Materials Sciences and Applications, 2011, 2, 141-150
doi:10.4236/msa.2011.23018 Published Online March 2011 (http://www.SciRP.org/journal/msa)
Gaseous and Electrochemical Hydrogen Storage
Properties of Nanocrystalline Mg2Ni-Type Alloys
Prepared by Melt Spinning
Zhihong Ma1,3, Bo Li1, Huiping Ren2, Zhonghu i Hou2, Guofang Zhang2, Yanghuan Zhang1,2
1Department of Functional Material Research, Central Iron and Steel Research Institute, Beijing, China; 2ElectedState Key Labora-
tory, Inner Mongolia University of Science and Technology, Baotou, China; 3Baotou Research Institute of Rare Earths, Baotou, China.
Received November 28th, 2010; revised January 17th, 2011; accepted January 21st, 2011.
ABSTRACT
A partial substitution of Ni by Cu has been carried out in order to improve the hydrogen storage characteristics of the
Mg2Ni-type alloys. The nanocrystalline Mg20Ni10-xCux (x = 0, 1, 2, 3, 4) alloys are synthesized by the melt-spinning
technique. The stru ctures of the as-cast and spun allo ys have been chara cterized by X-ray diffraction (XRD), scanning
electron microscope (SEM) and high resolution transmission electron microscope (HRTEM). The electrochemical per-
formances were evaluated by an automatic galvanostatic system. The hydrogen absorption and desorption kinetics of
the alloys were determined by using an automatically controlled Sieverts apparatus. The results indicate that the sub-
stitution of Cu for Ni does not alter the major phase Mg2Ni. The Cu substitution significantly ameliorates the electro-
chemical hydrogen storage performances of alloys, involving both the discharge capacity and the cycle stability. The
hydrogen absorption capacity of alloys has been observed to be first increase and then decrease with an increase in the
Cu contents. However, the hydrogen desorption capacity of the alloys exhibit a monotonous growth with an increase in
the Cu contents.
Keywords: Mg2Ni-Type Alloy, Cu Substitution, Melt Spinning, Hydr o gen Storage Prope rt y
1. Introduction
Mg and Mg-based alloys in the form of metallic hydrides
such as MgH2 and Mg2NiH4 have been considered as
potential materials for solid state hydrogen storage. The
theoretical hydrogen storage capacities of MgH2 and
Mg2NiH4 are 7.6 wt% and 3.6 wt% [1,2] respectively.
Unfortunately, the applications of this kind of materials
are marred by their poor sorption/desorption kinetics and
high dissociation temperature. Therefore, during the re-
cent years, the main focus of research in this area has
been to find the ways to substantially ameliorate the hy-
dration kinetics of Mg-based alloys. In the past, various
efforts such as mechanical alloying (MA) [3], GPa hy-
drogen pressure method [4], melt spinning [5], gravity
casting [6], hydriding combustion synthesis [7], surface
modification [8], alloying with other elements [9,10], and
adding catalysts [11] have been undertaken to improve
the activation and hydriding properties.
Zaluska et al. [12] have demonstrated the excellent
absorption/desorption kinetics of a milled mixture of
Mg2NiH4 and MgH2 at 220-240˚C and claimed a maxi-
mum hydrogen concentration of more than 5 wt%.
Hanada et al. [13] have reported a hydrogen storage ca-
pacity of 6.5 wt% after doping of MgH2 with nanosized-
Ni in a temperature range of 150-250˚C. Recham et al.
[14] have concluded that the hydrogen absorption char-
acteristics of ball-milled MgH2 can be enhanced by add-
ing NbF5, and MgH2 + NbF5 composite has been found
to desorb 3 wt% H2 at 150˚C. Dobrovolsky et al. [15]
have synthesized a MgH2 (50 wt%) + TiB2 (50 wt%)
composite by intensive mechanical milling and found
that TiB2 additions lower the dissociation temperature of
the MgH2 hydride by about 50˚C. The results reported by
Cui et al. [16] have testified the capability of amorphous
and/or nanocrystalline Mg–Ni-based alloys to electro-
chemically absorb and also desorb a large amount of hy-
drogen at ambient temperatures. Kohno et al. [17] have
documented a large discharge capacity of 750 mA·h/g at
a current density of 20 mA/g for modified Mg2Ni alloys.

Gaseous and Electrochemical Hydrogen Storage Properties of Nanocrystalline Mg2Ni-Type Alloys
Prepared by Melt Spinning
142
Ball-milling indubitably is a very effective method for
the fabrication of nanocrystalline and amorphous Mg and
Mg-based alloys. Particularly, it is quite appropriate to
solubilize the particular elements into MgH2 or Mg2NiH4
above the thermodynamic equilibrium limit. This may
facilitate the destabilization of MgH2 or Mg2NiH4 [18].
However, the milled Mg and Mg-based alloys exhibit
very poor hydrogen absorbing and desorbing stability on
account of the evanishment of the metastable structures
formed by ball milling during the multiple hydrogen ab-
sorbing and desorbing cycles [19]. Alternatively, the
melt-spun technique has not only overcome the afore-
mentioned shortcoming but also prohibits the significant
degradation of the hydrogen absorbing and desorbing
cyclic characteristics of Mg and Mg-based compounds
[20]. Furthermore, the melt-spinning technique is a bene-
ficial method to yield a nanocrystalline structure and has
been regarded to be the most appropriate for the mass-
production of the nanocrystalline Mg-based alloys. It has
also been testified that the nanocrystalline alloys pro-
duced by melt-spinning method can exhibit excellent
hydriding characteristics even at ambient temperatures,
which is similar to that of the alloys fabricated by the
MA process. Spassov et al. [21] have prepared Mg2 (Ni, Y)
hydrogen storage alloy with possessing the composition
of Mg63Ni30Y7 by rapid solidification process to yield a
maximum hydrogen absorption capacity of about 3.0
wt%. In addition, the hydrogenation kinetics of the melt-
spun Mg2 (Ni, Y) have been observed to exceed those of
the conventionally prepared polycrystalline Mg2Ni alloys
and also found to be comparable to that of the nanocrys-
talline ball-milled Mg2Ni. Huang et al. [22] have con-
cluded that the amorphous and the nanocrystalline Mg-
based alloy (Mg60Ni25)90Nd10 prepared by melt-spinning
technique displays the highest discharge capacity of 580
mAh/g and the maximum hydrogen capacity of 4.2 wt%
H.
Our previous work has confirmed that the substitution
of Co for Ni significantly improves the hydriding and
dehydriding kinetics of the Mg2Ni-type alloys [23].
Therefore, it is very desirable to investigate the influence
of substituting the Ni with Cu on the hydrogen storage
characteristics of Mg2Ni-type alloys prepared by melt-
spinning. The objective of present work is to synthesize
the Mg-Ni-based ternary nanocrystalline alloys by melt
spinning and to examine their structures and hydrogen
storage characteristics.
2. Experimental
The nominal compositions of the experimental alloys
were Mg20Ni10-xCux (x = 0, 1, 2, 3, 4). For convenience,
the alloys were denoted with Cu content as Cu0, Cu1, Cu2,
Cu3 and Cu4, respectively. The alloy ingots were pre-
pared using a vacuum induction furnace in a helium at-
mosphere at a pressure of 0.04 MPa. A part of the as-cast
alloys was re-melted and spun by melt-spinning with a
rotating copper roller. The spinning rate was approxi-
mately expressed by the linear velocity of the copper
roller because it was too difficult to measure a real spin-
ning rate i.e. the cooling rate of the sample during spin-
ning. The spinning rates used in the experiment were 15,
20, 25 and 30 m/s.
The phase structures of the as-cast and spun alloys
were determined by XRD (D/max/2400). The diffraction,
with the experimental parameters of 160 mA, 40 kV and
10˚/min was performed with CuKα1 radiation filtered by
graphite. The morphologies of the as-cast alloys were
examined by SEM (Philips QUANTA 400). The thin
film samples of the as-spun alloys were prepared by ion
etching for observing the morphology with HRTEM
(JEM-2100F, operated at 200 kV), and for determining
the crystalline state of the samples with electron diffrac-
tion (ED).
The alloy ribbons were pulverized and then mixed
with carbonyl nickel powder in a weight ratio of 1:4. The
mixture was cold pressed into round electrode pellets of
10 mm in diameter and total mass of about 1 g with a
pressure of 35 MPa. A tri-electrode open cell, consisting
of a metal hydride electrode, a sintered NiOOH/Ni(OH)2
counter electrode and a Hg/HgO reference electrode, was
used for testing the electrochemical characteristics of the
experimental alloy electrodes. A 6 M KOH solution was
used as electrolyte. The voltage between the negative
electrode and the reference electrode was defined as the
discharge voltage. In every cycle, the alloy electrode was
first charged at a current density of 20 mA/g, after resting
for 15 min, it was discharged at the same current density
to −0.500 V cut-off voltages. The environment tempera-
ture of the measurement was kept at 30˚C.
The hydrogen absorption and desorption kinetics of
the alloys were monitored by an automatically controlled
Sieverts apparatus. The hydrogen absorption was con-
ducted at 1.5 MPa and the hydrogen desorption at a
pressure of 1 × 10−4 MPa was performed at 200˚C.
3. Results and Discussion
3.1. Microstructure Characteristics
The XRD profiles of the as-cast and spun Mg20Ni10-xCux
(x = 0-4) alloys are presented in Figure 1. The results
indicate that all the as-cast and spun alloys display a sin-
gle phase structure. The substitution of Cu for Ni does
not modify the phase structure. Table 1 lists the lattice
parameters, cell volume and full width at half maximum

Gaseous and Electrochemical Hydrogen Storage Properties of Nanocrystalline Mg2Ni-Type Alloys
Prepared by Melt Spinning
143
Table 1. The lattice parameters, cell volume and the FWHM values of the major diffraction peaks of the alloys.
FWHM values Lattice parameters and cell Volume
2θ (20.02°) 2θ (45.14°) a (nm) c (nm) V (nm3)
Alloys
As-cast 15 m/s As-cast 15 m/sAs-cast 15 m/s As-cast 15 m/s As-cast 15 m/s
Cu0 0.122 0.125 0.169 0.171 0.5210 0.5210 1.3244 1.3251 0.3113 0.3115
Cu1 0.133 0.155 0.178 0.194 0.5210 0.5212. 1.3252 1.3259 0.3115 0.3120
Cu2 0.148 0.181 0.183 0.207 0.5214 0.5216 1.3283 1.3293 0.3127 0.3132
Cu3 0.151 0.197 0.192 0.215 0.5215 0.5217 1.3297 1.3305 0.3132 0.3135
Cu4 0.165 0.232 0.204 0.241 0.5217 0.5220 1.3302 1.3311 0.3135 0.3141
(a) (b)
Figure 1. XRD profiles of the as-cast and spun alloys: (a) As-cast; (b) As-spun (15 m/s).
(FWHM) values of the main diffraction peaks of the
as-cast and spun (15 m/s) alloys, which are calculated by
software Jade 6.0. It is evident in Table 1 that the sub-
stitution of Cu for Ni intensifies the FWHM values of the
main diffraction peaks exhibited by the as-cast and spun
alloys. Furthermore, it leads to a sharp enlargement of
the lattice parameter and cell volume of the alloys, justi-
fies the successful alloying of Cu with Mg2Ni. Table 1
also demonstrates the enhancement in the FWHM values
of the main diffraction peaks of the alloys caused by the
melt spinning, which is ostensibly attributed to the re-
finement of grains and the accumulated stress in grains
rendered by the melt spinning. The crystallite size <Dhkl>
(nm) of the as-spun alloy has been calculated by utilizing
the FWHM values of the broad diffraction peak (203) in
Figure 1(b) by employing the Scherer’s equation. The
grain size of the as-spun alloys is found to be in the range
of 15-30 nm, which is consistent with the results reported
by Friedlmeier et al. [24]. It is noteworthy that for the
comparison purposes, the <D> values have been calcu-
lated by using the similar peak having the Miller indices
(203).
The SEM images of the as-cast Cu0 and Cu2 alloys are
illustrated in Figure 2. It is quite evident that the as-cast

Gaseous and Electrochemical Hydrogen Storage Properties of Nanocrystalline Mg2Ni-Type Alloys
Prepared by Melt Spinning
144
alloys exhibit a typical dendritic structure. The substitu-
tion of Cu for Ni, instead of changing the morphology of
the alloys, causes a significant refinement of the grains.
The result obtained by energy dispersive spectrometry
(EDS) reveals that the major phase of the as-cast alloys is
Mg2Ni phase (denoted by A). The Cu2 alloy clearly ex-
hibits some small massive particulates. The EDS analysis
confirms that these particulates are Mg2Cu phase (de-
noted by B). This result is contrary to the XRD observa-
tions depicted in Figure 1. This phenomenon may be
attributed to the fact that the amount of the Mg2Cu phase
is very little and therefore unable to be detected by the
XRD observation.
Figure 3 depicts the TEM micrographs and electron
diffraction patterns of as-spun Cu0 and Cu2 alloys. A na-
nocrystalline microstructure possessing an average crys-
tal size of about 20 nm is evident. TEM observations
clearly supplement the evidence of the presence of
strongly disordered and nanostructured phase of as-spun
alloys. This result agrees very well with the XRD obser-
vations shown in Figure 1.
The existence of crystal defects in the as-spun alloy
such as stacking faults (denoted as A), dislocations (de-
noted as B), sub-grain boundaries (denoted as C) and
twin-grain boundaries (denoted as D) is clearly depicted
in Figure 4.
3.2. Electrochemical Hydrogen Storage
Performances
3.2.1. Activation Capability and Discharge Capacity
Electrochemical galvanostatic charge/discharge is an
effective and time-saving method for determining the
hydrogen absorbing capacity as compared with a gaseous
technique. The influence of substituting Ni with Cu on
(a) (b)
Figure 2. SEM images of the as-cast alloys together with typical EDS spectra of sections A and B in Figure 2(b): (a) Cu0
alloy; (b) Cu2 alloy.

Gaseous and Electrochemical Hydrogen Storage Properties of Nanocrystalline Mg2Ni-Type Alloys
Prepared by Melt Spinning
145
(a) (b)
Figure 3. HRTEM micrographs and ED of the as-spun alloys (30 m/s): (a) Cu0 alloy; (b) Cu2 alloy.
(a) (b)
Figure 4. Crystal defects in the as-spun (30 m/s) Cu2 alloy taken by HRTEM: (a) Stacking fault; (b) Dislocations,
sub-grain boundaries and twin-gr a in boundar y.
the activation capability of the alloys with a charging-
discharging current density of 20 mA/g is shown in Fig-
ure 5. The figure demonstrates that all the alloys exhibit
excellent activation capability and attains their maximum
discharge capacities during the first charging-discharg-
ing cycle. The activation performances of the alloys are
not affected by the substitution of Cu for Ni. The dis-
charge capacity of the as-spun alloys first increases and
then decreases with the variation of Cu content. The Cu2
alloy presents the maximum discharge capacities of
135.8 mAh/g and 189.3 mAh/g corresponding with two
spinning rates of 20 m/s and 30 m/s, respectively. It must
be mentioned that the discharge capacity of alloys sub-
stituted by Cu is higher than that of the Cu-free alloy,
suggesting that the substitution of Cu for Ni has amelio-
rated the discharge capacity of Mg2Ni-type alloy. A
similar result has already been reported by Simičić et al.
[2]. The specific capacity and hydriding/dehydriding
kinetics of hydride electrode materials depend on their
chemical composition and crystalline structure. It has
been ascertained that the high hydride formation enthalpy
of Mg2Ni accounts for its low discharge capacity. The
partial substitution of some elements (Cu, Fe, V, Cr, Co)
for Ni in Mg2Ni compound may facilitate the destabiliza-
tion of the hydride and activate the Mg2Ni phase to ex-
hibit the reversible hydrogen storage properties in the
alkaline electrolyte [25]. On the other hand, the secon-
dary phase Mg2Cu probably acts as an efficient catalyst
to dissociate the H2 molecules and transferring the H
atoms to the surrounding Mg2Ni matrix [19].
3.2.2. Charging and Discharging Cycle Stability
The cyclic stability of the electrode alloy is a decisive
factor in determining the life of Ni-MH battery. The ca-

Gaseous and Electrochemical Hydrogen Storage Properties of Nanocrystalline Mg2Ni-Type Alloys
Prepared by Melt Spinning
146
(a) (b)
Figure 5. Evolution of the discharge capacity of the as spun alloys with the cycle number: (a) 20 m/s; (b) 30 m/s.
pacity retaining rate (Sn), which is introduced to evaluate
accurately the cyclic stability of the alloy, is defined as
Sn = Cn/Cmax × 100%, where Cmax is the maximum dis-
charge capacity and Cn is the discharge capacity of the
nth charge-discharge cycle. It has been observed that the
large capacity retaining rate (Sn) yields in the better cycle
stability of the alloy. The capacity retaining rates of
as-spun alloys as a function of cycle number are plotted
in Figure 6. The figure indicates that the substitution of
Cu for Ni has significantly enhanced the cyclic stability
of as-spun alloys. As the Cu contents x grow from 0 to 4,
the capacity retaining rate of the as-spun (20 m/s) alloy at
20th cycle increases from 31.3 to 57.2%, and from 27.1
to 51.1% for the as-spun (30 m/s) alloy. It is well known
fact that the main rationale for the capacity degradation
of Mg-based alloy electrodes is the severe corrosion of
Mg in the alkaline KOH solution. Especially, during the
discharging process, the anodicpolarization of alloys fa-
cilitates the faster corrosion rate [25]. On the other hand,
the vanishment of metastable structures formed by melt
spinning or ball milling during the multiple charging/
discharging cycles tend to enhance the capacity decay of
the alloys. Two reasons are responsible for the enhanced
cyclic stability of the Mg2Ni-type alloy subjected with
Cu substitution. Firstly, the improved performance re-
garding the cyclic life of substituted alloy electrode is
presumably attributed to preferential oxidation of Cu on
the alloy surface, which prevents the formation of
Mg(OH)2 passive layer. Secondly, the additions of a third
element significantly stabilize the nanostructure forma-
tion of Mg-Ni-based alloy [21], suggesting an increase of
the cyclic stability of alloy. Furthermore, the comparison
of Figures 6(a) and (b) reveals that the capacity retaining
rates of alloys slightly decline with rising spinning rate.
It implies that the melt spinning mildly impairs the cyclic
stability of alloys. The nanostructure exhibiting by the
alloys resulting from melt spinning has been considered
to be detrimental due to its corrosion in the electrolyte
during cycling on account of the fact that the intercrys-
talline corrosion is facilitated by the nanostructure for-
mation. This provides an illustration for the decline of
the cyclic stability of the Mg-Ni-Cu system alloy caused
by a higher spinning rate.
3.3. Hydriding and Dehydriding Characteristics
The hydrogen absorption kinetic curves of the as-spun
alloys are depicted in Figure 7. It is evident that the hy-
drogen absorption capacity of the as-spun alloys first
increases and then decreases with the variation of Cu
content. The Cu2 alloy demonstrates the maximum hy-
drogen absorption capacity at 200˚C. The evidence of
extremely fast kinetics of hydrogenation is provided by
the fact that the alloys acquire more than 95% of their
hydrogen capacities within the first 5 min. The excellent
hydriding kinetics is ascribed to the nanocrystalline
structure resulting in the high surface to volume ratios
(high specific surface area). In addition, the presence of
large number of grain boundaries in nanocrystalline al-
loys enhances the kinetics of hydrogen absorption/de-
sorption. The benefaction of Cu substitution on the hy-
drogen absorption capacity and kinetics of the alloy has
also been attributed to the increased cell volume and the
refined grain caused by Cu substitution. The enlargement
in the cell volume is highly beneficial to the hydrogen
absorption capacity, since, the grain boundary possesses
the capability of the largest hydrogen absorption [25]. It
is well known that the catalytic action of Ni on hydriding
is stronger than Cu. Therefore, it is justifiable that a su-
perfluous amount of Cu substitution (x > 2) leads to a
decrease of the hydrogen absorption capacity of the al-

Gaseous and Electrochemical Hydrogen Storage Properties of Nanocrystalline Mg2Ni-Type Alloys
Prepared by Melt Spinning
147
(a) (b)
Figure 6. Evolution of the capacity retaining rate of the alloys with cycle number: (a) 20 m/s; (b) 30 m/s.
(a) (b)
Figure 7. Hydrogen absorption kine tic curv e s of the as-spun alloys: (a) 20 m/s; (b) 30 m/s.
loys.
The hydrogenation kinetics and storage capacity of all
the as-spun nanocrystalline Mg2Ni-type alloys have been
found to be superior to those of conventional polycrystal-
line materials possessing the similar composition. The
enhanced hydrogenation property generated by melt
spinning is doubtlessly associated with the refinement of
grains produced by melt spinning [26]. Upon refining the
microstructure, a lot of new crystallites and grain bounda-
ries evolve, which may act as fast diffusion paths for
hydrogen absorption.
The hydrogen desorption kinetic curves of the as-spun
alloys are plotted in Figure 8. An essential characteristic
of the dehydrogenation process in the alloys is very fast
hydrogen desorption at the initial stages, followed by a
slack increase in the amount of hydrogen absorbed. The
specific capacity and hydriding/dehydriding kinetics of
hydride materials depend on their chemical composition
and crystalline structure [27]. The observed differences
from the previous documented results about the hydrid-
ing/dehydriding kinetics of the melt-spun nanocrystalline
Mg2Ni type alloys may be associated with the composi-
tion of alloys, whereas, the differences in their micro-
structure may be ascribed to the various spinning rates. It
has been already reported that the high surface to volume
ratios (high specific surface area) and the existence of
large number of grain boundaries in nanocrystalline al-
loys enhance the kinetics of hydrogen absorption/de-
sorption [21]. Zaluski et al. [28] and Orimo et al. [29]
have confirmed the exhibition of low temperatures (lower
than 200˚C) hydriding/dehydriding characteristics of na-
nocrystalline Mg2Ni alloys prepared by mechanical al-

Gaseous and Electrochemical Hydrogen Storage Properties of Nanocrystalline Mg2Ni-Type Alloys
Prepared by Melt Spinning
148
(a) (b)
Figure 8. Hydrogen desorption curves of the as-spun alloys: (a) 20 m/s; (b) 30 m/s.
loying. They have testified that a reduction in the grain
size (20-30 nm) enhances the hydriding/dehydriding cha-
racteristics owing to the hydrogen occupation in the dis-
ordered interface phase. Two reasons are chiefly respon-
sible for the impact action of Cu substitution on the de-
hydriding kinetics of the alloys. Firstly, the partial sub-
stitution of element Cu for Ni in Mg2Ni compound
weakens the stability of hydride and renders the desorp-
tion reaction easier [30]. Secondly, the presence of
Mg2Cu phase apparently presents the catalytic effects for
the hydriding and dehydriding reactions of Mg and
Mg-based alloys [19].
4. Conclusions
1) All the as-spun Mg20Ni10-xCux (x = 0, 1, 2, 3, 4) alloys
exhibit the nanocrystalline structures without showing
any presence of amorphous phase. The substitution of Cu
for Ni does not vary the major phase of Mg2Ni-type in
the alloy. On the contrary, the substitution leads to a sig-
nificant refinement of grains in the as-cast alloys.
2) The Cu substitution has significantly enhanced the
electrochemical hydrogen storage performances of alloy.
It ameliorates the discharge capacity and the cycle stabil-
ity by lowering the stability of hydride. This improve-
ment is mainly attributed to the enlargement in the cell
volume and the refinement of grains caused by the Cu
substitution.
3) Furthermore, the substitution of Cu for Ni renders
the hydrogen absorption capacity of the alloys first in-
creased and then decreased. But overall it enhances the
hydrogen desorption capacity and dehydriding rate of the
alloys.
5. Acknowledgements
This work is supported by National Natural Science
Foundations of China (50871050 and 50961009), Natural
Science Foundation of Inner Mongolia, China (2010ZD05)
and Higher Education Science Research Project of Inner
Mongolia, China (NJzy08071).
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