Origin of the highly enhanced porosity of styryl LDH hybrid-type carbon replicas and study of a subsequent fluorination at low-temperature

Abstract

To understand the textural properties of carbon replicas obtained from the charring of styryl-based LDH hybrid assemblies, the calcination process was carefully studied within a temperature domain between 500 and 700 °C. Supported by several characterizations such as nitrogen gas adsorption, X-ray diffraction recorded in situ, Raman and ESR, the drastic change in the porosity (micro- and meso-porosity) should be explained by the thermal process combined to the acid-leaching procedure responsible for the opening of the porosity and more specifically by the intimate mixing between the organic and inorganic components. Indeed, a temperature of 600 °C is found to ensure the carbonisation of the organic part, as well as supplying small inorganic particles. Their departure during the acid-leaching is believed to create the well defined mesoporosity, and higher temperatures of treatment induce a crystallisation of the particles, and ill defined mesopores for the relative carbon replica. The carbon replica is found to be reactive against fluorine gas, with a nominal uptake dependant of the temperature and of up to 1.0 F per carbon atom. The fluorination is highly deleterious for the texture with a drastic decrease in the specific surface area. The higher the temperature of fluorination, the lower is the corresponding BET surface area. The mechanism of fluorination is studied by ESR and ¹⁹F solid state NMR. Against lithium ion the fluorinated carbon replicas exhibit an intricate electrochemical behavior, and an unexpected reversible process is observed at 2.5 V vs Li for the carbon replica fluorinated at 300 °C.