Effect of TiH2 and Mg2Ni additives on the hydrogen storage properties of magnesium hydride

Article abstract of DOI:10.1016/j.jallcom.2010.03.128

A study was done to determine the effect of TiH₂ and Mg₂Ni additives on the hydrogen sorption behavior of MgH₂. A series of mixtures were made in which MgH₂ was ball milled with various amounts of TiH₂, Mg₂Ni or combinations of both. Temperature programmed desorption (TPD) analysis showed that the addition of increasing amounts of TiH₂ to MgH₂ resulted in a progressive reduction of the onset temperature of MgH₂ by as much as 70 °C for a mixture containing 50 mol% TiH₂. However the hydrogen yield also decreased from a high of 7.5 wt% for pure MgH₂ to a low of 2.4 wt% for the mixture containing 50 mol% TiH₂. The rates of H₂ reaction with MgH₂ increased with increasing amounts of TiH₂. Pressure composition isotherms were also made for each of the mixtures at several temperatures and van't Hoff plots were constructed. The results showed that the enthalpies for hydrogen absorption systematically decreased with increasing amounts of TiH₂, from a high of 76 kJ/mole H₂ for pure MgH₂ to a low of 65 kJ/mole H₂ for a mixture containing 50 mol% TiH₂. This indicated that thermodynamic stability can be reduced by TiH₂ additions. When Mg₂Ni was added to these mixtures, the desorption temperatures were reduced dramatically. The addition of 6 or 10 mol% of Mg₂Ni to MgH₂ lowered the onset temperature from 330 to 190 °C with a hydrogen yield of 6 wt%. The addition of both TiH₂ and Mg₂Ni catalysts did not result in any further reduction in the onset temperatures. However, the mixture of both catalysts was more effective at increasing reaction rates than either of them alone.

Full text of DOI:10.1016/j.jallcom.2010.03.128

Journal of Alloys and Compounds 499 (2010) 35–38
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Effect of TiH2 and Mg2Ni additives on the hydrogen storage properties of
magnesium hydride
S.T. Sabitu, G. Gallo, A.J. Goudy∗
Department of Chemistry, Delaware State University, 1200 N. Dupont Highway, Mishoe Science Center 301, Dover, DE 19901, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
A study was done to determine the effect of TiH2 and Mg2Ni additives on the hydrogen sorption behavior
of MgH2. A series of mixtures were made in which MgH2 was ball milled with various amounts of TiH2,
Mg2Ni or combinations of both. Temperature programmed desorption (TPD) analysis showed that the
addition of increasing amounts of TiH2 to MgH2 resulted in a progressive reduction of the onset tempera-
ture of MgH2 by as much as 70 ◦C for a mixture containing 50 mol% TiH2. However the hydrogen yield also
decreased from a high of 7.5 wt% for pure MgH2 to a low of 2.4 wt% for the mixture containing 50 mol%
TiH2. The rates of H2 reaction with MgH2 increased with increasing amounts of TiH2. Pressure composi-
tion isotherms were also made for each of the mixtures at several temperatures and van’t Hoff plots were
constructed. The results showed that the enthalpies for hydrogen absorption systematically decreased
with increasing amounts of TiH2, from a high of 76 kJ/mole H2 for pure MgH2 to a low of 65 kJ/mole H2
for a mixture containing 50 mol% TiH2. This indicated that thermodynamic stability can be reduced by
TiH2 additions. When Mg2Ni was added to these mixtures, the desorption temperatures were reduced
dramatically. The addition of 6 or 10 mol% of Mg2Ni to MgH2 lowered the onset temperature from 330 to
190 ◦C with a hydrogen yield of 6 wt%. The addition of both TiH2 and Mg2Ni catalysts did not result in any
further reduction in the onset temperatures. However, the mixture of both catalysts was more effective
at increasing reaction rates than either of them alone.
Received 6 January 2010
Received in revised form 9 March 2010
Accepted 14 March 2010
Available online 18 March 2010
Keywords:
Hydrogen storage
Magnesium hydride
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
modynamic properties of MgH2 by using additives to lower the
temperature of the hydriding and dehydriding process. One tech-
There is great interest in developing suitable materials for
hydrogen storage applications. A number of materials ranging from
interstitial metal hydrides to complex hydrides such as alanates,
borohydrides and imides have been extensively studied [1–7].
Interstitial hydrides such as LaNi5H6 can absorb hydrogen rapidly
and reversibly at moderate temperatures and pressures [8–10]
but their low hydrogen-holding capacity limits their potential for
hydrogen storage. Complex hydrides such as LiBH4 and LiNH2 have
very high hydrogen holding capacities but the release of hydrogen
is often accompanied by the release of byproducts such as B2H6 and
NH3.
Magnesium hydride is an attractive material for hydrogen stor-
age because it has a high hydrogen-holding capacity of 7.6 wt% and
the release of hydrogen occurs via a simple one-step process with
no byproducts [11–13]. However, a major obstacle to its useful-
ness for hydrogen storage is the high temperature required for it
to release hydrogen. Efforts have been made to improve the ther-
nique that has been used is alloying Mg with other metals. Reilly
and Wiswall [14,15] demonstrated that the thermodynamic stabil-
ity of magnesium hydride can be modified by alloying magnesium
with Cu or Ni. They showed that Mg2Cu and Mg2Ni alloys formed
less stable hydrides with lower reaction temperatures than pure
Mg. However, the hydrogen holding capacities of these materials
were considerably less than that of MgH2.
Attempts have also been made to lower the reaction tempera-
ture of MgH2 by ball milling it with various additives such as metal
oxides and transition metals [16–20]. For example, Hanada et al.
[16] used Nb2O5, prepared by ball milling under a hydrogen atmo-
sphere, to act as a catalyst and found that approximately 6 wt%
hydrogen could be desorbed from MgH2 in the 200–250 ◦C range.
Oelerich et al. [17] used a variety of metal oxides (e.g. Nb2O5, Fe3O4,
V2O5, Mn2O3, Cr2O3, TiO2, Sc2O3, Al2O3, CuO, and SiO2) to improve
reaction rates of MgH2 and found that as little as 0.2 mol% was suf-
ficient to provide fast sorption kinetics. Huot et al. [18] ball milled a
mixture of Mg and Ni under hydrogen atmosphere and found that
the presence of Ni lowered the onset temperature for desorption
of hydrogen from MgH2 from 440.7 to 225.4 ◦C. However, the pres-
ence of Mg2Ni slowed the decomposition kinetics of MgH2. Liang
∗
Corresponding author. Tel.: +1 302 857 6534; fax: +1 302 857 6539.
E-mail address: agoudy@desu.edu (A.J. Goudy).
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.03.128

36
S.T. Sabitu et al. / Journal of Alloys and Compounds 499 (2010) 35–38
et al. [19] studied the catalytic effect of transition metals (e.g. Ti, V,
Mn, Fe and Ni) on hydrogen sorption by MgH2. They found that the
formation enthalpy of MgH2 was not altered by milling with transi-
tion metals. In another study, Liang et al. [20] studied the hydrogen
storage properties of Mg1.9Ti0.1Ni, made by mechanical alloying,
and found that it absorbs more than 3 wt% H2 in 2000 s at 423 K.
This material showed better kinetics than ball milled Mg2Ni and
cast Mg2Ni.
In this work, an attempt has been made to compare the hydro-
gen sorption behavior of MgH2 ball milled with TiH2, Mg2Ni or a
mixture of both. The results would lead to a better understanding of
the role that thermodynamic stability has on reaction temperatures
and rates.
2. Experimental details
The starting materials used in this research were obtained from Sigma–Aldrich.
The MgH2 powder was hydrogen storage grade and according to the analysis
provided by the supplier, the total amount of trace metal contaminants was
less than 0.09%. The major contaminants were Ca = 199.0 ppm, Al = 157.8 ppm,
Mn = 126.6 ppm, Fe = 126.2 ppm and K = 109.7 ppm. All other contaminants were less
than 70 ppm. The TiH2 powder was 99.98% pure. All sample handling, weighing and
loading were performed in a Vacuum Atmospheres argon-filled glove box to prevent
contamination from air and moisture. The glove box was capable of achieving less
than 1 ppm oxygen and moisture. Prior to analysis, each sample mixture was milled
for up to 10 h in a SPEX 8000M Mixer/Mill that contained an argon-filled stainless
steel milling pot with four small stainless steel balls. The ball-to-powder ratio in the
pot was 5:1. X-ray powder diffraction analysis was used to determine whether new
phases were formed from milling the different substances. A Panalytical X’pert Pro
MPD Analytical X-ray Diffractometer Model PW 3040 Pro was used for this analysis.
The samples were covered with a Mylar film to keep them from air and moisture.
Pressure composition isotherm (PCI) analyses and temperature programmed des-
orption (TPD) were used to evaluate the hydrogen desorption properties of each
reaction mixture. These analyses were done in a gas reaction controller—PCI unit.
This stainless steel apparatus was manufactured by the Advanced Materials Corpo-
ration in Pittsburgh, PA. The unit was fully automated and was controlled by a Lab
View-based software program. The amount of hydrogen absorbed or released from
samples was determined by monitoring the pressure changes in a 50 cm3 calibrated
volume. The pressure was measured by Heise HPO pressure transducers that had
an accuracy of 0.05% of full scale, a repeatability of 0.005% of full scale and a
resolution of 0.01% of full scale. The temperature was controlled to 0.1 ◦C with
an Omega CN616 Series temperature controller. This unit accepted signals from a
CN616TC1 thermocouple. The thermocouple was inside of a stainless steel reactor
with its tip in contact with the center of the sample bed. PCI analyses were done
on freshly ball milled materials and no activation procedure was necessary. The
MgH2 was found to absorb and release hydrogen reversibly under the conditions
used and it was stable when subjected to repeated cycling. The TPD analyses were
done on freshly ball milled materials. The wt% hydrogen released during the runs
was determined by the software based on the pressure rise in the calibrated volume
at constant temperature. Scans were done in the 30–450 ◦C range at a temperature
ramp of 4 deg/min. Absorption rate measurements were also carried out in the gas
reaction controller. In each run, the sample was fully evacuated and hydrogen was
allowed to flow from the calibrated volume into the stainless steel sample reac-
tor. The initial pressure was set at 100 atm and the reaction rate was determined
by measuring the rate of pressure decrease in the calibrated volume. High purity
hydrogen gas of 99.999% purity was used throughout the analyses.
Fig. 1. X-ray diffraction patterns for MgH2, TiH2 and MgH2 + 4 mol% TiH2.
observed by Shang et al. [21] who did a study on MgH2 mechanically
alloyed with various transition metals. Fig. 2 contains XRD patterns
for mixtures of MgH2 with different mole percentages of TiH2 (i.e.
4, 10, and 50). The diffraction patterns show a progressive decline
in the peak corresponding to MgH2 in the region of 54◦ as the mol%
of TiH2 in the mixture increases. There is also the emergence of
prominent TiH2 peaks in the regions of 60◦ and 70◦ as the amount
of TiH2 in the reaction mixture increases. This is a further indication
that a new phase was starting to form.
Temperature programmed desorption measurements were
done on a series of MgH2 mixtures containing 0, 4, 10, and 50 mol%
TiH2 in order to determine the effect of TiH2 on the hydrogen des-
orption properties of MgH2. The profiles in Fig. 3 show the effect of
various mole fractions of TiH2 on the hydrogen desorption temper-
atures of MgH2. In the case of pure MgH2, the onset temperature
was about 330 ◦C. This temperature systematically decreased to
250 ◦C as the relative amount of TiH2 in the mixture increased to
50 mol%. The plots also show that as the mol% of TiH2 increases
the amount of H2 released decreases from a high of about 7.5 wt%
for pure MgH2 to a low of about 2.4 wt% for the mixture contain-
ing 50 mol% TiH2. This reduction in hydrogen weight percentage
is most likely due to the fact that TiH2 does not release its hydro-
gen in the temperature range used in this study. Temperatures in
excess of the 450 ◦C used in these experiments must be reached
before TiH2 begins to release hydrogen. Thus, there is an increasing
weight penalty that occurs as the percentage of TiH2 in the mixture
increases.
3. Results and discussion
A series of MgH2–TiH2 mixtures containing various amounts of
TiH2 ranging from 4 to 50 mol% were studied to determine the effect
of TiH2 on the hydrogen sorption properties of MgH2. After each
mixture was ball milled for 10 h, XRD measurements were used
to determine if any alloying had taken place. Fig. 1 contains XRD
patterns for MgH2, TiH2 and a mixture containing MgH2 + 4 mol% of
TiH2. Scan (c) was done on the mixture before ball milling and scan
(d) was done on the same mixture after ball milling. A comparison
of the patterns in scans (c) and (d) shows the disappearance and
emergence of some of the Mg and Ti reflections between 25◦ and
75◦. This indicates that some alloying of the Mg and Ti had most
likely taken place. It is also evident that the diffraction peaks for
the major phase, MgH2, in the ball milled mixture are broader as
the result of smaller particle size. This type of behavior was also
Fig. 2. X-ray diffraction patterns for several MgH2–TiH2 mixtures. All mixtures were
ball milled for 10 h.

S.T. Sabitu et al. / Journal of Alloys and Compounds 499 (2010) 35–38
37
Fig. 3. TPD profiles for MgH2 and several TiH2–MgH2 mixtures.
Fig. 5. Absorption isotherms for MgH2 and several TiH2–MgH2 mixtures.
Since adding large amounts of TiH2 to MgH2 to lower reaction
temperatures is accompanied by an excessive weight penalty, it
was desirable to test another material to see if similar temperature
lowering could be achieved with less weight penalty. Since nickel
is known to be a good hydrogenation catalyst, the Ni-containing
alloy, Mg2Ni, was studied to determine if it would be more effec-
tive at lowering the reaction temperature. A mixture containing
10 mol% Mg2Ni in MgH2 was made by ball milling and the TPD curve
is shown in Fig. 4. It can be seen that the onset temperature for the
mixture containing 10 mol% Mg2Ni is 195 ◦C, which is 55◦ lower
than that for the mixture containing 50 mol% TiH2, shown in Fig. 3.
Just as importantly, the weight penalty is significantly lower than
the mixture containing 50 mol% TiH2. This mixture releases about
6.3 wt% H2, which is considerably better than the 2.4 wt% that was
observed in the case of the 50 mol% TiH2 mixture. Based on this, it
appears that Mg2Ni is a more effective catalyst than TiH2. In order
to determine if a combination of both catalysts might yield even
better results, another mixture containing 4 mol% TiH2 and 6 mol%
Mg2Ni, a total of 10 mol% catalyst, was also studied. The curves in
Fig. 4 show that the mixed catalyst causes about the same tem-
perature lowering as the mixture containing 10 mol% Mg2Ni. Thus,
in this case, the presence of TiH2 does not produce any significant
improvement in the results.
Fig. 6. Van’t Hoff absorption plots for MgH2 and several TiH2–MgH2 mixtures.
since Mg2NiH4 releases only 4.45 wt% hydrogen, its hydrogen stor-
age potential is very limited. It is interesting to note that a similar
phenomenon was also observed in the MgH2 mixtures containing
TiH2. The TiH2 is stable up to temperatures in excess of 500 ◦C. But
when it is ball milled with MgH2, the new phase releases hydrogen
at a temperature which is lower than that of either constituent.
Since TiH2 and Mg2Ni are both able to lower reaction temper-
ature of MgH2, it was also of interest to determine their effect on
the thermodynamic stability of MgH2. Reilly and Wiswall [15] had
already established that Mg2NiH4 had a lower stability than MgH2
and thus it was expected that incorporating TiH2 might produce a
similar effect. Pressure-composition-isotherms were constructed
for the MgH2–TiH2 mixtures shown in Fig. 3. Fig. 5 shows the
absorption isotherms for these mixtures at 350 ◦C. It is evident
from the curves that the plateau pressure increases with increasing
TiH2 content. It is also evident that the hydrogen-holding capacity
decreases as the TiH2 content increases. This was also observed in
the TPD profiles. Pressure composition isotherms were constructed
for each mixture at several temperatures and thus it was possible
to construct the van’t Hoff plots shown in Fig. 6. The value of ꢀH for
each mixture could be determined from the slopes of these plots.
Table 1 gives the values of ꢀH for each mixture. It is evident that
Since Mg2Ni has such a large effect on the onset temperature
for hydrogen desorption from MgH2, a TPD profile was done on
sample of pure Mg2NiH4 to determine if it might have an even
lower onset temperature than the MgH2–Mg2Ni mixtures. Surpris-
ingly, the TPD curves in Fig. 4 for hydrogen desorption from the
MgH2–Mg2Ni mixtures all show a lower onset temperatures than
the Mg2NiH4, which has an onset temperature of 245 ◦C. In addition,
Table 1
Thermodynamic parameters obtained for pure MgH2 and MgH2–TiH2 mixtures.
Composition
Onset temp. (◦C) wt% Pm (at 350 ◦C) ꢀH (kJ/mol)
MgH2
MgH2 + 4 mol% TiH2
MgH2 + 10 mol% TiH2 265
MgH2 + 50 mol% TiH2 250
346
276
7.50
7.60
6.25
7.49
7.79
8.81
76.0
75.1
72.2
65.2
Fig. 4. TPD profiles for pure MgH2, pure Mg2NiH4, and several mixtures containing
various amounts of TiH2 and/or Mg2Ni.
2.40 12.93

38
S.T. Sabitu et al. / Journal of Alloys and Compounds 499 (2010) 35–38
Table 2
Reacted fractions obtained for pure MgH2 and mixtures containing MgH2, TiH2
and/or Mg2Ni after 1000 s of reaction time. All reactions were done at 350 ◦C.
Composition
Reacted fraction(1000)
MgH2
0.138
0.185
0.229
0.522
0.430
0.551
MgH2 + 4 mol% TiH2
MgH2 + 10 mol% TiH2
MgH2 + 20 mol% TiH2
MgH2 + 10 mol% Mg2Ni
MgH2 + 4 mol% TiH2 + 6 mol Mg2Ni
reaction rates. This is significant because it indicates that using an
appropriate mixture of catalysts is a promising way to make MgH2 a
suitable material for hydrogen storage purposes. These findings are
in agreement with those of Lu et al. [22]. They studied the hydro-
gen storage behavior of MgH2 mechanically alloyed with Ti and
Ni catalysts and found that a combination of Ti and Ni is more
effective catalyst for increasing reaction rates than Ti or Ni alone
(Table 2).
Fig. 7. Reaction rate plots for MgH2 and several TiH2–MgH2 mixtures.
the values of ꢀH systematically decrease from a high of 76 kJ/mol
for pure MgH2 to a low of 65 kJ/mol for the mixture containing
50 mol% TiH2. This indicates that the thermodynamic stability of
MgH2 mixtures decreases with increasing TiH2 content. This find-
ing is somewhat different than that reported by Liang et al. [19].
They studied the catalytic effect of transition metals on hydrogen
sorption by MgH2 and found that the formation enthalpy of MgH2
was not altered by milling with transition metals.
4. Conclusions
This research has shown that TiH2 and Mg2Ni are both effec-
tive catalysts for lowering the reaction temperature of MgH2 and
increasing reaction rates, with Mg2Ni being the more effective of
the two. The research has also shown that a mixed catalyst is bet-
ter at increasing reaction rates than a single catalyst. This indicates
that an optimum amount of two or more catalysts is the most
promising way to make MgH2 a suitable material for hydrogen
storage purposes. The research has also demonstrated that the
enthalpy for the reaction of hydrogen with MgH2 decreases with
the addition of TiH2. Thus it appears that the thermodynamic sta-
bility and reaction rates can be affected by ball milling MgH2 with
TiH2.
In addition to lowering reaction temperatures, it is also impor-
tant to have fast reaction rates. Huot et al. [20] reported that adding
Mg2Ni to MgH2 actually decreases reaction rates. Therefore a series
of experiments were done in order to determine the effect of addi-
tives on the reaction rates of H2 with magnesium. Fig. 7 contains
plots of reacted fraction versus time for the uptake of hydrogen by
the MgH2–TiH2 for mixtures containing 0, 4, 10, and 20 mol% TiH2.
In these experiments the pressure in the reaction chamber was ini-
tially set to 100 atm. Then the pressure decrease in the constant
volume system was monitored as the sample mixture absorbed
hydrogen. It can be seen that reaction rates increase with increasing
percentage of TiH2 in each mixture. The reaction rates of hydrogen
in mixtures containing both TiH2 and Mg2Ni were also measured.
Fig. 8 contains the rate curves for mixtures containing 10 mol% TiH2,
10 mol% Mg2Ni, or a mixed catalyst containing 4 mol% TiH2 + 6 mol%
Mg2Ni. From the curves it is evident that the Mg2Ni is more effec-
tivethan TiH2 inincreasingreactionrate. This is somewhatdifferent
than the findings of Huot et al. who reported a decrease in reaction
rates of desorption reactions. It is also evident that the mixed cat-
alyst is more effective than the individual catalysts at increasing
Acknowledgements
This work was financially supported by grants from the Depart-
ment of Energy, the US Department of Transportation and the BP
Foundation.
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