Pine-cone morphology and pseudocapacitive behavior of nanoporous nickel oxide

Article abstract of DOI:10.1016/j.electacta.2010.07.042

Nanoporous pine-cone structured NiO powder was prepared by hydrothermally heated homogeneous precipitation method using cetyltrimethylammonium bromide surfactant (CTAB) as a template and urea as hydrolysis controlling agent. The NiO powder sample was characterized by thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD), Brunauer-Emmet-Teller (BET) isotherm, scanning electron microscopy (SEM) and electrochemical measurements. The prepared NiO was found to be crystalline and highly porous in nature with high specific surface area and pore volume. The pseudocapacitance behavior of this material was investigated using cyclic voltammetry, chronopotentiometry and impedance spectroscopic studies employing a three-electrode system in the single cell mode. The SEM analysis reveals hierarchically porous pine-cone morphology for NiO which shows good specific capacitance (～337 F g^-1) measured by cyclic voltammetry. The galvanostatic charge-discharge cycles obtained in chronopotentiometric measurements indicate that the NiO sample exhibits good electrochemical stability. The columbic efficiency of NiO was found to be about 99% after 100 galvanostatic charge-discharge cycles. The impedance spectroscopic studies confirmed that the pseudocapacitance behavior of the porous NiO was a result of OH^- ion diffusion processes in the system. In this study, a correlation has been made between the specific capacitance values and physicochemical properties as well as the unique surface morphology of NiO.

Full text of DOI:10.1016/j.electacta.2010.07.042

Electrochimica Acta 55 (2010) 8388–8396
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Pine-cone morphology and pseudocapacitive behavior of nanoporous
nickel oxide
Sumanta Kumar Meher, P. Justin, G. Ranga Rao∗
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Nanoporous pine-cone structured NiO powder was prepared by hydrothermally heated homogeneous
precipitation method using cetyltrimethylammonium bromide surfactant (CTAB) as a template and urea
as hydrolysis controlling agent. The NiO powder sample was characterized by thermogravimetric analy-
sis (TGA), powder X-ray diffraction (PXRD), Brunauer–Emmet–Teller (BET) isotherm, scanning electron
microscopy (SEM) and electrochemical measurements. The prepared NiO was found to be crystalline and
highly porous in nature with high specific surface area and pore volume. The pseudocapacitance behavior
of this material was investigated using cyclic voltammetry, chronopotentiometry and impedance spec-
troscopic studies employing a three-electrode system in the single cell mode. The SEM analysis reveals
Received 6 March 2010
Received in revised form 11 July 2010
Accepted 17 July 2010
Available online 23 July 2010
Keywords:
Porous NiO
Hydrothermal method
Pine-cone morphology
Electrochemical property
Supercapacitor
hierarchically porous pine-cone morphology for NiO which shows good specific capacitance (∼337 F g−1
)
measured by cyclic voltammetry. The galvanostatic charge–discharge cycles obtained in chronopo-
tentiometric measurements indicate that the NiO sample exhibits good electrochemical stability. The
columbic efficiency of NiO was found to be about 99% after 100 galvanostatic charge–discharge cycles.
The impedance spectroscopic studies confirmed that the pseudocapacitance behavior of the porous NiO
was a result of OH− ion diffusion processes in the system. In this study, a correlation has been made
between the specific capacitance values and physicochemical properties as well as the unique surface
morphology of NiO.
© 2010 Elsevier Ltd. All rights reserved.
specific capacitance theoretically in the range of 1300–2220 F g−1
and experimentally the highest specific capacitance reported is
1. Introduction
1580 F g−1 in acidic electrolyte [8–13]. RuO2 also shows good elec-
trochemical stability and is a better material than conventional
carbon materials [14] and electronically conducting polymer mate-
rials [15]. But, apart from being toxic, RuO2·xH2O is quite expensive
for extensive commercial applications and its 1 V voltage win-
dow limits their application to smaller electronic devices [16]. This
has initiated a search for inexpensive alternate electrode mate-
rials having good capacitive characteristics with longer cycle life
time and good reversibility which can be prepared from cheaper
precursors as well as reproducible, reliable and easy techniques.
Transition metal oxides and hydroxides clearly fit into these criteria
and have variable oxidation states which results in redox tran-
sitions and psudocapacitance storage within the potential range
of water decomposition. In this context, Liu and Anderson [17]
in their early report found that nano-sized NiO/Ni particles are
active for electrochemical capacitors producing comparatively low
specific capacitance values of 50–64 F g−1. Later Srinivasan and
Weidner [18] synthesized porous NiO by electrochemical pre-
cipitation method which showed a specific capacitance value of
59 F g−1. A number of studies have followed on other transition
metal oxides (e.g. oxides of Ti, V, Cr, Mn, Fe, Co, Cu, Mo, W, Re),
and hydroxides (e.g. Ni(OH)2, Co(OH)2) [19–27], which are mod-
Electrochemical capacitors (also known as supercapacitors,
pseudocapacitors or ultracapacitors) are vital devices for electric
vehicles and uninterrupted power supplies. These are high power
delivery (20–200 times) and high energy density devices with long
cycle life compared to batteries and conventional electrochemi-
cal double-layer capacitors, respectively [1–3]. The classification
of electrochemical capacitors is generally based on the concept of
non-faradaic and faradaic energy storage mechanisms. The capaci-
tor is classified as electrochemical double-layer capacitors (EDLCs)
when the energy storage occurs non-faradaically by charging an
electrochemical double layer at the interface between the porous
electrode and the electrolyte. If the energy storage occurs due to
faradaic reactions of the redox-active material then the capaci-
tor is classified as redox supercapacitor or pseudocapacitor [4–7].
Porous materials with high specific surface area (e.g. activated
carbon and its related materials) [4–6] are generally utilised to
prepare supercapacitors. However, supercapacitors based on RuO2
and nanocomposites of RuO2/C materials are capable of delivering
∗
Corresponding author. Tel.: +91 44 2257 4226; fax: +91 44 2257 0545.
E-mail address: grrao@iitm.ac.in (G.R. Rao).
0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.07.042

S.K. Meher et al. / Electrochimica Acta 55 (2010) 8388–8396
8389
erately active for electrochemical capacitor applications. There is
considerable effort in current literature to increase the capacitance
value of metal oxides and other materials for commercial applica-
tions [9,21,25,27,28]. Pseudocapacity behavior in these materials
originates from fast and reversible surface or near-surface phe-
nomena involved during the faradic reactions. So, high surface
area, suitable morphology and porosity are crucial to enhance the
active material–electrolyte interfacial area and faster ion trans-
fer inside the pore structure [29]. A good combination of all these
factors contributes to the pseudocapacitance behavior of an oxide
material [13,30,31]. The charge storage in pseudocapacitors occurs
within few nanometers of the surface region and hence the electro-
active material can be used effectively by decreasing its particle
size. In this context, nanostructured electrode materials show bet-
ter rate capabilities and pseudocapacitance behavior than the bulk
materials.
form a homogeneous green solution. 40 mmol of solid urea was
added to the resulting solution and again stirred for 3 h to get a
complete homogeneity. The solution was then transferred to a
Teflon lined autoclave of 250 mL capacity and subjected to heating
at a temperature of 120 ◦C for 24 h, which resulted in the formation
of a fluffy grass-green solid precipitate after auto-cooling the
autoclave to room temperature. The green material was then
separated by centrifugation at 3000 rpm with repeated washing
using triple distilled water followed by a mixture of absolute
ethanol and water, and finally with absolute ethanol. The green
material was then dried in a vacuum oven at 60 ◦C for 24 h. The
sample was then heated slowly at the rate of 10 ◦C min−1 from
room temperature to 300 ◦C and calcined for 3 h in flowing air to
obtain the final product.
2.2. Experimental measurements
Nanoscale NiO is a promising material with high specific sur-
face area and high bulk conductivity for supercapacitor applications
[17,18,27–29,32–36]. A number of chemical methods such as
sol–gel [35], hard templating [37], reverse micelle/microemulsion
[38] and precipitation routes [36,39] have been employed to syn-
thesize nanostructures of NiO. Sol–gel dip coating [17,40] and
electrochemical cathodic precipitation by potentiodynamic [41,42]
methods have produced thin and porous NiO films for enhanced
capacitive applications. Although the electrochemical techniques
can control the structure and morphology of nanoporous films
to a great extent but the NiO films obtained are very thin. How-
ever, deposition of higher amounts of electro-active NiO to increase
the electrode capacitance is not practical, since it limits the ion
diffusion due to the blockage of pores. This interrupts the fast
and reversible redox process and reduces the overall electrode
capacitance. Hence, there is ample scope to prepare and study the
nanostructured NiO having suitable surface morphology, high spe-
cific surface area as well as pore volume with appropriate pore size
distribution for electrochemical supercapacitor applications.
Synthesis and characterization of NiO using homogeneous pre-
cipitation method in the presence of urea has been reported in
the literature [43–47]. It is also known that hydrothermal method
gives better crystallinity and shape selectivity to oxides and fluo-
ride materials [37,45,48]. However, the application of a combined
synthetic approach of homogeneous precipitation and hydrother-
mal method in the presence of cost effective surfactants has been
very limited in the literature [36]. We find that the combined
method is effective to study the morphological and electrochemi-
cal characteristics of NiO. In this work, we have used the combined
synthetic approach to prepare porous NiO using urea to control the
hydrolysis and cetyltrimethylammonium bromide (CTAB) as a sur-
factant template. This simple method produced NiO sample with
hierarchical porous pine-cone morphology through nanofiber sur-
face assemblies. The cyclic voltammetry, chronopotentiometry and
impendance spectroscopic measurements carried out on the NiO
sample shows improved electrochemical pseudocapacitance and
good cyclability.
Multipoint nitrogen adsorption and desorption experiment was
carried out on an automatic Micromeritics ASAP 2020 analyzer at
77 K using Brunauer–Emmett–Teller (BET) gas adsorption method.
The sample was out gassed at 100 ◦C for 2 h followed by 150 ◦C
for 10 h in a dynamic vacuum before physisorption measurements.
The specific surface area, SBET was calculated using the BET equa-
tion and software of the instrument. The porosity distribution
was generated from desorption branch of the isotherm using BJH
model. Thermo gravimetric analysis of the sample was performed
on a TA make TGA Q500 V20.10 Build 36 instrument, in air flow
(20 mL/min) with a linear heating rate of 20 ◦C min−1, from room
temperature to 800 ◦C to identify the decomposition temperature
of the precursor. PXRD patterns were recorded using a Bruker
AXS D8 Advance diffractometer at room temperature using Cu K␣
(ꢀ = 0.15408 nm) irradiation generated at 40 kV, 30 mA and at a scan
rate of 0.01◦ s−1. The crystallite size of the NiO powder was esti-
mated using the Scherrer equation, D = Kꢀ/(ˇ cos ꢁ) where D is the
crystallite size, K is the shape factor usually taken as 0.89 for spher-
ical particles, ꢀ is the wavelength of the incident X-ray, ꢁ is the
diffraction angle and ˇ is the graphically measured full width at half
maximum height (FWHM) of the peak. The crystallite size was cal-
culated by taking (1 1 1), (2 0 0) and (2 2 0) peaks into consideration.
The final crystallite size was the average of crystallite sizes calcu-
lated using the above three peaks. FEI Quanta 200 scanning electron
microscope was used to study the surface morphology of the NiO
powder mounted on a carbon tape for analysis. The high resolution
transmission electron microscopy (HRTEM) measurements were
performed on a JEOL 3010 HRTEM with a UHR polepiece operated at
an accelerating voltage of 300 kV and a lattice resolution of 0.14 nm.
2.3. Electrochemical analysis
The working electrodes for evaluating the electrochemical prop-
erties of NiO were fabricated by mixing 75 wt% of NiO with
15 wt% acetylene black in an agate mortar. To this mixture, 10 wt%
polyvinylidene difluoride (PVdF) binder dissolved in 1-methyl-2-
pyrrolidinone (NMP) was added to form slurry. The slurry was
coated (area of coating: 1 cm2) on a pretreated battery-grade Ni
foil (0.2 mm thick) for electrical conductivity and oven dried at
60 ◦C for 8 h in air. Cyclic voltammetry (CV), chronopotentiome-
try (CP) and electrochemical impedance spectroscopy (EIS) studies
were performed using a CHI 7081C electrochemical workstation in
a three-electrode configuration with NiO coated Ni plate as working
electrode, Pt foil (1 cm × 2 cm) counter electrode and Hg/HgO (1.0 M
KOH) reference electrode using deoxygenated 2.0 M KOH aque-
ous electrolyte. Before electrochemical tests, 25 CV cycles were
performed in 2 M KOH between −2.0 V and 0.6 V to activate each
NiO electrode. The capacitance was measured after activation and
reaching the steady state in each electrode [28,41]. The scan rates of
2. Experimental
2.1. Preparation of porous NiO
Analytical
grade
Ni(NO3)2·6H2O
(CDH
Chemicals),
cetyltrimethyl ammonium bromide (CTAB, SD Fine Chemi-
cals) and urea (Thomas-Baker) were used as received to prepare
NiO samples. Triple distilled water was used during all the experi-
mental processes. 20 mmol of Ni(NO3)2·6H2O dissolved in 100 mL
of triple distilled water was added dropwise to 10 mmol of CTAB
dissolved in 100 mL triple distilled water and stirred for 1 h to

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Fig. 1. TGA and DTGA profiles of dried Ni(OH)2 sample.
Fig. 2. PXRD pattern of (a) dried ␣-Ni(OH)2 precursor and (b) NiO obtained by
calcining the precursor at 300 ◦C.
2 mV s−1, 5 mV s−1, 10 mV s−1 and 20 mV s−1 have been employed
in the CV study. Chronopotentiometric measurements were per-
formed at different current densities of 200 mA g−1, 500 mA g−1
,
adsorption–desorption isotherm of NiO is given in Fig. 3 with pore
size distribution in the inset. The isotherm demonstrates the porous
nature of NiO sample. There is a steep uptake of N2 at the begin-
ning of the measurement (p/po < 0.02) followed by an increased
uptake of gas and a hysteresis. The steep uptake of N2 in the very
beginning of the N2 adsorption–desorption process indicates the
presence of micropores in the sample. The isotherm in the p/po
of 0.4–1.0 shows the presence of meso as well as macropores in
the sample. The overall isotherm, however, is a characteristic of
type IV with H4 type hysteresis loop according to IUPAC classifi-
cation [52,53]. The isotherm confirms the porosity of NiO and the
presence of narrow slit like pores on its surface. These results com-
bined with pore size distribution suggests that the NiO synthesized
by CTAB–urea homogeneous precipitation method contains narrow
pore size distribution mainly in the mesopore range of 3.6 nm and
a wide distribution of macropores in the range of 10–50 nm. NiO
1000 mA g−1 and 2000 mA g−1. The stability of the catalyst was
evaluated using 100 galvanostatic charge–discharge test cycles. The
electrochemical impedance spectra were measured by imposing a
sinusoidal alternating voltage frequency from 10−2 Hz to 105 Hz
with amplitude of 5 mV at constant dc biased potential of 0.1 V,
0.3 V and 0.5 V.
3. Results and discussion
3.1. Thermogravimetric analysis
Thermogravimetric analysis was performed in air on the dried
sample to find the decomposition temperature. Fig. 1 shows the
TG curve and its differential curve (DTG) up to 800 ◦C. About 17%
weight loss has occurred in the temperature range from 100 ◦C to
200 ◦C due to the removal of chemisorbed water from the dried
precursor sample. The weight loss (sharp peak in the DTG curve
at 288 ◦C) in the temperature range from 200 ◦C to 400 ◦C is due to
the removal of crystalline water by the decomposition of Ni(OH)2 to
NiO [49]. No significant loss has occurred beyond this temperature
which indicates complete decomposition of Ni(OH)2 to NiO.
produced by this method shows BET surface area of 265 m2 g−1
,
higher pore volume of 0.269 cm3 g−1 and an average BJH pore diam-
eter of 4.19 nm. In a recent study, hydrothermal route has been
employed to synthesize nanobelts, nanowires and nanoplates of
3.2. Powder X-ray diffraction
The wide angle PXRD profiles in Fig. 2 further support the TGA
analysis. The dried and calcined samples are identified as Ni(OH)2
and NiO phases, respectively. The PXRD pattern of the as synthe-
sized product is similar to ␣-phase of Ni(OH)2 with peaks related to
(0 0 6), (1 0 1) and (1 1 0) planes [50,51]. After controlled calcination
at 300 ◦C under ambient pressure, the formation of pure and crys-
talline NiO phase is confirmed by major PXRD peaks at 2ꢁ values
of 37.2◦, 43.4◦ and 62.7◦ of the rock salt structure (JCPDS card no.
040835). There are no impurities detected in the PXRD profile of
NiO. The ␣-phase of Ni(OH)2 looses water at a calcination temper-
ature of about 300 ◦C to form NiO. The broad PXRD peaks suggest
the formation of nano-crystalline NiO phase. The crystallite size is
estimated to be ∼4.6 nm based on Scherrer calculation taking all
three peaks, (1 1 1), (2 0 0) and (2 2 0) into consideration.
3.3. BET analysis
The BET analysis was carried out at 77 K to probe the
physicochemical properties of NiO calcined at 300 ◦C. The N2
Fig. 3. BET isotherm of NiO sample. The inset shows pore size distribution, calcu-
lated from desorption branch of isotherm, using BJH method.

S.K. Meher et al. / Electrochimica Acta 55 (2010) 8388–8396
8391
Fig. 4. SEM images of NiO with (a) 5 ␮m, (b) 1 ␮m, (c) 500 nm and (d) 400 nm scale bars showing pine-cone morphology.
NiO which show surface area of ∼145 m2 g−1 and different pore
diameters ranging 7–12 nm [54]. However, the CTAB-urea homo-
geneous precipitation method employed by us produces NiO with
lesser pore diameter and higher surface area suitable for pseudo-
capacitor applications.
surface of porous NiO micro-balls. The TEM pictures in Fig. 5 show
random fibrous growth of NiO ball-like structure by the aggrega-
tion of nanofibers on the surface. The average diameter of porous
ball-like structures is ∼2 ␮m (Fig. 5a) and an approximate diam-
eter of surface nanofibers is ∼15 nm (Fig. 5b). The elimination of
water as well as surfactant residues during the calcination process
contributes to the porous nature of the hierarchical NiO prod-
uct. Using this method, we have been able to prepare uniformly
porous microstructure having nano-fiber assembled pine-cone sur-
face morphologies of NiO with good reproducibility.
3.4. Morphology study
The surface morphology of NiO sample is shown in SEM pic-
tures in Fig. 4. The low magnification SEM pictures show uniform
size hierarchical porous ball like structure. The high magnification
micrographs show pine-cone fibrous structures crowding on the
Homogeneous precipitation is a simple method to control and
attain high degree of supersaturation during synthesis. The mecha-
Fig. 5. TEM images of NiO with (a) 0.2 ␮m and (b) 50 nm showing the porous structures made of nanofibers.

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Fig. 7. Scan rate dependence of specific capacitance obtained from cyclic voltam-
metry measurements.
Fig. 6. Cyclic voltammograms of NiO electrode measured at different scan rates in
2 M KOH electrolyte. The inset shows the plot of anodic current density versus scan
rate.
occurring during the potential sweep of porous NiO electrode [33].
nism of the formation of Ni(OH)2 and final product of NiO involves
simple hydrolysis-precipitation process [43]. Urea acts as hydrol-
ysis controlling agent and a source of OH− ions which co-ordinate
with Ni2+ to form ␣-Ni(OH)2. The OH− ions slowly generated in the
reaction system control the rate of hydrolysis and crystal growth
processes under hydrothermal conditions. The porous NiO with
hierarchical structure is obtained in the presence of cetyltrimeth-
lammonium bromide (CTAB) surfactant. The NiO when prepared
without CTAB, exhibits great difference in size, less porosity and no
particular morphology. It is known that organic and inorganic addi-
tives adsorb onto certain crystallographic planes and modify the
relative order of surface energies during the crystal growth pro-
cess. This preferential adsorption lowers the surface energy and
directs the formation of a particular morphology of the material.
The CTAB acting as a shape controlling agent and the slow release
of OH− ions from urea hydrolysis are the crucial factors for the for-
mation of porous pine-cone morphology of NiO. The reaction steps
involved in the formation of ␣-Ni(OH)2 are given below [44,45].
charge
NiO + OH−
ꢀ
NiOOH + e−
discharge
The anodic peak (positive current density) is due to the oxi-
dation of NiO to NiOOH and the cathodic peak (negative current
density) is for the reverse process, which together are the signa-
tures of pseudocapacitance behavior of porous NiO [55,56]. The
anodic peak current densities are 1.1 A g−1 (scan rate 2 mV s−1),
2.6 A g−1 (scan rate 5 mV s−1), and 4.1 A g−1 (scan rate 10 mV s−1
)
and 6.8 A g−1 (scan rate 20 mV s−1). There are small changes in the
shape of the CV curves observed with scan rate. A small positive
shift of the oxidation peak potential and a negative shift of the
reduction peak potential have been observed with increased scan
rate. The positive sweeps of CV curves are not completely symmet-
ric to their corresponding negative sweeps, which indicates some
irreversibility in the system [11,12,19,20,22,25,30,32,34,56]. In fact
supercapacitor systems involving faradaic processes suffer polar-
ization effects, and ideal reversibility cannot be practically realized
in the kinetic sense for the forward and backward directions of
the processes [57]. Unlike true electrostatic capacitor, the ohmic
resistance due to electrolyte diffusion within the porous electrode
of the supercapacitors results in certain kinetic irreversibility of
the ions involved in the redox reaction. However, the lower ꢂEp
(ꢂEp = Ea − Ec where Ea and Ec are anodic and cathodic peak poten-
tials, respectively) values at lower scan rate than at higher scan
rate from Fig. 6 demonstrates a slightly better reaction reversibil-
ity at lower scan rates than at higher scan rates. The almost linear
(quasi-linear) relationship of the plot of anodic peak current density
versus the scan rate (inset in Fig. 6) indicates high power property
and the occurrence of the surface redox reactions which further
corroborates the pseudocapacitance behavior of the NiO electrode
[29,58].
NH2CONH2hyd−r→olysisNH4+ + NCO−
NCO− + 3H2O → HCO3− + NH4− + OH−
Ni2+ + 2OH− → Ni(OH)2
The pH of the solution after the reaction is 8.2, a much less value
than the pH (∼12.5) maintained in the precipitation route using
aqueous ammonia. The formation of relatively unstable ␣-Ni(OH)2
phase is favored at low pH which can transform to stable ␤-Ni(OH)2
under higher alkaline conditions [45]. Therefore, this method can
be conveniently used to synthesize less stable ␣-Ni(OH)2.
Specific capacitance can be calculated using the current density,
scan rate, and weight of the active NiO material. Fig. 7 shows typical
cyclic voltammograms of NiO where y-axis represents the specific
capacitance (Cs, F g−1) obtained by using the following equation
[33]:
3.5. Electrochemical study
3.5.1. Cyclic voltammetry measurements
Cyclic voltammetry has been successfully used as a tool to study
the capacitive behavior of electrode materials [19,26]. Fig. 6 shows
the cyclic voltammograms of hierarchically porous NiO electrode
measured at different scan rates of 2 mV s−1, 5 mV s−1, 10 mV s−1
and 20 mV s−1. A couple of electrochemical redox peaks have been
observed within the potential range from 0.1 V to 0.6 V (versus
Hg/HgO). These are due to fast quasi-reversible redox processes
ꢀ
V
f
1
Cs =
idV
ꢃ · w · ꢂV
V
i
Here Cs is the specific capacitance of nickel oxide electrode
(F g−1), w is the mass of nickel oxide (g), ␯ is the scan rate (V s−1), i

S.K. Meher et al. / Electrochimica Acta 55 (2010) 8388–8396
8393
Fig. 8. Chronopotentiometry of NiO electrode measured at different current densities in 2 M KOH electrolyte.
is the cathodic or anodic current (A) in the potential range of Vi
to Vf, and ꢂV is the applied potential window (V). The specific
capacitance values are calculated by graphical integration of the
potential versus specific capacitance curves shown in Fig. 7. The
specific capacitance curves do not have the characteristic shape
(nearly rectangular) of a pure double-layer capacitor [59,60], but
show similar behavior of a pseudocapacitor [61], in agreement with
the CV curves. For porous NiO, we obtained the capacitance values
of 337 F g−1, 279 F g−1, 223 F g−1 and 167 F g−1 by applying the scan
rates of 2 mV s−1, 5 mV s−1, 10 mV s−1 and 20 mV s−1, respectively.
Although sufficient OH− ions are present in the system, the diffu-
sion of OH− ions depends on scan rates. At lower scan rates, both
outer- and the inner-pore surfaces are effectively utilised for OH−
ion propagation, while at high scan rates mainly outer regions of the
pores are accessed by the ions [62]. Again, for a porous electrode,
current increases with scan rate but, due to the effect of the dis-
tributed resistance of the pores (associated with the resistance of
electrolyte in pores of the matrix and to some extent also with the
resistance of the electronically conducting oxide matrix), the actual
local potential at the electrode surface differs from that applied
externally to the electrode at a particular applied potential sweep
rate. At lower sweep rate, the current is small, so IR losses are neg-
ligible, and the current response is mostly that of a pure capacitor,
whereas at higher sweep rate, the current is more and the IR losses
become significant. This results in a decrease in the reduced current
and the response current becomes more resistive which is ‘capaci-
tative’ in form at low scan rate. Therefore, the specific capacitance
is high at lower scan rates and vice versa (Figs. 6 and 7) [63].
Hg/HgO). At different current densities, all the charge–discharge
curves are non-linear which clearly verify the pseudocapacitance
behavior of the material in this voltage range [56]. The specific
capacitance values at different current densities were calculated
from the discharge curve of the charge–discharge cycles, using the
following equation:
i
Cs =
m(ꢂV/ꢂt)
where, Cs (F g−1) is the specific capacitance, i (A) is the applied
current, m (g) is the mass of the electrode material and ꢂV/ꢂt
(V s−1) is the slope of the discharge curve. Good specific capacitance
values have been obtained from chronopotentiometry measure-
ments at different current densities which are calculated from the
second charge–discharge cycle (239 F g−1 at 200 mA g−1; 183 F g−1
at 500 mA g−1; 71 F g−1 at 1000 mA g−1; 33 F g−1 at 2000 mA g−1).
The easy accessibility of ions through the pores of the material
is very important for efficient faradaic reactions [64]. Hence pore
size is an important parameter for higher capacitance behavior of
the electrode materials. Simon and Gogotsi recently reported [16]
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 distri-
bution of ∼3.6 nm (Fig. 3), which satisfies the value of the required
pore size for efficient faradaic reactions. Therefore the OH− ions
easily diffuse through the fibrous structure of NiO microspheres
(Fig. 4) and access the inner space for more faradaic reactions to
occur at lower current density. This process contributes to the
enhancement of the capacitance value of NiO with pine-cone struc-
ture. It is observed that, the charging and discharging curves at all
current densities are not completely symmetrical and are more pro-
nounced at higher applied current densities due to relative increase
in kinetic irreversibility of the OH− ions on the porous NiO surface
[57,31]. Generally with increase in applied current density, a large
voltage (IR) drop is produced and finally the capacity decreases
3.5.2. Chronopotentiometry measurements
Chronopotentiometry measurements are performed at differ-
ent current densities to find out the specific capacitance values
and stability of the porous NiO. In Fig. 8, the galvanostatic
charge–discharge curves of NiO porous microspheres are plotted
at different current densities between 0.05 V and 0.50 V (versus

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S.K. Meher et al. / Electrochimica Acta 55 (2010) 8388–8396
Fig. 9. The specific capacitance and coulombic efficiency as a function of cycle num-
ber at a current density of 500 mA g−1. The inset shows 40 cyclic charge–discharge
curves of the NiO electrode at a current density of 500 mA g−1
.
Fig. 10. Complex plane impedance plots (Nyquist plots) of NiO electrode at different
bias potentials in 2 M KOH electrolyte. Inset shows the equivalent circuit.
[20,30,31,34]. In this study, the fast decay of capacitance at higher
current densities is observed which is attributed to the combina-
tion of large voltage (IR) drop and flaking off of porous NiO/C/PVdF
binder composite film from the smooth Ni electrode surface, due
to severe redox reactions on the electrode surface at higher cur-
rent density. So, the measured higher specific capacitance at lower
current density is attributable to better OH− ion diffusion where
as at higher current density, the decrease in specific capacitance is
an amalgamation of other factors like IR drop and distributed pore
resistance as discussed earlier.
nearly 99% after 100 cycles. This is again related clearly to
the increase 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.
3.5.3. Electrochemical impedance spectroscopy (EIS)
measurements
As electrochemical supercapacitors are power devices, the elec-
trode material should have higher capacitance value as well as low
electronic resistance for commercial applications. The porous NiO
material is subjected to electrochemical impedance spectroscopy
(EIS) measurements between 0.1 Hz and 105 Hz to quantitatively
evaluate the electrical resistance which can be correlated to the
microstructure and electrochemical properties of the material.
Nyquist plots (imaginary part, Zꢀꢀ versus real part, Zꢀ) of electro-
chemical impedance measured for porous NiO at different biased
potentials are shown in Fig. 10. The impedance behavior shows
three major processes occurring in the high- , medium- , and low-
frequency regions. The two partial semicircles at high and medium
In order to find the stability and service life of the electrochemi-
cal capacitor, the cyclic charge–discharge test has been carried out
on porous NiO electrode and the results are displayed in Fig. 9.
Although 1000 cycles of life test is required for practical applica-
tion, we have studied 100 cycles of repetitive charge–discharge
process to obtain preliminary information on the stability and
performance of the porous NiO electrode material [64]. There is
negligible fading of specific capacitance observed after 100 repeti-
tive charge–discharge cycles at current density of 500 mA g−1. This
shows that the NiO microspheres with rich hierarchical structure
are non-degradable at comparatively lower current densities. As
reported recently on hierarchical porous graphitic carbon [62], the
reason for the unique property of the porous NiO material is the
fibrous porous network which can shorten the diffusion path length
of OH− ions for the reversible redox process by acting as an “ion
buffering reservoir”. The interfacial area between the electrode and
the electrolyte is larger due to the assembled nanofibers on porous
NiO surface (Fig. 5) which results in higher diffusion rates and sig-
nificantly faster electronic kinetics. The pine-cone porous structure
improves the solid-state diffusion, intercalation/de-intercalation of
OH− ions, and the overall cyclic stability of NiO at a current density
of 500 mA g−1
.
The columbic efficiency “ꢄ” is a measure of the efficacy with
which charge is transferred in a system facilitating an electrochem-
ical reaction. High coulombic efficiency suggests that the process
requires lower energy to complete the reaction making the process
more feasible. Fig. 9 shows the coulombic efficiency of the porous
NiO electrode during 100 repetitive charge–discharge cycles. This
can be analyzed using the equation:
tD
ꢄ =
× 100
tC
where tD and tC represents galvanostatic discharging and charging
time, respectively [65]. It is found that the coulombic efficiency
slowly increases with increase in number of cycles and reaches
Fig. 11. Nyquist plots of NiO electrode at different bias potentials showing two semi-
circles at high frequency region. Inset shows the first semicircle at high frequency
region representing the grain boundary resistance of NiO.

S.K. Meher et al. / Electrochimica Acta 55 (2010) 8388–8396
8395
electronic charge-transfer resistances, (ii) intrinsic charge-transfer
resistance of the active NiO material, and (iii) diffusive as well as
contact resistance at the active material/current collector interface.
The experimental impedance data are also converted to specific
capacitance (Cs) using the following equation [19]:
1
Cs =
2ꢆfZꢀꢀ
and plotted against frequency, f in Fig. 12. The capacitance
inevitably decreases with biased potential which is consistent with
the scan rate dependent current density observed in the cyclic
voltammograms in Fig. 6. Further the specific capacitance calcu-
lated from impedance measurements at different biased potentials
(Fig. 12) is also in agreement with the results obtained from the
cyclic voltammetry measurements in Fig. 7.
4. Conclusions
A simple hydrothermal technique using homogeneous pre-
cipitation method has been employed to synthesize NiO with
nanoporous pine-cone morphology. The NiO samples have the
surface area of 265 m2 g−1, pore volume of 0.269 cm3 g−1 and aver-
age pore diameter of 4.19 nm. The pore size distribution analysis
shows that the material contains both meso- and macro-porosities.
The pore size distribution is quite narrow (∼3.6 nm) which falls
in the specified range of pore size [16] suitable for efficient
faradaic reactions. The electrochemical studies on this material
show good capacitance value (337 F g−1) attributed to its porous
surface of pine-cone morphology which acts as an “ion buffering
reservoir” to facilitate OH–ion mobility. The specific capacitance
values obtained from cyclic voltammetry, chronopotentiometry
and potential dependent impedance spectroscopy measurements
are in good agreement with each other.
Fig. 12. The specific capacitance calculated from impedance spectra as a function
of frequency at different bias potentials.
frequency region which are often attributed to different resistances
due to different processes occurring in the system as a result of
faradaic reactions. The expanded view of the semicircles at differ-
ent biased potentials is shown in the Fig. 11. The EIS spectra can be
evaluated through a simple circuit model, proposed by Hu et al. [28]
and is shown in the inset of Fig. 10. The partial semicircle at high
frequency region (shown as an inset in Fig. 11) is characteristic of
the processes occurring at the oxide–electrolyte interface. This can
be modeled as a double-layer capacitor Cd in parallel with an ionic
charge-transfer resistor, Rict. The Rict is a resistance due to different
conductivity in the solid oxide (electronic conductivity) and the
aqueous electrolyte phase (ionic conductivity). So, the resistance
is due to some discontinuity in the charge-transfer process at the
solid oxide/liquid electrolyte interface. Here, the resistance is inde-
pendent of applied biased potential due to steady electronic and
ionic conductivity of the solid oxide and liquid electrolyte, respec-
tively. The partial kinetic semicircles at medium frequency regions
correspond to the charge-transfer resistance due to faradaic redox
processes in the system involving the exchange of OH−. This is asso-
ciated with the surface phenomena of the porous NiO electrode.
This impedance characteristic can be modeled as a film impedance
due to the faradaic redox processes involving electron hopping in
the NiO particle and OH− ion diffusion. This is assumed to be con-
sisting of a film capacitor, Cf in parallel with an electron-transfer
resistor, Rect and is dependent upon the applied biased potentials.
With increase in biased potential, there is a decrease in impedance
due to the increase in OH− ion diffusion, thereby increasing the
feasibility of electron transfer. The linear part at lower frequen-
cies corresponds to the Warburg impedance, W, which is described
as a diffusive resistance of the OH− ion within the NiO electrode
pores. At a biased potential of 0.5 mV, the Nyquist plot of NiO
displays a nearly vertical line along the imaginary axis at lower fre-
quencies. When the dc potential is reduced to 0.3 mV and 0.1 mV,
a considerable deviation from near 90◦ slopes is observed at the
low-frequency region. This deviation along the imaginary axis is
ascribed to the pseudocapacitance, Cs, due to facile and reversible
faradaic redox reactions at the electrode–electrolyte interface and
an easy access of the OH− ions with the electro-active porous NiO
electrode under such low frequencies. The zero slope at higher
voltage is attributed to the effect of high operation voltages on
the supercapacitive behavior and more pronounced surface redox
reactions at electrode/electrolyte interface. For all biased potentials
at higher frequencies, the intercept at real part (Zꢀ) is 0.16 ꢅ. This
constant value is due to the identical combination of (i) ionic and
Acknowledgment
We thank the Ministry of New and Renewable Energy (MNRE),
New Delhi, for financial support through grant no. 102/28/2006-NT.
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