Morphological and structural studies of nanoporous nickel oxide films fabricated by anodic electrochemical deposition techniques

Article abstract of DOI:10.1016/j.electacta.2008.08.027

Porous nickel oxide films are directly deposited onto conducting indium tin oxide coated glass substrates by cyclic voltammetric (CV), galvanostatic, and potentiostatic strategies in a plating bath of sodium acetate, nickel sulfate, and sodium sulfate. By tuning the deposition parameters, it is possible to prepare nickel oxide films with various morphologies and structures. Film formation relies on the oxidation of dissolved Ni²⁺ to Ni³⁺, which further reacts with the available hydroxide ions from a slightly alkaline electrolyte to form insoluble nickel oxide/hydroxide deposits on the substrate. A compact film with particularly small pores is obtained by CV deposition in a potential range of 0.7-1.1 V. A galvanostatically deposited film is structurally denser near the surface of the substrate, and becomes less dense further away from the surface. Interestingly, a potentiostatically deposited film has pores distributed uniformly throughout the entire film. Therefore, for obtaining a uniform film with suitable pore size for electrolyte penetration, potentiostatic deposition technique is suggested. In addition, except for CV deposition, the deposited films resemble closely to cubic NiO when the annealing temperature exceeds 200 °C.

Full text of DOI:10.1016/j.electacta.2008.08.027

Electrochimica Acta 54 (2008) 155–161
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Morphological and structural studies of nanoporous nickel oxide films
fabricated by anodic electrochemical deposition techniques
Mao-Sung Wu∗, Chung-Hsien Yang, Min-Jyle Wang
Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 May 2008
Received in revised form 18 August 2008
Accepted 18 August 2008
Available online 23 August 2008
Porous nickel oxide films are directly deposited onto conducting indium tin oxide coated glass substrates
by cyclic voltammetric (CV), galvanostatic, and potentiostatic strategies in a plating bath of sodium acetate,
nickel sulfate, and sodium sulfate. By tuning the deposition parameters, it is possible to prepare nickel
oxide films with various morphologies and structures. Film formation relies on the oxidation of dissolved
Ni2+ to Ni3+, which further reacts with the available hydroxide ions from a slightly alkaline electrolyte to
form insoluble nickel oxide/hydroxide deposits on the substrate. A compact film with particularly small
pores is obtained by CV deposition in a potential range of 0.7–1.1 V. A galvanostatically deposited film
is structurally denser near the surface of the substrate, and becomes less dense further away from the
surface. Interestingly, a potentiostatically deposited film has pores distributed uniformly throughout the
entire film. Therefore, for obtaining a uniform film with suitable pore size for electrolyte penetration,
potentiostatic deposition technique is suggested. In addition, except for CV deposition, the deposited
films resemble closely to cubic NiO when the annealing temperature exceeds 200 ◦C.
Keywords:
Anodic deposition
Porous nickel oxide
Nanostructured materials
ITO glass
© 2008 Elsevier Ltd. All rights reserved.
1. Introduction
penetration. Wetting, which determines the electrochemical per-
formance of electrodes, in the porous electrode is controlled mainly
In recent years, nickel oxides have attracted much attention due
to their potential applications in thin-film batteries, electrochromic
films, optical materials, fuel cell electrodes, photocatalysts, elec-
trochemical capacitors, etc. [1–27]. Most of these useful functions
depend markedly on the composition, the morphology, and the
structure of the oxide deposits. Nanostructured nickel oxides
turned out to possess better properties than those of their bulk
counterparts [2]. A variety of preparation methods, such as chemi-
cal precipitation [1–7], thermal decomposition [8,9], hydrothermal
process [10,11], surfactant template [12–15], polymer template
[16,17], sol–gel method [18,19], and electrodeposition [20–27], have
been explored to fabricate nanostructured nickel oxides to effec-
tively improve the desired functionality.
When used in electrochemical applications, usually nickel oxide
powder is processed to form porous electrodes. Many of porous
nickel oxide electrodes are of considerable practical interest owing
to their charge storage capabilities, electrochromic activities, elec-
trocatalytic properties, and photoelectrochemical activities. Porous
electrodes may offer high-specific surface area for electrolyte
by the electrolyte penetration and spreading in pores [28]. In prac-
tice, a porous electrode composed of nano-sized materials is more
difficult to fabricate due to the poor dispersibility of nano-sized
materials in slurry (composed of organic solvent, nano-sized mate-
rials, polymer binder, conducting agent, etc.). Some of the above
methods for preparing nickel oxide powder suffer from difficulty in
the fabrication of well-dispersed nickel oxide electrodes for elec-
trochemical applications. In addition, it has been shown that the
electrochemical deposition of porous nickel oxide electrodes pro-
duces superior electrochemical performances compared to those
made by the conventional chemical loading process [5,23]. Thus,
it is more advantageous to have a porous electrode of nanostruc-
tured nickel oxide fabricated by electrochemical deposition, which
deposits the active material directly onto the substrate at room
temperature without templates.
Nanostructured materials are of great importance for the
electrochemical energy-storage devices because they have high-
specific surface area and fast redox reactions [15]. In electro-
chemical preparation of the nickel oxide electrodes, the cathodic
deposition process has been previously used in the preparation
of thin-film electrodes of nickel hydroxide in nitrate-based bath
[20–22], but the electrochemical anodic deposition has rarely
been studied. In addition, most of the electrochemically deposited
nickel oxides are nanoparticles. Our previous work showed that
∗
Corresponding author. Fax: +886 9 45614423.
E-mail address: ms wu@url.com.tw (M.-S. Wu).
0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2008.08.027

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the anodically deposited nickel oxide film at a current density of
0.25 mA cm−2 in a plating bath of sodium acetate, nickel sulfate,
and sodium sulfate is nanoporous and composed of intercon-
nected nanoflakes [25]. It was reported that surface morphology
of the electrodeposited manganese oxide depends on the depo-
sition conditions such as current and potential [29]. Therefore,
in this work, in order to achieve different morphologies and
structures of the nickel oxide/hydroxide films, the deposition
parameters such as deposition potential and current density are
varied in a plating bath consisting of sodium acetate, nickel
sulfate, and sodium sulfate. The proposed electrochemical depo-
sition technique has a great advantage over the others because
mass and thickness of the nickel oxide film can be easily con-
trolled by adjusting the potential, current, bath chemistry, and
temperature.
(GAXRD, Rigaku D/MAX2500, Japan) with a Cu K␣ target (wave-
length = 1.54056 Å).
3. Results and discussion
3.1. Cyclic voltammetric deposition
Fig. 1 shows cyclic voltammograms associated with the nickel
oxide/hydroxide deposition in a scanned potential range between
0 and 1.6 V. As evident from the inset of Fig. 1, the first cycle of a
CV with the specified anodic and cathodic potential limits is quite
different from the subsequent cycles. An oxidation peak labeled O2
and a reduction peak labeled R1 are not observed in the first scan
cycle, but these two peaks appear in the subsequent cycles. Clearly,
in the first anodic scan, the corresponding oxidation current starts
to occur at a potential of about 0.85 V vs. SCE, possibly due to an
oxidation reaction of Ni2+ to Ni3+. At higher anodic potentials, this
oxidation reaction is accompanied with the oxygen evolution reac-
tion. Initially formed Ni3+ ions react with hydroxide ions to form
an insoluble nickel oxy-hydroxy film presumably as NiOOH on the
surface of transparent ITO substrate [30]. R1 can be attributed to
a reduction reaction of Ni3+ to Ni2+. This reveals that most of the
Ni3+ ions in close vicinity of the electrode/electrolyte interface react
with the hydroxide ions to form insoluble NiOOH in the first anodic
scan. The peak current (R1) increases on each cycle, indicating that
some Ni3+ ions may not react with the hydroxide ions to form insol-
uble deposits during anodic scan and these unreacted Ni3+ ions in
close vicinity of the electrode/electrolyte interface are reduced to
Ni2+ ions in the subsequent cathodic scans.
In the cathodic scans, except for a cathodic peak R1, another
cathodic peak labeled R2 appears at about 0.52 V. Probably, this
current peak is due to a reduction reaction of NiOOH to Ni(OH)2
[30]. The corresponding oxidation reaction of Ni(OH)2 to NiOOH is
labeled with O2. These two peaks, R2 and O2, increase with each
cycle meaning that the current increases on each cycle as new Ni2+
is oxidized and deposited, and the oxide deposit is cycled through
the redox reactions between NiOOH and Ni(OH)2.
As mentioned above, the cathodic and anodic peaks, R2 and O2,
correspond to reduction and oxidation of the nickel oxy-hydroxy
film, which is deposited by the oxidation of Ni2+ to Ni3+ and precip-
itates on the electrode surface, presumably as NiOOH. The overall
2. Experimental
Nanostructured nickel oxide films were electrochemically
deposited at room temperature onto the ITO coated glass sub-
strates (1 cm × 1 cm) and the stainless steel (SS) foils (1 cm × 1 cm)
by galvanostatic, potentiostatic, and cyclic voltammetric strate-
gies, respectively. The plating solution consisted of 0.13 M sodium
acetate, 0.13 M nickel sulfate, and 0.1 M sodium sulfate [25,30]. The
pH of the plating solution was about 6.9. All chemicals were ana-
lytical grade and were used without further purification. Prior to
electrochemical deposition, SS foils were polished by emery paper
with grain size of about 65 ␮m, and rinsed by ultrasonic vibration
in acetone and de-ionized water, respectively. The transparent ITO
glass was cut into pieces of 1 cm × 4 cm, which were then soaked
in acetone and ultrasonically vibrated for 20 min to wash away any
contaminants from their surface. De-ionized water was then used
to rinse the ITO glass and SS foil in ultrasonic vibration for another
15 min. To control the reaction area, every ITO sample was taped by
PET (polyethylene terephthalate) film (Alliance Material S221-H,
Taiwan) to leave an exposed surface area of 1 cm × 1 cm. A pre-
process was applied to ensure the uniformity and adhesion of the
deposits: conduct a 2 mA cm−2 current through each ITO sample in
1 M NaOH for 30 s for anodic oxidation.
The plating solution was stirred by a Teflon stir bar on a mag-
netic hot plate during the course of deposition. After deposition,
the resultant brown-colored films were rinsed several times in de-
ionized water and then dried at different temperatures for 1 h in
air. All electrochemical nickel oxide deposition experiments were
carried out using a potentiostat (CH Instruments CHI 608, USA)
in a beaker-type electrochemical cell equipped with the working
electrode, a platinum counter electrode, and a saturated calomel
electrode (SCE). The geometric area of a platinum counter electrode
was 2 cm × 2 cm.
Electrochemical characteristics of the deposited films were
determined by galvanostatic charge and discharge in a three-
electrode cell with a 1 M KOH electrolyte. A platinum foil with
dimension of 2 cm × 2 cm was used as a counter electrode. Charge
and discharge tests were performed by a potentiostat (Princeton
Applied Research EG&G 362, USA) in a potential range of 0–0.45 V
at a current density of 2.8 A g−1. All data in EG&G 362 were acquired
through an interface card (Labjack, U12, USA) with LabVIEW soft-
ware. The amount of nickel oxide deposits for charge/discharge
tests was measured by a microbalance (Ohaus G160, USA) with an
accuracy of 0.01 mg and was kept almost the same (about 0.5 mg)
for each electrode by adjusting the deposition time.
Fig. 1. Cyclic voltammograms associated with the nickel oxide/hydroxide deposi-
tion on the ITO substrate in the scanned potential range between 0 and 1.6 V. Scan
rate was 10 mV s−1. The plating solution consisted of 0.13 M sodium acetate, 0.13 M
nickel sulfate, and 0.1 M sodium sulfate. The pH of the plating solution was about
6.9.
Surface morphology and structure of cross-sections of the films
were examined with a field-emission electron microscope (FE-
SEM, Hitachi S-4700I, Japan). The crystal structure of nickel oxide
coatings was identified by a glance angle X-ray diffractometer

M.-S. Wu et al. / Electrochimica Acta 54 (2008) 155–161
157
the redox reactions between Ni2+ and Ni3+; R2 and O2 come from
the redox reactions between Ni(OH)2 and NiOOH.
3.2. Galvanostatic deposition
The potential variations with time during galvanostatic depo-
sition with different current densities applied to the substrate
electrode were measured. The deposition current was varied from
0.05 to 1.0 mA cm−2. The related graph is not shown here. At the
beginning of galvanostatic deposition, the corresponding potential
decreases rapidly and then stabilizes in the ongoing deposition. The
higher the deposition current, the higher the established plateau
value of the deposition potential. Furthermore, for nickel oxide
deposition with a current density of 1 mA cm−2 an increase in the
deposition potential was observed about 1400 s after initiation. At
the same time, it was observed that some deposits begin to exfoliate
from the ITO substrate due to the oxygen evolution. The exfolia-
tion decreases the surface area of deposited oxide film available
for electrochemical deposition, therefore increasing the deposition
overpotential. On the other hand, some small oxygen bubbles may
be adsorbed on the surface of deposits leading to an increased depo-
sition overpotential. Consequently, a high-current density during
deposition is detrimental to the deposited oxide film.
Fig. 2. Cyclic voltammograms associated with the nickel oxide/hydroxide depo-
sition on the ITO substrate in a scanned potential range between 0.7 and 1.2 V.
Scan rate was 10 mV s−1. The plating solution consisted of 0.13 M sodium acetate,
0.13 M nickel sulfate, and 0.1 M sodium sulfate. The pH of the plating solution was
about 6.9.
reaction can be schematically represented as follows [30]:
3.3. Potentiostatic deposition
O1
Ni2+ꢀ Ni3+ + e−
(1a)
R1
The current variations with time during potentiostatic deposi-
tion at different potentials were measured. The deposition potential
was varied from 0.85 to 1.05 V. The related graph is not displayed
here. Results show that the deposition current increases with
increasing the deposition potential. At the beginning of poten-
tiostatic deposition, except for the lowest deposition potential
(0.85 V), the current density initially increases with deposition
potential before it stabilizes to about constant levels at longer times
of deposition. For the chosen potentials, the current density is par-
ticularly small when the oxide formation (deposition) is carried out
at 0.85 V. However, a little amount of deposit was obtained under
this condition after 1 h of potential application meaning that oxide
deposition takes place even at a low potential of 0.85 V.
Ni3+ + 3OH− → NiOOH ↓ + H2O
R2
NiOOH + H+ + e− ꢀNi(OH)2
O2
(1b)
(2)
Oxide deposits may be damaged and exfoliated from the ITO
substrate by the generation of oxygen bubbles, which clearly were
observed at higher anodic potentials. The oxygen evolution reaction
can be alleviated by limiting the anodic potential to lower val-
ues, therefore, cyclic voltammograms were scanned in a potential
range between 0 and 1.2 V. The shape of CVs in a scanned poten-
tial range of 0–1.2 V, which is not shown here, is similar to that of
CVs in Fig. 1, except that current peaks, O2 and R2, are significantly
decreased by lowering the positive limit potential from 1.6 to 1.2 V.
The result indicates that the oxidation reaction of Ni2+ to Ni3+ is
also mitigated by lowering the anodic potential from 1.6 to 1.2 V. As
a result, the lower the concentration of Ni3+, the lower the amount
of oxide deposits. Consequently, a decrease in redox peak currents
of reaction (2) is expected.
A dark brown deposit is observed in the first anodic scan, a
transparent deposit in the first cathodic scan. It was reported that
the nickel oxy-hydroxy films prepared by potential cycling tech-
nique appear to be uniform and smooth and exhibit electrochromic
properties rapidly turning from transparent to dark brown at well-
defined potentials [30]. The electrochromic properties of nickel
oxides were reported by the redox processes of NiOOH (dark brown)
and Ni(OH)2 (transparent) according to reaction (2) [31].
In order to distinguish reaction (1a) from (2), the potential is
scanned between 0.7 and 1.2 V, results are shown in Fig. 2. It is clear
that the couple of redox peaks, O2 and R2, disappear, but peak R1
as well as the oxidation current (the oxidation reaction of Ni2+ to
Ni3+) and the oxygen evolution remain. These results indicate that a
reduction peak R1 is ascribed to a reduction reaction of Ni3+ to Ni2+
according to reaction (1a). In addition, the deposited film remains
dark brown in a potential range between 0.7 and 1.2 V, a transpar-
ent deposit cannot be observed meaning that the electrochromic
effect of nickel oxide according to reaction (2) is not involved in
this scanned potential range. Consequently, R1 and O1 come from
3.4. Surface morphology of the deposits
Fig. 3 shows the surface morphologies of nickel oxide films
deposited at two different potential windows for 10 cycles. The
positive limit potential was held at 1.1 V, while the lower limit
potential was either 0.4 or 0.7 V. Obviously, the resulting nickel
oxide coatings show a significantly different surface morphology.
Scattered or dense deposits are observed with the potentials being
scanned between 0.4 and 1.1 V and 0.7 and 1.1 V, respectively. In
addition, the total amount of deposits obtained with a CV poten-
tial range of 0.4–1.1 V is much less than that gained through a
scan from 0.7 to 1.1 V, meaning that the deposition rate depends
strongly on the lower limit potential during CV deposition. As evi-
dent from Fig. 2, a film deposited at a potential range of 0.7–1.1 V
involves reactions (1a) and (1b). However, as a film is deposited at
a potential range of 0.4–1.1 V, in addition to reactions (1a) and (1b),
reaction (2) should also be involved (referring to Fig. 1). A reason
for such low deposition rate could be attributed to the formation of
Ni(OH)2 film via reaction (2). It was reported that pure Ni(OH)2
powder exhibits poorly electrical conductivity [6]. For instance,
the electrical conductivity of Ni(OH)2 electrode used in nickel-
metal hydride batteries is very low and insufficient for high-rate
charge/discharge; it may be improved by the addition of conduct-
ing additives such as cobalt [32], nickel [33], and carbon black [34].

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M.-S. Wu et al. / Electrochimica Acta 54 (2008) 155–161
Fig. 3. Surface morphology of a film deposited on the ITO substrate by CV strategy
at different potential windows for 10 cycles: (a) 0.4–1.1 V and (b) 0.7–1.1 V. Scan rate
was 10 mV s−1. The plating solution consisted of 0.13 M sodium acetate, 0.13 M nickel
sulfate, and 0.1 M sodium sulfate. The pH of the plating solution was about 6.9. All
films were annealed at 100 ◦C for 1 h.
Therefore, it is more difficult to deposit nickel oxy-hydroxy on the
Ni(OH)2 film resulting in a decrease in the deposition rate.
Fig. 4 shows the surface morphology of films deposited galvano-
statically at different current densities. Obviously, all the films are
porous and composed of interconnected nanoflakes of a thickness
of ca. 12–16 nm. Interestingly, a film deposited at lower current
density has bigger pores compared with those deposited at higher
current density, reflecting that pore size depends strongly on the
current density during the deposition. The lower the deposition
current density, the bigger the pore. Similar trend in pore size is
also found in the potentiostatic deposition shown in Fig. 5. It is
generally believed that the nucleation strongly depends on the
overpotential during electrochemical deposition. An increase in
the overpotential results in an increase in the nucleation rate and
a decrease in the radius of cluster on the substrate and deposit.
As a result of this, a compact film is formed, and the pore size of
film decreases with increasing the deposition potential. The pore
size of deposited films may play an important role in electrolyte
penetration. A bigger pore is easily accessible by the electrolyte,
while a smaller pore is unfavorable to the electrolyte penetration.
Therefore, it is reasonable that a nickel oxide film with intercon-
nected nanoflakes and bigger pores is in favor of its electrochemical
behavior.
Fig. 4. Surface morphology of the film deposited galvanostatically on the ITO sub-
strate at different current densities: (a) 0.05 mA cm−2, (b) 0.25 mA cm−2, and (c)
1.0 mA cm−2. The plating solution consisted of 0.13 M sodium acetate, 0.13 M nickel
sulfate, and 0.1 M sodium sulfate. The pH of the plating solution was about 6.9. All
films were annealed at 100 ◦C for 1 h.
3.5. Cross-sectional images of the films
Cross-sectional images of the films deposited by CV, gal-
vanostatic, and potentiostatic strategies are shown in Fig. 6a–c,
respectively. A compact film is obtained by CV deposition in a
potential range of 0.7–1.1 V for 150 scan cycles, the pores within

M.-S. Wu et al. / Electrochimica Acta 54 (2008) 155–161
159
Fig. 5. Surface morphology of the film deposited on the ITO substrate at different
potentials: (a) 0.9 V, (b) 1.0 V, and (c) 1.1 V. The plating solution consisted of 0.13 M
sodium acetate, 0.13 M nickel sulfate, and 0.1 M sodium sulfate. The pH of the plating
solution was about 6.9. All films were annealed at 100 ◦C for 1 h.
Fig. 6. Cross-sectional images of the films: (a) deposited by CV strategy in a
potential range between 0.7 V and 1.1 V, (b) deposited by galvanostatic strategy at
0.25 mA cm−2, and (c) deposited by potentiostatic strategy at 0.9 V. The plating solu-
tion consisted of 0.13 M sodium acetate, 0.13 M nickel sulfate, and 0.1 M sodium
sulfate. The pH of the plating solution was about 6.9. All films were deposited on the
ITO substrate and annealed at 100 ◦C for 1 h.
the deposited film are particularly small. A film deposited gal-
vanostatically at a current density of 0.25 mA cm−2 is much denser
near the surface of the ITO substrate, and becomes less dense
further away from the surface of the substrate. This is for the
reason that the deposition potential is high in the initial stage
of deposition and in course of the procedure decreases to a sta-
ble value. High overpotential is prone to form dense film, and
therefore a denser film is formed near the surface of the sub-
strate electrode during galvanostatic deposition. Interestingly, it
is evident that a film deposited galvanostatically at a poten-
tial of 0.9 V has bigger pores distributed uniformly over the
entire film. A porous film with bigger pores is beneficial to the

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M.-S. Wu et al. / Electrochimica Acta 54 (2008) 155–161
Fig. 7. XRD pattern of the film deposited galvanostatically on the SS substrate at
0.25 mA cm−2 after annealing at various temperatures: (a) substrate, (b) 100 ◦C, (c)
200 ◦C, (d) 250 ◦C, (e) 300 ◦C, (f) 400 ◦C, and (g) 500 ◦C. The plating solution consisted
of 0.13 M sodium acetate, 0.13 M nickel sulfate, and 0.1 M sodium sulfate. The pH of
the plating solution was about 6.9.
Fig. 8. (a) XRD patterns of the film deposited on the SS substrate at 0.9 V after
annealing at various temperatures; (b) XRD patterns of the film deposited on the
SS substrate by CV strategy in a potential range of 0.7–1.1 V after annealing at var-
ious temperatures. The plating solution consisted of 0.13 M sodium acetate, 0.13 M
nickel sulfate, and 0.1 M sodium sulfate. The pH of the plating solution was about
6.9.
ferent temperatures are shown in Fig. 8a. It is evident that XRD
patterns of the potentiostatically deposited film after annealing are
cubic NiO similar to those of galvanostatically deposited films. The
XRD patterns of films deposited by CV strategy after annealing are
shown in Fig. 8b. Interestingly, no detectable characteristic peak
of NiO is found in the XRD patterns even when annealed at a high
temperature of 400 ◦C, representing the amorphous structure of CV
deposited films.
electrolyte penetration leading to an increase in surface area avail-
able for electrochemical reactions. Therefore, for the purpose of
obtaining a uniform film with suitable pore size for electrolyte
penetration, the potentiostatic strategy is better than other two
strategies.
3.6. Crystal structure of the deposits
In order to obtain the required amount of deposits for determin-
ing the crystal structure of films, nickel oxide films were deposited
onto the SS substrate instead of the ITO coated glass substrate. This
is for the reason that a thick nickel oxide film is prone to exfo-
liate from the ITO substrate rather than the SS substrate during
deposition. The XRD patterns of galvanostatically deposited films
after annealing at various temperatures are shown in Fig. 7. Except
for the diffraction peaks of the SS substrate, there is no signifi-
cant peak in the XRD pattern of the film after 100 ◦C annealing,
representing an amorphous phase. As the annealing temperature
is 200 ◦C and above, in addition to the diffraction peaks of the SS
substrate, some characteristic diffraction peaks of the nickel oxide
appear. The XRD patterns obtained for deposits treated at 200,
250, 300, 400, and 500 ◦C resemble closely to cubic NiO (nickel
oxide, JCPDS 47-1049). Change in crystallinity after annealing at
higher temperature may be attributed to the removal of both the
water of hydration and the absorbed water from the solid phase
of nickel oxide structure, thereupon the amorphous nickel oxide
phase is converted into crystalline phase [35,36]. In addition, water
content within nickel oxide is known to affect the electrochem-
ical reactivity and the thermodynamic stability of nickel oxide
[37].
3.7. Charge/discharge of the potentiostatically deposited films
Fig. 9 shows the galvanostatic charge/discharge curves of the
nickel oxide films deposited potentiostatically at 0.9 and 1.1 V,
respectively. Prior to charge and discharge measurements, both
deposited films were annealed at 300 ◦C for 1 h. The charge and
The crystal structure of films deposited galvanostatically at
different current densities was measured. All galvanostatically
deposited films were annealed at 400 ◦C for 1 h before measure-
ments. There is no significantly difference in crystal structure of
the films deposited at different current densities varying from 0.05
to 1.0 mA cm−2, reflecting that the effect of deposition current den-
sity on the crystal structure can be negligible. All galvanostatically
deposited films resemble closely to cubic NiO (nickel oxide, JCPDS
47-1049) after annealing at 400 ◦C. Furthermore, XRD patterns of
the film deposited potentiostatically at 0.9 V after annealing at dif-
Fig. 9. Galvanostatic charge/discharge curves of the nickel oxide films deposited
on the SS substrate. The plating solution consisted of 0.13 M sodium acetate, 0.13 M
nickel sulfate, and 0.1 M sodium sulfate. The pH of the plating solution was about
6.9. Prior to electrochemical measurements, all films were annealed at 300 ◦C for
1 h. The amount of nickel oxide deposit for both films was about 0.5 mg. The charge
and discharge current densities were set at a constant current density of 2.8 A g−1
.

M.-S. Wu et al. / Electrochimica Acta 54 (2008) 155–161
161
discharge current densities were set at a constant current den-
Acknowledgment
sity of 2.8 A g−1. Total time for charging (from 0 to 0.45 V) and
discharging (from 0.45 to 0 V) processes are almost the same at
each cycle. Similar time length implies a high reversibility of the
Faraday’s reaction on the active surface of the nickel oxide grains.
Generally, the specific capacitance (C) of an electrode during gal-
vanostatic charge/discharge can be calculated by the following
equation:
The authors gratefully acknowledge a financial support from the
National Science Council, Taiwan, Republic of China (Project No:
NSC-96-2221-E-151-027).
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i ꢀt
C =
(3)
w ꢀV
where w is the mass of nickel oxide (g), ꢀV is the potential win-
dow (V), i is the discharge current (A) applied for time ꢀt (s). The
specific capacitance of film deposited at 0.9 V (320 F g−1) is much
higher than that of deposited at 1.1 V (136 F g−1). This reflects that
the pores within a nickel oxide deposited at 0.9 V are larger and
better for electrolyte penetration.
4. Conclusions
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Nickel oxide/hydroxide films are deposited anodically on the
ITO and SS substrates, respectively. The deposition conditions
such as potential and current influence both the morphology
and the structure of deposits. Results show that the film growth
rate is particularly low during CV deposition in a potential range
of 0.4–1.1 V, possibly due to the formation of poorly conduct-
ing Ni(OH)2 through the reduction of NiOOH. The film growth
rate is much improved by shifting the lower limit potential from
0.4 to 0.7 V, which mitigates the formation of Ni(OH)2. A film
deposited galvanostatically at a current density of 0.25 mA cm−2
is more compact near the surface of the substrate electrode, and
becomes less compact further away from the surface of the sub-
strate electrode. A high potential at the beginning of deposition
is responsible for forming a compact film near the surface of
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with uniform pore distribution can be achieved by potentiostatic
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0.9 V shows a higher specific capacitance compared with that of
deposited at 1.1 V, because film deposited at 0.9 V has larger pore
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