Three-dimensional metal-intercalated covalent organic frameworks for near-ambient energy storage

A critical review of COFs-based photocatalysis for environmental remediation

Covalent organic frameworks (COFs) are highly porous crystalline polymers formed through covalent bonding of molecular building blocks. Numerous fabrication strategies have been developed, including solvothermal, ionothermal, microwave, mechanochemical, and sonochemical methods, alongside ligand substitution and post-modification techniques, which allow for precise control over the structures and properties of COFs. The exceptional physicochemical stability, large specific surface area, broad visible light absorption, and extended π-conjugated systems have sparked significant interest in photocatalytic applications. Recently, COFs have shown remarkable efficacy in environmental remediation, demonstrating the ability to degrade a wide range of organic pollutants, including dyes, antibiotics, and drugs, as well as to reduce/oxidize heavy metals such as Cr(VI), U(VI), and As(III), in addition to targeting biological pollutants. This review comprehensively explores recent advancements in COFs-based photocatalysis, covering synthetic methods, COF types, modification method, theoretical calculations, environmental applications, and underlying mechanisms. Additionally, the challenges and opportunities for COFs as a robust, cost-effective technology in practical applications was discussed, and offering valuable insights for researchers in environmental remediation, materials science, and photocatalysis.

Latest advances in layered covalent organic frameworks for water and wastewater treatment

This review provides an overview of recent progress in the development of layered covalent organic frameworks (LCOFs) for the adsorption and degradation of pollutants in water and wastewater treatment. LCOFs have unique properties such as high surface area, porosity, and tunability, which make them attractive adsorbents and catalysts for water and wastewater treatment. The review covers the different synthesis methods for LCOFs, including self-assembly, co-crystallization, template-directed synthesis, covalent organic polymerization (COP), and solvothermal synthesis. It also covers the structural and chemical characteristics of LCOFs, their adsorption and degradation capacity for different pollutants, and their comparison with other adsorbents and catalysts. Additionally, it discussed the mechanism of adsorption and degradation by LCOFs, the potential applications of LCOFs in water and wastewater treatment, case studies and pilot-scale experiments, challenges, and limitations of using LCOFs, and future research directions. The current state of research on LCOFs for water and wastewater treatment is promising, however, more research is needed to improve their performance and practicality. The review highlights that LCOFs have the potential to significantly improve the efficiency and effectiveness of current water and wastewater treatment methods and can also have implications for policy and practice.

Advances and challenges in the development of nanosheet membranes

The development of highly efficient separation membranes utilizing emerging materials with controllable pore size and minimized thickness could greatly enhance the broad applications of membrane-based technologies. Having this perspective, many studies on the incorporation of nanosheets in membrane fabrication have been conducted, and strong interest in this area has grown over the past decade. This article reviews the development of nanosheet membranes focusing on two-dimensional materials as a continuous phase, due to their promising properties, such as atomic or nanoscale thickness and large lateral dimensions, to achieve improved performance compared to their discontinuous counterparts. Material characteristics and strategies to process nanosheet materials into separation membranes are reviewed, followed by discussions on the membrane performances in diverse applications. The review concludes with a discussion of remaining challenges and future outlook for nanosheet membrane technologies.

Two-Dimensional Polymers and Polymerizations

Synthetic chemists have developed robust methods to synthesize discrete molecules, linear and branched polymers, and disordered cross-linked networks. However, two-dimensional polymers (2DPs) prepared from designed monomers have been long missing from these capabilities, both as objects of chemical synthesis and in nature. Recently, new polymerization strategies and characterization methods have enabled the unambiguous realization of covalently linked macromolecular sheets. Here we review 2DPs and 2D polymerization methods. Three predominant 2D polymerization strategies have emerged to date, which produce 2DPs either as monolayers or multilayer assemblies. We discuss the fundamental understanding and scope of each of these approaches, including: the bond-forming reactions used, the synthetic diversity of 2DPs prepared, their multilayer stacking behaviors, nanoscale and mesoscale structures, and macroscale morphologies. Additionally, we describe the analytical tools currently available to characterize 2DPs in their various isolated forms. Finally, we review emergent 2DP properties and the potential applications of planar macromolecules. Throughout, we highlight achievements in 2D polymerization and identify opportunities for continued study.

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.

Computational Investigation on the Electronic Structure and Functionalities of a Thiophene-Based Covalent Triazine Framework

Using the state-of-the-art theoretical method, we have investigated the electronic and optical properties of a thiophene-based covalent triazine framework (TBCTF). We have found that TBCTF is a direct band gap semiconductor. Our calculations reveal that constitutional isomerism is a tool for band gap tuning. The variation of band gap can be achieved by the bilayer TBCTF formation and further can be tuned by the z-axial strain. We have designed a new two-dimensional van der Waals heterostructure g-ZnO/TBCTF, which shows type-II band alignment, ensuring effective separation of photogenerated electron-hole pairs. This composite system also exhibits enhanced absorption in the visible range compared to that of individual g-ZnO and TBCTF monolayers. Therefore, this composite system may find potential application in photovoltaic devices. We have also investigated the hydrogen adsorption ability of TBCTF through Li atom doping. Our results indicate that the calculated hydrogen adsorption energies lie in the range, which is suitable for reversible hydrogen storage under ambient conditions. Therefore, the lithium-doped TBCTF may be a potential candidate for the hydrogen storage material.

Intercalation of first row transition metals inside covalent-organic frameworks (COFs): a strategy to fine tune the electronic properties of porous crystalline materials

Covalent-organic frameworks (COFs) have emerged as an important class of nano-porous crystalline materials with many potential applications. Here we present an strategy to control their electronic properties.

Design Principles for Covalent Organic Frameworks in Energy Storage Applications

Covalent organic frameworks (COFs) are an exciting class of porous materials that have been explored as energy-storage materials for more than a decade. This review discusses efforts to develop these materials for applications in gas and electrical power storage. Some of the design strategies for developing the gas sorption properties of COFs and mechanistic studies on their formation are also discussed.

"H₂ sponge": pressure as a means for reversible high-capacity hydrogen storage in nanoporous Ca-intercalated covalent organic frameworks

We explore the potential and advantages of Ca-intercalated covalent organic framework-1 (CaCOF-1) as a 3-dimensional (3D) layered material for reversible hydrogen storage. Density functional theory calculations show that by varying the interlayer distance of CaCOF-1, a series of metastable structures can be achieved with the interlayer distance falling in the range of 4.3-4.8 Å. When four hydrogen molecules are adsorbed on each Ca, a high hydrogen uptake of 4.54 wt% can be produced, with the binding energy falling in the ideal range of 0.2-0.6 eV per H2. While H2 absorption is a spontaneous process under H2 rich conditions, tuning the interlayer distance by reasonable external pressure could compress CaCOF-1 to release all of the hydrogen molecules and restore the material to its original state for recyclable use. This provides a new method for gradual, controllable extraction of hydrogen molecules in covalent organic frameworks, satisfying the practical demand for reversible hydrogen storage at ambient temperatures.