Synthesis, characterization and electrochemical properties of capsule-like NiO nanoparticles

Article abstract of DOI:10.1149/2.106206jes

Capsule-like NiO nanoparticles were prepared by heat-treatment of the cathodically electrodeposited nickel hydroxide at 300C for three hours. The hydroxide precursor was deposited galvanostatically onto the stainless steel substrate from aqueous Ni(NO₃)₂ solution (0.01 M). Morphological, structural and chemical characteristics of the synthesized NiO were examined by means of scanning and transmission electron microscopy (SEM and TEM) as well as X-ray diffraction (XRD) and FT-IR spectroscopy. The XRD and TEM results revealed that the oxide products have well-crystallized cubic structure with capsule-like morphology of various length and diameters in the order of 20-70 nm and 10-30 nm, respectively. Capacitive performance of NiO nanocapsules were evaluated by cyclic voltammetry and galvanostatic chargedischarge measurements in a potential window between -0.2 and +0.5 V vs. AgAgCl. Electrochemical measurements revealed that the NiO nanocapsules have a specific faradaic pseudocapacitance as high as 307.2 F g^-1 in 1 M KOH solution and good cycle life (approximately displayed 86.4 of its maximum capacitance after 3000 cycle discharging). These results showed that the prepared NiO nanostructure can be recognized as a promising candidate for the supercapacitor applications.

Full text of DOI:10.1149/2.106206jes

E132
Journal of The Electrochemical Society, 159 (6) E132-E138 (2012)
0
013-4651/2012/159(6)/E132/7/$28.00 © The Electrochemical Society
Synthesis, Characterization and Electrochemical Properties
of Capsule-Like NiO Nanoparticles
a
b,z
Hamid Mohammad Shiri and Mustafa Aghazadeh
a
Department of Chemistry, Payame Noor University, Iran
Laser and Optics Research School, NSTRI, Tehran, Iran
b
◦
Capsule-like NiO nanoparticles were prepared by heat-treatment of the cathodically electrodeposited nickel hydroxide at 300 C
for three hours. The hydroxide precursor was deposited galvanostatically onto the stainless steel substrate from aqueous Ni(NO3)2
solution (0.01 M). Morphological, structural and chemical characteristics of the synthesized NiO were examined by means of
scanning and transmission electron microscopy (SEM and TEM) as well as X-ray diffraction (XRD) and FT-IR spectroscopy. The
XRD and TEM results revealed that the oxide products have well-crystallized cubic structure with capsule-like morphology of
various length and diameters in the order of 20–70 nm and 10–30 nm, respectively. Capacitive performance of NiO nanocapsules
were evaluated by cyclic voltammetry and galvanostatic charge/discharge measurements in a potential window between −0.2 and
+
0.5 V vs. Ag/AgCl. Electrochemical measurements revealed that the NiO nanocapsules have a specific faradaic pseudocapacitance
−1
as high as 307.2 F g in 1 M KOH solution and good cycle life (approximately displayed 86.4% of its maximum capacitance after
000 cycle discharging). These results showed that the prepared NiO nanostructure can be recognized as a promising candidate for
the supercapacitor applications.
2012 The Electrochemical Society. [DOI: 10.1149/2.106206jes] All rights reserved.
3
©
Manuscript submitted November 22, 2011; revised manuscript received April 4, 2012. Published April 23, 2012.
−
1
Nanostructured nickel oxide is an important technological mate-
rial of interest, due to its high-specific surface area and fast redox
solid-state (HTSS) and microwave (MW) methods (74–186 F g at
−
1 30
scan rate of 30 mV s ), and highly porous NiO prepared via a com-
1–8
−1 36
reactions. NiO can be used in energy storage devices like batteries,
bination of sol-gel and supercritical drying method (75–125 F g ).
9
–11
12–14
dye-sensitized photocathode’s,
portant catalyst material for photocatalytic production of hydrogen
from water. Other catalytic reactions include reduction of NO , desul-
supercapacitors
and is an im-
Moreover, the specific capacitance of NiO nano-capsules obtained
in this paper is comparable with those published for other NiO ma-
terials synthesized by electrodeposition route. Anodically deposited
nanoporous NiO flaky structures has been reported to have specific
x
phurization of hydrocarbons, partial and complete oxidation reactions
of various organic substances. In combination with other metals or
metal oxides, it can act as a conductor, high temperature insulator,
−
1
−1
capacitance of 162 F g at the scan rate of 10 mV s in a potential
38
−1
window between −0.2 to +0.45 V and 178 F g at a scan rate of
1
4
−1 39
temperature dependant resistors and gamma radiation detectors.
25 mV s
.
Three-dimensional nanowhiskers of NiO, prepared by
1
5–17
Various methods including chemical precipitation,
thermal
potentiodynamically deposition of nickel hydroxide on stainless steel
1
8–21
22–26
◦
decomposition/oxidation,
thermal hydrothermal process,
sur-
(SS) and its thermal treatment in air at 300 C for 3 h reported that to
2
7–29
30,31
32–34
−1
−1 43
. Wu et al.
factant template,
solid state,
solvothermal
and sol–
have a specific capacitance of 458 F g at 20 mV s
have performed comparative studies on capacitances of nickel oxide
3
5–37
gel
have been reported for the synthesis of nanostructured
45
nickel oxide. Electrochemical route i.e. electrochemical synthesis
has been also used for the preparation of nickel oxide and demon-
strated that electrolytic NiO can be employed in electrochemical
films electrodeposited on different substrates. They showed that the
prepared NiO films on nickel foil and graphite can deliver specific
capacitances as high as 135 and 195 F g , respectively. Previously,
we have found that cathodic electrodeposition of nickel hydroxide
from an additive-free chloride bath at temperature of 10 C resulted
the formation of ultrafine nanoparticles of Ni(OH) , which provided
much more activated surface area for electron conduction and short-
ens proton diffusion paths within the bulk of solid nickel hydroxide.
In this work, NiO nanocapsules were synthesized by cathodic elec-
trodeposition of Ni(OH) from nitrate bath and then its heat-treatment
in air at 300 C. The selection of the annealing temperature of 300 C
was based on Refs. 43–45, which resulted the smallest crystal size
and high capacitance. To the best of our knowledge, there is no report
on the NiO nanocapsules.
−
1
3
8–49
supercapacitors.
In a comparative study, the nanostructured elec-
◦
trolytic NiO has proven to be superior in electrochemical performance
25
compared to those synthesized via conventional methods. The nickel
oxide electrode prepared by electrochemical route has not only a
comparable specific capacitance to those made via other mentioned
techniques but also the cost of electrochemical route is lower than sol-
gel technology. Furthermore, the technical variables such as current
density, electrolyte concentration, heating conditions and so on, are
easy to be controlled. Another advantage of electrochemical route is
that the additives can be easily doped via co-deposition technology.
Our previous works on the cathodic electrodeposition showed that
this method can be considered as an effective and facile route for the
2
56
2
◦
◦
5
0–57
preparation of nanostructured metal hydroxide/oxides.
Experimental
Up to now, a variety of NiO nanostructures such as nanoparticles,
nanotubes, nanoflakes, and nanowiskers have been grown electrolyti-
cally and their capacitive properties were investigated by means of dif-
ferent electrochemical methods. These are including nanoporous thin
Preparation of NiO.— Commercial purity Ni(NO
3
)
2
· 6H
2
O was
purchased from Merck and used as starting material. The electro-
chemical cell includes a stainless steel cathode (316 L, 100 × 50
× 0.5 mm) centered between two parallel graphite counter electrodes.
Prior to each deposition, steel substrates have given a galvanostati-
−1
NiO film prepared by chemical precipitation (580 F g at scan rate
−
1 15
of 5 mV s ), nanocrystalline NiO synthesized by a simple liquid-
−
2
−
1
−1 12
cally electropolishing treatment at a current density of 0.5 A cm for
phase process (300 F g at scan rate of 5 mV s ), nanoflakes
◦
−1
5 min in a bath (70 C) containing 50 vol.% phosphoric acid, 25 vol.%
prepared by hydrothermal method (337.7 F g at scan rate of
−
1
23
sulfuric acid and balanced deionized water. Electrodeposition exper-
iments were performed galvanostatically at a current density of 0.1
1
0 mV s ), nanocrystalline NiO prepared by electrophoretic
−
1
−1 24
method (112 F g at scan rate of 10 mV s ), mesoporous NiO
−
2
◦
−1
mA cm at a fixed bath temperature of 10 C. After drying at room
temperature for 48 h, the deposited hydroxide sample was scraped
from the steel electrode and the obtained powder was used for further
characterization and heat-treatment. Heat treatment was conducted at
obtained by surfactant template method (206 F g at scan rate of
−
1 27
1
0 mV s ), nanocrystalline NiO synthesized by high temperature
zE-mail: mustafa.aghazadeh@gmail.com
◦ ◦ −1
00 C in air and a heating rate of 10 C min for 3 hours.
3
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Journal of The Electrochemical Society, 159 (6) E132-E138 (2012)
E133
[4-2]
2
+
−
−
Characterization.— The phase and crystal structure of the pre-
pared NiO powder were determined by X-ray diffraction (XRD) with
a diffractometer (Phillips, PW-1800) using monochromatized Cu Kα
radiation. These measurements were conducted in the range of diffrac-
tion angles (2θ) from 5 to 80 degree at scan rate of 5 degree/min. The
morphology of NiO powder was examined using a scanning electron
microscopy (Philips 515). Transmission electron microscopy (TEM)
images were taken using a Phillips EM 2085 transmission electron
microscope, with an accelerating voltage of 100 kV. FT-IR spectra
were obtained by a Bruker Vector 22 FT-IR spectrometer within the
Ni (aq) + (2-x)OH (aq) + x NO + y H O
3
2
→
[Ni(OH)(2
-
x) (NO
3
)
x
.yH
2
O] ↓
The reactions of 3-1 to 3-3 result in an increase of local pH at the
cathode surface. This process is known as electrogeneration of base.
By increasing the OH concentration to the required conditions in the
reactions of 4-1 or 4-2, Ni(OH)
It is worthy to note that the formation of Ni
is possible on the cathode surface before deposition of Ni(OH) on
the cathode occurs:
−
2
will form and deposit on the cathode.
4
+
soluble complex
4
(OH)
4
−
1
range of 400–4000 cm wave numbers. The measurement of specific
surface area for the prepared samples was performed through measur-
2
ing N
2
adsorption-desorption isotherms at 77 K with a Quanta-chrome
Ni2+ + 4OH− → Ni
4+
4
4
(OH)4
[5]
[6]
NOVA-2200e system.
(OH)4 + 4OH− → 4Ni(OH)
4
+
Ni
4
2
Electrochemical measurements.— Cyclic voltammetric (CV) and
chronopotentieometry measurements were carried out using an Auto-
lab workstation system (AUTOLAB, Eco Chemie, PGSTAT 30). KOH
aqueous solution (1 M) was used as the electrolyte (the electrolyte
solutions were degassed with purified nitrogen gas before the elec-
trochemical measurements). In a three-electrode cell configuration, a
The latter may diffuse away from the reaction interface and lead
to insufficient utilization efficiency of electrochemically generated
OH . A decrease in the temperature of the bath may be an impor-
tant influential factor to reduce the diffusion coefficient of the latter
process.
−
2
platinum wire, a Ag/AgCl and a glassy carbon (GC) with 0.31 cm
We observed gas evolution on the cathode surface at the time of
electrodeposition. The same effect was observed in our previous work
on a two-step method to grow yttrium oxide nanospheres on steel
area were used as counter, reference and working electrodes, respec-
tively. The working electrode was formed by mixing 80 wt% active
material (NiO powder), 15 wt% carbon black as conductive filler,
wt% PTFE as binder and distilled water as a solvent. The prepared
mixture was pasted into glassy carbon electrode and then dried in oven
for 12 h at 120 C. The loading density of the electroactive materials
was 0.1 mg cm . Cyclic voltammetry (CV) studies were performed
51
substrate. This suggests that formation of gas bubbles on the elec-
trode starts to occur simultaneously after some degree of reduction,
which may opposed to transport of nitrate ion to the cathode surface
as well as electrogeneration of base. In this case, no deposits may
be obtained at all, as observed during the cathodically deposition of
some metal hydroxide like yttrium hydroxide especially at higher ca-
thodic current density or at high concentration metal salt solutions.
5
◦
−
2
within a range of −0.2 V to +0.5 V vs. Ag/AgCl at the scan rates of
−
1
5
, 10, 25, 50, 100 and 200 mV s . The specific capacitance (SC) was
calculated from integrating CV curves area as follows:
The transport of nitrate ions to the cathode probably represents the
limiting factor of the cathodic current.
at lower bath temperature, the hydrogen overvoltage is increased, the
formation of gas bubbles is lowered, and the reaction is no more un-
der mass transport control. This condition may facilitate the diffusion
phenomena including the transport of nitrate ions to the electrode sur-
face as well as the kinetics of the Ni(OH) deposition. The impeding
effect of the decreasing temperature of the bath results in the decrease
hydrogen bubbling rather than the nitrate ion diffusivity in the bulk
solution.
ꢀ
Vc
58,59
1
It could be supposed that
C = mυ(V
I(V )dV
[1]
a
− V
c
)
Va
where, C is specific capacitance of NiO (F g−1) measured in the
potential range of V to V , m is the mass of NiO (g), ν is the scan rate
a
c
−
1
30
(
V s ) and I(V) is a current response depending on sweep voltage.
Galvanostatic charge−discharge cycling in the potential range of
2
−
0.2 to +0.5 V vs. Ag/AgCl was performed at a constant current
−
1
density of 1 A g . The obtained charge/discharge curves were used
for the calculation of SC via Eq. 2:
It may be assumed that the induced physico-chemical changes
into the hydroxide precursor may modify various physico-chemical
features of NiO, which could be obtained via thermal annealing of
C
m
1 × ꢀt
C
m
=
=
[2]
ꢀV × m
2
Ni(OH) precursor:
where I is the applied current (A), ꢀV is potential window (V), m is
mass of the active material (g), and ꢀt is time of a discharge cycle (s).
Ni(OH)
2
→ NiO
[7]
As mentioned above, the properties of deposited hydroxide films
on the steel substrate may differ significantly under various experi-
mental conditions. Compared to RT, a decrease in the temperature of
Results and Discussion
Electrodeposition mechanism.— Formation of the nickel hydrox-
ide deposit on the cathode surface can be explained on the base of
electrochemical−chemical (EC) mechanism as follows:
1) Electrochemical step (base electrogeneration):
+
2
the bath during conversion process of Ni to Ni(OH) , could be re-
2
51,58
garded as an important influential parameter, leading to the formation
of NiO nanocapsules.
(
−
O + 2e− → NO − + 2OH− E = −0.21 V vs. Ag/AgCl
o
Structural, spectral and morphological characterizations.—
XRD.—The crystal phase and structural information on the NiO were
NO3 + H
2
2
[
3-1]
obtained by XRD measurements. Fig. 1 shows XRD pattern of NiO
◦
after annealing of electrodeposited Ni(OH)
2
at 300 C for 3 h. The
O + 4e− → 4OH− E = 0.26 V vs. Ag/AgCl
o
[3-2]
observed indexed peaks in this XRD pattern are fully matched with the
corresponding pure cubic-structured crystalline NiO. Several peaks
O
2
+ 2H
2
◦
◦
◦
◦
◦
o
are observed in the pattern at 2θ = 37.1 , 43.3 , 62.8 , 75.2 , and 79.3
2H
2
O + 2e− → H
2
+ 2OH− E = −1.08 V vs. Ag/AgCl
can be assigned to the (111), (200), (220), (311), and (222) crystal
planes, respectively. The indexed peaks are fully consistent with the
cubic-structured crystalline NiO (JCPDS 47-1049). In addition, no
[
3-3]
(
2) Chemical step (deposit formation):
2
peaks from any impurities of Ni(OH) or other phases were observed
in the pattern which further confirmed the purity of the obtained
crystalline phase of the cubic NiO. Moreover, the observed peaks are
2
+
−
Ni (aq) + 2OH (aq) + y H
2
O → [Ni(OH)
2
.yH
2
O] ↓[
4-1]
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E134
Journal of The Electrochemical Society, 159 (6) E132-E138 (2012)
5
4
3
2
1
00
00
00
00
00
2
00
2
20
1
11
Figure 1. XRD pattern of NiO nanocapsules annealed
at 300 C for 3 h.
◦
3
11
2
22
1
0
20
30
40
50
60
70
80
2
(degree)
sharp which confirmed the well-crystallization of the obtained NiO
structures.
vibrations of the adsorbed water and (O–H) deformation vibrations,
respectively. The peaks at about 1487 cm and 1054 cm in both
samples can be attributed to the carbonate groups, which originate
−
1
−1
IR.—FT-IR analysis of the synthesized samples is important both for
understanding the composition of the products and reaction mecha-
nism. Fig. 2 shows the FT-IR transmission spectra of the NiO annealed
from the reaction of the samples with CO
sis procedure.
2
from air during the analy-
◦
at 300 C. Metal oxides such as NiO generally give absorption bands
SEM, TEM and AFM.—Fig. 3 shows SEM images of nickel oxide
representing a smooth surface with approximately homogeneous and
compact morphology. In fact the prepared NiO powder has been com-
posed exclusively of disoriented bundles of nanocapsules. It is ob-
servable from this sub-micrometer scale that the compact structure
−
1
below 800 cm arising from inter-atomic vibrations. The peaks at
60 cm and 520 cm show Ni–O stretching vibration and are clear
evidences about the presence of the crystalline NiO. The bands ap-
proximately at 3460 and 1640 cm are due to the (O–H) stretching
−1 −1
4
30
−1
1
0
0
0
0
.0
.8
.6
.4
.2
Figure 2. FTIR spectrum of NiO nanocapsules.
Ni-O-Ni modes
OH stretching mode
4
000
3500
3000
2500
2000
1500
1000
500
-
1
Wavenumber (cm )
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Journal of The Electrochemical Society, 159 (6) E132-E138 (2012)
E135
Figure 3. SEM images of NiO nanocapsules.
is an aggregate of particles arranged in different orientations. The
roughly oriented capsule-like particles tend to grow in groups which
lead to the formation of bundles of capsules (as seen in TEM images,
Fig. 4). The capsules are not uniform in size; rather they are 40–70
nm in length and 10–20 nm in diameter, as seen in TEM images
(
Fig. 4). In accordance to SEM and TEM images, AFM shows that
NiO is rather homogeneously distributed (Fig. 5) and has compact
morphology. Furthermore, BET analysis (Fig. 6) showed that the pre-
pared NiO nanocapsules possess a narrow microporous distribution at
2
around 2–4 nm and they have surface area of about 48.9 m /g. These
results clearly confirmed the compact morphology of NiO capsules.
Although, capsule like particles have large surface area but lack of
porous morphology in bulk may impede the electrolyte penetration in
these structures.
Electrochemical Measurements
Cyclic voltammetry.— Fig. 7 shows the cyclic voltammogram of
the prepared nickel oxide electrode in 1 M KOH at a scan rate of 5 mV
Figure 4. TEM images of NiO nanocapsules.
−
1
s
(
. Redox peaks are observed at +0.45 V (oxidation) and +0.35 V
reduction) in the potential range of −0.2 to +0.5 V vs. Ag/AgCl ref-
erence electrode in 1 M KOH solution. The thermodynamic instability
of NiO when immersed in aqueous electrolyte leading to formation of
60
hydroxide layer at the surface of NiO grains and agglomerates:
NiO + H O → Ni(OH) (at the surface of NiO)
2
2
[8]
A second step involves the electrochemical reaction based on
surface faradaic Ni2 /Ni redox process in alkaline solution, which
+
3
corresponds to the oxidation of the hydroxide into an oxyhydroxide
60
layer:
Ni(OH)
2
+ OH− → NiOOH + H
2
O + e−
[9]
However, electrochemical reactions take place at the bound-
ary where there is a transition between ionic conduction and elec-
tronic conduction. Since Ni(OH) is predominantly an ionic conduc-
2
tor (a solid electrolyte), the electrochemical reactions occur at the
−
Ni(OH)
2
/NiOOH interface, where neither H
2
O nor OH are present.
Therefore the electrochemical reaction of 9 should be written more
61,62
properly as follows:
Ni(OH)
2
− H+ → NiOOH + e−
[10]
The protons may transport away from the interface by the following
reaction occurred at the electrolyte/Ni(OH)
2
interface:
−
+ H+
OH(
→ H
2
O
[11]
Figure 5. AFM image of NiO nanocapsules.
electrolyte)
Ni(OH)2
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E136
Journal of The Electrochemical Society, 159 (6) E132-E138 (2012)
8
7
6
5
4
3
2
1
0
0
.008
Adsorption
Desorption
0
0
0
0
0
0
0
0.007
0.006
0.005
0.004
Figure 6. BET analysis of the prepared NiO nano-capsules. Inset
shows the BJH pore size distribution.
0.003
0
2
4
6
8
10
12
14
16
Pore Diameter (nm)
0
.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
1
1
5
(
a)
ORR
Hence, oxidation of Ni2 , leading to the formation of an oxyhy-
droxide (NiOOH), occurs at the electrode surface of NiO. High surface
area of capsule-like nanoparticles facilitates the anodic reaction. How-
ever, the reverse cathodic reaction involves solid state proton insertion
from the electrolyte into the crystal lattice. In this case, presence of
electrolyte is inevitable, which is more probably facilitated inside a
porous matrix.
+
NiO + OH
NiOOH+e
GC+CB+NiO
GC+CB
0
5
0
5
For potentials lower than +0.35 V vs. Ag/AgCl, the roughly rect-
angular mirror image with respect to the zero-current line indicates
purely capacitive current, which results from the electrical double-
layer capacitance (Fig. 7a). At higher potentials, the CV represents
redox (faradaic) reactions, through which, much more capacity can be
stored in the bulk of nickel oxide than in the electrical double-layer.
The contribution of the carbon black to the specific capacitance of
the electrode can be determined from CV curve in Fig. 7a (dashed
line). This measurement was done for carbon black on glassy carbon
electrode with equal weight and under the same experimental condi-
tions. From Fig. 7a, the specific capacitance of the carbon black was
-
NiOOH + e
NiO + OH
-
10
-
0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Potential/ V vs Ag/AgCl
−1
calculated to be approximately 3.5 F g . Thus the contribution of car-
bon black is negligible to the total capacitance of the NiO electrode
−
1
2
2
2
1
1
80
40
00
60
20
(415.2 F g ). In fact carbon black mainly acts as conductive filler
for the active material of electrode. The average specific capaci-
tance of the NiO was estimated from the CV curve in Fig. 7a using
Eq. 1. The specific capacitance of NiO capsules was calculated to be
(
b)
5
1
mV/S
0 mV/S
25 mV/S
− −1
1
4
15.2 F g at a scan rate of 5 mV s . Also, the observed high re-
5
1
2
0 mV/S
00 mV/S
00 mV/S
dox current could be attributed to an increased electrochemical active
surface area due to capsule-like nanostructures of NiO. Fig. 7b shows
the CVs of the NiO nanocapsules at various scan rates in the potential
range of −0.2–0.5 V. Generally, scan rates affects the specific capaci-
tance of an electrochemical capacitor rather than an ideal double-layer
8
4
0
0
0
1
8
capacitor. Current response in Fig. 7b is proportionally increased by
increasing the scan rate during CV scans. The specific capacitance of
NiO nanocapsules was calculated to be 415.2, 337.1, 246.8, 191.5,
−
1
1
64.3 and 146.8 F g at scan rates of 5, 10, 25, 50, 100 and 200 mV
s , respectively, (Fig. 7b). An extraordinary decrease in capacity
-
-
40
80
−1
−
1
−1
−1
from 415.2 F g (v = 5 mV s ) to 150 F g at high scan rates could
be attributed to a rather low porous, compact arrangement of capsule-
like nanoparticles and so lower penetration of electrolyte. Consid-
ering the calculated capacitances at the high scan rates of 100 and
-
0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Potential/ V vs. Ag/AgCl
−
1
2
00 mV s , it seems that the prepared nanocapsules are not suitable
for the high rate supercapacitor applications whereas at low scan rates
these nanostructures show acceptable capacitances which are compa-
rable with the reported ones for the other nanostructures of NiO i.e.
Figure 7. Cyclic voltammogram of (a) NiO nanocapsules compared with
blank electrode and (b) its CVs at various scan rates. GC: Glassy carbon and
CB: Carbon black.
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Journal of The Electrochemical Society, 159 (6) E132-E138 (2012)
E137
(
a)
sible presence of micro-pores as confirmed by BET analysis, which
cannot be wetted by the electrolyte or due to self-discharge reactions
according to (Eq. 12). Further work is necessary to optimize condi-
tions in order to increase the capacitive performance of capsule-like
NiO particles, which is depend on the composition and pH value of
the electrolyte medium.
0
0
0
0
0
0
.5
.4
.3
.2
.1
.0
Charge/discharge tests.— The galvanostatic charge/discharge cy-
cling (3000 cycles) was recorded at a constant current density of 1 A
−
1
g
and the first 10 cycles is presented in Fig. 8a. The shapes of the
charge–discharge curves do not display the characteristic of a pure
electric double layer capacitor, but mainly pseudocapacitance, which
corresponds with the result of the CV tests. The specific capacitance
of NiO electrode was calculated by the Eq. 2. The specific capacitance
-
-
-
0.1
0.2
0.3
th
−1
value (based on the 10 cycle) was calculated to be 307.2 F g which
is very close to that of calculated from the CV curves (Fig. 7b). The
specific capacitances of NiO electrode after 500 and 3000 discharg-
−
1
0
500
1000 1500 2000 2500 3000 3500 4000 4500 5000
Time (s)
ing cycles were also calculated to be 290.2 and 265.4 F g which
show 5.5% and 13.6% capacity losses (Fig. 8b). Thus NiO particles
have a relative high stable capacitance during cycling, and therefore
can be suitable for long-time capacitor applications in KOH. These
results showed that the prepared NiO nanostructure can be recognized
a promising candidate for supercapacitor applications.
3
00
50
00
50
00
(b)
2
Conclusions
2
1
1
Nanostructured electroactive NiO was prepared by electrochem-
ical cathodic deposition of nickel hydroxide on steel, and its post-
◦
heating at 300 C for 3 hours in air. The XRD results showed that
the obtained product has a cubic phase of NiO. Morphological char-
acterizations revealed that the product has compact morphology with
capsule-like structures having the average particle diameter and length
of 20 nm and 50 nm, respectively. Electrochemical performances such
as specific capacitance, charge/discharge, and cycle life of the NiO
capsules were investigated in 1 M KOH electrolyte at potential range
of −0.2–0.5 V vs. Ag/AgCl. The specific capacitance of NiO nanocap-
5
0
0
−
1
−1
sules was calculated to be 337.1 F g at a scan rate of 10 mV s .
−
1
0
100 200 300 400 500 1000
Cycle number
1500
2000
2500
3000
Cycle-life stability studies at current density of 1 A g showed that
the NiO electrode is more electrochemically stable where it maintains
approximately 86.4% of its maximum capacitance after 3000 cycles.
In conclusion, the synthesized NiO nanocapsules can be considered
as a promising candidate for used in electrochemical capacitors.
Figure 8. Galvanostatic charge/discharge curves (first 10 cycles) of NiO
nanocapsules at a current density of 1 A g (a) and their cycle life (b).
−1
− −1
1
15
12
5
3
3
g
80 F g at scan rate of 5 mV s for nanoporous thin NiO film,
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