Hollow Covalent Organic Framework Architectures: Template-Free Synthesis, Mechanistic Insights, and Advanced...

Owing to their enhanced functionality, hollow covalent organic frameworks (HCOFs) have garnered significant research attention across various fields, including energy storage, catalysis, and biomedicine. While template-based synthesis methods for HCOFs are conceptually straightforward, they are plagued by challenges such as low yields, cumbersome procedures, and the use of hazardous substances like HF and NaOH for template removal. As a result, template-free synthetic routes (TFRs) for HCOFs have emerged as a promising alternative. This review provides an overview of recent advancements in the template-free fabrication of HCOFs and their diverse applications. The template-free synthetic routes are summarized, alongside discussions of the hollowing mechanisms underlying cavity formation, such as Ostwald ripening, Kirkendall-like conversion, self-assembly, and selective etching. For the advanced applications of HCOFs, the unique relationship between their cavity structure and functional properties is emphasized. It is evident that HCOFs with thin shells and built-in cavities not only enhance active sites and reduce mass transfer distances but also mitigate volume expansion, ensuring superior structural stability. Finally, the challenges and future perspectives for innovation in HCOF synthesis and applications are outlined.