Facile approach to prepare hollow core-shell NiO microspherers for supercapacitor electrodes

Article abstract of DOI:10.1016/j.jssc.2013.04.009

A facile lamellar template method (see image) has been developed for the preparation of uniform hollow core-shell structure NiO (HCS-NiO) with a nanoarchitectured wall structure. The prepared NiO was found to be highly crystalline in uniform microstructures with high specific surface area and pore volume. The results indicated that ethanol interacted with trisodium citrate played an important role for the formation of hollow core-shell spheres. On the basis of the analysis of the composition and the morphology, a possible formation mechanism was investigated. NiO microspheres with hollow coreshell showed excellent capacitive properties. The exceptional cyclic, structural and electrochemical stability with ～95% coulombic efficiency, and very low ESR value from impedance measurements promised good utility value of hollow core-shell NiO material in fabricating a wide range of highperformance electrochemical supercapacitors.

Full text of DOI:10.1016/j.jssc.2013.04.009

Journal of Solid State Chemistry 203 (2013) 60–67
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Facile approach to prepare hollow core–shell NiO microspherers
for supercapacitor electrodes
a,b
b
a
a,c,
n
a
a
Dandan Han , Pengcheng Xu , Xiaoyan Jing , Jun Wang
Milin Zhang
a
, Dalei Song , Jingyuan Liu ,
a
Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, Harbin Engineering University, Harbin 150001, PR China
College of Chemistry and Pharmaceutical Engineering, Jilin Institute of Chemical Technology, Jilin 132022, PR China
Institute of Advanced Marine Materials, Harbin Engineering University, Harbin 150001, PR China
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 February 2013
Received in revised form
A facile lamellar template method (see image) has been developed for the preparation of uniform hollow
core–shell structure NiO (HCS–NiO) with a nanoarchitectured wall structure. The prepared NiO was
found to be highly crystalline in uniform microstructures with high specific surface area and pore
volume. The results indicated that ethanol interacted with trisodium citrate played an important role for
the formation of hollow core–shell spheres. On the basis of the analysis of the composition and the
morphology, a possible formation mechanism was investigated. NiO microspheres with hollow core–
shell showed excellent capacitive properties. The exceptional cyclic, structural and electrochemical
stability with ∼95% coulombic efficiency, and very low ESR value from impedance measurements
promised good utility value of hollow core–shell NiO material in fabricating a wide range of high-
performance electrochemical supercapacitors.
5
April 2013
Accepted 10 April 2013
Available online 17 April 2013
Keywords:
Nickel oxide
Hollow core–shell structure
Electrochemical property
Supercapacitor
&
2013 Elsevier Inc. All rights reserved.
1
. Introduction
better materials for supercapactor applications due to their higher
surface area, good redox property, controllable size and shape, and
structural identities [12–14]. Pseudocapacity behavior in these
materials originates from fast and reversible surface or near-
surface phenomena involved during the faradic reactions. More-
over, electrode structure, conductivity, and crystallinity are three
important parameters in electrode design for achieving high-
performance pseudocapacitors. So, designed electrode with tailored
electrode structure with high ion accessibility is vital to achieve
maximum performance [15]. Various morphologies of NiO show
clear difference in their electrochemical performance due to dis-
similarity in the material electrolyte interface properties and ion
transfer rate during the charge storage process [12,16–18]. Although
various structures/morphologies such as nanoparticles, fibers, and
wires have been demonstrated to be quite feasible for high-
performance and stable electrodes [19], hollow core–shell structure
materials with high surface area and narrow electrode pore size
distribution are shown to be electrochemically preferable because
of their uniform pores and ordered mesoporous structure [20].
In this context, hollow micro/nanostructure metal oxides with
highly synergic nanometer-sized building blocks have been widely
reported in the literature for high performance electrochemical
applications and energy storage [21,22]. Moreover, most of the
synthesis methods are explored including hydrothermal mediated
Development of sustainable and renewable energy future has
been the most important challenge for scientists and engineers,
internationally, for addressing the rapidly depleting fossil fuels [1].
In this context, supercapacitors comprising a characteristic combi-
nation of high power and reasonable energy density with faster
response time and near-infinite life cycle in a wide operational
temperature range, can complement other energy storage devices
like conventional capacitors, batteries and fuel cells [2–5]. Among
the array of electrode materials for supercapacitor applications,
transition metal oxides are widely studied due to variable oxidation
states of metal ions which facilitate redox transitions and higher
charge storage within the potential range of water decomposition
2 2
[6]. Various forms of RuO and hydrous RuO are found to be
remarkable in supercapactor applications due to their ultrahigh
theoretical capacitance (∼2000 F g− ) in a wider applied potential
window of ∼1.4 V, higher room temperature electrical conductivity
and significant chemical and thermodynamic stability under the
usual electrochemical environment [7–11]. However, toxicity and
expensiveness are the major short comings of RuO for its wide-
2
spread commercial applications. Among the other transition-metal
oxides, different crystallographic forms of NiO are identified as
1
[
23], Localized Ostwald ripening [24], removable templates [25],
n
Corresponding author at. Harbin Engineering University, College of Material
and sacrificial carbon core generated by hydrothermal treatment of
aqueous solutions of glucose and polysaccharides [26]. Recently,
Lou and co-workers [27] have successfully prepared double-shelled
Science and Chemical Engineering, Nantong Street 145, Harbin 150001, PR China.
Fax: +86 451 8253 3026.
E-mail address: zhqw1888@sohu.com (J. Wang).
0
022-4596/$ - see front matter & 2013 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jssc.2013.04.009

D. Han et al. / Journal of Solid State Chemistry 203 (2013) 60–67
61
SnO
silica nanotemplates. Zhou’ group [28] synthesized hollow core-
shell TiO microspheres by a hydrothermal method using polyethy-
lene glycol (PEG, MW 2000) as the soft template. In our previous
paper, we demonstrated La -doped NiO microspheres with porous
structure using colloidal carbon spheres as hard template via a
hydrothermal method followed by calcination process [29]. How-
ever, it still remains a major challenge to develop facile one-step
methods for the preparation of metal oxide microspheres with
hollow core–shell structures and their application for ECs.
2
hollow colloids by shell-by-shell hydrothermal deposition on
spectroscopy (EIS) measurements were carried out using conven-
tional three-electrode configuration on a CHI 660 D electrochemical
workstation. The Ni foam coated with NiO was used as the working
2
2
electrode, and platinum foil (1 cm ) and a saturated calomel elec-
3+
trode (SCE) were used as the counter and reference electrodes. All
the experiments were carried out using freshly prepared 5.0 M
aqueous KOH electrolyte in double distilled water. The EIS measure-
ments were carried out in the frequency range from 100 kHz to
0.05 Hz at open circuit potential with an ac perturbation of 5 mV.
In this study, we report a novel approach for preparing high-
uniform hollow core–shell NiO microspheres under non-hydrothermal
and a facile lamellar template model. This simple method produces
NiO sample with uniform surface morphology and ordered mesopor-
ous structure. The cyclic voltammetry, chronopotentiometry and
impendance spectroscopic measurements carry out on the NiO sample
show improved electrochemical pseudocapacitance and good cyclicity.
3
. Results and discussion
3.1. Material characterization
The XRD pattern of black powder obtained after the calcination
process was shown in Fig. 1. The diffraction peaks are indexed at
θ¼37.21, 43.21, 62.81, 75.41, and 79.41, which correspond to the
1 1 0), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) reflections of NiO
2
(
2
. Experimental section
structures (JCPDS card no. 47-1049). A temperature of 300 1C is
sufficient to drive to the conversion to NiO, and no peaks of
impurities were observed.
2.1. Synthesis procedure
To investigate the morphologies of the obtained NiO, the
FESEM images of the hollow core-shell NiO were shown in
Fig. 2a–c at different magnifications. It is clear from Fig. 2a that
the particles exhibit a uniform and rather spherical shape with
diameters in the range of 1–1.5 μm. At a high magnification
All reagents were used as received without any further pur-
ification. In a typical alkalization process, 45 mmol nickel sulfate
Ni(SO O) was dissolved in 150 mL distilled water. 45 mmol
ꢀ 6H
hexamethylenetetramine (C ), 12.24 mmol trisodium citrate
dihydrate (C Na O) and 150 mL ethanol was then added
ꢀ 2H
to the Ni(SO solution under stirring. The mixture was then
(
4
)
2
2
6 12 4
H N
6
H
5
3
O
7
2
(Fig. 2b), there is a mouth-like opening on some of the spheres.
4 2
)
Through the opening, the inner hollow structure and inside
smaller NiO sphere can be clearly seen, the zoomed-in view of
the surface (Fig. 2c) confirms this. Moreover, this HCS–NiO with
hollow core–shell structure reveals most vividly by a sphere with
partially broken shell (inset in Fig. 1a). As can be seen in Fig. 2c, the
thickness of the outer NiO shell is about 100 nm. Moreover, there
existed a large number of mesopores on the shell surface (the
inference can be verified by TEM observation). 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. As shown in Fig. 2d, the spectra of energy dispersive
spectroscopy (EDS) demonstrated that the core and shell of NiO
spheres all consisted of carbon, nickel and oxygen, which is in
agreement with those observed in the XRD pattern.
heated at 90 1C for 2 h with the sealed flask. After the solution was
cooled naturally to room temperature, the green precipitate was
separated, washed with double distilled water several times, and
dried in a vacuum at 60 1C for 12 h. Finally, the hollow core–shell
NiO (HCS–NiO) was obtained by calcination in flowing air at a
heating ramp of 5 1C min− from room temperature to 300 1C and
kept at this temperature for 3 h.
1
2.2. Characterization
X-ray diffraction (XRD) measurement was examined on a Rigaku
D/max TTR-III diffractometer using CuKα radiation (λ¼0.15405 nm).
Thermogravimetric analyses (TG) were performed at a heating rate of
1
1
0 1C min− in air on a thermal analyzer (TA SDT-Q600, American).
Fourier transform infrared spectroscopy (FT-IR) was measured on a
Perkin-Elmer 580B IR spectrophotometer. The morphologies of the
samples were inspected on a scanning electron microscope (SEM,
The interesting nanostructure of hollow core–shell mesoporous
spheres was further corroborated by transmission electron micro-
scopy (TEM). The low-magnification TEM image in Fig. 3a shows
JSM-6480A, Japan Electronics). Transmission electron microscopy
2
(
TEM) micrographs were performed on a FEI Tecnai G 20S-TWIN
transmission electron microscope with a field emission gun operat-
ing at 200 kV. N adsorption/desorption was performed on a Micro-
2
meritics ASAP 2010 instrument. The specific surface area was
calculated by the Brunauer-Emmett-Teller (BET) method.
2.3. Preparation of electrodes and electrochemical characterization
To evaluate the electrochemical properties of the NiO microstruc-
tures, working electrodes were prepared as follows. The as-prepared
materials, graphite, acetylene black, and poly (tetrafluoroethylene)
(PTFE) were mixed in a mass ratio of 75:10:10:5 and dispersed in
ethanol to produce a homogeneous paste. Graphite, acetylene black
and PTFE were used as the conductive agent and binder, respectively.
2
Then the mixture was coated onto the nickel foam substrate (1 cm )
with a spatula. Finally, the fabricated electrodes were dried at 60 1C
for 1 day in a vacuum oven. The loading level of the active material
on the current collector is typically 0.01 g cm− . Cyclic voltammetry
1
0
20
30
40
50
60
70
80
90
2Theta (degree)
2
(CV), chronopotentiometry (CP), and electrochemical impedance
Fig. 1. XRD patterns of the hollow core–shell NiO spheres.

62
D. Han et al. / Journal of Solid State Chemistry 203 (2013) 60–67
0
03
8
7
6
5
4
3
2
1
10
20
30
40
50
60
70
80
Ni
O
Ni
C
9
0
Ni
0
0
.00 0.90 1.80 2.70 3.60 4.50 5.40 6.30 7.20 8.10 9.00
keV
Fig. 2. (a–c) FESEM images of the hollow core–shell NiO spheres at different magnifications and (d) EDS spectra of the hollow core–shell NiO spheres.
several core–shell spheres with clearly visible hollow interiors. The
TEM image of a single sphere (Fig. 3b) shows that there is a
distinguishable sphere structure inside the second sphere shell,
which is well in agreement with the FESEM pattern in Fig. 2a. The
thickness of the inside core is about 300 nm and the measured outer
shell thickness of 100 nm is consistent with the FESEM determina-
tion. Moreover, the walls of the hollow sphere are highly porous
aggregated with nanoparticles (Fig. 3c). The HRTEM image exhibits
clear lattice fringes of hollow core–shell NiO (Fig. 3d). Homogeneous
and parallel lattice fringes imply the single crystalline behavior of the
prepared samples. The distance between the adjacent lattice fringes
is 0.24 nm, which is calculated from the FFT pattern (inset) corre-
sponding to the HRTEM image and can be ascribed to the interplanar
distance of NiO (1 1 1) plane, agreeing approximately with the (1 1 1)
d spacing of the literature value (JCPDS No. 47-1049).
groups. Ethanol with alkyl chains could be inserted into the layered
arrangements with the possible interaction: an ion–dipole interac-
tion between the citrate anion and the hydroxyl group of the
ethanol [30]. This effect is similar to extra- and intra-lamellar
template model of layered zinc hydroxide with SDS/long alkyl chain
alcohols as template [31–33]. In conclusion, the lamellar template is
the layered collective arrangement of citrate to make a unique core–
shell structure. Also, through the alkalization process, citrate anions
in the lamellar template are intercalated into nickel hydroxide
layers and finally became a component part of the precursors. After
final calcinations, the template is removal and the HCS–NiO
obtained.
In order to verify the model described above, the intermediate
products with different reaction time were characterized by TEM
and XRD (Fig. 6). At the early reaction stage (1 h), α-Ni(OH)
nanostructure could be synthesized due to the reaction between
2
To have a better characterization of the porous nature of the
2
+
HCS–NiO samples obtained after firing at 300 1C, N
2
adsorption/
HMT and Ni ions which is chelated by the citrate anions layers,
and the ions are reprecipitated onto the high-energy crystal-
lographic facet of the particles, leading to the growth of large
particles. Simultaneously, the layered citrate arrangement with the
desorption isotherms are collected as shown in Fig. 4. The BJH pore
size distribution obtained from the adsorption–desorption iso-
therms indicates the highly mesoporous nature of the synthesized
NiO spheres. The NiO spheres show a nitrogen adsorbed volume of
2
alkyl chain enthanol around the α-Ni(OH) nanospheres, formed
P/P
0
¼0.2 and an average pore size of 4 nm. The BET surface area
the core–shell intermediate products. After calcination at 300 1C,
the primary template is removal and the porous residual layer
with NiO nanoparticles is observed (Fig. 6a). When the reaction is
prolonged to 1.5 h, the layered collective arrangement of citrate
with the alkyl chain continue to pack and the immediate surface
layer forms, resulting in a homogeneous multilayered core–shell
structure (two-layers) with the diameter about 0.6–1 μm (Fig. 6b).
Finally, when the dwell time is processed for 2 h, the amount of α-
and the total pore volume of the sample are found to be
2
−1
3
−1
196 m g and 0.2 cm g , respectively. The high BET surface area
and large porosity illustrate the mesoporous structure of the
sample which should be favorable to improve the capacitance
behavior of an electroactive material due to unhindered diffusion
and accession of electrolyte ions into the inner space/matrix [13].
3
.2. Growth mechanism of NiO microstructures
Ni(OH)
tion time, resulting in the increase of the size and shell thickness
of the α-Ni(OH) layer and much more thinner template layers.
2
around the microspheres increased along with the reac-
The synthetic mechanism of hollow core–shell NiO microspheres
2
could be explained by a lamellar template model, as shown in Fig. 5.
The lamellar template consists of a few of citrate anion bilayers in
which binding to the Ni surfaces through the –COO– and –OH
Therefore, according to the diameter of the sphere, it is thus
reasonable to observe a single layer core–shell structure in the
TEM (Fig. 6c).
2+

D. Han et al. / Journal of Solid State Chemistry 203 (2013) 60–67
63
Fig. 3. TEM images of hollow core–shell NiO spheres: (a) and (b) overview of product, (c) edge of a NiO shell, and (d) HRTEM image of the shell and the insert is
corresponding FFT pattern.
1
1
1
40
20
00
8
6
4
2
0
0
0
0
Adsorption
Desorption
Fig. 5. Schematic illustration of the theoretical model for lamellar template and the
morphology evolution of the hollow core–shell NiO.
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0)
The CV responses of the HCS–NiO samples carried out at different
Fig. 4. N
NiO hollow core–shell spheres.
2
adsorption/desoprtion isotherms and pore size distributions (insets) of
−1
scan rates (5–50 mV s ) in a fixed potential range of 0–0.4 V in
aqueous 5 M KOH electrolyte are shown in Fig. 7a. The HCS–NiO
electrodes were subjected to one time stabilization/activation by
repeated CV cycles (20 numbers) at sweep rates of 5 mV s− in the
similar potential range, before performing the CV experiments for
accessing the electrochemical performance. The CV patterns of both
the samples show identical shapes, which are typical of pseudoca-
pacitance behavior due to occurrence of Faradaic redox reactions in
the system. The respective anodic and cathodic peaks at the positive
1
3.3. Electrochemical behavior
To explore the potential applications in electrochemical energy
storage, the hollow core–shell NiO samples were used to make
supercapacitor electrodes and characterized with cyclic voltammetry,
galvanostatic charge/discharge and impedance measurements.

64
D. Han et al. / Journal of Solid State Chemistry 203 (2013) 60–67
Fig. 6. TEM images of the hollow core–shell NiO microspheres calcined at 300 1C and the precursors prepared for various reaction times (a) 1 h, (b) 1.5 h, (c) 2 h.
1
0
8
6
4
2
0
a
b
a
b
After 500th charge/discharge cycles
Before 500th charge/discharge cycles
1
0
5
0
5
a
b
c
d
5
2
0mV/s
0mV/s
a
Cs: 555 Fg-
Cs: 455 Fg
1
-
1
10mV/s
5mV/s
b
d
c
b
-
-2
a
-
-
4
6
-
10
0.0
0.1
0.2
0.3
0.4
0.0
0.1
0.2
0.3
0.4
Potential(V vs.SCE)
Potential(V vs.SCE)
5
00
110
d c
b
a
a
0.5A/g
1A/g
2A/g
4A/g
0
.4
.3
.2
.1
.0
b
c
d
450
400
1
9
8
00
0
0
3
3
2
2
1
50
00
50
00
50
0
0
0
0
70
0
100 200 300 400 500 600 700 800 900
Time (s)
0
1
2
3
4
Current density (A/g)
Fig. 7. (a) Cyclic voltammetry curves of hollow core–shell NiO at scan rate of 5, 10, 20 and 50 mV s−1; (b) comparison of cyclic voltammograms at a scan rate of 10 mV s
−1
−
1
before and after 500 charge–discharge cycles; (c) the charge/discharge curves of hollow core–shell NiO between voltage limits of 0 to 0.4 V at rates varied from 0.5 A g to
−1
4
A g and (d) current density dependent specific capacitance and capacitance retention plots of the samples.
and negative current density correspond to following charge-
discharge phenomena [16].
using the equation [34,35]:
Z
vc
C
s
¼ ð1=vwðΔVÞÞ
idV
ð2Þ
−
−
va
NiO þ zOH ↔zNiOOH þ ð1−zÞNiO þ ze
ð1Þ
where ΔV (V) is the applied potential window (V
a
to V
c
) and
The anodic peak at roughly 0.28 V is related to the oxidation of
NiO to NiOOH, and the cathodic peak at about 0.09 V is due to its
reverse process similar to previous reports. Therefore, the two
redox reaction peaks in the CV curves are responsible for the
pseudocapacitance of the NiO microstructures. As the scan rate
increased, the potential of the anodic and cathodic peaks shifted in
more positive and negative directions, due to the limitation of the
ion diffusion rate to satisfy electronic neutralization during the
redox reaction. From the CV measurements before and after long
life cycling, the specific capacitance (Cs, F g− ) values are estimated
w (g) is the weight of the active material, At potential scan rates of
−
1
10 mV s , the specific capacitance values for HCS–NiO sample
calculated before charge/discharge cycles is found to be 455 F g−
(Fig. 7b). Similarly, at the same scan rates the specific capacitance
values for the sample calculated after charge/discharge cycles is
555 F g . Clear enhancement in the specific capacitance value is
observed which is possibly due to the structural activation and
pore opening of the sample during the charge/discharge process. It
is known that the characteristics of redox reactions are strongly
dependent on the shape and dimensionality of the electrode
1
−
1
1

D. Han et al. / Journal of Solid State Chemistry 203 (2013) 60–67
65
material at the nanoscale level [10,36]. Two main factors, (1) higher
specific area and (2) suitable porosity for easy insertion/de-
insertion of ions into/from the electrode matrix, mostly contribute
to the number, efficiency and ease of redox reactions. The HCS–NiO
sample consists of uniform multilayer shell–core structure with
porosity shell as compared to the lowly porous bulk spheres
sample which has higher efficiency specific area and promotes
higher number of redox reactions resulting in higher specific
capacitance. However, the peak potentials (ΔEOR) between the
oxidation and reduction became slightly larger after the 500th
charge/discharge cycles. It is therefore speculated that the
declined reversibility maybe caused by the loss of adhesion of
some active materials or composite crack with the current collec-
tor during the long term charge/discharge cycling.
1
1
.2
.0
after cycling
before cycling
0.8
0.6
0.4
0
.20
0.15
0
0
0
.10
.05
.00
0
0
.2
.0
0
.25
0.30
0.35
0.40
Z'(Ω)
High rate discharge capability is crucial for an electrode
material to be used efficiently for supercapacitor applications.
The hollow core and shell NiO has been investigated for the first
charge–discharge cycle study at current rates of 0.5, 1, 2, and
0.3
0.4
0.5
Z'(Ω)
0.6
0.7
0.8
Fig. 8. Nyquist plots of experimental impedence data (scattering dot) and fitting
results (solid line) for hollow core–shell NiO before and after 500 charge–discharge
cycles. Inset shows the electrical equivalent circuit used for fitting impedance
spectra.
−
1
4
A g within the potential range of 0 to 0.4 V, and the results are
shown in Fig. 7c. The nonlinearity in the discharge curves, unlike
battery [37] and double-layer capacitors [38], shows representa-
tive pseudocapacitance behavior of NiO resulting from the elec-
trochemical adsorption/desorption or redox reaction at the
electrode electrolyte interface. The specific capacitance values
have been calculated from the applied current density (i), mass
of the NiO sample (m), discharge time (Δt), and operating potential
(Z′′) and real part (Z′) of impedance. Fig. 8 shows Nyquist plots of
the capacitive electrode before and after 500 cycles at an applied
potential of 5 mV (vs. Ag/AgCl). The impedance characteristics
were analyzed by the CNLS fitting method based on a Randles
equivalent circuit, as depicted insert Fig. 7. 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. 7a,
(
ΔV) using the following equation [39]:
¼ i=mðΔV=ΔtÞ
The specific capacitance values are found to be 448, 402, 380,
C
s
ð3Þ
and 342 F g− at specific current densities of 0.5, 1, 2, and 4 A g−1
1
,
respectively. To our knowledge, the specific capacitance values
from CV and charge/discharge measurements reported here are
better than the reported NiO materials prepared by tedious
experimental procedures with various morphologies [34,40–44].
The easy accessibility of ions through the pores of the material is
very important for efficient faradaic reactions [45]. Hence pore size
is an important parameter for higher capacitance behavior of the
electrode materials. Simon and Gogotsi recently reported [9] that a
pore size distribution in the range of 2–5 nm is favorable to
improve the capacitance behavior of an electro-active material.
In this study, the hierarchically porous NiO exhibits a pore size
distribution of ∼4.0 nm (Fig. 4), which satisfies the value of the
required pore size for efficient faradaic reactions. Therefore, the
where R
s
is the solution resistance of the electrochemical system,
Cdl is a double layer capacitor, Cps is a Faradaic pseudocapacitor, W
is Warburg impedance, and Rct is Faradaic interfacial charge
transfer resistance. It is known that the equivalent series resis-
tance (ESR) is a combination of ionic resistance of electrolyte,
intrinsic resistance of active materials, and contact resistance at
the active material/current collector interface, which is an impor-
tant parameter of supercapacitor measured at the high frequency
region, where the impedance curve intercepts on the real axis. The
higher ESR value indicates the lower electrical conductivity of the
sample and vice versa [49]. It can be seen that the ESR values in
the Nyquist plot of the NiO electrode are very small and remain
almost the same before and after long charge–discharge process.
In addition, after 500 cycles the slope at the low-frequency region
is still close to 901, and there is near absence of semicircles at the
high-frequency region. This indicates that the capacitive behavior
of hollow core–shell NiO is almost identical and suggests no
appreciable change in structural and electrochemical characteris-
tics of the hollow core–shell NiO during high rate cycling. The
overall impedance characteristics suggest very good accessibility
of the OH ions through the layered structure and explicit
contribution of the pseudocapacitance to energy storage perfor-
mance of hollow core–shell NiO material.
The coulombic efficiency ηis a measure of the efficacy with which
charge is transferred in a system facilitating an electrochemical
reaction. High coulombic efficiency suggests that the process requires
lower energy to complete the reaction making the process more
feasible. Fig. 9a shows the coulombic efficiency of the porous NiO
electrode during 500 repetitive charge–discharge cycles. This can be
analyzed using the equation:
−
OH ions easily diffuse through the fibrous structure of NiO
microspheres (Fig. 3) and access the inner space for more faradaic
reactions to occur at lower current density. This process contri-
butes to the enhancement of the capacitance value of NiO with
hollow core–shell structure. It is observed that the charging and
discharging curves at all current densities are not completely
symmetrical and more pronounced at higher applied current
densities due to relative increase in kinetic irreversibility of the
−
−
OH ions on the NiO surface [46,47].
Generally, an increase in the discharge current leads to a large
voltage (iR) drop which results in decrease in capacitance value
[
48]. The specific capacitance value of the hollow core–shell NiO
−
1
shows∼76% retention even at 4 A g as compared to the value at
0
.5 Ag− (Fig. 7c). This suggests prominent capacitive performance
1
of hollow core–shell NiO under very high current density opera-
tion conditions which is significant in practical supercapacitor
applications. The considerable enhancement of the supercapaci-
tive performance is essentially attributable to better utilization of
electroactive surface of multilayer core–shell NiO material.
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
η ¼ t
D
=t
C
ꢁ 100
and t
ð4Þ
where t
D
C
represent galvanostatic discharging and charging time,
respectively [50]. It is found that the coulombic efficiency reaches
nearly 95% after 500 cycles. This is again related clearly to the increase

66
D. Han et al. / Journal of Solid State Chemistry 203 (2013) 60–67
current density due to the facile electrolyte penetration, lowered
ion diffusion resistance and better utilization of the electroactive
surface for Faradaic reactions. The excellent cyclic, structural, and
electrochemical stability at higher current rate with ∼95% cou-
lombic efficiency may be very useful for possible utilization of this
hollow core–shell material in fabricating a wide range of electro-
chemical supercapacitors.
1
00
8
6
4
2
0
0
0
0
0
Acknowledgments
This work was supported by Special Innovation Talents of
Harbin Science and Technology (2011RFQXG016), Fundamental
Research Funds of the Central University (HEUCFZ), Key Program
of the Natural Science Foundation of Heilongjiang Province
0
100
200
Cycle number
300
400
500
(ZD201219), Program of International S&T Cooperation special
project (S2013ZR0649), Special Innovation Talents of Harbin
Science and Technology (2012RFXXG104). The 12th five-year plan
in Science and Technology of the Education Department of Jilin
Province (2012290) and “ChunMiao” talents special project in
Science and Technology of the Education Department of Jilin
Province.
1
1
1
8
6
40
20
00
0
7
50
00
50
00
50
6
4
3
1
References
[1] D.R. Rolison, J.W. Long, J.C. Lytle, A.E. Fischer, C.P. Rhodes, T.M. McEvoy, M.
E. Bourg, A.M. Lubers, Chem. Soc. Rev. 38 (2009) 226–252.
[
2] P.J. Hall, M. Mirzaeian, S.I. Fletcher, F.B. Sillars, A.J.R. Rennie, G.O. Shitta Bey,
G. Wilson, A. Cruden, R. Carter, Energy Environ. Sci. 3 (2010) 1238–1251.
3] R. Kötz, M. Carlen, Electrochim. Acta 45 (2000) 2483–2498.
4] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and
Technological Applications, Kluwer Academic/Plenum Publishers, New York,
0
[
[
40
1
999.
5] X.H. Xia, J.P. Tu, Y.Q. Zhang, X.L. Wang, C.D. Gu, X.B. Zhao, H.J. Fan, ACS Nano 6
2012) 5531–5538.
6] P.J. Hall, M.S. Mirzaeian, I. Fletcher, F.B. Sillars, A.J.R. Rennie, G.O. Bey,
G. Wilson, A. Cruden, R. Carter, Energy Environ. Sci. 3 (2010) 1238–1241.
7] C.C. Hu, K.H. Chang, M.C. Lin, Y.T. Wu, Nano Lett. 6 (2006) 2690–2695.
0
100
200
Cycle number
300
400
500
[
[
[
(
Fig. 9. (a) The 20 cyclic charge–discharge curves of the NiO electrode and (b) the
specific capacitance and coulombic efficiency as a function of cycle number at a
current density of 2 A g−1 in 5.0 M KOH solution.
[8] R. Liu, S. Cho, S.B. Lee, Nanotechnology 19 (2008) 215710–215714.
9] P. Simon, Y. Gogotsi, Nat. Mater. 7 (2008) 845–854.
[
[
[
[
10] W. Deng, X. Ji, Q. Chen, C.E. Banks, RSC Adv. 1 (2011) 1171–1178.
11] G. Wang, L. Zhang, J. Zhang, Chem. Soc. Rev. 41 (2012) 797–828.
12] X. Zhang, W. Shi, J. Zhu, W. Zhao, J. Ma, S. Mhaisalkar, T.L. Maria, Y. Yang,
H. Zhang, H.H. Hng, Q. Yan, Nano Res 3 (2010) 643–652.
in reversibility of charge–discharge cycles due to pore opening and
activation of the hierarchically porous NiO electrode material during
the faradaic reaction in galvanostatic charge–discharge process.
Long cyclic stability is one of the most important requirements
for practical competence of supercapacitor devices. The cyclic-
specific capacitance performance corresponding capacitance
retention plots of HCS–NiO sample for 500 cycles at current
density of 2.0 A g− is presented in Fig. 9b. It can be observed that
the sample exhibits striking cyclic stability. After surface activation
and minor pore opening (150 cycles), the capacitance gradually
[13] S.K. Meher, P. Justin, G. Ranga Rao, Electrochim. Acta 55 (2010) 8388–8396.
[14] P. Justin, S.K. Meher, G. Ranga Rao, J. Phys. Chem. C 114 (2010) 5203–5210.
[15] Q. Lu, J.G. Chen, J.Q. Xiao, Angewandte Chemie 52 (7) (1882-1889).
[16] S.K. Meher, P. Justin, G. Ranga Rao, Nanoscale 3 (2011) 683–692.
[17] K. Juodkazis, J. Juodkazyte, R. Vilkauskaite, V. Jasulaitiene, J. Solid State
Electrochem. 11 (2008) 1469–1479.
[18] J.H. Zhu, J. Jiang, J.P. Liu, R.M. Ding, H. Ding, Y.M. Feng, G.M. Wei, X.T. Huang, J.
1
Solid State Chem. 184 (2011) 578–583.
[19] Y. Gao, S. Chen, D. Cao, G. Wang, J. Yin, J. Power Sources 195 (2010) 1757–1760.
[
20] X.H. Huang, J.P. Tu, C.Q. Zhang, F. Zhou, Electrochim. Acta 55 (2010)
981–8985.
21] M.M. Titirici, M. Antonietti, A. Thomas, Chem. Mater. 18 (2006) 3808–3812.
8
increases, after 500 cycles, the pseudocapacitance reaches
[
1
4
55 F g− , 20% increase of the first cycle value, indicating its
[22] L. Wang, Y.J. Hao, Y. Zhao, Q.Y. Lai, X.Y. Xu, J. Solid State Chem. 183 (2010)
576–2581.
23] P. Yu, X. Zhang, D.L. Wang, L. Wang, Y.W. Ma, Cryst. Growth Des. 9 (2009)
28–533.
[24] W.S. Wang, L. Zhen, C.Y. Xu, B.Y. Zhang, S.Z. Wen, J. Phys. Chem. B 110 (2006)
3154–23158.
25] X.Y. Han, F.Q. Jiang, X.C. Zhong, P.A. Xin, L.C. Yu, J. Phys. Chem. C 111 (2007)
4067–14071.
[26] D. Deng, J.Y. Lee, Chem. Mater. 20 (2008) 1841–1846.
2
excellent pseudocapacitance retention capability. The obtained
capacitance value of the hollow core–shell material at current
density 2.0 A g− during long cycling is due to high-order distribu-
tion of the pore on the shell or efficiency specific area of the
multilayer shell which results in higher diffusion rates and
significantly faster electronic kinetics.
[
5
1
2
[
1
[
[
[
27] X.W. Lou, C. Yuan, L.A. Archer, Small 3 (2007) 261–265.
28] Y.M. Cui, L. Liu, B. Li, X.F. Zhou, N.P. Xu, J. Phys. Chem. C 114 (2010) 2434–2439.
29] D.D. Han, X.Y. Jing, J. Wang, P.P. Yang, D.L. Song, J.Y. Liu, J. Electroanal. Chem.
4
. Conclusion
682 (2012) 37–44.
[
[
[
[
30] J.W. Lee, W.C. Choi, J.D. Kim, Cryst. Eng. Commun. 12 (2010) 3249–3254.
31] S.G. Oh, D.O. Shah, Langmuir 7 (1991) 1316–1318.
32] A. Patist, T. Axelberd, D.O. Shah, J. Colloid Interface Sci. 208 (1998) 259–265.
33] J.W. Lee, T. Ahn, J.H. Kim, J.M. Ko, J.D. Kim, Electrochim. Acta 56 (2011)
4849–4857.
In this work, we have demonstrated the characteristic of better
surface properties like uniform surface morphology, high surface
area and pore volume of hollow core–shell NiO samples synthe-
sized with a new preparation method in an ethanol/trisodium
citrate aqueous mixture. The electrochemical studies show the
high capacitance reported so far for the single phase NiO at high
[34] V. Srinivasan, J.W. Weidner, J. Power Sources 108 (2002) 15–20.
[35] S.K. Meher, G. Ranga Rao, J. Phys. Chem. C 115 (2011) 15646–15654.
[36] O. Ghodbane, J.L. Pascal, ACS Appl. Mater. Interfaces 1 (2009) 1130–1139.
[37] J.Y. Shi, C.W. Yi, K. Kim, J. Power Sources 195 (2010) 6860–6866.

D. Han et al. / Journal of Solid State Chemistry 203 (2013) 60–67
67
[38] W. Xiong, M.X. Liu, L.H. Gan, Y.K. Lv, Y. Li, L. Yang, Z.J. Xu, Z.X. Hao, H.L. Liu, L.
[44] K. Byoungwoo, C. Gerbrand, Nature 458 (2009) 190–193.
W. Chen, J. Power Sources 196 (2011) 10461–10464.
39] Y.G. Wang, Y.Y. Xia, Electrochim. Acta 51 (2006) 3223–3227.
40] D.C. Wang, W.B. Ni, H. Pang, Q.Y. Lu, Z.J. Huang, J.W. Zhao, Electrochim. Acta 55
[45] C. Yuan, X. Zhang, L. Su, B. Gao, L. Shen, J. Mater. Chem. 19 (2009) 5772–5777.
[46] X.Z. Wu, W. Xing, L. Zhang, S.P. Zhuo, J. Zhou, G.Q. Wang, S.Z. Qiao, Powder
Technol. 224 (2012) 162–167.
[
[
(
2010) 6830–6835.
[47] Y. Liang, M.G. Schwab, L. Zhi, E. Mugnaioli, U. Kolb, X. Feng, K. Müllen, J. Am.
Chem. Soc. 132 (2010) 15030–15037.
[48] P. Ragupathy, H.N. Vasan, N. Munichandraiah, J. Electrochem. Soc. 155 (2008)
A34–A40.
[49] X.J. Zhu, H.L. Dai, J. Hu, L. Ding, L. Jiang, J. Power Sources 203 (2012) 243–249.
[50] B.C. Jones, A.F. Hollenkamp, S.W. Donne, J. Electrochem. Soc. 157 (2010)
A551–A557.
[
[
[
41] Y. Zhang, Y.H. Gui, X.B. Wua, H. Fenga, A.Q. Zhang, L.Z. Wang, T.C. Xia, Int. J.
Hydrogen Energy 34 (2009) 2467–2470.
42] Y. Zhang, Y.H. Gui, X.B. Wua, H. Fenga, A.Q. Zhang, L.Z. Wang, T.C. Xia, Int. J.
Hydrogen Energy 34 (2009) 2467–2470.
43] Y.Q. Zhang, X.H. Xia, J.P. Tu, Y.J. Mai, S.J. Shi, X.L. Wang, C.D. Gu, J. Power
Sources 199 (2012) 413–417.