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

Bonding iron chalcogenides in a hierarchical structure for high-stability sodium storage

Surface Stabilization of O3-type Layered Oxide Cathode to Protect the Anode of Sodium Ion Batteries for Superior Lifespan.

The design and synthesis of boronate affinity materials that show high efficiency, high selectivity, and high enrichment performance have gained significant attention. The principle of boronate affinity relies on the reversible covalent reactions, including the formation of stable five-membered or six-membered cyclic esters with cis-diol-containing compounds in alkaline aqueous media and dissociation of cyclic esters in an acidic surrounding to release cis-diol-containing compounds. Recently, various boronate affinity materials have been synthesized and utilized for selective enrichment of these compounds. Metal organic frameworks (MOFs) and covalent organic frameworks (COFs) have been widely used in chromatographic separation and sample pretreatment because of their adjustable pore size, high porosity, high specific surface area, tunable skeleton structure, and favorable chemical and thermal stability. To promote the enrichment selectivity of MOFs and COFs for cis-diol-containing compounds, boronic acid-functionalized MOFs and COFs with various structures and categories have been synthesized. This review summarizes more than 80 investigations into the categories, synthetic strategies, and applications of boronic acid-functionalized MOFs and COFs from the Science Citation Index. These synthesis methods include metal ligand-fragment co-assembly, post-synthetic modification, and bottom-up modification of boronic acid-functionalized porous materials. Although two modification strategies (post-synthetic and metal ligand-fragment co-assembly) have been introduced for the preparation of boronic acid-functionalized MOFs, the latter is more commonly adopted as it improves the enrichment selectivity and enrichment efficiency of MOFs. The common limitations of MOFs such as aggregation and aperture issues were also resolved. Boron affinity MOFs possessing favorable properties according to the characteristics of cis-diol-containing compounds, have also been synthesized. Furthermore, to facilitate enrichment and separation, many boronic acid-functionalized magnetic material MOFs have been developed for the enrichment and analysis of cis-diol-containing compounds. Additionally, the luminescent properties of Ln-MOFs have been used in combination with boronic acid affinity for the enrichment, separation, and subsequent detection of cis-diol-containing compounds. Post-synthetic modification and the bottom-up strategy are the primary methods for the preparation of boronic acid-functionalized COFs. Boronic acid-functionalized COFs are less investigated than boronic acid-functionalized MOFs, likely due to the greater complexity of COF synthesis. This work aims to summarize the research advances, synthesis ideas, and synthesis methods related to boric acid-functionalized porous organic frameworks, which will provide theoretical guidance and technical support for its applications while accelerating the commercialization of such organic frameworks.

Covalent organic frameworks (COFs) are an emerging material family having several potential applications. Their porous framework and redox-active centers enable gas/ion adsorption, allowing them to function as safe, cheap, and tunable electrode materials in next-generation batteries, as well as CO2 adsorption materials for carbon-capture applications. Herein, we develop four polyimide COFs by combining aromatic triamines with aromatic dianhydrides and provide detailed structural and electrochemical characterization. Through density functional theory (DFT) calculations and powder X-ray diffraction, we achieve a detailed structural characterization, where DFT calculations reveal that the imide bonds prefer to form at an angle with one another, breaking the 2D symmetry, which shrinks the pore width and elongates the pore walls. The eclipsed perpendicular stacking is preferable, while sliding of the COF sheets is energetically accessible in a relatively flat energy landscape with a few metastable regions. We investigate the potential use of these COFs in CO2 adsorption and electrochemical applications. The adsorption and electrochemical properties are related to the structural and chemical characteristics of each COF, giving new insights for advanced material designs. For CO2 adsorption specifically, the two best performing COFs originated from the same triamine building block, which—in combination with force-field calculations—revealed unexpected structure–property relationships. Specific geometries provide a useful framework for Na-ion intercalation with retainable capacities and stable cycle life at a relatively high working potential (>1.5 V vs Na/Na+). Although this capacity is low compared to conventional inorganic Li-ion materials, we show as a proof of principle that these COFs are especially promising for sustainable, safe, and stable Na-aqueous batteries due to the combination of their working potentials and their insoluble nature in water.

Covalent organic frameworks for fluorescent sensing: Recent developments and future challenges

Covalent organic frameworks (COFs) are periodic porous polymers with relatively high crystallinity, which have become a newly emerging family of materials. The COF materials are constructed by covalent bonds (B–O, C–C, C–N etc.) of light elements, and by using various organic building blocks we can create versatile COFs with diverse topology and functionalities. The COFs often possess the characteristics of low density, high porosity and large specific surface area, and have therefore been widely applied in gas adsorption and separation, energy chemistry and the environmental fields. To develop COF materials with appreciable functionalities, extensive experimental and theoretical techniques are being employed. In particular, multiscale simulation methods have been proven to be an efficient tool to design and investigate the properties of new COFs. This article mainly introduces two rational strategies to design new COF materials. One is linker replacement or the node replacement strategy to achieve high performance gas storage COF materials with large surface areas and pore volumes. The other is functional group modification or the metal doping strategy to design functionalized COFs with strong affinity towards gases. For the two design strategies, we also give a corresponding example for further analysis. Finally, we summarize the applications of COFs in hydrogen and methane storage, as well as CO2 capture.

Unraveling the atomic-level manipulation mechanism of tin-based ternary anodes via hetero-anion engineering for stable sodium ion storage

Metal-organic framework derived FeS/MoS2 composite as a high performance anode for sodium-ion batteries

Preparation of nitrogen- and phosphorous co-doped carbon microspheres and their superior performance as anode in sodium-ion batteries

Due to their high structural tunability, remarkable internal surface areas, readily accessible pore space, and host of possible applications, covalent organic frameworks (COFs) remain at the forefront of materials science research. Unfortunately, many COFs suffer from structural distortions or pore collapse during activation, which can lead to a substantial loss of crystallinity and functionality. Thus, herein, we demonstrate a facile method to address this issue by introducing polymer guests. The polymer adheres to the COF internal pore wall, acting as a supporting pillar during activation and effectively preserving the COF porosity and crystallinity. In fact, the surface area of one COF/polymer composite, known as TAPB-TA/PDA, was boosted by a factor of 16 when compared to the parent COF, TAPB-TA. More importantly, the now robust COF structure was able to resist layer shifting and order loss during both solvent immersion and removal. The introduction of functional polymer guests not only solidifies the COF structure and preserves its high porosity but is also shown to enhance the transport and separation of photogenerated charge carriers, thereby facilitating hydrogen evolution during photocatalytic water splitting. Molecular dynamics simulations further support experimental observations that the incorporation of PDA within the COF pores reinforces the walls, preventing its collapse. The proposed mechanism is based on the adsorption of PDA oligomers along the c direction of the unit cell, fastening the COF layers in place via van der Waals interactions. This kind of interaction locks -N═CH-Ph-CH═N- units in a trans-configuration in the COF pores.

Biomass carbon micro/nano-structures derived from ramie fibers and corncobs as anode materials for lithium-ion and sodium-ion batteries

Covalent organic frameworks (COFs) have been recognised as an excellent candidate for gas-phase storage, separation and catalysis due to their high porosity and stability and has been explored as a versatile and robust molecular platform, but the use of interchangeable substituents to enhance porosity and stability while retaining a functional framework has yet to be deeply explored. Herein, we report the properties and synthesis of a fluorinated COF, showing improved porosity as well as modifiable linkers for modular functionality. This was accomplished via imine condensation between a non-fluorinated monomer (5,10,15,20-tetrakis (4-aminophenyl)porphyrin) and a fluorinated monomer (2,3,5,6-tetrafluoroterephthaldehyde) to produce the COF in good yield. This compound was then compared with related COFs in a two-way comparison to relate their properties, such as the non-fluorinated analogues of the porphyrin-based COF, as well as COFs incorporating tris(aminophenyl)benzene. Upon analysis with PXRD and UV–Vis spectroscopy, correlating UV–Vis absorbance with pore size and thus porosity, it was concluded that the incorporation of fluorine in the compound indeed resulted in higher porosity in the products, possibly owing to the formation of electron-deficient tetrafluorobenzene centres in the COF, inducing coordination with the electron-rich porphyrin and triphenylbenzene units.

Covalent organic frameworks (COFs) are crystalline and porous organic materials attractive for photocatalysis applications due to their structural versatility and tunable optical and electronic properties. The use of photocatalysts (PCs) for polymerizations enables the preparation of well-defined polymeric materials under mild reaction conditions. Herein, we report two porphyrin-based donor–acceptor COFs that are effective heterogeneous PCs for photoinduced electron transfer-reversible addition–fragmentation chain transfer (PET-RAFT). Using density functional theory (DFT) calculations, we designed porphyrin COFs with strong donor–acceptor characteristics and delocalized conduction bands. The COFs were effective PCs for PET-RAFT, successfully polymerizing a variety of monomers in both organic and aqueous media using visible light (λmax from 460 to 635 nm) to produce polymers with tunable molecular weights (MWs), low molecular weight dispersity, and good chain-end fidelity. The heterogeneous COF PCs could also be reused for PET-RAFT polymerization at least 5 times without losing photocatalytic performance. This work demonstrates porphyrin-based COFs that are effective catalysts for photo-RDRP and establishes design principles for the development of highly active COF PCs for a variety of applications.

Recently, sodium ion battery (SIB) has attracted considerable attentions and been regarded as the promising alternative to Li ion battery (LIB) for electric energy storage (EES), owing to the nature abundance of sodium sources [1]. Graphite, the typical LIBs anode has been studied as the SIBs anode, but the specific capacity is low as it cannot store sufficient Na+ between its layers. Therefore, searching for appropriate anode material is a hot topic and vital for developing high performance SIBs. The 2D layer structured TMDCs (MoS2, WS2, and VS2 et al.), have attracted great interests [2-4], due to their sandwiched framework can facilitate the electrons and Li+/Na+ ions transportation. However, the (de)sodiation process might induce a phase transition of the TMDs and fast degradation of electrochemical performance over cycling [2]. In this study, NbS2 nanosheets have been synthesized by the combination of solid state sulfuration and chemical exfoliation method (ce-NbS2). The ce-NbS2 flakes seem to be diminished and homogeneous dispersed, they are much thinner and smaller than the bulk NbS2 (b-NbS2) flakes (Fig. 1A). HRTEM in the edge areas of ce-NbS2 is shown in Fig. 1B, it suggests that the NbS2 flakes after chemical exfoliation were stacked up with only 14 to 15 single layers. An inter-planar spacing of 6.65 nm has been found for the ce-NbS2 flakes. In situ XRD of ce-NbS2 has been performed at selected charged/discharged states in the first two-cycle between 0.01-3.0 V at 0.1 A g-1 (Fig. 1C and D). On the sodiation process, the main peak of CE-NbS2 located at 15° is gradually shifted towards left, indicating the crystal lattice is expanded as a consequence of Na+ ions insertion. A new remarkable peak has been observed at 13.5°, which is perceived to be the NaxNbS2, a solid evidence of Na+ ions insertion into the NbS2 framework. After discharging to 0.01 V, the peak at 15° almost disappears, while the intensity of the peak located at 13.5o becomes stronger, demonstrating the further insertion of Na+ ions. This process is reversed in the followed charging process. After charging to 3.0 V, the peaks of NbS2 recover. In the second cycle, similar phenomena have been observed, demonstrating a high reversible Na+ ions intercalation reaction in NbS2 nanosheets. Therefore, the possible reaction can be expressed as follows, NbS2+ x Na+ + xe- ⇔ Na x NbS2 (1) Therefore, the ce-NbS2 nanosheets are expected to maintain the same configurational phase upon sodiation. The initial discharge capacity of ce-NbS2 nanosheets and b-NbS2 reach to 377 and 324 mAh g-1 (Fig. 1E and F), respectively. Followed by the first cycle, ce-NbS2 delivers a high reversible capacity of 205 mAh g-1 at 0.1 A g-1. But that of b-NbS2 drops rapidly to 181 mAh g-1 after five cycles at 0.1 A g-1. After 100 cycles test at 0.5 A g-1, ce-NbS2 still can deliver a capacity of 157 mAh g-1, while the b-NbS2 exhibits a fast capacity drop from 173.3 to 81.6 mAh g-1 (Fig. 3G). Fig. 1. SEM (A) and HRTEM (B) images of ce-NbS2. First two-cycle charge-discharge curves (C) and the corresponding in situ XRD patterns collected at the selected charged/discharged states between 0.01 and 3.0 V at 0.1 A·g-1 (D) of ce-Nb2S nanosheets based anode. Initial charge/discharge profiles from 0.01 to 3.0 V vs. Na+/Na at 0.1 A g-1, long-term cycling performance at a cycling rate of 0.5 A g-1 of b-NbS2 (E and G) and ce-NbS2 nanosheets (F and G), respectively. In summary, the ce-NbS2 nanosheets deliver a reversible specific capacity (205 mAh g-1 at 100 mA g-1), and excellent cycling stability (a high capacity of 157 mAh g-1 at 0.5 A g-1 after 100 cycles). In situ X-ray diffraction test demonstrates that ce-NbS2 nanosheets can maintain its configuration upon soidation/desodiation, which is a meaningful character of long cycle life anode for SIBs.

Free-standing COF nanofiber in ion conductive membrane to improve efficiency of vanadium redox flow battery