Flexible High-Aspect-Ratio COF Nanofibers: Defect-Engineered Synthesis, Superelastic Aerogels, and Uranium Extraction Applications

Abstract

The lack of macro-continuity and mechanical strength of covalent organic frameworks (COFs) has significantly limited their practical applications. Here, we propose an “alcohol-triggered defect cleavage” strategy to precisely regulate the growth and stacking of COF grains through a moderate reversed Schiff base reaction, realizing the direct synthesis of COF nanofibers (CNFs) with high aspect ratio (L/D = 103.05) and long length (> 20 μm). An individual CNF exhibits a biomimetic scale-like architecture, achieving superior flexibility and fatigue resistance under dynamic bending via a multiscale stress dissipation mechanism. Taking advantages of these structural features, we engineer CNF aerogels (CNF-As) with programmable porous structures (e.g., honeycomb, lamellar, isotropic) via directional ice-template methodology. CNF-As demonstrate 100% COF content, high specific surface area (396.15 m2 g−1) and superelasticity (~0% elastic deformation after 500 compression cycles at 50% strain), outperforming most COF-based counterparts. Compared with the conventional COF aerogels, the unique structural features of CNF-A enable it to perform outstandingly in uranium extraction, with an 11.72-fold increment in adsorption capacity (920.12 mg g−1) and adsorption rate (89.9%), and a 2.48-fold improvement in selectivity (U/V = 2.31). This study provides a direct strategy for the development of next-generation COF materials with outstanding functionality and structural robustness.

1. Introduction

Covalent organic frameworks (COFs) have emerged as a novel class of crystalline porous polymers for advanced molecular technologies due to high specific surface area (SSA), abundant micro/mesopores, and designable functionalities. However, their transition from particulate powders to macroscopic functional materials remains impeded by intrinsic brittleness and structural discontinuity [1–5]. While composite engineering through substrate hybridization offers partial solutions, compromised active site accessibility and interfacial stress concentration fundamentally limit their practical performances [6–10]. Moreover, achieving stable and massive integration with the supporting substrate in the composites necessitates precise control of COF particle sizes at the nanoscale with high uniformity, which is a persistent challenge in solvothermal synthesis. Thus, attempts have been made to increase the COF dimensions and to directly shape them into macro-continuous materials such as self-standing films and monoliths. Although interfacial synthesis enables the fabrication of COF films, their structural integrity critically depends on the chemical composition of the COF and synthetic parameters, deviations from optimal conditions usually result in crack formation [11–13]. The dense packing of COF grains in the vertical direction (Z-axis) in the film inherently limits their application performances.

Keeping this challenge in mind, COF aerogels with 3D porous architecture have been engineered through a sol–gel strategy coupled with supercritical CO2 drying [14–16]. Nevertheless, these aerogels possess irregular porous structures that resist precise manipulation or pre-fabrication design. More critically, most reported COF aerogels demonstrate insufficient elasticity and restricted structural adaptability, which are fundamental drawbacks impeding practical application.

Fibers, with their high aspect ratio (L/D) and capability for textile design, represent a unique material form that combines structural adaptability and multifunctional synergy. Crystalline materials exhibit exceptional mechanical robustness and physiochemical properties due to ordered lattice arrangements, but their inherent brittleness constrains their utility in flexible devices. Emerging strategies to circumvent these limitations focus on engineering crystalline materials into fibrous morphologies by controlling crystallization dynamics and microstructural topology. The one-dimensional continuity of fibers enables precise regulation of stress distribution via lattice orientation and defect engineering. A notable example includes the puzzle-like polycrystalline stacking in oxide ceramics.