Electrochemically deposited nanowhiskers of nickel oxide as a high-power pseudocapacitive electrode

Article abstract of DOI:10.1063/1.1814816

High performance pseudocapacitive characteristics of potentiodynamically deposited NiO are reported here. Three-dimensional nanowhiskers of NiO were prepared by potentiodynamically depositing nickel hydroxide on stainless steel (SS) and heating the hydroxide in air at 300°C for 3 h. The scan rate used for the deposition was 200 mV s^-1 and the electrolyte solution used was 100 mM NiCl₂·6H₂O at a pH 8.0. The NiO-covered SS electrodes were characterized by cyclic voltammetry (CV) in 1 M KOH electrolyte for the redox supercapacitor application. A maximum specific capacitance (SC) of 138 F g^-1 was obtained from the CV at a scan rate of 100 mV s^-1. A SC value of 82 F g^-1 was obtained even at the very high CV scan rate of 500 mV s^-1, demonstrating the high-power characteristics of NiO. Long cycle-life of the NiO nanowhiskers was also demonstrated. High specific capacitance, high power, high stability, and low cost of the electrode materials are favorable factors for commercial applications.

Full text of DOI:10.1063/1.1814816

Electrochemically deposited nanowhiskers of nickel oxide as a high-power
pseudocapacitive electrode
K. Rajendra Prasad and Norio Miura
Citation: Applied Physics Letters 85, 4199 (2004); doi: 10.1063/1.1814816
View online: http://dx.doi.org/10.1063/1.1814816
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APPLIED PHYSICS LETTERS
VOLUME 85, NUMBER 18
1 NOVEMBER 2004
Electrochemically deposited nanowhiskers of nickel oxide as a high-power
pseudocapacitive electrode
K. Rajendra Prasada) and Norio Miurab)
Art, Science and Technology Center for Cooperative Research, Kyushu University, Kasuga-shi,
Fukuoka 816-8580, Japan
(Received 9 April 2004; accepted 7 September 2004)
High performance pseudocapacitive characteristics of potentiodynamically deposited NiO are
reported here. Three-dimensional nanowhiskers of NiO were prepared by potentiodynamically
depositing nickel hydroxide on stainless steel (SS) and heating the hydroxide in air at 300 °C for
3 h. The scan rate used for the deposition was 200 mV s−1 and the electrolyte solution used was
100 mM NiCl2·6H2O at a pH 8.0. The NiO-covered SS electrodes were characterized by cyclic
voltammetry (CV) in 1 M KOH electrolyte for the redox supercapacitor application. A maximum
specific capacitance (SC) of 138 F g−1 was obtained from the CV at a scan rate of 100 mV s−1. A
SC value of 82 F g−1 was obtained even at the very high CV scan rate of 500 mV s−1, demonstrating
the high-power characteristics of NiO. Long cycle-life of the NiO nanowhiskers was also
demonstrated. High specific capacitance, high power, high stability, and low cost of the electrode
materials are favorable factors for commercial applications. © 2004 American Institute of Physics.
[DOI: 10.1063/1.1814816]
The development of emission-free electrical vehicles
(EVs), which can help to alleviate environmental concerns
arising from the generation of greenhouse gases and NOx by
vehicles burning carbonaceous fuels, has created the need for
a reliable secondary source to provide peak electrical power.
The idea of using an extra battery is rejected, since modern
batteries are expensive and incapable of providing high
power without performance degradation. This fact has led to
extensive research in the area of electrochemical capacitors
(ECCs)1,2 and the development of EVs being powered by a
combination of a rechargeable battery and an ECC. ECCs
can be rapidly and repeatedly cycled through their charge/
discharge processes and hence they operate at high power
densities. Two basic types of electrochemical capacitors can
be realized using different charge-storage mechanisms:1,2 (i)
electrical double-layer capacitors, which utilize the capaci-
tance arising from charge separation at an electrode/
electrolyte interface, and (ii) redox supercapacitors, which
utilize the charge-transfer pseudocapacitance arising from re-
versible Faradaic reactions occurring at the electrode surface.
The electrodes of redox supercapacitors consist of elec-
troactive materials with several oxidation states. These types
of capacitors have been under extensive investigation in re-
cent years due to their high-capacitance and high-energy
characteristics. Since the pseudocapacitance comes from the
reversible redox transitions of the electroactive materials,
transition metal oxides3–9 and conducting polymers10–12 with
various oxidation states are considered to be promising ma-
terials for redox supercapacitor applications. Recently, a hy-
drous form of RuO2 in aqueous H2SO4 was found to possess
very high specific capacitance ͑720 F g−1͒, which is associ-
ated with various redox reactions. Although RuO2 provides
high specific capacitance (SC), it has the disadvantages of
high cost and toxicity. NiO has been considered to be a
promising material for ECCs, as well as for other applica-
tions such as fuel cells.13–17 Liu and Anderson13 have re-
ported an ECC using nickel oxide prepared by the sol-gel
method. A SC value of 65 F g−1 was reported. However, the
sol-gel method is very tedious and is difficult to control. A
less expensive, more controllable electrochemical method of
preparing porous nickel oxide capacitors was reported by
Srinivasan and Weidner.14 A SC value of 60 F g−1 was re-
ported from CV at a scan rate of 20 mV s−1.
In the present study, NiO, prepared by potentiodynamic
deposition was studied for the electrochemical redox super-
capacitor application. The deposition was carried out on an
inexpensive stainless steel (SS) substrate. Before deposition,
a 1ϫ10 cm SS foil (grade 304; thickness, 0.2 mm) was pol-
ished with emery paper to a rough finish, washed free of
emery particles, and then air-dried. The area of SS used for
the potentiodynamic deposition was 1 cm2. The electro-
chemical cell used in this study was a four-electrode cell,
having provisions for a working electrode, a reference elec-
trode, and two auxiliary electrodes. The working electrode
(SS or NiO/SS) was placed symmetrically between the two
Pt auxiliary electrodes. An Ag/AgCl reference electrode
(with saturated KCl as the filling solution) was used, and all
potentials in the present study are reported versus this elec-
trode. First, nickel hydroxide was potentiodynamically de-
posited by cycling the SS electrode between 0.4 and 1.0 V
versus Ag/AgCl in an electrolyte solution of 100 mM
NiCl2·6H2O at pH 8.0. The scan rate used for deposition
was 200 mV s−1. After deposition, the electrode was first
rinsed with pure water and then dipped into a beaker con-
taining pure water that was mechanically stirred for 5 min.
Subsequent to cleaning, the Ni͑OH͒2/SS electrode was
heated in air at 300 °C for 3 h to form NiO. The microstruc-
ture of the samples was observed by means of a JEOL scan-
ning electron microscope model JSM-6340F. Cyclic voltam-
metry was recorded by the use of a Hokuto Denko
potentiostat/galvanostat model HSV-100.
a)Also at: Japan Science and Technology Agency.
b)Electronic mail: miura@astec.kyushu-u.ac.jp
Figure 1 shows the SEM image of NiO prepared in the
present study. Under high magnification, the structure is
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0003-6951/2004/85(18)/4199/3/$22.00 4199 © 2004 American Institute of Physics
130.18.123.11 On: Sun, 21 Dec 2014 13:21:38

4200
Appl. Phys. Lett., Vol. 85, No. 18, 1 November 2004
K. R. Prasad and N. Miura
FIG. 1. SEM image of NiO prepared in the present study by a potentiody-
namic method at a scan rate of 200 mV s−1 at room temperature.
composed of nanowhiskers or small nanowires of NiO with
diameters in the range of 7–8 nm and lengths on the order of
50–60 nm. Since the successful synthesis of semiconducting
oxide nanobelts,18–20 research in the area of oxide nanostruc-
tures has attracted considerable attention. Various synthetic
methods have been reported in the literature.21 The morphol-
ogy of the deposit clearly shows a highly nanostructured and
three-dimensional network. These types of highly amorphous
and highly porous metal oxides can be used as active elec-
trode materials for redox supercapacitors. To confirm that the
nanowhiskers in Fig. 1 are from NiO, a SEM image was also
recorded with a SS electrode cycled in a solution of pH 8.0
(without Ni-chloride) and heated at 300 °C for 3 h. The im-
age has not shown any structure, which indicates the forma-
tion of nanowhiskers of NiO on the surface of SS by the
present method.
Cyclic voltammetry (CV) is considered to be a suitable
tool to indicate the capacitive behavior of any material. A
large magnitude of current and a rectangular-type of voltam-
mogram, symmetric in anodic and cathodic directions, are
the indications of ideal capacitive behavior for any material.
The NiO-deposited SS electrodes were characterized by CV
in 1 M KOH electrolyte for the application of electrochemi-
cal redox supercapacitors. Figure 2 shows the typical CVs of
NiO/SS electrodes in 1 M KOH at various scan rates. All of
the curves were rectangular in shape and exhibited mirror-
image characteristics. Cyclic voltammograms were also re-
corded in 1 M KOH with an SS electrode cycled in a solu-
tion of pH 8.0 (without Ni-chloride) and heated at 300 °C.
These CVs indicated no capacitive behavior, with negligible
current values compared to those shown in Fig. 2. A capaci-
tor voltage of 0.5 V was reported in the literature14 for NiO.
In the present study, however, a capacitor voltage of 0.7 V
FIG. 3. Variation of (a) specific capacitance and (b) cyclic voltammetric
current density of NiO/SS electrode as a function of scan rate of cyclic
voltammograms. NiO was deposited onto SS by a potentiodynamic method
at a scan rate of 200 mV s−1 at room temperature.
was observed. SC values were calculated using the current,
the CV scan rate, and the weight of the material. Figure 3
shows the variation of SC and the CV current density with
the CV scan rate. It is noteworthy from Fig. 3(a) that a maxi-
mum SC of 138 F g−1 was obtained at a CV scan rate of
100 mV s−1. This SC value is more than twice that of the
60 F g−1 reported13,14 for NiO at the low scan rate of
25 mV s−1. It is even more interesting to note that a SC value
of 84 F g−1 was obtained at the very high CV scan rate of
500 mV s−1. This value at such a high CV scan rate indicates
that NiO can be a promising material for redox supercapaci-
tors used in automobile applications. A linear variation of the
voltammetric current with the CV scan rate [Fig. 3(b)] indi-
cates the high-power properties of the NiO prepared in the
present study.
A large decrease in the SC value (more than 70%) was
reported5 for some other low-cost metal oxides when the CV
scan rate was increased to just 100 mV s−1. However, in the
present study, a 40% decrease in SC was observed by in-
creasing the CV scan rate from 100 to 500 mV s−1 and a
decrease of just 15% was observed between the CV scan
rates of 200 and 500 mV s−1. This relatively small decrease
in SC with increasing CV scan rate also indicates the high
power characteristics of NiO. Moreover, the present method
of synthesis and the materials used are highly advantageous
from a commercial and economic point of view. The poten-
tiodynamic deposition at a high scan rate is an important
factor for the formation of microporous and nanostructured
NiO. As a result, there was an increased electrolyte access
through the micropores of the NiO deposit and hence an
increased electrochemical activity of the material.
A cycle-life test of NiO was performed by subjecting an
NiO/SS electrode to CV for a larger number of cycles. The
cycling was performed at a scan rate of 500 mV s−1 for 1000
cycles. Figure 4 shows the variation of SC as a function of
cycle number. There was a small decrease in SC in the first
100 cycles, but then it remained almost constant. A decrease
FIG. 2. Cyclic voltammograms of the NiO (deposited by a potentiodynamic
method at a scan rate of 200 mV s−1 at room temperature) deposited SS
electrodes in 1 M KOH at a scan rate of (1) 100, (2) 300, and (3)
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500 mV s−1
.
of 8% of the SC was observed after 1000 cycles. This indi-
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Appl. Phys. Lett., Vol. 85, No. 18, 1 November 2004
K. R. Prasad and N. Miura
4201
Science and Technology (CREST)” under the project “De-
velopment of advanced nanostructured materials for energy
conversion and storage.”
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rate and the electrodes were characterized for redox superca-
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The present work was supported by Japan Science and
Technology (JST) through “Core Research for Evolutional
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