Facile approach to prepare loose-packed NiO nano-flakes materials for supercapacitors

Article abstract of DOI:10.1039/b800264a

The nickel oxide nano-flakes materials prepared by a facile approach maintain high power density at high rates of discharge and have excellent cycle life, suggesting their potential application in supercapacitors. The Royal Society of Chemistry.

Full text of DOI:10.1039/b800264a

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COMMUNICATION
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Facile approach to prepare loose-packed NiO nano-flakes materials
for supercapacitorsw
Jun-Wei Lang, Ling-Bin Kong,* Wei-Jin Wu, Yong-Chun Luo and Long Kang
Received (in Cambridge, UK) 8th January 2008, Accepted 28th May 2008
First published as an Advance Article on the web 15th July 2008
DOI: 10.1039/b800264a
The nickel oxide nano-flakes materials prepared by a facile
approach maintain high power density at high rates of discharge
and have excellent cycle life, suggesting their potential applica-
tion in supercapacitors.
and nano-flake structure. We demonstrate that the unique
microstructure can accommodate the electroactive species in
solid bulk electrode materials. The maximum specific capaci-
tance of 942 F gÀ1 is the highest report of NiO for super-
capacitors, which shows better rate capability and great
potential as electrode materials for supercapacitors.
Supercapacitors, also called electrochemical capacitors, are
unique energy-storage devices exhibiting 20–200 times greater
capacitance than conventional capacitors.1 Growing demands
in electric vehicles, and other devices that require electrical
energy at high power levels in relatively short pulses have
prompted considerable interest in supercapacitors.2 Moreover,
the energy density of the relatively new pseudocapacitor
devices that are based on faradaic processes has been reported
many times, because it has been recognized that the energy
density based on pseudo-faradaic processes is many times
greater than that of the traditional double-layer capacitors.3
In particular, supercapacitors based on hydrous ruthenium
oxides exhibit much higher specific capacitance than conven-
tional carbon materials and better electrochemical stability
than electronically conducting polymer materials. However,
the high cost of RuO2ÁxH2O makes this material unsuitable for
commercial applications.4b,c Hence, much effort has been made
to at search for alternative inexpensive electrode materials with
good capacitive characteristics, such as NiO,4d CoOx,4e
MnO2,4f Ni(OH)2,4g Co(OH)2,4a etc. Among the transitional
base metal oxides, nickel oxides have been extensively studied
for supercapacitors due to its large surface area and high
pseudo-capacitive behavior.5
The preparation procedure of the NiO materials is described
as follows. All of the chemicals were of analytical grade and
used without further purification, and the nickel chloride
hydrate used was from Strem Chemicals. In the first step,
nickel chloride hydrate was dissolved in distilled water (Ni
concentration = 1.0 M) in a glass beaker, using a magnetic stir
bar. The nickel chloride hydrate solution was slowly adjusted
to pH 9 by dropwise addition of 5 wt% aqueous NH3 for 2 h
at a temperature of B10 1C. The resulting suspension was
stirred at this temperature for an additional 3 h. Then the solid
was filtered off, washed with a copious amount of distilled
water, and heated at different temperatures in air for 8 h, viz.,
250, 300, 350 1C.
The working electrodes were prepared according to the
method reported in the literature.4f 80 wt% of NiO powder
was mixed with 7.5 wt% of acetylene black (499.9%) and
7.5 wt% of conducting graphite in an agate mortar until a
homogeneous black powder was obtained. To this mixture,
5 wt% of poly(tetrafluoroethylene) was added with a few
drops of ethanol. After briefly allowing the solvent to evapo-
rate, the resulting paste was pressed at 10 MPa to a nickel
gauze with a nickel wire for an electric connection. The
electrode assembly was dried for 16 h at 80 1C in air. Each
electrode contained about 8 mg of electroactive material and
had a geometric surface area of about 1 cm2. Electrochemical
measurements were carried out in 2 M aqueous KOH in a half-
cell setup configuration at room temperature. A platinum
gauze electrode (1 Â 1 cm2) and a saturated calomel electrode
served as the counter electrode and the reference electrode,
respectively. According to literature,10 one plausible reaction
can occur as a quasi-reversible redox processes during the
potential sweep of the NiO electrode:
There are several methods used to prepare nickel oxides
such as chemical precipitation,6 electrodeposition7 and sol-gel
technique.8 Among these synthetic methods, chemical preci-
pitation technique has one advantage over the others: it’s a
facile method. The sol–gel dip-coating method and cathodic
precipitation method will inevitably encounter a serious fall in
capacitance during scaling-up,9 the specific capacitance of
devices ranging from 50 to 696 F gÀ1 (for single-electrode
system). Therefore, a lot of research work has been done to
improve the electrochemical performance of nickel oxides.
In this communication, we report a facile chemical precipi-
tation method to prepare nickel oxides materials. The studies
show that the as-prepared NiO material has a low crystallinity
NiO + OHÀ 2 NiOOH + eÀ
(1)
The XRD patterns of the Ni(OH)2 precursor and nickel oxide
produced by heating the Ni(OH)2 at different temperatures for
6 h in air are shown in Fig. 1. The precursor corresponds to a
layered a-Ni(OH)2 structure of low crystallinity. Upon heating
at 250 1C for 6 h, almost all of the Ni(OH)2 precursor were
converted to NiO of low crystallinity. The wide-angle XRD
patterns exhibit the characteristic peaks of rocksalt NiO at
2y = 37, 43 and 631, which first appeared in the resultant
materials at 250 1C, and all these diffraction peak positions
State Key Laboratory of Gansu Advanced Non-Ferrous Metal
Materials, Lanzhou University of Technology, Lanzhou, 730050,
China. E-mail: konglb@lut.cn; Fax: +86 931-2976578;
Tel: +86 931-2976579
w Electronic supplementary information (ESI) available: Calculation
method of measured specific capacitance, the schematic formation
mechanism of the as-prepared Ni(OH)2 precursor, TG and electro-
chemical impedance analysis. See DOI: 10.1039/b800264a
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Fig. 3 Electrochemical properties of NiO in 2 M KOH solution: (a)
CV curves at different scan rates; (b) discharging curves at different
discharging currents
Fig. 1 XRD patterns of the Ni(OH)2 precursor and nickel oxide
materials produced from this.
increases. As shown in Fig. 2 both particle and nano-flake
phases appeared in the NiO samples.
and their relative intensities are in good accordance with the
standard spectrum (PDF, card no 44-1159).5,9
CV and chronopotentiometry measurements have been used
to evaluate the electrochemical properties and quantify the
specific capacitance of the as-prepared NiO electrodes.
Fig. 3(a) shows the CV curves of the NiO obtained at 250 1C.
The shapes of the CV reveal that the capacitance characteristic is
very distinct from that of electric double-layer capacitance in
which the shape is normally close to an ideal rectangular shape.
For this electrochemical system, a couple of redox peaks were
observed within the potential range À0.2 to 0.55 V, the anodic
peak P1 is due to the oxidation of NiO to NiOOH and the
cathodic peak P2 is for the reverse process.10 This indicates that
the capacity mainly results from the pseudo-capacitance, which is
based on a redox mechanism. It should be noted that with the
sweep rate increased, the shape of the CV changed, the anodic
peak potential and cathodic peak potential shift in more anodic
and cathodic directions, respectively, and the capacitance, in-
evitably, decreased, which is in agreement with the result of
chronopotentiometry measurement.
SEM and TEM measurement were performed on the sample
obtained at a temperature of 250 1C. Two morphologies
(a: particles and b: nano-flakes) can be seen in Fig. 2(a) and
(b). It is noteworthy that the network-like structure (which
consists of interconnected nano-flakes) shows anisotropic
morphology characteristics and the formation of a loosely
packed microstructure in the nanometer scale. The unique
structure plays a basic role in the morphology requirement for
electrochemical accessibility of electrolyte OHÀ to the NiO
active material and a fast diffusion rate within the redox phase.
It is believed that the unique structure provides an important
morphological foundation for the extraordinary high specific
capacitances.
To understand the reason why this particular network
structure of NiO phase can be formed during the synthesis
process, a formation mechanism from aggregation growth to
surface growth in the solution can be summarized as follows.
The initial precipitations provide numerous Ni(OH)2 nuclea-
tion centers. Because of the anisotropy of a-Ni(OH)2 crystals,
the growing point of each crystal is located at certain direc-
tions, so nano-flakes will be formed. These a-Ni(OH)2 nano-
flakes contain a lot of interbedded water, which is distributed
at certain directions. When the sizes of these flakes become
larger, their tendency to aggregate is increased. As a result,
microspheres of nano-flake aggregates will be formed due to
the strong hydrogen bonding of H2O. A the reaction proceeds,
new Ni(OH)2 nano-flakes will grow up around the nucleation
centers at the surface of the microsphere. The surface growth
leads to the continuous increase in size and density of the
microsphere to form a network structure.11 When Ni(OH)2 is
transformed to NiO at the subsequent annealing process,
however, some of the nano-flakes grow to a stacking structure
because H2O will be lost as the annealing temperature
In practice, the ability to discharge at high rate is crucial in
capacitors. The relationship between specific capacitance and
current density was therefore investigated and Fig. 3(b) shows
the results obtained for the NiO (calcined at 250 1C) electrode in
the potential range of 0–0.4 V in 2 M KOH at various current
rates (5, 10, 20 and 30 mA). The corresponding specific capaci-
tance was calculated using the formula provided in ESI.w In this
way, the specific capacitance of the NiO materials at 5, 10, 20
and 30 mA were 942, 804, 696 and 613 F gÀ1. This result
indicates that the nano-flake structure increases the specific
capacitance of NiO greatly. The shape of the discharge curves
does not show the characteristics of a pure double-layer capa-
citor, but mainly pseudo-capacitance, which is in agreement with
the result of the CV curves.12 Fig. 3(b) also shows that the NiO
sample obtained at 250 1C exhibits the best rate capability. As
the discharge current increases, a large voltage drop is produced,
and finally the capacitance decreases. This phenomenon may be
explained by referring to the OHÀ ion diffusion processes during
the charging/discharging for the electrode. At high sweep rates
with high current density significant OHÀ ions are required to
intercalate swiftly at the interface of electrode/electrolyte, how-
ever, the relatively low concentration of OHÀ ions can not meet
this demand and the processes would be controlled by the ion
diffusion.13 Despite this, the discharge capacitance at 10 mA is
85.4% of that discharged at 5 mA, so the excellent rate
capability of the sample makes it attractive particularly for a
practical application.
Fig. 2 SEM and TEM image of as-prepared NiO materials.
4214 | Chem. Commun., 2008, 4213–4215
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and applied its microstructure for supercapacitors. XRD,
SEM and TEM studies show that the as-prepared NiO
material has a low crystallinity and a nano-flake structure.
The unique microstructure can accommodate the electroactive
species in the solid bulk electrode materials. The specific
capacitances of the NiO material at 5, 10, 20 and 30 mA were
942, 804, 696 and 613 F gÀ1, which shows better rate capability
and great potential for electrode materials for supercapacitors.
The maximum specific capacitance is the highest reported of
NiO for supercapacitors. Even though we do not fully under-
stand the fundamental structure of the NiO materials, the
strategy reported here should be viable to extend to other
transition metal oxides systems.
Fig. 4 Electrochemical properties of NiO samples heated at different
temperatures in 2 M KOH solution: (a) discharging curves at the
discharging current of 5 mA and (b) specific capacitance as a function
of discharge currents.
The authors acknowledge the financial support by the
National Natural Science Foundation of China (No.
50602020) and the National Basic Research Program of China
(No. 2007CB216408).
Notes and references
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Fig. 5 Cycle life of as-prepared NiO electrode at a discharge current
of 5 mA in 2 M KOH electrolyte. The inset is the charge/discharge
curves of the NiO electrode.
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To evaluate the electrochemical capacitance of the NiO
samples as a function of the heating temperatures, chrono-
potentiometry measurements were conducted on NiO samples
with different annealing temperature. As shown in Fig. 4(a)
and (b), with increasing annealing temperature, the decline of
the specific capacitance is similar to that found for increasing
discharge currents. Once the oxide phase is formed, further
heat-treatment at higher temperature may cause the increase in
crystal size, and accordingly a decrease in specific surface area,
and also possibly its reactivity for surface chemical processes,
thereby leading to a decreased capacitance.4d Just as in the
case of RuO2, the NiO amorphous phase exhibited far greater
specific capacitance than the crystalline counterpart.14
and D. Belanger, Chem. Mater., 2004, 16, 3184; (g) L. Cao, L. B.
´
As long cycle life is very important in supercapacitors, cycle
charge/discharge testing was employed to examine the service
life of the NiO electrode (the NiO was obtained at an anneal-
ing temperature of 250 1C) employing a discharging current of
5 mA in 2 M KOH electrolyte. Fig. 5 shows approximately
1.5% loss of capacitance after 1000 cycles, and the lowest
capacitance between the 1000 cycles also remains 82% of the
maximum capacitance. This demonstrates that, within the
voltage window 0.0–0.4 V, the charge and discharge processes
do not seem to induce significant structural or micro-structural
changes of the nano-flakes NiO electrode materials as expected
for pseudo-capacitance reactions. The long-term stability im-
plies that the flake-like NiO is an excellent electrode material
for supercapacitors.
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¨
In summary, we have synthesized loose-packed flake-like
NiO materials using a facile chemical precipitation method,
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Chem. Commun., 2008, 4213–4215 | 4215