Improving H-sorption in MgH2 powders by addition of nanoparticles of transition metal fluoride catalysts and mechanical alloying

Article abstract of DOI:10.1016/j.jallcom.2004.07.042

Mechanical alloying of nanograined MgH₂ with addition of small amounts of FeF₃ can produce, via a fluoride transfer reaction, a nanocomposite powder containing both the transition metal and fluorine as catalysts. The solid-state phase transformations and microstructural changes during milling and/or during heating were studied by X-ray diffraction during in-situ heating, transmission electron microscopy, differential scanning calorimetry and magnetization measurements. The fluoride transfer reaction occurs partially during milling and can be completed during the first heating cycle. An improved H-desorption behavior was observed in such nanocomposite powder in comparison with the best reported values for MgH₂-based composites.

Full text of DOI:10.1016/j.jallcom.2004.07.042

Journal of Alloys and Compounds 389 (2005) 270–274
Improving H-sorption in MgH powders by addition of nanoparticles of
2
transition metal fluoride catalysts and mechanical alloying
a,b,∗
b
b
a
a,b
J.F.R. de Castro
, A.R. Yavari , A. LeMoulec , T.T. Ishikawa , W.J. Botta F
a
Departamento de Engenharia de Materiais, Universidade Federal de S a˜ o Carlos, CP 676, CEP 13565-905, S a˜ o Carlos, S a˜ o Paulo, Brazil
b
LTPCM, Institut National Polytechnique de Grenoble, LTPCM CNRS umr 5614, BP 75, St-Martin d’H e` res Campus 38402, France
Received 11 June 2004; accepted 15 July 2004
Abstract
2 3
Mechanical alloying of nanograined MgH with addition of small amounts of FeF can produce, via a fluoride transfer reaction, a nanocom-
posite powder containing both the transition metal and fluorine as catalysts. The solid-state phase transformations and microstructural changes
during milling and/or during heating were studied by X-ray diffraction during in-situ heating, transmission electron microscopy, differential
scanning calorimetry and magnetization measurements. The fluoride transfer reaction occurs partially during milling and can be completed
during the first heating cycle. An improved H-desorption behavior was observed in such nanocomposite powder in comparison with the best
reported values for MgH
2
-based composites.
©
2004 Elsevier B.V. All rights reserved.
Keywords: Hydrogen storage materials; Nanostructures; Mechanical alloying
1
. Introduction
material. However, application has been hindered by high
sorption temperatures and slow kinetics [2].
The greenhouse effect and climate changes due to hu-
When Mg is milled to reduce its grain size to nanometric
range, the kinetics of gas-phase sorption are enhanced [4,5]
although the hydrogen desorption kinetics remain too slow
man activity have resulted in environmental protection and
in search of ecologically correct form of energy storage. Hy-
drogen is the ideal means of energy storage, transportation
and conversion in a clean-energy concept due to the fact that
its combustion generates water [1,2].
Metal hydrides offer a safe alternative medium for trans-
mission and storage of hydrogen energy and can be used
in fuel cells, metal-hydride batteries, thermal engines, air-
conditioning systems, transformers of low quality thermal
energy (for example from waste heat and solar energy),
thermosorption compressors (TSC) and other environment-
friendly applications [2,3]. Among these hydrides, Mg-
based metal hydrides with volumetric energy density of
◦
for applications; even at 300 C desorption occurs in about
30 min or more. Introduction of small volume fractions of
transition metal (TM) nanoparticles catalyst such as Ni, Ti,
V, Cr, Nb and others have been shown to improve sorption
kinetics of H2 in nanostructured Mg to varying degrees [6–8].
All these transition metals elements are mixed valence and
immiscible with Mg, the latter property allowing them to
maintain their integrity during heating cycles [8].
Improved results have also been obtained with the intro-
duction of nanoscale oxide particles of transition metals with
mixed valence such as Cr2O3, Fe3O4, Mn2O3, V2O and
5
1
.5–1.6 kWh/l, low cost and high abundance may be used
Nb2O [9]. However, the oxide particles are likely to be re-
5
as safe environmentally sound energy storage and transport
duced to metal particles by the Mg such that the catalytic
effect may be the same as for the introduction of the transi-
tion metal particle directly.
∗
In another type of development for hydriding alloys, the
reaction between F ions in aqueous solution and the metal
Corresponding author. Tel.: +33 04 76 826516; fax: +33 04 76 826516.
E-mail address: pjfrc@iris.ufscar.br (J.F.R.d. Castro).
−
0
925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2004.07.042

J.F.R. de Castro et al. / Journal of Alloys and Compounds 389(2005) 270–274
271
atoms was shown to create a highly reactive and protective
fluorinated surface for H uptake [10]. In these F-treated hy-
drides, the original oxide and/or hydroxide layer is cleaned-
up from the surface and the created fluoride layer enhances
activation. However, there is no report so far concerning the
addition of TM-fluorides and its effect in reducing sorption
temperatures.
In the present paper, we report an important catalytic ef-
fect observed with the simultaneous introduction of transi-
tion metal (TM) nanoparticles and fluorine into fine-grained
MgH2 by the mechanical alloying of a few volume per cent
of nanograined transition metal fluorides and nanograined
MgH2.
2
. Experimental procedure
MgH2 nanocomposite powder containing transition metal
and fluoride catalysts was obtained by mechanical alloying
of a mixture of nano-grained MgH2 and FeF3.
Fig. 1. XRD patterns for the composite MgH2–3%FeF3 in the as-milled
condition and during heating at 250 C.
◦
The nanograin size of the initial reactants were obtained
by milling separately, under argon atmosphere, commercial-
grade MgH2 and FeF3 in a vibrating mill containing a single
protective intergranular MgF2 and a fine nanoparticle disper-
sion of Fe on the Mg or MgH2 particles.
5
cm diameter steel ball. The two powders were then mixed in
the proportions of MgH2–3 mole %FeF3 and milled together
for mechanical alloying with a ball/material mass ratio of
3MgH2(nano) + 2FeF3(nano)
⇒
3MgF2(intergranular) + 2Fe(nano) + 3H2
(1)
2
00:1 for 72 h.
This procedure usually leads to partial occurrence of the
fluoride transfer reaction although the complete reaction may
occur during milling depending on the milling device and
impact energy used. The reaction is in any case completed
during the first heating cycle.
All the powder manipulation was carried out in a clean
glove-box under argon gas, which also was used for filling
the steel vials for the milling processing.
The heats of formation, ꢀH , of MgH2, FeF3 and MgF2
f
are, respectively, −75.3, −1042.3 and −1123.39 kJ/mole
such that the total enthalpy change associated with reaction
(
1) would be −353.7 kJ/mole of MgH2.
In fact, in the as-milled composite the following phases
could be identified: tetragonal ␤-MgH2 (JCPDS 12-697),
MgO (JCPDS 78-0430), FeF3 (JCPDS 75-0451), Fe (JCPDS
The as milled material was characterized by synchrotron
X-ray diffraction with a wavelength λ = 0.69668 under Ar
atmosphere. The fluoride transfer reaction and generation of
Fe particles was monitored by magnetization measurements
with applied field up to 600 kA/m in a vibrating sample mag-
netometer (VSM). Transmission electron microscopy (TEM)
was carried out in a Philips CM120 microscope. Particle size
and phase characterization was carried out also by TEM with
dark-field observations and selected area electron diffraction
pattern (SAEDP). The dehydrogenation behavior was studied
by differential scanning calorimetry (DSC) and thermogravi-
metric analysis (TGA), using a heating rate of 10 K/min.
3
. Results and discussion
Fig. 1 shows XRD patterns of the nanocomposite
MgH2–3%FeF3 as-milled and after heating to a temperature
◦
close to 250 C, when desorption has already started.
Depending on the milling conditions a partial fluoride
transfer reaction of the type of Eq. (1) is induced, forming
Fig. 2. Magnetization loops for as-milled powder during heating showing
sharp change between 225 and 275 C.
◦

2
72
J.F.R. de Castro et al. / Journal of Alloys and Compounds 389(2005) 270–274
Fig. 5. DSC curve for the milled MgH2–3%FeF3 nanocomposite. Heating
rate of 10 K/min.
Fig. 3. Saturation magnetization vs. temperature during heating showing
◦
sharp rise around 250 C.
the magnetization measurements. Fig. 2 shows magnetiza-
tion loops obtained on the as-milled powder during heat-
ing. It is seen that saturation of the magnetization ␴ oc-
curs at applied fields H near 150 kA/m and its value rises
sharply between 225 and 275 C. Fig. 3 plots the saturation
values (at H = 550 kA/m) of magnetization ␴ as a func-
8
7-0722) and Mg (JCPDS 04-0770). The appearance of Fe
peak thus indicates that some degree of transfer of F from
FeF3 to MgF2 occurred during the milling. An increase in
the intensity of the Fe peak and a decrease in the intensity
of the FeF3 peak are observed in the XRD pattern at higher
temperatures. This indicates that the fluoride transfer reaction
continues during heating. The presence of MgF2 could not
be clearly detected by XRD due to its low volume fraction
although a small shoulder on the left side of the first MgH2
peak (as marked in Fig. 1) may suggest its presence. How-
ever, the presence of MgF2 after milling was confirmed by
adding a larger quantity of FeF3 in the initial mixture (not
shown here).
◦
tion of temperature during heating of the as-milled pow-
der. Since FeF is non-magnetic at room temperature, the
3
initial non-zero value σ > 4.5 emu/g in the as-milled pow-
der is indicative of partial occurrence of reaction (1) during
milling. The subsequent sharp rise from this level to about
◦
8 emu/g between 225 and 275 C signals the completion of
the transfer reaction and pure Fe generation at these tempera-
tures.
These results are consistent with a larger presence of the
free Fe product of the transfer reaction in the XRD pattern of
Further confirmation that the fluoride transfer reaction
1) continues during the first heating cycle was obtained by
◦
the powder heated to 250 C (Fig. 1).
(
Fig. 4. TEM micrographs of the 72 h milled MgH2–3%FeF3 nanocomposite. (a) Bright-field, (b) dark-field.

J.F.R. de Castro et al. / Journal of Alloys and Compounds 389(2005) 270–274
273
Fig. 6. Thermogravimetric analysis for the milled MgH2–3%FeF3
nanocomposite.
Fig. 7. DSC curves comparing the desorption behavior of the milled
MgH2–3%FeF3 nanocomposite with MgH2, MgH2–5%Fe and MgH2–2%
Nb2O5. Heating rates of 10 K/min.
Fig. 4a and b show bright-field and dark-field TEM im-
ages, respectively, from the as milled MgH2–3%FeF3 com-
posite. The dark-field image of Fig. 4a was obtained using
reflections from the first diffraction ring, which correspond
both to the (200) reflection of ␤-MgH2 (JCPDS 12-697) and
the (111) reflection of MgF2 (JCPDS 72-1150). The parti-
cle sizes, measured from the dark-field images were mostly
within the range of 5 to 10 nm.
TMFx addition, as shown in reaction (2), with subsequent
hydrogen charging or with H-charging during the milling
[11].
Mg(nano) + NiF2(nano)
⇒ Ni(nano) + MgF (intergranular)
2
(2)
In Fig. 5 the hydrogen desorption behavior of the milled
MgH2–3%FeF3 nanocomposite is followed by the DSC
calorimetry. A first-stage endothermic reaction is seen to start
The catalytic effect of metal fluoride addition to MgH2,
with the corresponding lowering of H-sorption temperatures
by 50 C can be a critical step towards the development of a
new family of application-enabled Mg-based composite hy-
drides for energy storage and transport.
◦
◦
◦
around 225 C and to peak around 270 C.
Thermogravimetric (TGA) trace of Fig. 6 for the milled
nanocomposite powder signals desorption starting around
◦
◦
2
9
35 C and reaching about 60% completion at 300 C and
◦
0% completion around 350 C.
4. Conclusions
To emphasize the important catalytic effect from the si-
multaneousintroductionoftransitionmetalnanoparticlesand
fluorine into fine-grained MgH2, Fig. 7 presents DSC results
comparing milled commercial MgH2 with MgH2 milled with
addition of pure Fe transition metal, transition metal oxide,
Mechanical alloying of a mixture of MgH2 with few per
cent FeF3 results in a fluoride transfer reaction from Fe to
Mg and the formation of Fe nanoparticles (and intergranular
MgF2) within a nanograined MgH2 powder. Improved H-
sorption kinetics have been observed in such nanocomposite
Nb2O as well as MgH2–3%FeF3, all prepared with the same
5
milling procedure as described in the previous section. The
DSC curves have been obtained with the same heating rate of
◦
powders with desorption temperatures 50 C lower than the
best reported values for MgH2 containing other catalysts.
1
0 K/min and the sorption temperature of the MgH2–3%FeF3
◦
nanocomposite is at least 50 C lower than the other Mg-
based composites. It is also important to notice that in the
temperature range 235–275 C, significant desorption ac-
◦
Acknowledgements
tivity occurs only in the milled MgH2–3%FeF3 composite
powder.
We would like to thank the financial support of the Brazil-
ian Institutions CNPq, CAPES and FAPESP. J.F.R de Castro
and W.J. Botta F. thank CNPq for scholarships during vis-
iting periods at LTPCM. This work was in part supported
The nanoparticles responsible for the catalytic effect re-
ported here are generated by the transfer reaction of Eq. (1),
which occurs partially during milling and is completed near
◦
by the EU RTN Network “H-sorption in MgH ” coordinated
2
70 C during the first heating. Similar processes also apply
2
by A.R. Yavari. Thanks are due to Dr Gavin Vaughan of
the ESRF for leading the in-situ annealing experiments that
yielded the data of Fig. 1.
to fluorides of other transition metals (TMFx) such as NiF2
and others or their mixtures. In addition, similar advanta-
geous results have been obtained by first milling Mg with

2
74
J.F.R. de Castro et al. / Journal of Alloys and Compounds 389(2005) 270–274
[6] G. Liang, J. Huot, S. Boily, A. Van Neste, R. Schulz, J. Alloys
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