Hydrogen storage property of MgH2 synthesized by hydriding chemical vapor deposition

Article abstract of DOI:10.1016/j.jallcom.2007.04.020

As reported previously, MgH₂ synthesized by hydriding chemical vapor deposition (HCVD) was made of single crystals with nano-fibrous figures. Therefore, the hydrogen storage property of the HCVDed MgH₂ is expected to reflect its structural character. In this study, we examined the hydrogen storage property of MgH₂ synthesized by HCVD. Results of pressure-composition-isotherm (PCT) measurement showed that the HCVDed MgH₂ reversibly absorbed and desorbed hydrogen as much as 7.6 mass% without any activation treatment retaining the fiber shape. Each hydrogenation/dehydrogenation rate was great because hydrogen diffused through a short pathway in a radial direction of the fiber.

Full text of DOI:10.1016/j.jallcom.2007.04.020

Journal of Alloys and Compounds 446–447 (2007) 80–83
Hydrogen storage property of MgH synthesized
2
by hydriding chemical vapor deposition
a,∗
b
b
a
I. Saita , T. Toshima , S. Tanda , T. Akiyama
a
Center for Advanced Research of Energy Conversion Materials, Hokkaido University, Kita 13 Nishi 8,
Kita-ku, Sapporo, Hokkaido 060-8628, Japan
b
Faculty of Engineering, Hokkaido University, Kita-13 Nishi-8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan
Received 26 October 2006; received in revised form 30 March 2007; accepted 3 April 2007
Available online 8 April 2007
Abstract
As reported previously, MgH
2
synthesized by hydriding chemical vapor deposition (HCVD) was made of single crystals with nano-fibrous
is expected to reflect its structural character. In this study, we examined
figures. Therefore, the hydrogen storage property of the HCVDed MgH
the hydrogen storage property of MgH synthesized by HCVD. Results of pressure-composition-isotherm (PCT) measurement showed that the
HCVDed MgH reversibly absorbed and desorbed hydrogen as much as 7.6 mass% without any activation treatment retaining the fiber shape. Each
hydrogenation/dehydrogenation rate was great because hydrogen diffused through a short pathway in a radial direction of the fiber.
2007 Elsevier B.V. All rights reserved.
2
2
2
©
Keywords: Metal hydrides; Nano-structured materials; Vapor deposition; Microstructure
1
. Introduction
gas phase is expected to alleviate those problems. For that rea-
son, chemical vapor deposition (CVD) under highly pressurized
hydrogen, known as hydriding CVD (HCVD), was used to syn-
thesize magnesium hydride from the mixture of high-pressure
H2 and magnesium vapor, and obtained the highly pure prod-
uct of MgH2 [2]. The HCVD method indicates the attractive
advantage that both the activation treatment and the rate-limiting
step in the solid–gas reaction are removed completely because
large collision frequency between gaseous metals and hydro-
gen molecules quickly synthesizes metal hydrides and because
as-synthesized metal hydride deposits quickly on the cool sub-
strate. However, in spite of its importance, the hydrogen storage
property of HCVDed product of MgH2 has not been evaluated
precisely. Therefore, in this study, we evaluated the hydrogen
storage property of the HCVDed MgH2. From the viewpoint of
the structural character of HCVDed MgH2, which was made of
single crystals with fibrous shapes, the hydrogen storage prop-
erty is expected to differ from that of the current product, which
inherently becomes polycrystalline because of large strain stress
during the hydrogenation and dehydrogenation.
Magnesium hydride, MgH2, is attractive from the viewpoint
of an engineering perspective because its raw materials are abun-
dant and it has great hydrogen density of 7.6 mass%. However, it
usually requires at least ten times or more activation treatments,
which are quite time and energy intensive [1]; such treatments
includebothhydridinganddehydridingtreatmentsundercontrol
of temperature and hydrogen pressure. Even after the activation
treatments, magnesium is not hydrided completely because of
the low reaction rate of hydrogenation and the only-slightly-
recoverable poisoned part. For marketed products, the product
purityofMgH2 is98%atmost. TheactivationdifficultyofMgH2
remains as a practical obstacle against its use as a hydrogen
storage material.
The problem of activation difficulty occurs because of the
low rate of hydrogen diffusion and its unrecoverable poisoned
part. The gas–solid reaction between magnesium and hydrogen
includes a reaction step of hydrogen diffusion in solid mag-
nesium, which is sufficiently slow that it is the rate-limiting
step of the reaction. Therefore, synthesizing metal hydride in
2
. Experiment
∗
Corresponding author. Tel.: +81 11 706 6842; fax: +81 11 726 0731.
A sample of MgH2 was prepared by HCVD as reported elsewhere [2]. Com-
E-mail address: itoko@eng.hokudai.ac.jp (I. Saita).
mercial magnesium (99.9% purity) was heated to 870 K to vaporize in hydrogen
0
925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2007.04.020

I. Saita et al. / Journal of Alloys and Compounds 446–447 (2007) 80–83
81
Fig. 1. XRD pattern of the product.
atmosphere at a pressure of 4 MPa. The product of the hydrogen and the mag-
nesium vapor was deposited on the substrate of cooled Inconel. The deposited
product was collected and investigated using X-ray diffractometry (XRD) and
scanning electron microscopy (SEM). Both the hydrogen storage and release
behavior and the pressure-composition-isotherm (PCT) were measured using a
PCT measurement apparatus (Suzuki Shokan Co., Ltd., Japan).
3
. Results and discussion
Fig. 1 shows the XRD pattern of the product. The XRD pat-
tern was measured shortly after the product was removed from
the HCVD apparatus to alleviate oxidization of the product in
air because the HCVDed product is readily oxidized in air and
forms oxidized phase [2]. Consequently, all peaks were identi-
fied with MgH2, indicating high purity of HCVDed MgH2; in
addition, expected impurities, as observed in a previous report
Fig. 3. PCT of HCVDed MgH2 at 578 K.
capacity of magnesium, even without activation treatment. The
measurement finished in 1 and 2 h, respectively, for hydrogena-
tion and dehydrogenation. The reaction rates were much faster
than that of conventional Mg, which is only slightly activated
by the current activation treatment. The results revealed that
the HCVDed MgH2 was not poisoned in air, thereby had great
activity for hydrogen storage.
[
2], were not detected in this study.
Fig. 2 shows an SEM image of the as-HCVDed MgH2. The
product showed two shapes: curved fibers; straight, needle-like
fibers, which a previous study [2] has identified as single crys-
tals of MgH2. The curved fibers were also revealed as MgH2
according to the XRD results. Each fiber had less than 1 ␮m
diameter and each was ten or more micrometers long.
The plateau pressure in hydrogen absorption and desorption
was slightly lower than the reported value; that is, HCVDed
MgH2 was stable in comparison to the conventional product of
MgH2. That stability was considered to be attributable to the
great crystallinity of the HCVDed MgH2 because hysteresis is
usually affected by the irreversible loss during hydrogenation
and dehydrogenation. The difference of plateau pressures was
more apparent at temperatures lower than 578 K. Fig. 4 shows
the PCT of HCVDed MgH2 at temperatures of 560 and 543 K.
Hydrogen desorption pressure of MgH2 at 560 K is reported to
be 0.1 MPa. However, HCVDed MgH2 desorbed hydrogen at a
pressure less than 0.1 MPa. In addition, the pressure showed a
remarkable super-decrease (see the blank square under the dot-
ted line in Fig. 4). The HCVDed MgH2 did not desorb hydrogen
until the pressure decreased to 0.04 MPa, but it recovered at
pressure greater than 0.06 MPa and started desorption. The same
phenomenon was observed at 543 K: the HCVDed product did
not desorb hydrogen, even at a pressure of less than 0.01 MPa.
It is difficult to determine equilibrium pressures in hydro-
gen desorption. Therefore, those in absorption were used for the
van’t Hoff’s plot. Fig. 5 shows the result. Although the pres-
sures were slightly smaller than the reported value, the slope
of the line was well fitted (the correlation coefficient in least
square method was 99.99%) and was almost parallel to the
Fig. 3 shows the PCT of the product measured at 578 K.
It is noteworthy that the product was not treated in any pro-
cess but was evacuated for 2 h at a temperature of 578 K before
the measurement. The product reversibly absorbed hydrogen
approximately 7.6 mass%, as much as the theoretical maximum
Fig. 2. SEM image of as-HCVDed MgH2.

8
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I. Saita et al. / Journal of Alloys and Compounds 446–447 (2007) 80–83
reported one. The enthalpy change and entropy change of the
reaction, ꢀH and ꢀS, were as much as −73.4 kJ/mol H2 and
129.0 J/mol H2, respectively. The differences in comparison to
−
reported values (ꢀH = −74.4 kJ/mol H2, ꢀS = −135.1 J/mol H2
[3]) were, respectively, within 1.5% and 4.5%.
Fig. 6 shows hydrogen sorption behavior of HCVDed MgH2.
In absorption, HCVDed product completed 80% of the reaction
in 600 s at a pressure of 4 MPa H2. In desorption, it completed
80% of the reaction in 300 s at a pressure of 0.6 MPa H2. Each
reactionratewasgreaterthanthoseofpreviousreports[4], which
examined the magnesium particles of approximately 10 ␮m.
Regarding the dimensions of the samples, the HCVDed MgH2
remained at original fiber lengths greater than 10 ␮m, even after
the evaluation of hydrogen storage properties, as described later.
Therefore, the greater reaction rate of HCVDed MgH2 was con-
sidered to result from the reaction through the radial pathway
of the fibers. The reaction rate of the HCVDed MgH2 was even
large compared to the other reports [5–23] on modified magne-
sium such as catalyzed magnesium, ball milled one, film-shaped
one, and the complex of them, in which ball milled magnesium
recordsthefastesthydrogenationcompleting95%ofhydrogena-
tion in 400 s. Most of those ball milling and catalyst addition
use high-energy mill and produce nano-ordered magnesium or
magnesium composites. However, the nano-ordered materials
are not stable in air and easily lose those effects. Film structured
magnesium is relatively stable, but not so reactive. Therefore,
the HCVDed MgH2, which actively reacted even after being
treated in air, is useful from the viewpoint of engineering per-
spective.
Fig. 4. PCT of HCVDed MgH2 at 560 and 543 K.
Fig. 7 shows an SEM image of the HCVDed MgH2 after
evaluation of hydrogen storage properties. The sample remained
in its original fibrous shape. Neither cracks nor fissures were
observed; moreover, the fiber length did not change. Metal
hydrides usually generate some cracks to release strain during
hydrogen storage cycles. Therefore, the morphological stability
of MgH2 fiber might have imparted great stress during hydrogen
absorption and desorption, but that remains a matter of specu-
lation. The fibrous shape is a clue for studying the difference
of plateau pressures between conventional MgH2 and HCVDed
MgH2, as shown in Figs. 3 and 4.
Fig. 5. van’t Hoff plot.
Fig. 6. Hydrogen absorption and desorption behavior of HCVDed MgH2 at a temperature of 615 K. (a) Hydrogen absorption at a pressure of 4 MPa; (b) hydrogen
desorption at a pressure of 0.6 MPa.

I. Saita et al. / Journal of Alloys and Compounds 446–447 (2007) 80–83
83
Ministry of Education, Culture, Sports, Science and Technology,
Japan, andbyFellowshipsoftheJapanSocietyforthePromotion
of Science for Young Scientists.
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