5
44
Published on the web April 24, 2010
Electrochemical Capacitor Properties of NiO in Ionic Liquids
1
1
1,2
Sho Makino, Yoshio Takasu, and Wataru Sugimoto*
Faculty of Textile Science and Technology, Shinshu University, 3-15-1 Tokida, Ueda 386-8567
Collaborative Innovation Center for Nanotech Fiber, Shinshu University, 3-15-1 Tokida, Ueda 386-8567
1
2
(
Received February 12, 2010; CL-100151; E-mail: wsugi@shinshu-u.ac.jp)
The electrochemical capacitor properties of dip-coated NiO/
Ni were studied in six different ionic liquids and compared with
a typical nonaqueous electrolyte and an alkaline electrolyte. A
V working potential for the ionic liquids leads to a larger
energy density when compared to KOH electrolyte, with the
highest energy density obtained in EMI-BF4.
imidazolium bis(trifluoromethanesulfonyl)imide (EMI-TFSI),
N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluo-
roborate (DEME-BF4), N,N-diethyl-N-methyl-N-(2-methoxy-
ethyl)ammonium bis(trifluoromethanesulfonyl)imide (DEME-
TFSI), and 1-butyl-3-methyl-imidazolium hexafluorophosphate
(BMI-PF6). 1 M tetraethylammonium tetrafluoroborate/propyl-
ene carbonate (TEA-BF4/PC) was used as the nonaqueous
electrolyte. 1 M KOH was also studied as the alkaline electro-
lyte. All reagents were used as-received. X-ray diffraction
(XRD) was conducted with a Rigaku RINT2500HF/PC, and a
Hitachi S-5000 was used for scanning electron microscopy
(SEM).
3
Metal oxides have been studied as potential candidates as
electrodes for electrochemical capacitors. Various oxides have
been proposed so far, including oxides of Ru, Ir, V, Mn, Mo, and
1
Ni. As a light and abundant material, nickel oxide is a potential
candidate, showing electrical double layer capacitor like behav-
SEM images of the surface of the NiO/Ni electrode
revealed fine nanoparticles constituting a mud-crack-like porous
network (Figure S1). The XRD pattern of the prepared NiO/
2
ior between 0.871.27 V vs. RHE in alkaline electrolytes.
¹
1
2,3
11
Specific capacitance of 200300 F g
has been reported,
which is comparable to commercial activated carbon electrodes.
Although the high capacitance is inspiring, the narrow electro-
chemical window (ca. 0.5 V) limits the use of NiO electrodes
Ni electrode reveals a broad peak at 2ª = 38°, which can be
11
indexed as the (111) reflection of NiO (Figure S2). The particle
size estimated from this peak was ca. 4 nm, in accordance with
the SEM images.
for many applications, as the energy density of a capacitor is
2
1
/2CV . Thus, it would be beneficial if the electrochemical
The cyclic voltammogram of NiO/Ni in 1 M KOH is typical
of a polarizable electrode, characterized by a rectangularly
shaped voltammogram with no appreciable faradaic process,
which indicates that the charge storage is dominated by the
potential range could be widened.
Ionic liquids have attracted increased interest as a new
electrolyte system with high conductivity and wide electro-
chemical window. Only a few studies have been conducted so
far regarding the capacitive behavior of oxide electrodes in ionic
liquids. RuO2 electrodes have been reported to exhibit lower
11
electrical double-layer charging (Figure S3). The specific
¹
1
¹1
capacitance was 100 F g
at 2 mV s
and decreased to
¹
1
¹1
60 F g at 500 mV s . The specific capacitance values are in
accordance with reported literature values
4
,5
1,7,8
specific capacitance in ionic liquids compared to sulfuric acid.
and are compara-
A study using a nickel-based rare earth oxide prepared by
oxidation of a nickel-based misch metal reports capacitance
ble to porous carbons under similar experimental conditions.
However, the energy density, 8 kJ kg , is considerably low due
to the narrow potential window of 0.4 V.
¹1
¹
1
of 360 F g with a 2 V electrochemical window in 1-butyl-3-
6
methylimidazolium hexafluorophosphate. Here, we report
Cyclic voltammogram of NiO/Ni in 1 M TEA-BF /PC at
4
¹
1
the capacitive properties of NiO in various ionic liquids and
compare them to the behavior in alkaline and nonaqueous
electrolytes.
2 mV s is shown in Figure 1a. The voltammograms reveal no
evidence of redox reactions, typical of a non-Faradaic ideally
polarizable electrode. The capacitance is 19 F g at 2 mV s
¹
1
¹1
,
NiO electrodes were prepared by dip-coating following
literature procedures. Ni(CH3COO)2¢4H2O (Aldrich) was dis-
which is much lower than the capacitance in 1 M KOH but
2
¹1
higher than the capacitance of 8 F g reported for RuO2 in 1 M
5
solved in distilled water, and magnetically stirred for 3 days
under ambient conditions to obtain a sol. The green precipitate
was centrifugally collected and dispersed in distilled water. A Ni
TEA-BF /PC. Although the capacitance is low compared to
4
1 M KOH, the 2.5 V potential window results in a sevenfold
¹
1
greater energy density of 58 kJ kg
.
2
plate (1 © 1 cm , 0.1 mm thick) was dipped into this dispersion
Voltammograms of NiO/Ni in six different ionic liquids are
shown in Figures 1b1g. The specific capacitance and energy
¹
1
at a rate of 0.1 cm s . After drying, the electrode was heat-
treated at 300 °C in air to obtain NiO/Ni. The weight of the
11
density are summarized in Table S1. The specific capacitance
¹
2
¹1
coating, typically 0.2 mg cm , was measured using a micro-
balance (AEG-45SM, Shimadzu).
in ionic liquids ranged from 12 to 33 F g , with EMI-BF4
affording the highest capacitance. These values are considerably
¹
1
A three electrode cell was employed for electrochemical
characterization. The counter electrode was Pt and the reference
higher than the reported 6.5 F g
for RuO2 in the same
4
electrolyte EMI-BF4. Redox peaks suggestive of any faradaic
contribution from the cation or the anion was not observed, thus
we believe that the capacitive behavior for NiO in organic
electrolytes is mainly due to electrical double layer charging. It
should be noted that redox peaks due to faradaic reactions are
clearly observed when the upper potential limit is increased to
+
electrode was a Ag/Ag (BAS). All electrochemical measure-
ments were conducted in a N2-filled dry box (dew point
¹90 °C). The ionic liquids used were 1-ethyl-3-methylimida-
<
zolium tetrafluoroborate (EMI-BF ), 1-ethyl-3-methylimidazoli-
4
um trifluoromethanesulfonate (EMI-TFMS), 1-ethyl-3-methyl-
Chem. Lett. 2010, 39, 544545
© 2010 The Chemical Society of Japan
Makino, Sho
