ls9"> an alloy with high di scharge capacit y
and good cycling stability. The A2B7-type La0.75xZrxMg0.25Ni3.2
Co0.2Al0.1 (x=0 -0.2) alloys were prepared by melt spinning, and
a systematic investigation on the effects of the substitution of
Zr for La and the melt spinning on the electrochemical cycle
stability and kinetics of the electrode alloys has been per-
formed.
2. Experimental
The chemical compositions of the alloys were La0.75-xZrxMg0.25
Ni3.2Co0.2Al0.1 (x=0, 0.05, 0.1, 0.15, 0.2). For convenience, the
alloys were denoted with Zr content as Zr0, Zr0.05, Zr0.1, Zr0.15
and Zr0.2, respectively. The alloy ingots were prepared using a
vacuum induction furnace in a helium atmosphere under a
pressure of 0.04 MPa. A part o f the as-cast alloys was r e-melted
and spun by melt-spinning with a rotating copper roller. The
spinning rate was approximately expressed b y the l inear veloc-
ity of the copper roller. The spinning rates used in the experi-
ment were 5, 10, 15 and 20 m/s, respecti vely.
The phase structures and compositions of the as-cast and
*Supported by National Natural Science Foundation
s of China (51161015 and
50961009), National 863 plans projects of China (2011AA03A408)
, Natural
Science Foundatio n of Inner Mongo lia, China (2011Z D10 and 2010ZD05).
Y.-H. ZHANG ET AL.
Copyright © 2012 SciRes. AMPC
79
spun alloys were characterized by XRD (D/max/2400). The
thin film samples of the as-spun alloys were prepared by ion
etching for observing the morphology with HRTEM (JEM-
2100F).
The round electrode pellets in a 15 mm diameter were pre-
pared by cold pressing a mixture of alloy powder and carbonyl
nickel powder in the weight ratio of 1:4 under a 35 MPa pres-
sure. Aft er d ried for 4 h, th e electrod e pellets were immers ed in
a 6 M KOH solution for 24 h in order to wet the electrodes
fully befor e the electr ochemical measu r ement.
Electrochemical measurements were performed at 30°C by
using a tri-electrode open cell, consisting of a working elec-
trode (the metal h ydride elect rode), a sintered Ni(OH)2/NiOOH
counter electrode and a Hg/HgO reference electrode, which
were immersed in a 6M KOH el ectrol yte. The vol tage between
the negative electrode and the reference electrode was defined
as the disch arge voltage. In every cycle, t he alloy electro de was
firstly charged with a constant current density of 300 mA/g.
After restin g 15 min, it was discharged at th e same current den-
sity to cut-off vo ltage of –0.500 V.
The electrochemical impedance spectra (EIS), the hydrogen
diffusion in alloy bulk and the Tafel polarization curves of the
alloys were measured by an electrochemical workstation
(PARSTAT 2273). The EIS of the alloy electrodes were meas-
ured in the frequency range from 10 kHz to 5 mHz at 50%
depth of discharge (DOD). The Tafel polarization curves were
measured i n t he p o tenti al ran ge of 1.2 to +1.0 V (vs. Hg/HgO)
with a scan rate of 5 mV/s also at 50% depth of discharge. For
the potentiostatic discharge, the test electrodes in the fully
charged state were discharged at 500 mV potential steps for
5000s, using the electrochemistry corrosion software (Corr-
Ware).
3. Results and Discussion
3.1. Structural characteristics
The XRD pattern s o f the as-cast and spun La0.75-xZrx Mg0.25Ni3.2
Co0.2Al0.1 (x=0-0.2) alloys are presented in Fi gure 1. It is evi-
dent that the melt spinning and Zr substitution bring on an ob-
vious broadening of the major diffraction peaks of the alloys, to
be ascribed to the refined grain and the stored stress in the
grains by melt spinning and Zr substitution. It is found that the
as-cast and spun alloys hold a multiphase structure, composing
of t wo major phases ( La, Mg)2Ni7 and LaN i5 as well a s a resid-
ual phase LaN i2. The structures of the alloys maintain almost
unaltered after partially substituted by Zr. The lattice parame-
ters together with the abundances of the (La, Mg)2Ni7 and
LaNi 5 majo r phases in the alloys, which were calculated by
Jade 6.0 software based on the XRD d ata, are listed in Ta ble 1 .
It is found that the Zr substitution arouses an increase of the
LaNi 5 phase and a decrease of the (La, Mg)2Ni7 p hase. Mean-
while, it also leads to a d ecrease of th e lat tice co n stan ts an d cell
volume of the two major phases, which is due to the fact that
the atom radius of Zr is smaller than that of La. Furthermore,
the shrink of the cell volume, incurred by the substitution, just i-
fies the successful alloying of Zr with (La, Mg)2Ni7 and LaNi5
major phases.
The TEM micrographs of the as-spun (10 m/s) La0.75-xZrx
Mg0.25Ni3.2Co0.2Al0.1 (x=0-0.2) alloys are demons trated in Fig-
ure 2. It can be seen from Figure 2 th at the ch ange of th e mor-
phologies of the alloys seems to be equivocal with the various
of Zr content. However, the amplified morphologies of Figure
2 reveal that the as-spun Zr0 and Zr0.1 alloys display an entire
crystalline structure. And some crystal defects such as subgrains
and grain boundaries can be seen clear ly. It is evid ent that Z r0.2
alloy displays an obvious amorphous-like structure, and the
nanocrystalline region, the amorphous region as well as the
transition region can be clearly viewed from the amplified
morphology of Figure 2(c), which seems to be conflicting with
the result in Figure 1 due to no amorphous phase is d etected by
XRD observation. One probable reason is that the amorphous-
like phase for ms at so me s elective locations in the as-spun alloy
and its amount is very small, thus, the XRD patterns could not
detect i ts presence.
Figure 1.XRD profiles of the as-cast spun alloys: (a) Zr0.1, (b)
As-spun (10 m/s)
Ta ble 1. Lattice parameters, abundances of LaNi5 and (La, Mg)2Ni7
major phases.
Spinni ng
rate Alloys
Major
phases
Lattice c onsta nts
Cell volume
Phase
abund-ance
A(nm) C(nm) V (nm3) (wt.%)
10 m/s
Zr0 (La, Mg)2Ni7
0.5199 2.4406 0.5712 72.31
LaNi5 0.5192 0.4180 0.0976 25.24
Zr0.05 (La, Mg)2Ni7
0.5142 2.4362 0.5578 66.03
LaNi5 0.5130 0.4107 0.0936 31.02
Zr0.1 (La, Mg)2Ni7
0.5071 2.4308 0.5413 64.25
LaNi5 0.5059 0.4082 0.0905 34.08
Zr0.15 (La, Mg)2Ni7
0.5031 2.4251 0.5315 60.97
LaNi5 0.5011 0.4053 0.0881 36.50
Zr0.2 (La, Mg)2Ni7
0.4963 2.4174 0.5156 58.02
LaNi5 0.4933 0.4021 0.0847 39.32
Figure 2. TEM micrographs of the as-spun (10 m/s) alloys: (a) Zr 0
alloy, (b) Zr0.1 alloy, (c ) Zr0.2 alloy.
Y.-H. ZHANG ET AL.
Copyright © 2012 SciRes. AMPC
80
3.2. Electroche mical Cycle Stability and Kine t ics
The capacity retaining rate (RN) is introduced as a token of the
electrochemical cycle stability of an alloy electrode, which is
defined as RN = CN/Cmax ×100%, where Cma x is the maximum
disch arge capacity while CN is the disch arge capacity at the nth
chargingdischarging cycle with a current density of 300 mA/g,
respecti vely. The evolution of the capacity retaining rates (RN)
of the La0.75-xZrxMg0.25Ni3.2Co0.2 Al0.1 (x=0-0.2) alloys with the
cycle number is described in Figure 3. The slopes of the cur ves
in Figure 3 prefigure the degradation rate of the discharge ca-
pacity during the charge-discharge cycling. The smaller the
slope of the curve is, the better the cycle stability of the alloy
will be. It is evident that the degradatio n rate of the discharge
capacity of the alloys visibly declines with rising the spinning
rate and the Zr content. In order to establish the relationship
between th e capacity retaini ng rates (RN) with the spinning rate
and the Zr content, taking the capacity retaining rate (R100) at
100th cycling as a benchmark, the evolution of the R100 of the
alloys with the spinning rate and the amount of Zr substitution
is also inserted in Figure 3. It is found that the R100 values of
the alloys markedly augment with growing the spinning rate
and the Zr content. The R100 value of the Zr0.1 alloy grows from
73.21% to 82.07% as the spinning rate increases from 0 (as-
cast was defined as the spinning rate of 0 m/s) to 20 m/s and
that of the as-spun (10 m/s) alloys augments from 69.25% to
83.09% as Zr content rises from 0 to 0.2.
Figure 3. Evolutio n of the capaci ty re taini ng ra tes (RN) of the alloy s
with the cycle number: (a) Zr0.1 alloy, (b) As-spun (10 m/s).
It is convinced that the pulverization and oxidation of the al-
loy during charging-discharging cycle are the fundamental
reason for the capacity decay of the el ectrode alloy. The lattice
stress and the expansion of the cell volume, which are inevita-
ble when hydrogen atoms entering into the interstitials of the
lattice, are the real driving force lead to the pulverization and
oxidation. The positive impact of the melt spinning on the cycle
stability of the alloy is primarily ascribed to the remarkable
refinement of the grains induced by melt spinning. The
anti-pulverization capability of the alloy basically depends on
its grain size. Therefore, it is understandable that the cycle sta-
bility of the alloy increases with growing the spinning rate. The
benefaction of the Zr substitution on the cycle stability of the
as-spun alloys is primarily ascribed to the formation of an
amorphous phase induced by the melt spinning due to an
amorphous phase improves not only anti-pulverization ability
but also anti-corrosion and anti-oxidation abilities of the alloy
electrode in a corrosive electrolyte [5].
The electrochemical hydrogen storage kinetics of an alloy
electrode, which has been considered to be quite important for
the practical application of hydride electrode in power battery,
is symbolized by its high rate discharge ability (HRD), being
calculated by formula: HRD=Ci /C100 ×100%, where Ci and
C100 are the maximum discharge capacities of the alloy elec-
trode charged-discharged at the current densities of i and 100
mA/g respectivel y. The current density dependence of the HRD
values of the allo ys is illustrated in Figure 4. In order to estab-
lish the relationship between the electrochemical hydrogen
storage kinetics of the alloys with the spinning rate and the
amount of Zr substitution, taking the 900 mA/g current density
as a benchmark to calculate the HRD of the alloys, the evolu-
tion of the HRD (i=900 mA/g) of the alloys with the spinning
rate and the Zr content is also inset in Figure 4. It is evident
that the melt spinning gives rise to a slight increase in the HRD
of Zr0.1 alloy as the spinning rate grows from 0 to 5 m/s, but
with further increase of the spinning rate, it brings on a dramat-
ic decline of the HRD. The HRD of the as-spun (10 m/s) sub-
stantially falls with rising the Zr content, indicating that the Zr
substitution impairs the electrochemical kinetics of the alloys.
It was well known that the high rate discharge ability of a
metal hydrid e electrod e is principally dominated by the ch arge-
transfer rate on the surface of an alloy electrode and the hydro-
gen diffusion capability in the alloy bulk. Hence, it is essential
to investigate the charge-transfer rate and the hydrogen diffu-
sion coefficient [6]. The charge-transfer rate on the surface of
an alloy electrod e is qualitatively evaluated by its electrochem-
ical impedan ce sp ectrum (EIS). The EIS of the as-cast and spun
La0.75-xZrxMg0.25Ni3.2Co0.2Al0.1 (x=0-0.2) alloys electrodes are
depicted in Figure 5. It is found that each EIS spectrum com-
prises two semicircl es corresponding to two different frequency
regions. As elucidated by K uriyama et al . [7], the smaller semi-
circle in the high frequency region corresponds to the contact
resistan ce between the all oy powder and the co nduct ive materi-
al, while the larger semicircle in the low frequency region
equat es to the charge-trans fer resistance on the alloy surface.
Hence, the larger the radius of the semicircle in the low fre-
quency region is, the h igher the charge-transfer resistance of th e
alloy electrode will be. It is evident that, with the rising of the
spinning rate, the radii of th e large semicircles of the Zr0.1 alloy
Y.-H. ZHANG ET AL.
Copyright © 2012 SciRes. AMPC
81
in the low frequency first shrink and then expand, and those of
the as-spun (10 m/s) alloys always swell with growing the Zr
content. The hydrogen diffusion coefficients in the alloys can
be derived by measuring the semilogarithmic curves of anodic
curren t versus working duration o f an alloy as depicted in Fig-
ure 6. Based on the model founded by White et al. [8], the dif-
fusion coefficient (D) of the hydrogen atoms in the bulk of the
alloy can be calculated by following formulae:
2
0
22
6
log log()2.303
s
FD D
iCC t
da a
π

=±−−


(1)
2
2
2.303 logad i
Ddt
π
= −
(2)
In (2),
logdi
dt
is the slope of the linear region of Figure 6,
which can be gained by origin 7.5 software in a walk. The alloy
particle radius (a) is supposed to be a=15 μm. Thus, hydrogen
diffusion coefficient D can be easily o btained, and the results
are also presented in Figure 6. It is evident that with the rising
of the spinning rate, the D values of the Zr0.1 alloy first mount
up and then fall, and those of the as-spun (10 m/s) alloys mo-
notonously drop with rising the Zr content.
Figure 4. Evolution of the high rate discharge ability (HRD) of the
alloys with the discharge current density: (a) Zr0.1 alloy, (b) As-
spun (10 m/s).
Figure 5. Electrochemical impedance spectra (EIS) of the alloy
electrodes: (a) Zr0.1 alloy, (b) As spun (10 m/s).
Figure 6. Semilogarithmic curves of anodic current vs. time res-
ponses of the alloys: (a) As-spun (10 m/s), (b) Zr0.15 alloy.
Y.-H. ZHANG ET AL.
Copyright © 2012 SciRes. AMPC
82
The limiting current density (IL), another important electro-
chemical kinetics parameter which is mainly dominated by the
hydrogen diffusion in the bulk of the alloy during anodic pola-
rization [9], can be obtained by measuring the Tafel polariza-
tion curves of an alloy. The Tafel po larizatio n curves of La0.75-x
ZrxMg0.25Ni3.2Co0.2Al0.1 (x=0-0.2) alloys are depicted in Figure
7, from which a clear inflection point in each anodic polariza-
tion curve can be seen, there being a limiting value of the cur-
rent density which is defined as limiting current density (IL). It
indicates that an oxidation reacti on took place on the sur face of
the alloy electrode, and the generated oxidation product resists
further penetration of hydrogen atoms [10]. The IL va lues of the
alloys as a function of the spinning rate and the Zr content are
also inset in Figure 7. It is visible that th e IL values of the Zr0.1
alloy first rise and then decline, and those of the as-spun (10
m/s) alloys always decrease wit h growing t he Zr co ntent.
Figure 7. Tafel polarization curves of the as-cast and spun alloys: (a)
Zr0.1 alloy, (b) As-spun (10 m/s).
Based on the investigation of the electrochemical kinetics,
some elucidation s can be provided as the reasons why the HRD
of the Zr0.1 alloy has a ma xi mum val u e with the variation of the
spinning rate. Upon the refined microstructure by melt spinning,
a lot of new crystallites and grain boundaries are generated,
which may act as the fast diffusion paths for hydrogen absorp-
tion [11], enhancing the HRD of the alloy. However, it must be
mentioned that the refinement of the grains resulted from the
melt spinning severely impairs the charge-transfer rate on the
alloy surface due to the fact that the refined grains effectively
prohibit the pulverization of the alloy particles, a lower new
surface o f the al loy electrode being formed, decreasing th e rate
of charge transfer at the alloy-electrolyte interface. It is the
above-mentioned contrary impacts engendered by melt spin-
ning that lead to a max imum HRD of the alloys. Furthermore,
the d ecreased HRD of th e as-spun (10 m/s) alloys by Zr substi-
tution is ascribed to the formation of a amorphous-like structur e
which not only increases the charge-transfer resistance of the
alloy electrodes but also hinders the hydrogen diffusion from
inner of the bulk to the surface, and subsequently results in the
drop of the electroch emical ki netic property.
4. Conclusions
The La-Mg-Ni system A2B7-type La0.75-xZrxMg0.25Ni3.2Co0.2
Al0.1 (x=0, 0.05, 0.1, 0.15, 0.2) electrode alloys were success-
fully synthesized by melt spinning. The as-spun Zr0 alloy exhibits
an enti re n anocr ystalli ne an d micr o-crystall ine struct ure, where as
the as-spun alloy substituted by Zr exhibits a amorphous-like
structure. Both the melt spinning and the substitution of Zr for
La markedly enhance the electrochemical cycle stability of the
electrode alloys. The electrochemical kinetics, including the
HRD, the hydrogen diffusion coefficients (D) as well as the
limiting current density (IL) of the Zr0.1 alloy first increase and
then decline with growing the spinning rate, and those of the
as-spun (10 m/s) alloys monotonously fall with rising the Zr
content, for which the refinement of the grains and the forma-
tion of the amorphous-like structure are principally responsible.
5. Acknowledgements
This work is supported by National Natural Science Founda-
tions of China (51161015 and 50961009), National 863 plans
projects of China (2011AA03A408), Natural Science Founda-
tion of Inner Mongolia, China (2011ZD10 and 2010ZD05).
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