Preparation of ordered mesoporous nickel oxide film electrodes via lyotropic liquid crystal templated electrodeposition route

Article abstract of DOI:10.1016/j.electacta.2007.07.053

A novel electrochemical route to fabricate ordered mesoporous metal oxide film electrodes has been investigated with particular reference to nickel oxide. Ordered mesoporous nickel oxide films are successfully synthesized by templated electrodeposition of H_I-e nickel hydroxide and followed by heat-treatment in air at various temperatures. The films are characterized physically by thermogravimetry (TG), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The applicability of this film as inexpensive and high-performance supercapacitor electrode material is demonstrated by the electrochemical characterization using cyclic voltammetry (CV) and chronopotentiometry technique. The specific capacitance of the nickel oxide film depends on the annealing temperature, showing a maximum value of 590 F g^-1 when the as-deposited film is heat-treated at 250 °C for 1.5 h.

Full text of DOI:10.1016/j.electacta.2007.07.053

Available online at www.sciencedirect.com
Electrochimica Acta 53 (2008) 2699–2705
Preparation of ordered mesoporous nickel oxide film electrodes
via lyotropic liquid crystal templated electrodeposition route
∗
Dan-Dan Zhao, Mao Wen Xu, Wen-Jia Zhou, Jin Zhang, Hu Lin Li
College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
Received 4 April 2007; received in revised form 25 July 2007; accepted 25 July 2007
Available online 31 July 2007
Abstract
A novel electrochemical route to fabricate ordered mesoporous metal oxide film electrodes has been investigated with particular reference to
nickel oxide. Ordered mesoporous nickel oxide films are successfully synthesized by templated electrodeposition of H -e nickel hydroxide and
I
followed by heat-treatment in air at various temperatures. The films are characterized physically by thermogravimetry (TG), transmission electron
microscopy (TEM) and X-ray diffraction (XRD). The applicability of this film as inexpensive and high-performance supercapacitor electrode
material is demonstrated by the electrochemical characterization using cyclic voltammetry (CV) and chronopotentiometry technique. The specific
−
1
capacitance of the nickel oxide film depends on the annealing temperature, showing a maximum value of 590 F g when the as-deposited film is
◦
heat-treated at 250 C for 1.5 h.
©
2007 Published by Elsevier Ltd.
Keywords: Ordered mesoporous nickel oxide film; Hexagonal phase liquid crystal template; Electrodeposition; Annealing temperature; Electrochemical capacitors
(
ECs)
1
. Introduction
inexpensive electrode materials with good capacitive charac-
teristics, e.g. NiO [9–21], CoOx [22], MnO2 [23], Ni(OH)2
[24], Co(OH)2 [25], etc. Among these candidates, nickel oxide
has attracted considerable attention due to its lower fabrication
cost and high pseudocapacitive behavior. As an environmen-
tal friendly material, nickel oxide is also applied as catalyst,
electrochromic films, fuel cell electrodes and gas sensors, etc.
[26].
Electrochemical capacitors utilize both electrical double-
layer and interfacial redox processes to store energy at an
electrode/electrolyte interface. Accordingly, structural proper-
ties are strongly related to the electrochemical properties of
nickel oxide for supercapacitors. It is well known that the pore
size at the range of 2–50 nm (mesopores) is considered to be
highly desirable for the electrochemical supercapacitors, and
the ordered mesopores make ionic motion easier compared
with conventional mesoporous material in which the pores are
randomly connected [27]. Up to now, many routes have been
developed to prepare the ordered mesoporous nickel oxide as
electrochemical capacitors’ electrode materials. In the case of
nickel oxide powder prepared by using SBA-15 (a kind of silica
molecular sieve with high ordered mesoporous structure) [20]
as the hard template or using anionic surfactant sodium dodecyl
In recent years, interest in electrochemical capacitors (ECs)
has risen due to their higher power density and longer cycle life
compared to secondary batteries and higher energy density than
conventional electrical double-layer capacitors. [1–3] They play
increasingly important roles in power source applications such
as auxiliary power sources for hybrid electric vehicles, short-
term power sources for mobile electronic devices, etc. [4]. The
electrochemical capacitors based on noble metal oxides, such
as RuO2 and IrO2, exhibit much higher specific capacitance
than conventional carbon materials and better electrochemical
stability than electronically conducting polymer materials. For
example, hydrous ruthenium oxides displaying remarkably high
specific capacitance varying from 658 to 760 F g have been
reported earlier [5–8] (Note that all capacitance values reported
in this paper are for a single electrode.). However, the high cost
of these noble metal materials limits it from commercialization.
Hence, much effort has been aimed at searching for alternative
−
1
∗
Corresponding author. Tel.: +86 931 891 2517; fax: +86 931 891 2582.
E-mail address: lihl@lzu.edu.cn (H.L. Li).
0
013-4686/$ – see front matter © 2007 Published by Elsevier Ltd.
doi:10.1016/j.electacta.2007.07.053

2
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D.-D. Zhao et al. / Electrochimica Acta 53 (2008) 2699–2705
sulfate [21] as the structure-directing agent, the specific capac-
as the cathode with a spiral platinum rod (1.0 mm in diameter) as
the counter electrode, and a saturated calomel electrode (SCE)
as the reference electrode. After deposition, the nickel hydroxide
films were rinsed consecutively with methanol, 2-propanol and
high purity water to remove adhering template mixture and dried
under ambient conditions. Ordered mesoporous nickel oxide
films studied here were formed by heating the as-deposited HI-e
Ni(OH)2 films in air at specified temperatures for 1.5 h with a
−
1
itance was observed to be about 120 F g . These nickel oxide
powders generally have an ordered mesoporous structure with
uniform pore size of 3–7 nm, which greatly increases the elec-
trode/electrolyte interfacial area and facilitates ion transfer in
the pore system.
In the last few years, nickel oxide films have been pre-
pared by sol/gel method [11], heat-treatment of electrodeposited
Ni(OH)2 (electrochemical route) [12–16,28], and electrostatic
spray deposition technique [17]. The high capacitance as high
◦
ramp rate of 5 C/min in muffle stove.
−1
as 278 F g is attributed to the increase in true surface area [13]
as well as to the non-stoichiometric nature of nickel oxide films
2.2. Characterization
[
16]. Among the existing synthetic approaches to film materials,
Before electrodeposition, the type of lyotropic phases was
examined to ensure that the lyotropic liquid crystal template was
in the hexagonal phase by polarized optical microscopy (POM)
technique using a COEIC XSZ-HS7 optical polarizing micro-
scope (Chongqing) equipped with a digitally controlled heating
stage. Thermogravimetry and differential thermogravimetry
(TG–DTG) was carried out on the scraped nickel hydroxide with
␣-Al2O3 as the reference substance using a Setarm TGDTA92A
electrochemical techniques show unique principles and flexibil-
ity in the control of the structure and morphology of the ordered
mesoporous films [29–37]. However, so far, there are few works
intheliteraturereportingonthesynthesisoforderedmesoporous
nickel oxide films via the electrochemical route. For example,
Nelson et al. [31] and Nelson and Owen [32] electrodeposited
HI-e Ni films by lyotropic liquid crystal templating technique
and studied the Ni(OH)2/NiOOH electrochemistry behaviors for
supercapacitor application. Nevertheless, the low capacitance
due to the formation of just a few monolayers of Ni(OH)2 on
metallic nickel surface has been an impediment to its practical
application.
In this paper, we report for the first time a relatively inex-
pensive and highly controllable method to prepare an ordered
mesoporous NiO film by heating HI-e Ni(OH)2 film electrode-
posited from the Brij 56-Ni(NO3)2 liquid crystal template (a
homogeneous mixture of Brij 56 and Ni(NO3)2 aqueous solu-
tion). The lyotropic liquid crystalline templates and the ordered
mesoporous NiO films are characterized by polarized-light
optical microscopy (POM), XRD and TEM. Electrochemi-
cal capacitance performance of the films is investigated by
the means of cyclic voltammetry (CV) and galvanostatic
charge–discharge tests in 3 wt.% KOH aqueous solution elec-
◦
−1
(France) at a heating rate of 5 C min . The crystalline struc-
ture of the samples was determined by X-ray diffraction (XRD)
measurements performed on a Rigaku D/MAX 2400 diffrac-
˚
tometer (Japan) with Cu K␣ radiation (λ = 1.5418 A) operating
at 40.0 kV, 60.0 mA. Low-angle X-ray diffractograms recorded
◦
over the 2θ range of 1.0–10 were used to provide primary evi-
dence for the formation of the hexagonal liquid crystal template
and the nanostructured films. The surface morphology of the
nanostructured films was investigated by transmission electron
microscopy (TEM) using a JEOL JEM-1230 (Japan) equipped
with a GATAN BioScan Camera 792 (USA). Specimens for
TEM images were prepared by scraping films from the working
electrodes onto the carbon-coated copper grid using a razor.
Electrochemical characterization was carried out in a three-
electrode electrochemical cell containing 3 wt.% KOH aqueous
solution as electrolyte. The freshly prepared nickel oxide film on
titaniumsubstratewasusedasworkingelectrodewithaplatinum
−
1
trolyte. A maximum specific capacitance of 590 F g
is
◦
2
achieved by annealing at 250 C for 1.5 h, suggesting that this
mesoporous nickel oxide film is a promising electrode material
for electrochemical capacitors.
foil (area 1 cm ) as counter electrode, and a Ag/AgCl electrode
as reference electrode. Cyclic voltammetric and chronopoten-
tiometric measurements were performed at room temperature
using a Chenhua CHI760B model Electrochemical Worksta-
tion (Shanghai) to determine the electrochemical properties and
specific capacitance of the nickel oxide films.
2
. Experimental
2
.1. Materials preparation
3. Results and discussion
Hexagonalliquidcrystaltemplatewaspreparedfromahomo-
geneous mixture of a 50 wt.% nonionic surfactant Brij 56 (main
component, C [EO]10, viz. polyoxyethylene (10) cetyl ether)
Considering the nanostructured film is a direct cast of the
structure of the lyotropic phase used to template the deposition
[30], it is important to investigate the electrolyte phase struc-
ture before electrodeposition. Fig. 1 shows the representative
polarized light micrograph for the 50 wt.% Brij 56-Ni(NO3)2
electrolyte. The hexagonal (HI) liquid crystal is anisotropic and
displays focal conic fan textures in the temperature range of
16
and a 50 wt.% aqueous solution of 1.8 M Ni(NO3)2 with 0.075 M
NaNO3 as supporting electrolyte. The HI-e Ni(OH)2 films were
electrodeposited from nickel nitrate dissolved in the aqueous
domains of the hexagonal liquid crystal template at a constant
potential of −0.70 V versus SCE using a Chenhua CHI760B
model Electrochemical Workstation (Shanghai). The electrode-
position was carried out in a standard three-electrode glass cell,
◦
32.2–77.5 C [38].
The HI-e Ni(OH)2 film is electrodeposited at −0.70 V versus
SCE from the hexagonal phase liquid crystal template. Thermo-
gravimetry and differential thermogravimetry (TG–DTG) has
◦
which was immersed in a water bath held at 40 C. A titanium
2
substrate with geometric surface area of about 1 cm was used

D.-D. Zhao et al. / Electrochimica Acta 53 (2008) 2699–2705
2701
◦
At 243 C, an experimental weight loss of 14.6% is
observed.
◦
◦
(
ii) From 243 to 277 C, Ni(OH)2 converts to NiO. At 277 C,
the experimental mass loss of 28.5% (approximates the
weight loss deduced from the following reaction equation,
28.7%), is attributed to the conversion of the hydroxide to
the oxide [39]:
Ni(2H)0.33O2.67H2.67→NiO + 1.67H2O
iii) When the temperature is above 277 C, the weight
decreases slowly because of the removal of a small amount
of hydroxyl-group residue during development of the NiO
(3)
◦
(
◦
phase [15]. When the temperature reaches 500 C, a weight
loss of 35.7% is observed, which is very close to the
stoichiometric weight loss, 36.7% and the film becomes
stoichiometric NiO.
Fig. 1. POM image of a hexagonal (HI) liquid crystal formed from a deposition
electrolyte consisting of 50 wt.% Brij 56 and 50 wt.% aqueous solution of 1.8 M
Ni(NO3)2 and 0.075 M NaNO3.
The wide-angle XRD patterns of the as-deposited HI-e
Ni(OH)2 film and ordered mesoporous NiO films formed by
heating the as-deposited films at different temperatures are
shown in Fig. 3. The XRD pattern of the as-deposited HI-e
been carried out on the as-deposited HI-e Ni(OH)2 to investigate
the thermal behavior and estimate the mass loss. The electro-
chemically deposited nickel hydroxide films are very defective,
which can be represented stoichiometrically as [13]
◦
◦
Ni(OH)2 film consists of broad peaks at 12.46 (0 0 1), 23.72
◦ ◦
(
0 0 2), 33.64 (1 1 0) and 59.92 (3 0 0). Except the substrate
peaks, the XRD result corresponds to the well-known layered
nickel hydroxide hydrate (JCPDS no. 22-0444) [40]. The film
2
3
Ni(2H)0.33O2.67H2.67 · XWH2O≡Ni(OH) · H2O · XWH2O
2
◦
heat-treated at 200 C still sustains ␣-Ni(OH)2 structure, how-
(1)
ever, the peaks shift slightly to higher angles, resulting in a
decrease of the ‘c’ spacing of the unit cell, which is due to water
removal from planes perpendicular to the c axis of Ni(OH)2 crys-
where XW is the moles of water. This water can be structurally
bound to the crystal or in the film between the crystallites [39].
In contrast, the H2O in the “2/3 H2O” is not molecular water but
rather two protons sitting on a nickel vacancy with an oxygen
atom associated with it [13]. As seen in Fig. 2, the TG–DTG
curves show three weight-loss regions:
◦
tals [39]. At temperatures at or above 250 C, the XRD patterns
◦
exhibit the characteristic peaks of rock salt NiO at 2θ = 37.41 ,
43.36 , 62.94 and 75.43 . And all the peak positions and their
relative intensities are in good agreement with the standard spec-
◦
◦
◦
◦
(
i) Below 243 C, both structural water and the water between
crystallites are lost from the active material according to
the following reaction [39]:
Ni(2H)0.33O2.67H2.67 · XWH2O
dehydration
−
→
Ni(2H)0.33O2.67H2.67 + XWH2O
(2)
Fig. 3. Wide-angle XRD spectra of the as-deposited HI-e Ni(OH)2 film obtained
at −0.70 V vs. SCE on titanium substrate from the hexagonal liquid crystal
template; and the films annealed at different temperatures for 1.5 h.
Fig. 2. TG–DTG curves for the as-deposited HI-e Ni(OH)2. Heating rate:
◦
−1
.
5
C min

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D.-D. Zhao et al. / Electrochimica Acta 53 (2008) 2699–2705
width can be attributed to the distortion of lyotropic liquid crys-
talline phase which is a soft solid [41] and the dynamic disorder
present in the liquid crystalline phase at elevated temperature
[42]. The (1 0 0) peak of the NiO film shifting slightly to higher
angle implies slight contraction of the framework during the
annealing process [43].
The surface morphology of the nanostructured films has been
investigated using TEM technique. Typical TEM images of
the as-deposited HI-e Ni(OH)2 film obtained from the hexag-
onal liquid crystal template and the ordered mesoporous NiO
◦
film heat-treated at 250 C for 1.5 h are shown in Fig. 5. TEM
study of the as-deposited HI-e Ni(OH)2 film reveals a well-
ordered nanostructure consisting of cylindrical pores arranged
on a hexagonal lattice (Fig. 5a). The film is estimated to have a
pore center-to-center distance of about 7.0 nm, with a uniform
pore diameter of about 2.5 nm and a nickel hydroxide wall thick-
ness of about 4.5 nm. These values are consistent with previous
observations of other mesoporous materials electrodeposited
from the HI phase of the same surfactant Brij 56 [31,32,34–37].
The TEM image of the NiO film, in the main, shows an ordered
mesoporous structure (Fig. 5b). The average diameter of the
pores is calculated to be about 2.3 nm and the center-to-center
pore spacing is about 6.8 nm, so that the nickel oxide walls are
about 4.5 nm thick. On the basis of the pore size and wall thick-
ness, a high specific surface area inside these pores would be
expected. Even after the annealing process, there seemed to be
no significant change in the pore system of the films except
for slight contraction of the framework and appearance of some
cracks. This trend is in good agreement with the low-angle XRD
data.
Fig. 4. Low-angle XRD spectra for the hexagonal liquid crystal template; the as-
deposited HI-e Ni(OH)2 film obtained at −0.70 V vs. SCE on titanium substrate
from the hexagonal liquid crystal template; and the ordered mesoporous NiO
◦
film formed by annealing the as-deposited film at 250 C for 1.5 h.
trum (JCPDS no. 04-0835), indicating that the nickel oxides are
crystalline with face-centered cubic structure. With the increase
of annealing temperature, the peaks become narrower and the
peak intensities increase as well, which indicates that the degree
of crystallization of NiO increases. Therefore, the wide-angle
XRD data indicates that the annealing temperature of about
In order to investigate the effect caused by annealing tempera-
ture on the specific capacitance of the samples, the performance
of the active materials were examined by chronopotentiome-
try. Fig. 6 shows the charge/discharge curves of the ordered
mesoporous NiO films electrodes heat-treated at different tem-
peratures for 1.5 h measured in the potential range of 0–0.55 V
in 3% KOH at a galvanostatic current of 2.5 mA. The working
electrode each contains 5 mg NiO and has a geometric sur-
◦
2
50 C may be the threshold temperature for the conversion of
Ni(OH)2 to NiO and suggests that annealing temperature is an
important factor for controlling the crystalline structure of NiO.
Low-angle XRD investigations show peaks at small angles,
which indicate the existence of a size-controlled nanoporosity.
Fig. 4 shows the low-angle XRD spectra for the hexagonal
(
HI) phase of the 50 wt.% Brij 56-Ni(NO3)2 electrolyte, an
2
HI-e Ni(OH)2 film electrodeposited from the template, and an
face area of 1 cm . The specific capacitance can be calculated
◦
−1
ordered mesoporous NiO film produced by annealing at 250 C
according to C = Iꢀt/(ꢀVm), where C (F g ) is the specific
for1.5 h, respectively. Thelow-angleXRDpatternsofthehexag-
onal liquid crystal template display diffraction lines at around
capacitance, I (mA) the applied galvanostatic current, ꢀt (s)
the discharging time, ꢀV (V) the potential drop during dis-
charge, and m (mg) is the mass of the NiO film [22]. The
specific capacitance values calculated from the whole discharg-
◦
◦
◦
◦
2
θ = 1.52 , 2.82 , 4.15 and 5.53 , corresponding to the (1 0 0),
(
1 1 0), (2 1 0) and (2 2 0) planes of the P6mm space group. The
−
1
˚
˚
d100-spacing of 58.1 A and d110-spacing of 31.3 A are in the ratio
1
ing curves are 590, 106 and 47 F g for the films heat-treated
√
◦
:1/ 3 as expected for the 2D hexagonal mesophase. The pore-
at 250, 300 and 350 C, respectively. Obviously, the ordered
◦
to-pore distance for this hexagonal array, given by d100/cos 30,
mesoporous NiO film obtained at 250 C exhibits the maxi-
˚
is 67.1 A. The low-angle XRD pattern of the as-deposited HI-e
mum specific capacitance. However, increasing the annealing
temperature from this optimum leads to a decrease in the spe-
Ni(OH)2 film shows a well defined intense peak correspond-
◦
˚
ing to a d100-spacing of 80.2 A and a pore-to-pore distance of
cific capacitance until at 400 C the electrode does not store
˚
9
2.6 A. For the NiO film, the low-angle XRD pattern also shows
any useful capacity. This can be explained based on the defect
healing effect [16] and the decrease in the electroactive sur-
face area with increasing annealing temperature [13]. Natarajan
et al. reported that the electrochemically prepared Ni(OH)2
a single broad reflection (d100) corresponding to a lattice peri-
˚
˚
odicity of 78.8 A and a pore-to-pore distance of 91.0 A. The
appearance of low-angle peaks indicates that mesoscopic order
in the liquid crystalline template is preserved in the as-deposited
HI-e Ni(OH)2 film and even in the NiO film heat-treated. The
presence of only one scattering peak as well as its broad line
◦
heated to ∼250 C or above is converted to non-stoichiometric
nickel oxide (Ni1−xO), and due to the presence of Ni vacan-
cies, Ni3 exists in the nickel oxide in order to balance the
+

D.-D. Zhao et al. / Electrochimica Acta 53 (2008) 2699–2705
2703
Fig. 5. TEM images for the nanostructured films, from end-on view of pores: (a) the as-deposited HI-e Ni(OH)2 film obtained at −0.70 V vs. SCE from the hexagonal
◦
liquid crystal template and (b) the ordered mesoporous NiO film formed by annealing the as-deposited film at 250 C for 1.5 h.
charge [44]. Heating the highly defective nickel oxide at higher
temperature leads to the corresponding atomic rearrangement,
heals the defects and approaches the stoichiometric NiO struc-
ture [16]. The non-stoichiometric nature (or defective nature)
capability of the NiO film electrode, if the proper surface mor-
phology and physicochemical properties are well controlled. For
the NiO film prepared via hexagonal liquid crystal templated
electrodeposition route, it exhibits both high surface area and
nano-scale microstructure constituted by the ordered mesopores
◦
of the nickel oxide heat-treated at 250 C is thought to be
˚
beneficial to its pseudocapacitance, therefore, control of non-
stoichiometry by heat-treatment could be utilized to improve its
specific capacitance for supercapacitor application. The elec-
troactive surface area of the nickel oxide film in contact with
the KOH solution is another important factor to be considered
in explaining the change in its specific capacitance with heat-
treatment temperature. It appears that for temperatures above
the optimal temperature, sintering and crystallization effects set
in and impact negatively on the active area of the film [13,14].
In recent studies on the NiO films as supercapacitor electrode
materials, the surface Faradaic reaction in alkaline solution has
been expressed as follows [13]:
with a uniform diameter of about 23 A. This nanostructure cre-
ates the fast electrochemical accessibility of the electrolyte and
−
OH ions to the bulk of the NiO phase, providing an impor-
tant morphological foundation for a high specific capacitance.
Based on the above discussions, we propose that the hexagonal
liquid crystal template plays a crucial role in optimizing superior
capacitive performance of the NiO film.
Further electrochemical investigation is focused on the
◦
ordered mesoporous NiO film electrode heat-treated at 250 C
for 1.5 h. In 3% KOH solution, cyclic voltammograms and
chronopotentiometric measurements have been used to eval-
charge
NiO + zOH−
ꢀ
zNiOOH + (1 − z)NiO + ze
(4)
discharge
Reaction (4) suggests that only a fraction of the nickel sites,
z, are involved in the redox reaction. The value of z can be
estimated from the specific capacitance of the oxide film by
z = (CMꢀV)/F, where z is the fraction of nickel atoms involved
−
1
in the redox process, C (F g ) denotes the specific capacitance,
−1
M (g mol ) is the molecular weight of nickel oxide, ꢀV (V)
represents the potential drop during discharge, and F is Faraday
constant (96,487 C/equiv.) [13]. Assuming stoichiometric NiO
−
1
(
withamolecularweightof74.692 g mol ), thespecificcapaci-
tance from Fig. 6 gives a z value of 0.251, 0.045 and 0.020 at 250,
◦
3
00 and 350 C, respectively, about 10 times larger than that of
NiO film obtained from aqueous solution without liquid crystal
template [13]. In other words, at these respective temperatures
about 25.1%, 4.5% and 2.0% of the nickel atoms participate in
the reaction. This suggests that the surface adsorption and redox
Fig. 6. Charge/discharge curves of the ordered mesoporous NiO film electrodes
annealed at different temperatures for 1.5 h measured in the potential rage from 0
to 0.55 V vs. Ag/AgCl at a galvanostatic current of 2.5 mA in 3% KOH solution.
−
2
reaction of OH ions contribute greatly to the charge-storage
The working electrode is 1 cm containing 5 mg NiO.

2
704
D.-D. Zhao et al. / Electrochimica Acta 53 (2008) 2699–2705
◦
Fig. 7. Electrochemical properties of the ordered mesoporous NiO film electrode annealed at 250 C for 1.5 h measured in 3% KOH solution: (a) cyclic voltammetric
(
CV) curves at different scan rates within a potential window of 0 to 0.65 V vs. Ag/AgCl; (b) charge/discharge curves in the potential rage from 0 to 0.55 V at different
2
galvanostatic currents; (c) variation of specific capacitance and coulombic efficiency with cycle number at 2.5 mA. The working electrode is 1 cm containing 5 mg
NiO.
uate the electrochemical properties and quantify the specific
capacitance. The shape of the CV curves reveals that the capac-
itive characteristic is very distinguished from that of electric
double layer capacitance in which case it is normally close to
an ideal rectangular shape (Fig. 7a). The current response is
not exactly linearly proportional to the sweep rate compared
to that an ideal capacitor. Since solution and electrode resis-
tance can distort current response at the switching potential and
this distortion is dependent upon the scan rate [45], the shape
of the CV has changed when the scan rate increased. Fig. 7b
shows the charge/discharge curves of the ordered mesoporous
indicates a typical pseudo-capacitance behavior, which resulted
from the electrochemical adsorption/desorption or redox reac-
tion at an interface between electrode and electrolyte[7]. The
shape of the discharge curves does not show the characteristics
of a pure double layer capacitor but mainly pseudo-capacitance,
which is in agreement with the result of the CV curves. These
results indicate that the measured capacitance is mainly Faradaic
pseudo-capacitance arising from fast, reversible electrosorption
and redox processes occurring at the non-stoichiometric nickel
oxide electrode surface.
Since long cycle life is very important to the electro-
chemical capacitor, the cycle charge/discharge test has been
employed to examine the service life of the ordered meso-
◦
NiO film electrode formed at 250 C measured in the potential
range of 0–0.55 V. The specific capacitance values calculated
−
1
◦
from the whole discharging curves are 590, 534 and 410 F g
porous NiO film electrode formed at 250 C. As shown in
corresponding to the discharging currents of 2.5, 5 and 10 mA,
respectively. As discharge current increases, the voltage (IR)
drop produces, and finally the capacitance decreases. The dis-
charging curves obviously display two variation ranges; a linear
variation of the time dependence of the potential (below about
Fig. 7c, approximately 4.7% loss of specific capacitance after
500 cycles indicates that the repetitive charge–discharges do not
induce noticeable degradation of the mesoporous structure. The
coulomb efficiency η (%) of the same NiO film electrode dur-
ing charge/discharge is also shown in Fig. 7c, which can be
evaluated from η = (tD/tC) × 100, where tD and tC are the time
for galvanostatic discharging and charging, respectively. It can
be found that the coulombic efficiency increases with growth
of cycle numbers and almost approaches 99% after 500 cycles,
0
.27 V) indicates the double-layer capacitance behavior, which
is caused by the charge separation taking place between the
electrode and the electrolyte interface, and a slope variation
of the time dependence of the potential (from 0.55 to 0.27 V)

D.-D. Zhao et al. / Electrochimica Acta 53 (2008) 2699–2705
[10] Y. Wang, Q.Z. Qin, J. Electrochem. Soc. 149 (2002) A873.
2705
implyingthatthereversibilityofcharge/dischargeincreases. The
[
[
[
[
11] K.C. Liu, M.A. Anderson, J. Electrochem. Soc. 143 (1996) 124.
12] V. Srinivasan, J.W. Weidner, J. Electrochem. Soc. 144 (1997) L210.
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has good cycle properties as an excellent electrode material for
electrochemical capacitor.
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(
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NiO film annealed at 250 C for 1.5 h. The results demonstrate
the versatility of fabricating a wide range of ordered mesoporous
transition metal oxide films via this lyotropic liquid crystal tem-
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