A comparative study of the role of additive in the MgH2 vs. the LiBH4–MgH2 hydrogen
storage system

A. Fernándeza, E. Depreza, O. Friedrichsb

a

Instituto de Ciencia de Materiales de Sevilla, Avda. Américo Vespucio 49,
41092 Seville, Spain
b Empa, Swiss Federal Laboratories for Materials Testing and Research,
Hydrogen & Energy, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland
Abstract
The objective of the present work is the comparative study of the behaviour of the Nband Ti-based additives in the MgH2 single hydride and the MgH2 + 2LiBH4 reactive
hydride composite. The selected additives have been previously demonstrated to
significantly improve the sorption reaction kinetics in the corresponding materials. XRay Diffraction (XRD), X-Ray Absorption Spectroscopy (XAS), X-Ray Photoelectron
Spectroscopy (XPS) and Electron Microscopy (TEM) analysis were carried out for the
milled and cycled samples in absence or presence of the additives. It has been shown
that although the evolution of the oxidation state for both Nb- and Ti-species are
similar in both systems, the Nb additive is performing its activity at the surface while
the Ti active species migrate to the bulk. The Nb-based additive is forming pathways
that facilitate the diffusion of hydrogen through the diffusion barriers both in
desorption and absorption. For the Ti-based additive in the reactive hydride
composite, the active species are working in the bulk, enhancing the heterogeneous
nucleation of MgB2 phases during desorption and producing a distinct grain refinement
that favours both sorption kinetics. The results are discussed in regards to possible
kinetic models for both systems.
Keywords

Hydrogen storage; Magnesium hydride; Reactive hydride composites; Kinetic
improvement by additives
1. Introduction
Light metal hydrides are the favoured materials for solid hydrogen storage
applications. In particular magnesium hydride is a good candidate with a theoretical
gravimetric storage capacity of 7.6%. Its limitation for practical application lies mainly
in the slow hydrogen sorption kinetics and in the high thermodynamic stability of its
hydride [1]. The reversible reaction for hydrogen storage occurs according to following
reaction:
(1)
MgH2 ↔ Mg + H2, ΔH = −78 kJ/mol H2
Major progress toward technical application has been achieved by using magnesium
hydride in nanocrystalline form obtained by mechanical milling [2] and by using
transition-metal oxides during the milling process as additives [3], [4], [5] and [6]. This
has led to big improvements in the hydrogen sorption kinetics. In particular, Nb 2O5 has
been shown to behave as one of the best additives [5], [6], [7] and [8] at the present
time.
More recently, Vajo et al. [9] and [10] and Barkhordarian et al. [11] and [12]
introduced the concept of reactive hydride composite (RHC) combining MgH 2 and
borohydrides. As a representative example the reversible system with LiBH4 operates
according to the following reaction:
(2)
2LiBH4 + MgH2 ↔ 2LiH + MgB2 + 4H2, ΔH = −46 kJ/mol H2
In this system the chemical reaction between the two hydrides lowers the overall
reaction enthalpy while the gravimetric hydrogen storage capacity remains high
(10.5 wt%). However, although the reaction enthalpy is lowered, desorption and
absorption processes occur at high temperatures with a relatively slow two step kinetic
[13] and [14]. A significant improvement can be also observed in this system upon the
addition of transition-metal-based additives. In particular, Ti-isopropoxide has been
shown to behave as one of the best additives [13], [14], [15] and [16] at the present
time. In this optimized system working with Ti-based additives at temperatures above
the melting point of LiBH4, hydrogen storage capacities of around 8 wt% have been
repetitively found during several cycles [16]. Reaction (2) has been determined to
extensively occurring by X-ray diffraction analysis.
Interesting new results have been recently reported [17] and [18] in which the
LiBH4 + MgH2 RHC system has been activated by high energy ball milling at liquid
nitrogen temperature with addition of graphite. In these systems operation was
possible at temperatures below the melting point of LiBH4 although re-hydriding is
mainly due to MgH2. Therefore cycling at this operation temperature is dominated by
MgH2 and limited H2 storage capacity was found upon cycling. The presented study has
been therefore carried out at high temperature conditions were the concept and
operation of the RHC strategy by formation of MgB2 can be fulfilled for the purpose of
our investigations.
Several works have elucidated the role of the additives in the individual systems.
Chemical and microstructural effects have been proposed as the origin of kinetic
improvement. For pure MgH2 both Nb metal and Nb2O5 have been investigated [7],
[19] and [20]. The formation of ternary Nb-Mg-oxide phases upon milling and heating
has been proposed to be the key point for kinetic improvement [5], [6], [21] and [22].
In fact these phases are formed at the grain boundaries and migrate to the surface of
Mg/MgH2 grains opening pathways for hydrogen diffusion [5] and [6]. The formation of
short-lived metastable Nb-hydride phases was also proposed to improve kinetics for
hydrogen absorption and desorption inside the gateways during hydrogen diffusion
[23] and [24].
For the RHC system based on the MgH2–LiBH4 couple, several additives based on Ti
have been studied including TiCl3, TiF3 and Ti-isopropoxide [13], [14], [25], [26],
[27] and [28]. Particularly for the Ti-isopropoxide additive, it was proposed the
formation of dispersed TiB2 phases that favours the heterogeneous nucleation of the
MgB2 phases during desorption [15], [29] and [30].
In this work both systems, single hydride (SH) and RHC, will be compared to
understand the effect of each additive. The chemical state and changes during the
reaction as well as the microstructural distribution of the additives are investigated
and compared. Analogies and differences will be correlated with the actual proposed
kinetic models for the hydrogen sorption kinetics in both systems. It will be shown how
the adequate additive is operating for each system according to the corresponding
kinetic model.
2. Experimental
2.1. Samples preparation
For the RHC system the initial microcrystalline powders, LiBH4 (95% purity) and MgH2
(98% purity, the rest being Mg) and the titanium isopropoxide (99.995% purity) were
purchased from Alfa Aesar. MgH2 powder used in the MgH2/Nb2O5 system was
purchased from Goldschmidt AG with a purity of 95% (the rest being Mg). As milling
additive Nb2O5 micropowder was purchased from Sigma Aldrich with purity higher
than 99.99%.
Both samples (SH and RHC) were prepared by high energy ball milling which is the
most common procedure used to activate the hydride materials for hydrogen storage.
Samples of 2LiBH4 + MgH2 composites were prepared in a Spex 8000 mixer mill using a
ball (steel) to powder ratio of 10:1. The MgH2 was premilled for 5 h before being mixed
to LiBH4 (with or without 10 (or 5) mol% Ti-iso) for a further 5 h milling. The MgH2
samples were synthesized in a Fritsch P5 planetary mill also with 10:1 ball to powder
ratio. MgH2 was milled for 5 h and additionally milled (with and without 17 (or 10) wt%
additive) for an additional milling time of another 5 h.
To prepare the samples at different sorption stages, hydrogen cycling was performed
using a thermovolumetric Sieverts apparatus designed by Hydro Quebec/HERA
Hydrogen Storage System. For samples of 2LiBH4–MgH2, desorption reactions were
performed at 400 °C under 5 bar hydrogen (to achieve the RHC effect with formation
of MgB2[13] and [29]), whereas the absorption reactions were measured at 350 °C
under 50 bar hydrogen. For MgH2 system measurements were performed at 300 °C at
10 bar of hydrogen for absorption and 0.01 bar for desorption. The samples were
prepared and handled under continuously purified argon or nitrogen atmosphere in
gloveboxes.
2.2. X-ray diffraction
For the system 2LiBH4 + MgH2/Ti-iso XRD measurements were done in a powder
diffractometer Siemens D5000-D with a goniometer for transmission geometry and
carried out with Cu-Kα radiation. The samples were sealed in glass capillaries inside the
glovebox. Data acquisition was performed for 2θ angle range of 20–80°.
For the system MgH2/Nb2O5 XRD analysis was carried out in a Bruker D8 Advance X-ray
diffractometer for all “ex-situ” prepared samples. For “in situ” heating experiments a
Phillips X′Pert diffractometer was used operating with Cu-Kα radiation. This
diffractometer was equipped with a high temperature chamber, which was evacuated
during heating experiments to a vacuum of 1×10 −5 mbar. The applied heating ramp
consisted of a first heating to 140 °C followed by an X-ray measurement, and further
heating in steps of 25 °C up to the final temperature of 440 °C with measurement after
each heating step. A heating rate of 10 °C/min was used in all cases.
2.3. Transmission Electron Microscope
TEM analysis and electron diffraction (ED) was performed in a Philips CM200
microscope operating at 200 kV. Samples were diluted into toluene inside the
glovebox, dropped onto a copper grid and introduced into the microscope. A vacuum
gate valve allowed the sample to be isolated in a pre-chamber for complete
evaporation of toluene prior to transfer for analysis.
2.4. X-ray photoelectron spectroscopy
For the system 2LiBH4+MgH2/Ti-iso XPS spectra were recorded with an SPECS
Phoibos150 electron spectrometer and a Delay Line Detector in the 9 segmented
mode, using Al Kα radiation (1486.6 eV) in an ultrahigh vacuum chamber at a base
pressure of 6×10−10 mbar. Ti 2p core level XPS spectra were acquired with 20 eV pass
energy and 0.5 eV energy step at normal emission take-off angle. The binding energy
reference was taken as the main component of the C1s peak at 284.6 eV for a mixture
of adventitious carbon.
For the system MgH2/Nb2O5, X-ray photoelectron spectroscopy (XPS) was carried out
in a VG Escalab 210 spectrometer with a five-channel hemispheric analyzer for
photoelectron registration. The data acquisition took place in constant energy analyzer
mode with pass energy of 30 eV. The radiation used was Al Kα (1486.6 eV) from a non
monochromatic aluminum X-ray source. The binding energy calibration was performed
on the Mg2p peak with energy of 51.2 eV corresponding to a MgO or MgH2 phase
appearing at this binding energy.
Samples were deposited onto adhesive Cu substrates inside the glovebox and
transported in N2 atmosphere to the XPS spectrometer. In both systems samples have
been introduced in the spectrometers without being exposed to air.
2.5. X-ray absorption spectroscopy
XAS measurements for 2LiBH4+MgH2/Ti-iso system have been performed in
transmission mode at BM29 beamline at the European Synchrotron Radiation Facility
(ESRF, Grenoble, France). The fixed exit monochromator was equipped with two
Si(111) crystals and harmonic rejection was achieved by the use of Si double system
mirror. Spectra have been collected at Ti K-edge in the energy ranges 4850–6200 eV
under vacuum conditions at ambient temperature. TiB2, Ti2O3, TiO2 anatase and TiO
samples were purchased from Sigma Aldrich, to be used as a reference for XAS
measurements.
EXAFS (X-ray Absorption Fine Structure) and XANES (X-ray Absorption Near Edge
Structure) data processing has been carried out by the software ATHENA and
ARTHEMIS two interactive graphical utility based on the IFFEFIT library of numerical
and XAS algorithms. XANES data reduction has been performed subtracting the preedge background and normalizing the edge jump to one. The obtained EXAFS spectra
were analyzed by the non-linear least square fit algorithm implemented in ARTHEMIS
software using the phase shift φj(k) and backscattering amplitude functions, Fj(k),
calculated with the FEFF 6L code. To fit the spectra, the single scattering paths (SS)
corresponding to each coordination shell were used.
The characterization of MgH2/Nb2O5 samples was performed at beamline ID24 of the
European Synchrotron Radiation Facility (ESRF, Grenoble, France) using a setup that
combines angle-dispersive XRD and EDXAS. This arrangement enabled in situ
measurements during hydrogen sorption cycles. The XAS measurements on the Nb Kedge (18 986 eV) were carried out in transmission mode. Nb2O5, NbO and NbO2 were
purchased from Sigma Aldrich to be used as a reference for XANES analysis. These data
were collected on a phosphor marked FReLoN2k CCD camera with a readout time of
approximately 800 μs and using a time resolution of 0.5 s by averaging 100 spectra of
5 ms each.
3. Results
In a first study XRD was carried out for the SH and RHC with and without additives.
Fig. 1 shows the data for the samples after milling and after 1 cycle. Main peaks
correspond to the hydrides phases: MgH2 and LiBH4. Considering the Scherrer method
crystallite sizes could be evaluated and are summarized in Table 1. It is clear that the
milling process, with its introduction of defects and grain boundaries, is decreasing
strongly the crystallite size as compared to the as received materials. The effect is even
more visible in the presence of additives. After one cycle the crystallite size increases
strongly in both SH and RHC systems for the MgH2 phase, giving similar values for both
systems under repetitive cycling conditions. The presence of the additive is however
producing a significant refinement of crystallite size mainly for the LiBH 4 phase after
one cycle in the RHC system. The role of milling seems to be very important to achieve
a high dispersion of the additives in the samples [16] as similar kinetic where measured
in successive cycling.
In the case of the Ti-isopropoxide additive in the RHC system (Fig. 1c and d), no
indications are obtained from XRD regarding Ti-based phases in both as milled and
cycled samples. The only observation is an increase in Mg-oxide phases for the sample
2LiBH4+MgH2/Ti-iso after cycling due to the extra addition of oxygen in the form of Tiisopropoxide. Another technique is needed here to follow the evolution of the Ti-base
additive phases.
Regarding the additive in the SH sample, the Nb2O5 phase can however be detected in
the XRD pattern of the as milled material. The small peaks at 23 to 26 2θ angles
correspond to the 2 mol% (17 wt%) content of the additive in this sample. Cycling
produces the disappearance of the niobium oxide peaks indicating the reaction and/or
dispersion of the additive. New peaks appear now in the regions corresponding to the
magnesium oxide phase. This result is expected as we are introducing extra oxygen in
the sample through the addition of the additive. However, a close look to the new
peaks corresponding to magnesium oxide phases allowed us to detect the formation of
ternary Mg–Nb oxide phases in addition to pure Mg-oxide. Fig. 2 shows the evolution
of the XRD pattern during the de-hydrogenation of MgH2 with and without additive.
The presence of the new phase, identified as a ternary Mg-Nb-oxide is clearly observed
in the presence of additive [31]. This results shows the evolution of the additive to the
real active species which has been proposed to be a mixed NbMgxOy phase [31].
Fig. 3a and c shows the morphological TEM analysis of both SH and RHC samples as
prepared by ball milling in the hydrided state. The MgH2 big particles (0.1–1 μm size)
appear composed of nanocrystals of a few tens of nanometers separated by grain
boundaries in both samples. Fig. 3b is showing a high resolution TEM image of the pure
MgH2 sample after ball milling where nanocrystals of MgH2 can be identified as well as
the MgO surface oxidation layer and the grain boundaries. These results are in
agreement with the crystallite size reduction determined by the XRD peaks broadening
after ball milling. MgH2 particles appear therefore to be bigger than crystallites in the
initial as prepared materials for both samples. The selected area electron diffraction
pattern in Fig. 3c for the RHC sample also shows the diffuse rings characteristic of
MgH2 nanocrystals of a few tens of nanometer in size. In the case of the RHC the MgH 2
grains appear surrounded by LiBH4. The LiBH4 phase has been shown to be very
sensible to the electron beam irradiation. Similar morphology has been observed in
the presence or absence of the additive for the majority phases (MgH2 and LiBH4).
After cycling the crystallite size of MgH2 increases as shown in Fig. 3d for the cycled
RHC sample. In fact the effect can be clearly observed with dark particles in the size
range of 100 nm appearing now to be single crystals (see also Table 1 and the selected
area electron diffraction pattern in Fig. 3d).
No information for the phases of the additive was obtained in the reported TEM
analysis. Higher resolution facilities, including elemental mapping analysis, may be
employed. In addition the Nb phases appear highly dispersed after cycling and the Ti
phases were not detected by XRD. To obtain additional data it was necessary to follow
Ti and Nb K-edges in the XAS spectra, from which oxidation state and local structure is
obtained. EXAFS analysis was also possible in the case of titanium edge for these
samples.
The shape and edge position of both Nb and Ti K-edges are represented in Fig. 4 for
the near edge structure (XANES) region. The features can be interpreted as finger
prints being the shift to lower values in the absorption edge correlated with decreasing
oxidation states [31] and [32]. Comparison to reference compounds is undertaken.
In Fig. 4 can be seen that the Nb2O5 additive changes already during milling to more
reduced Nb species in the SH system. The evolution is complete after the first
desorption leading to a state more reduced than +4 for Nb in NbO 2 and slightly over +2
for Nb in NbO. Formally it is around +2.5 indicating a superposition of different states
[31]. The situation remains stable under further cycling.
In the case of the RHC the XANES analysis of Ti additive is very important as the XRD
analysis gives no information. Fig. 4 shows the data of the as prepared milled sample
together with data for samples desorbed. The results reveal that also in this case
irreversible changes in the oxidation state and local structure of the additive occur
during the initial stages of the first desorption. The spectrum of the milled sample
presents a first pre-edge at 4970 eV and comparable edge absorption position to the
fully oxidized Ti(IV). A significant shift to lower energy is observed for the desorbed
sample. Additionally the pre-edge observed with the original milled material is strongly
decreased. The Ti-species are clearly reduced upon heating although the metallic Ti
phase is not reached since the absorption edge is in an intermediate state between the
Ti(II) (TiB2 or TiO) and the Ti(III) (Ti2O3) oxidation states. This can be explained by a
mixture of different chemical states of Ti after cycling. The EXAFS analysis (see below)
will further support the latter hypothesis.
The FT data of the EXAFS oscillations at the Ti K-edge for the fully desorbed RHC
sample is represented in Fig. 5 in comparison to relevant references. The reduction
process from fully oxidized Ti(IV) in the ball milled sample as observed in the XANES
region (Fig. 4) can be also detected in the EXAFS data. In the case of FT data it is
possible to observe three peaks in the 0.5 to 2.2 Å region. The two first, located at
around 1.15 Å and 1.55 Å (without phase shift corrections), correspond to Ti–O bond
distances in the first coordination shell of the Ti2O3 reference material. The intensity of
these two peaks decreases progressively during first desorption indicating a further
reduction process. The third peak located at 1.9 Å (without phase shift correction),
corresponds to Ti–B distances in the first coordination shell of TiB2 and is progressively
increasing across this process. We therefore propose that the evolution of the Ti-iso
additive during desorption is leading to a final state which is mainly a mixture of Ti(III)
as Ti2O3 and Ti(II) as TiB2. The presence of remained TiO2 like phase cannot be fully
disregarded.
Both Nb-based and Ti-based additives start from a fully oxidized state (+5 and +4
respectively). Both evolve to a reduced state to form the actual active species, namely
reduced oxides and borides, which remain unchanged upon further cycling.
The XAS analysis has shown a similar concept of formation of reduced oxides and
borides as active phases for both SH and RHC systems. However, the surface analysis
by XPS of the additive phases lead to a different behaviour of the two kinds of
additives. According to Fig. 6 in the as prepared samples, Nb is not present at the
surface while there is a remarkable Ti signal. After desorption and absorption cycles
the reduced Nb species migrate from the bulk to the surface while the Ti compounds
are not detected anymore at the sample surface.
The XPS analysis is also giving the oxidation of the elements at the surface. The Ti
signal after ball milling correspond to an oxidized +4 state in agreement with previous
XANES data. In the case of Nb oxidized and partially reduced phases are found also in
agreement with the XANES data.
4. Discussion
According to the results presented above we propose a view of the microstructure of
the SH and RHC samples as prepared after ball milling (see Fig. 7). For the SH system it
is expected that the softer MgH2 phase is covering the harder Nb2O5 oxide crystals [33]
(no Nb signal detected at the surface by XPS). In the case of the RHC system, and
according to TEM results, the LiBH4 phase appears surrounding the MgH2 while the
liquid Ti-isopropoxide impregnates the solid and evolves during milling to an
amorphous, soft and highly dispersed Ti(IV) phase (Ti signal detected at the surface by
XPS).
Upon first desorption it has been shown that the reduced Nb species (ternary Mg-Nboxides) migrate to the surface opening pathways that facilitate the diffusion of
hydrogen through diffusion barriers (namely Mg hydride and oxide) [6] and [34]. A
schematic representation is included in Fig. 7. This situation is in fact in agreement
with a contracting volume kinetic model that has been previously proposed for the
Mg/MgH2 system [35]. Hydrogen diffuses from the surface to the bulk during charging
and from the bulk to the surface during de-charging. For Ti-based additives in complex
hydride materials like NaAlH4 (sodium alanate), the formation of active TiAl3 species on
the surface has been proposed as the origin of kinetic improvements in the presence
of the additive [36]. A contracting volume model is therefore operating at/from the
surface also in this case.
For the RHC system the first desorption occurs, under the present experimental
conditions without additive, in two steps [13]. First desorption of MgH2 occurs and
then desorption of LiBH4 starts (due to slower kinetics). The desired coupling of the
MgH2 decomposition to the LiBH4 desorption is a reaction that starts at the Mg/MgH2–
LiBH4 interface. This reaction is favoured in the presence of the additive by
enhancement, at the TiB2 sites, of the heterogeneous nucleation of the MgB2 product
during desorption. It should be emphasized here that the LiBH4 is in a liquid state at
the operation temperature. Therefore the H2 desorption reaction from LiBH4 and the
simultaneous formation of solid MgB2 occurs at the liquid–solid interface.
A heterogeneous nucleation kinetic model is therefore proposed for the desorption
process in the RHC system [30]. Already in the first step (see Fig. 4 for 1/3 desorbed
sample), the Ti-based additive is reduced to highly dispersed TiB2 (and reduced oxides)
and migrates to the bulk. This migration should be favoured by the presence of liquid
LiBH4. A schematic representation of the microstructure is included in Fig. 7. In fact
heterogeneous nucleation of MgB2 was demonstrated to be the limiting step of the
reaction rate in the RHC materials [30].
The re-absorption reaction is occurring in one step and needs again the reaction at the
interfaces between the two phases LiH and MgB2. Hydrogen needs however also to be
diffused from the surface and this process seems to occur with a contracting volume
kinetic model [30].
It is clear that additives operate according to the kinetic model playing their role to
improve kinetic of the rate limiting step. Crystallographic parameters are very
important to determine why an additive is adequate for a certain system. In fact for
the SH material the MgO and the NbO reduced oxide present the same NaCl structure
with very similar cell volumes and lattice constants. Therefore the formation of mixed
Mg-Nb ternary oxides is highly favoured. For the RHC system in order to favour the
heterogeneous nucleation of MgB2 it is necessary to have a lattice match with the
additive active species. This is in fact the case of TiB2 and other additives like ZrB2[29].
In both cases it is important to achieve a good dispersion of the additive, which favours
the formation of active phases in the form of nanoparticles/nanocrystals for improved
kinetics [16] and [37].
5. Conclusions
A comparative study has been presented of the evolution and behaviour of additives in
both a single hydride (MgH2/Nb2O5) and a reactive hydride composite
(2LiBH4 + MgH2/Ti-iso). It has been shown that in both cases the additive evolves from
a fully oxidized state (Nb2O5 and TiO2) to a reduced situation with formal oxidation
state near +2 by formation of NbMgxOy and TiB2 respectively. This evolution can occur
partially during ball milling and is completed after the first desorption leading to a
stable situation of the active phases of the additive.
These active species, in the form of dispersed nanoparticles, are working for kinetic
improvement according to different mechanisms. For the single hydride system a
contracting volume mechanism is operating and the additive is favouring the diffusion
of hydrogen from the surface to the bulk and from the bulk to the surface. Nb-based
species could be detected at the surface by XPS.
In the case of the reactive hydride composite, the desorption process is limited by the
heterogeneous nucleation of the MgB2 phase. In this case the additive is working at
the LiBH4–Mg interfaces in the bulk of the material where TiB2 species act as
nucleation sites for the formation of MgB2. Ti-based species could not be detected at
the surface by XPS. As the LiBH4 phase is liquid at the operation temperatures the
migration of solid TiB2 species to the LiBH4–Mg interface is favoured.
An important aspect is that crystallographic parameters are controlling the capacity of
each additive to improve kinetics. For the SH system the lattice match of NbO and
MgO favours the formation of ternary NbMgxOy phases that migrate to the surface
opening pathways for hydrogen diffusion at grain boundaries. For the RHC system the
lattice match of TiB2 and MgB2 allows the nanoparticles of titanium boride to act as
nucleation sites for the formation of MgB2 in the heterogeneous reaction of LiBH4 with
Mg.
The adequate additives are therefore adapted to each system. However, much work is
needed in order to understand each particular system in detail.
Acknowledgements
Authors thank the financial support from the European Marie Curie training networks
program (HPRN-CT-2002-00208 and MRTN-CT-2006-035366) and the collaboration
with the GKSS research centre. The help of the personnel at BM29 and ID24 in ESRF
are gratefully acknowledged. Financial support of the Spanish MICINN (CTQ200913440) and the “Junta de Andalucía” is also acknowledged.
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Table 1. Crystallite sizes calculated by Scherrer of MgH2, MgH2/Nb2O5,
2LiBH4+MgH2 and 2LiBH4+MgH2/5Ti-iso after milling and 1 cycle.
Peak
Samples
Crystallites sizes
(nm)
Samples
Crystallites
sizes (nm)
(200) MgH2
MgH2 as received
56.0
(310) LiBH4
LiBH4 as received
73.5
(200) MgH2
MgH2 milled
12.0
MgH2/Nb2O5
milled
7.6
(200) MgH2
MgH2 – 1 cycle
69.4
MgH2/Nb2O5 – 1
cycle
63.7
(200) MgH2
(310) LiBH4
25.5
2LiBH4+MgH2 milled
(200) MgH2
(310) LiBH4
40.8
58.9
2LiBH4+MgH2 – 1 cycle
39.1
2LiBH4+MgH2/5Ti- 9.8
iso milled
24.4
2LiBH4+MgH2/5Ti- 62.0
iso – 1 cycle
19.7
Figure captions
Figure 1. XRD pattern after ball milling and one cycle of MgH2 (a), MgH2/17 wt% Nb2O5 (b),
2LiBH4+MgH2 (c) and 2LiBH4+MgH2/5 mol% Ti-iso (d). Peak positions of the references
(NbMgxOy, monoclinic Nb2O5, hexagonal Mg, cubic MgO, tetragonal MgH2, hexagonal MgH2,
orthorhombic LiBH4 and anatase TiO2) are indicated above. ∗ Indicate the MgH2 (200) and
LiBH4 (310) peaks used for calculation of crystallite size in Table 1.
Figure 2. XRD pattern of MgH2 and MgH2/17 wt% Nb2O5 as a function of temperature.
Positions of MgO and NbMgxOy diffraction peaks are indicated by a dash line.
Figure 3. TEM micrographs of MgH2 after ball milling (a) and (b) and 2LiBH4+MgH2 after ball
milling (c) and after one cycle (d). The insets in (c) and (d) correspond to the selected area
electron diffraction pattern for the big particles with a darker contrast.
Figure 4. XANES data at the Nb (a) and Ti K-edge (b) for milled and desorbed samples (with
10 wt% Nb2O5 and 10 mol% Ti-iso respectively for SH and RHC system) as compared to
reference materials.
Figure 5. Fourier Transform curves of the EXAFS oscillations for 2LiBH4+MgH2/10 mol% Ti-iso
milled, 1/3 desorbed and fully desorbed as compared to reference materials (TiB2, TiO2, Ti2O3
and Ti).
Figure 6. Ti 2p photoemission peak in 2LiBH4+MgH2/5 mol% Ti-iso after ball milling, desorption
and 1 cycle and Nb 3d photoemission peak in MgH2/17 wt% Nb2O5 after milling, 1 cycle and 6
cycles.
Figure 7. Scheme of the microstructure of MgH2/Nb2O5 and 2LiBH4+MgH2/Ti-iso.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7