. Nevertheless, only anhydride monomers with C2 symmetry are generally used, and limited selectivity of electron-deficient monomers has become a major obstacle in the development of materials. The introduction of polycyclic aromatic hydrocarbons (PAHs) is a very effective method to regulate the structure-activity relationship of PI-COFs due to their excellent stability and electrical properties. Over the past two years, various star-shaped electron-deficient PAH building blocks possessing different compositions and topologies have been successfully fabricated, greatly improving the monomer selectivity and electrochemical performances of PI-COFs. This paper systematically summarizes the recent highlights in PI-COFs based on these building blocks. Firstly, the preparation of anhydride (or phthalic acid) monomers and PI-COFs related to different star-shaped PAHs is presented. Secondly, the applications of these PI-COFs in energy conversion and storage and the corresponding factors influencing their performance are discussed in detail. Finally, the future development of this meaningful field is briefly proposed.

Chemical Sensors Based on Covalent Organic Frameworks

Covalent organic frameworks (COFs) are a type of crystalline porous polymer composed of light elements through strong covalent bonds. COFs have attracted considerable attention due to their unique designable structures and excellent material properties. Currently, COFs have shown outstanding potential in various fields, including gas storage, pollutant removal, catalysis, adsorption, optoelectronics, and their research in the sensing field is also increasingly flourishing. In this review, we focus on COF-based sensors. Firstly, we elucidate the fundamental principles of COF-based sensors. Then, we present the primary application areas of COF-based sensors and their recent advancements, encompassing gas, ions, organic compounds, and biomolecules sensing. Finally, we discuss the future trends and challenges faced by COF-based sensors, outlining their promising prospects in the field of sensing.

Covalent Organic Framework-based Solid-State Electrolytes, Electrode Materials, and Separators for Lithium-ion Batteries

The increasing global energy consumption has led to the rapid development of renewable energy storage technologies. Lithium-ion batteries (LIBs) have been extensively studied and utilized for reliable, efficient, and sustainable energy storage. Nevertheless, designing new materials for LIB applications with high capacity and long-term stability is highly desired but remains a challenging task. Recently, covalent organic frameworks (COFs) have emerged as superior candidates for LIB applications due to their high porosity, well-defined pores, highly customizable structure, and tunable functionalities. These merits enable the preparation of tailored COFs with predesigned redox-active moieties and suitable porous channels that can improve the lithium-ion storage and transportation. This review summarizes the recent progress in the development of COFs and their composites for a variety of LIB applications, including (quasi) solid-state electrolytes, electrode materials, and separators. Finally, the challenges and potential future directions of employing COFs for LIBs are also briefly discussed, further promoting the foundation of this class of exciting materials for future advances in energy-related applications.

Dendritic sp Carbon-Conjugated Benzothiadiazole-Based Polymers with Synergistic Multi-Active Groups for High-Performance Lithium Organic Batteries

Green organic materials composed of C, H, O, and N elements are receiving more and more attention worldwide. However, the high solubility, poor electrical conductivity, and long activation time limit the development of organic materials in practice. Herein, two stable covalent organic materials with alkynyl linkage between benzene rings and benzothiadiazole groups with different amounts of fluorine atoms modification (defined as BOP-0F and BOP-2F), are designed for lithium-ion batteries. Both BOP-0F and BOP-2F can achieve superior reversible capacities of ≈719.8 and 713.5 mAh g-1 over 100 cycles on account of the redox activity of alkynyl (two-electron involved) and benzothiadiazole units (five-electron involved) in these organic materials. While BOP-2F electrodes exhibit much more stable cycling performance than BOP-0F electrodes, especially without pronounced capacity ascending during initial cycling. It can be assigned to the synergy effect of alkynyl linkage and fluorine atom modification in BOP-2F. The lithium storage and activation mechanism of alkynyl, benzothiadiazole, and fluorine groups have also been deeply probed by a series of material characterizations and theoretical simulations. This work could be noteworthy in providing novel tactics for the molecular design and investigation of high-efficiency organic electrodes for energy storage.

Linkage conversions in single-crystalline covalent organic frameworks

Single-crystal X-ray diffraction is a powerful characterization technique that enables the determination of atomic arrangements in crystalline materials. Growing or retaining large single crystals amenable to it has, however, remained challenging with covalent organic frameworks (COFs), especially suffering from post-synthetic modifications. Here we show the synthesis of a flexible COF with interpenetrated qtz topology by polymerization of tetra(phenyl)bimesityl-based tetraaldehyde and tetraamine building blocks. The material is shown to be flexible through its large, anisotropic positive thermal expansion along the c axis (αc = +491 × 10-6 K-1), as well as through a structural transformation on the removal of solvent molecules from its pores. The as-synthesized and desolvated materials undergo single-crystal-to-single-crystal transformation by reduction and oxidation of its imine linkages to amine and amide ones, respectively. These redox-induced linkage conversions endow the resulting COFs with improved stability towards strong acid; loading of phosphoric acid leads to anhydrous proton conductivity up to ca. 6.0 × 10-2 S cm-1.

Organic electrodes based on redox-active covalent organic frameworks for lithium batteries

Electroactive organic materials have received much attention as alternative electrodes for metal-ion batteries due to their high theoretical capacity, resource availability, and environmental friendliness. In particular, redox-active covalent organic frameworks (COFs) have recently emerged as promising electrodes due to their tunable electrochemical properties, insolubility in electrolytes, and structural versatility. In this Highlight, we review some recent strategies to improve the energy density and power density of COF electrodes for lithium batteries from the perspective of molecular design and electrode optimisation. Some other aspects such as stability and scalability are also discussed. Finally, the main challenges to improve their performance and future prospects for COF-based organic batteries are highlighted.

Synergetic Coupling of Redox-Active Sites on Organic Electrode Material for Robust and High-Performance Sodium-Ion Storage

Organic electrode materials (OEMs), valued for their sustainability and structural tunability, have been attracting increasing attention for wide application in sodium-ion batteries (SIBs) and other rechargeable batteries. However, most OEMs are plagued with insufficient specific capacity or poor cycling stability. Therefore, it's imperative to enhance their specific capacity and cycling stability through molecular design. Herein, we designed and synthesized a heteroaromatic molecule 2,3,8,9,14,15-hexanol hexaazatrinaphthalene (HATN-6OH) by the synergetic coupling of catechol (the precursor of ortho-quinone)/ortho-quinone functional groups and HATN conjugated core structures. The abundance of catechol/ortho-quinone and imine redox-active moieties delivers a high specific capacity of nine-electron transfer for SIBs. Most notably, the π-π interactions and intermolecular hydrogen bond forces among HATN-6OH molecules secure the stable long-term cycling performance of SIBs. Consequently, the as-prepared HATN-6OH electrode exhibited a high specific capacity (554 mAh g-1 at 0.1 A g-1 ), excellent rate capability (202 mAh g-1 at 10 A g-1 ), and stable long-term cycling performance (73 % after 3000 cycles at 10 A g-1 ) in SIBs. Additionally, the nine-electron transfer mechanism is confirmed by systematic density functional theory (DFT) calculation, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), and Raman analysis. The achievement of the synergetic coupling of the redox-active sites on OEMs could be an important key to the enhancement of SIBs and other metal-ion batteries.

Observation of Interpenetrated Topology Isomerism for Covalent Organic Frameworks with Atom-Resolution Single Crystal Structures

Rational control and understanding of isomerism are of significance but still remain a great challenge in reticular frameworks, in particular, for covalent organic frameworks (COFs) due to the complicated synthesis and energy factors. Herein, reaction of 3,3',5,5'-tetra(4-formylphenyl)-2,2',6,6'-tetramethoxy-1,1'-biphenyl (TFTB) with 3,3',5,5'-tetrakis(4-aminophenyl)bimesityl (TAPB) under different reaction conditions affords single crystals of two 3D COF isomers, namely, USTB-20-dia and USTB-20-qtz. Their structures with resolutions up to 0.9-1.1 Å have been directly solved by three-dimensional electron diffraction (3D ED) and synchrotron single crystal X-ray diffraction, respectively. USTB-20-dia and USTB-20-qtz show rare 2 × 2-fold interpenetrated dia-b nets and 3-fold interpenetrated qtz-b frameworks. Comparative studies of the crystal structures of these COFs and theoretical simulation results indicate the crucial role of the flexible molecular configurations of building blocks in the present interpenetrated topology isomerism. This work not only presents the rare COF isomers but also gains an understanding of the formation of framework isomerism from both single crystal structures and theoretical simulation perspectives.

Recent Progress in Design Principles of Covalent Organic Frameworks for Rechargeable Metal-Ion Batteries

Covalent organic frameworks (COFs) are acknowledged as a new generation of crystalline organic materials and have garnered tremendous attention owing to their unique advantages of structural tunability, frameworks diversity, functional versatility, and diverse applications in drug delivery, adsorption/separation, catalysis, optoelectronics, and sensing, etc. Recently, COFs is proven to be promising candidates for electrochemical energy storage materials. Their chemical compositions and structures can be precisely tuned and functionalized at the molecular level, allowing a comprehensive understanding of COFs that helps to make full use of their features and addresses the inherent drawback based on the components and functions of the devices. In this review, the working mechanisms and the distinguishing advantages of COFs as electrodes for rechargeable Li-ion batteries are discussed in detail. Especially, principles and strategies for the rational design of COFs as advanced electrode materials in Li-ion batteries are systematically summarized. Finally, this review is structured to cover recent explorations and applications of COF electrode materials in other rechargeable metal-ion batteries.

Dual-Interface Engineering in Perovskite Solar Cells with 2D Carbides

Passivating the interfaces between the perovskite and charge transport layers is crucial for enhancing the power conversion efficiency (PCE) and stability in perovskite solar cells (PSCs). Here we report a dual-interface engineering approach to improving the performance of FA0.85 MA0.15 Pb(I0.95 Br0.05 )3 -based PSCs by incorporating Ti3 C2 Clx Nano-MXene and o-TB-GDY nanographdiyne (NanoGDY) into the electron transport layer (ETL)/perovskite and perovskite/ hole transport layer (HTL) interfaces, respectively. The dual-interface passivation simultaneously suppresses non-radiative recombination and promotes carrier extraction by forming the Pb-Cl chemical bond and strong coordination of π-electron conjugation with undercoordinated Pb defects. The resulting perovskite film has an ultralong carrier lifetime exceeding 10 μs and an enlarged crystal size exceeding 2.5 μm. A maximum PCE of 24.86 % is realized, with an open-circuit voltage of 1.20 V. Unencapsulated cells retain 92 % of their initial efficiency after 1464 hours in ambient air and 80 % after 1002 hours of thermal stability test at 85 °C.

A phthalocyanine-based porous organic polymer for a lithium-ion battery anode

Porous organic polymers (POPs) are a novel class of polymeric materials with high flexibility and designability for building structures. Herein, a phthalocyanine-based porous organic polymer (PcPOP) was constructed in situ on copper foil from H2Pc(ethynyl)4 [Pc(ethynyl)4 = 2(3),9(10),16(17),23(24)-tetra(ethynyl)phthalocyanine] by the coupling reaction. Benefiting from the uniformly distributed electron-rich nitrogen atoms in the Pc structure and the sp-hybridized carbons in the acetylenic linkage, Li intercalation in the porous organic polymer would be improved and stabilized. As a result, PcPOP showed remarkable electrochemical performance in lithium-ion batteries as the anode, including high specific capacity (a charge capacity of 1172 mA h g-1 at a current density of 150 mA g-1) and long cycling stability (a reversible capacity of 960.1 mA h g-1 can be achieved even after 600 cycles at a current density of 1500 mA g-1). The result indicates that the intrinsic doping of electron-rich sites of the building molecules is beneficial for the electrochemical performance of the porous organic polymer.

Electric-Field-Induced Alignment of Functionalized Carbon Nanotubes Inside Thermally Conductive Liquid Crystalline Polyimide Composite Films

The positive liquid crystals, 4'-heptyl-4-biphenylcarbonitrile (7CB), are used to functionalize carbon nanotubes (LC-CNT), which can be aligned in the liquid crystalline polyimide (LC-PI) matrix under an alternating electric field to fabricate the thermally conductive LC-CNT/LC-PI composite films. The efficient establishment of thermal conduction pathways in thermally conductive LC-CNT/LC-PI composite films with a low amount of LC-CNT is achieved through the oriented alignment of LC-CNT within the LC-PI matrix. When the mass fraction of LC-CNT is 15 wt %, the in-plane thermal conductivity coefficient (λ∥ ) and the through-plane thermal conductivity coefficient (λ⊥ ) of the LC-CNT/LC-PI composite films reach 4.02 W/(m ⋅ K) and 0.55 W/(m⋅K), which are 90.5 % and 71.9 % higher than those of the intrinsically thermally conductive LC-PI films respectively, also 28.8 % and 5.8 % higher than those of the CNT/LC-PI composite films respectively. Meanwhile, the thermally conductive LC-CNT/LC-PI composite films also possess excellent mechanical and heat resistance properties. The Young's modulus and the heat resistance index are 2.3 GPa and 297.7 °C, respectively, which are higher than the intrinsically thermally conductive LC-PI films and the thermally conductive CNT/LC-PI composite films under the same amount of CNT.

Single Site Coordination Enabled Construction of Metal-Diketimine-Linked Covalent Organic Frameworks for Boosted Electrooxidation of Biomass Derivative

Aromatic aldehydes are widely used for the construction of covalent organic frameworks (COFs). However, due to the high flexibility, high steric hindrance, and low reactivity, it remains challenging to synthesize COFs using ketones as building units, especially the highly flexible aliphatic ones. Here, the single nickel site coordination strategy is reported to lock the configurations of the highly flexible diketimine to transform discrete oligomers or amorphous polymers into highly crystalline nickel-diketimine-linked COFs (named as Ni-DKI-COFs). The strategy has been successfully extended to the synthesis of a series of Ni-DKI-COFs by the condensation of three flexible diketones with two tridentate amines. Thanks to the ABC stacking model with high amount and easily accessible single nickel (II) sites on their 1D channels, Ni-DKI-COFs are exploited as well-defined electrocatalyst platforms for efficiently electro-upgrading biomass-derived 5-Hydroxymethylfurfural (HMF) into value-added 2,5-furandicarboxylic acid (FDCA) with a 99.9% yield and a 99.5% faradaic efficiency as well as a high turnover frequency of 0.31 s-1 .

Tuning electron delocalization of hydrogen-bonded organic framework cathode for high-performance zinc-organic batteries

Stable cathodes with multiple redox-active centres affording a high energy density, fast redox kinetics and a long life are continuous pursuits for aqueous zinc-organic batteries. Here, we achieve a high-performance zinc-organic battery by tuning the electron delocalization within a designed fully conjugated two-dimensional hydrogen-bonded organic framework as a cathode material. Notably, the intermolecular hydrogen bonds endow this framework with a transverse two-dimensional extended stacking network and structural stability, whereas the multiple C = O and C = N electroactive centres cooperatively trigger multielectron redox chemistry with super delocalization, thereby sharply boosting the redox potential, intrinsic electronic conductivity and redox kinetics. Further mechanistic investigations reveal that the fully conjugated molecular configuration enables reversible Zn2+/H+ synergistic storage accompanied by 10-electron transfer. Benefitting from the above synergistic effects, the elaborately tailored organic cathode delivers a reversible capacity of 498.6 mAh g-1 at 0.2 A g-1, good cyclability and a high energy density (355 Wh kg-1).

Covalent Organic Framework with Multiple Redox Active Sites for High-Performance Aqueous Calcium Ion Batteries

Organic materials are promising for cation storage in calcium ion batteries (CIBs). However, the high solubility of organic materials in an electrolyte and low electronic conductivity remain the key challenges for high-performance CIBs. Herein, a nitrogen-rich covalent organic framework with multiple carbonyls (TB-COF) is designed as an aqueous anode to address those obstacles. TB-COF demonstrates a high reversible capacity of 253 mAh g-1 at 1.0 A g-1 and long cycle life (0.01% capacity decay per cycle at 5 A g-1 after 3000 cycles). The redox mechanism of Ca2+/H+ co-intercalated in COF and chelating with C═O and C═N active sites is validated. In addition, a novel C═C active site was identified for Ca2+ ion storage. Both computational and empirical results reveal that per TB-COF repetitive unit, up to nine Ca2+ ions are stored after three staggered intercalation steps, involving three distinct Ca2+ ion storage sites. Finally, the evolution process of radical intermediates further elucidates the C═C reaction mechanism.

Covalent Organic Frameworks as Model Materials for Fundamental and Mechanistic Understanding of Organic Battery Design Principles

Redox-active covalent organic frameworks (COFs) have recently emerged as advanced electrodes in polymer batteries. COFs provide ideal molecular precision for understanding redox mechanisms and increasing the theoretical charge-storage capacities. Furthermore, the functional groups on the pore surface of COFs provide highly ordered and easily accessible interaction sites, which can be modeled to establish a synergy between ex situ/in situ mechanism studies and computational methods, permitting the creation of predesigned structure-property relationships. This perspective integrates and categorizes the redox functionalities of COFs, providing a deeper understanding of the mechanistic investigation of guest ion interactions in batteries. Additionally, it highlights the tunable electronic and structural properties that influence the activation of redox reactions in this promising organic electrode material.

Highly Stable Metal-Organic Framework with Redox-Active Naphthalene Diimide Core as Cathode Material for Aqueous Zinc-Ion Batteries

Recently, metal-organic frameworks (MOFs) as the cathode materials for aqueous zinc-ion batteries (ZIBs) received growing attention. Herein, a novel MOF, Ni-Ndi-trz (Ndi-trz=2,7-di(4H-1,2,4-triazol-4-yl)benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone) was synthesized through a solvothermal method. Its rational design using a naphthalene diimide (Ndi) core allowed the formation of a four-fold interpenetrated pcu (primitive cubic) topology. The as-synthesized Ni-Ndi-trz is highly stable over a wide pH range (0-12) for 30 days, which is critical to ensure the decent cyclability of zinc-ion batteries (ZIBs). When used as the cathode material of ZIBs, it shows a high initial specific capacity of 90.7 mAh g^-1 and excellent cycling stability. Remarkably, three-electrode system tests, ex situ FTIR, UV/Vis and XPS spectra revealed that the Ndi core of Ni-Ndi-trz undergoes a reversible interconversion between the keto and enol forms when interacting with Zn²⁺ ions. This work may shed light on the feasibility of designing novel MOFs and exploring their mechanisms for zinc ion batteries.

2D Covalent Organic Frameworks Based on Heteroacene Units

Covalent organic frameworks (COFs) are a unique new class of porous materials that arrange building units into periodic ordered frameworks through strong covalent bonds. Accompanied with structural rigidity and well-defined geometry, heteroacene-based COFs have natural advantages in constructing COFs with high stability and crystallinity. Heteroacene-based COFs usually have high physical and chemical properties, and their extended π-conjugation also leads to relatively low energy gap, effectively promoting π-electron delocalization between network units. Owing to excellent electron-withdrawing or -donating ability, heteroacene units have incomparable advantages in the preparation of donor-acceptor type COFs. Therefore, the physicochemical robust and fully conjugated heteroacene-based COFs solve the problem of traditional COFs lacking π-π interaction and chemical stability. In recent years, significant breakthroughs are made in this field, the choice of various linking modes and building blocks has fundamentally ensured the final applications of COFs. It is of great significance to summarize the heteroacene-based COFs for improving its complexity and controllability. This review first introduces the linkages in heteroacene-based COFs, including reversible and irreversible linkages. Subsequently, some representative building blocks are summarized, and their related applications are especially emphasized. Finally, conclusion and perspectives for future research on heteroacene-based COFs are presented.

An In Situ Fabricated Graphene/Bipolar Polymer Hybrid Material Delivers Ultralong Cycle Life over 15 000 Cycles as a High-Performance Electrode Material

Organic electrode materials are promising for the future energy storage systems owing to their tunable structures, abundant resources, and environmental friendliness. Many advanced lithium-ion batteries with organic electrodes have been developed and show excellent performance. However, developing organic materials with overall superior performance still faces great challenges, such as low capacity, poor stability, inferior conductivity, and low utilization of active sites. To address these issues, a bipolar polymer (Fc-DAB) is designed and further polymerized in situ with three-dimensional graphene (3DG), offering a hybrid material (Fc-DAB@3DG) with a variety of merits. Fc-DAB possesses stable polymer backbone and multiple redox-active sites that can improve stability and capacity simultaneously. The embedded highly conductive 3DG network endows Fc-DAB@3DG with stable conductive framework, large surface area, and porous morphology all together, so the fast diffusion of ions/electrons can be achieved, leading to high utilization of active sites and enhanced electrochemical performance. As a result, Fc-DAB@3DG cathode delivers capacity of ≈260 mA h g^-1 at 25 mA g^-1 , ultra-long cycle life over 15 000 cycles at 2000 mA g^-1 with retention of 99.999% per cycle, and remarkable rate performance. The quasi-solid Li-metal battery and full cell fabricated using this material also exhibit superior electrochemical performance.