Structural and Electrochemical Properties of the La0.7Mg 0.3Ni2.975-xCo0.525Mnx Hydrogen Storage Electrode Alloys

Article abstract of DOI:10.1149/1.1643070

The effect of partial substitution of Mn for Ni on the structural and electrochemical properties of the La_0.7Mg_0.3Ni _2.975-xCo_0.525Mn_x (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) hydrogen storage alloys has be

Full text of DOI:10.1149/1.1643070

A374
Journal of The Electrochemical Society, 151 ͑3͒ A374-A380 ͑2004͒
0
013-4651/2004/151͑3͒/A374/7/$7.00 © The Electrochemical Society, Inc.
Structural and Electrochemical Properties
of the La Mg Ni2.975ÀxCo0.525Mn Hydrogen Storage
0
.7
0.3
x
Electrode Alloys
,z
Hongge Pan,* Yongfeng Liu, Mingxia Gao, Yunfeng Zhu, Yongquan Lei,
and Qidong Wang
Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027,
People’s Republic of China
The effect of partial substitution of Mn for Ni on the structural and electrochemical properties of the La0.7Mg0.3Ni2.975ϪxCo0.525Mnx
(
x ϭ 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) hydrogen storage alloys has been investigated systematically. The results of X-ray powder
diffraction and Rietveld analyses showed that all alloys consisted of the (La, Mg͒Ni phase and the LaNi phase, and the content
3
5
of the (La, Mg͒Ni phase first remained unchanged ͑ϳ77 wt %͒ and then decreased, but the content of the LaNi phase increased
3
5
progressively with increasing x. Meanwhile, the lattice parameters and cell volumes of the (La, Mg͒Ni phase and the LaNi phase
3
5
all increased with increasing Mn content. The pressure composition isotherms showed that the hydrogen storage capacity first
remained almost unchanged and then decreased with increasing x from 0.0 to 0.5, and the equilibrium pressure decreased from
0
͑
.51 atm to 0.06 atm. The electrochemical measurements indicated that the maximum discharge capacity first remains unchanged
ϳ400 mAh/g͒ with increasing x from 0.0 to 0.2 and then decreased when x increased further. Moreover, the high rate discharge-
ability, the exchange current density I , the limiting current density I , and the hydrogen diffusion coefficient D of the alloy
0
L
electrodes all increased first and then decreased with increasing x, which indicates that the kinetics of hydriding/dehydriding of the
La0.7Mg0.3Ni2.975ϪxCo0.525Mnx (x ϭ 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) hydrogen storage alloys increased first up to x ϭ 0.1 and then
decreased with further increasing x.
©
2004 The Electrochemical Society. ͓DOI: 10.1149/1.1643070͔ All rights reserved.
Manuscript submitted April 28, 2003; revised manuscript received September 18, 2003. Available electronically January 26, 2004.
Hydrogen storage alloys have been successfully used as negative
element substitution is a very effective method. As for rare earth-
electrodes in nickel/metal-hydride ͑Ni/MH͒ secondary batteries. Be-
sides the merits of higher energy density, higher charge and dis-
charge ability, and longer charge/discharge cyclic life, the Ni/MH
secondary batteries also have smaller memory effect and cause less
environmental pollution compared with the rechargeable nickel/
cadmium ͑Ni/Cd͒ batteries.1
based hydrogen storage electrode alloys, Mn is commonly present
and has been found to be beneficial in many respects.1
4-18
The par-
tial substitution of Ni with Mn on the B side can increase the lattice
parameters and the cell volume, and decrease the plateau equilib-
1
4,19,20
rium pressure of hydrogen.
In more recent studies, Mn is re-
-4
ported effective in improving the electrochemical properties of rare
earth-based hydrogen storage electrode alloys, particularly the high
Recently, R-Mg-Ni (R ϭ rare earth element, Ca or Y͒ system
hydrogen storage electrode alloys for Ni/MH batteries have attracted
our attention because of their higher hydrogen storage capacities
1
5-18,20
rate dischargeability ͑HRD͒.
So we believe that Mn element
would also play an important role in the La-Mg-Ni-Co system hy-
drogen storage electrode alloys.
In the present study, the structural and electrochemical properties
of the La0.7Mg0.3Ni2.975ϪxCo0.525Mnx (x ϭ 0.0, 0.1, 0.2, 0.3, 0.4,
than those of R-Ni alloys.5 Chen et al. found that the maximum
-9
8
discharge capacity of LaCaMgNi alloy reached 356 mAh/g when
9
worked on the structural and electrochemical characteristics of
LaCaMg͑NiM)9 (M ϭ Al, Mn). Kohno et al.10 have studied the
electrochemical properties and structures of La-Mg-͑NiCo)x
0.5) hydrogen storage electrode alloys were studied systematically.
Experimental
(
x ϭ 3-3.5) system alloys and found that the discharge capacity of
the La0.7Mg0.3Ni2.8Co0.5 alloy reached 410 mAh/g, much higher than
Alloy
preparation.—Alloy
samples
of
those of the conventional AB -type electrode alloys. Baddour-
La0.7Mg0.3Ni2.975ϪxCo0.525Mnx
(x ϭ 0.0, 0.1, 0.2, 0.3, 0.4, 0.5)
5
Hadjean et al.1 have studied the structural and electrochemical
properties of the R-Y-Ni (R ϭ La, Ce) system electrode alloys and
found the maximum discharge capacity to be 260 mAh/g. Moreover,
in our previous work,12 the structural and electrochemical properties
of La0.7Mg0.3(Ni0.85Co0.15)x (x ϭ 2.5-5.0) hydrogen storage alloy
were investigated, and the results indicated that the maximum dis-
charge capacity of La0.7Mg0.3(Ni0.85Co0.15)3.5 electrode alloys
reached 396 mAh/g. The study of the effect of the annealing
treatment on the electrochemical properties and microstructure of
1
were prepared from component metals with the purity all above 99%
by vacuum magnetic levitation melting under argon atmosphere. To
assure the homogeneity of the alloy, ingots were turned over and
remelted twice. Then, the ingots were mechanically crushed and
ground into the powder of 300 mesh size for electrochemical and
X-ray diffraction ͑XRD͒ measurements. The average particle size of
the resulting powder as measured by Malvern particle analyzer Mas-
tersizer2000 is 26.24 ␮m.
Structure analyses and Pressure-composition isotherms (PCT)
measurement.—The crystal structures, phase abundance and the lat-
tice parameters were determined by X-ray diffraction with Cu K␣
the La-Mg-͑NiCo) (x ϭ 3.0 ϳ 3.5) system alloys revealed that the
x
discharge capacity of the La0.7Mg0.3Ni2.8Co0.5 alloy annealed at
1
123 K increased to 414 mAh/g.13
21
radiation by the Rietveld method using Rietan97 software. P-C-Ts
Although the La-Mg-Ni-Co system electrode alloys have high
hydrogen storage capacity, the plateau pressure of absorption/
desorption hydrogen is too high and the cyclic stability is too low
for a good electrode alloy. On account of those inadequacies, we
have continued our study to improve their overall electrochemical
properties. From previous studies, we have noticed that the partial
were measured with an automatic Sieverts-type apparatus at 303 K
after each alloy powder sample ͑about 1 g͒ has been activated by
four hydriding/dehydriding cycles.
Electrochemical measurements.—A hydrogen storage alloy elec-
trode was prepared each time by mixing a specific alloy powder
with carbonyl nickel powder in the weight ratio of 1:4 and then cold
pressing the mixture into a pellet under a pressure of 16 MPa. Elec-
trochemical measurements were performed at 303 K in a standard
open trielectrode electrolysis cell consisting of a working electrode
*
Electrochemical Society Active Member.
E-mail: honggepan@zjuem.zju.edu.cn
z
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Journal of The Electrochemical Society, 151 ͑3͒ A374-A380 ͑2004͒
A375
Figure 1. Rietveld refinement of the XRD profiles of the La0.7Mg0.3Ni2.975ϪxCo0.525Mn (x ϭ 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) hydrogen storage alloys. The gray line
x
is the calculated intensity and the points superimposed on it ͑ϩ͒ are observed intensities. The tick marks below the profile indicate the positions of all allowed
K␣1 and K␣2 peaks for the structure models adopted. The bottom solid line shows the difference between calculated and observed intensities. ͑a͒ x ϭ 0.0, ͑b͒
x ϭ 0.1, ͑c͒ x ϭ 0.2, ͑d͒ x ϭ 0.3, ͑e͒ x ϭ 0.4, and ͑f͒ x ϭ 0.5.
amplitude of 5 mV under the open-circuit condition. The linear po-
larization curves and Tafel polarization curves of the electrodes were
measured on a Solartron SI1287 potentiostat by scanning the elec-
trode potential at the rate of 0.1 mV/s from Ϫ5 to 5 mV ͑vs. open
circuit potential, OCP͒ and 5 mV/s from Ϫ500 to 500 mV ͑vs. OCP͒
at 50% DOD, respectively. For the potentiostatic discharge, the test
electrodes in the fully charged state were discharged at ϩ600 mV
potential steps for 3600 s on a Solartron SI1287 potentiostat.
͑
the MH pellet electrode for studying͒, a sintered Ni͑OH) /NiOOH
2
counter electrode, and a Hg/HgO reference electrode immersed in
the 6 M KOH electrolyte. The discharge capacity and the cycle
stability of each test electrode were determined by the galvanostatic
method. Each electrode was charged at 100 mA/g for 5 h followed
by a 10 min rest and then discharged at 60 mA/g to the cutoff
potential of Ϫ0.6 V vs. the Hg/HgO reference electrode. For inves-
tigating the HRD, the discharge capacities at several specific dis-
charge current densities were measured. Electrochemical impedance
spectroscopy ͑EIS͒ studies were conducted at 50% depth of dis-
charge ͑DOD͒ using a Solartron SI1287 electrochemical interface
with 1255B frequency response analyzer. Before EIS measurements,
the electrodes were first completely activated by charging/
discharging for 5 cycles. The EIS spectra of the electrodes were
obtained in the frequency range of 10 kHz to 5 mHz with an ac
Results and Discussion
Crystal structures.—Figure 1 shows the XRD patterns and the
Rietveld analysis patterns of the La0.7Mg0.3Ni2.975ϪxCo0.525Mnx
(x ϭ 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) hydrogen storage alloys. Besides
the very small amounts of some particular phases, each alloy is com-
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A376
Journal of The Electrochemical Society, 151 ͑3͒ A374-A380 ͑2004͒
a
Table I. The characteristics of alloy phases in the La0.7Mg0.3Ni2.975ÀxCo0.525Mn „x Ä 0.0, 0.1, 0.2, 0.3, 0.4, 0.5… hydrogen storage alloys.
x
Space group
Phase abundance
Lattice parameter
Cell volume
3
Samples
Phases
͑no.͒
͑wt %͒
͑Å͒
͑Å ͒
x ϭ 0.0
(La, Mg͒Ni3
LaNi5
LaNi
(La, Mg͒Ni3
LaNi5
(La, Mg͒Ni3
LaNi5
(La, Mg͒Ni3
LaNi5
(La, Mg͒Ni3
LaNi5
R-3m ͑166͒
P6/mmm ͑191͒
Fd3m ͑227͒
R-3m ͑166͒
P6/mmm ͑191͒
R-3m ͑166͒
P6/mmm ͑191͒
R-3m ͑166͒
P6/mmm ͑191͒
R-3m ͑166͒
P6/mmm ͑191͒
Pnma ͑62͒
R-3m ͑166͒
P6/mmm ͑191͒
Pnma ͑62͒
78.9
18.9
2.2
77
23
75.2
24.8
65.9
34.1
62.9
36.6
0.5
56.5
42.4
1.1
a ϭ 5.041
a ϭ 5.034
a ϭ 3.849
a ϭ 5.055
a ϭ 5.04
a ϭ 5.07
a ϭ 5.049
a ϭ 5.077
a ϭ 5.054
a ϭ 5.097
a ϭ 5.064
a ϭ 9.29
a ϭ 5.107
a ϭ 5.067
a ϭ 9.29
c ϭ 24.21
c ϭ 3.99
4.36
532.93
87.55
b ϭ 10.825
181.67
537.25
88.162
541.31
88.58
543.75
89.03
562.86
89.79
119.83
565.369
90.134
119.92
x ϭ 0.1
x ϭ 0.2
x ϭ 0.3
x ϭ 0.4
c ϭ 24.273
c ϭ 4.0
c ϭ 24.32
c ϭ 4.012
c ϭ 24.36
c ϭ 4.024
c ϭ 25.01
c ϭ 4.042
4.545
MnO2
(La, Mg͒Ni3
LaNi5
b ϭ 2.838
b ϭ 2.838
x ϭ 0.5
c ϭ 25.033
c ϭ 4.054
c ϭ 4.546
MnO2
a
The Rietveld refinement program Rietan97 was used.
ingly. In the present investigation, the change of hydrogen storage
capacity agreed well with the change of phase abundance obtained
from Rietveld analysis.
prised of a ͑La, Mg͒Ni phase with the PuNi -type rhombohedral
3
3
structure and a LaNi phase with the CaCu -type hexagonal struc-
5
5
ture. The phase abundance, lattice parameters, and cell volumes for
the La0.7Mg0.3Ni2.975ϪxCo0.525Mn hydrogen storage alloys are listed
x
Electrochemical characteristics.—Figure 3a shows the discharge
capacity vs. the cycle number of the La0.7Mg0.3Ni2.975ϪxCo0.525Mnx
hydrogen storage alloy electrodes. Table II summarizes the electro-
in Table I. It can be seen that the lattice parameters and cell volumes
of the ͑La, Mg͒Ni3 phase and LaNi5 phase all increase with the
increase of Mn content ͑as can be seen in Table I͒, which can be
attributed to the larger atomic radius of Mn ͑1.79 Å͒ than that of Ni
chemical properties of the La0.7Mg0.3Ni2.975ϪxCo0.525Mn alloy elec-
x
trodes. All alloy electrodes can be activated very easily, namely
within two cycles. The maximum discharge capacities of the alloy
electrodes first remain almost unchanged ͑ϳ400 mAh/g͒ and then
decrease to 362.5 mAh/g when the x for Mn content increases from
͑
1.62 Å͒. Thus the increase of Mn content with increasing x leads to
the crystal lattice expansion of both the ͑La, Mg͒Ni phase and the
3
LaNi phase and the increase of phase abundance of LaNi phase.
5
5
Moreover, from Table I, it can be seen that the content of
0.3 to 0.5, which agrees with the result of the P-C-T measurement.
͑
La, Mg͒Ni3 phase first remains almost unchanged ͑ϳ77 wt %͒
Figure 3b shows the capacity retention rate of the
when x increases from 0.0 to 0.2. However, as x increases further,
the content of ͑La, Mg͒Ni3 phase decreases from 65.9 wt %
La0.7Mg0.3Ni2.975ϪxCo0.525Mn hydrogen storage alloy electrodes af-
x
ter 60 charge/discharge cycles. It can be seen that the capacity re-
tention of alloy electrodes remains almost unchanged ͑ϳ46%͒ after
60 cycles when x increases from 0.0 to 0.4 (Mn/Ni ϭ 0.00
ϳ 0.11, atomic ratio͒ and then increases to 54.5% when x reaches
0.5 (Mn/Ni ϭ 0.20), which can be attributed to increasing of the
(
x ϭ 0.3) to 56.5 wt % (x ϭ 0.5). In contrast, the content of LaNi5
phase increases progressively from 18.9 to 42.4 wt % with increas-
ing x from 0.0 to 0.5, which also confirms that Mn promotes the
formation of LaNi phase in the alloy.
5
In a word, the increase of Mn element content affected the phase
composition of alloy only moderately, but increased the lattice pa-
rameters and cell volumes of the main phases and thus affected the
overall electrochemical properties of the alloy electrodes noticeably.
P-C-Ts.—Figure 2 shows the hydrogen absorption/desorp-
tion
P-C-Ts
of
the
La0.7Mg0.3Ni2.975ϪxCo0.525Mnx
(x
ϭ 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) hydrogen storage alloys at 303 K. As
shown in Fig. 2, the equilibrium pressure of hydrogen desorption
decreases from 0.51 atm to 0.06 atm as x for Mn content increases
from 0.0 to 0.5, which is in good agreement with the results reported
previously about the rare earth-based hydrogen storage alloys by
1
4
Lartigue et al. The hydrogen storage capacity decreases with in-
creasing x. To make the comparison more exact, the maximum hy-
drogen desorption capacity Cmax is defined as the hydrogen des-
orbed from the alloy at 303 K between 5 atm and 0.1 atm. It can be
seen that the hydrogen desorption capacity maintains almost un-
changed at first with x increasing from 0.0 to 0.2. Then, the hydro-
gen desorption capacity decreases while x increasing from 0.3 to
0.5. The change of hydrogen storage capacity can be attributed to
the change of the phase abundance of the alloys. According to the
work of Oesterreicher22 and Takeshita, the hydrogen storage ca-
23
pacity of the LaNi phase is larger than that of the LaNi system
3
5
Figure
2. The
absorption/desorption
P-C
isotherms
of
the
alloy. Therefore, with the decrease of the ͑La, Mg͒Ni phase in the
La0.7Mg0.3Ni2.975ϪxCo0.525Mnx (x ϭ 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) hydrogen
3
alloys, the absorption/desorption capacity would decrease accord-
storage alloys at 303 K.
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Journal of The Electrochemical Society, 151 ͑3͒ A374-A380 ͑2004͒
A377
Figure 4. ͑a͒ The high rate dischargeability ͑HRD͒ of the
La0.7Mg0.3Ni2.975ϪxCo0.525Mnx (x ϭ 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) alloy elec-
Figure 3. ͑a͒ Discharge capacity vs. cycle number for the
La0.7Mg0.3Ni2.975ϪxCo0.525Mnx (x ϭ 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) alloy elec-
trodes at 303 K. ͑b͒ The capacity retention of the alloy electrodes after 60
charge/discharge cycles at 303 K.
trodes at 303 K. ͑b͒ The HRD of the alloy electrodes with I ϭ 1500 mA/g.
d
form a permeable Mg͑OH) passive film, which results in the rapid
capacity degradation during cycling. In the present study, accord-
ing to Rietveld analyses, as x increased, the content of ͑La, Mg͒Ni3
phase at first remained almost unchanged and then decreased with
2
content of LaNi phase. Previous studies have ascertained that the
26
5
Mg atoms only occupy 6c sites if La is partially substituted by Mg
2
4
in PuNi -type rhombohedral structure, and that Mg atoms do not
3
25
occupy the La sites in LaNi phase. As it is generally accepted that
5
further increase of x, and the content of LaNi phase increased pro-
5
the Mg element would be corroded very easily in KOH solution and
gressively, so the effect of Mg corrosion weakened, and the cycling
stability of the alloy improved noticeably when x ϭ 0.5.
Figure 4a shows the HRD of the La0.7Mg0.3Ni2.975ϪxCo0.525Mnx
alloy electrodes. Figure 4b shows the HRD of the alloy electrodes at
the very high discharge current density of 1500 mA/g. It can be seen
that the HRD of the alloy electrodes first increases from 66.7%
Table
II. The
electrochemical
properties
of
the
La0.7Mg0.3Ni2.975ÀxCo0.525Mn „x Ä 0.0, 0.1, 0.2, 0.3, 0.4, 0.5… al-
x
loy electrodes.
(
x ϭ 0.0) to 70.2% (x ϭ 0.1) and then decreases to 50.0% with
Cmax
C60 /Cmax
͑%͒
HRD1500b
further increase of x, and the La0.7Mg0.3Ni2.875Co0.525Mn0.1 alloy
Naa
electrode shows the highest value of HRD even when I ϭ 1500
Samples
͑mAh/g͒
͑%͒
d
mA/g. It is well known that, for the rare earth-based hydrogen stor-
age electrode alloys, Mn partial substitutions for Ni improves the
HRD because the dissolution of Mn into electrolyte leads to the
x ϭ 0.0
x ϭ 0.1
x ϭ 0.2
x ϭ 0.3
x ϭ 0.4
x ϭ 0.5
406.4
404.2
400.1
398.0
2
2
2
1
1
1
47.1
47.2
44.6
46.1
48.6
54.5
66.7
70.2
61.9
58.8
51.8
50.0
1
7
increase of Ni content on the alloy surface, which has good elec-
trocatalytic activity for the electrochemical decomposition of H O
387.8
2
on the surface of the electrode. In the present study, the total Ni
content in the alloy decreased when the Mn content increased. It is
believed that an optimum ratio between Mn element and Ni element
in the alloys could be found for improving the HRD of the alloy
electrodes by means of a plot as shown in Fig. 4b.
362.5
a
The cycle numbers needed to activate the electrodes.
The high rate dischargeability with discharge current density Id
ϭ 1500 mA/g.
b
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A378
Journal of The Electrochemical Society, 151 ͑3͒ A374-A380 ͑2004͒
Figure
5. Electrochemical
impedance
spectra
of
the
La0.7Mg0.3Ni2.975ϪxCo0.525Mnx (x ϭ 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) alloy elec-
trodes measured at the 50% DOD and 298 K.
Figure 5 shows the electrochemical impedance spectra of the
La0.7Mg0.3Ni2.975ϪxCo0.525Mn alloy electrodes at 50% DOD at 298
x
K. It can be seen that, for all alloy electrodes, each EIS spectrum is
consisted of two circular arcs and a sloped straight line. According
to the model of Kuriyama et al.,27 the smaller arc in the high-
frequency region represents the contact impedance between the
foamed Ni substrate and the alloy particles, the larger arc in the
low-frequency region represents the charge-transfer resistance on
the alloy surface, and the low-frequency straight line is attributed to
the Warburg impedance. From Fig. 5, it can be seen that the contact
impedance remains almost unchanged for the alloy electrodes with
different values of x, but the radius of the larger arc in the low-
frequency region decreases first and then increases with increasing
x, which indicates that the charge-transfer resistance of the alloy
electrode decreased first and then increased with increasing Mn con-
tent in alloys. There is an optimum value of Mn substitution for Ni
Figure
6. ͑a͒
Linear
polarization
curves
for
the
La0.7Mg0.3Ni2.975ϪxCo0.525Mnx (x ϭ 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) alloy elec-
trodes with a scan rate 0.1 mV/s measured at the 50% DOD and 298 K. ͑b͒
The exchange current density of the alloy electrodes at 298 K.
(
x ϭ 0.1) for the kinetics of electrochemical reaction of the alloy
electrodes.
Figure 6a shows the linear polarization curves of the
La0.7Mg0.3Ni2.975ϪxCo0.525Mn alloy electrodes. From the slope of
x
the specific line, the polarization resistance of any alloy electrode
can be determined, which is listed in Table III. The polarization
resistance of the alloy electrode decreases first from 141.0 m⍀
content at which the kinetics of the electrochemical hydrogen reac-
tion in alloy electrodes is the highest as shown in Fig. 6b.
Figure
La0.7Mg0.3Ni2.975ϪxCo0.525Mnx alloy electrodes. It can be seen in Fig.
that each Tafel polarization curve contains an anode polarization
7 shows the Tafel polarization curves of the
(
x ϭ 0.0) to 119.9 m⍀ (x ϭ 0.1) and then increases with further
2
8
7
increase of x to 148.1 m⍀ (x ϭ 0.5). According to Notten et al.,
branch, which corresponds to the hydrogen desorption, and a cath-
ode polarization branch, which corresponds to the hydrogen absorp-
the exchange current density I , generally used to measure the ki-
0
netics of the electrochemical hydrogen reaction, can be calculated
by the following formula
tion. The anode polarization slope (B ) and the cathode polarization
a
slope (B ) obtained from the curves are listed in Table III. Both Ba
c
IdRT
and Bc increase first with x increasing from 0.0 to 0.1 and then
decrease with further increasing x. The exchange current density I0
can be obtained from Tafel polarization curves according to the
Tafel equation
I0 ϭ
͓1͔
F␩
where R is the gas constant, T the absolute temperature, I the ap-
d
plied current density, F the Faraday constant and ␩ the total over-
potential. The exchange current density I0 calculated with Eq. 1 is
listed in Table III. Figure 6b also shows the variation of the ex-
change current density I0 with increasing the atomic ratio between
Mn and Ni. As shown in Table III, the exchange current density I0
of the alloy electrodes increases first from 182.1 mA/g (x ϭ 0.0) to
EϪE /B
ϪEϪE /B
0
0
a
c͒
I ϭ I ͑10
ϩ 10
͓2͔
0
in which I the measured current (Ianodic-Icathodic), I0 the exchange
current density, E is the applied potential, E the open circuit poten-
tial ͑OCP͒, B the anodic Tafel slope, B the cathodic Tafel slope,
respectively. The exchange current density I obtained from Eq. 2 is
0
a
c
214 mA/g (x ϭ 0.1) and then decreases to 173.3 mA/g (x ϭ 0.5)
0
with increasing x, which indicates that the kinetics of the electro-
chemical hydrogen reaction in alloy electrodes increases first and
then decreases with increasing Mn content in alloys. There also
exists a most optimum ratio between the Mn content and the Ni
also listed in Table III. It can be seen that the I0 increases from
181.8 mA/g (x ϭ 0.0) to 221.0 mA/g (x ϭ 0.1) and then decreases
to 166.2 mA/g when x reaches 0.5, which is consistent with the
result obtained from the line polarization curves. Moreover, all the
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Journal of The Electrochemical Society, 151 ͑3͒ A374-A380 ͑2004͒
A379
Table III. The polarization resistance RD , the exchange current density I , the limiting current density I and the hydrogen diffusion
0
L
coefficient D of La0.7Mg0.3Ni2.975ÀxCo0.525Mn „x Ä 0.0, 0.1, 0.2, 0.3, 0.4, 0.5… alloy electrodes.
x
Linear polarization
exchange current density
Tafel slope
͑mV/dec͒
Tafel polarization
exchange current density
Limiting
current density
IL
Polarization
resistance RD
͑m ⍀͒
Hydrogen diffusion
I0
͑mA/g͒
I0
͑mA/g͒
coefficient D
Ϫ11
2
Samples
Ba
Bc
͑mA/g͒
(ϫ10
cm /s͒
x ϭ 0.0
x ϭ 0.1
x ϭ 0.2
x ϭ 0.3
x ϭ 0.4
x ϭ 0.5
141.0
119.9
133.1
133.3
146.4
148.1
182.1
214.0
192.7
192.6
175.4
173.3
169
206
182
174
173
170
155
187
179
174
169
165
181.8
221.0
206.6
199.8
171.5
166.2
2098.5
2399.4
2002.6
2000.3
1552.9
1400.1
9.49
9.85
8.57
7.28
7.23
6.61
curves show that, during the anode polarization process, the anodic
current increases first with increasing overpotential and finally
reaches a maximum, which is defined as the limiting current density
IL , which represents the hydrogen diffusion ability of the alloy elec-
trode. The limiting current density IL of the electrodes made of
different alloys ͑Table III͒ increases first from 2098.5 mA/g
where i, D, C , C , a, d and t are the diffusion current density
0 s
2
͑A/g͒, the hydrogen diffusion coefficient ͑cm /s͒, the initial hydro-
3
gen concentration in the bulk of the alloy ͑mol/cm ͒, the hydrogen
3
concentration on the surface of the alloy particles ͑mol/cm ͒, the
alloy particle radius ͑cm͒, the density of the hydrogen storage alloy
3
͑g/cm ͒, and the discharge time ͑s͒, respectively. By taking that the
average particle radius as 13.12 ␮m, the values of hydrogen diffu-
sion coefficient, D, of alloys with different x values are calculated by
Eq. 3 and tabulated in Table III. The hydrogen diffusion coefficient
(
x ϭ 0.0) to 2399.4 mA/g (x ϭ 0.1) and then decreases to 1400.1
mA/g (x ϭ 0.5), indicating that the hydrogen diffusion in alloy
increases first and then decreases with increasing x, and the variation
in hydrogen diffusivity is in agreement with the variation of the
exchange current density and the HRD.
Ϫ11
2
Ϫ11
increases from 9.49 ϫ 10
cm /s (x ϭ 0.0) to 9.85 ϫ 10
2
Ϫ11
2
cm /s (x ϭ 0.1) and then decreases to 6.61 ϫ 10
cm /s (x
Figure 8 shows the semilograrithmic curves of anodic current vs.
time responses of the La0.7Mg0.3Ni2.975ϪxCo0.525Mnx alloy elec-
trodes. As shown in Fig. 8, it can be distinguish the current re-
sponses in two time regions. The first one is the shorter time region
in which the current density declines rapidly, and the other is the
longer time region in which the current density decreases slowly in
ϭ 0.5), which is consistent with the result of the Tafel polarization
examination, with the electrochemical kinetics of the alloy elec-
trodes increasing first and then decreasing with increasing x from
0.0 to 0.5.
As the performance of a hydrogen storage alloy electrode is de-
termined by both the kinetics of the processes at the alloy/electrolyte
interface and the rate of hydrogen diffusion in the bulk of the alloy,
the exchange current density I , the limiting current density I , and
2
9-30
29
a linear.
Using a spherical diffusion model, the linear response
in the longer time region can be treated as the finite diffusion of
hydrogen inside the alloy particle. In this case, the diffusion coeffi-
cient of the hydrogen atoms in the bulk of the alloy can be estimated
according to following expression31
0
L
the D are the three important parameters for charactering the kinet-
ics of alloy hydriding. In the present study, the exchange current
density I , the limiting current density I , and the D of the alloy
0
L
electrodes all increased first and then decreased with increasing Mn
content due to the increasing ratio of Mn content and Ni content in
the alloy. From the above results, the kinetics of hydriding of the
La0.7Mg0.3Ni2.975ϪxCo0.525Mn (x ϭ 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) hy-
x
␲2
drogen storage alloys is affected by changing x, with the optimum
composition around x ϭ 0.1, which is consistent with the variation
of HRD.
6
FD
D
log i ϭ log
ͫ da2
͑C Ϫ C ͒
Ϫ
t
͓3͔
0
s
ͬ
2
.303 a
Figure 7. Tafel polarization curves for the La0.7Mg0.3Ni2.975ϪxCo0.525Mnx
x ϭ 0.0,0.1,0.2,0.3,0.4,0.5) alloy electrodes with a scan rate 5 mV/s mea-
sured at the 50% DOD and 298 K.
Figure 8. Semilograrithmic plots of anodic current vs. time responses of the
La0.7Mg0.3Ni2.975ϪxCo0.525Mnx (x ϭ 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) alloy elec-
trodes at 298 K.
(
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A380
Journal of The Electrochemical Society, 151 ͑3͒ A374-A380 ͑2004͒
Conclusions
The structural and electrochemical properties of the
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Acknowledgment
This work was supported by the National Nature Science Foun-
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