Block copolymer assisted synthesis of porous α-Ni(OH)2 microflowers with high surface areas as electrochemical pseudocapacitor materials

Article abstract of DOI:10.1039/c2cc32945j

Porous α-Ni(OH)₂ microflowers are successfully synthesized via a one-step aqueous-phase reaction assisted by block copolymers under mild conditions. The electrochemical measurement demonstrates that the α-Ni(OH)₂ microflowers calcined at 200 °C are capable to deliver a specific capacity of 1551 F g^-1 in 6 M KOH solution, suggesting their high potential as a novel electrochemical pseudocapacitor.

Full text of DOI:10.1039/c2cc32945j

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COMMUNICATION
Block copolymer assisted synthesis of porous a-Ni(OH) microflowers
2
with high surface areas as electrochemical pseudocapacitor materialsw
a
b
b
c
Bishnu Prasad Bastakoti, Hou-Sheng Huang, Lin-Chi Chen,* Kevin C.-W. Wu* and
ade
Yusuke Yamauchi*
Received 25th April 2012, Accepted 18th July 2012
DOI: 10.1039/c2cc32945j
Porous a-Ni(OH) microflowers are successfully synthesized via a
Ni(OH) microstructures have attracted considerable attention,
2
2
one-step aqueous-phase reaction assisted by block copolymers
under mild conditions. The electrochemical measurement demon-
because of short diffusion path lengths for both electrolyte ions
and electrons, favouring high diffusion and easy migration of
7
electrolyte ions during the rapid charge–discharge process.
strates that the a-Ni(OH)
2
microflowers calcined at 200 8C are
À1
8
capable to deliver a specific capacity of 1551 F g in 6 M KOH
solution, suggesting their high potential as a novel electrochemical
pseudocapacitor.
Most of the previous works on Ni(OH)
Ni(OH) has two polymorphs; a- and b-Ni(OH)
contains stacked and positively-charged Ni(OH)
2
2
are on b-Ni(OH) .
2
2
. The a-phase
layers with
2
exchangeable anions and water molecules intercalated into the
interlayer space to balance the positive charges. The b-phase
structurally consists of an ordered stacking of well-oriented
Supercapacitors have attracted considerable attention in recent
years, because they offer extremely high power density, long cycle
1
life, short charging time, and minimal safety concerns. Various
9
Ni(OH) layers without any intercalated species. In general,
2
a-Ni(OH) shows better electrochemical properties than b-Ni(OH) .
materials such as transition metal oxides (e.g., RuO ) have been
2
2
2
However, the a-Ni(OH)
2
phase is hard to prepare due to its easy
extensively investigated for possible applications as super-
2
capacitors. However, those materials are not suitable for industrial
10
phase transformation into the b-Ni(OH)
Here, we demonstrate a simple, cost-effective, and solution-based
approach for the synthesis of a-Ni(OH) microflowers with the
assistance of a commercially available triblock copolymer (F127)
Fig. 1). The obtained a-Ni(OH) microflowers consist of assembled
nanosheets in random directions. The as-prepared a-Ni(OH)
2
phase.
applications due to very high cost. Considerable effort is being
devoted to developing ‘‘inexpensive’’ electrode materials.
2
2
Ni(OH) is a promising alternative electrode material due to
(
2
its low cost, high theoretical specific capacitance, well-defined
electrochemical redox activity, easy availability and high stability
3
in alkaline electrolytes. To date, numerous works have been
2
microflowers were then calcined at different temperatures to tune
the crystal phases, crystallinity, and surface areas (Fig. S1, ESIw),
which would affect their electrochemical performance.
developed to synthesize Ni(OH) with different morphologies,
2
because the electrochemical performances of Ni(OH) are
2
4
directly affected by its morphology and particle size. Tao et al.
In this experimental work, a-Ni(OH)2 microflowers were
3 2
prepared by mixing Ni(NO ) , F127, and urea. F127 works as a sort
synthesized Ni(OH) microtubes by a template-free wet-chemical
2
5
method. Microrods, microplates, microspheres of Ni(OH)
of structure directing agent, whereas the urea is a precipitating
agent. The slow hydrolysis of urea upon heating causes precipitation
2
have
been also synthesized and their electrochemical properties have
6
been studied. Among the several morphologies, flower-shaped
of Ni(OH)
ammonia can form a complex [Ni(NH
coordinated NH molecules interact with polyethylene oxide chains
PEO) of F127 units through hydrogen bonding, which is a
2
. The slow hydrolysis of urea releases ammonia. The
2
+
2+
3
)
x
] with Ni . The
3
a
World Premier International (WPI) Research Center for Materials
(
Nanoarchitectonics (MANA), National Institute for Materials
Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.
E-mail: Yamauchi.Yusuke@nims.go.jp
11
driving force for the interaction of metal ions with the polymer.
b
Department of Bio-Industrial Mechatronics Engineering,
National Taiwan University, No. 1, Sec. 4, Roosevelt Road,
Taipei 10617, Taiwan. E-mail: chenlinchi@ntu.edu.tw
Department of Chemical Engineering, National Taiwan University,
c
No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan.
E-mail: kevinwu@ntu.edu.tw
PRESTO, Japan Science and Technology Agency (JST), Saitama,
d
Japan
Faculty of Science and Engineering, Waseda University,
e
3
-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan
w Electronic supplementary information (ESI) available: Details of the
experimental procedure, SEM images, TG-DTA curves, N adsorption
isotherms of the samples at various temperatures. See DOI: 10.1039/
c2cc32945j
2
Fig. 1 (a) SEM image and (b) TEM image of porous a-Ni(OH)
microflowers calcined at 200 1C.
2
9
150 Chem. Commun., 2012, 48, 9150–9152
This journal is c The Royal Society of Chemistry 2012

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Yuan et al. synthesized Ni(OH)
2
microparticles via amino-based
(fcc) NiO phase (JCPDS No.47-1049). It has been generally
2+
complex metal ions. The combination of NH
3
with Ni ions
decreases free Ni ions concentration, which ultimately reduces
2
known that the decomposition of a-Ni(OH) into NiO com-
2+
15
pletes at around 300 1C, which coincided with the present
study. The thermal decomposition of Ni(OH) took place at a
12
the rate of unnecessary crystal growth.
2
1
6
The concentration of the F127 surfactant is very important to
tune the morphology of microstructures. When the concentration
lower temperature than 300 1C, but the transformation of
Ni(OH) into NiO was not complete at such a low temperature.
2
À1
of the polymer was decreased up to 0.1 g L , several irregular
2
For example, for the a-Ni(OH) microflowers calcined at 200 1C,
structures appeared (Fig. S2a, ESIw). In this case, the polymer
a very small (111) peak derived from the NiO phase was also
observed. With further increase of the calcination temperature
(from 300 to 600 1C), the FWHM (full width at half maximum)
of each peak decreased, indicating that the crystal grains of NiO
became larger. From SEM observation (Fig. S1, ESIw), it was
proved that the original flower morphology of the as-prepared
material was retained till the calcined temperature reaches
400 1C. When the calcination temperature was further increased,
the flower morphology gradually disappeared, indicating that the
NiO nanosheets were not stable at high temperatures.
amount was not enough to direct the flower-shaped a-Ni(OH)
2
structure. In contrast, when the concentration of the polymer
À1
became very high (30 g L ), the flower-like structure was
observed. But, several particles were aggregated with each other
(
Fig. S2b, ESIw). At high concentration, the polymer forms
13
multimicellar aggregates due to intermicellar association, which
might hinder good dispersion of the particles. Thus, the F127
concentration is a critical factor to achieve the well-developed
flower-shaped structures. Hereafter, we fix the F127 concentration
À1
at 5 g L throughout the study.
2
The phase transformation of a-Ni(OH) to NiO was also
Fig. 2a shows wide-angle XRD profiles of as-prepared and
calcined samples. The peaks of the as-prepared sample were
examined by TG-DTA analysis. The initial slope on the TG
curves (Fig. S3, ESIw) was desorption of physically-absorbed and/
or structurally-bonded water molecules. The endothermic peak
centered at around 205 1C is due to the combustion of the block
copolymer. Whereas the endothermic peak around 300 1C can be
1
4
2
assignable to a-Ni(OH) (JCPDS No:38-0715). The sharp
diffraction peaks confirmed the high crystallinity of the
as-prepared material. The average crystallite size estimated by
the Scherrer equation from the (003) peak was around 4 nm. No
peaks of any other impurities were detected, suggesting high
2
assigned to the phase transformation of a-Ni(OH) into NiO by
loss of water produced by dehydroxylation of hydroxide sheets,
which was also confirmed by XRD measurement (Fig. 2a).
The BET surface areas of all the samples were characterized
purity of the turbostratic phase of a-Ni(OH) . In order to tune
2
the crystalline phases and crystallinities, the as-prepared sample
was calcined at different temperatures. The crystalline phase of
by N adsorption–desorption isotherms. The surface areas of
2
a-Ni(OH)
00 1C. Calcination at higher temperature ( Z 300 1C) induced
transformation from a-Ni(OH) to NiO, as confirmed by
XRD profiles in Fig. 2a. The peaks derived from a-Ni(OH)
2
was retained until the temperature was increased to
all the samples calcined at different temperatures are shown in
Fig. S1 and S4 (ESIw). When the calcination temperatures were
higher than 300 1C, the crystal grains of the nickel oxides grew
(as observed in Fig. 2a) and the pore networks shrunk, thereby
seriously decreasing the surface areas (inset of Fig. S1, ESIw). The
2
2
2
completely disappeared and new peaks (111, 200, 220, and 311)
appeared, which are all consistent with the face-centered cubic
high magnification SEM and TEM images of the a-Ni(OH)
2
microflowers calcined at 200 1C (with the highest surface area
among other a-Ni(OH) samples) are shown in Fig. 1. The thin
2
Ni(OH)2 nanosheets were strongly interconnected to form
flower-like structure. Raman spectroscopy was also performed
to obtain further information on the sample. The characteristics
À1
À1
peaks at 452 cm and 1040 cm in Fig. 2b confirmed the
1
7
2
a-Ni(OH) formation.
We evaluated the electrochemical performance of the
synthesized materials as pseudocapacitors (Fig. 3). Typical
2
CV curves of a-Ni(OH) microflowers (calcined at 200 1C)
consisted of a pair of strong redox peaks, as shown in Fig. 3a.
2+
One peak is anodic during the oxidation reaction of Ni
to
3+
Ni , and the other is cathodic during the reverse process,
indicating that the capacitance characteristics are mainly supplied
À
by the faradaic reaction a-Ni(OH) + OH 2 g-NiOOH +
2
À 18
H O + e .
2
Such a peak potential difference can be regarded as quasi-
reversible. The shape of the CV curves was basically constant even
at high scan rates (not shown), which resulted from the improved
19
mass transportation within the electrode material.
For example, our a-Ni(OH)
2
microflowers (calcined at 200 1C)
were enabled to deliver a specific capacity of 469 F g and
À1
Fig. 2 (a) XRD patterns of the samples calcined at different tempera-
À1 À1 À1
2
29 F g at the scan rate of 5 mV s and 25 mV s in 1 M
tures. In the reference, blue-coloured peaks indicate the a-Ni(OH)
phase, while red-coloured peaks indicate the NiO phase. (b) Raman
spectra of porous a-Ni(OH) microflowers calcined at 200 1C.
2
KOH solution, respectively. The capacitance decreased while
the scan rate increased, which can be attributed to more
2
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 9150–9152 9151

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accordingly decreased the surface area (inset of Fig. S1, ESIw),
thereby seriously decreasing the capacitance (Fig. 3c).
2
5
In conclusion, we successfully synthesized well-defined porous
a-Ni(OH) microflowers by using commercial block copolymers
2
(F127) followed by controlled heat treatment. The finding is very
encouraging in view of easy synthetic routes and our material would
be a very promising electrode material for future pseudocapacitors.
Notes and references
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(
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Fig. 3 (a) CV curve of porous a-Ni(OH)
2
microflowers calcined at
À1
200 1C (6 M KOH). Scan rate is 5 mV s . (b) Capacitance retention
4
(a) Y. Li, B. Tan and Y. Wu, Chem. Mater., 2008, 20, 567;
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2
%) versus the cycle number of porous a-Ni(OH) microflowers
calcined at 200 1C. (c) Summary of specific capacitance values of the
samples calcined at different temperatures. Two different electrolytes
1
09, 1125; (c) B. Li, M. Ai and Z. Xu, Chem. Commun., 2010, 46, 6267.
5 F. Tao, M. Guan, Y. Zhou, L. Zhang, Z. Xu and J. Chen, Cryst.
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(1 M KOH and 6 M KOH) are compared.
6
7
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difficult penetration and diffusion of the electrolyte (i.e., some parts
20
of the electrode surface become inaccessible) at high scan rates.
8
We also examined the capacitance in 6 M KOH. Commonly, 6 M
KOH has been selected as the electrolyte, because the high ion
20, 479; (c) S. Zhang and H. C. Zeng, Chem. Mater., 2009, 21, 871.
2
1
concentration can enhance the conductivity of the electrolyte. The
capacitance values for all the samples are shown in Fig. 3c. The
9
(a) C. Faure, C. Delmas and P. Willmann, J. Power Sources, 1991,
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À1
006, 17, 4278.
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maximum capacitance value was observed to be 1551 F g for
1
the a-Ni(OH)
2
sample (i.e., calcination temp.: 200 1C, scan rate:
mV s , electrolyte: 6 M KOH), as calculated from the CV curve
shown in Fig. 3a. The high capacitance value was supposed to be
(
À1
5
1
12 C. Yuan, X. Zhang, L. Su, B. Gao and Laifa Shen, J. Mater.
Chem., 2009, 19, 5772.
24
resulted from the structurally bonded water on the sample.
1
3 (a) S. Guragain, B. P. Bastakoti, S. Yusa and K. Nakashima,
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copolymers, Oxford University Press, New York, USA, 1998.
Comparison of our data with various published data on
NiO/Ni(OH) -based supercapacitors is summarized in Table S1
2
(
ESIw). Long-term cycling stability of electrode material is a
critical requirement for practical application (Fig. 3b). Our
00 1C-calcined Ni(OH) micro-flowers showed excellent cycle
14 (a) S. Shang, K. Xue, D. Chen and X. Jiao, CrystEngComm, 2011,
3, 5094; (b) S. Yang, X. Wu, C. Chen, H. Dong, W. Hu and
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1
2
2
stability. The capacitance retention slightly increased to 7%
1
13, 14440; (b) N. D. Hoa and S. A. El-Safty, Chem.–Eur. J.,
after the first 200 cycles, instead of decreasing as in the most
3
2
2011, 17, 12896; (c) H. W. Ha, T. W. Kim, J. H. Choy and
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16 J. W. Lang, L. B. Kong, W. J. Wu, Y. C. Luo and L. Kang, Chem.
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cycling stability test in the previous literature. The capacitance
loss after 1000 cycles was only 10%.
When the as-prepared samples were calcined over 300 1C,
their capacitances greatly decreased, as shown in Fig. 3c. The
XRD results in Fig. 2a indicated that the 300 1C-calcined sample
1
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(
was no longer in the a-Ni(OH) phase and it transformed into the
NiO phase. In addition, the CV curve of the NiO phase was
18 S. Yang, X. Wu, C. Chen, H. Dong, W. Hu and X. Wang, Chem.
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2
1
9 J. Yan, W. Sun, T. Wei, Q. Zhang, Z. Fan and F. Wei, J. Mater.
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very different from that of a-Ni(OH) . For example, unlike the
2
a-Ni(OH)2 phase exhibiting a couple of redox peaks in its
CV curve, the NiO phase showed almost no redox peaks in the
CV curve. This is because the NiO phase was inert to the
20 R. R. Salunkhe, K. Jang, S. Lee and H. Ahn, RSC Adv., 2012, 2, 3190.
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1
6,22
electrolyte and its faradaic reaction was unlikely to occur.
The different electrochemical CV behavior indicated that the
capacitance characteristics are mainly supplied by the faradaic
23 (a) C. C. Hu, K. H. Chang and T. Y. Hsu, J. Electrochem. Soc.,
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2
reaction of a-Ni(OH) and by the electrical double layer of NiO.
After the NiO phase was formed, further increase of the calcination
2
4 V. Ganesh and V. Lakshminarayanan, Electrochim. Acta, 2004,
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temperature caused the increase in the crystal sizes (Fig. 2a) and
9
152 Chem. Commun., 2012, 48, 9150–9152
This journal is c The Royal Society of Chemistry 2012