F. Lantelme

Synthesis and Structural Characterization of Carbon Powder by Electrolytic Reduction of Molten Li2 CO 3 ­ Na2 CO 3 ­ K 2 CO 3

A new method was proposed for the preparation of carbon powders based on the electrochemical reduction of a fused eutectic mixture of lithium-sodium-potassium carbonates at 450°C. Transmission electron microscopy observations have revealed the presence of three forms of carbon in the powder samples depending on the deposition potential values: amorphous carbon, graphite, and fibers. The presence of graphite and amorphous carbon was confirmed by X-ray diffraction measurements. The d 002 values were rather constant whatever the sample and close to 0.34 nm. Moreover, surface analyses by X-ray photoelectron spectroscopy revealed the presence of nearly metallic and ionic lithium belonging to lithium oxides. The specific areas were measured by the Brunaucr-Emmett-Teller (BET) method. Whatever the deposit potential values, the total BET specific surface area is much higher than that corresponding to external one because of the pore walls contribution. It has been shown that, for carbon powder dried at 400°C under vacuum, the specific surface area decreased as the deposition potential became more cathodic: from about 850 m 2 g -1 at -2.4 V vs. CO 2 -O 2 to about 500 m 2 g 1 at -6.0 V vs. CO 2 -O 2 . With increasing negative values of the potential, the nucleation and the growth of carbon competes with the formation of nearly metallic lithium and/or lithium oxides. The presence of the latter induces the closure of some nanopores.

Internal reference systems for fused electrolytes

The transient oxidation of the electrode material or a constituent of the electrolyte has been used to generate an internal reference system. This technique is convenient for standardizing the reference electrodes used in fused electrolytes at high temperatures. Applications are described for the model case of fused NaCl + KCl, where the internal reference system is either the couple Pt2+/Pt (platinum working electrode) or the couple Cl2/Cl− (oxidation of the constitutive Cl− ions at a carbon electrode). The standard potentials of the internal systems are determined from cyclic voltammetry.

Study of the Electrochemical Reduction of Zr4 + Ions in Molten Alkali Fluorides

The electrochemical behavior of Zr 4+ in potassium-free fused alkali fluorides has been studied in the temperature range of 773-1123 K. With an inert tungsten or molybdenum electrode, a well-defined oxydo-reduction wave was observed during cyclic voltammetry experiments. The reduction of Zr 4+ is a single four-electron reversible step, and the process is diffusion controlled. Chronopotentiometry measurements give rise to the diffusion coefficient values of Zr 4+ ions. For instance, a value 2.92 X 10 -5 cm 2 s -1 was found at 1010 K. The reduction of Zr 4+ ions was also studied using a nickel electrode for which Ni-Zr alloys formation was pointed out as revealed by cyclic voltammetry. The analysis of these deposits by scanning electron microscopy (SEM) and energy dispersive X-ray showed the presence of a multilayered Ni-Zr alloy with different Zr contents. On graphite electrodes, zirconium carbides have been evidenced on cyclic voltammograms and the formation of Zr metal was observed by SEM.

Study of the concentration-dependent diffusion in lithium batteries

Abstract A mathematical analysis of the diffusion process is carried out to examine the validity of the classical treatment when the diffusion coefficient depends on the concentration. Investigations concerning the analysis of intercalation processes by transient electrochemical techniques are examined. A digital simulation programme is used to obtain an accurate description of the transport mechanism. Examples of lithium intercalation in a Nb 2 O 5 matrix are given.

Electrodeposition of niobium from fluoroniobate K2NbF7 solutions in fused NaCl-KCl

Abstract The mechanism of the electrodeposition of niobium from fluoroniobate solution in molten NaCl-KCl at 720 °C was studied by voltammetry and chronopotentiometry. It was shown that the reaction proceeds in two steps: first a reversible redox reaction, Nb(V)+e↔Nb(IV), followed by a quasi-reversible reduction, Nb(IV)+4e→Nb(0). The standard potentials of the redox couples Nb(V)/Nb(IV) and Nb(IV)/Nb(0) are E 5 4 0 =0.394 V and E 4 0 0 = −0.178 V vs. the Ni/Ni2+ reference electrode. The diffusion coefficients of the two electroactive species are DNb(V)=1.83×10−5 cm2 s−1 and DNb(IV)=2.34×10−5 cm2 s−1. On a nickel electrode the electrodeposition of well-crystallized metallic niobium occurred at a potential of −0.5 V vs. Ni/Ni2+. Better adherence and cohesion were achieved when pre-reduced solutions of niobium(IV) were used. At potentials lower than −0.7 V, a black powder was obtained. X-ray analysis indicated the presence of metallic niobium and of a new phase containing potassium and niobium. In solutions containing niobium(IV) a thin layer of niobium carbide formed spontaneously on carbon. Voltammetric investigations indicated that the formation of niobium carbide occurred before the metal deposition; the redox reaction corresponding to the formation of the niobium carbide was found to be effectively irreversible.

Electrochemical Behavior of Solutions of Niobium Chlorides in Fused Alkali Chlorides

The electrochemical properties of solutions of niobium chlorides in the fused eutectic LiCl-KCl were studied by transient electrochemical techniques. The fundamental role of temperature was determined. At temperatures ranging from 380-600 o C, two reversible redox reactions were detected: Nb(V)+e↔Nb(IV), and Nb(IV)+e↔Nb(III). Reduction of Nb(III) led to metallic niobium, however the deposition was perturbed by the formation of insoluble non-stoichiometric niobium subhalides (mean oxidation state between 2 and 3) at the electrode surface

Electrochemical deposition of niobium on vitreous carbon and silicon carbide fibres in fused alkali chlorides

The electrodeposition of metallic niobium on the surface of a carbon electrode in fused electrolytes is studied by transient techniques. The initial step of the process involves the formation of a thin layer of carbide. The growth of this layer is controlled by niobium-carbon interdiffusion. The transport process is shown to be more rapid in the initial atomic layers. This technique was used to examine niobium deposition on silicon carbide fibres coated with amorphous carbon.

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.

Characteristics of the Fluorocarbon Surface Film Generated on Carbon Anode during F2 Evolution in Molten KF-2HF

During the fluorine evolution reaction occurring in the electrolysis of molten KF-2HF, a thin, solid fluorocarbon layer is formed on the carbon anode. The high anodic overvoltage which characterized this process is mainly due to the presence of such a film which drastically changes the wettability of the electrode by the melt. Activation of carbon at 40 V in KF-2HF modifies strongly the surface morphology (scanning electron microscopy) and composition (energy-dispersive X-ray and X-ray photoelectron spectroscopy) of the surface fluorocarbon film. In addition, it enhances the electrochemical performances of the carbon electrodes vs fluorine evolution. Local microconductivity measurements carried out by atomic force microscopy have revealed heterogeneities of composition of the carbon-fluorine film due to the presence of conducting fluorine-graphite intercalation compounds and insulating graphite fluorides (CF x ) within the film. Finally, the peculiar shape of the fluorine bubbles observed during electrolysis of molten KF-2HF was studied, taking into account the presence of these two fluorocarbon compounds on the surface.