Materials Sciences and Applications, 2010, 1, 168-176
doi:10.4236/msa.2010.13027 Published Online August 2010 (http://www.SciRP.org/journal/msa)
Hydrogen Absorption and Electrochemical
Properties of As-Quenched Nanocrystalline
Mg20Ni10 – xCux (x = 0 – 4) Alloys
Jinliang Gao1, Zhonghui Hou2, Qilu Ge1, Dongliang Zhao1, Shihai Guo1, Yanghuan Zhang1,2
1Department of Functional Material Research, Central Iron and Steel Research Institute, Beijing, China; 2School of Material, Inner
Mongolia University of Science and Technology, Baotou, China.
Received April 20th, 2010; revised May 18th, 2010; accepted May 19th, 2010.
ABSTRACT
Nanocrystalline Mg2Ni-type alloys with nominal compositions of Mg20Ni10 – xCux (x = 0, 1, 2, 3, 4) were synthesized by
rapid quenching technique. The microstructures of the as-cast and quenched alloys were characterized by XRD, SEM
and HRTEM. The hydrogen absorption and desorption kinetics of the alloys were measured using an automatically
controlled Sieverts apparatus. The electrochemical hydrogen storage performances were tested by an automatic gal-
vanostatic system. The results show that all the as-quenched alloys hold a typical nanocrystalline structure, and the
rapid quenching does not change the major phase Mg2Ni. The hydrogen absorption and desorption capacities of the
alloys significantly increase with rising quenching rate. Additionally, the rapid quenching significantly improves the
electrochemical hydrogen storage capacity of the alloys, but it slightly impairs the cycle stability of the alloys.
Keywords: Mg2Ni-type Alloy, Rapid Quenching, Nanocrystalline, Hydrogen Absorption, Electrochemical Properties
1. Introduction
Among the known alloys with a potential use in hydrog-
en storage, Mg and Mg-based metallic hydrides are consi-
dered to be more promising materials for hydrogen stor-
age because of their highest hydrogen capacity and low
price [1,2]. Unfortunately, some shortcomings of these
kinds of metal hydrides, such as slow sorption/desorption
kinetics, high dissociation temperature and poor electroc-
hemical cycling properties, limit their practical applica-
tion. Therefore, finding ways of improving the hydration
kinetics of Mg-based alloys has been one of the main ch-
allenges faced by researchers in this area. Various attem-
pts, involving mechanical alloying (MA) [3], GPa hydr-
ogen pressure method [4], melt spinning [5], gravity ca-
sting [6], polyol reduction [7], hydriding combustion sy-
nthesis [8], surface modification [9], alloying with other
elements [10,11], adding catalysts [12] etc, have been un-
dertaken to improve the activation and hydriding proper-
ties.
Zaluska et al. [13] reported that a milled mixture of
Mg2NiH4 and MgH2 shows excellent absorption/desorp-
tion kinetics at 220-240˚C and a maximum hydrogen
concentration of more than 5 wt.%. Hanada et al. [14]
obtained a hydrogen storage capacity of 6.5 wt.% after
doping MgH2 with nanosized-Ni in a temperature range
of 150-250˚C. Recham et al. [15] found that the hydrog-
en absorption property of ball-milled MgH2 can be enha-
nced by adding NbF5, and MgH2 + 5wt.%NbF5 compos-
ite desorbs 3 wt.% of H2 at 150˚C. Dobrovolsky et al. [16]
synthesized a MgH2 (50 wt.%) + TiB2 (50 wt.%) com-
posite by intensive mechanical milling and found that
TiB2 addition decreases the dissociation temperature of
the MgH2 hydride by about 50˚C. The result obtained by
Cui et al [17] confirmed that amorphous and/or nanocr-
ystalline Mg-Ni-based alloys can electrochemically abs-
orb and also desorb large amounts of hydrogen already at
room temperature. Kohno et al. [18] obtained a large
discharge capacity of 750 mAh/g at a current density of
20 mA/g for modified Mg2Ni alloys.
Ball-milling, indubitably, is a quite powerful method
for the preparation of nanocrystalline and amorphous Mg
and Mg-based alloys. Particularly, it is suitable to solubi-
lize particular elements into MgH2 or Mg2NiH4 above the
thermodynamic equilibrium limit. This is helpful to des-
tabilize MgH2 or Mg2NiH4 [19]. However, the milled Mg
and Mg-based alloys show very poor hydrogen absorbing
and desorbing stability due to the fact that the metastable

Hydrogen Absorption and Electrochemical Properties of As-Quenched 169
Nanocrystalline Mg20Ni10 - xCux (x = 0 - 4) Alloys
structures formed by ball milling tended to vanish during
multiple hydrogen absorbing and desorbing cycles [20].
Alternatively, rapid quenching technique can overcome
the above mentioned shortcoming and effective avoiding
the significant degradation of hydrogen absorbing and de-
sorbing cycle properties of Mg and Mg-based [21]. Ad-
ditionally, the rapid quenching technique is an effective
method to obtain a nanocrystalline structure and is very
suitable for mass-production of nanocrystalline Mg-bas-
ed alloys. It was also confirmed that nanocrystalline al-
loys produced by rapid quenching could have excellent
hydriding characteristics even at room temperature, sim-
ilar to the alloys produced by the MA process. Spassov
et al. [22] prepared Mg2(Ni,Y) hydrogen storage alloy
with exact composition Mg63Ni30Y7 by rapid solidifica-
tion, and its maximum hydrogen absorption capacity
(about 3.0 wt.%) and hydrogenation kinetics of the as-
quenched Mg2(Ni, Y) were found to exceed those of the
conventionally prepared polycrystalline Mg2Ni alloys
and to be comparable to the hydrogen absorption charac-
teristics of nanocrystalline ball-milled Mg2Ni. Huang et al.
[23] found that amorphous and nanocrystalline Mg- based
alloy (Mg60Ni25)90Nd10 prepared by rapid quenching ob-
tained the highest discharge capacity of 580 mAh/g and
the maximum hydrogen capacity of 4.2 wt.% H.
The objective of this work is to produce the Mg-Ni-
based ternary nanocrystalline alloys by rapid quenching
and to examine the hydrogen absorption and electro-
chemical properties of the nanocrystalline Mg20Ni10-xCux
(x = 0 – 4) alloys.
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. Part of the as-cast
alloys was re-melted and quenched by melt-spinning
with a rotating copper roller. The quenching rate was
approximately expressed by the linear velocity of the
copper roller because it is too difficult to measure a real
quenching rate i.e. cooling rate of the sample during
quenching. The quenching rates used in the experiment
were 15, 20, 25 and 30 m/s, respectively.
The phase structures of the as-cast and quenched al-
loys were determined by XRD diffractometer (D/max/
2400). The diffraction, with the experimental parameters
of 160 mA, 40 kV and 10˚/min respectively, was per-
formed with CuKα1 radiation filtered by graphite. The
thin film samples of the as-quenched alloys were pre-
pared by ion etching for observing the morphology with
high resolution transmission electronic microscope (HR-
TEM) (JEM-2100F, operated at 200 kV), and for deter-
mining the crystalline state of the samples with electron
diffraction (ED). The morphologies of the as-cast alloys
were examined by scanning electronic microscope (SEM)
(Philips QUANTA 400).
The hydrogen absorption and desorption kinetics of
the alloys were measured by an automatically controlled
Sieverts apparatus. The hydrogen absorption was con-
ducted at 1.5MPa and the hydrogen desorption in a vac-
uum (1 × 10-4 MPa) at 200˚C. 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 electr-
olyte. 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 voltage. The environment temperature of the me-
asurement was kept at 30˚C.
3. Results and Discussion
3.1 Microstructure Characteristics
The XRD profiles of the as-cast and quenched Cu2 and
Cu4 alloys are presented in Figure 1, showing that all the
as-cast and quenched alloys display a single phase struc-
ture. The rapid quenching does not change the phase str-
ucture. Listed in Table 1 are the lattice parameters, the
cell volume and the full width at half maximum (FWHM)
values of the main diffraction peaks of the as-cast and
quenched Cu2 and Cu4 alloys which were calculated by
software of Jade 6.0. It can be derived from Table 1 that
the rapid quenching causes the FWHM values of the
main diffraction peaks of the alloys significantly incre-
ased and the lattice parameters and cell volume of the
alloys cleverly enlarged, which is doubtless attributed to
the refinement of the average grain size and stored stress
in the grains produced by the rapid quenching. The crys-
tallite size < Dhkl > (Ǻ) of the as-quenched alloy was
calculated from the FWHM values of the broad diffrac-
tion peak (203) in Figure 1(b), using Scherrer’s equation.
The grain sizes of the as-quenched alloys are in a range
of 2-6 nm, consistent with results reported by Friedlmeier
et al. [24]. It is important to notice that < D > values
were calculated on the same peak having Miller indices
(203) due to better possibility of mutual comparison.
Figure 2 shows the HRTEM micrographs and electron

Hydrogen Absorption and Electrochemical Properties of As-Quenched
Nanocrystalline Mg20Ni10 – xCux (x = 0 – 4) Alloys
170
diffraction pattern of the as-quenched Cu2 and Cu4 all-
oys, which display a nanocrystalline microstructure, with
an average crystalline size of about 2-5 nm. From HR-
TEM observations there is some evidence that the as-
quenched alloys were strongly disordered and nanostruc-
tured, but they are free of amorphous phase. This result
agrees very well with the XRD observation shown in Fi-
gure 1. The crystal defects in the as-quenched alloy, sta-
cking faults (denoted as A), twin-grain boundaries (deno-
ted as B), dislocations (denoted as C) and sub-grain bou-
ndaries (denoted as D) generated by rapid quenching, can
clearly be seen in Figure 3.
(403) (204)
(220) (118)
(215)
(206)
(212)
(211)
(203)
(113) (105)
(200)
(112)
(103)
(102)
(101)
(003)
△ △
△
△
△
△
△ △
△
△
△ △
△
△
△
△
△As-cast
15 m/s
20 m/s
25 m/s
(a)
△-Mg2Ni 30 m/s
(403)
(204)
(220)
(118)
(215) (206)
(212)
(211)
(203)
(105)
(200)
(112)
(103)
(102)
(101) (003)
△△
△
△
△
△△△
△
△
△
△
△
△
△
△As-cast
15 m/s
20 m/s
25 m/s
(b) △-Mg2Ni 30 m/s
2 θ (degree)
20 30 40 50 60 70 80 90 10020 30 40 50 60 70 80 90 100
2 θ (degree)
△ － Mg
2
Ni △ － Mg
2
Ni
30 m/s
25 m/s
30 m/s
25 m/s
20 m/s20 m/s
15 m/s15 m/s
As-cast As-cast
(a) (b)
Figure 1. XRD profiles of the as-cast and spun alloys: (a) Cu2 alloy; (b) Cu4 alloy
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)
Quenching
rates
(m/s) Cu2 Cu4 Cu2 Cu4 Cu2 Cu4 Cu2 Cu4 Cu2 Cu4
0 0.148 0.165 0.183 0.2040.52140.52171.32831.33020.3127 0.3135
15 0.181 0.232 0.207 0.2410.52160.52201.32931.33110.3132 0.3141
20 0.232 0.286 0.223 0.2520.52160.52201.33071.33170.3135 0.3143
25 0.258 0.292 0.242 0.2730.52170.52211.33111.33230.3138 0.3145
30 0.274 0.305 0.259 0.2850.52190.52221.33161.33310.3141 0.3148
(a) (b)
Figure 2. HRTEM micrographs and ED of the as-spun alloys (30 m/s) (a) Cu2 alloy (b) Cu4 alloy

Hydrogen Absorption and Electrochemical Properties of As-Quenched 171
Nanocrystalline Mg20Ni10 - xCux (x = 0 - 4) Alloys
Shown in Figure 4 are the SEM images of the as-cast
alloy, displaying a typical dendrite structure. The substit-
ution of Cu for Ni does not change the morphology of the
alloys but it causes a significant refinement of the grains.
The result obtained by energy dispersive spectrometry
(EDS) indicates that the major phase of the as-cast alloys
is Mg2Ni phase (denoted as A), but Cu substitution leads
to the secondary phase Mg2Cu formed (denoted as B).
This seems to be contrary with the result of XRD observ-
ation shown in Figure 1. It is most probably associated
with the fact that Mg2Ni and Mg2Cu hold completely id-
entical structure and nearly same lattice constants. On the
other hand, the amount of the Mg2Cu phase is very little
so that the XRD observation can not detect the presence
of the Mg2Cu phase.
3.2 Hydriding and Dehydriding Characteristics
Figure 5 shows the hydrogen absorption capacity and ki-
netics of the as-cast and quenched Cu2 and Cu4 alloys. It
can be seen that all hydriding kinetic curves of the as-
quenched alloys show an initial fast hydrogen absorption
stage after which the hydrogen content is saturated at
longer hydrogenation time, indicating that the rapid que-
nching significantly improves the hydrogen absorption
property of the alloys. The hydrogen absorption capacity-
ies of the alloys increase with rising quenching rate. Wh-
en the quenching rate grows from 0 (As-cast was defined
as quenching rate of 0 m/s) to 30 m/s, the hydrogen ab-
sorption capacity of the Cu2 alloy in 10 min rises from
2.33 to 3.24 wt.%, and from 1.54 to 2.72 wt.% for the
Cu4 alloy.
(a) (b) (c)
Figure 3. Crystal defects in the as-spun (30 m/s) Cu4 alloy taken by HRTEM
20 μm
A
(a)
(b)
B
A
20 μm
Section A
0
Mg
Ni
keV
Ni
Cu Cu
2 46 8
Section B
M
g
Cu
Ni
Cu
02 468 10
k
eV
Figure 4. SEM images of the as-cast alloys with typical EDS spectra of Sections A and B in Figure 3 (b): (a) Cu0, (b) Cu4

Hydrogen Absorption and Electrochemical Properties of As-Quenched
172
Nanocrystalline Mg20Ni10 – xCux (x = 0 – 4) Alloys
0
The hydrogenation kinetics and storage capacity of all
the as-quenched nanocrystalline Mg2Ni-type alloys stud-
ied are superior to those of conventional polycrystalline
materials with the same composition. Wu et al. [25] pre-
pared Mg-10Ni-2Mm (at.%) (Mm = Ce, La-rich Misch-
metal) alloy using melt spinning technology, finding that
the rapid quenching treatment markedly ameliorates the
kinetics of H-absorption/desorption of the alloy. A very
similar result was reported by Song et al. [20]. The Mg-
based alloys with the compositions of Mg–23.5wt% Ni
and Mg–23.5wt% Ni–5wt% Cu were fabricated by rapid
quenching technology. It was found that the hydriding
rates of the alloys are quite high, even at 200℃. The en-
hanced hydrogenation property is undoubtedly associated
with the refinement of the grains produced by rapid
quenching [26]. By refining the microstructure, a lot of
new crystallites and grain boundaries are created which
can act as fast diffusion paths for hydrogen absorption.
Based on the result reported by Orimo and Fujii [27], the
distribution of the maximum hydrogen concentrations in
three nanometer-scale regions, i.e. grain region and grain
boundary region as well as amorphous region, have been
experimentally determined to be 0.3 wt.% H in the grain
region of Mg2Ni, 4.0 wt.% H in the grain boundary and
2.2 wt.%H in the amorphous region. It revealed that the
hydrides mainly exist in grain-bound- ary region and the
amorphous phase region. The impro- ved hydrogenation
characteristics can be explained with the enhanced hy-
drogen diffusivity in the nanocrystalline microstructure
as the nanocrystalline leads to an easier access of hydro-
gen to the nanograins, avoiding the long- range diffusion
of hydrogen through an already formed hydride, which is
often the slowest stage of absorption. It is known that the
nanocrystalline microstructures can accommodate higher
amounts of hydrogen than polycrystalline ones. The large
number of interfaces and grain boundaries available in
the nanocrystalline materials provide easy pathways for
hydrogen diffusion and promote the absorption of hy-
drogen.
Figure 6 shows the hydrogen desorption capacity and
kinetics of the as-cast and quenched Cu2 and Cu4 alloys,
indicating that the dehydriding capability of the alloys ob-
viously meliorates with rising quenching rate. When the
quenching rate increases from 0 to 30 m/s, the hydrogen
0 2040608010012
0. 0
0. 5
1. 0
1. 5
2. 0
2. 5
3. 0
3.5 (a)
Hydrogen absorption (wt%)
Time (min)
As-cast
15 m/s
20 m/s
25 m/s
30 m/s
020 40 60 80100120
0.0
0.5
1.0
1.5
2.0
2.5
3.0 (b)
Hydrogen absorption (wt%)
Time (min)
As-cast
15 m/s
20 m/s
25 m/s
30 m/s
(a) (b)
Figure 5. Hydrogen absorption kinetic curves of the as-cast and quenched alloys. (a) Cu2 alloy; (b) Cu4 alloy
020 40 60 80100120
0.0
0.4
0.8
1.2
1.6
(a)
Hydrogen desorption (wt%)
Time (min)
As-cast 25 m/s
15 m/s 30 m/s
20 m/s
020 40 60 80100120
0.0
0.4
0.8
1.2
1.6
2.0 (b)
Hydrogen desorption (wt%)
Time (min)
As-cast 25 m/s
15 m/s 30 m/s
20 m/s
(a) (b)
Figure 6. Hydrogen desorption kinetic curves of the as-cast and spun alloys. (a) Cu2 alloy; (b) Cu2 alloy

Hydrogen Absorption and Electrochemical Properties of As-Quenched 173
Nanocrystalline Mg20Ni10 - xCux (x = 0 - 4) Alloys
desorption capacity of the Cu2 alloy in 20 min rises from
0.42 to 1.35 wt.%, and from 0.65 to 1.68 wt.% for Cu4
alloy, respectively. The nanocrystalline Mg2Ni-type all-
oys produced by rapid quenching exhibit higher H-abso-
rption capacity and faster kinetics of hydriding/dehyd-
riding than crystalline Mg2Ni. A similar result was repo-
rted by Spassov et al [28,29]. The specific capacity and
hydriding/dehydriding kinetics of hydride materials de-
pend on their chemical composition and crystalline stru-
cture [30]. The observed essential differences in the hyd-
riding/dehydriding kinetics of the as-quenched nanocry-
stalline Mg2Ni type alloys studied most probably have to
be associated with the composition of the alloys as well
as with the differences in their microstructure due to the
different quenching rates. It was reported that the high
surface to volume ratios, i.e. high specific surface area,
and the presence of large number of grain boundaries in
nanocrystalline alloys enhance the kinetics of hydrogen
absorption/desorption [28]. Zaluski et al. [31] and Orimo
et al. [32] confirmed that the hydriding/dehydriding char-
acteristics at low temperatures (lower than 200˚C) of na-
nocrystalline Mg2Ni alloys prepared by mechanical al-
loying can be improved by reducing the grain size (20-
30 nm), due to hydrogen occupation in the disordered
interface phase.
3.3 Electrochemical Hydrogen Storage
Characteristics
3.3.1 Activation Capability and Discharge Capacity
Electrochemical galvanostatic charge/discharge is a more
effective and less time-consuming method for determin-
ing the absorbing hydrogen capacity than a gaseous tech-
nique. The influences of rapid quenching on the activa-
tion capability of the alloys were shown in Figure 7, and
the charging-discharging current density being 20 mA/g.
The figures show that all the alloys have excellent acti-
vation capabilities and attain their maximum discharge
capacities at first charging-discharging cycle. The rapid
quenching does not affect the activation performances of
the alloys. The evolution of the maximum discharge ca-
pacities of the alloys with the quenching rate is shown in
Figure 8. It can be derived in Figure 8 that the discharge
capacity of the alloys increases with rising quenching
rate. When quenching rate increases from 0 to 30 m/s,
the discharge capacity of the Cu2 alloy enhances from
65.9 to 189.3 mAh/g, and from 53.3 to 140.4 mAh/g for
Cu4 alloy. A similar result was reported by Simičić et al
[2]. It must be mentioned that the discharge capacity of
the alloys containing Cu are higher than that of Cu-free
alloy, suggesting that the substitution of Cu for Ni en-
hances the discharge capacity of the Mg2Ni-type alloy.
Two reasons are mainly responsible for this result. Firstly,
the partial substitution of Cu for Ni in Mg2Ni compound
0246810121416182022
40
80
120
160
200 (a)
Discharge capactiy (mAh/g)
Cycle number, n
As- cas t
15 m/s
20 m/s
25 m/s
30 m/s
(a)
0246810121416182022
40
60
80
100
120
140(b)
Discharge capactiy (mAh/g)
Cycle number, n
As-cast
15 m/s
20 m/s
25 m/s
30 m/s
(b)
Figure 7. Evolution of the discharge capacity of the alloys
with the cycle number: (a) Cu2 alloy; (b) Cu4 alloy
0510 15 20 25 30
25
50
75
100
125
150
175
200
Discharge capacity (mAh/g)
Quenching rate (m/s)
Cu0
Cu1
Cu2
Cu3
Cu4
Figure 8. Evolution of the discharge capacity of the alloys
with the quenching rate
may help to destabilize the hydride and activate the
Mg2Ni phase to absorb/desorb reversibly hydrogen in the
alkaline electrolyte [2]. On the other hand, the secondary
phase Mg2Cu probably act as an efficient catalyst for
dissociating H2 molecules and transferring the H atoms
to the surrounding Mg2Ni matrix [20]. The observed es-
sential differences in the discharge capacity of the alloys
caused by rapid quenching most probably have to be as-
sociated with the differences in their microstructures.

Hydrogen Absorption and Electrochemical Properties of As-Quenched
174
Nanocrystalline Mg20Ni10 – xCux (x = 0 – 4) Alloys
The crystalline material, when rapidly quenched, beco-
mes at least partially disordered and its structure changes
to nanocrystalline. Consequently, high densities of crys-
tal defects such as dislocations, stacking faults and grain
boundaries are introduced. The densities of the crystal
defects mainly depend on the quenching rate. The higher
the quenching rate, the larger the densities of the crystal
defects are. The large number of interfaces and grain
boundaries available in the nanocrystalline materials
provide easy pathways for hydrogen diffusion and accel-
erates the hydrogen absorbing/desorbing process. Addi-
tionally, as a result of the defects introducing distortion
of crystal lattice, the stored sufficient energy as chemical
disorder and the introduced defects (including both stac-
king faults as well as grain boundaries) will produce in-
ternal strain. It was concluded by Northwood et al. [33]
that the exchange current density and H-diffusion coeffic
ient are directly proportional to the internal strain. The-
refore, it is understandable that the introduction of de-
fects, disordering and internal strain leads to an increas-
ing hydriding/dehydriding rates and capacity.
3.3.2 Charging and Discharging Cycle Stability
The cycle stability of the electrode alloy is a decisive fac-
tor of the life of the Ni-MH battery. The capacity retain-
ing rate (Sn), which was introduced to evaluate accurately
the cycle stability of the alloy, is defined as Sn= Cn/Cmax ×
100%, where Cmax is the maximum discharge capacity and
Cn is the discharge capacity of the nth charge-discharge
cycle, respectively. According to the above menti- oned
definition, it can be known that the larger the cap- acity
retaining rate (Sn), the better the cycle stability of the
alloy. The evolution of the capacity retaining rate (S20) of
the alloys with quenching rate is illustrated in Figure 9.
The figure show that the capacity retaining rate (S20) of
the alloys clearly declines with rising quenching rate.
When quenching rate increases from 0 to 30 m/s, the
capacity retaining rate of the Cu2 alloy after 20 cycles
falls from 58.6 to 42.3%, and from 74.4 to 51.1% for the
Cu4 alloy. It can also be seen in Figure 9 that, for a fixed
quenching rate, the capacity retaining rate of the alloys
mounts up with rising Cu content, reflecting that the sub-
stitution of Cu for Ni enhances the cycle stability of the
alloys. In order to clearly see the process of the capacity
degradation of the alloy electrode, the evolution of the
capacity retaining rate of the as-cast and quenched Cu2
and Cu4 alloys with the cycle number is shown in Figure 10.
A rough tendency can be seen in the Figure 10 that the
rapid quenching causes an increase of the decay rates of
the discharge capacities of the alloys, suggesting that the
rapid quenching impairs the cycle stability of the alloys.
It was well known that the essential reason of which
leads to the capacity degradation of the Mg-based alloy
electrodes is the severe corrosion of Mg in the alkaline
KOH solution. Especially, during the discharging process,
the alloys are anodically polarized so that corrosion wo-
uld be faster [2]. On the other hand, the metastable str-
uctures formed by rapid quenching or ball milling tended
to vanish during multiple charging/discharging cycles,
which is an important factor for the capacity decay of the
alloys. Two reasons are responsible for the enhanced cy-
cle stability of the Mg2Ni-type alloy by Cu substitution.
0510 15 20 25 30
20
30
40
50
60
70
80
C20/Cmax (%)
Quenching rate (m/s)
Cu0 Cu3
Cu1 Cu4
Cu2
Figure 9. Evolution of the capacity retaining rate (S20) of the
alloys with the quenching rate
246810121416182022
40
50
60
70
80
90
100 (a)
Cn/Cmax (%)
Cycle number, n
As-cast
15 m/s
20 m/s
25 m/s
30 m/s
(a)
246810 12 1416 18 2022
50
60
70
80
90
100 (b)
Cn/Cmax (%)
Cycle number, n
As-cast
15 m/s
20 m/s
25 m/s
30 m/s
(b)
Figure 10. Evolution of the capacity retaining rate of the
as-cast and quenched alloys with cycle number (a) Cu2 alloy;
(b) Cu4 alloy

Hydrogen Absorption and Electrochemical Properties of As-Quenched 175
Nanocrystalline Mg20Ni10 - xCux (x = 0 - 4) Alloys
02.
Firstly, the improved performance in the cycle life of
substituted alloy electrodes is presumably attributed to
preferential oxidation of Cu on the alloy surface and the
prevention of the formation of the Mg(OH)2 passive
layer. Secondly, the additive of a third element significa-
ntly stabilizes the nanostructure of Mg-Ni-based alloy
[28], reflecting an increase of the cycle stability of the
alloy. The nanostructure of the alloys formed by rapid
quenching was detrimental for corrosion in the electro-
lyte during cycling due to the fact that intercrystalline
corrosion is inevitable. Therefore, it is comprehensible
why rapid quenching leads to a decline of the cycle sta-
bility of the Mg-Ni-Cu system alloy.
4. Conclusions
The structures and hydrogen storage characteristics of the
nanocrystalline Mg20Ni10 – xCux (x = 0 – 4) alloys were
investigated, and the conclusions obtained are summa-
rized as follows:
1) All the as-quenched Mg20Ni10 – xCux (x = 0 – 4) al-
loys hold nanocrystalline structures and are free of
amorphous phase. The rapid quenching does not change
the major phase of Mg2Ni-type in the alloy, but it leads
to an increment of the lattice parameters and cell volume
as well as the FWHM values of the major diffraction
peaks of the alloys.
2) Rapid quenching significantly improves the hydrid-
ing and dehydriding properties of the alloys. Hydriding
and dehydriding capacities and rates of the alloy mark-
edly rise with increasing quenching rate.
3) Additionally, rapid quenching considerably enha-
nces the electrochemical discharge capacity of the alloys,
whereas it slightly weakens the charging/discharging cy-
cle stability of the alloys, for which the nanocrystalline
structure formed by rapid quenching is basically respon-
sible.
5. Acknowledgements
This work is supported by Hi-Tech Research and Devel-
opment Program of China (2007AA03Z227), National
Natural Science Foundations of China (50871050 and
50701011), Natural Science Foundation of Inner Mongo-
lia, China (200711020703) and Higher Education Scien-
ce Research Project of Inner Mongolia, China (NJzy
08071).
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