Chemical Storage of Elemental Fluorine in Nanostructured Cerium Fluorides

Fluorine possesses remarkable properties that ensure its enduring and indispensable role in both academic and industrial development across diverse domains of our daily lives. Nevertheless, fluorine has become a growing environmental concern, leading to the consideration of molecular fluorine (F 2 ) as an alternative fluorinating agent due to its low environmental impact compared to hydrofluorocarbons (HFCs) or perfluorinated compounds (PFCs). However, its pronounced toxicity, corrosiveness, and hazardous nature are problematic when handling F 2 gas cylinders. Solid storage through chemisorption via the CeF 4 /CeF 3 transformation appears to be a promising approach to overcome its intrinsic problems. This article introduces a fundamental study exploring the impact of the chemical composition of precursor materials, CeF 3 or CeO 2, and the nanostructuration in the form of nanoparticles or macroporous structures on fluorination/defluorination temperatures, redox process reversibility, and the nature of the released gas, a parameter not systematically examined in previous studies. Through a deep investigation via X-ray diffraction (XRD) and electron microscopies (SEM, TEM), we demonstrate the benefit of both the pristine phase (CeO 2 ) and the nanostructuration into a macroporous structure (OPIF) on the limitation of crystalline growth during the fluorination process. The defluorination process, monitored by TGA and gas-phase IR spectroscopy, revealed that when CeO 2 -OPIF undergoes fluorination to form CeF 4 followed by consecutive vacuum defluorination without exposure to ambient air, incomplete decomposition of CeF 4 into CeF 3 results in the partial release of F 2 . Conversely, exposure of the fluorinated material to air results in partial formation of a hydrate, CeF 4 ·0.33H 2 O, confirmed by solid-state NMR, and promotes the defluorination (enhanced yield and release kinetics) through the formation of CeF 3 with both HF and F 2 release, increasing the cyclability performance from 1 cycle to at least 8 cycles.