Hydrogen storage in 2NaBH4 + MgH2 mixtures: Destabilization by additives and nanoconfinement

Article abstract of DOI:10.1016/j.jallcom.2011.12.042

We focus on the H₂ desorption properties of the 2NaBH ₄ + MgH₂ system destabilized by different methods. Nanostructured powder mixtures were prepared by ball milling the starting hydrides and nanoconfined reactive composit

Full text of DOI:10.1016/j.jallcom.2011.12.042

Journal of Alloys and Compounds 536S (2012) S236–S240
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Hydrogen storage in 2NaBH + MgH mixtures: Destabilization by additives and
4
2
nanoconfinement
a,∗
b
a
a
c
b
d
G. Mulas , R. Campesi , S. Garroni , E. Napolitano , C. Milanese , F. Dolci , E. Pellicer ,
d
c
D. Baró , A. Marini
Dipartimento di Chimica, Università di Sassari and INSTM, Via Vienna 2, I-07100 Sassari, Italy
JRC-IE, Westernduinweg 3, 1755 ZG Petten, The Netherlands
Pavia H2 Lab, C.S.G.I. & Dipartimento di Chimica, Sezione di Chimica Fisica, Università di Pavia, Viale Taramelli 16, I-27100 Pavia, Italy
Departament de Física, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Spain
a
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 July 2011
Received in revised form 6 December 2011
Accepted 12 December 2011
Available online 21 December 2011
We focus on the H desorption properties of the 2NaBH + MgH system destabilized by different methods.
2
4
2
Nanostructured powder mixtures were prepared by ball milling the starting hydrides and nanoconfined
reactive composites were obtained by melting infiltration of the hydrides into a Si-based SBA-15 support.
NbF5 was tested as catalyst in both the preparations.
Structural characterization by X Ray Diffraction and Transmission Electron Microscopy allowed
evaluating the successful synthesis of SBA15 matrix, the microstructural features of ball milled and
nanoconfined hydrides as well as the success of infiltration process. The evaluation of the sorption
properties, by manometric Sievert-type apparatus and thermal desorption spectroscopy, revealed the
efficiency of the hydride destabilization, obtained by the different routes, in decreasing the hydrogen
release temperature and improving desorption kinetics.
Keywords:
Hydrogen absorbing materials
Metal hydrides
Mechanochemical processing
nanostructured materials
Transmission electron microscopy (TEM)
X-Ray diffraction
© 2012 Elsevier B.V. All rights reserved.
1
. Introduction
Solid state hydrogen storage in light-weight materials repre-
NaBH –MgH2 multi-component system shows a high gravimet-
ric capacity, high volumetric hydrogen density and a rather low
4
dehydrogenation temperature compared to the single borohydride
component [6]. Recently, Garroni et al. proved that the addition of
MgH2 to NaBH4 contributes to decrease the desorption tempera-
sents both a scientific and technological challenge [1,2]. Among
these materials, complex borohydrides have been extensively stud-
ied as potential hydrogen storage systems for on-board application
in hydrogen-fueled cars, thanks to both their very high gravimet-
ric and volumetric hydrogen density [3]. In particular, NaBH4 is
an interesting material due to its hydrogen gravimetric and vol-
◦
ture of the complex borohydride to 450 C, and partial reversibility
for the whole system was also achieved [7,8]. However, the dehy-
drogenation temperature is still too high for future application
purpose. It is well known that transition metal chlorides and fluo-
rides have a promoting effect on kinetics performances of complex
hydrides [9,10]. Recently, the enhancement of the hydrogen sorp-
−
3
umetric contents of 10.8 wt.% and 115 kg of H2
that match some of the targets set by the US Department of Energy
m
respectively,
(
DoE). However, it is very stable and the temperature of hydrogen
tion properties of the NaBH + 2MgH2 multi-component system
doped with NbF5, was investigated [11].
4
◦
release under static vacuum is around 500 C, too high for practical
automotive application [4].
Another chance to improve significantly the hydrogen uptake
kinetics and the thermodynamic of complex hydrides is repre-
sented by their confinement in nanoporous materials, and several
works have been recently put forward on the beneficial effects of
the nanosizing obtained by the insertion in such matrices [12–15].
Conversely, the contribution towards the dispersion of multi-
component systems, as RHC, into scaffold is quite scarce. Along
Multi-component hydrogen storage systems, also referred as
reactive hydride composites (RHC), comprise more than one
phase in the hydrogen charged state in order to decrease the
dehydrogenation temperature and, at the same time, keep the
high gravimetric capacity of the system [5]. Among them, the
this way Nielsen et al. recently synthesized LiBH –MgH2 nanopar-
4
ticles embedded into a nanoporous carbon aerogel matrix, and
showed that the nanocomposites improved hydrogen desorption
∗ Corresponding author. Tel.: +39 079 229524; fax: +39 079 229559.
E-mail address: mulas@uniss.it (G. Mulas).
0
925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2011.12.042

G. Mulas et al. / Journal of Alloys and Compounds 536S (2012) S236–S240
S237
Table 1
kinetics and released hydrogen at lower temperature than bulk
LiBH –MgH [16]. Within this frame, in this paper we investigated
Weight (%) fraction and crystallite size of the phases present in the XRPD patterns
plotted in Fig. 1.
4
2
the hydrogen desorption release from 2NaBH + MgH2 RHC pre-
4
Samples
Phases
Crystallite size
Weight (%) (± 5)
pared either in bulk state by mechanical treatment or in nanosized
range by supporting the RHC in a silica based mesoporous scaffold,
SBA-15. The latter is a high surface area, amorphous matrix charac-
terized by hexagonally arranged cylindrical mesochannels, which
is often used to confine active phases such heterogeneous catalysts,
with the additional possibility to tailor the size of the pores in the
nano range. The effect of the addition of NbF5 on the hydrogen des-
orption properties (temperature, kinetics) was also tested in the
samples prepared by both routes.
(
nm) (± 5)
Sample A
Sample B
Sample C
NaBH4
MgH2
NaBH4
MgH2
NaBH4
MgO
Mg2Si
MgSiO3
NaBH4
MgO
55
50
38
55
191
75
10
75
124
71
98
76
24
78
22
67
12
16
≤5
70
17
8
Sample D
Mg
Mg2SiO3
2
. Experimental details
100
≤5
Commercial powders of NaBH4 (95% purity) and NbF5 (95%
purity) were purchased from Sigma–Aldrich, while MgH2 (95%
purity) was supplied by Tego Magnan company. As for the synthe-
sis by mechanochemical processing, two different mixtures were
prepared, namely 2NaBH + MgH2 and 2NaBH + MgH + 0.05NbF5
named sample A and B, respectively. The stoichiometric coefficients
indicate the relative molar abundance of the reactants. For each
mixture, 8 g of powder were sealed in a stainless steel vial and sub-
sequently ball-milled for 15 h by means of a Spex mixer/mill model
XRD patterns of the ball milled 2NaBH –MgH2 (sample A) and
4
2
NaBH + MgH + 0.05NbF5 (sample B) systems. The Rietveld analy-
4 2
sis confirms previous findings on such stoichiometry, as reported in
6]: the mechanical treatment did not induce the formation of new
mixed phases containing Mg, Na and B, while only NaBH4 (JCPD
5-0208) and ␥-MgH2 (JCPD 12-0697) can be indentified. Similar
4
4
2
[
7
results were observed for the sample with NbF5 added, which also
showed a slight broadening of the NaBH4 peaks, due to a decrease
of the borohydride mean crystallite size (see Table 1).
Then, as already stated in the introduction, two other com-
posites have been synthesized, by melting impregnation of
8
000. All sample handling and vial sealing were performed under
high purity Argon atmosphere inside a glove box (MBraun- 20-G),
with O2 and H O levels below 0.1 ppm. The mechanical treatment
2
was carried out using a single stainless steel ball (8 g) and a rotation
speed of 875 rpm.
For the preparation of nanoconfined hydride materi-
als, the SBA-15 matrix was first synthesized starting from
tetraethyl orthosilicate (TEOS) and Triblock polymer P123 (Alfa
Aesar) according to the procedure by Zhao et al. [17]. After-
2
NaBH + MgH2 and 2NaBH + MgH + 0.05NbF5 blends into SBA15
4 4 2
mesoporous structure, indicated in the following as sample C
and sample D, respectively. The thermal treatment conditions,
◦
4
50 C for 1 h under Ar atmosphere, were selected to avoid the
NaBH4 decomposition. Fig. 1b shows the XRD patterns of the melt-
impregnated samples C and D. From the Rietveld analysis (data
reported in Table 1) it can be noted that the main phases present
wards, the two nanocomposites SBA–15/2NaBH –MgH2 and
4
SBA–15/2NaBH –MgH –0.05NbF5 (named sample
C and D,
4
2
in sample C are NaBH , MgO, Mg Si and MgSiO , indicating that,
4
2
3
respectively) were prepared by melt impregnation: the SBA-15,
during the heating process, MgH2 decomposed to Mg and, at least
hydrides and additive (for sample D) were thermally treated at
in part, reacted with the support, leading to the formation of Mg Si
◦
2
4
50 C, under purified Ar atmosphere, for 1 h. The weight ratio of
and MgSiO . Similar findings were already reported by Kelly et al.
3
the hydride phases to SBA-15 matrix was 4:5, hence the amount of
hydride phases was 44.4% in the total mass of the powder system.
The structural characterization of the samples was carried out by
X-ray diffraction XRD (Philips X’pert, Braun Bruker D8), and trans-
mission electron microscopy TEM (Jeol-JEM 2011, 200 kV). For the
TEM measurements, samples were prepared by dispersing a small
amount of powder in tetrahydrofuran (THF) under Ar atmosphere
in a glove box. Just before their observation in the TEM, a holey
carbon supported grid was soaked into the suspension.
The weight percentage of the different phases and their
microstructural parameters were evaluated by fitting the XRD
patterns using the MAUD (materials analysis using diffraction)
Rietveld refinement software [18].
[
19]. The diffraction pattern of sample D displays additional peaks
corresponding to pure Mg while no phases containing Nb or F can be
detected, confirming that the additive did not interact either with
NaBH4 or with MgH . Finally the absence in the pattern D of the
2
MgSi phase could also be in some way due to an action of NbF5.
However we could not leave out the presence of some amount
of MgSi, still in nanostructured conditions, which makes hardly
detectable the Bragg reflections over the pattern background.
The production of nanosized materials for hydrogen storage has
been widely investigated by dispersing active phases within amor-
phous structured support as C aerogels [20], active carbon AC [21]
and C nanotubes [22], all of them characterized by the lacking of
long range order [23]. The structural characteristics of the scaffold
used in the present work are different: SBA-15 is characterized by a
highly ordered, two-dimensional, hexagonal (P6mmm), mesostruc-
The sorption properties of the samples were studied by means
of a manometric Sievert type apparatus (PCT-Pro 2000 Setaram &
Hy-Energy, pressure measurement accuracy: 1% of reading). Des-
orption was carried out by heating about 100 mg of each mixture
ture, with thick uniform SiO walls [17], and, additionally, chemical
2
short range order is not present at all. Such characteristics are con-
firmed in the TEM analysis of the matrix shown in Fig. 2a, that
displays the regularity of the hexagonal pores as well as the perfect
alignment of the channels, whose average diameter size is around
◦
◦
from room temperature to 450 C at 10 C/min under static vac-
−
3
◦
uum (10 mbar), followed by 5 h isothermal step at 450 C. Using
the same condition the hydrogenation properties of each sample
have been also investigated by thermal desorption spectroscopy,
TDS (Hiden, Isochema).
5
nm. Hexagonal long range order of the SBA-15 porous struc-
ture was also evidenced by the three main peaks observed in the
small angle X-ray scattering pattern (SAXS), here not shown. SAXS
and TEM analyses thus proved that the synthesis of SBA-15 was
successful and in good agreement with literature data [17,24,25].
TEM analyses also allow evaluating the success of infiltration pro-
cess. In Fig. 2b the TEM image of the sample C is shown: aligned
3
. Results and discussion
As previously mentioned, the synthesis of bulk composites
was carried out by mechanical treatment. Fig. 1a displays the

S238
G. Mulas et al. / Journal of Alloys and Compounds 536S (2012) S236–S240
Fig. 1. (a) XRD patterns of the ball milled (15 h) 2NaBH4 + MgH2 (sample A, bottom) and 2NaBH4 + MgH2 + 0.05 mol NbF5 (sample B, top) mixtures. (b) XRD patterns of the
melt impregnated samples SBA–15/2NaBH4 + MgH2 (sample C, bottom) and SBA–15/2NaBH4 + MgH2 + 0.05 mol NbF5 (sample D, top).
channels of different grey tones are clearly visible, together with
dark grey spherical particles: image contrast and optical consid-
erations on the tilted specimen indicate that the area inside the
circle corresponds to the filled pores, while the particle pointed
with the arrow is outside the pores. It can be concluded that the
melting process resulted in a partial insertion of the hydrides within
the silica mesochannels, thus reducing the hydride particles to the
nanosized range thanks to the confinement. Remarkably, although
amount and the H2 theoretical content) with respect to sample A
(88% vs 91%).
In Fig. 3b the desorption kinetic curves of sample C (filled
squares) and D (open circles) are shown, as well as the temper-
ature profile (dark line). Even in the case of nanosized specimens,
the NbF5-containing sample (i.e. sample D) starts to release hydro-
gen before the homologous sample free from additive (i.e. sample
C), and after 1.5 h the desorbed amount reached 2.8 and 2.3,
respectively. This trend is maintained also when the full desorp-
tion capacity is attained: after 3 h the sample D displays a higher
total desorption capacity, 3.4 wt.%, compared to that of sample C,
2.4 wt.%. Such values are smaller than those shown by the bulk sam-
ples A and B, because of the silica host weight. Then, assuming that
no cooperative mechanism takes place during the dehydrogena-
tion process between active phases and matrix, hydrogen capacity
data of all the samples should account for the relative amount of
hydride phases into the porous matrix. Considering also that Mg
◦
the temperature used for the infiltration process, 450 C, was lower
than the melting point of pure bulk NaBH4 and Mg [11], the suc-
cess of embedding, witnessed by the TEM images (Fig. 2) and EDX
analyses (not shown here), suggests that the melting occurred
during the infiltration process in our experimental conditions. To
this, the reduction in the melting temperature could be ascribed
to the chemical interaction between Mg and NaBH4 during heat
treatment. However, the detailed mechanism of the embedding
remains, at present, not definitely clear.
As already mentioned, the hydrogen storage properties of the
ball milled samples have been investigated either by Sievert appa-
ratus and by TDS. Fig. 3a shows the hydrogen desorption kinetic
curves (H2 wt.% vs time, hours, h) of sample A (filled squares) and
sample B (open circles) as well as the temperature profile (dark
line). It can be easily seen that sample B starts to release hydro-
is present in the matrix instead of MgH , previous data should
2
be scaled by a factor 0.4 to estimate the exact hydrogen release
from the hydride phase. The observations from the kinetic curves
were confirmed by the results of the TDS spectra of all the samples
shown in Fig. 4a and b, respectively. To this regard it is impor-
tant to highlight that the desorption phenomena occurring during
TDS measurement and kinetic analyses carried out by Sievert type
apparatus, can follow different reaction paths, due to the possible
effect of hydrogen backpressure desorbed in closed systems, and
gen before sample A, the first onset being observable at about
◦
1
56 C. After 1.5 h the desorbed amount reaches 7.2 and 4.8 wt.%,
respectively. Despite increased kinetics, sample B displays a lower
desorption yield (ratio between the experimental H2 desorption
confirmed in recent papers for the LiBH /MgH2 system [26–29].
4
Fig. 2. TEM image of the SBA-15: the hexagonal pores as well as the aligned channel are displayed (a). TEM image of sample C (b): the area inside the circle corresponds to
the filled pores while the particle pointed with the arrow is outside the pores.

G. Mulas et al. / Journal of Alloys and Compounds 536S (2012) S236–S240
S239
Fig. 3. (a) Kinetic desorption curves (H2 wt.% vs time h) of the milled (15 h)
NaBH4 + MgH2 (sample A, squares) and 2NaBH4 + MgH2 + 0.05 mol NbF5 (sample
Fig. 4. a. TDS desorption spectra (m/z = 2) of the milled (15 h) samples
NaBH4 + MgH2 (sample A, squares) and 2NaBH4 + MgH2 + 0.05 mol NbF5 (sample B,
circles). The temperature profile is also shown (dark line). (b) TDS desorption spectra
m/z = 2) of the milled (15 h) samples SBA–15/2NaBH4 + MgH2 (sample C, squares)
and SBA–15/2NaBH4 + MgH2 + 0.05 mol NbF5 (sample D, circles). The temperature
profile is also shown (dark line, Y axis in the right side of the picture).
2
2
B, circles). The temperature profile is also shown (dark line, Y axis in the right side
of the picture). (b) Kinetic desorption curves (H2 wt.% vs time h) of the milled (15 h)
SBA–15/2NaBH4 + MgH2 (sample C, squares) and SBA–15/2NaBH4 + MgH2 + 0.05 mol
NbF5 (sample D, circles) mixtures. The temperature profile is also shown (dark line).
In the insets, the enlargement of the graphs is shown, to highlight the onset of the
desorption phenomena.
(
mixture: NbF5 resulted more efficient than the other tested fluo-
rides, TiF4 and MgF2 [11].
As for the embedded hydrides, it emerges that the presence of
NbF5 gives additional strength to the action of the reduced size
of the hydride particles and their dispersion over the matrix, both
of them concurring in improving the desorption kinetics. To this
regard it should be stressed that these last materials presented
desorption temperature and properties much promising than the
corresponding bulk systems. The chemical nature of the observed
hydrogen release steps is currently under study.
However it seemed to us safe to say that there is a correspondence
between the two desorption phenomena which occurred during
kinetics and TDS analyses.
All the curves refer to the mass to charge signals equals to 2, cor-
responding to the H molecule. In detail, sample B displays 3 peaks
2
◦
◦
◦
centered at around 200 C, 300 C, and 400 C, respectively. The TDS
spectrum of sample A shows only two peaks at around 300 C and
10 C, respectively. The comparison of the two traces suggests that
◦
◦
4
the NbF5 additive plays an active role in favoring the hydrogen des-
orption process of the 2NaBH + MgH2 system. As for the samples
4. Conclusions
4
confined into SBA-15, sample D (Fig. 4b, top), displays 3 peaks cen-
◦
◦
◦
tered at around 134 C, 323 C and 354 C, respectively, while the
TDS spectrum of sample C (Fig. 4b, bottom) shows two peaks cen-
The pattern of data here presented allows gaining some new
hints in the study of the NaBH –MgH2 system. The use of an addi-
4
◦
◦
tered at around 150 C and 386 C, respectively. The presence of
different peaks in all the TDS analyses, indicates that H2 release is
promoted at different energy levels, and whose processes require
different activation energies. Such findings, already observed in
previous works on bulk systems of similar stoichiometry, have been
a valuable tool to investigate reaction mechanism [11]. In particular
the efficiency of the NbF5 as destabilizing agent towards dehydro-
genation, and as catalyst for the borohydride desorption process,
tive and the nanoconfinement of the composite were successful
in obtaining hydrides destabilization and improvements of their
desorption properties. In accordance with previous results NbF5
appears to be a very promising additive, since it reduced the NaBH4
decomposition temperature and increasing the desorption kinetics.
Moreover, nanoconfinement by melt infiltration could represent a
promising way for tuning the thermodynamic and kinetic proper-
ties of the multi-component system, leading to a drastic decrease
in the desorption temperature. It is true that the results here
confirms previous observations in the NaBH –2MgH2 ball milled
4

S240
G. Mulas et al. / Journal of Alloys and Compounds 536S (2012) S236–S240
presented do not allow an exhaustive comprehension of the char-
acteristics of the confined system, but it can be taken as a first
approach. A more in-depth investigation will focus on the evalu-
ation of the fraction of embedded hydrides, and on a more detailed
structural and thermodynamic characterization, which will hope-
fully allow us to define the reaction mechanism and to further
improve the sorption properties.
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Acknowledgments
[
5
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versity and Research) in the framework of the PRIN Project
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