542
H. Wang et al. / Journal of Alloys and Compounds 470 (2009) 539–543
tions (3)–(5) are in the following order:
From a thermodynamics point of view, the hydrolysis of Mg is eas-
ier than MgH2 since ꢀG1 < ꢀG12. However, it was observed that
the hydrolysis of MgH2 is more severe than for Mg regardless of
whether it is in water or in an alkaline solution. Possible reasons
for this behaviour as follows: There is a volume expansion of 32%
the bonds in MgH2 are greatly weakened. In addition, MgH2 is an
ionic compound, which is more easily affected by the ionic water.
ular tetrahedral [NiH4] complex surrounded by a distorted cube of
magnesium ions [20]. H atoms prefer to be located in the neighbour-
hood of the Ni atoms and are covalently bonded with Ni forming a
complex of nominal composition NiH4. The NiH4 is ionically bonded
to magnesium [21]. The ionic characteristic of Mg2NiH4 may be the
reason for the release of hydrogen on immersion in water.
The hydrolysis characteristics of Mg2Ni and Mg2NiH4 in water
and in alkaline solutions may be the reason that Mg2Ni exhibited a
rapid degradation and a poor cycle life in alkaline solution. Hence,
Mg2Ni is not suitable for use as electrodes in rechargeable batter-
ies. In addition, their electrochemical discharge capacities cannot
be improved through generating nano-crystalline Mg2Ni since
reducing the Mg2Ni particle size will accelerate its hydrolysis rate.
After removal of Mg(OH)2 in the hydrolysis product obtained
in distilled water using dilute hydrochloric acid, Ni nanoparticles
can be obtained. Therefore, the hydrolysis of Mg2Ni and Mg2NiH4
provides with a new and relatively simple method for producing
nickel nanoparticles. Compared with other more conventional syn-
thesis methods for Ni nanoparticles, this method offers a potential
to produce Ni nanoparticles on a large scale. In addition, there are
many similar binary magnesium intermetallic compounds, such
as Mg2Cu, Mg3Ag, Mg3Au, Mg3Pt and Mg3Pd that had been suc-
cessfully used to produce these transition metal nanoparticles by a
similar hydrolysis technique [22].
ꢀG4 < ꢁG3 < ꢁG5
(9)
Therefore, Reaction(4) ismorefavourablethan Reaction (3). In addi-
tion, if there is an intermediate Reaction (3), MgNi2 should not start
to hydrolyze until the hydrolysis is complete for all Mg2Ni since
Mg2Ni is more active than MgNi2. Both Mg2Ni and Ni, but no MgNi2
were found in the hydrolysis product. Thus, Mg2Ni probably directly
hydrolyzed into Ni.
When the ball-milled Mg2Ni particles were immersed in the dis-
tilled water, a small amount of the surplus Mg and a larger amount
of Mg2Ni gave rise to many small cells with small cathodes and large
anodes. Thus, Reaction (1) is enhanced, and the hydrolysis reaction
for Mg2Ni was inhibited until all the Mg was consumed. At that
stage, the hydrolysis of Mg2Ni would begin.
There is not a large difference in the thermodynamics for hydrol-
ysis between the ball-milled Mg2Ni particles in water and in the
6 M KOH solution. However, the activities of Mg(OH)2 and H2O are
significantly affected.
Reaction (4) can be written as two ionic reactions:
Mg2Ni(s) + 4H+ → 2Mg2+ + Ni + 2H2(g)
Mg2+ + 2OH− ↔ Mg(OH)2(s)
(10)
(11)
The solubility of product constants of Mg(OH)2 is 5.6 × 10−12. The
OH− concentration is estimated as 2.237 × 10−4. The corresponding
pH value is 10.4, which agrees with the pH value (10–11) of the
solution in the samples immersed in distilled water. Kuji et al. [8],
reported that the pH value is 11.2. The pH value of the 6 M KOH
solution is higher than 14. Hence, the concentration of the H+ ions in
the samples immersed in the 6 M KOH solution is much lower than
that of the samples immersed in water. Therefore, the hydrolysis
rate of Mg2Ni in the 6 M KOH solution is much slower that that in
water. In other words, if the pH value of the solution is reduced
(adding some acid), the hydrolysis rate of Mg2Ni will be greatly
increased.
5. Conclusions
The main conclusions from this study are as follows:
MgH2 and Mg2NiH4 in the hydrogenated samples can react with
water and form Mg(OH)2, and release hydrogen gas. The reaction
can be written as follows:
1. When Mg2Ni is immersed in water or in an alkaline solution, it
will spontaneously react with water to form Mg(OH)2, Ni and
hydrogen.
2. When Mg2NiH4 is immersed in water or in an alkaline solution,
it will spontaneously first dissociate into Mg2Ni and hydrogen.
The Mg2Ni will then further hydrolyze into Mg(OH)2 and Ni.
3. Reducing the pH value of the solution (adding an acid) will accel-
erate the hydrolysis of Mg2Ni and Mg2NiH4.
MgH2(s) + 2H2O(l) → Mg(OH)2(s) + 2H2(g)
(12)
The XRD patterns for the hydrolysis product of Mg2NiH4 in water
and the alkaline solution suggest that Mg2NiH4 was first dissociated
into Mg2Ni and hydrogen. The dehydrogenation of Mg2NiH4 can be
written as:
4. The hydrolysis characteristics of Mg2Ni and Mg2NiH4 suggest
that they are not suitable for use as electrodes in rechargeable
batteries.
Mg2NiH4(s) → Mg2Ni(s) + 2H2(g)
(13)
The Mg2Ni will further hydrolyze into Mg(OH)2 and Ni in water
and in an alkaline solution (Reaction (4)). The overall reaction for
the Mg2NiH4 hydrolysis is:
5. Ni particles resulting from hydrolysis of Mg2Ni are roughly spher-
ical in shape and have a very small particle size (nano range).
6. The hydrolysis of Mg2Ni and Mg2NiH4 provides a new and rela-
tively simple method for producing nano-size nickel particles.
Mg2NiH4(s) + 4H2O(l) → 2Mg(OH)2(s) + Ni + 4H2(g)
(14)
Acknowledgements
The standard free energy change for MgH2 and Mg2NiH4 are
−35.9 kJ/mol, −64.4 kJ/mol, respectively. The free energy changes
ꢀG12, ꢀG13 and ꢀG14 for Reactions (12)–(14) are given as:
This research was financially supported by the Natural Sciences
and Engineering Research Council of Canada (NSERC) through Dis-
covery Grants awarded to Drs. D.O. Northwood and H. Hu.
2
H
˛
P
Mg(OH)
2
2
ꢀG12 = −323.52 kJ/mol + RT ln
(15)
2
˛
H
˛
MgH
O
2
2
References
2
˛
P
MgNi
H
2
ꢀG13 = 12.5 kJ/mol + RT ln
(16)
(17)
˛
MgNiH
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4
2
4
˛
˛ P
Ni
H
Mg(OH)
2
2
ꢀG14 = −654.44 kJ/mol + RT ln
4
˛
H
˛
Mg NiH
O
2
4
2