the ionic mobility of these materials was tested. Here it was shown that the ionic mobility follows the trends of the hydrogen uptake/release properties of these materials with the iodides having the highest mobility, the lowest temperature of hydrogen release and the most favourable conditions for hydrogen uptake.
4.7
Discussions and further work
The work discussed in these first two results chapters was undertaken to investi- gate how the incorporation of halide anions within the lithium amide and imide structures affects the hydrogen storage properties of this system. It is there- fore noteworthy that all of the amide-halide materials released hydrogen at lower temperatures than lithium amide; while the imide-halide materials hydrogenated more quickly than lithium imide. It should also be noted that this work has shown a potential link between the speed of hydrogen release/uptake and the ionic mobility of a material.
It is also seen that with reaction of LiH or MgH2 there was a significant
decrease in the amount of NH3 compared to LiNH2. It has been seen previously
that magnesium halides can form stable ammoniate complexes which could trap any released NH3 [114, 115]. However, it is seen in this work that the lithium
only containing systems performed better in this respect; this suggests a different explanation. A possibility is that phases exhibit improved ion diffusion, favouring a topotactic mechanism [52], rather than one mediated by NH3. This would in
fact be the reverse of Li2NH hydrogenation which is assumed does not involve
NH3.
It cannot of course be ignored that any improvements in the hydrogen re- lease/uptake properties through the addition of a halide anion are gained at the cost of the gravimetric storage capacity. Figure 4.36 shows the gravimetric penalty for the introduction of halide ions in the amide system with both LiH
4.7. DISCUSSIONS AND FURTHER WORK 149 and MgH2. The arrows show the stoichiometric compositions studied in this
work. In this work the lower solid solution limits of the different materials were investigated. It can be seen from figure 4.36 that relatively small changes in the stoichiometry of these materials can have a significant effect on the gravimetric capacity of the system.
Figure 4.36: Gravimetric penalty for introducing halide anions into the lithium amide system upon reaction with LiH (full line) and MgH2(dashed line). Arrows
indicate the compositions investigated in this study.
This work has shown the potential for amide- and imide-halides as good hy- drogen storage materials, however, further work could be done to fully understand the mechanism for hydrogen release and uptake. Several of the TPD-MS traces show multiple hydrogen release peaks, suggesting a multistep desorption process. Deconvolution of these TPD-MS traces through anex-situX-ray diffraction study could provide important information on the steps in these decomposition path- ways. Along with this an in depth IGA study could be undertaken to attempt to understand the hydrogenation process. While this work has shown a link between the hydrogen uptake and release properties of a material and its ionic mobility, a further, indepth conductivity study would be required to confirm and understand this link. Finally, while not in the remit of this study, work could be carried out into the optimisation of these materials for hydrogen storage. Im-
4.7. DISCUSSIONS AND FURTHER WORK 150 provements could be made in the kinetics of both hydrogen release and uptake through nanostructuring of these materials or through the use of catalysts.
Chapter 5
Reactions of LiBH
4
with
LiNH
2
, Li
2
NH and Li
3
N
5.1
Introduction
In the Li-N-H system Li2NH acts as an intermediate between LiNH2 and Li3N
as shown in equation 3.1. This makes the system more readily reversible as it is possible to cycle between LiNH2 and Li2NH without going to Li3N which has
less favourable thermodynamics for desorption and slow hydrogenation kinetics. Reaction of LiBH4 with LiNH2 is known to form pure phases at ratios of 1:1
and 1:3; the products from these reactions are Li2BH4NH2[72] and Li4BH4(NH2)3[73],
respectively. In the Li-B-N-H system, unlike the Li-N-H system, there are no known hydrogen deficient intermediates and the decomposition product of Li3BN2
(equation 5.1) is currently known to have limited reversibility[74]. Li3BN2 is
known to form three polymorphs, the P21/c[116] and P42212[117] polymorphs,
which can be formed from Li3N and BN, and theI41/amd[118] polymorph formed
from milled LiBH4 and LiNH2.
LiBH4+ 3LiNH2Li4BH4(NH2)3 Li3BN2+ LiNH2+ 4H2↑ (5.1)
5.2. EXPERIMENTAL 152 In this section I have investigated reactions of LiBH4 with LiNH2 at varying
stoichiometries and temperatures, and studied the decomposition properties and products. The reactions of LiBH4with Li2NH and Li3N were also be investigated
to look at these previously unstudied parts of the system in the hope of finding new intermediates of interest.
5.2
Experimental
Samples of ground mixtures of lithium borohydride (Sigma-Aldrich, 95% purity) and either lithium amide (Sigma-Aldrich, 95% purity), lithium imide (made from lithium amide and lithium hydride) or lithium nitride (Sigma-Aldrich, 95% pu- rity) were prepared in an argon filled glove box (<10 ppm O2, <1 ppm H2O).
The samples were prepared in various stoichiometries and heated in quartz tubes under flowing argon at temperatures up to 600◦C for up to 12 hours.
Powder X-ray diffraction data were collected using a Bruker D8 instrument in transmission geometry with a wavelength of 1.54059 ˚A (chapter 2.4.1). Raman spectroscopy, scanning electron microscopy (SEM) and temperature programmed desorption coupled to a mass spectrometer (TPD-MS) were also performed on the samples (chapter 2).
5.3
xLiBH
4+ yLiNH
25.3.1 Powder X-ray Diffraction
The powder diffraction data were analysed using the computer programme Topas[108]. A zero point error and an 18 point Chebyshev background were refined for each diffraction pattern. For each of the known phases present, lattice parameters, along with a pseudo-Voigt peak shape were refined. Where available, atomic po- sitions and thermal parameters were taken from reported structures and a full Rietveld refinement was performed on the structures. The estimated weight per-
5.3. XLiBH4+YLiNH2 153