Nickel oxide film with open macropores fabricated by surfactant-assisted anodic deposition for high capacitance supercapacitors

Article abstract of DOI:10.1039/c0cc02180f

Nickel oxide film with open macropores prepared by anodic deposition in the presence of surfactant shows a very high capacitance of 1110 F g^-1 at a scan rate of 10 mV s^-1, and the capacitance value reduces to 950 F g^-1 at a high scan rate of 200 mV s^-1.

Full text of DOI:10.1039/c0cc02180f

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Nickel oxide film with open macropores fabricated by surfactant-assisted
anodic deposition for high capacitance supercapacitorsw
Mao-Sung Wu* and Min-Jyle Wang
Received 29th June 2010, Accepted 26th July 2010
DOI: 10.1039/c0cc02180f
Nickel oxide film with open macropores prepared by anodic
deposition in the presence of surfactant shows a very high
capacitance of 1110 F gꢀ1 at a scan rate of 10 mV sꢀ1, and
the capacitance value reduces to 950 F gꢀ1 at a high scan rate of
The nickel oxide films were electrodeposited directly onto
the SS substrate by applying an anodic potential of 0.9 V
versus a saturated Ag/AgCl electrode. The plating solution
consisted of 0.13 M sodium acetate, 0.13 M nickel sulfate,
0.1 M sodium sulfate, and 0.001 M surfactant (CTAB) at
room temperature.15 After deposition, the film was rinsed
several times in enthanol to remove the surfactant, and then
the film was annealed at 300 1C for 1 h in air. The capacitive
behavior of films was characterized using cyclic voltammetry
in a three-electrode cell with 1 M KOH electrolyte. The
potential was cycled at various scan rates using a potentiostat
in a potential range of 0–0.45 V. The electrochemical impedance
measurements were performed by means of a potentiostat
which was coupled to a frequency response analyzer under
the open-circuit condition.
200 mV sꢀ1
.
Supercapacitors have attracted a great deal of attention from
researchers because of their high power density and long life
cycle compared to traditional batteries.1 Previous reports have
demonstrated that metal oxides such as ruthenium oxide,2
nickel oxide,3 manganese oxide,4 and cobalt oxide5 can meet
the requirements of capacitor materials. Among these oxide
materials, nickel oxide is a promising candidate for alkaline
supercapacitors due to its unique physical and chemical
properties in alkaline electrolyte.
Porous structure is a crucial factor in determining the
capacitive behavior of the nickel oxide. The mesoporous
structure has been shown to increase the capacitance of nickel
oxides due to the larger effective surface area of the meso-
porous structures.6–8 It was reported that the capacitance value
of the ordered mesoporous nickel oxide can reach as high as
590 F gꢀ1.8 It is important for ions to be transported rapidly
through the porous film during high rate charging and dis-
charging. The nickel oxide film with open macropores fabricated
by polystyrene sphere template has turned out to be effective in
improving the capacitance of film (351 F gꢀ1).9 The open
macropores may enhance the electrolyte penetration and ion
migration, therefore increasing the utilization of nickel oxide film.
The electrical conductivity also plays an important role in
increasing the capacitive behavior of nickel oxide/hydroxide
films. Electrodepositions of nickel hydroxides and oxides on
high electronic conductivity substrates such as nickel foam,10
carbon fiber,11 and carbon nanotube12 have shown an enhance-
ment in their specific capacitance.
Fig. 1 shows the TEM images of nickel oxide films after
annealing at 300 1C for 1 h. The nickel oxide film deposited in
the absence of CTAB possesses a compact film structure
(Fig. 1a). Our previous work has demonstrated that the anodic
deposited nickel oxide film is made up of flaky nickel oxide.16
A compact nickel oxide film may consist of very small pores,
some of which may be closed. The existence of closed pores is
an important drawback because the closed pores make it
difficult for electrolyte to enter into the interior of the film.
Interestingly, a nickel oxide film deposited in the presence
of CTAB has open macropores with a pore diameter of ca.
150–200 nm (Fig. 1b).
Fig. 2 shows a schematic illustration of the formation of
porous nickel hydroxide films. When Ni2+ ions arrive at the
electrode surface via diffusion, the film formation depends on
the oxidation of 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 anode
substrate.16 A compact film with particularly small pores is
obtained by anodic deposition in absence of CTAB. When
adding amphiphilic molecules (surfactants) to the plating bath,
they may adsorb and desorb at the solid/solution interface
depending on the applied potential. CTAB has a positively
charged head group and a hydrophobic hydrocarbon chain.
During anodic electrodeposition (positive potential), the
positively charged head groups of CTAB may be repelled
from the anode surface, while the hydrophobic chains will be
attracted to the anode surface. The adsorbed CTAB may cover
a part of the electrode surface and hinder the access of nickel
ions to the electrode surface. Therefore, the deposited film has
macropores distributed throughout the entire film.
Generally, the high-rate capability of an electrode is con-
trolled by the ion diffusion resistance within the crystal
structure of active material. Diffusion resistance in the solid
phase can be mitigated by shortening the diffusion path.
Nickel oxide films with nanoflakes have been previously
demonstrated to effectively improve the capacitive behavior
of films.13,14 In this work, we used a cationic surfactant,
cetyltrimethylammonium bromide (CTAB), as a template for
anodic electrodeposition of nickel oxide nanoflakes with open
macropores onto the stainless steel (SS) substrate.
Department of Chemical and Materials Engineering,
National Kaohsiung University of Applied Sciences, Kaohsiung 807,
Taiwan. E-mail: ms_wu@url.com.tw; Fax: 886-9-45614423
w Electronic supplementary information (ESI) available: Experimental
details, surface morphology, crystal characterization, and life cycle
stability. See DOI: 10.1039/c0cc02180f
Fig. 3a shows the cyclic voltammograms (CVs) of nickel
oxide films at a scan rate of 25 mV sꢀ1. Clearly, both films
have redox peaks during the CV scan. Compared to the
electrical double layer capacitance, much more capacitance
c
6968 Chem. Commun., 2010, 46, 6968–6970
This journal is The Royal Society of Chemistry 2010

View Article Online
Fig. 3 (a) CV curves of the nickel oxide films at a scan rate of 25 mV
sꢀ1 and (b) specific capacitance of the films at various scan rates.
can be stored in the bulk of nickel oxide by means of redox
reactions. The redox couple of nickel oxide films in alkaline
solution can be expressed as follows:17,18
Fig. 1 TEM images of the nickel oxide films deposited (a) without
and (b) with CTAB.
NiO + OHꢀ 2 NiOOH + eꢀ
(1)
Obviously, the peak current densities of a nickel oxide film
with open macropores deposited in the presence of CTAB
are much higher than those of a compact film. The higher
the current density, the larger is the specific capacitance of
the film. The specific capacitance (C) was calculated by the
equation:
Z
0:45
1
C ¼
i ꢁ dV
ð2Þ
v ꢁ DV
0
where i is the current density (A gꢀ1), DV is the potential
window (V), V is the applied potential (V), and v is the scan
rate (V sꢀ1). The specific capacitance of a film with open
macropores at a scan rate of 25 mV sꢀ1 reaches as high as
1106 F gꢀ1, which is 4.5 times higher than that of a compact
film (243 F gꢀ1). This reflects that a porous film with bigger
open pores may facilitate the electrolyte transport through the
film. A film with open macropores can shorten the ion migra-
tion and diffusion paths, leading to an increase in the effective
surface area. While for a compact film without open macro-
pores, the migration and diffusion resistances of ions are
significantly pronounced, especially in the regions near the
SS substrate.
Fig. 2 Schematic illustration of the formation of porous nickel
hydroxide film during electrodeposition (a) without and (b) with
CTAB.
Fig. 3b shows the effect of CV scan rate on the specific
capacitance of nickel oxide films. Obviously, both films have
c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 6968–6970 6969

View Article Online
nickel oxide film turns out to be significantly affected by the
open macropores. The specific capacitance of nickel oxide film
with open macropores reaches as high as 1110 F gꢀ1 at a scan
rate of 10 mV sꢀ1, which is much higher than that of a compact
film (254 F gꢀ1). Possibly, a highly porous film with bigger open
macropores offers large surface area for fast charge storage
and redox reactions and is capable of delivering high power.
The prepared nickel oxide film with open macropores also
exhibits a high capacitance and a stable capacitance retention
during galvanostatic cycling (see Fig. S2 in ESIw), and is there-
fore suitable for long-term capacitor applications in alkaline
solution.
The authors gratefully acknowledge financial support from
the National Science Council, Taiwan, Republic of China
(Project No: NSC 98-2221-E-151-032).
Fig. 4 Nyquist plots of the nickel oxide films deposited with and
Notes and references
without CTAB. The enlarged figure is shown in the inset of Fig. 4.
1 B. E. Conway, J. Electrochem. Soc., 1991, 138, 1539.
2 (a) J. P. Zheng and T. R. Jow, J. Electrochem. Soc., 1995, 142, L6;
(b) H. S. Kim, N. Branko and B. N. Popov, J. Power Sources, 2002,
104, 52; (c) C. C. Hu, K. H. Chang, M. C. Lin and Y. T. Wu, Nano
Lett., 2006, 6, 2690.
3 (a) K. C. Liu and M. A. Anderson, J. Electrochem. Soc., 1996, 143,
124; (b) V. Srinivasan and J. W. Weidner, J. Electrochem. Soc.,
1997, 144, L210; (c) E. E. Kalu, T. T. Nwoga, V. Srinivasan and
J. W. Weidner, J. Power Sources, 2001, 92, 163; (d) K. W. Nam and
K. B. Kim, J. Electrochem. Soc., 2002, 149, A346.
higher specific capacitance at low scan rate than those at high
scan rate. A nickel oxide film with open macropores has a
specific capacitance of 1110 F gꢀ1 at a low scan rate of
10 mV sꢀ1, and the value reduces slightly to 950 F gꢀ1 at a high
scan rate of 200 mV sꢀ1. A small decrease in specific capacitance
with increasing scan rate indicates the high power charac-
teristics of a nickel oxide film with open macropores. High
capacitance retention at high scan rate reveals that the faradaic
reaction occurring on the active sites of the nickel oxide
surface is fast enough to maintain the current response during
high rate scanning. Possibly, the interconnected nanoflakes
provide many more paths for electron conduction, and the
flake-like structure shortens the ion diffusion paths within the
solid nickel oxide. More importantly, the open macropores
benefit electrolyte transport and provide larger surface area
for charge-transfer reactions.
4 (a) M. Toupin, T. Brousse and D. Belanger, Chem. Mater., 2004,
´
16, 3184; (b) C. C. Hu and C. C. Wang, J. Electrochem. Soc., 2003,
150, A1076; (c) C. Xu, Y. Zhao, G. Yang, F. Li and H. Li, Chem.
Commun., 2009, 7575.
5 C. Lin, J. A. Ritter and B. N. Popov, J. Electrochem. Soc., 1998,
145, 4097.
6 M. W. Xu, S. J. Bao and H. L. Li, J. Solid State Electrochem.,
2006, 11, 372.
7 Y. G. Wang and Y. Y. Xia, Electrochim. Acta, 2006, 51,
3223.
8 D. D. Zhao, M. W. Xu, W. J. Zhou, J. Zhang and H. L. Li,
Electrochim. Acta, 2008, 53, 2699.
Fig. 4 shows the effect of pore structure on the Nyquist plot
of nickel oxide films. At very high frequencies, the intercept at
real part (Z0) is a combinational resistance of electrolyte resis-
tance, intrinsic resistance of substrate, and contact resistance
at the active material/current collector interface.19 This value
is approximately the same for both films. A major difference is
the semicircle in the high-frequency range, which corresponds
to the charge-transfer resistance caused by the faradaic reaction
and the double-layer capacitance on the nickel oxide surface.
The charge-transfer resistance of a film with open macropores
was approximately 0.26 O, which is much lower than that of a
compact film (0.57 O). The charge-transfer resistance depends
on the electroactive surface area of film for the faradaic
reaction to occur. A film with low charge-transfer resistance
9 M. S. Wu, M. J. Wang and J. J. Jow, J. Power Sources, 2010, 195,
3950.
10 G. W. Yang, C. L. Xu and H. L. Li, Chem. Commun., 2008,
6537.
11 (a) M. S. Wu and M. J. Wang, Electrochem. Solid-State Lett., 2010,
13, A1; (b) M. S. Wu, C. Y. Huang and J. J. Jow, Electrochem.
Commun., 2009, 11, 779.
12 (a) K. W. Nam, E. S. Lee, J. H. Kim, Y. H. Lee and K. B. Kim,
J. Electrochem. Soc., 2005, 152, A2123; (b) M. S. Wu, C. Y. Huang
and K. H. Lin, Electrochem. Solid-State Lett., 2009, 12,
A129.
13 J. W. Lang, L. B. Kong, W. J. Wu, Y. C. Luo and L. Kang, Chem.
Commun., 2008, 4213.
14 M. S. Wu, Y. A. Huang and C. H. Yang, J. Electrochem. Soc.,
2008, 155, A798.
15 D. Tench and L. F. Warren, J. Electrochem. Soc., 1983, 130, 869.
16 M. S. Wu, C. H. Yang and M. J. Wang, Electrochim. Acta, 2008,
54, 155.
17 C. Natarajan, H. Matsumoto and G. Nogami, J. Electrochem.
Soc., 1997, 144, 121.
is more effective for fast charge and discharge responses.20
A
nickel oxide film having open macropores possesses a larger
electroactive surface area possibly due to an increase in the
electrolyte accessible area.
18 P. Delichere, S. Joiret, A. Hugot-Le Goff, K. Bange and B. Hetz,
J. Electrochem. Soc., 1988, 135, 1856.
19 J. Gamby, P. L. Taberna, P. Simon, J. F. Fauvarque and
M. Chesneau, J. Power Sources, 2001, 101, 109.
20 J. E. Park, M. Saikawa, M. Atobe and T. Fuchigami, Chem.
Commun., 2006, 2708.
In summary, nickel oxide film with nanoflakes and open
macropores has been successfully prepared by anodic deposition
in the presence of CTAB template. The capacitive behavior of
c
6970 Chem. Commun., 2010, 46, 6968–6970
This journal is The Royal Society of Chemistry 2010