A Molecular Pillar Approach To Grow Vertical Covalent Organic Framework Nanosheets on Graphene: Hybrid Materials for Energy Storage

Covalent Organic Framework/Graphene Hybrids: Synthesis, Properties, and Applications

To address current energy crises and environmental concerns, it is imperative to develop and design versatile porous materials ideal for water purification and energy storage. The advent of covalent organic frameworks (COFs), a revolutionary terrain of porous materials, is underscored by their superlative features such as divinable structure, adjustable aperture, and high specific surface area. However, issues like inferior electric conductivity, inaccessible active sites impede mass transfer and poor processability of bulky COFs restrict their wider application. As a herculean stride forward, COF/graphene hybrids amalgamate the strengths of their constituent components and have in consequence, enticed significant scientific intrigue. Herein, the current progress on the structure and properties of graphene-based materials and COFs are systematically outlined. Then, synthetic strategies for preparing COF/graphene hybrids, including one-pot synthesis, ex situ synthesis, and in situ growth, are comprehensively reviewed. Afterward, the pivotal attributes of COF/graphene hybrids are dissected in conjunction with their multifaceted applications spanning adsorption, separation, catalysis, sensing, and energy storage. Finally, this review is concluded by elucidating prevailing challenges and gesturing toward prospective strides within the realm of COF/graphene hybrids research.

Tuning the Topology of Two-Dimensional Covalent Organic Frameworks through Site-Selective Synthetic Strategy

Tuning the topology of two-dimensional (2D) covalent organic frameworks (COFs) is of paramount scientific interest but remains largely unexplored. Herein, we present a site-selective synthetic strategy that enables the tuning of 2D COF topology by simply adjusting the molar ratio of an amine-functionalized dihydrazide monomer (NH2 -Ah) and 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde (Tz). This approach resulted in the formation of two distinct COFs: a clover-like 2D COF with free amine groups (NH2 -Ah-Tz) and a honeycomb-like COF without amine groups (Ah-Tz). Both COFs exhibited good crystallinity and moderate porosity. Remarkably, the clover-shaped NH2 -Ah-Tz COF, with abundant free amine groups, displayed significantly enhanced adsorption capacities toward crystal violet (CV, 261 mg/g) and congo red (CR, 1560 mg/g) compared to the non-functionalized honeycomb-like Ah-Tz COF (123 mg/g for CV and 1340 mg/g for CR), underscoring the pivotal role of free amine functional groups in enhancing adsorption capacities for organic dyes. This work highlights that the site-selective synthetic strategy paves a new avenue for manipulating 2D COF topology by adjusting the monomer feeding ratio, thereby modulating their adsorption performances toward organic dyes.

Redox-Active Azulene-based 2D Conjugated Covalent Organic Framework for Organic Memristors

As a conjugated and unsymmetric building block composed of an electron-poor seven-membered sp² carbon ring and an electron-rich five-membered carbon ring, azulene and its derivatives have been recognized as one of the most promising building blocks for novel electronic devices due to its intrinsic redox activity. By using 1,3,5-tris(4-aminophenyl)-benzene and azulene-1,3-dicarbaldehyde as the starting materials, an azulene(Azu)-based 2D conjugated covalent organic framework, COF-Azu, is prepared through liquid-liquid interface polymerization strategy for the first time. The as-fabricated Al/COF-Azu/indium tin oxide (ITO) memristor shows typical non-volatile resistive switching performance due to the electric filed induced intramolecular charge transfer effect. Associated with the unique memristive performance, a simple convolutional neural network is built for image recognition. After 8 epochs of training, image recognition accuracy of 80 % for a neutral network trained on a larger data set is achieved.

A nanostructured covalent organic framework with readily accessible triphenylstibine moieties for high-performance supercapacitors

A pristine, redox-active triphenylstibine based COF (Sb-COF) exhibits well-uniform nanostructures which could provide sufficient electron conduction pathways and minimize the ion transport lengths, making triphenylstibine moieties readily accessible by the electrolyte. The assembled Sb-COF//rGO thus provides an excellent energy density of 69 W h Kg^-1.

Ionic Covalent Organic Frameworks for Energy Devices

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

Single-Phase Covalent Organic Framework Staggered Stacking Nanosheet Membrane for CO2 -Selective Separation

Two-dimensional covalent organic frameworks (2D COFs) are considered as potential candidates for gas separation membranes, benefiting from permanent porosity, light-weight skeletons, excellent stability and facilely-tailored functionalities. However, their pore sizes are generally larger than the kinetic diameters of common gas molecules. One great challenge is the fabrication of single-phase COF membranes to realize precise gas separations. Herein, three kinds of high-quality β-ketoenamine-type COF nanosheets with different pore sizes were developed and aggregated to ultrathin nanosheet membranes with distinctive staggered stacking patterns. The narrowed pore sizes derived from the micro-structures and selective adsorption capacities synergistically endowed the COF membranes with intriguing CO₂ -philic separation performances, among which TpPa-2 with medium pore size exhibited an optimal CO₂ /H₂ separation factor of 22 and a CO₂ permeance of 328 gas permeation units at 298 K. This membrane performance reached the target with commercial feasibility for syngas separations.

Self-Assembly of Polyoxometalate-Resorcin[4]arene-Based Inorganic-Organic Complexes: Metal Ion Effects on the Electrochemical Performance of Lithium Ion Batteries

With their adjustable structures and diverse functions, polyoxometalate (POM)-resorcin[4]arene-based inorganic-organic complexes are a kind of potential multifunctional material. They have potential applications for lithium ion batteries (LIBs). However, the relationship between different coordinated metal ions and electrochemical performance has rarely been investigated. Here, three functionalized POM-resorcin[4]arene-based inorganic-organic materials, [Co₂ (TMR4 A)₂ (H₂ O)₁₀ ][SiW₁₂ O₄₀ ]⋅2 EtOH⋅4.5 H₂ O (1), [Ni₂ (TMR4 A)₂ (H₂ O)₁₀ ][SiW₁₂ O₄₀ ]⋅4 EtOH⋅13 H₂ O (2), and [Zn₂ (TMR4 A)₂ (H₂ O)₁₀ ][SiW₁₂ O₄₀ ]⋅2 EtOH⋅2 H₂ O (3), have been synthesized. Furthermore, to enhance the conductivities of these compounds, 1-3 were doped with reduced graphene oxide (RGO) to give composites 1@RGO-3@RGO, respectively. As anode materials for LIBs, 1@RGO-3@RGO can deliver very high discharge capacities (1445.9, 1285.0 and 1095.3 mAh g^-1 , respectively) in the initial run, and show discharge capacities of 898, 665 and 651 mAh g^-1 , respectively, at a current density of 0.1 A g^-1 over 100 runs. More importantly, the discharge capacities of 319, 283 and 329 mAh g^-1 were maintained for 1@RGO-3@RGO even after 400 cycles at large current density (1 A g^-1 ).

Synergizing Functional Nanomaterials with Aptamers Based on Electrochemical Strategies for Pesticide Detection: Current Status and Perspectives

Owing to the high toxicity and large-scale use of pesticides, it is imperative to develop selective, sensitive, portable, and convenient sensors for rapid monitoring of pesticide. Therefore, the electrochemical detection platform offers a promising analytical approach since it is easy to operate, economical, efficient, and user-friendly. Meanwhile, with advances in functional nanomaterials and aptamer selection technologies, numerous sensitivity-enhancement techniques alongside a widespread range of smart nanomaterials have been merged to construct novel aptamer probes to use in the biosensing field. Hence, this study intends to highlight recent development and promising applications on the functional nanomaterials with aptamers for pesticides detection based on electrochemical strategies. We also reviewed the current novel aptamer-functionalized microdevices for the portability of pesticides sensors. Furthermore, the major challenges and future prospects in this field are also discussed to provide ideas for further research.

A new hydrazone-linked covalent organic framework for Fe(iii) detection by fluorescence and QCM technologies

A new stable hydrazone-linked covalent organic framework was synthesized, which can be used as either a fluorescent probe or a COF-based QCM sensor for the identification of metal ions, showing high selectivity and sensitivity toward Fe(iii) ions.

Beyond Frameworks: Structuring Reticular Materials across Nano-, Meso-, and Bulk Regimes

Reticular materials are of high interest for diverse applications, ranging from catalysis and separation to gas storage and drug delivery. These open, extended frameworks can be tailored to the intended application through crystal-structure design. Implementing these materials in application settings, however, requires structuring beyond their lattices, to interface the functionality at the molecular level effectively with the macroscopic world. To overcome this barrier, efforts in expressing structural control across molecular, nano-, meso-, and bulk regimes is the essential next step. In this Review, we give an overview of recent advances in using self-assembly as well as externally controlled tools to manufacture reticular materials over all the length scales. We predict that major research advances in deploying these two approaches will facilitate the use of reticular materials in addressing major needs of society.

Covalent Organic Framework (COF-1) under High Pressure

COF-1 has a structure with rigid 2D layers composed of benzene and B3 O3 rings and weak van der Waals bonding between the layers. The as-synthesized COF-1 structure contains pores occupied by solvent molecules. A high surface area empty-pore structure is obtained after vacuum annealing. High-pressure XRD and Raman experiments with mesitylene-filled (COF-1-M) and empty-pore COF-1 demonstrate partial amorphization and collapse of the framework structure above 12-15 GPa. The ambient pressure structure of COF-1-M can be reversibly recovered after compression up to 10-15 GPa. Remarkable stability of highly porous COF-1 structure at pressures at least up to 10 GPa is found even for the empty-pore structure. The bulk modulus of the COF-1 structure (11.2(5) GPa) and linear incompressibilities (k[100] =111(5) GPa, k[001] =15.0(5) GPa) were evaluated from the analysis of XRD data and cross-checked against first-principles calculations.

Ultrastable Triazine-Based Covalent Organic Framework with an Interlayer Hydrogen Bonding for Supercapacitor Applications

Covalent organic frameworks (COFs) with redox-active units are a class of ideal materials for electrochemical-energy-storage devices. A novel two-dimensional (2D) PDC-MA-COF with redox-active triazine units was prepared via aldehyde-amine condensation reaction by using 1,4-piperazinedicarboxaldehyde (PDC) and melamine (MA) as structural units, which possessed high specific surface area (S_BET = 748.2 m² g^-1), narrow pore width (1.9 nm), large pore volume (1.21 cm³ g^-1), and high nitrogen content (47.87%), for pseudocapacitance application. The interlayer C-H···N hydrogen bonding can "lock" the relative distance between two adjacent layers to avoid an interlayer slip, which is more conducive to maintaining the ordered pore structure of the COF and improving a fast charge transfer between the electrode interface and triazine units. The PDC-MA-COF exhibited an excellent electrochemical performance with the highest specific capacitance of 335 F g^-1 along with 19.71% accessibility of the redox-active triazine units in a three-electrode system and 94 F g^-1 in a two-electrode system at 1.0 A g^-1 current density. Asymmetric supercapacitor of PDC-MA-COF//AC assembled using PDC-MA-COF and activated carbon (AC) as positive and negative electrode materials, respectively, exhibited a high energy density of 29.2 W h kg^-1 with a power density of 750 W kg^-1. At the same time, it also showed an excellent cyclic stability and could retain 88% of the initial capacitance after 20 000 charge-discharge cycles, which was better than those of the most of the analogous materials reported previously. This study provided a new strategy for designing redox-active COFs for pseudocapacitive storage.

Tailored Polyimide-Graphene Nanocomposite as Negative Electrode and Reduced Graphene Oxide as Positive Electrode for Flexible Hybrid Sodium-Ion Capacitors

Redox-active polyimide materials hold a great promise for electrochemical energy storage applications, especially for flexible energy storage devices. However, the low utilization efficiency due to poor electrical conductivity of the materials remains one of the greatest challenges. In this work, we designed and prepared polyimide-graphene composite materials and tested their electrochemical properties in sodium-ion capacitors. By manipulating the interfacial chemistry and interactions between the polyimide and graphene, composite electrode materials with different polyimide particle sizes and morphologies were obtained. Sodium-ion storage capacity was significantly improved, from ∼50 mAh g^-1 for pure polyimide to 225 mAh g^-1 for a polyimide-graphene composite. A hybrid sodium-ion capacitor fabricated with freestanding polyimide-graphene composite as the negative electrode and reduced graphene oxide as the positive electrode delivered energy densities of 55.5 and 21.5 Wh kg^-1 at power densities of 395 and 3400 W kg^-1, respectively. A flexible sodium-ion capacitor with outstanding mechanical properties was also demonstrated.

More Than 12 % Polarization and 20 Minute Lifetime of 15 N in a Choline Derivative Utilizing Parahydrogen and a Rhodium Nanocatalyst in Water

Hyperpolarization techniques are key to extending the capabilities of MRI for the investigation of structural, functional and metabolic processes in vivo. Recent heterogeneous catalyst development has produced high polarization in water using parahydrogen with biologically relevant contrast agents. A heterogeneous ligand-stabilized Rh catalyst is introduced that is capable of achieving ¹⁵ N polarization of 12.2±2.7 % by hydrogenation of neurine into a choline derivative. This is the highest ¹⁵ N polarization of any parahydrogen method in water to date. Notably, this was performed using a deuterated quaternary amine with an exceptionally long spin-lattice relaxation time (T₁ ) of 21.0±0.4 min. These results open the door to the possibility of ¹⁵ N in vivo imaging using nontoxic similar model systems because of the biocompatibility of the production media and the stability of the heterogeneous catalyst using parahydrogen-induced polarization (PHIP) as the hyperpolarization method.

Supramolecular Reassembly of Self-Exfoliated Ionic Covalent Organic Nanosheets for Label-Free Detection of Double-Stranded DNA

Ionic covalent organic nanosheets (iCONs), a member of the two-dimensional (2D) nanomaterials family, offer a unique functional platform for a wide range of applications. Herein, we explore the potential of an ethidium bromide (EB)-based covalent organic framework (EB-TFP) that self-exfoliates in water resulting in 2D ionic covalent organic nanosheets (EB-TFP-iCONs) for the selective detection of double-stranded DNA (dsDNA). In an aqueous medium, the self-exfoliated EB-TFP-iCONs reassemble in the presence of dsDNA resulting in hybrid EB-TFP-iCONs-DNA crystalline nanosheets with enhanced fluorescence at 600 nm. Detailed steady-state and time-resolved emission studies revealed that the reassembly phenomenon was highly selective for dsDNA when compared to single-stranded DNA (ssDNA), which allowed us to use the EB-TFP-iCONs as a 2D fluorescent platform for the label-free detection of complementary DNA strands.