André Hamwi

Fluorination of carbon nanotubes

Abstract Carbon nanotubes were prepared by thermal decomposition of acetylene over silica-supported cobalt catalysts. Subsequent chemical treatments with concentrated hydrofluoric acid and dilute nitric acid, respectively, eliminated the silica substrate and the remaining catalyst. The complete removal of cobalt was very difficult due to encapsulation of part of it in the shells, but a graphitization at 2800 °C eliminated the rest of cobalt and the structural defects. The reactivity of carbon nanotubes with fluorine was studied and compared to that of graphite. Important modification of the nanotubes structure was observed after reaction at high temperature as against the behaviour observed during reaction at room temperature. Compounds were analysed by means of transmission electron microscopy, X-ray diffraction and Fourier transform infrared spectroscopy.

Fluorine reactivity with graphite and fullerenes. fluoride derivatives and some practical electrochemical applications

Abstract The reactivity of fluorine with graphite depends strongly on reaction temperature and purity of the fluorine atmosphere. The presence of fluoride impurities accelerates the reaction even at room temperature. The CF bond character ranges from covalent to ionic or semi-ionic. The electrochemical performance of fluorinated graphite products as cathode material in lithium cells is related to lamellar structure, ionicity and fluorine content. Unlike graphite, fullerenes react readily with pure fluorine even at room temperature. Highly fluorinated C 60 compounds of high crystallinity are obtained at 300 °C. A fully fluorinated product C 60 F 60 can also be isolated.

Some chemical and electrochemical properties of graphite oxide

Abstract Graphite oxide is a very hygroscopic compound and can accommodate up to 12wt% of water in its structure. Specific treatments afforded derivatives that present definitely no affinity for water and in which the hydroxy/epoxy ratio may be modified according to the treatment. These graphite oxide derivatives were characterized by X-ray powder diffraction, and IR and NMR spectroscopies. Electrochemical measurements were performed with these compounds as cathode material in liquid electrolyte lithium batteries.

Hybrid-type graphite fluoride as cathode material in primary lithium batteries

A graphite fluoride (CF(LT)-T) series with singular physicochemical properties were prepared by a refluorination treatment between 100 and 600°C of a low-temperature (LT) fluorinated graphite. The most striking feature ofthese materials is the coexistence of sp 2 - and the sp 3 -type hybridizations of the carbon atoms, which gives the materials an unusual hybrid covalent/ semicovalent character. This hybrid structure leads to outstanding electrochemical performances as cathode materials in primary lithium batteries. In particular a very high energy density of 2277 Wh kg - 1 was achieved with the CF(LT)550 material. The discharge voltage and capacity are refluorination temperature dependent.

Perfluorofullerenes: Characterization and structural aspects

Abstract Characterization of highly fluorinated fullerenes (perfluorofullerenes) has revealed a wide range of x values. High reaction temperatures are limited by volatilization of the compounds. A good compromise is obtained for a reaction at 300 °C for several hours leading to a white C 60 F 54 compound with a narrow range of x values (± 2). X-ray powder diffraction reveals a f.c.c. lattice. Sublimation of C 60 F x compounds is carried out under static vacuum and separate fractions are condensed successively. As sublimation temperature is increased, fractions move from the liquid state to solids. 19 F NMR spectroscopy reveals a single narrow peak for liquid fractions, with a chemical shift of −72ppm/ CF 3 COOH thus suggesting a highly symmetrical species. The presence of oxygen is also detected when the starting material is exposed to air. The first fraction of air exposed compounds gives rise to a NMR signal at −55ppm. A relatively large quantity of liquid phase is obtained by direct condensation during C 60 fluorination. The NMR spectra is similar to that of the first sublimation fraction, exhibiting a peak at −72ppm.

Graphite fluorides prepared at room temperature 2. A very good electrochemical behaviour as cathode material in lithium non-aqueous electrolyte cell

Abstract New graphite fluoride compounds prepared at room temperature, CF x (RT), were used as the cathode material in a lithium battery based on anhydrous propylene carbonate- 1 M LiClO 4 electrolyte. They were found to offer better performances than CF x (HT), conventional graphite fluoride prepared at high temperature. Among the advantages are a higher and constant discharge voltage (≈3 V), faster reaction kinetics even under high current density, and higher specific energy density of about 2000 W h/kg.

Low-temperature carbon fluoride for high power density lithium primary batteries

Abstract Quantitative fluorination of graphite has been achieved at ambient temperature by the preliminary intercalation of iodine fluoride in HF + F 2 atmosphere. This fluorinated graphite LT-CF x was used as the cathode material in liquid electrolyte lithium cells; rate capability tests were carried out. Results are presented in comparison with those obtained with conventional HT-CF x . An outstanding behavior with LT-CF x is found, especially at high rate.

The Effect of Carbon Starting Material on Carbon Fluoride Synthesized at Room Temperature: Characterization and Electrochemistry

A series of carbon fluorine compounds (C x F) was prepared by catalyzed, room-temperature reactions of natural graphite, petroleum coke, and coal-tar-pitch coke with fluorine in the presence of HF and IF 5 . Elemental analysis indicated that most of the compounds had carbon-to-fluorine ratios between 1 and 2. X-ray diffraction, X-ray photoelectron spectroscopy, and infrared spectroscopy suggest that carbon-fluorine bond strengths were intermediate between ionic and covalent. The C x F materials exhibit higher reduction potentials when compared to commercially available CF x . Although the discharge capacities were lower for C x F compounds, their greater discharge voltage yielded higher energy densities than for analogous CF x cells. Discharge capacities for C x F increased slightly when prepared from petroleum coke heated to 1700°C compared with coke heated to 2600°C. This suggests that better discharge performance may be achieved from carbons with lower degrees of graphitization. C x F synthesized from natural graphite with particle sizes x F prepared from natural graphite with particle sizes between 20 and 40 µm. Materials prepared from coal-tar-pitch coke exhibited poor discharge capacities.

Electrochemical Properties of Carbon Nanotube Fluorides in a Lithium Cell System

Abstract Carbon nanotubes synthesized by decomposition of acetylene over silica-supported cobalt catalysts were fluorinated in different conditions : at room or high temperature, ca. 500°C. The electrochemical behaviour of these carbon nanotube fluorides as electrode materials in a lithium cell was investigated using a liquid electrolyte. Results are compared to those obtained with graphite fluoride compounds which present a well known high electrochemical performance.

Preparation and Characterization of Fluorinated Fullerenes

Abstract Fluorination of pure C60 and a C60/C70 mixture has been studied under different experimental conditions: reaction time, temperature, fluorine pressure. The products obtained have been characterized by IR and NMR spectroscopy, X-ray diffraction, and mass spectrometry analysis by positive and negative ions.