Journal of New Materials for Electrochemical Systems 8, 101-108 (2005)
c J. New. Mat. Electrochem. Systems
Electrochemical study of nanostructured multiphase nickel hydroxide
Xianyou Wang1∗ , P.J. Sebastian2,4 , Ari-Carman Millan3 , P.V. Parkhutik3 , S.A. Gamboa4
1 Chemistry
College, Xiangtan University, Hunan 411105, China
Cell Group, CIE-UNAM, 62580, Temixco, Morelos, Mexico
3 Alcoy Higher School of Engineering, Universidad Politecnica de Valencia, 46022 Valencia, Spain
4 IMP, Eje Central Lázaro cárdenas 152, 07730, D.F., Mexico
2 Solar-Hydrogen-Fuel
( Received September 24, 2004 ; received in revised form December 5, 2004 )
Abstract: The high density nano-crystalline multiphase nickel hydroxide containing at least three doping elements has been synthesized and its
electrochemical characteristics studied. It was found that the structure of the material is a mixed phase of α-Ni(OH)2 and β-Ni(OH)2 . It was
also found that they have a stabilized structure same as α-Ni(OH)2 during long-term charge/discharge process. The electrochemical behavior of
the high density spherical multiphase α-Ni(OH)2 was studied and found that they have much better redox reversibility, a much lower oxidation
potential of Ni(II) than the corresponding oxidation state in the case of β-Ni(OH)2 , and a much higher reduction potential. They exchange one
electron during electrochemical reaction and have a higher proton diffusion coefficient. The mechanism of the electrode reaction is still found to be
controlled by proton diffusion, and the proton diffusion coefficient is 5.67×10−10 cm2 s−1 . Moreover, they reveal a higher discharge capacity than
β-Ni(OH)2 /β-NiOOH because they exchange one-electron per nickel atom during charge/discharge process.
Key words : nickel hydroxide, electrochemical performance, voltammogram, Ni-MH battery
1.
Nickel hydroxide possesses two polymorph states denoted αNi(OH)2 and β-Ni(OH)2 [2, 3]. Both forms crystallize in the
hexagonal system with the brucite-type structure with Ni(OH)2
layers stacked along the c-axis. Each Ni(OH)2 layer consists of
a hexagonal planar arrangement of octahedrally oxygen-coordinated Ni(II) ions. The main difference between the β- and αtype Ni(OH)2 phase is the mode of stacking of the layers along
the c-axis. For the β-Ni(OH)2 these layers are perfectly stacked
along the c-axis with an inter-lamellar distance of 4.6Å while
for the α-Ni(OH)2 they are disoriented relatively to one other
forming turbo-stratic phase with the presence of water molecules
and anionic species in the van der waals gap [4]. The interlamellar distance for the α-Ni(OH)2 is about 7.8Å.
INTRODUCTION
The Ni–MH batteries exhibit great potential for electric and hybrid vehicle (EHV) applications due to their high specific energy, good cycle life and power density. Although consumer
size Ni–MH cell technology is maturing, and prismatic large
capacity modules are on their way to commercialization, the
potential for large energy storage and vehicle-related applications heavily relies on their ability to reliably deliver high power
and cycle life. The electrochemical active Ni(OH)2 electrode is
used as positive electrode of Ni-MH batteries and the nickel hydroxide electrode is essentially the limiting factor in the overall
battery energy density. So, there is a great scientific and technological interest in the study of the nickel hydroxide synthesis
and characterization, due to its application as positive electrodes
of alkaline batteries [1].
∗ To whom correspondence should be addressed:
[email protected]
email:
In the α-Ni(OH)2 lattice, the plane (001) is separated by intercalated water molecules and alkali metal ions. α- Ni(OH)2 is
unstable in the presence of alkali, and by aging, that is, with water and alkali metal ions elimination it becomes β-Ni(OH)2 [5].
wxi-
101
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X. Wang et al./ J. New Mat. Electrochem. Systems 8, 101-108 (2005)
Due to α-Ni(OH)2 instability in alkaline solutions, β-Ni(OH)2
is frequently used as a precursor material in alkaline batteries.
Electrochemical reactions of nickel hydroxide during the
charge/discharge cycling were summarized by Bode et al. [6]
as
α-Ni(OH)2 ⇔ γ-NiOOH
⇓⇑
β-Ni(OH)2 ⇔ β-NiOOH
Routinely, the active material of the nickel hydroxide batteries has always been β-Ni(OH)2 , especially spherical particles of
β-Ni(OH)2 [7]. Conventionally, electrode reaction of β-Ni(OH)2
was considered to be a one-electron process involving oxidation of divalent nickel hydroxide to trivalent nickel oxyhydroxide during the charging stage and subsequent discharge of trivalent nickel oxyhydroxide to divalent nickel hydroxide. Thus,
β-Ni(OH)2 was found to possess a maximum theoretical
specific capacity of 289 mAh/g. It is well known that more
than one electron transfer could be realized by deviating from
β-Ni(OH)2 /β-NiOOH structures and by cycling between the
highly oxidized γ-NiOOH and β-NiOOH phases or the mixture of them [7]. However, due to the difference in the volumetric densities of β-Ni(OH)2 and γ-NiOOH materials there
is expansion and contraction of the material during charge and
discharge. The expansion and contraction can cause swelling
and deforming of the cathode, thus reducing the lifetime of
the battery. So, it was conventionally recognized that such γNiOOH formation destroyed reversible structural stability and
therefore cycle life was unacceptably degraded. A large number of patents and publications in the technical literature have
reported modifications of the nickel hydroxide material in order
to diminish destructive mechanical pressure effects accompanying the formation of the γ-NiOOH.
It has also been reported that α-Ni(OH)2 has better electrochemical properties than β-Ni(OH)2 [8-14]. The α-Ni(OH)2 can be
oxidized to γ-NiOOH at lower potential compared with
β-Ni(OH)2 , and has a higher discharge capacity than
β-Ni(OH)2 /β-NiOOH since the nickel valence in the γ-NiOOH
is known to exceed 3, due to Ni4+ defects (3.3 to 3.7). Kamath
et al. reported that a stabilized α-Ni(OH)2 electrode has much
higher discharge capacity than β-Ni(OH)2 [8]. Moreover, in the
α-Ni(OH)2 /γ-NiOOH phase with water and alkali anions intercalated between nickel hydroxide layers, the transformation
of α-Ni(OH)2 into γ-NiOOH occurs reversibly and without any
mechanical deformation and swelling of the electrode during
cycling, unlike the β-Ni(OH)2 /β-NiOOH case [8, 9].
The limitation of using α-Ni(OH)2 in batteries is related with
its limited stability to highly alkaline electrolyte – the material
transforms itself into β-Ni(OH)2 . There have been made a vari-
ety of attempts to improve the stability of the α-Ni(OH)2 [8-14].
Dixit et al. synthesized an aluminum-substituted α-Ni(OH)2
by electrochemical impregnation and found that the material attains a reversible discharge capacity of 450 mAh/g [9]. Faure et
al [10]. studied pasted electrodes with cobalt-doped α-Ni(OH)2
and found that it has a capacity of 345 mAh/g. Ezhov et al.
[11] found that zinc additive could stabilize the thermodynamically unstable α-Ni(OH)2 . Guerlou-Demourgues et al. studied
chemical and physical properties of new manganese-substituted
nickel hydroxide containing inter-lamellar water and prepared
by precipitation technique [12,13]. Very recently Dai et al.[14]
investigated the structure, morphology and electrochemical performance of nano-structured aluminum-stabilized nickel hydroxides. These reports on α-Ni(OH)2 showed that the morphology of the particles are neither spherical nor quasi-spherical in
their shape and have a low tap-density when being filled-in into
porous nickel foam. So, low volumetric specific capacity of αNi(OH)2 material made it not prospective for commercial batteries.
We have earlier reported the synthesis of high-density spherical
multiphase α-Ni(OH)2 material by adding at least three modifier elements [15]. It is suitable for stable α-Ni(OH)2 /γ-NiOOH
cycling during charge/discharge process with improved capacity, electrochemical performance and battery life. In this paper,
we report the electrochemical performance of the high density
spherical multiphase α-Ni(OH)2 .
2.
EXPERIMENTAL
The material was synthesized by co-precipitation of several
metal ions from the aqueous solution. The resulting precipitate substance was filtered by a laboratory suction filter, washed
with distilled water until it was free form neutral salts and then
dried at 80◦ C.
The structure of material was examined using powder X-ray
diffraction analysis [15]. The X-ray powder diffractometer
(XRD, PW1830, Philips Co.) with Cu Kα radiation was used to
identify the phase. Scanning electron microscopy (SEM) measurements were carried out with a JSM6300 scanning electron
microscope. The doped metal ions are most preferably Co, Zn,
and Mn. For comparison all samples have the same Co and Zn
amount, but only Mn amount was changed.
The electrochemical behavior of the spherical multiphase nickel
hydroxide in 6 M KOH electrolyte was studied. A typical threeelectrode one-compartment electrolysis cell was used for the experimental measurements. The working electrode was a powder
microelectrode with a 200 µm diameter platinum. Nickel sheet
counter-electrode was placed in the side and the working electrode was positioned in the center. A Luggin capillary with a
salt bridge was used to connect the cell to the reference elec-
Electrochemical study of nanostructured multiphase nickel hydroxide / J. New Mat. Electrochem. Systems 8, 101-108(2005)
103
trode. All potentials were measured and quoted with respect to
a Ag/AgCl (3 M NaCl) reference electrode.
Cyclic voltammetry experiments were performed using an EG&G
PAR-273A Potentiostat/galvanostat under the control of CorrWare 2.5 software.
3.
3.1
RESULTS AND DISCUSSION
The structure and morphology of multiphase nickel
hydroxide
Fig.1 shows the SEM of the nanostructural multiphase nickel
hydroxide with different magnifications. The SEM examinations show that the samples are made up of monolithic particles
and some of the particles agglomerated together to form a quasispherical shape. The single particles agglomerated in quasispherical shape through physical or chemical process when they
were synthesized. The tap-density measurements showed that it
had a tap-density between 1.7 – 1.9 g/cm3 . The size of these
particles varied from a few decade nanometer to a few hundred
nanometer. This explains the reason why the tap-density of the
nanostructural multiphase nickel hydroxide is nearly same like
that of commercial spherical nickel hydroxide and higher than
that of α-Ni(OH)2 reported in the literature.
Fig. 2 shows the powder X-ray diffraction pattern of the nanostructured multiphase nickel hydroxide and β-Ni(OH)2 . In the
XRD patterns no diffraction pattern attributable to impurities
was observed, it was assumed that they were dissolved in the
nickel hydroxide to form a solid solution. Comparing Fig.2(a)
with Fig.2(b), the appearance of the new peak at (003) in
Fig. 2(a) implies the presence of α-Ni(OH)2 , and XRD pattern in Fig.2(a) indicates the nanostructural multiphase nickel
hydroxide has a mixed phase structure as compared to unsubstituted α-Ni(OH)2 and β-Ni(OH)2 reported in the literature [14,
15]. In addition, nanostructural multiphase nickel hydroxide
shows broad asymmetric band in the 2.2 Å to 2.6 Å, this is the
typical structure of turbostratic structure.
3.2
Influence of the manganese content on the characteristics of charge/discharge process
Fig.3 shows the typical cyclic voltammogram for various amounts
Mn doped multiphase nickel hydroxide microelectrodes after
activation by 15 cycles at a scan rate of 5 mV/s. Anodic oxidation peaks for the electrodes with various manganese content
appears between 0.34 and 0.38 V before oxygen evolution potential. Reduction peaks at about 0.162- 0.177 V is observed
on the reverse sweep. As shown in Fig.3, the potential of oxidation peak that corresponds to the formation of higher-valence
nickel species decreases with increase of Mn content, while the
reduction potential remains nearly unchanged. Similar voltammogram is observed for the electrode without Mn [15], thought
Figure 1: The SEM of the nanostructural multiphase nickel hydroxide. (a) X5000, (b) X16000
the anodic oxidation peak is more positive and cathodic peak is
more negative as compared with those of the electrode with Mn.
The results shown in Fig. 3 are further presented in Table 1.
The difference between the anodic and cathodic peak positions
(∆Ea,c ), is taken as an estimate of the reversibility of the redox
reaction. Oxygen evolution is a side reaction during charging
of the nickel electrode. To compare the effect of multiphase αNi(OH)2 on oxygen evolution reaction, the difference between
the oxidation peak potential and the oxygen evolution potential
(DOP) on the return sweep required to produce 5×10−5 A of
anodic current is also estimated from voltammograms. For this
reason, it was assumed that the return sweep provided the best
approximation of the steady state condition since there would
be less interference from the nickel hydroxide redox reaction
[16, 17].
Table 1 shows that multiphase nickel hydroxide allows the electrode to charge at a significantly less positive potential (387 mV,
352 mV, 346 mV) instead of 412 mV [15]. In addition, the
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X. Wang et al./ J. New Mat. Electrochem. Systems 8, 101-108 (2005)
Figure 2: The X-ray diffraction patterns for (a) the nanostructural multiphase nickel hydroxide and (b) β-Ni(OH)2
charge process appears to occur more reversibly (∆Ea,c is 210
mV, 168 mV, 184 mV instead of 254 mV). Moreover, their oxygen evolution overpotential has nearly the same value. Thus,
these results show that multiphase nickel hydroxide electrodes
allow the charge process to occur more easily and more reversibly, suggesting that much more active material can be utilized during charge. Besides, as the oxygen evolution overpotential remains nearly the same and the oxidation peak potential
of nickel hydroxide is lower, the charge efficiency of the electrode improves essentially due to greater discharge capacity of
the electrode.
Table 1: Parameters obtained for various Mn contents in nickel
hydroxide electrodes
Mn content
E pa
E pc
∆Ea,c
EO2
(weight %)
(V)
(V)
(V)
(V)
0
0.412 0.158 0.254 0.423
10
0.387 0.177 0.21 0.422
15
0.352 0.184 0.168 0.423
20
0.346 0.162 0.184 0.426
Figure 3: Cyclic voltammograms for (a) β-Ni(OH)2 , 0wt.% Mn
and (b) multiphase nickel hydroxide after 15 charge/discharge
cycles containing different amounts of Mn: (1) 10wt.% (2)
15wt.% (3) and 20wt.%
Similar results have been reported for nickel hydroxide substituted by cobalt, manganese and aluminum [8-14]. Corrigan et
al. [16] have observed that co-precipitation of cobalt or manganese in nickel hydroxide decreases the oxidation potential
with respect to that of the un-substituted nickel hydroxide.
Our results are in agreement with those of Guerlou-Demourgues
et al. [12] in reduction potential increase when Mn amount
changes from 10% to 15% and its decrease to 0.162 V when
Mn amount attains 20%. The cited work assumed that the cell
potential change is related to the difference in size between the
substituting cations and the nickel ion. On the basis of the ionic
radii established by Shannon and Prewitt [18], smaller size of
the Mn4+ ions (rMn4+ = 0.54 Å) against that of the Ni3+ ions
(rNi3+ = 0.56 Å) must induce a decrease in the cell potential
with increasing substituting amount.
Electrochemical study of nanostructured multiphase nickel hydroxide / J. New Mat. Electrochem. Systems 8, 101-108(2005)
3.3
Influence of the potential sweep rate on the
charge/discharge characteristics
Although oxidation potentials are sensitive to Mn content they
are nearly independent of the sweep rate. Fig.4 illustrates this
effect in the case of the electrode with 15% Mn content. Similar
results are obtained for all other electrode compositions.
105
From Figures 3-5 it can be seen that the nickel hydroxide microelectrode with different Mn contents has a single diffusion
limited peak. Theoretically, potential difference of peak to peak
on a voltammogram for a reversible electrode reaction should
be equal to 59 mV/n (where n is the number of electrons transferred), however the values in Table 1 are noticeably higher than
the theoretical value. Therefore, electrode reaction of nickel hydroxide has been considered as quasi-reversible [19].
The dependence of E pa and E pa/2 can be expressed using Nicholson and Shain equation, [20], which can be used to calculate
the number of electrons (n) participating in the rate determining
stages
E pa − E pa/2 = 1.857RT /(αnF)
(1)
Where E pa/2 is the potential of half peak height, R is the universal gas constant, T is the absolute temperature, and α is the
charge transfer coefficient.
Figure 4: Cyclic voltammograms of 15% Mn electrode at various sweep rates
Fig.5 shows the dependence of the oxidation and reduction potentials on the potential sweep rate. Note that anodic peak has
only slightly shifted to more positive potentials at faster scan
rates while the potential of cathodic peak remained relatively
constant.
Figure 5: Effect of the sweep rate on the anodic and cathodic
potentials for 15% Mn electrode
Following Eq. 1, the results shown in Figure 3 means that the
number of electrons participating in the electrode reaction are
different for different Mn doping levels of nickel hydroxide microelectrode, which can be calculated and are shown in Table 2.
As far as the oxygen evolution is a side reaction interfering with
the charging of nickel electrode and it will obscure the basic line
of reduction peak current. Fig. 6 shows that the anodic peak
current (i pa ) in Fig. 4 depends linearly on the square root of
the potential sweep rate. As seen in Fig.6, there exists a linear
relationship between the peak current and the potential sweep
rate square root.
Figure 6: Dependence of the anodic peak current on potential
sweep rate for 15% Mn electrode
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X. Wang et al./ J. New Mat. Electrochem. Systems 8, 101-108 (2005)
Table 2: Parameters calculated for various Mn contents in nickel hydroxide electrodes
Mn content
(weight %)
0
10
15
20
3.4
E pa
(V)
0.412
0.387
0.352
0.346
E pa/2
(V)
0.307
0.301
0.286
0.258
I pa
(A)
2.82×10−4
1.34×10−4
9.4×10−5
8.99×10−5
Kinetics of the proton diffusion during charge/discharge
cycles
The charge-discharge processes of the nickel hydroxide in alkaline solution can be described as a proton deintercalationintercalation mechanism:
discharge
NiOOH + H2 O + e−
Ni(OH)2 + OH −
charge
(E 0 = 0.291V vs.Ag/AgCl) (2)
It has been found that the movement of protons and electrons in
and out of the bulk of the solid phase follows diffusion kinetics
[20]. MacArthur [21] identified that the solid-state proton diffusion from the oxidation state to the electrode-electrolyte interfaces was a rate-limiting process. The oxidation process may
be represented as [21]
Ni(OH)2 ⇔ H + (s0 ) + NiOOH(s0 ) + e− (s0 )
(3)
H + (s0 ) ⇔ H + (s)
(4)
−
OH(aq)
+ H+
⇔ H2 O
(5)
Reaction (3) is the charge-transfer process involving the reduction of one of the higher valence species of active material in the
lattice (in the present case - NiOOH); reaction (4) involves diffusion of the proton from site (s) of the electrode to the charge
transfer site (s’); and reaction (5) represents the formation of a
proton at catalytic site (s) at the electrode/electrolyte interface.
Indeed, Ni(OH)2 and NiOOH do not correspond ideal +2 and
+3 oxidation states. Rather, the active material is composed of
a mixture of oxidation states lying between 3.6 and 2.0 [21].
Therefore, the number of electrons transferred in Eq. 2 is not
necessarily equal to 1 (see Table 2).
As a stationary electrode, a linear relationship between peak
current and the square root of potential sweep rate (V1/2 ) over
the range of sweep rate in Fig.4 can be represented as[21]
1/2
I p = 2.99 × 105 n(αn)1/2 ADo CooV 1/2
(6)
n
0.90
1.255
1.44
1.08
D
(cm2 /s)
3.21×10−11
4.47×10−11
6.67×10−10
9.94×10−11
Where Do is the proton diffusion coefficient in nickel hydroxide, Coo is the concentration of hydrogen in the solid (assuming
the density of Ni(OH)2 is 3.5g/cm3 and thus the maximum concentration of hydrogen in the lattice of electrochemically active
material, Coo is 0.0383 mol/cm3 [21]), the other coefficients have
the usual meaning. According to the data obtained and Eq.6, the
H+ diffusion coefficient for 15% Mn doped multiphase nickel
hydroxide is 5.67×10−10 cm2 /s. This value as well as those
corresponding to other electrodes is tabulated in Table 2.
Motupally et al. [22] found that the proton diffusion coefficient
is sensitive to the state of charge of the electrode and decreases
by approximately three orders of magnitude (from 3.4×10−8 to
6.4×10−11 cm2 /s ) when electrode is discharged from the complete charged state. MacArthur [23] measured the diffusion coefficient of 3.1×10−10 for the charge process. In Table 2, the
proton diffusion coefficient in the multiphase nickel hydroxide
exhibits a higher value, which was higher than Al-substituted
α-Ni(OH)2 , 3.6×10−11 [24].
As seen from Fig.3 and Tables 1, 2 the multiphase α-Ni(OH)2
doped with different amounts of Mn reveal better electrochemical performance than β-Ni(OH)2 (lower oxidation potential of
nickel (II), higher electrochemical reversibility, and higher exchange electron number), thus indicating that the material has
greater discharge capacity and therefore is suitable for application as active material. Furthermore, the reduction peak potential for these electrodes is higher; this implies that the half discharge potential of the cell, which is an important parameter of
judging battery performance, will be improved effectively. With
increasing Mn amount in the nickel hydroxide, the reduction
peak potential significantly increased and oxidation peak potential apparently decreased (nickel hydroxide electrode doped
with 20% Mn shows different behavior and will be discussed
in a forthcoming paper). Guerlou-Demourgues et al. [12] had
explained a similar behavior by assuming that too small Mn
content in nickel hydroxide causes worsening of the stability
of material on cycling.
Both structures of α-Ni(OH)2 and β-Ni(OH)2 consist of brucitetype layers well ordered along the C-axis (β-Ni(OH)2 ) or randomly stacked along the C-axis (α-Ni(OH)2 ). In the latter, incorporation of water molecules and anionic species within the
Electrochemical study of nanostructured multiphase nickel hydroxide / J. New Mat. Electrochem. Systems 8, 101-108(2005)
Van der Waals gap results in a c-spacing of 7.8 Å. Compared to
4.6 Å for the β-Ni(OH)2 , α-Ni(OH)2 bonding strength with the
brucite layer is also poor. This accounts for the poor stability
of α-Ni(OH)2 in alkali media. Introducing Mn and other metal
ions in the nickel hydroxide lattice, with the purpose of enhancing the intercalated anion content successfully stabilizes the αNi(OH)2 structure in alkali media and improving ionic and electronic conductivity. The charge excess due to Mn3+ is compensated by the insertion of carbonate between the hydroxide slabs
[8,9]. The anions strongly anchor the positively charged brucite
layers and stabilize the structure in a variety of stressful conditions. However, the sample with lower anion content (about 5%
Mn ) is less stable in alkali media. As seen from Fig. 3 and
Table 2, the multiphase nickel hydroxide electrode, which was
doped with 15% Mn, has a much better electrochemical performance, much more exchange electron number, and indicating
that the electrode has greater discharge capacity than the theoretical capacity of β-Ni(OH)2 (289 mAh/g). As a consequence,
it can be concluded that α-Ni(OH)2 doped with 15% Mn is an
excellent positive active material for alkaline rechargeable MH
batteries.
4.
CONCLUSION
The nickel hydroxide active material doped with at least three
modifying elements revealed improved proton diffusion coefficient, excellent electrochemical performance and more efficient
use of the available storage sites. The nickel hydroxide doped
with 15% Mn has much lower oxidation peak potential and
much higher reduction peak potential compared with the undoped material. The optimum content of Mn doping in nickel
hydroxide is about 15%. When Mn content is too low, the electrode reaction is no longer stable in form on cycling, while too
high Mn amount will also affect discharge capacity. The presence of Mn in nickel hydroxide lattice can increase the electronic and protonic conductivity of the material, thus improving
the performance of the electrode (more efficient utilization of
active material, greater discharge capacity and better reversibility of Ni(II)/Ni(III) redox reaction).
5.
ACKNOWLEDGMENTS
This project was supported by Universidad Politecnica de Valencia, Spain; Excellent Youth Fund of Hunan Province Educational Committee, China, and DGAPA-UNAM (IN102100),
Mexico. The authors appreciate the assistance of Mr. Maunel
Plaues of Electron Microscopy Center, Polytechnic University
of Valencia.
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