Nanosheets based mesoporous NiO microspherical structures via facile and template-free method for high performance supercapacitors

Article abstract of DOI:10.1016/j.electacta.2011.02.116

A novel template-free method to synthesize NiO microstructures is reported and the electrochemical properties of the fabricated microstructures are evaluated. Applying hexamethylenetetramine hydrolysis under a refluxing condition for 2 h, α-Ni(OH)₂ microstructures were synthesized, subsequent calcination at 300 °C yielded NiO microstructures that retained the morphologies of their predecessors. The fabricated NiO microstructures had high specific surface area, large pore volume, and narrow pore size distribution making them ideal candidates for supercapacitor applications. The NiO microstructures have the high specific capacitance and displayed a good retention for more than 1000 cycles in a cycling test. The results suggest that NiO microstructures are a promising supercapacitor electrode material.

Full text of DOI:10.1016/j.electacta.2011.02.116

Electrochimica Acta 56 (2011) 4849–4857
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Nanosheets based mesoporous NiO microspherical structures via facile and
template-free method for high performance supercapacitors
a
a
a
b
a,∗
Jeong Woo Lee , Taebin Ahn , Jong Hun Kim , Jang Myoun Ko , Jong-Duk Kim
a
Department of Chemical and Biomolecular Engineering, Center for Energy and Environment Engineering, Korea Advanced Institute of Science and Technology (KAIST), 373-1,
Guseong-dong, Yuseong-gu, Daejeon, P. O. 305-701, Republic of Korea
Department of Applied Chemistry and Biotechnology, Hanbat National University, San 16-1, Dukmyung-dong, Yuseong-gu, Daejeon, P. O. 305-719, Republic of Korea
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel template-free method to synthesize NiO microstructures is reported and the electrochemical
properties of the fabricated microstructures are evaluated. Applying hexamethylenetetramine hydrolysis
under a refluxing condition for 2 h, ␣-Ni(OH)2 microstructures were synthesized, subsequent calcination
Received 1 November 2010
Received in revised form 7 January 2011
Accepted 23 February 2011
Available online 6 March 2011
◦
at 300 C yielded NiO microstructures that retained the morphologies of their predecessors. The fabricated
NiO microstructures had high specific surface area, large pore volume, and narrow pore size distribution
making them ideal candidates for supercapacitor applications. The NiO microstructures have the high
specific capacitance and displayed a good retention for more than 1000 cycles in a cycling test. The results
suggest that NiO microstructures are a promising supercapacitor electrode material.
Keywords:
NiO
Supercapacitor
©
2011 Elsevier Ltd. All rights reserved.
2584 Fg 1 within 0.5 V [20], suggesting its potential application as
an electrochemical material. In general, the specific capacitance of
pseudocapacitors arises from the interface of the electrode. There-
fore, it is desirable to produce electrode materials with high specific
surface area as well as suitable pore size distribution. High specific
surface area induces the formation of many electrical double layers
and employs more electroactive species in pseudocapacitive reac-
tions. Appropriate narrow pore size distribution (2–5 nm) helps the
mass transfer of electrolytes within the pores for fast redox reac-
tions and double layer charging/discharging reaction [11,12,14].
Thus, most studies have aimed at enhancing the electrochemical
properties of NiO.
To date, various routes to fabricate NiO electrodes have been
reported including film-type using electrodeposition methods
[21–23] and chemical bath deposition [24]. Powder-type elec-
trodes have also been produced, using a soft or hard template,
[12–14] hydrothermal processes [12,25], and precipitation meth-
ods [12–14,26–31]. NiO film can be obtained by tuning deposition
parameters, but the product mass is quite small, thus limiting large-
scale production of electrode materials. In the case of the template
method, additional cost is incurred and complex procedures are
needed to remove the template. The hydrothermal method is effec-
tive in terms of controlling the morphology and size of products.
However, the hydrothermal process in an autoclave limits fine con-
trol over reactions and large-scale production.
−
1
. Introduction
Supercapacitors, also known as electrochemical capacitors or
ultracapacitors have garnered considerable attention over the past
decade due to their high power density and longer cycle ability
compared to secondary batteries and conventional dielectric capac-
itors [1,2]. Many studies have accordingly been carried out during
in the past few years in effort to improve the performance of elec-
trochemical capacitor electrodes and to develop replacements for
batteries. Supercapacitors are generally classified into two types,
electrical double layer capacitors (EDLCs) and pseudocapacitors,
depending upon the charge storage mechanism. EDLCs function on
the basis of charge the separation at electrode and an electrolyte
interface. The most commonly used materials for EDLCs are acti-
vated carbons [3,4]. Pseudocapacitors, meanwhile, are based on a
fast, reversible redox process occurring at or near the active elec-
trode surface [5,6]. Among the pseudocapacitors, various types of
ruthenium oxides with high capacitance have been extensively
studied for active electrode applications [7–10]. However, they are
limited to commercial products due to their costliness and toxic
nature. Hence, many efforts have been exerted to replace RuO2 by
other transition metal oxide such as NiO [11–15], Co O [16], MnO
3
4
2
[
17], SnO2 [18], Fe O [19], etc.
3 4
Among transition metal oxides, NiO has been widely studied
due to its high surface area, high specific capacitance, and low
cost. Furthermore, the theoretical specific capacitance of NiO is
Precipitation methods are simple as well as effective to obtain
−
desired products. Depending on the method of supplying of OH ,
the precipitation method generally can be classified into two-types:
(i) coprecipitation methods, which generally control pH of a solu-
tion containing precursors, where NH4OH [15,26–28], KOH [29],
∗ Corresponding author. Tel.: +82 042 350 3921; fax: +82 042 350 3910.
E-mail address: jdkim@kaist.ac.kr (J.-D. Kim).
0
013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.02.116

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J.W. Lee et al. / Electrochimica Acta 56 (2011) 4849–4857
NaOH [30], and LiOH [31] are used as alkaline sources to produce
−
the OH group; (ii) homogeneous precipitation methods, where the
solution is rendered to have a basic condition, preventing sudden
change of the pH by slow and progressive hydrolysis, where urea
[
13,14,25] and hexamethylenetetramine (HMT) are widely used as
hydrolysis agents.
In this article, we report on the template-free synthesis of NiO
spherical microstructures by HMT hydrolysis employing a reflux-
ing condition for 2 h without any template agents or seed. To
the best of our knowledge, this is the first report of powder-type
NiO electrodes synthesized by HMT hydrolysis. The fabricated NiO
microstructures have high specific surface area, large pore vol-
ume area, and narrow pore size distribution, constituting ideal
supercapacitive properties, thus suggesting that HMT is an effec-
tive alkaline source to fabricate NiO electrodes. From the results of
electrochemical experiments, the NiO microstructures show spe-
−1
−1
cific capacitance of 403 Fg at 20 mV s and good retention after
000 cycles in a cycling test. Based on this high specific capacitance
1
and electrochemical stability, they hold promise for application as
supercapacitor electrode.
Fig. 1. XRD pattern of NiO microstructures.
solvent. The obtained paste was incorporated into nickel foam
2
. Experimental
(1 cm × 1 cm). The mass of active materials in working electrode
was 0.8 mg. Prepared working electrodes were dried at room tem-
perature for 1 day, and then under vacuum at room temperature
overnight. Dried working electrodes were pressed, and its thickness
was 0.7 mm.
2.1. Preparation of NiO microstructures
Nickel sulfate (NiSO ·6H O) and hexamethylenetetramine
4
2
(
HMT, C H N ) were purchased from Sigma–Aldrich. All reagents
6 12 4
were used as received without any further purification. Deion-
ized water was used throughout the experiment. A mixture of
NiSO ·6H O (0.01 mol) and HMT (0.1 mol) was dissolved in 400 mL
2
.4. Electrochemical measurements
4
2
Electrochemical studies were carried out by measurement of
of water, and the solution was heated at refluxing temperature
under continuous magnetic stirring. The refluxing period was set
at 2 h. After the reaction, the solution was cooled naturally to room
temperature. The green precipitate was separated, washed with
water several times, and dried overnight at room temperature
under a vacuum condition. NiO microstructures were obtained by
cyclic voltammetry (CV) and impedance of half cells using AUTO-
LAB (ECO CHEMIE, PGSTAT 100). A beaker type three-electrode cell
equipped with a NiO microstructures current collector on nickel
foam as a working electrode, an Ag/AgCl electrode (Metrohm AG
9
trode, and a Pt plate (2 cm × 2 cm) as a counter electrode was used.
A Luggin capillary tip was kept as close as 1–2 mm to the work-
ing electrode, and was set to minimize error due to iR drop in the
electrolyte. Electrochemical impedance measurements were con-
ducted for the working electrode in a frequency range of 100 kHz to
◦
101 Herisau, 3 M KCl, 0.222 V vs. SHE at 25 C) as a reference elec-
◦
calcination of green powder in air at 300, 400, and 500 C for 3 h,
which are denoted as N300, N400, and N500, respectively.
2.2. Sample characterization
0
.01 Hz with ac perturbation of 5 mV. For all electrochemical char-
The crystal structures were analyzed by X-ray diffrac-
tion (XRD, Rigaku D/MAX–IIIC) equipped with Cu-K␣ radiation
ꢀ = 1.50405 A˚ ). To investigate the morphology of the structures,
acterizations, a test solution of 6 M KOH was used as an electrolyte
solution. The specific capacitance was calculated from the cyclic
voltammetry (CV) curves based on the following equation [32,33]:
(
field emission scanning electron microscopy (FE-SEM) images were
obtained by using a Nova 230 (FEI Company). Transmission electron
microscopy (TEM) images and selected electron diffraction (SAED)
pattern were taken on a JEM–2100F at 200 kV. A Thermogravimetry
qa + qc
C =
(1)
2
mꢁV
where qa, qc, m, and ꢁV are the sums of anodic and cathodic voltam-
metric charges on the anodic and cathodic scans, the mass of the
active material and and potential window of cyclic voltammetry,
respectively.
◦
−1
analysis (TGA) was performed at a heating rate of 10 C min in air
on a thermal analyzer (NETSCH TG 209 F3, Germany). The specific
surface area, total pore volumes, and pore size distribution were
obtained from a N2 adsorption/desorption analysis conducted at
7
7 K on a Micromeritics Tristar II 3020 system. The specific surface
3. Results and discussion
area was calculated by the Brunauer–Emmett–Teller (BET) method,
and pore size distribution was obtained from a desorption plot by
a Barrett–Joyner–Halenda (BJH) analysis.
3.1. Characterization of NiO microstructures
The XRD pattern of black powder obtained after the calcina-
tion process is shown in Fig. 1. The diffraction peaks are indexed
at 2ꢂ = 37.2 , 43.2 , 62.8 , 75.4 , and 79.4 , which correspond to the
2.3. Electrode preparation
◦
◦
◦
◦
◦
To evaluate the electrochemical properties of the NiO
(1 1 0), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) reflections of NiO structures
(JCPDS card no. 47–1049). A temperature of 300 C was sufficient
◦
microstructures, working electrodes were prepared as follows. The
prepared NiO microstructures, vapor grown carbon fiber (VGCF,
to drive to the conversion to NiO, and no peaks of impurities were
observed.
To investigate the morphologies of the obtained NiO, SEM mea-
surement was performed, and SEM images of NiO are shown in
Fig. 2. Spherical-type structures were found over the samples, and
2
−1
Showa Denko K.K., Japan, specific area: 13 m g , aspect ratio:
7) as a conducting material and polyvinylidene fluoride (PVdF) as
6
a binder, respectively, were mixed in a weight ratio of 80:10:10
to yield a paste. N-methyl-2-pyrrolidone (NMP) was used as a

J.W. Lee et al. / Electrochimica Acta 56 (2011) 4849–4857
4851
Fig. 2. SEM images of NiO microstructures at different magnifications.
appeared to be massed with each other. At a high magnification
Fig. 2d), thin and highly vented nanosheets were disorderly inter-
larger and broader with a bimodal size distribution at 5.1 and
7.9 nm due to expansion of pores. With further increase in calci-
nation temperature (500 C), the bimodal size distribution became
(
◦
connected in a disorderly arrangement. The estimated thickness of
the nanosheets was around 10–30 nm. It was speculated that these
unique structures might have high surface area, and consequently,
could provide high specific capacitance due to easy access of the
active species in the redox process at the interface of the electrode.
To estimate the surface area of NiO microstructures, BET mea-
surements were carried out, and nitrogen adsorption/desorption
isotherms of N300, N400, and N500, respectively are shown in
Fig. 3a–c. BET surface area and total pore volume of N300, N400, and
N500 are shown in Table 1. According to the IUPAC classification, all
isotherm can be classified as type IV with H4 type hysteresis behav-
ior. The type IV isotherm is obtained with mesoporous (2 nm < pore
size < 50 nm) materials. Meanwhile, the hysteresis loop is associ-
ated with the secondary process of capillary condensation, which
starts at about P/P = 0.4 and expends almost to P/P = 1, suggest-
apparent, namely, at 4–6 nm and 7–11 nm. The total pore volumes
3
−1
,
of N300, N400, and N500 were 0.633, 0.692, and 0.640 cm g
respectively. As the calcination temperature increased, the spe-
cific surface area was dramatically reduced due to collapse or
sintering small pore walls. In conclusion, the BJH pore size distri-
bution of N300 is a single modal centered at at 3.8 nm, and the
NiO microstructures has a uniform framework with both a regu-
lar mesopore size distribution and the optimal pore size range of
2–5 nm, applicable for NiO supercapacitor applications [11,12,14].
3.2. Characterization of precursor of NiO microstructures
Since most properties such as morphologies and surface area
of NiO are related to the NiO precursor, synthesis and character-
ization about the NiO precursor are necessary. Fig. 4a shows the
0
0
ing complete filling of the mesopores. This indicates that NiO
microstructures are mesoporous materials with a high fraction
of textual porosity. The measured BET specific surface area of
Table 1
2
−1
Surface properties of prepared N300, N400, and N500.
N300, N400, and N500 were 258.52, 227.13, and 125.39 m g
respectively, which is higher than those of reported NiO particles
11,13,15]. The pore size distribution of sample N300 (Fig. 3d) is
,
Sample
BET surface area
Total pore volume
(cm
2
−1
3
−1
g )
[
(m
g
)
narrow with a single modal centered at 3.8 nm. Such small pores
are presumably generated from pore opening when the physi-
cal absorbed water or intercalated water were removed between
a
N300
258.52
227.13
125.39
0.633
0.692
0.640
a
N400
a
N500
◦
a
Ni(OH)2 layers. At high temperature (400 C), pore size became
Samples were obtained by calcination temperature for 3 h.

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J.W. Lee et al. / Electrochimica Acta 56 (2011) 4849–4857
Fig. 3. Nitrogen adsorption-desorption isotherms (a)–(c) with corresponding the pore size distributions (d)–(f) of N300 (a) and (d), N400 (b) and (e), and N500 (c) and (f),
respectively.
XRD pattern of the green precipitate obtained by HMT hydroly-
sis. In a low angle range, a diffraction peak around 10.34 was
observed, which can be assigned to the (0 0 3) plane of ␣-Ni(OH)2
SEM images of obtained ␣-Ni(OH)2 are shown in Fig. 5a and b,
revealing similarity to the NiO microstructure (Fig. 2). This indicates
that the pore framework was not destroyed during the calcination
process. Fig. 5d shows a selected area electron diffraction (SAED)
◦
(
JCPDS card no. 38–0715). Two broad and asymmetric peaks were
◦
◦
observed at about 33.6 and 60.0 corresponding to non-basal
spacing, which is present in turbostratic materials. Turbostratic
disorder can be explained on the basis of the stacking order of
a brucite-like structure parallel and equidistant along the c-axis
of the hexagonal structure but randomly twisted against each
other. This turbostratic behavior has also been observed in other
pattern taken at the nanosheet of ␣-Ni(OH) . The SAED displays
2
hexagonally arranged spots as a (h k 0) reflection on the (0 0 l) plane,
which was well matched with the hexagonal ␣-Ni(OH)2 phase. It
is thought that hierarchical ␣-Ni(OH)2 consists of numerous thin
nanosheets forming a single crystalline state.
␣
-Ni(OH)2 [34]. During thermal decomposition, ␣-Ni(OH)2 has
3
.3. Growth mechanism of ˛-Ni(OH)2 microstructures
multistep and larger mass losses compared to ␤-Ni(OH)2 (gener-
ally 18%), which does not have any anions or water species [34].
TG data (Fig. 4b) shows two-step weight loss at temperatures of
around 60 C and 300 C. The first-weight loss arose from removal
of intercalated water or absorbed water, and the second-weight
loss resulted from the decomposition of intercalated anions and
dehydration of Ni(OH)2.
The mechanism underlying the formation of the hierarchically
◦
◦
porous morphology was investigated with ␣-Ni(OH)2 collected
after reaching the refluxing condition. Fig. 6 shows SEM images
of ␣-Ni(OH)2 powder: (a) 5 min and (b) 30 min, respectively. The
porous shapes are partially observed on the surface of ␣-Ni(OH)2

J.W. Lee et al. / Electrochimica Acta 56 (2011) 4849–4857
4853
Fig. 4. (a) XRD pattern and (b) TG curve of ␣-Ni(OH)2.
(
Fig. 6a), and as reaction time proceeded, more porous morpholo-
phologies on the surface. This proposed mechanism is supported
by SEM images of broken ␣-Ni(OH)2 microstructures obtained
after 2 h under a refluxing condition. (Fig. 7) Compared to the
porous surface, densely packed regions are observed in the inte-
rior. In conclusion, porous ␣-Ni(OH)2 microstructures are thought
to result from both Ostwald ripening and oriented attachment.
The proposed formation mechanism of ␣-Ni(OH)2 microstruc-
tures is illustrated in Fig. 8. Obtained ␣-Ni(OH)2 were transformed
to NiO via calcination. Morphologies of NiO microstructures are
different from reported other NiO [11,13,14,28,30]. During the cal-
cination, the hierarchical morphologies of NiO did not changed,
but maintained the high specific area and narrow pore size dis-
tribution observed by BET and BJH results. The large surface
gies are observed, as seen in Fig. 6b. On the basis of the evaluation
of the time-dependent morphologies, the formation mechanism of
porous structures could be attributed to anisotropic Ostwald ripen-
ing. At the initial stage of reaction, the precursors tend to dissolve
into the solution due to their relatively high surface energy. By
supplied energy in the system, the ions are reprecipitated onto
the high energy crystallographic facet of the particles, leading to
the growth of large particles. Simultaneously, through the attach-
ment and coalescence of particles with favorable crystallographic
planes, oriented attachments proceed, resulting in the formation
of large aggregated spherical forms. Further growth on the surface
of particles progress steadily, forming hierarchically porous mor-
Fig. 5. TEM and SEM images of ␣-Ni(OH)2: (a) and (b) SEM images ␣-Ni(OH)2 at different magnifications, (c) TEM image of nanosheet of ␣-Ni(OH)2, and (d) SAED pattern
taken from a nanosheet of ␣-Ni(OH)2.

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J.W. Lee et al. / Electrochimica Acta 56 (2011) 4849–4857
Fig. 6. SEM images of ␣-Ni(OH)2 obtained after reaching refluxing temperature for (a) 5 min and (b) 30 min.
area of hierarchical structures between active materials and elec-
trolyte, consequently resulted in the high specific capacitance (vide
post).
Thus far, in accordance with the employed route of synthesis, i.e.,
use of a template, alkaline source species, and synthetic methods,
hexagonal plate [35], nanoribbon [36], hollow-type [37], fibrous-
type [38], and flower-type [39] ␣-Ni(OH)2 have been reported. It
is thought that unique morphologies such as these fabricated ␣-
3.4. Electrochemical properties
The electrochemical properties and specific capacitance of the
NiO microstructures were determined by cyclic voltammetry (CV)
to evaluate their potential utility in supercapacitor applications.
−
1
Fig. 9 shows the CV curves of N300, N400, and N500 at 20 mV s
,
respectively. Two strong current peaks were observed, indicating
that the capacitance of NiO mainly resulted from pseudocapacitive
capacitance based on a fast redox mechanism. Two strong current
peaks correspond to a redox reaction according to the following
equation.
Ni(OH) microstructures could be synthesized with the aid of HMT.
2
HMT has been used as a hydrolysis agent in the preparation of
␣
-Ni(OH)2 [34,35]. The following reaction might occur in solution
[
40]:
NiO + OH− ↔ NiOOH + e−
(4)
C6H12N + 6H O → 6CH O + 4NH
(2)
(3)
4
2
2
3
The anodic peak at roughly 0.31 V is related to the oxidation
of NiO to NiOOH, and the cathodic peak at about 0.21 V is due to
its reverse process similar to previous reports. Therefore, the two
strong redox reaction peaks in the CV curves are responsible for
the pseudocapacitance of the NiO microstructures. As increased the
calcinations temperature, both cathodic and anodic current den-
sity under CV curves decreased. As redox reaction occurs at the
interface of electrolyte and NiO particles, high specific capacitance
can be obtained under condition of high specific surface area of
active materials and narrow pore distribution with many channels
NH + H O → NH + OH−
+
3
2
4
Thus, in our synthetic procedure, the slow and progressive
hydrolysis of only HMT lends the solution a basic condition
and induces homogeneous precipitation of porous ␣-Ni(OH)2
microstructures.. In addition, nickel sulfate was replaced with
nickel nitrate, nickel acetate, and nickel chloride, respectively, but
porous ␣-Ni(OH)2 microstructures could not be obtained. Sulfate
anions are thought to affect the formation of porous morphologies,
but further study is needed.
−
for diffusion of OH ions. Therefore, specific capacitance of N300 is
Fig. 7. SEM images of broken ␣-Ni(OH)2 at different magnifications.

J.W. Lee et al. / Electrochimica Acta 56 (2011) 4849–4857
4855
Fig. 8. Proposed growth mechanism for ␣-Ni(OH)2 microstructures.
in more positive and negative directions, due to the limitation of
the ion diffusion rate to satisfy electronic neutralization during the
redox reaction.
Since long cycle-life is in the most important criterion for a
supercapacitor, an endurance test of NiO microstructures was con-
ducted using CV 1000 times between 0.0 and 0.45 V at a nominal
−
1
scan rate of 20 mV s . The CV curves and the specific capacitances
as a function of cycle number are plotted in Fig. 11a and b. All
the CV curves almost overlapped with each other, indicating good
cycling stability. The degradation of specific capacitance was about
5
.2% after 1000 cycles. It is believed that the unique mesoporous
−
NiO microstructures allow easy access of OH to the NiO elec-
trode surface, and thereby offer high specific capacitance and less
degradation during the cycle test.
The detailed characteristics of a capacitive electrode can be
observed by electrochemical impedance spectroscopy (EIS) ana-
lyzed using a Nyquist plot, which represents the imaginary part
Fig. 9. Cyclic voltammetry curves of N300, N400, and N500.
Table 2
Calculated parameter, qa, qc, qa + qc, and specific capacitance of N300 at different
scan rate.
highest among prepared samples. This observation is in agreement
with the results reported earlier [11,12,14].
Scan rate (mV s−1)
C (F)a
qa (C)
qc (C)
qa + qc (C)
Fig. 10a shows the decay in specific capacitance as a function of
2
5
10
0.3204
0.2041
0.1774
0.1654
0.1965
0.1344
0.1255
0.1253
0.5169
0.3385
0.3029
0.2907
718
470
420
403
−
1
the scan rate. Specific capacitance at 2, 5, 10, and 20 mV s scan
−1
rate is 718, 470, 420, and 403 Fg , respectively, which is shown
in Fig. 10b. Calculated parameter, qa, qc, and specific capacitance of
N300 at different scan rates are tabulated in Table 2. As the scan rate
increased, the potential of the anodic and cathodic peaks shifted
20
a
Specific capacitances were calculated by C = (q + q )/2mꢁV, where
(g) = 0.0008, ꢁV (V) = 0.45.
a
c
m

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J.W. Lee et al. / Electrochimica Acta 56 (2011) 4849–4857
−
1
Fig. 10. (a) Cyclic voltammetry curves of N300 at scan rate of 2, 5, 10, and 20 mV s
and (b) specific capacitance as a function of scan rate of N300.
Fig. 11. Cycle test of N300: (a) cyclic voltammetry curves of 10 cycles to 1000 cycles
−
1
at a 20 mV s scan rate and (b) variation in the specific capacitance as a function of
−
1
ꢀꢀ ꢀ
Z ) and real part (Z ) of impedance. Fig. 12 shows Nyquist plots of
the cycle number at a 20 mV s scan rate.
(
the capacitive electrode before and after 1000 cycles at an applied
potential of 0.25 V (vs. Ag/AgCl). The impedance spectra are almost
similar in shape, which consist of one semicircle at high frequency,
and followed by a straight line at the low frequency range. Such
a pattern can be fitted by an equivalent circuit for impedance
analysis, and is shown in the upper right inset of Fig. 12a, where
Rs is the solution resistance of the electrochemical system, Cdl is
a double layer capacitor, CF is a Faradaic pseudocapacitor, W is
Warburg impedance, and Rct is Faradaic interfacial charge transfer
resistance. The solution resistance Rs and the charge transfer resis-
tance Rct can be obtained from the Nyquist plot, where the high
frequency semicircle intercepts the real axis at Rs and (Rs + Rct),
respectively. As shown in Fig. 12b, the solution resistance Rs are
Fig. 12. Nyquist plots of N300 electrode before and after 1000 cycles at an applied potential of 0.25 V (vs. Ag/AgCl). The right inset (a) is the proposed equivalent circuit for
the EIS spectrum and (b) indicates enlarged Nyquist plots in the high frequency region.

J.W. Lee et al. / Electrochimica Acta 56 (2011) 4849–4857
4857
almost same, namely, 0.1 ꢃ, for two electrodes. However, there
is a major difference about the charge-transfer resistance (Rct).
After 1000 cycles, the charge-transfer resistance for the capaci-
tive electrode is increased. As the charge-transfer resistance (Rct) is
normally resulting from ionic resistance of electrolyte, the intrin-
sic resistance of the active materials, and the contact resistance
at the active material/current collector interface, increase in the
charge-transfer resistance (Rct) is caused by the loss of adhesion of
some active materials or composite crack with the current collector
during the long term charge/discharge cycling.
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[
[
[
[
4
. Conclusion
We have demonstrated a simple and template-free method for
fabricating NiO microstructures. ␣-Ni(OH)2 microstructures were
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[
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◦
tion at 300 C yielded NiO microstructures that retained the original
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morphologies of the ␣-Ni(OH) . The prepared NiO microstructures
2
had large BET specific surface area, narrow pore size distribu-
tion, and high pore volume, for characteristics that are ideal for
supercapacitor applications. The NiO microstructures produced
high specific capacitance and showed good retention for more
than 1000 cycles in a cycling test. This article is the first report
about synthesis of NiO electrode by HMT hydrolysis. We expect
to obtain NiO with unique morphologies by other method using
HMT, which could result in superior candidates for supercapacitor
applications.
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This work was supported by the BK 21 Program and Basic Sci-
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