F. Henn

ELECTROCHEMICAL INTERCALATION OF LITHIUM INTO MULTIWALL CARBON NANOTUBES

Abstract Electrochemical intercalation of lithium into carbon electrodes containing multiwall nanotubes produced by the arc-electric technique has been carried out in button cells. X-ray diffraction (XRD) and 7 Li NMR spectroscopy coupled with electrochemical characterisation allow us to study the structural modifications induced by the intercalation of the alkali metal between the graphene shells and the nature of interactions between lithium species and the host material. Furthermore, direct observation of individual nanotubes by transmission electron microscopy (TEM) reveals that while the lithiated nanotubes are intercalated very inhomogeneously, the electrochemical intercalation process seems to be reversible and to proceed through the graphene walls via structural defects.

X-ray diffraction and micro-Raman spectroscopy analysis of new nickel hydroxide obtained by electrodialysis

Abstract Industrial nickel hydroxide samples produced by electrodialysis have been characterized both by X-ray diffraction (XRD) and by micro-Raman spectroscopy. A comparison with some commercial products of β-type Ni(OH) 2 is made. Aged in KOH solution, these nickel hydroxides are transformed to the β-Ni(OH) 2 phase and undergo Ostwald ripening, leading to crystallite sizes ranging between 260 and 380 A with a reduction in defects. The anisotropic broadening of the XRD peaks in the β-Ni(OH) 2 pattern is interpreted in terms of crystalline lattice micro-strains and crystallite size. For the first time, Rietveld refinement has been performed on industrial Ni(OH) 2 . The results indicate that the micro-strain effects increase the peak width according to the sequence of families ( hk 0)→(00 l )→( h 0 l ). This study shows that the presence of proton vacancies leads to a lower degree of distortion in the crystalline lattice than does the presence of stacking faults or the adsorption of inorganic species at the border of the crystallites. A very high concentration of point defects inducing strong distortions in the crystal lattice is correlated with the presence of high energy Ni–O bond vibrations in the Raman spectra, with a frequency shift towards unusually high values.

Electrochemical lithium intercalation into multiwall carbon nanotubes : a micro-Raman study

Abstract The electrochemical intercalation of lithium into carbon electrodes containing multiwall carbon nanotubes produced by electric arc technique was carried out in button cells in different electrolytes. An exfoliation of graphene layers was observed with the electrolyte LiPF 6 (1M) dissolved in ethylene carbonate (EC), propylene carbonate (PC) and dimethyl carbonate (DMC) (1:1:3 by volume). Raman spectra were recorded to elucidate the lithium intercalation mechanisms of multiwall nanotubes. The spectral changes of the Raman E 2g band showed that the lithium was intercalated between graphene layers of carbon nanotubes without the formation of n -staged phases with n higher than 2 in contrast to the intercalation into graphite which proceeds via the formation of staged graphite intercalation compounds.

Analysis of thermally stimulated currents measured on ionic conductors

We propose a new method for analyzing the experimental results obtained from thermally stimulated depolarization current experiments on ionic conductors. This method is based on a model developed to explain polarization in these materials. The fundamental assumptions about the elementary mechanisms responsible for polarization is assumed to be due to the “free” volume created by thermal fluctuations occurring in solids near the equilibrium state. The consequence of these fluctuations is a statistical distribution of the relaxation times or energies, which can be evidenced by the thermal windowing technique, and then analyzed by relaxation map analysis. Applied to two typical ionic compounds, the method described in this work allows us to determine more realistic values of energy and τ0, corresponding to the measured relaxation times.

Discrimination between dipolar and space-charge relaxation by thermally stimulated current spectroscopy: application to several alkali-exchanged mordenites

Thermally stimulated current (TSC) spectroscopy is widely used to understand dielectric relaxation in complex solid systems. However, when these systems contain free and bound charges, it appears that TSC spectra exhibit multicomponent signals, due both to dipolar and to space-charge relaxation. In this work, we propose an experimental method to identify clearly and then separate the space-charge contribution from the dipolar current, in ionically conducting solids. This method is based on the comparison between global and fractional thermally stimulated current procedures and is applied to analyse the dielectric response of several alkali-exchanged mordenites (with exchange of , and for ).

Conductivity and Dielectric Relaxation in Various Ni ( OH ) 2 Samples

Conductivity and dielectric relaxation are measured on several uncycled Ni(OH) 2 powders whose composition and degree of crystallization differ. The role played by water molecules intercalated in the a motifs of the interstratified structure is pointed out. Further, it is shown that coprecipitated dopants such as Co(II) and Zn(II) lower conductivity. Finally, a schematic model based on protonic conduction is proposed to account for the electrical and dielectrical properties of these solids.

Estimation of the breathing energy of flexible MOFs by combining TGA and DSC techniques.

The energetics of the breathing phenomenon of MIL-53(Cr) have been estimated through a combined TGA and DSC water desorption analysis of the rigid MIL-47(V) and flexible MIL-53(Cr) and MIL-53(Fe) systems.

WATER AND ETHANOL DESORPTION IN THE FLEXIBLE METAL ORGANIC FRAMEWORKS, MIL-53 (Cr, Fe), INVESTIGATED BY COMPLEX IMPEDANCE SPECTROCOPY AND DENSITY FUNCTIONAL THEORY CALCULATIONS

The breathing behaviour of MIL-53(Cr) and MIL-53(Fe) upon water and ethanol desorption has been investigated by combining complementary experimental techniques including ThermoGravimetry Analysis (TGA), Differential Scanning Calorimetry (DSC) and Complex Impedance Spectroscopy (CIS). It was shown that two stages of solvent departure are involved in the desorption process, as revealed by (i) a change of the weight loss gradient in the TGA curve, (ii) the existence of a second endothermic peak in the DSC signal and (iii) a sudden drop and/or profile change of the ac conductivity in CIS. All these features are observed around a typical temperature T(c), for which the framework contractions, caused by the solvent desorption, occur. Moreover, it is shown that these modifications are more pronounced when the magnitude of the breathing is higher, as illustrated by the comparison of the water/MIL-53(Cr), ethanol/MIL-53(Cr) and water/MIL-53(Fe) systems. CIS data were further analyzed in the light of DFT calculations which provided the preferential arrangements of the molecules within the pores and the resulting host/guest interactions. It could then be proposed that (i) the polarization conductivity results from the local re-orientation of the μ(2)-OH dipoles bonded to the metal atom from the hybrid solid, i.e. Fe or Cr, and (ii) that dc conductivity, which can be ascribed to a proton propagation via a Grotthus mechanism, is favoured when the solvent molecules form strong hydrogen bonds between each other.

Complex permittivity in ionically conducting solids : a hopping model

Abstract Using both the correlated barrier hopping model accounting for the dielectric losses and the Anderson and Stuart model describing the potential barrier in ionically conducting glasses, the calculation of the dielectric processes in ionic solids is extended to the complex permittivity and the results are compared with the behaviour described by the empirical Cole-Davidson function. Experimental results for the ac conductivity obtained on Na 2 O-3SiO 2 glass are fitted by the theoretical behaviour predicted by this model.

COOPERATIVE OR NONCOOPERATIVE DIELECTRIC RELAXATION IN IONIC SOLIDS : A DISCRIMINATING EXPERIMENTAL APPROACH

Calculations, confirmed by experimental evidence on a polymer and on two ionically conducting solids, show that the variation of thermally stimulated depolarization currents (TSDC) with the heating rate depends on the polarization decay function that is assumed. This permits discrimination between models usually proposed for describing the non-Debye behavior of low-frequency dielectric relaxation in solids. It is also demonstrated that consideration of the kinetics involved in nonequilibrium TSDC spectroscopy is an essential, and complementary, experimental tool for dielectric measurements carried out under isothermal conditions in the frequency domain in establishing models of relaxation.