Covalent–Organic Framework (COF)-Core–Shell Composites: Classification,Synthesis, Properties,and Applications

Core–shell structures, where the “guest” material is encapsulated within a protective shell, integrate the advantages of different materials to enhance the overall properties of the composite. Covalent–organic frameworks (COFs) are favorable candidates for composing core–shell structures due to their inherent porosity, good activity, excellent stability, and other advantages. In particular, COFs as shells to encapsulate other functional materials are becoming increasingly popular in the fields of environmental remediation and energy conversion. However, there is a lack of reviews on COF-based core–shell materials. In this context, this review provides a systematic summary of the current research on COF-based core–shell composites. First, a simple classification is made for COF-based core–shell composites. The second part of the review describes the main synthesis methods. The changes brought about by the COF shell and core–shell structures on the properties of the composites and their applications in photocatalysis, electrocatalysis, adsorption, sensing, and supercapacitors are then emphasized. Finally, new perspectives on the future development and challenges of composites are presented. The purpose of this study is to provide future insights into the design and application of COF-based core–shell composites.     

In Situ Deformation Topology of COFs with Shortened Channels and High Redox Properties for Li–S Batteries

Covalent organic frameworks (COFs) with various topologies are typically synthesized by selecting and designing connecting units with rich shapes. However, this process is time-consuming and labour-intensive. Besides, the tight stacking of COFs layers greatly restrict their structural advantages. It is crucial to effectively exploit the high porosity and active sites of COFs by topological design. Herein, for the first time, inducing in situ topological changes in sub-chemometric COFs by adding graphene oxide (GO) without replacing the monomer, is proposed. Surprisingly, GO can slow down the intermolecular stacking and induce rearrangement of COFs nanosheets. The channels of D- [4+3] COFs are significantly altered while the stacking of periodically expanded framework is weakened. This not only maximizes the exposure of pore area and polar groups, but also shortens the channels and increases the redox activity, which enables high loading while enhancing host-guest interactions. This topological transformation to exhibit the structural features of COFs for efficient application is an innovative molecular design strategy.     

Use in Photoredox Catalysis of Stable Donor–Acceptor Covalent Organic Frameworks and Membrane Strategy

Optoelectronic attributes notwithstanding donor–acceptor covalent organic frameworks (D–A COFs) are not durable photocatalysts in many cases. Herein, a stabilization strategy of D–A COFs by intramolecular hydrogen (H)-bonds and a membrane-based mass transfer strategy for photocatalytic modulation are reported. The crystalline stability design of COF is cored at the strong π–π interactions and the H-bonds of adjacent tetrakis(4-formylphenyl)pyrene and naphthalenediimide units and the D–A charge transfer is designed for efficiency optimization. The well-defined, stable structure and charge dynamics of D–A COF, and the structure-controlled reactive oxygen species yields are confirmed. In two photoredox models, the COF presents both robust activity and stability and is further integrated with the mass transfer optimization of the COFs/polyvinylidene fluoride membrane. The membrane is recycled at least 15 times, and the turnover frequency value of g-scale amine coupling is as high as 62.4 h⁻¹. This work offers a facile approach to the stabilization design of D–A COFs and explores a general membrane-based mass transfer strategy for photocatalysis.     

Bifunctional Covalent Organic Framework-Derived Electrocatalysts with Modulated p-Band Centers for Rechargeable Zn–Air Batteries

Fine control over the physicochemical structures of carbon electrocatalysts is important for improving the sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in rechargeable Zn–air batteries. Covalent organic frameworks (COFs) are considered good candidate carbon materials because their structures can be precisely controlled. However, it remains a challenge to impart bifunctional electrocatalytic activities for both the ORR and OER to COFs. Herein, a pyridine-linked triazine covalent organic framework (PTCOF) with well-defined active sites and pores is readily prepared under mild conditions, and its electronic structure is modulated by incorporating Co nanoparticles (CoNP-PTCOF) to induce bifunctional electrocatalytic activities for the ORR and OER. The CoNP-PTCOF exhibits lower overpotentials for both ORR and OER with outstanding stability. Computational simulations find that the p-band center of CoNP-PTCOF down-shifted by charge transfer, compared to pristine PTCOF, facilitate the adsorption and desorption of oxygen intermediates on the pyridinic carbon active sites during the reactions. The Zn–air battery assembled with bifunctional CoNP-PTCOF exhibits a small voltage gap of 0.83 V and superior durability for 720 cycles as compared with a battery containing commercial Pt/C and RuO₂. This strategy for modulating COF electrocatalytic activities can be extended for designing diverse carbon electrocatalysts.