β-Keto-Enamine-Functionalized Nitrogen-Rich Covalent Organic Frameworks for Metal-Free H₂ Sensor at Room Temperature

Hydrogen (H 2 ) is a promising clean energy carrier, but its high flammability and explosive nature necessitate highly sensitive and reliable detection at trace levels. Conventional hydrogen sensors often require high operating temperatures and suffer from slow response times and limited sensitivity. Covalent organic frameworks (COFs), with their intrinsic porosity, tunability, conductive, and high stability, offer an excellent platform for efficient, chemiresistive H 2 sensing at room temperature. This study presents a series of nitrogen-rich Schiff-based COFs (COF-1, COF-2, and COF-3) functionalized with keto-enamine linkages, as confirmed by the presence of peaks at ∼529.8 eV (C═O), ∼402.2 eV (C═N), ∼180 ppm (C═O), and ∼168 ppm (C═N) by XPS and 13 C CP-MAS SS NMR analyses. Among these, COF-3 shows exceptional performance, revealing a sensitivity with a rapid response time (6 s), and recovery times (4 s) at 1 ppm H 2 concentration. The density functional theory (DFT) calculations were performed to support experimental findings, and they revealed the strong electronic interactions between COF-3 and H 2 molecules (−168.03 kJ/mol). These results highlight the potential of Schiff-based COFs, particularly COF-3, as a promising sensor for next-generation H 2 sensors operating efficiently in ambient environments.