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The Time of Flight experiments on IN5
Part of the ToF measurement were performed on the spectrometer IN5B (ILLGrenoble) [46]. Each sample has been measured at T=293 K with a wavelength of 8 Å, using a cylindrical cell of thickness of 0.2 mm. This configuration allowed to probe the dynamics with an energy resolution around 10 eV on a wave vector range going from 0.005 to 1.45 Å1. This means that, with this spectrometer configuration, we could observe characteristic times included between 0.7 and 65 ps. At each raw spectrum was firstly subtracted the contribution of the empty cell. Then each signal was normalized to the spectrum of a cylindrical vanadium reference sample (measured in the same spectrometer configuration) for correcting the efficiency variation of the different detectors. Finally it was applied a Q grouping for improving the signal statistics and energy rebinning with a step of 0.01 meV.
The Backscattering measurements on IRIS
To probe the cationic dynamic occurring at tens of picoseconds time scale, we performed some measurements on the ToF spectrometer IRIS (ISIS-Didcot) [48]. We setted the instrument in a back-scattering (BS) configuration. This allowed to achieve a better energy resolution (E 17.5 eV ) than a classic ToF machine. We choose a wavelength equal to 1.1 Å which allowed to probe an energy range included between -0.5 and 0.5 meV and Q range coming from 0.48 to 1.07 Å1. All acquisitions were done at room temperature. Each spectra was normalized to the signal of vanadium sample for the correction of the efficiency of the detector. After that the spectra were subjected to Q-grouping for increasing the signal statistic.
The neutron spin-echo measurements on IN11
The spin-echo measurements were performed on the spectrometer IN11C (ILLGrenoble) [49]. For these measures we used a wavelength of 5.5 Å and an angle between the two procession coils equal to 20 and 50. This apparatus has a energy resolution much higher than a typical ToF machine (E 1 neV ), which allowed us to probea time scale coming from 10 to 1000 ps and the Q range included between 0.1 to 1.2 Å1.
We acquired each IL spectra at room temperature (T= 300 K). For correcting the signal for the spectrometer resolution (see section C.5), we divided the room temperature spectra for those measured on the sample at low temperature (T=2 K).
It is important to notice that with NSE we don’t measure the dynamical structure factor S(Q; !) as ToF or BS. In fact because of the specific characteristics of the technique (see appendix C), with NSE the measured quantity is the intermediate scattering function I(Q; t) which is the Time Fourier Transform of S(Q; !) [1]: I(Q; t) = Z +1 1 S(Q; !)ei!t d! (1.6) So in this case, the observed correlation function is defined in the space of the time and the wave vector. Before acquiring the sample scattering function, we performed a polarisation analysis for choosing which Q values to probe. In fact, how it is explained in section C, by NSE the sample scattering function is obtained measuring the polarisation of neutron beam after it interaction with the sample. So the presence of polarisation after the scattering event is a necessary condition for detecting a signal. Seen that the sample is not magnetic, the polarised neutrons may interact in two different ways with nuclei: they can scatter coherently or incoherently. The first scattering doesn’t cause a neutron spin-flip while the second one does. So this latter can be a detrimental effect because it could decrease the polarisation of the beam. Being the incoherent scattering an isotropic event, a 1 reversal which doesn’t change the spin direction. So if we define as SF the number of scattered neutrons which had a spin-flip, while with NSF the number of those which didn’t had it, these quantities will be given by the equations: NSF = Ncoh(Q) + 1 3 Ninc(Q).
Effects of the cation-anion couple on ILs dynamics
The first step of this work was the determination of the influence of the ionic couple on the ILs dynamics. We will show that the anion nature seems to have an important role on the ion dynamics at the local level.
For this purpose we studied the same four ILs that we have analysed by SANS. An example of the measured spectra (normalized to the sample mass) is shown in figure 1.14, where the samples are grouped by the cation. From the plot we can notice that for the same cation the spectra have the same intensity for null energy transfer S(Q; ! = 0). This latter is proportional to the sample scattering cross section (see appendix A), which, in the case of protonated samples can be approximated to its incoherent part. Since in the samples considered in this study the hydrogen is only in the cation, the incoherent cross section is approximated to the cationic one. This means that the systems with the same cation have the same cross section. When we consider the quasi-elastic signal (S(Q; !) with ! 6= 0) for the liquids in the same groups, it seems to have different width. This difference suggests the cation is not a key parameter to define the dynamical features of the samples. Another feature that we can observe from the comparison of the spectra in the figure 1.14, is that the width of the quasi-elastic signal seems to be narrower in the case of ILs which have the BF4 as anion. Since the characteristic time of the dynamics is proportional to the width of the quasi-elastic signal, we can conclude that for this latter group of liquids the dynamics is slower. This effect could be explained by delocalisation of the TFSI electric charge which causes a weakening of the coupling anion-cation inducing greater freedom to the cationic dynamics. This effect seems to agree with the results obtained by different NMR measurements performed on the same liquids. In fact if we use the values reported in the works of Tokuda [41,42] and Harris [50] for calculating the cation self-diffusion coefficient at T=293 K we find the following cationic self-diffusion coefficient: DOMIMTFSI = 9 108 cm2=s, DBMIMTFSI = 2.2 107 cm2=s, DOMIMBF4 = 2.6 108 cm2=s and DBMIMBF4 = 1.1 107 cm2=s, which show the same trend observed by our neutron study.
Table of contents :
Introduction
Why this thesis?
Ionic Liquids: a safe electrolyte, but not efficient enough
A promising route for better conductivity: nanometric confinement
Dimensionality: confinement of a molecular liquid in a solid matrix: from
powders to nano-pipes
1 Ionic liquids in the bulk state
1.1 Historical hints
1.2 Self-organization at the mesoscale
1.2.1 State of the art
1.2.1.1 Ions ordering
1.2.1.2 Self-organisation at the mesoscale
1.2.2 Self-organization: a SANS analysis
1.3 Dynamical properties of bulk ILs probed by QENS measurements .
1.3.1 State of the art
1.3.1.1 ILs dynamic at the molecular scale
1.3.1.2 Ion diffusion at the microscopic scale
1.3.2 Experimental part
1.3.2.1 Choice of experimental techniques
1.3.2.2 The measurements
1.3.2.2.1 The Time of Flight experiments on IN5 .
1.3.2.2.2 The Time of Flight experiments on LET .
1.3.2.2.3 The Backscattering measurements on IRIS
1.3.2.2.4 The neutron spin-echo measurements on IN11
1.3.3 Effects of the cation-anion couple on ILs dynamics
1.3.4 Study of the cationic diffusion OMIM-BF4 at the molecular scale: a phenomenological approach
1.3.5 Characterization of the cationic dynamics at the molecular scale using a combination of QENS techniques: the case of the OMIM-BF4
1.3.5.1 Point of the departure: the dynamics at the picoseconds scale probed by ToF spectroscopy
1.3.5.2 A new multi-components model
1.3.5.2.1 Modelling of the cation diffusion: the generalised
Gaussian model
1.3.5.2.2 Modelling the molecular re-orientation.
1.3.5.2.3 The total dynamical structure factor
1.3.5.3 Analysis procedure
1.3.5.4 Results and discussion
1.3.5.4.1 Re-orientation dynamics
1.3.5.4.2 Diffusion inside the aggregate
1.3.5.4.3 Long range diffusion
1.3.6 Test of model robustness with the selective deuteration: the case of the BMIM-TFSI
1.3.6.1 Re-orientation dynamics
1.3.6.2 Diffusion inside the aggregate
1.3.6.3 Long range diffusion
1.3.7 Conclusions and comparison with the previous results
2 Properties of the ionic liquids confined in anodic aluminium oxide membranes
2.1 State of the art on the ILs confinement: the ionogels
2.2 The porous anodic aluminium oxide membranes
2.2.1 The AAO synthesis
2.2.2 The AAO morphology
2.2.3 Choice and preparation of the sample
2.2.4 Check of the IL confinement by contrast variation
2.3 ILs under confinement: thermodynamical aspects
2.3.1 A brief introduction about the ILs thermodynamical properties
2.3.1.1 Bulk IL phase diagram
2.3.1.2 Confinement effects on the ILs phase transition .
2.3.2 The case of the confined BMIM-TFSI: a DSC study
2.3.2.1 A double glass transition in the confined state
2.3.2.2 Crystallisation temperature
2.3.2.3 Melting point depression
2.4 Confinement effect on the IL self-organisation behaviour by WAXS
2.4.1 Introduction: Surface effects
The surface effects
2.4.2 Experimental part
2.4.3 The liquid structure factor determination
2.4.4 Phenomenological analysis
2.4.5 Conclusion
2.5 Confinement effect on the ILs dynamics at molecular level
2.5.1 Confinement effects on the ILs dynamics in the literature .
2.5.2 Experimental part
2.5.3 Characterization of the confined IL dynamical behaviour .
2.5.3.1 Long range diffusion
2.5.3.1.1 NSE vs BS scenarios
2.5.3.1.2 Results
2.5.3.2 Diffusion inside the aggregate
2.5.3.3 The re-orientation dynamics
2.5.4 Conclusions
2.6 The IL leaks: a NMR 1D tomography study
2.6.1 The NMR 1-D tomography
2.6.1.1 The basis of the NMR tomography
2.6.1.2 The instrument and its calibration
2.6.2 Results
3 Properties of the ionic liquids confined in carbon nanotubes membranes
3.1 Sample preparation
3.1.1 What are carbon nanotubes?
3.1.2 Experimental protocol for the CNTs membranes production
3.1.2.1 Carbon nanotubes synthesis
3.1.2.2 The membrane formation
3.1.2.3 The membrane opening
3.1.3 Membrane filling
3.2 Dynamical effect of the confinement
3.2.1 Cationic dynamics at molecular scale probed by QENS
3.2.2 Ionic conductivity probed by impedance spectroscopy
3.2.3 Ions dynamics probed by NMR spectroscopy
Conclusions and perspective
A The neutron scattering theory (from [1, 2])
A.1 Fundamental quantities in a scattering experiment
A.2 Van Hove formalism
A.3 Single particle dynamics probed by incoherent quasi-elastic scattering
A.4 Quasi-elastic neutron scattering analysis: model for the atomic motio
A.4.1 Translational motion model
A.4.1.1 Free diffusion model (Fick’s law)
A.4.1.2 Jump diffusion model
A.4.2 Rotation motion models
A.4.2.1 Jump model between two equivalent sites
A.4.2.2 Jump model between three equivalent sites
B Time of flight technique
B.1 ToF spectrometer
B.2 Direct geometry ToF spectrometer
C Neutron spin-echo spectroscopy (from [3]) 163
C.1 The spin dynamics in a magnetic field
C.2 The spin-echo principle
C.3 The transmission of a analyser
C.4 Determination of the beam polarisation from a spin-echo measure .
C.5 The case of quasi-elastic neutron scattering measure
D Ion pairs and the concept of ionicity
E ILs applications
E.1 ILs as green solvents
E.2 Energy management by electrochemical devices
Energy management by electrochemical devices
F Résumé en français
Bibliography
