Kim et al - Electrochemical properties of nickel hydroxide films deposited galvanostatically.pdf

Kim et al - Electrochemical properties of nickel hydroxide films deposited galvanostatically.pdf
Journal of Ceramic Processing Research. Vol. 13, No. 6, pp. 688~692 (2012)
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Ceramic
Processing Research
Electrochemical properties of nickel hydroxide films deposited galvanostatically
S.-J. Kima, M.-K. Hongb, B.C. Kimc, J.-K. Chungb, G.G. Wallaced and S.-Y. Parka,b,*
Department of Ceramic Engineering, Gangneung-Wonju National University Gangneung, 210-702, Korea
Technology Innovation Center for Fine Ceramic, Gangneung-Wonju National University, Gangneung, 210-702, Korea
c
Natural Science Research Institute, Department of Chemistry, Dongguk University, Seoul, 100-715, Korea
d
ARC Centre of Excellence for Electromaterials Science, Intelligent Polymer Research Institute, AIIM Facility, Innovation
Campus, University of Wollongong, Wollongong, NSW 2522, Australia
a
b
A high specific capacitance was obtained for Ni(OH)2 galvanostatically deposited onto a stainless-steel foil. The structure and
surface morphology of the Ni(OH)2 were studied using X-ray diffraction(XRD) analysis and scanning electron microscopy(SEM).
A loosely packed structure containing individual Ni(OH)2 particles with an average size of 10-20 nm was obtained. The thickness
of the deposit was several micro met. The capacitive characteristics of the Ni(OH)2 electrodes were investigated using cyclic
voltammetry(CV) in a 1 M KOH electrolyte solution. A maximum specific capacitance of 1233 F g-1 was obtained for the Ni(OH)2
electrode with a voltage range 0-0.5 V. The effect of deposition conditions, such as the current density, and 0.1M aqueous
solution of Ni(NO3)2 • 6H2O on the electrochemical capacitance of the deposited Ni(OH)2 films are discussed in detail.
Key words: Nickel hydroxide films, Galvanostatic deposition, Redox capacitior, Electrochemical properties.
behavior of Ni(OH)2 have appeared [19, 20]. The use
of electrochemical techniques to produce Ni(OH)2
provides control over the structure and morphology of
the films deposited [21, 22]. Tan et al. electrochemically
deposited mesoporous Ni(OH)2 films from dilute
surfactant solutions using potentiostatic deposition [23].
Here we studied the electrochemical properties of
Ni(OH)2 films on stainless-steel(SS) foil using galvanostatic
deposition at various current densities. The microstructure as well as the electrochemical properties of the
synthesized Ni(OH)2 films were investigated.
Introduction
Recently, electrochemical capacitors (ECs) have
received a great deal of attention due to the high power
density and longer life cycle attainable compared to
secondary batteries. Electrochemical capacitors also exhibit
a high energy density compared with the conventional
electrical double-layer capacitors (EDLC) [1-3]. They
have many practical applications such as auxiliary power
sources in combination with fuel cells, batteries for
hybrid electric vehicles, back up and pulse power
sources for mobile electric devices, etc [4].
Electrochemical capacitors (ECs) based on hydrous
ruthenium oxides exhibit much higher specific
capacitance than conventional carbon materials with
remarkably high specific capacitance values ranging
from 658 to 760 F g-1 (from a single electrode) [5-7].
However, the high cost of this noble metal material
limits practical use. Therefore, much effort has been
aimed at searching for alternative inexpensive electrode
materials with good capacitive characteristics. Materials
investigated include NiO [8, 9], CoOx [10], MnO2 [11],
Ni(OH)2 [12], and Co(OH)2 [13]. Amongst these electrode
materials, Ni(OH)2 is considered to be the most promising
for applications in energy/power storage devices, due to its
low cost and well-defined electrochemical redox activity
[14-18]. To date, only limited reports on the capacitive
Experimental
All chemical reagents were of analytical grade from
Aldrich. A research grade stainless steel(SS) sheet
(grade 304, t = 0.2 mm) was used as the current
collector. The stainless steel(SS) sheet was cut into the
required size (10 mm × 30 mm). Stainless steel(SS) foil
was polished with emery paper to a rough finish and
then washed in acetone, ethyl alcohol and distilled
water in turn for 10 minutes. A three-electrode electrochemical
cell was assembled in which the stainless steel(SS) foil,
platinum plate (20 mm × 20 mm) and a Ag/AgCl (in 1M
KCl) were used as the working, counter and reference
electrode, respectively. All electrochemical depositions
of nickel hydroxide films were performed using a
potentiostat (IM6 Instrument, Zanher electric, Germany).
Ni(OH)2 was electrochemically deposited onto stainless
steel(SS) foil in an electrolyte solution of 0.1M
Ni(NO3)2 • 6H2O under galvanostatic conditions.
Galvanostatic deposition was carried out at various
*Corresponding author:
Tel : +82-33-655-2502
Fax: +82-33-655-2506
E-mail: [email protected]
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Electrochemical properties of nickel hydroxide films deposited galvanostatically
current densities. The deposited electrodes were
washed in distilled water and then dried at room
temperature for 24 h. The weight of the electrode was
determined by means of a micro-balance. Deposited
weight was calculated by weighing the substrate
before and after deposition experiments, and after by
drying at room temperature for 24 h.
The phase analysis of specimens was examined by
X-ray diffraction (XRD) (D/MAX-2500 V, RIGAKU)
with Cu Kα radiation (λ = 1.5406 Å). The microstructures
of the specimens were evaluated by field emission
scanning electron microscopy (FE-SEM) (S-4700,
HITACHI, Japan).
All electrochemical measurements were tested in a
three-electrode arrangement. Cyclic voltammetry(CV)
and electrochemical impedance spectroscopy(EIS) were
carried out using an IM6 Instrument electrochemical
work station (IM6ex, ZHANER Elektrik, Germany) at
room temperature. The impedance measurements were
conducted by means of an IM6 Instrument and an AC
perturbation amplitude of 5 mV at 0.3 V was applied in
the frequency range between 100 mHz and 100 kHz.
Results and Discussion
Fig. 1 shows the variation of the working electrode
potential during galvanostatic deposition at various current
densities. Galvanostatic depositions were performed in the
range of 1-5 mA/cm2. With current densities up to 4 mA/
cm2, the potential decreased with reaction time indicating
deposition of a conductive layer on the electrode.
Corrigan and Bendert suggested that the electrodeposition of the Ni(OH)2 films from the Ni(NO3)2
precursor involves the following steps [24]: Nitrate
ions are reduced on the cathodic surface to produce
hydroxide ions according to Equation (1).
The generation of OH- at the cathode raises the local
pH, resulting in the precipitation of α-Ni(OH)2 at the
electrode surface according to Equation (2):
Fig. 1. Variation of the potential of the electrodes during
galvanostatic deposition at different anodic current densities.
NO3- + 7H2O + 8e- → NH4+ + 10OH-
(1)
Ni2+ + 2OH- → Ni(OH)2↓
(2)
Others[25] have shown that the predominant species
present in concentrated Ni(NO3)2 aqueous solutions
(ca. 1.8 M in our work) is the polymeric Ni4(OH)44+.
The polymeric species Ni4(OH)44+ combines with OHto form a Ni(OH)2 deposit as given in Equation (3):[26]
Ni4(OH)4
4+
+ 4OH- → 4Ni(OH)2↓
(3)
These reactions are likely to occur simultaneously
during the electro-deposition process.
Fig. 2 shows the scanning electron microscope(SEM)
images of the Ni(OH)2 films formed at 3 mA/cm2. As
the current density increased, the deposited mass and
area also increased. At higher magnification, deposited
particles are shown to have agglomerated. The average
diameter of nano-sized nickel hydroxide was 10 ~ 20 nm.
It possesses a loosely packed structure, which is
advantageous for the electrolyte ions to access the
active materials for Faradaic reactions, and for the H+ or
OH formed to migrate in time; which may contribute to
an enhancement of capacitive performance.
Fig. 3 shows the X-ray diffraction(XRD) patterns of
as-deposited nickel hydroxide before and after a cyclic
voltammetry(CV) test. The diffraction pattern of as-
Fig. 2. Scanning electron microscope(SEM) images of Ni(OH)2
electrodes prepared at a current density of (a) 3 mA/cm2, (b) high
magnification of (a).
Fig. 3. X-Ray Diffraction(XRD) patterns of (a) before CV test, (b)
after CV test of Ni(OH)2 .
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S.-J. Kim, M.-K. Hong, B.C. Kim, J.-K. Chung, G.G. Wallace and S.-Y. Park
Fig. 4. Cyclic voltammograms(CV) of the Ni(OH)2 electrodes
prepared at a current density of 3 mA/cm2.
Fig. 5. Cyclic voltammograms(CV) of the Ni(OH)2 electrodes
prepared with different current densities.
deposited nickel hydroxide can be indexed to the
diffraction data of the Ni(OH)2 • 0.75H2O (JCPDS No.
38-0715, rhombohedral) (Fig. 3 (a)). It can be assigned
as the typical reflection of α-type Ni(OH)2. However,
after several cycles, γ-phase (JCPDS No. 84-1459,
orthorhombic) and β-phase (JCPDS No. 74-2075,
hexagonal) appeared (Fig.3 (b)). This implies that the
surface state of the electrode would not return to its
original state under the experimental conditions. It
follows that the surface state of the electrode will
hardly be restored to its original state as the cycle
number increases. Also the X-ray diffraction(XRD)
results shows that the crystallization of the γ-phase is
induced by oxidation of Ni(OH)2 to NiOOH for Ni.
Cyclic voltammetry(CV) is considered to be a suitable
tool to indicate the capacitive behavior of any material.
There is a pair of redox peaks, as a result of the Faradaic
reaction of the Ni(OH)2. For the Ni(OH)2 electrode
material, it is well known that the surface Faradaic
reactions will proceed according to Equation (4)[27]:
the second cyclic voltammetry(CV) curves was to be
1233 Fg-1. It was observed that the anodic peak potential
shifts to a low potential with increasing cycles. At the
same time, capacity was reduced as a whole. According
to Ref. [30], the α-type material is first oxidized into
γ(III) as similar inter-sheet distances characterize both
α(II) and γ(III) phases. However, after one cycle, the
discharged material shows the β(II) structural type. They
have shown that the following sequence occurs during
the several first electro-chemical cycles:
Ni(OH)2 + OH– ↔ NiOOH + H2O + e–
(4)
The anodic peak is due to the oxidation of Ni(OH)2
to NiOOH, and the cathodic peak is for the reverse
process. One quasi-reversible electron transfer process is
visible in the cyclic voltammetry(CV) curve, indicating
that the measured capacitance is mainly based on a
redox mechanism [28].
Cyclic voltammetry(CV) curves at a scan rate of
2 mVs1 for the nickel hydroxide electrode deposited at
a 3 mA/cm2 are presented in Fig. 4. One quasi-reversible
electron transfer process was observed in the cyclic
voltammetry(CV) curves, indicating that the measured
capacitance is mainly based on a redox mechanism.
The anodic peak is due to the oxidation of Ni(OH)2 to
NiOOH, and the cathodic peak is for the reverse
process [29]. Cyclic voltammetry(CV) measurements
show that the specific capacitance calculated from the
first cyclic voltammetry(CV) curves was 2168 Fg-1 and
α(II) / γ(III) → β(II)ex α / γ(III) → β(II) / β(III)
(5)
Therefore, as seen in Fig. 5, it is clear that the reduction
of the anodic peak potential may be related to a phase
conversion with increasing electrochemical cycles.
Cyclic voltammetry(CV) for the Ni(OH)2 electrodes in
1 M KOH electrolyte, at a scan rate of 10 mVs1, are
presented in Fig. 5. The shape of the curves shows that
the capacitance characteristic was distinct from that of
the electric double layer capacitor, which would produce
a cyclic voltammetry(CV) curve that is usually close to
an ideal rectangular shape. By overlapping the various
cyclic voltammetry(CV) curves, it can be seen that the
films at a 3 mA/cm2 current density exhibit a cyclic
voltammetry(CV) curve that encloses a greater area and
thus these films have larger capacitance values. Gupta et
al.(31) successfully synthesized α-Co(OH)2 by potentiostatically depositing it onto a stainless-steel electrode from
a 0.1 M Co(NO3)2 electrolyte at -1.0 V vs. Ag/AgCl. The
specific capacitance of 860 F g-1 was obtained for a
0.8 mg/cm2 mass. Moreover, the deposited α-Co(OH)2
showed very stable specific capacitance values even for a
large deposited mass[31]. However, our results exhibit a
drastic decrease at a high deposited mass.
Fig. 6 shows the dependence of specific capacitance
on the deposited mass and current density in the
Ni(OH)2 films. The specific capacitance increases and
peaks at 1233 F g-1, when the current density increases
and peaks at 3 mA/cm2, and then decreases with
Electrochemical properties of nickel hydroxide films deposited galvanostatically
691
Fig. 8. The complex plane impedance plots of unit cells of the
Ni(OH)2 films prepared at different current densities.
Fig. 6. (a) Plot of specific capacitance and deposited mass for
Ni(OH)2 films as a function of the current density. (b) Specific
capacitance as a function of scan rate; for Ni(OH)2 films
galvanostatically deposited at different current densities.
increasing current density. The deposited weight increased
with increasing current density for the electro-deposition
of Ni(OH)2 films. At a current density of 5 mA/cm2, the
weight increased to 0.6 mg while the specific capacitance
decreased to 297 F g-1. In general, capacitance increases
with increasing current density, due to the deposited
weight of the films. However, capacitance per unit
weight decreased at current densities of 4 mA/cm2, and
5 mA/cm2. This could be considered to be due to the
increase in weight of the Ni(OH)2 films, and reduced
surface area due to cracking (Fig. 7) This is in keeping
with the work by Nagarajan et al. on a MnOx electrode
prepared by potentiostatic deposition that behaved as
an ideal capacitor within the window of 0-0.9 V [32];
but also found that the specific capacitance decreased
with increasing sample weight. The specific
capacitance measured at the scan rate of 20 mV s-1 was
found to be 328, 260 and 223 F g-1 for deposited
weights of 0.12, 0.18 and 0.30 mg/cm2, respectively.
Fig. 7 shows the scanning electron microscope(SEM)
images of Ni(OH)2 electrodes, formed at various current
densities, after testing by cyclic voltammetry. The
morphology of each deposit is nano-structured with
different surface morphologies. The deposited dimensions
and cracks of the Ni(OH)2 films vary depending on the
current density which can lead to different capacitive
behaviors. The nano-structured morphology of the
Ni(OH)2 films prepared at 3 mA/cm2 possessed higher
capacitance. However, the cracked morphology of the
Ni(OH)2 film prepared at 5 mA/cm2 gave a relatively
lower capacitance, as shown in Fig. 5.
The complex-plane impedance plots of unit cells of
the Ni(OH)2 films are shown in Fig. 8. The intercept at
the real impedance (Z’) axis of these plots indicates the
combined series resistance of electrodes, electrolyte,
and current collectors. The intercept for the Ni(OH)2
film (prepared at 3 mA/cm2) is about 4, whereas this
Fig. 7. Scanning electron microscope(SEM) images of Ni(OH)2 electrodes prepared at different current densities after cyclic voltammetry
tests : (a) 1 mA/cm2, (b) 3 mA/cm2, (c) 5 mA/cm2.
692
S.-J. Kim, M.-K. Hong, B.C. Kim, J.-K. Chung, G.G. Wallace and S.-Y. Park
value for the Ni(OH)2 film (prepared at 5 mA/cm2) is
over 20. The resistance of the Ni(OH)2 film (prepared
at 5 mA/cm2) was increased by the loss of adhesion of
some active materials with the current collector or
cracking during the galvanostatic deposition.
From the capacitance of the Ni(OH)2 films (prepared
at 3 mA/cm2) in 1 M KOH, it can be calculated that an
energy density of about 21.4 Wh. kg-1 and a power
density of 3 kW. kg-1 are attainable on a single cell
device. Gupta et al. reported that, for an Al-substituted
á-cobalt hydroxide, the specific energy density increased
from 11.3 to 18.7 Whkg-1 by the substitution of just 8
atomic percentage aluminum [33]. This indicates that the
Ni(OH)2 films are good and competitive candidates for
the fabrication of pseudo-capacitors.
Conclusions
We have investigated supercapacitor electrodes based
on Ni(OH)2 films that were galvanostatically deposited
onto stainless-steel substrates. The specific capacitance
was found to depend on the weight and surface
morphology of the Ni(OH)2 films. As the current density
increased, the deposited mass also increased but so did the
impedance of the deposited film, because of the cracking
of the deposited area. For the film prepared at a current
density of 3 mA/cm2, its specific capacitance was higher
than the others and its impedance was lower. This
indicates that the film manufactured at a current density of
3 mA/cm2 may be more stable than the others. Wu et al.
showed that a film deposited galvanostatically at a low
current density of 0.25/cm2 was more compact than at a
high current density; especially near the surface of the
substrate, and the film then becomes less compact further
away from the surface of the substrate electrode [34].
Thus, due to the increase in connectivity and adhesion
between particle/particle, and particle/substrate, the
highest specific capacitance (1233 F g-1) was obtained
from the film prepared at a current density of 3 mA/cm2.
Acknowledgement
This research was financially supported by the Ministry
of Education, Science Technology (MEST), Gangwon
Province, Gangneung City, Gangneung Science Industry
Foundation (GSIF) as the R & D Project for Gangneung
science park promoting program. Partially funding
from New South Wales Government and the Australian
Research Council is also gratefully acknowledged.
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