Electrochromic properties of porous NiO thin film as a counter electrode for NiO/WO3 complementary electrochromic window

Article abstract of DOI:10.1016/j.electacta.2011.01.078

Highly porous nickel oxide (NiO) thin films were prepared on ITO glass by chemical bath deposition (CBD) method. SEM results show that the as-deposited NiO film is constructed by many interconnected nanoflakes with a thickness of about 20 nm. The electrochromic properties of the NiO film were investigated in a nonaqueous LiClO₄-PC electrolyte by means of optical transmittance, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements. The NiO film exhibits a noticeable electrochromic performance with a variation of transmittance up to 38.6% at 550 nm. The CV and EIS measurements reveal that the NiO film has high electrochemical reaction activity and reversibility due to its highly porous structure. The electrochromic (EC) window based on complementary WO₃/NiO structure shows an optical modulation of 83.7% at 550 nm, much higher than that of single WO₃ film (65.5% at 550 nm). The response time of the EC widow is found to be about 1.76 s for coloration and 1.54 s for bleaching, respectively. These advantages such as large optical modulation, fast switch speed and excellent cycle durability make it attractive for a practical application.

Full text of DOI:10.1016/j.electacta.2011.01.078

Electrochimica Acta 56 (2011) 4281–4286
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrochromic properties of porous NiO thin film as a counter electrode for
NiO/WO complementary electrochromic window
3
a
a
a,∗
a
a
b
b
H. Huang , J. Tian , W.K. Zhang , Y.P. Gan , X.Y. Tao , X.H. Xia , J.P. Tu
a
College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou 310014, China
Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 August 2010
Received in revised form 18 January 2011
Accepted 22 January 2011
Available online 28 January 2011
Highly porous nickel oxide (NiO) thin films were prepared on ITO glass by chemical bath deposition
CBD) method. SEM results show that the as-deposited NiO film is constructed by many interconnected
nanoflakes with a thickness of about 20 nm. The electrochromic properties of the NiO film were inves-
tigated in a nonaqueous LiClO4–PC electrolyte by means of optical transmittance, cyclic voltammetry
(
(CV) and electrochemical impedance spectroscopy (EIS) measurements. The NiO film exhibits a notice-
able electrochromic performance with a variation of transmittance up to 38.6% at 550 nm. The CV and
EIS measurements reveal that the NiO film has high electrochemical reaction activity and reversibility
due to its highly porous structure. The electrochromic (EC) window based on complementary WO3/NiO
structure shows an optical modulation of 83.7% at 550 nm, much higher than that of single WO3 film
(65.5% at 550 nm). The response time of the EC widow is found to be about 1.76 s for coloration and 1.54 s
for bleaching, respectively. These advantages such as large optical modulation, fast switch speed and
excellent cycle durability make it attractive for a practical application.
Keywords:
Nickel oxide
Electrochromism
Thin film
Electrochromic window
Lithium intercalation
©
2011 Elsevier Ltd. All rights reserved.
1
. Introduction
electrochromism and electrochemical performance of NiO films in
nonaqueous electrolyte solutions [15,16].
Electrochromic (EC) materials are able to change their optical
A high performance EC device should present high optical con-
trast, good optical memory and chemical stability to electrochromic
cycles. NiO films are being developed for application in dynamic
or “smart” windows that are at the forefront of emerging energy-
saving advances in building technologies [17]. NiO can be used as a
counter electrode to WO3 electrode in EC window, where the opti-
cal modulation increases because they are quite complementary
[18,19]. However, in the fabricating process of EC window, they are
chemically incompatible when NiO and WO3 have to be immersed
in acidic or alkaline aqueous electrolytes. The aqueous electrolyte
can be replaced by nonaqueous solvents such as propylene carbon-
ate (PC) and ␥-butyrolactone. It has been revealed that WO3 has
properties reversibly and persistently by an external voltage.
The transition metal oxide thin films, e.g. oxides of W, Ir, V, Ti
and Ni [1–4], are widely studied for their electrochromic behav-
ior. Among these materials, NiO is of considerable interest due
to high coloration efficiency (CE), good cyclic reversibility and
low material cost [5,6]. In the past years, many studies on the
electrochemical and electrochromic properties of NiO have been
conduced in aqueous electrolytes, where the optical modulation
depends on proton/electron transport [7–10]. Additionally, the
+
electrochromic effect during the injection/extraction of Li in/from
NiO films has been observed in water-free organic system [11–14].
Considering practical applications, high coloration efficiency
and long-term durability are required for NiO film. Therefore,
nonaqueous organic solvents are more suitable as the electrolyte
solution in comparison with aqueous electrolyte solution because
the nonaqueous electrolyte has a wider potential window for
electrochemical reactions. However, there are a few reports on the
a very reversible electrochromic behavior in the cited LiClO –PC
4
electrolyte [20–22]. The use of WO3 as the primary electrode in EC
window requires a counter electrode, which is reversible and elec-
trochromic in a complementary mode. The porous structure of NiO
film is believed to be helpful to the enhancement of electrochromic
performance. In our previous work, we found that the NiO film pre-
pared by chemical bath deposition (CBD) method had highly porous
structure and exhibited excellent EC properties in KOH electrolyte
[
23]. However, to the best of our knowledge, few investigations
∗ Corresponding author at: College of Chemical Engineering and Materials Sci-
ence, Zhejiang University of Technology, Zone 6, Chaohui, Hangzhou, Zhejiang
have been conducted in organic systems. In the present work, we
employed a simple CBD method to prepared highly porous NiO
film and investigated the EC properties of NiO films as the counter
3
10014, China. Tel.: +86 571 88320394; fax: +86 571 88320394.
E-mail address: msechem@zjut.edu.cn (W.K. Zhang).
0
013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.01.078

4
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H. Huang et al. / Electrochimica Acta 56 (2011) 4281–4286
electrode of an EC window based on NiO/WO3 complementary
structure.
*
NiO
*
β
-Ni(OH)2
*
*
γ
-NiOOH
2
. Experimental
*
*
d
c
b
The porous NiO film was prepared by a chemical bath deposition
(
CBD) method [24]. The solution for CBD was obtained by mixing
8
2
0 ml of 1 M nickel sulfate, 60 ml of 0.25 M potassium persulfate and
0 ml of aqueous ammonia (25–28% NH ) in a 250 ml pyrex beaker
3
at room temperature. Clean indium–tin oxide (ITO, sheet resistance
·
·
2
5 ꢀ/ꢀ, 4 cm × 5 cm in sizes)-coated glass substrates were masked
with polyimide tape to prevent deposition on the nonconductive
sides. The ITO samples were placed vertically in the freshly solution
·
·
·
· a
·
·
◦
and kept at 20 C for 10 min to deposit the precursor film under
constant vigorous stirring. Then they were washed with deionized
water. After removing the tape masks, the coated samples were
1
0
20
30
40
50
2θ/ deg
60
70
80
90
◦
◦
dried at 75 C and then annealed at different temperatures (300 C,
◦
◦
4
00 C and 500 C) in air for 1.5 h.
Fig. 1. XRD patterns of powders from the annealed films (a) as-deposited precursor
film and the films annealed at (b) 300 C, (c) 400 C, (d) 500 C.
The structure of the samples was examined by X-ray diffraction
◦
◦
◦
(
XRD, philips PC-APD) using Cu K␣ radiation. The surface mor-
phology was observed using a scanning electron microscopy (SEM,
HITACHI S-4700 П). The electrochromic properties were evalu-
ated by UV–visible absorption spectra, which was measured on a
spectrophotometer (UV-2550, SHIMADZU) equipped with an inte-
grating sphere.
The characteristics of the electrochromic processes were inves-
tigated by cyclic voltammetry (CV), and electrochemical impedance
spectroscopy (EIS), which were carried out in a three-electrode
system using a CHI650B electrochemical workstation. 1 M non-
aqueous solution of LiClO4 in propylene carbonate (PC) was
employed as the electrolyte. A platinum foil was used as the counter
electrode and Ag/AgCl as the reference electrode.
The EC window was realized by facing a activated nickel oxide
ITO-coated glass electrode with a pristine tungsten oxide ITO-
coated glass electrode in the LiClO –PC electrolyte. The WO3 film
4
used in this case was obtained by electrodeposition method [25,26].
The two-electrode window was assembled with epoxy glue in the
dry box. The distance between the electrodes was 2.0 mm. The per-
formances of the EC device were tested by double-potential-step
experiment and by optical transmission spectra.
3
. Results and discussion
Hydroxide precursor thin film was prepared using chemical bath
deposition. The chemical reactions may occur as follows [27]:
Ni(H O) (NH )x] + 2OH− → Ni(OH) + (6 − x) H O + xNH
2
+
[
2
6−x
3
2
2
3
(1)
2−
→ 2NiOOH + 2SO42− + 2H+
2
Ni(OH) + S O
(2)
2
2
8
The XRD patterns of powders from the as-deposited precursor
and annealed films on ITO substrate are presented in Fig. 1. All the
peaks of pattern a in Fig. 1 indicate that the as-deposited precur-
sor film contains ␤-Ni(OH)2 (JCPDS 14-0117) and ␥-NiOOH (JCPDS
06-0075). The typical peaks of cubic NiO phase (JCPDS 4-0835) can
be found in Fig. 1(b–d), indicating that polycrystalline NiO films
have formed after heat treatment. Although the XRD patterns of the
◦
NiO films annealed at 400 and 500 C are similar to that of the film
◦
annealed at 300 C, there is a slight difference, the relative intensi-
ties of the former are stronger than those of the latter. It indicates
that higher annealing temperature is favorable to the formation of
compact crystalline NiO.
◦
Fig. 2 shows the SEM images of NiO films annealed at 300 C,
◦
◦
◦
Fig. 2. SEM images of NiO films annealed at (a) 300 C, (b) 400 C and (c) 500 C.
◦ ◦
4
00 C and 500 C. It can be seen that the morphologies of the films

H. Huang et al. / Electrochimica Acta 56 (2011) 4281–4286
4283
a
Bleached
1
00
3
1st cycle
8
6
4
2
0
0
0
0
0
2
c2
1
c1
Colored
0
o
3
00 C
o
a1
400 C
o
-1
5
00 C
a2
-
3
-2
-1
0
1
2
3
2
00
300
400
500
600
700
800
900
Potential / V (vs. Ag/AgCl)
Wavenumbers / nm
b
5
4
3
2
1
0
1
c2 20th
Fig. 3. Optical transmittance spectra of NiO thin films annealed at different tem-
peratures.
have no distinctive differences after annealing at different tem-
peratures. The NiO film is porous, and it is constructed by many
interconnected NiO nanoflakes with a thickness of about 20 nm.
These nanoflakes grow vertically on the substrate, forming a net-
like structure and leaving pores with sizes of about 50–250 nm. It
is generally believed that the highly porous structure will allow
better penetration of electrolyte to promote lithium ions diffusion.
During the electrode reaction, the diffusion length of lithium ions
in the film is short, resulting in fast insertion/extraction reaction
and thus obtaining good performance.
c1
1st
-2
1
st
a1
-
-2
a2
-3
-3
-1
0
1
2
3
Potential / V (vs. Ag/AgCl)
The electrochromic properties of the NiO films were measured
after the film electrodes had been subjected to cyclic voltammetric
test for 20 cycles in LiClO4–PC electrolyte. The NiO film electrodes
were colored by applying a step potential of −1 V for coloration and
c
5
4
3
2
1
0
o
3
4
5
00 C
o
00 C
−
2 V (vs. Ag/AgCl) for bleaching. The color of the NiO film changes
o
00 C
from brown black (colored state) to transparent (bleached state).
Fig. 3 shows transmittance spectra of the annealed NiO films in the
colored and bleached states. It clearly reveals that the transmit-
tance variation decreases as the annealing temperature increases.
◦
The NiO film annealed at 300 C presents the best electrochromic
performance, the transmittance variation between colored and
bleached states is high up to 38.6% at 550 nm, which is higher than
that of the films annealed at 400 and 500 C. The XRD results reveal
-1
-2
◦
that the high annealing temperature contributes to the formation of
compact and well-crystallined NiO. This is in accord with the result
obtained by Koro sˇ ec et al. [28]. The electrochromic processes are
associated with the double injection (extraction) of lithium ions
and electrons into (from) the film. However, a fully crystalline film
may be too dense for ions intercalation, and it will result in low
electrochromic performance.
-
3
-3
-2
-1
0
1
2
3
Potential / V (vs. Ag/AgCl)
Fig. 4. Cyclic voltammograms of NiO thin films. Scan rate: 10 mV/s.
[12]. The simplified electrochemical reaction can be expressed as
the following:
Fig. 4 shows the CV curves of NiO films in 1 M LiClO /PC elec-
4
trolyte at a scan rate of 10 mV/s. In the first cycle of the NiO film
NiO + yLi+ + ye− → LiyNiO
(3)
◦
annealed at 300 C (Fig. 4(a)), two cathodic (insertion) peaks (c ,
1
c ) at −0.72 and −1.98 V can be found in the CV curve, meanwhile,
The reproducible c /a redox peaks can be ascribed to a fast and
2 2
reversible insertion and extraction process of lithium ions:
2
two anodic (extraction) peaks (a , a ) located at +2.26 and −1.04 V
1
2
are observed, respectively. In the subsequent cycles as shown in
LiyNiO + xLi + xe− → Li(
+
NiO
(4)
x+y)
(bleached)
Fig. 4(b), the c /a1 peaks gradually disappear, however, the c /a2
1
2
(
colored)
peak currents increase with an increase in the number of cycles,
implying that the pair of c /a2 peaks is highly reversible and the
reaction activity of the NiO film increases with cycling. After the
which is accompanied by an electrochromic effect. Fig. 4(c)
2
◦
◦
shows the 20th CV curves of the NiO films annealed at 300 C, 400 C
◦
2
0th cycle, the peak potentials and curve shape remain virtually
and 500 C, respectively. The cathodic and anodic peak currents
◦
unchanged. It has been reported that the c1 cathodic peak corre-
sponds to an initial and irreversible intercalation of lithium ions,
which may activate the electrode by somewhat widening the host
structures, favoring further and reversible lithium intercalation
of the NiO film annealed at 300 C are much higher than those of
the NiO film annealed at 400 C and 500 C. Moreover, the anodic
peak potential of the NiO film annealed at 300 C moves slightly
◦
◦
◦
to lower direction in comparison with the other samples, leading

4
284
H. Huang et al. / Electrochimica Acta 56 (2011) 4281–4286
80
70
60
50
40
30
20
10
0
100
o
a 300 C
9
8
7
6
0
0
0
0
5
0
4
-
o
3
00 C
-
2
0
2
4
-
-
2V
1V
0
10
20
30
40
Z' (Ω)
50
60
70
80
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
4
3
3
2
2
1
1
00
50
00
50
00
50
00
Potential / V (vs. Ag/AgCl)
o
b 400 C
Fig. 6. Cyclic voltammogram (bottom) and in situ optical transmittance (top) of the
◦
−1
.
NiO thin film annealed at 300 C. Scan rate: 10 mV s
of electrode material. It can be seen that the NiO film annealed
at 300 C exhibits much smaller semicircle in the high frequency
◦
region and lower slope in the low frequency region, compared to
◦
◦
those annealed at 400 C and 500 C. According to former reports
29], the smaller semicircle means the lower charge-transfer resis-
-
2V
[
5
0
-1V
tance, and the lower slope signifies the higher diffusion rate. It can
be concluded that the NiO film annealed at 300 C has the high
◦
0
charge-transfer rate and ion diffusion rate for lithium ion insertion
and extraction.
0
50
100
150
200
250
300
350
400
Z' (Ω)
Fig. 6 shows the cyclic voltammertry and in situ transmittance of
◦
the NiO film annealed at 300 C. The transmittance was measured
8
00
00
00
00
00
00
00
00
0
at the wavelength of 550 nm. During the cathodic scan, the NiO
film is almost transparent and the transmittance increases, while
in the following anodic scan, the film gradually becomes brown
black and the transmittance decays. The film shows good and repet-
itive optical response, as well as the fast switching time, which
is very important for the realization of efficient EC window based
on the WO3 as main electrochromic material. The highly porous
structure of NiO film facilitates electrolyte penetrating into the par-
ticles and shortens the ions diffusion path within the bulk of nickel
oxide. Meanwhile, the intercrossing network provides much more
paths for the double injection (extraction) of ions and electrons to
o
7
6
5
4
3
2
1
c 500 C
-
-
2V
1V
(from) the film. All these contribute to the improvement of optical
0
100 200 300 400 500 600 700 800
and electrochromic performances. These properties make lithiated
NiO film of consistent interest for the realization of efficient EC
window.
Z' (Ω)
Fig. 5. Electrochemical impedance spectroscopy (EIS) of NiO film electrodes
recorded at the peak potentials of −2 V and −1 V, respectively.
Commonly, the EC window use WO3 as the main electrochromic
electrode. Fig. 7 shows optical transmittance spectra of the WO3
film at the 10th and 1000th cycle. The electrode was colored by
applying a step potential of −1 V for 10 s and bleached by a potential
to a smaller potential separation between the oxidation peak and
the reduction peak. It is well-known that the peak potential sep-
aration is used as a measure of reversibility. It indicates that the
of +2 V (vs. Ag/AgCl) for 10 s, respectively. The color of the WO film
3
changes from blue (colored state) to near transparent (bleached
state). From Fig. 7, it can be seen that the optical transmittance of
◦
NiO film annealed at 300 C has high electrochemical reversibility
the WO film is 87.6% in the bleached state and 22.1% in the colored
3
and activity, which is believed to be responsible for the superior
electrochromic performance.
state at 550 nm. The transmittance variation is about 65.5%, imply-
ing that the WO3 has good electrochromic properties. Moreover,
there is no significant difference in the transmittance between the
10th and 1000th cycle, indicating that the WO3 film shows highly
reversible electrochromic reaction and is suitable for EC window
applications. Fig. 8 shows optical transmittance spectra of the EC
◦
Fig. 5 shows the EIS of NiO film electrodes annealed at 300 C,
◦
◦
4
00 C and 500 C. The EIS plots were recorded at the redox peak
potentials of −2 V and −1 V, which corresponds to the insertion
extraction) processes of lithium ions, respectively. The EIS is com-
(
posed of a depressed semicircle in the high frequency region and a
sloping line in the low frequency region. Generally, the semicircle
represents the charge-transfer resistance and the sloping line with
window based on WO /NiO complementary structure applied at
3
+2 V and −2 V for 10 s in the 10th and 1000th cycle, respectively.
The transmittance variation of the EC window reaches about 83.7%
at 550 nm, which is much higher than the single WO3 film. The
◦
a 45 angle is attributed to the diffusion of lithium ions into the bulk

H. Huang et al. / Electrochimica Acta 56 (2011) 4281–4286
4285
a
12
8
Bleached
8
6
4
2
0
0
0
0
0
4
1
1
000 CA cycles
0 CA cycles
0
-
-
4
8
Colored
-12
3
00
400
500
600
700
800
900
0
10
20
30
40
50
60
70
80
Wavelength / nm
Time / s
Fig. 7. Optical transmittance spectra of the WO3 films in the colored and bleached
states.
b
10
transmittance of EC window in the bleached state is similar to that
5
0
of single WO film (90.1% at 550 nm), however in the colored state,
3
the transmittance of EC window is as low as 7.0% at 550 nm. Obvi-
ously, the improved electrochromic performance of EC window is
attributed to the NiO layer. In addition, the optical transmittance of
EC window has no obvious difference in the 10th and 1000th cycle,
indicating that the EC window presents excellent reversibility and
stability.
-5
10
The switching speed from one state to another state and cycling
stability of EC window is of great importance to determine its
potential application. The time required for full bleaching from
full colored state is defined as the response time. In this case, a
double-potential-step cycling experiment was performed on the EC
device (E = 2.5 V, E = −2.5 V), and the corresponding current–time
-
0
500
1000
1500
2000
2500
3000
Time / s
1
2
Fig. 9. Current–time response curves of the EC window during the double-potential-
step cycling (a) 1 cycle (b) 50 cycles.
response curves are shown in Fig. 9. From Fig. 9(a), it is concluded
that the response time for coloration and bleaching is found to be
about 1.76 and 1.54 s, respectively. The switching speed is much
faster than the results reported by others [30,31]. Furthermore, the
coloring current is almost the same as the bleaching current dur-
ing the double-potential-step process and the current–time curves
remain unchanged upon cycling. Indeed, the EC window based on
4
. Conclusion
The NiO films with high porosity were prepared by a simple CBD
method. XRD indicates that polycrystalline NiO films have been
formed after annealing. SEM reveals a porous structure of NiO,
which will allow better penetration of electrolyte to promote Li+
ions diffusion and provide good electrochemical kinetics for injec-
tion/extraction of Li ions. The NiO film annealed at 300 C exhibits
the best electrochromic performance with the variation of trans-
mittance up to 38.6% at 550 nm. The assembled EC window based
complementary WO /NiO structure has efficient and reproducible
3
behavior, since the current–time curves remain unchanged for
many cycles (see Fig. 9(b)).
+
◦
1
00
on WO /NiO complementary structure shows large optical mod-
Bleached
3
ulation of 83.7% at 550 nm, much higher than that of single WO3
film (65.5% at 550 nm), fast switch speed and excellent cycle dura-
bility, which make it very attractive for a practical application. The
improved electrochromic performance of EC window is attributed
to the NiO layer.
8
6
4
2
0
0
0
0
0
1
1
000 CA cycles
0 CA cycles
Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (20673100 and 51002138), the ‘Qianjiang Scholars’
program of Zhejiang Province, the Natural Science Foundation of
Zhejiang Province (Y4090420), the Qianjiang Project of Zhejiang
Province (2010R10029).
Colored
800
3
00
400
500
600
700
900
Wavelength / nm
References
Fig. 8. Optical transmittance spectra of the EC window at the 10th and 1000th CA
cycle.
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