Electrochemistry of non-oxide ceramics: some new ideas for energy storage and conversion

International audienc

Une étude des PEMFC et de leurs membranes

Dans la première partie, des électrodes métalliques recouvertes d'un ou de plusieurs films polymères pour piles à combustible (PAC)sont étudiées théoriquement et expérimentalement. On montre qu'il n'est pas possible de déterminer par spectroscopie d'impédance électrochimique (SIE) les paramètres clés de ces systèmes : coefficients de diffusion dans les films (D), épaisseurs des films (L) et coefficients de partage à chaque interface (gamma). Seuls les paramètres gamma D/L et L*L/D sont déterminables à l'aide de cette technique.Dans la seconde partie, une nouvelle méthode de mesure de l'impédance de chaque élément d'une PAC en cours de fonctionnement re el est proposée. On montre, pour une PAC directe au méthanol commerciale, que l'impédance de l'anode où a lieu l'electro-oxydation du méthanol est environ 20 fois plus petite que l'impédance de la cathode où a lieu l'electro-réduction du dioxygène. On montre aussi, pour un stack de PAC dihydrogène-dioxygène lui aussi commercial, que les quatre éléments de ce stack ne fonctionnent pas de manière identique selon leur position par rapport à l'arrivée des gaz.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Electrooxidation of substoichiometric titanium nitrides, TiN1−x, 0<x<0.5

We show that stable titanium oxynitride, TiN1−xOx, may be easily obtained by aqueous electrochemical oxidation of substoichiometric titanium nitride, TiN1−x, for all compositions x comprised between 0 and 0.5. This happens at lower potentials than oxidation into titanium dioxide. Hence, titanium oxynitride free from TiO2 may be synthesized. A two-step mechanism is proposed for the complete oxidation of the nitride into dioxide: first, the oxynitride is formed on the electrode surface by oxidation of nitrogen vacancies. Then, once formed, the oxynitride can be fully oxidized, to give the dioxide. Measurements of charges associated to voltammetric peaks corroborate this mechanism. Keywords: Titanium nitride, Titanium oxynitride, Substoichiometry, Nitrogen vacancies, Non-oxide ceramics, Electrochemical oxidatio

Electrochemical hydrogen storage in TiC0.6, not in TiC0.9

We demonstrate that hydrogen can be electrochemically loaded, at room temperature, in the non-oxide ceramic TiC0.6. A capacity of 2.9 wt.% is easily reached, corresponding to the formula TiC0.6H1.6. We also demonstrate that hydrogen absorption was not possible in TiC0.9. This surprising result is interpreted in terms of absence of long-range ordered carbon vacancies, contrary to TiC0.6. It seems that TiC0.6 behaves like an intercalation compound for hydrogen. Keywords: Titanium carbide, Non-oxide metallic ceramic, Electrochemical hydrogen storage, Substoichiometry, Long- and short-range ordered vacancie

High quality, substoichiometric titanium carbide (TiC~0.6) thin films grown on cost-efficient BK7 glass substrates

International audienc

Purification of hot-pressed ZrCO into ZrC by a laser treatment

International audiencePellets of zirconium oxycarbide ZrC0.82O0.14, obtained by hot pressing of zirconia and zirconium carbide, were irradiated by an ytterbium-doped fibre laser in argon atmosphere (incident power density = 24.7 kW cm-2, beam diameter 0.7 mm). The surface of the samples, heated at 3300 C and more, i.e. near the melting, released its oxygen, leading to the carbide ZrC0.75 despite the presence of traces of oxygen in the cell of treatment (PO2 estimated around 1 Pa). A mechanism is proposed for explaining this result, based on the thermodynamical stability of the carbide, higher at these temperature and oxygen pressure than the oxide. Oxygen of the oxycarbide evolved in the form of the gaseous species ZrO, while the grains of the oxycarbide, converted into the carbide, grew from 2 to 20 lm. Similarly, the traces of dioxygen present inside the treatment cell react with the carbide, giving ZrO (gas), and do not form any oxidised solid phase. This opens interesting future prospects in the field of the production of oxygen-free zirconium carbide powders

Surface densification of porous ZrC by a laser process

International audienceThin films of 100%-dense zirconium carbide, ZrC, on top of ca. 30%-porous ZrC are obtained by using an ytterbium-doped fibre laser under argon atmosphere. It is shown experimentally and theoretically that oxidation into zirconia or oxycarbide is completely avoided at the working temperature of at least 3420 ◦C. This type of highly refractory materials – dense at the surface and porous in the bulk – could be used in high temperature applications where both diffusion and ained by thermal carboreduction of oxides

Unusual electrochemical behaviour of AuBr4− in ionic liquids. Towards a simple recovery of gold( iii ) after extraction into an ionic liquid

International audienceThe electrochemistry of AuBr4− complexes extracted into ionic liquids [C8PYR][NTf2] or [C8MIM][NTf2] saturated with water and gas has been studied by cyclic voltammetry on a glassy carbon macro electrode and by linear voltammetry on a platinum microelectrode. Unlike AuCl4−, the reduction of AuBr4− to Au(0) is achieved following a one reduction step involving three electrons. The deposition of Au(0) from AuBr4− is carried out at a potential above that of water, leading to the simple and easy deposition of gold and subsequently to the extraction of AuBr4− into an ionic liquid

Surface densification of porous ZrB2-39 mol.% SiC ceramic composites by a laser process

International audience40% porous composites ZrB2-39 mol.% SiC were irradiated under a mobile laser beam, with a low power of 90W, in a protective atmosphere of flowing argon. Quasi-full densification of the surface zone was obtained, with thicknesses of more than 20 m. Temperature reached ca. 2500 ◦C and a fusible phase rich in SiC appeared, which partially dissolved the ZrB2, favouring its sintering. The liquid phase filled the porosity present between the ZrB2 grains, and migrated towards the surface and the bulk of the samples. After cooling, the liquid phase demixed, according to the ZrB2-SiC phase diagram, and the phases ZrB2 and SiC interweaved in a eutectic-like solid phase, co-existing with granular zones mainly composed of ZrB2. Only few cracks were observed. Traces of free carbon were found at the surface, while oxygen never penetrated inside the samples, despite the presence of traces of this element in the surrounding atmosphere. These two last observations were explained by a thermodynamic study. The pellets so obtained could find applications in the fields of aerospace and of low-temperature protonic ceramic fuel cells

Hydrogen insertion in titanium carbide based thin films (nc-TiC x /a-C:H) - comparison with bulk TiC x

International audienceNanocomposites composed of titanium carbide nanosized grains embedded in an amorphous hydrogenated carbon matrix (nc-TiCx/a-C:H) are prepared by hybrid Magnetron Sputtering - PECVD process using a titanium metal target and gaseous C6H6. By controlling the benzene flow rate, thin films with different carbon content are obtained. The structures of nc-TiCx/a-C:H materials are analyzed by X-ray diffraction, X-ray photoelectron and Raman spectroscopic methods. The electrochemical hydrogen insertion, as studied by cyclic voltammetry, strongly depends on the carbon content in the thin films. The correlation between the hydrogen insertion ability and the structure of materials are discussed. Furthermore, we show that the hydrogen insertion in these thin films reaches values much more significant than in bulk substoichiometric titanium carbide obtained by reactive sintering