1. Preparation of mesoporous microspheres of NiO with high surface area
and analysis on their pseudocapacitive behavior
Syed Asad Abbasa,b
, Kwang-Deog Junga,b,
*
a
Center for Clean Energy and Chemical Engineering, Korea Institute of Science and Technology, Hwarangno 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic of
Korea
b
Clean Energy and Chemical Engineering, University of Science and Technology, 217 Gajeong-ro Yuseong-gu, Daejeon, Republic of Korea
A R T I C L E I N F O
Article history:
Received 25 August 2015
Received in revised form 26 January 2016
Accepted 9 February 2016
Available online 11 February 2016
Keywords:
Mesoporous microspheres of NiO
a-Ni(OH)2
Oleylamine method
hydrothermal method
Pseudocapacitor
A B S T R A C T
Nickel oxide with a high surface area showing high capacitance is reported here. Mesoporous
microspheres (MMS) of 250NiO, 300NiO, 350NiO, 400NiO and 500NiO are synthesized by calcining
mesoporous a-Ni(OH)2 at 250
C, 300
C, 350
C, 400
C, and 500
C, respectively. The mesoporous a-Ni
(OH)2 was prepared by a hydrothermal method. 250NiO has the highest specific surface area of 295 m2
/g,
and a high specific capacitance of 1,140 F gÀ1
at a current density of 10 A gÀ1
from galvanostatic discharge
measurements. The cyclic voltammetry, galvanostatic discharge measurement and electrochemical
impedance analysis exhibited that the pseudocapacitive behavior is more clarified for NiO prepared at
higher calcination temperature. Apparently, the high specific capacitance of 250NiO results from the
mesoporous pores and high specific surface area enhancing the transportation of ions during the
charging and discharging process to store high energy. The power density and energy density of
the 250NiO are 2.5 kW kgÀ1
and 59 W h kgÀ1
respectively at the current density of 10 A gÀ1
.
ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction
Recently, supercapacitors have received increasing attention
due to their ability to store massive amounts of energy [1–3].
Supercapacitors have distinct advantages over the other storage
devices such as batteries in terms of high power delivery, long life
span, high cycle efficiency and wide working temperature range
[1]. Due to these advantages, supercapacitors meet the energy
storage requirements of electric devices requiring high electric
density such as electric vehicles [4]. Supercapacitors are also useful
for the applications such as backing-up for uninterruptible power,
absorbing power during the short periods of generation such as
regenerative braking, and storing renewable energies such as wind
and solar energy [5].
Based on energy storage principals, an electrochemical capaci-
tor (EC) can be categorized as either a pseudocapacitor or an
electric double layer capacitor [6,7]. Transition metal oxides,
conducting a reversible redox reaction, are generally used as
pseudocapacitors due to their excellent pseudocapacitance
behaviour [8]. RuO2, MnO2, Fe3O4 and Co3O4 have been intensively
studied in the past as peudocapacitance materials [9,10], RuO2 has
been the most extensively studied due to its wide potential
window, long life cycle and high specific capacitance, but it is too
expensive to be used for general purposes [11].
Theoretically, NiO has been known to be a good candidate
to replace RuO2, because the theoretical capacitance of NiO
(2,573 F gÀ1
) is much higher than that of RuO2 (1,360 F gÀ1
for
RuO2Á0.5H2O). NiO has not only a high theoretical capacitance, but
also good thermal and chemical stability [12]. Recently, it was
proposed that NiO or Ni(OH)2 should not be classified as
pseudocapacitive material [13]. On the other hand, nanostructured
Ni(OH)2 was treated as a pseudocapacitor by identifying the charge
storage mechanism rather than basing the claim on the material
type alone [14,15]. Nonetheless, a lot of papers have been published
with the pseudocapacitive nature of NiO. Different kinds of Nickel
oxide morphologies are reported to achieve improved capacitance
values [16]. A high capacitance of 405 F gÀ1
at a current density of
0.5A gÀ1
is reported by Shenglin et al. through mesoporous
nanotubes [17]. Similarly, Shujiang et al. achieved specific
capacitance of 415 F gÀ1
at a current density of 3A gÀ1
using
hollow spheres of NiO with a specific surface area of 62 m2
gÀ1
[18].
Recently, NiO prepared by a microwave-assisted gas/liquid method
showed specific capacitance of 770 F gÀ1
and 585 F gÀ1
at discharge
currents of 2 A gÀ1
and 5 A gÀ1
[19].
* Corresponding author at: Center for Clean Energy and Chemical Engineering,
Korea Institute of Science and Technology, Hwarangno 14-gil 5, Seongbuk-gu, Seoul
136-791, Republic of Korea. Tel.:+ +822 958 5218; fax: +822 958 5219.
E-mail addresses: jkdcat@kist.re.kr, jkdyym@hanmail.net (K.-D. Jung).
http://dx.doi.org/10.1016/j.electacta.2016.02.054
0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
Electrochimica Acta 193 (2016) 145–153
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2. In the current work, we prepared mesoporous microspheres
(MMS) of NiO with ultra-high capacitance. Alfa nickel hydroxide
spheres are first prepared by a hydrothermal process, followed by
calcination at 250
C, 300
C, 350
C, 400
C, and 500
C to produce
MMS of NiO. The prepared NiO samples are expected to have high
specific capacitance because of their high specific surface area and
mesoporous structures. These characteristics will allow better
contact between electrolyte and electrode for the transfer of
electrons and ions, presumably leading to high energy storage
capacity. The cyclic voltammetry, galvanostatic discharge meas-
urements and electrochemical impedance spectroscopy were
performed to investigate the pseudocapacitive behaviour of the
MMS of NiO.
2. Experimental
2.1. Material Preparation
2.1.1. Synthesis of a-Ni(OH)2
In a typical procedure, 4 mmol (1.164 g) of nickel nitrate
hexahydrate Ni (NO3)2.6H2O was dissolved in 80 mL of ethanol
(99.9%) under continuous magnetic stirring for 10 minutes.
Next, 8 mL of oleylamine (Aldrich, 70%) and 40 mL of ethanol
was added simultaneously and under constant stirring to get a
homogeneous solution. The mixed solution was then trans-
ferred and sealed in a 200 mL Teflon coated autoclave and
placed in a convection oven for 15 hours at 180
C. In this study
we also prepared alfa nickel hydroxide at 160
C and 200
C, and
studied the effect of cooking time as well. The resulting green
a-Ni(OH)2 powder was collected after cooling the autoclave at
room temperature. The powder was washed vigorously with
distilled water, cyclohexane and ethanol to remove organics,
ions and other remnants followed by drying at 60
C under
vacuum for four hours.
2.1.2. Synthesis of NiO mesoporous microspheres
Five NiO samples were produced simply by heating a-Ni(OH)2
at the rate of 2
C minÀ1
in air, to five different temperatures:
250
C, 300
C, 350
C, 400
C, and 500
C. The samples prepared at
these different temperatures are designated as 250NiO, 300NiO,
350NiO, 400NiO and 500NiO.
2.2. Materials characterization
In-situ X-ray diffraction (HT-XRD, Pananalytical X’pert Pro,
Netherlands) of powder samples were recorded with Cu Ka
radiation (l= 1.5406 Å) to confirm the formation of NiO. The
morphology and structure of the samples were characterized by
in-situ transmission electron microscopy (TEM, Model No: JEM
3011 (HR), JOEL Ltd, Japan). Specific surface area, pore size
distribution and pore volume were measured by a BET analyzer
(Belsorp II mini, BEL Japan, Inc.). The thermal behavior of the
samples were characterized by a thermogravimetric analysis (TGA,
SDT Q 600 U.S.A.) at a ramping rate of 2
C minÀ1
in air from room
temperature to 700
C.
2.3. Electrochemical Measurements
The working electrode was prepared by mixing 70 wt % of the
prepared NiO (active material), 20 wt % acetylene black (conduct-
ing agent) and 10 wt % polyvinylidene difluoride (PVDF, Aldrich) as
binder. This mixture was then pressed on a glassy carbon electrode
and dried at 60
C for two hours. Electrochemical measurements
were carried out in a typical three-electrode cell.
The cyclic voltammetry (CV) analysis of prepared electrodes
was performed in the potential range from 0.0 V to 0.5 V in aqueous
solution of 2 M KOH with Ag/AgCl as a reference electrode and
platinum foil as a counter electrode.
Galvanostatic charge and discharge measurements were
carried out to determine the electrochemical capacitance in the
three-electrode system at current densities of 3, 5 and 10 A gÀ1
.
Active material as the working electrode, Pt foil electrode as the
counter electrode and Ag/AgCl electrode as the reference electrode
were used in the potential range of 0 V to 0.5 volts in aqueous
solution of 2 M KOH.
Electrochemical impedance spectroscopy (EIS) measurements
were performed at 0.4 V vs Ag/AgCl from 100 kHz to 0.1 Hz in
aqueous solution of 2 M KOH.
Fig. 1. XRD patterns and TEM images of a-Ni(OH)2 samples prepared by hydrothermal reaction at (a) 160
C, (b) 180
C and (c) 200
C.
146 S.A. Abbas, K.-D. Jung / Electrochimica Acta 193 (2016) 145–153

3. 3. Results and discussion
3.1. Preparation and Characterization of a-Ni(OH)2
Alfa nickel hydroxide was prepared at three different temper-
atures of 160
C, 180
C and 200
C in an autoclave. Fig. 1 shows the
XRD patterns and TEM images of the prepared samples at 160,
180 and 200
C. All three samples show the same XRD patterns. The
characteristic peaks at 2u values of 11.4
(003), 22.8
(006) and
33.5
(006) in the XRD patterns correspond to a-Ni(OH)2 with a
rhombohedral structure (JCPDS card no. 380715). The mesoporous
NiO spherical particles with hairs were resulted from the
hydrothermal reaction at 160
C and 180
C, while the NiO hollow
spheres were prepared at 200
C. The mechanism from oleylamine
in nanoparticle synthesis was suggested as the hollow sphere
appeared first and the core could be filled, increasing the reaction
time [20]. On the other hand, the morphology did not change with
respect to the hydrothermal reaction time here (Fig. S1), but could
be decided by the reaction temperature (Fig.1). Nam et al. prepared
solid CoO at 200
C and hollow Co nanoparticles at 290
C [21]. The
formation of the hollow spheres can be related to the decomposi-
tion reaction of oleylamine as suggested by Nam et al. The resulting
a-Ni(OH)2 samples (Fig.S2) follows a type IV isotherm, which is
typical for mesoporous material [22]. BET surface areas of the
a-Ni(OH)2 samples at 160
C, 180
C and 200
C are 109 m2
gÀ1
,
176 m2
gÀ1
and 109 m2
gÀ1
, respectively. Pore volumes of the
a-Ni(OH)2 samples at 160
C, 180
C and 200
C are 0.20 cm3
/g,
0.44 cm3
gÀ1
, 0.27 cm3
gÀ1
respectively. It should be noted that the
surface area of mesoporous micro sphere (MMS) of a-Ni(OH)2 at
180
C is the highest among the prepared a-Ni(OH)2 samples.
3.2. Preparation and characterization of NiO
Fig. 2 shows thermogravimetric and differential thermal
analysis (TGA and DTA) curves of MMS of a-Ni(OH)2 prepared
by hydrothermal process at 180
C, which had the highest surface
area of 176 m2
/g.
TGA and DTA analysis were conducted with a-Ni(OH)2 sample
prepared at 180
C from room temperature to 700
C. The weight
loss of 3.7 wt % up to 130
C is due to dehydration of adsorbed water
on the a-Ni(OH)2 sample and the DTA curve in that temperature
range show a broad endothermic curve. The weight loss of 24.9 wt
% from 130
C is due to dehydration and dehydroxylation of the
a-Ni(OH)2ÁxH2O. The x value is estimated to be about 0.4.
However, DTA shows both exothermic and endothermic curves
in the temperature range from 150 to 350
C. The exothermic curve
may be due to oxidation of the organic compounds. It was shown
that the oleylamine was decomposed and the functional groups
were complexed with metal during the nanoparticle formation
using oleylamine [23]. Major weight loss is observed at 200
C
$ 260
C, which is due to phase transformation of nickel hydroxide
to nickel oxide according to the following equation [24]:
NiðOHÞ2 ! NiO þ H2O ð1Þ
There are no weight losses in the temperature ranges above 500
C.
Fig. 3 shows XRD patterns of the a-Ni(OH)2 particles prepared
by a hydrothermal method. The XRD patterns were obtained by an
in-situ XRD instrument from 50 to 500
C with steps of 50
C. The
structure of a-Ni(OH)2 did not change up to 200
C, indicating that
the weight loss up to 200
C in the TGA/DTA analysis is not due to
dehydroxylation, but due to water bonded to a-Ni(OH)2.
It is shown that a-Ni(OH)2 is transformed into NiO at 250
C. At
250
C, characteristic peaks of NiO clearly appear and all five
possible peaks are observed on XRD pattern, which are in good
match with JCPDS file (04-005-9695). Individual peaks at 2u values
of 37
, 43
, 62.4
, 74.7
and 78.8
Can be identified as (111), (200),
(220), (311) and (222) planes of NiO, respectively. FWHM (full
width half maximum) of the peak at 43
decreased with increasing
calcination temperature.
The Scherer equation (t= 0.9l/B cosuB) was used to find the
crystal size of NiO prepared at different calcination temperatures.
The crystal size of NiO increased with an increase in calcination
temperature. The crystal sizes of 250NiO, 300NiO, 350NiO, 400NiO
and 500NiO are 1.5 nm, 5.4 nm, 9.9 nm, 15.7 nm and 35.4 nm,
respectively.
Fig. 4 shows the TEM images of the prepared NiO samples using
in-situ transmission electron microscopy (TEM). The morphology
of 250NiO is similar to that of a-Ni(OH)2, although a-Ni(OH)2 is
transformed into NiO as shown in XRD patterns (Fig. 3). The
particle size of 250NiO spheres is ranging from 1.5 mm to 3 mm. The
shape and size of NiO spheres are not changed until the calcination
temperature reached 250
C. The MMS spheres starts to be
agglomerated at the temperature higher than 250
C. The spherical
shape is distorted at 400
C. In HR-TEM (Fig. S3), (111) planes are
mainly observed. The observed lattice spacing of NiO is estimated
to be 0.24 nm. The lattice distance for all the differently calcined
NiO samples is identical. Selected area electron diffraction (SAED)
patterns (Fig. S4) of 250NiO, 300NiO, 350NiO, 400NiO and 500NiO
show that the crystallinity of NiO crystals increased with an
increase in the calcination temperature, corresponding with the
results of XRD analysis.
Isotherm measurements of NiO samples were conducted to
investigate the calcination effect on the specific surface area and
pore size distribution. Fig. 5 shows the isotherms and pore size
Fig. 2. TGA and DTA curves of of a-Ni(OH)2 samples prepared by hydrothermal
reaction at 180
C.
Fig. 3. In situ XRD patterns of of a-Ni(OH)2 samples prepared by hydrothermal
reaction at (a) 50
C, (b) 100
C, (c) 150
C, (d) 200
C, (e) 250
C, (f) 300
C (g) 350
C,
(h) 400
C, (i) 450
C and (j) 500
C.
S.A. Abbas, K.-D. Jung / Electrochimica Acta 193 (2016) 145–153 147

4. distributions of NiO samples calcined at 250, 300, 350, 400, and
500
C. The pore structure of 250NiO (Fig. 5(a)) is similar to that of
a-Ni(OH)2, indicating that 250NiO also has ink-bottle pores. The
lower closure point increases with increasing calcination temper-
ature, indicating that the pore diameter of NiO samples from
desorption branch increases with increasing the calcination
temperature. The pore diameters of 250NiO, 300NiO, 350NiO,
400NiO and 500NiO are 9.2 nm, 7.1 nm, 7.1 nm, 9.2 nm, and
18.6 nm, respectively as shown in Fig. 5(f). No tensile stress effect
in the isotherms of the NiO samples in Fig. 5 was confirmed by
measuring the isotherms of the NiO samples using Ar isotherm.
However, the PSDs of NiO samples (Fig. 5(f)) are plotted from
adsorption branch for the purpose of comparison with those of
a-Ni(OH)2. Table 1 shows BET surface areas and average pore
volumes of the NiO samples. The surface area and pore volume of
the NiO samples decreased with increasing the calcination
temperature. Interestingly, the BET surface area of MMS of
250NiO is exceptionally large, 295m2
gÀ1
, which was more than
that of a-Ni(OH)2 (176 m2
gÀ1
). During the phase transformation
from Ni(OH)2 to NiO, the apparent sphere size was not much
decreased as shown in Fig. 4. The skeletal Ni(OH)2 volume was
maintained during the removal of H2O and the removed H2O left
pores in NiO, resulting in the increase of the pore volume
(0.44 cm3
gÀ1
of a-Ni(OH)2 and 0.64 cm3
gÀ1
of 250NiO) and the
surface area of the sphere particles. It has been known that
electrochemical capacitance of NiO samples is closely related to
the specific surface area and pore structure [2,25]. Therefore, the
dependency of the electrochemical capacitance on the surface area
and pore structure of the prepared mesoporous NiO samples is
investigated.
3.3. Electrochemical performance
3.3.1. CV measurements
Fig. 6 shows the cyclic voltammetry (CV) analysis of prepared
NiO electrodes. During the anodic and cathodic sweeps, reversible
redox peaks are clearly observed in the all CV curves. These redox
peaks are associated with the reversible reaction (NiO + OHÀ
$
NiOOH + e-) for pseudocapacitance of NiO [26]. In a redox reaction,
Ni2+
is transformed to Ni3+
during charging (oxidation) and during
discharging (reduction) returns back from Ni3+
to Ni2+
[27]. The CV
measurements with 250NiO were conducted at scan rates of 5,
10 and 25 mV sÀ1
as shown in Fig. 6(a). Cathodic sweeps are not in
complete symmetry with the corresponding anodic sweeps due to
some electrochemical irreversibility during the redox process [28].
During the faradaic redox reactions various thermodynamic and/or
ion transport barriers occurred, which cause the non-ideal
behavior of pseudocapacitance. Ohmic loss resulting from the
electrolyte diffusion within the porous electrode or electrolyte
concentration depletion can cause the irreversibility of the redox
reaction [29]. The irreversibility is also ascribed to the indication of
phase transformation. In batteries, the cyclic voltamograms are
characterized by faradaic redox peaks, often with rather large
potential separation (greater than 0.1 to 0.2 V) [14,15]. Table 2
shows oxidation potential (EO), reduction potential (ER) and
potential separation (EO-ER). The potential separation of 250NiO
is in the range from 0.18 V to 0.31 V, which indicates that 250NiO
can be treated with battery-type as well as pseudocapacitor. The
potential difference was lower with NiO sample at higher
calcination temperature, which indicates that the capacitance of
NiO at higher calcination temperature corresponds to higher
Fig. 4. TEM images taken (a) 25
C, (b) 200
C, (c) 250
C, (d) 300
C, (e) 400
C and (f) 500
C during in-situ TEM analysis of a-Ni(OH)2 samples from room temperature to
500
C.
148 S.A. Abbas, K.-D. Jung / Electrochimica Acta 193 (2016) 145–153

5. pseudocapacitive contributions from surface or near-surface
charge storage.
Fig. 6(b) shows the cyclic voltammetry (CV) measurements
with 300NiO, 350NiO, 400NiO and 500NiO at a scan rate of
10mVsÀ1
. All the curves in Fig. 6(b) also show clear redox peaks, but
the current density drops as the calcination temperature is
increased. This decrease in current (i) values can be associated
with a decrease in specific surface area and total pore volume as
shown in Table 1, causing fewer active sites to be available for redox
reaction. The specific capacitances from CV measurements can be
calculated using the following equation:
CCV ¼
1
v  m  DV
Z Vc
Va
idV ð2Þ
Where i is cathodic or anodic current in ampere; DV is the applied
potential window (Va-Vc); Va is anodic potential; Vc is cathodic
potential; v is scan rate in V/s; m is the mass of active material in
grams. Table 3 shows the specific capacitances of the prepared NiO
samples at scan rates of 5 mV sÀ1
, 10 mV sÀ1
and 25 mV sÀ1
. CCV
values are large at the low scan rate of 5 mVsÀ1
and low at
25 mV sÀ1
. The specific capacitance values of 250NiO at the scan
rate of 5 mV sÀ1
, 10 mV sÀ1
and 25 mV sÀ1
were 1200 F gÀ1
, 1100 F
gÀ1
and 806 F gÀ1
, respectively. The specific capacitance decreased
with an increase in scan rate. This decrease of specific capacitance
at the high scan rate has been explained by the ion exchange
mechanism [27]. The OHÀ1
ions have enough time to diffuse into
the NiO electrode at low scan rate, while they did not at high scan
rate. The less accessibility at high scan rate resulted in low
capacitance (Table 3).
Fig. 5. N2 adsorption-desorption isotherms of (a) 250NiO, (b) 300NiO, (c) 350NiO, (d) 400NiO, (e) 500NiO and (f) pore size distribution of all five samples.
Table 1
Specific surface area and pore volume of MMS of NiO.
250NiO 300NiO 350NiO 400NiO 500NiO
Specific Surface Area
(m2
gÀ1
)
295 196.85 133.41 91.79 10.51
Pore Volume
(cm3
gÀ1
)
0.64 0.41 0.30 0.23 0.11
S.A. Abbas, K.-D. Jung / Electrochimica Acta 193 (2016) 145–153 149

6. 3.3.2. Galvanostatic Measurements
Fig. 7(a) shows galvanostatic discharge curves of 250NiO at
discharge current density of 3 A gÀ1
, 5 AgÀ1
and 10 AgÀ1
in the
potential range of 0.0–0.5 V. The nonlinear discharge curves
indicate the faradaic redox behaviour of NiO [17], which was
ascribed to the pseudocapacitive characteristics of the electrode
[29]. The galvanostatic discharge curve from 0.0 V to 0.2 V was due
to the charge separation between the electrode/electrolyte
interface. The slope change of discharge curve from 0.2 to 0.5 V
was ascribed to the contribution of the redox reaction of NiO with
the electrolyte. The slope at ca. 0.25 V with 250NiO can be assigned
to a battery-type potential plateau [15]. Then, the sloping regions
at potentials above and below $0.25 V correspond to pseudocpa-
citive contributions from surface or near-surface charge storage.
From galvanostatic measurements, capacitance was estimated
by the following equation [30]:
CGM ¼ I Â Dtð Þ= DV Â mð Þ ð3Þ
Where I is the discharge current in amperes; V is the potential
change during the discharge in volts; m is the mass of active
material in grams; t is the time to discharge in seconds.
Fig. 6. Cyclic voltammetry of MMS of NiO: (a) CV curves of MMS of 250NiO at 5 mV sÀ1
, 10 mV sÀ1
and 25 mV sÀ1
. (b) CV curves of MMS of NiO calcined at different
temperatures at a scan rate of 10 mV sÀ1
.
Table 2
Oxidation potential (EO), reduction potential (ER) and (EO-ER) for MMS of NiO with
respect to scan rate.
Sample Scan Rate (mV sÀ1
) EO (V) ER (V) EO À ER (V)
NiO250 5 0.38 0.20 0.18
10 0.41 0.18 0.23
25 0.46 0.15 0.31
NiO300 5 0.34 0.24 0.10
10 0.37 0.23 0.14
25 0.41 0.2 0.21
NiO350 5 0.35 0.23 0.12
10 0.38 0.22 0.16
25 0.42 0.2 0.22
NiO400 5 0.35 0.28 0.07
10 0.36 0.28 0.08
25 0.38 0.28 0.10
NiO500 5 0.35 0.30 0.05
10 0.35 0.30 0.05
25 0.37 0.29 0.08
Table 3
Specific capacitance (CCV) of MMS of NiO from cyclic voltammetry.
250NiO 300NiO 350NiO 400NiO 500NiO
CCV at 5 mV sÀ1
(F gÀ1
) 1200 668 425 230 70
CCV at 10 mVsÀ1
(F gÀ1
) 1100 591 391 191 58
CCV at 25 mVsÀ1
(F gÀ1
) 806 553 348 144 50
Fig. 7. Galvanostatic discharge measurements of MMS of NiO: (a) galvanostatic discharge curves of MMS of 250NiO at 3 A gÀ1
, 5 A gÀ1
and 10 A gÀ1
, (b) galvanostatic discharge
curves of MMS of NiO at a current density of 3 AgÀ1
.
Table 4
Specific capacitance (CGM) of MMS of NiO from galvanostatic discharge measure-
ments.
250NiO 300NiO 350NiO 400NiO 500NiO
CGM at 3 A gÀ1
(F gÀ1
) 1560 816 656 342 53
CGM at 5 A gÀ1
(F gÀ1
) 1370 748 636 336 50
CGM at 10 A gÀ1
(FgÀ1
) 1140 656 592 324 44
150 S.A. Abbas, K.-D. Jung / Electrochimica Acta 193 (2016) 145–153

7. The specific capacitances of 250NiO are 1,560 F gÀ1,
1,370 F gÀ1
and 1,140 F gÀ1
at scan rates of 3, 5 and 10 A gÀ1
, respectively. The
specific capacitance of 250NiO in this report is the highest value of
any reported capacitance values of NiO as far as we know. Ccv of
250NiO at 25 mV sÀ1
(40 s) was 806 F gÀ1
and the equivalent CGM
(discharge time of 40 s) is estimated to be 1085 F gÀ1
by
extrapolation. Capacitance of CCV is not much different from
CGM. Galvanostatic charge-discharge measurements were also
conducted for 300NiO, 350NiO, 400NiO and 500NiO at the current
density of 3 A gÀ1
to investigate the effect of calcination on specific
capacitance as shown in Fig. 7(b). Table 4 shows the capacitances
of the NiO samples prepared at 250
C, 300
C, 350
C, 400
C and
500
C from galvanostatic measurements. The specific capacitance
value decreased with increasing calcination temperature, which
was due to a decrease in specific surface area and pore volume, and
consequently there are fewer sites of active material available for
redox reaction.
3.3.3. Electrochemical impedance measurements
Fig. 8 shows Nyquist plots, which are fitted by Z-view software
and the parameters are tabulated in Table 5. Equivalent circuit
diagram is shown in Fig. 8 (a). Nyquist plots consist of a semicircle
in the high frequency region while a straight line in low frequency
region. In the equivalent circuit diagram, Rs is the resistance of
electrolyte; Rct is the resistance of charge transfer; W is the
Warburg impedance; Cdl is the double layer capacitance; CPE is the
constant phase element of capacitance (CPE). Rct increased as the
calcination temperature for preparing NiO was increased as shown
in Fig. 8(b) and Table 5. Rct value increased from 0.2 V with
250NiO to 0.33 V with NiO500. Diffusion coefficient (D) of the
samples was calculated using the following relation [31]:
D ¼ R2
T2
=2A2
n4
F4
C2
s2
ð4Þ
Where R is the gas constant; T is the absolute temperature; A is the
surface area of the electrode; n is the number of electrons; C is the
concentration of (OH)À
ions; s is the Warburg factor. The Warburg
factor, s, can be obtained from the slope of Randle’s plot, the
relation of angular frequency vÀ1/2
and Zre. The values of the
calculated diffusion coefficient are 1.08 Â 10À8
cm2
sÀ1
, 1.28 Â 10À8
cm2
sÀ1
, 7.29 Â10À9
cm2
sÀ1
and 7.43 Â10À9
cm2
sÀ1
for 250NiO,
350NiO, 400NiO and 500NiO, respectively. The diffusion coeffi-
cient for NiO is closely related to pore structure. NiO sample with
high porosity and mesopores shows high diffusion coefficient.
Fig. 8(c) shows vertical lines in the Nyquist presentation. The
vertical line of a 90
phase angle indicates an ideal capacitor. The
deviation from the vertical line to phase angles of 90
can
indicate pseudocapacitive behavior. The phase angle is often
presented by a constant phase element (CPE) in Fig. 8(a):
ZCPE ¼
1
CPE À T Â ðjwÞCPEÀP
ð5Þ
CPE-P has been used as an indication of pseudocapacitive
behaviour. When CPE-P is 1, this is ideal capacitor and when CPE-P
is 0.5, this is semi-infinete diffusion [15]. In Table 5, all the CPE-P
values are close to 1.0, indicating that NiO has both the double layer
and pseudocapacitive feature. However, the low Cdl values of NiO
make the double layer contribution of NiO doubtful.
The specific capacitance (Cs) values of NiO electrodes were
calculated from the impedance measurements using the following
relation [32]:
CEIS ¼ 1= 2pfZ
00À Á
ð6Þ
Fig. 8. Electrochemical impedance spectroscopy of MMS of NiO: (a) equivalent circuit model for Nyquist plots. (b) Nyquist plots of NiO, (c) enlarged scale of Nyquist plots of
NiO with enlarged scale, (d) limit capacitance with respect to frequency.
Table 5
Values of parameters from fitted Nyquist plots using equivalent circuit model in
Fig. 8(a).
Sample Rs (V) Cdl (F gÀ1
) Rct (V) W0
W0-R W0-T
CPE
CPE-T CPE-P
250 NiO 1.30 0.084 0.20 0.62 0.25 0.65 0.90
350 NiO 1.31 0.077 0.26 0.57 0.4 0.28 0.94
400 NiO 1.26 0.095 0.27 3.77 2.08 0.15 0.95
500 NiO 1.29 0.062 0.33 1.37 0.14 0.080 0.97
S.A. Abbas, K.-D. Jung / Electrochimica Acta 193 (2016) 145–153 151

8. Where f is the frequency and Z00
is corresponding imaginary part.
The specific capacitance values of NiO from EIS are 371 F gÀ1
,
298 F gÀ1
, 169.8 F gÀ1
and 65.15 F gÀ1
at 0.1 Hz for 250NiO, 350NiO,
400NiO and 500NiO, respectively. The specific capacitance values
from EIS are very low as compared with those from cyclic
voltammetry and gavanostatic discharge measurements, which is
due to inhomogeneities at the atomic level and deeply trapped
counter ions during the impedance analysis [33]. Dependence of
limit capacitance with respect to frequency is shown in Fig. 8(d).
3.3.4. Stability
Turning to the electrochemical stability of the electrode, the
specific discharge capacitance of 250NiO was plotted against the
number of charge/discharge cycles for up to 1,000 cycles at the
high current density of 50 A gÀ1
as shown in Fig. 9(a). The first
10 cycles of the charge/discharge voltage profile are shown in
Fig. 9(b). The specific capacitance of the 250NiO electrode is
maintained at 680 F gÀ1
at the end of 1,000 cycles, which indicates
100% retention of its initial capacitance. This result suggests that
MMS of NiO have excellent stability during the insertion/extraction
of OHÀ
ions through the NiO lattice of the electrode. It is noticed
that during the first 100 cycles, the specific capacitance of MMS of
NiO increased from 650 F gÀ1
to 700 F gÀ1
which indicates the
activation process of NiO electroactive material. These galvano-
static testing results suggest that the prepared MMS of NiO have
high specific capacitance and very good capacitance retention as
well, indicating that NiO is a suitable candidate for supercapacitor
applications.
We also calculated the power density and the energy density,
which were 2.5 kW KgÀ1
and 59 Wh KgÀ1
at a current density of
10 A gÀ1
.
4. Conclusions
MMS of NiO with a high specific surface area could be prepared
from MMS of a-Ni(OH)2 with the specific surface area of 176 m2
/g.
The morphology and structure did not change during the
transformation from a-Ni(OH)2 to NiO by calcining the a-Ni(OH)2.
The MMS of NiO were obtained by calcination of a-Ni(OH)2 at
different temperatures and NiO calcined at 250
C (250NiO) has a
high specific surface area of 295 m2
gÀ1
. Cyclic voltammetry,
galvanostatic measurements and electrochemical impedance
analysis of NiO samples prepared with different calcination
temperatures were conducted to determine the electrochemical
pseudocapacitive behavior. The indication of pseudocapacitance of
NiO was more clarified for NiO sample with higher calcination
temperature. 250NiO has very high capacitance of 1,140 F gÀ1
at a
current density of 10 A gÀ1
which is ascribed to its high surface area
and mesopore structure. The power density and energy density of
250NiO were 2.5 kW KgÀ1
and 59WhKgÀ1
respectively at a current
density of 10 AgÀ1
.
Acknowledgements
This work was financially supported through Korea Institute of
Science and Technology (KIST) and the Korea CCS RD Center
(2014M1A8A1049293) by Ministry of Science, ICT Future
Planning.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.electacta.2016.02.
054.
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