Didier Devilliers

Structure and composition of passive titanium oxide films

The thickness and composition of several kinds of titanium oxide films formed on a titanium substrate were determined by surface analysis techniques: X-ray photoelectron spectroscopy, Rutherford back scattering, X-ray diffraction and atomic force microscopy. Most titanium oxide samples were prepared by anodisation, using a galvanostatic procedure. The films were shown to be composed of an amorphous TiO2 outer layer (10–20 nm thick) and an intermediate TiOx layer, in contact with the TiO2 layer and the metallic substrate. The outer layer is sensitive to the environment: its thickness usually decreases with ageing in a corrosive solution. A stabilisation procedure was proposed in order to improve its ability to withstand corrosion.

Surface study of a titanium-based ceramic electrode material by X-ray photoelectron spectroscopy

The electrochemical behaviours of the Magneli phase titanium oxides with the general formula TixO2x-1 have been investigated. Surface analysis of these ceramic materials was performed using X-ray photoelectron spectroscopy in order to determine the surface composition. It was shown that the surface layers contain mainly TiIV. When these materials are used as an anode for oxygen evolution, in sulfuric acid, the XPS spectrum shows considerable modification in the O1s region, due to an important contribution of hydroxyl groups and the adsorption of sulfate anions.

Dielectric properties of anodic oxide films on tantalum

Abstract The electrochemical behaviour of Ta-TaO x structures was investigated by impedance spectroscopy. It was shown that, after the polishing treatment, the electrode is covered by an “initial” oxide film containing tantalum monoxide, TaO, and that “electrogenerated” oxides are only composed of tantalum pentoxide, Ta 2 O 5 (⩾ 97 mol%). However, the dielectric properties of the structures strongly depend on the thickness of the oxide, d ox , because of the influence of a sub-stoichiometric oxide of TaO in thin layers, in agreement with previous results obtained by XPS measurements. The variation of the reciprocal total capacitance of the oxide, C ox , with the quantity of electricity, Q a , involved in the formation of the oxide exhibits two linear parts. The breakdown of the slope of that curve is interpreted by a variation of the relative permittivity, e r , of the oxide with its thickness: for thin films ( d ox ⩽ 19nm), ϵ r ≈ 18.5 and for thick films ( d ox > 19 nm) ϵ r ≈ 27.5.

Electrochemical behaviour and surface analysis of crude and modified carbon electrodes for fluorine production

During the electrolytic preparation of elemental fluorine in molten KF-2HF using carbon anodes, the electronic transfer is usually inhibited by a passivating layer of graphite fluorides, CFx, at the surface of the electrode. Significant improvements may be obtained if a conducting ternary carbon-aluminium-fluorine compound, ie a graphite intercalation compound denoted G.I.C., is formed at the interface instead of graphite fluorides. “Modified” carbon electrodes were prepared from crude carbon samples by impregnation technique. Their electrochemical behaviour was studied by cyclic voltammetry and impedance measurements. The chemical composition of the surface was determined by ex situ XPS. It is shown that the electronic transfer is enhanced, so as the wettability of the electrodes by the melt. Thus, the anodic overvoltage is significantly decreased.

Kinetics of fluorine evolution reaction on carbon anodes: influence of the surface CF films

Abstract During the fluorine evolution reaction by electrolysis of molten KF–2HF, a thin solid fluoro-carbon layer is formed on carbon anodes. This film is mainly composed of fluorine–graphite intercalation compound and a small amount of insulating graphite fluorides. Impedance measurements performed in mercury and cyclic voltammetry studies in aqueous solution containing a redox couple have shown that the surface film behaves like an electronic conductor and cannot constitute a high energy barrier for the electron transfer in electrochemical reactions. However, the kinetics of the fluorine evolution reaction is strongly dependent on the water content in KF–2HF, indeed, water contributes to the formation of graphite fluorides which limit the fluorine evolution reaction. STM measurements performed on crude and passivated highly oriented pyrolitic graphite (HOPG) samples have pointed out the heterogeneities of composition of this carbon–fluorine film and the influence of water. It has been shown that, if the passivation of the carbon electrodes was performed in molten KF–2HF containing a high amount of water, the hexagonal symmetry of the images obtained with HOPG is lost.

Origin of the anodic overvoltage observed during fluorine evolution in KF-2HF

The kinetics of the fluorine evolution reaction was studied in molten KF-2HF with a horizontal disk electrode in a large potential window. A new model was proposed recently for representing the electrode/electrolyte interface; it includes the presence of a fluidized layer between the surface carbon-fluorine film (C-F) generated on the carbon anode during electrolysis and the fluorine gas film. This model was confirmed here to he consistent. The fluidized layer is composed of liquid KF-2HF melt and dissolved fluorine gas. With our electrode geometry, it was shown that the main electroactive surface area is located at the lateral side of the disk electrode. Finally, the contributions of the C-F film, η C+F . and of the fluidized layer. η fluid . to the total anodic overvoltage, η T , were studied using a numerical calculation method. It was shown that both contributions must he taken into account for a global understanding of the fluorine evolution process.

Effect of chemical fluorination of carbon anodes on their electrochemical properties in KF-2HF

Two kinds of carbon-fluorine surface layers on carbon electrodes were prepared and the electrochemical properties of these electrodes were studied in the KF-2HF melt. The first film was prepared in situ by electrochemical oxidation of carbon in KF-2HF during the fluorine evolution reaction. The surface layer is mainly composed of conducting graphite intercalation compounds, C x (HF) y F, usually denoted C x ,F. The second film was prepared ex situ by chemical fluorination of carbon with fluorine gas at 275 C, leading to a mixture of C x F and graphite fluorides, CF x . The fluorine evolution reaction is thus completely inhibited in KF-2HF XPS investigations have shown that ionic and covalent carbon-fluorine bonds are present in the C x F and CF x compounds, respectively. When doping aluminum particles were inserted into carbon before chemical fluorination, the fluorine evolution reaction occurred due to the presence of C x F-AlF 3 phases randomly scattered on the electrode surface and within the film. It is supposed that the electron transfer occurs either directly from the electrolyte to the substrate or by tunnel effect via intermediate states. These particles also facilitate fluorine bubble detachment from the surface of the electrode.

Electrolytic generation of fluorine with KF—2HF metls containing AlF3 or LiF

Abstract Cyclic voltammetry experiments have been performed in order to point out the effect of the addition of LiF or AlF 3 into KF—2HF melts on the behaviour of the anodic carbon material during the preparation of elemental fluorine. These additions are effective to reduce the passivation of the anode, only if colloidal suspension of the metallic fluoride are used. This effect is attributed to the formation, at the interface, of a conducting ternary compound similar to those which have been chemically synthesized by other authors. This compound is thought to enhance electron transfer at the interface and to improve the wettability of the electrode by the melt.

A Gas Electrode: Behavior of the Chlorine Injection Electrode in Fused Alkali Chlorides

The electroreduction of chlorine in fused alkali chlorides was studied by transient electrochemical techniques on graphite and carbon electrodes. Two types of experiments were carried out. First, on massive electrodes, the separate contributions of dissolved chlorine and of chlorine from bubbles at the electrode surface were detected. Second, injection electrodes were used to improve the efficiency of the reduction; the chlorine gas introduced inside the electrode flowed through the wall of the electrode. A model is presented to describe the behavior of this gas electrode

Electrochemical reduction of some graphite fluorides in propylene carbonate

Abstract The reduction of graphite fluorides has been carried out in propylene carbonate by the carbon paste electrode technique. Our results confirm the insertion mechanism of Li + ions during the reduction of CF x in lithium batteries. The shift of the reduction peak towards more negative potentials when LiClO 4 is replaced by nBu 4 NClO 4 as the supporting electrolyte is attributed to the difficulty of the bulky cation to intercalate in the host structure. In this paper, the electrochemical behaviour of several CF x samples is compared, in relation to their crystallinity and composition.