Supercapacitor electrodes with high-energy and power densities prepared from monolithic NiO/Ni nanocomposites

Article abstract of DOI:10.1002/anie.201101083

Impressive energy storage and delivery: A simple, cost-effective, and potentially scalable technique is described for fabricating support- and additive-free NiO/Ni nanocomposite electrodes (see picture) for electrochemical supercapacitors. Maximum performances of energy storage and delivery were simultaneously achieved by developing a slow-charging and fast-discharging procedure. Copyright

Full text of DOI:10.1002/anie.201101083

DOI: 10.1002/anie.201101083
Energy Storage
Supercapacitor Electrodes with High-Energy and Power Densities
Prepared from Monolithic NiO/Ni Nanocomposites**
Qi Lu, Michael W. Lattanzi, Yunpeng Chen, Xiaoming Kou, Wanfeng Li, Xin Fan,
Karl M. Unruh, Jingguang G. Chen, and John Q. Xiao*
The depletion of traditional energy resourcesas well as the
desire to reduce high CO2 emissions associated with their use
has led to significant interest in developing sustainable and
clean energy products,[1–4] such as electricity produced from
wind- or solar-based technologies. Because of the intermittent
availabilityof these resources the realization of their full
potential will also require the development of new and
advanced energy-storage and delivery systems. Supercapaci-
tors, as a new class of energy storage devices, are now
attracting intensive attention[2] because of their ability to store
energy comparable to certain types of batteries, but with the
advantage of delivering the stored energy much more rapidly
than batteries.[3] This property makes supercapacitor ideal to
augment traditional batteries in many different applications.
However, to become primary devices for power supply,
supercapacitors must be developed further to improve their
abilities to deliver, simultaneously, high energy and power.[5]
To realize this objective, nanostructured electrodes have
been developed from a variety of different functional
materials.[6–10] Despite significant progress, however, most of
the processes for the fabrication of electrodes are either too
delicate,[11–14] which makes them less viable for large-scale
industrial applications, or require additives,[15,16] which dete-
riorates the performance of the electrodes. In addition,
previously reported electrode materials with the desirable
specific capacitance typically show high resistances,[9,13,17]
which not only restrict the power performance but also
prevent the utilization of thick electrodes. Based on these
considerations, the goal of the present work was to build an
advanced supercapacitor electrode using a simple and scal-
able fabrication technique and to optimize the electrode
performance using a controlled functional material and a
well-defined electrode network with minimum resistivity.
First, Ni nanoparticles were synthesized using a modified
polyol process.[18] After a simple mechanical compaction of
the as-prepared (AP) nanoparticles and a subsequent low-
temperature annealing process, monolithic and mechanically
robust, stable, and low-resistivity NiO/Ni nanoporous com-
posite electrodes were obtained with both maximized energy
and power densities.
The structure of the AP Ni particles was characterized by
X-ray diffraction (XRD; Figure 1a) and electron diffraction
(ED; Figure 1b). The particle size estimated from the
Scherrer method was 4.4 nm, and several particles formed
larger aggregates with a diameter smaller than 20 nm (Fig-
ure 1b); these findings are in agreement with the measured
Figure 1. Characterization of the structure: a) XRD pattern of the as-
prepared Ni nanoparticles. b) Transmission electron microscopic
(TEM) image and ED pattern of the as-prepared Ni nanoparticles.
Scanning electron microscopic images at the c) surface and d) cross-
section of prototype electrodes.
Brunauer–Emmett–Teller (BET) surface area of 40 m2 gꢀ1.
The AP particles were then mechanically compacted into
monolithic pellets and used as prototype electrodes. These
pellets are stable, easy to handle, and did neither require
additives nor a supporting substrate. Scanning electron
microscopic images obtained at both the surface and cross-
section (Figure 1c,d) of these electrodes revealed a highly
porous structure formed by a network of Ni nanoparticles.
BET measurements of these pellets (ca. 37 m2 gꢀ1) suggested
that most of the surface area of the nanoparticles was not
affected by the preparation technique.
[*] Q. Lu, M. W. Lattanzi, Y. Chen, Dr. X. Kou, Dr. W. Li, Dr. X. Fan,
Prof. Dr. K. M. Unruh, Prof. Dr. J. Q. Xiao
Department of Physics and Astronomy
University of Delaware, Newark, DE 19716 (USA)
Fax: (+1)302-831-1637
After sputter deposition of an 80 nm thick Pt layer on one
side of the pellets which serves as a current collector, we
performed a thermal annealing process (in atmospheric air)
to create NiO shells around the pellets that are the active
layer in their application as supercapacitors. Figure 2 shows
E-mail: jqx@udel.edu
Prof. Dr. J. G. Chen
Department of Chemical Engineering, University of Delaware (USA)
[**] We acknowledge financial support from the Office of Naval
Research under grant number N00014-08-1-0432.
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Communications
Figure 3b display the reversible process of insertion and
extraction of OHꢀ anions according to Equation (1)
NiIIO þ OHꢀ Ð NiIIIOOH þ eꢀ
ð1Þ
The electrodes oxidized at 2508C show a significantly better
performance than their high-temperature counterparts. The
average specific capacitance (SC) was calculated to be
910 Fgꢀ1 from cyclic voltammetry (CV) and 905 Fgꢀ1 from
cyclic chronopotentiometry (CC) based on the mass of NiO
(ca. 300 Fgꢀ1 based on the total electrode mass), and the SC
drops to 348 Fgꢀ1 at 3008C and further to 72 Fgꢀ1 at 3508C.
The SC performance of the sample annealed at 2508C is
remarkable for two reasons: first, the high density of the grain
boundaries as a result of the very small particle size of the
NiO shell creates efficient diffusion paths for the OHꢀ ions,
which significantly enhances the intercalation of the electro-
lyte ions and, therefore, the utilization of the electrode
material.[19,20] Second, the conductive network of metal cores
is well-maintained since the oxide shell is estimated to be
about 3 nm. More intense oxidation at higher temperatures
tends to isolate the conductive cores and gives rise to a much
higher ohmic resistivity of the electrode as shown in Fig-
ure 4a. The ohmic resistivity of the electrode reduces the
Figure 2. Characterization of the oxidation process: a) XRD patterns
and b) ED patterns of the NiO/Ni nanoparticles oxidized at different
temperatures.
the XRD and ED patterns of the NiO/Ni nanocomposites
oxidized at different temperatures. At 2508C the appearance
of broad peaks at diffraction angles of about 37 and 438
displays the presence of a NiO phase consisting of very small
crystallites. At higher annealing temperatures these peaks
decreased in width and increased in intensity. Similar results
were also observed in the corresponding ED pattern. Vibrat-
ing sample magnetometer (VSM) measurements were also
performed on the samples to determine the saturation
magnetizations. The ratio of Ni to NiO was determined by
comparing the magnetization loss of the annealed and the AP
samples and used to calculate the specific capacitance of the
samples.
Working electrodes were fabricated for electrochemical
characterization by attaching Pt wires to the Pt-coated
annealed pellets. An aqueous 1m KOH solution was chosen
as electrolyte because NiO is stable in this solution. Fig-
ure 3a,b shows the cyclic voltammograms and cyclic chro-
nopotentiometric plots of the electrodes prepared at different
annealing temperatures. The peaks of the redox currents in
Figure 3a and the transition periods of the potentials in
Figure 4. Characterization of the conductivity: a) Resistivities of sam-
ples prepared at different thermal conditions. Inset: I–V curves and
associated linear fits within the range of ꢀ0.1 to 0.1 V. At 2508C the
sample behaves like a metal, whereas at 3508C it shows a tunneling
effect because of the isolated conductive metal cores. b) Reconstructed
cyclic chronopotentiometric curves of discharge (measured at 1 Agꢀ1
)
with respect to the normalized charge. At each potential of discharge,
electrodes with the lowest ohmic resistivity show the highest utilization
of the electrode material (NHE=normal hydrogen electrode).
utilization of the electrode material because of polarization
loss.[21,22] This phenomenon can be observed by reconstructing
the cyclic chronopotentiometric curves from the electrode
potential and the normalized charge (stored/released charge
scaled to the total capacity). The utilization of the electrode
material can thus be qualitatively compared at each potential
of the electrode (Figure 4b). High utilization of the electrode
is observed for a sample oxidized at low temperature,
indicating that the electrode performance is enhanced by a
small ohmic resistivity originating from the conductive net-
work of metal cores. Another advantage of the conductive
network of metal cores is that the resistivity of the electrode
does not depend on the size of the electrode itself, which
facilitates its commercial production at large scale.
Figure 3. Electrochemical characterization: a) Cyclic voltammometric
curves at a scan rate of 5 mVsꢀ1 and b) cyclic chronopotentiometric
curves with a charge/discharge current density of 1 Agꢀ1 for the
electrodes prepared at different temperatures. c) Cyclic chronopoten-
tiometric curves of a sample at 2508C for various charge/discharge
currents and d) accordingly derived energy and power densities.
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The most important advantage of supercapacitors is their
much higher power density relative to that of ordinary
batteries. Therefore, the high-power performance of the
electrodes was characterized by a series of cyclic chronopo-
tentiometric measurements with charge/discharge current
densities up to 28.6 Agꢀ1 (equivalent to 10 kWkgꢀ1 of
power density; Figure 3c). The reversible redox reaction
between NiO and OHꢀ [Eq. (1)], is a highly diffusion-
controlled process.[23] Therefore, it can be expected that the
SC, and thus the obtained energy density, will decrease at fast
charge/discharge rate. This phenomenon has been observed in
most of the previous studies.[24–27] In Figure 3d the energy
density as well as the power density is plotted versus the
In summary, the current study reports a simple, cost-
effective, and potentially scalable technique for fabricating
monolithic NiO/Ni nanocomposite electrodes for electro-
chemical supercapacitors. The electrodes annealed at 2508C
showed a remarkably high specific capacitance (ca. 900 Fgꢀ1
)
because of the highly activated NiO surface layer and the
conductive network of metal cores. High-energy (ca.
60 Whkgꢀ1) and high-power (10 kWkgꢀ1) densities were
achieved with slow and fast charge/discharge rates, respec-
tively. For longer times of energy storage, the delivery of
energy density was not affected by the output power.
current density. At a slow charge/discharge rate (1 Agꢀ1) a Experimental Section
high-energy density of 62 Whkgꢀ1 (equivalent to 905 Fgꢀ1 for
the SC) was observed. However, the power density achieved
at this rate (ca. 0.4 kWkgꢀ1) was small relative to that of
electrochemical double-layer capacitors (EDLCs) which
The Ni nanoparticles were synthesized by a modified polyol process.
Solid NiCl2·6H2O (Alfa Aesar, 1.0 g) was dissolved in ethylene glycol
(Alfa Aesar, 250 mL) at room temperature by mechanical stirring.
The solution was then heated to reflux at (195 ꢁ 2)8C. Once a stable
temperature had been reached, solid NaBH4 (Strem Chemicals, 2.0 g)
was added to the solution as a reducing agent. The mixture was
subsequently maintained at reflux for 30 min and then cooled to room
temperature. The resulting particles were magnetically isolated,
repeatedly washed in a sonicated bath with acetone and ethyl alcohol,
and then dried in vacuum at 1008C overnight.
Electrodes for the characterization of the supercapacitor were
prepared by mechanically compacting a specified mass (5 mg) of Ni
powder in a hydraulic press to produce thin pellet disks of 4 mm
indiameter. A Pt layer (80 nm) was then splutter coated on one side of
the pellet to serve as the current collector. The electrodes were
thermally annealed in air at different temperatures (for 1 hour) to
create a NiO shell with supercapacitance.
show
a fast mechanism for the storage of surface
charges.[18,28,29] As the charge/discharge rate was increased
to 28.6 Agꢀ1, a high-power density of 10 kWkgꢀ1 was
achieved. Although the corresponding energy density drop-
ped to 26 Whkgꢀ1 (equivalent to 380 Fgꢀ1 for the SC), it still
is one of best performances reported so far.[3]
In most applications that require an energy storage system
the process of energy collection is usually slow (e.g., wind or
solar-power plant), but the stored energy must be released
rapidly to meet the power demands of these applications.
Therefore, the NiO/Ni nanocomposite electrodes were first
charged at a small current density (1 Agꢀ1) and then dis-
charged at a series of higher discharge rates (Figure 5a). The
power densities of discharge, average specific capacitances of
discharge, and calculated energy densities of discharge were
obtained from the discharge portion of the cyclic chronopo-
tentiometric curves. As shown in Figure 5b, the energy
density of discharge is only slightly affected by the discharge
rates. Outstanding performances of high-energy (ca.
60 Whkgꢀ1) and high-power densities (10 kwkgꢀ1) were
simultaneously achieved.
The electrochemical measurements were carried out in
a
beakertype electrochemical half-cell setup equipped with an Ag/
AgCl (saturated KCl) reference electrode (Fisher Scientific) and a
platinum-plate counterelectrode. A KOH solution (1m) was used as
electrolyte. The working electrode was impregnated with the electro-
lyte for 30 min to ensure that the NiO/Ni nanocomposite electrode
was thoroughly wet. Cyclic voltammometry and cyclic chronopoten-
tiometric measurements were performed on a potentiostat/galvano-
stat (PRA 263A) to determine the electrochemical properties.
Average specific capacitance values determined from the cylic
voltammometric curves were calculated according to Equation (2),
R
Idt
mDV
ð2Þ
C ¼
where I is the oxidation/reduction current, dt is the time differential,
m is the mass of the active electrode material, and DV is the voltage
range of one scanning segment. The specific capacitance, energy
density (de), and power density (dp) were also calculated from the
cyclic chronopotentiometric curves according to Equations (3–5),
IDt
mDV
ð3Þ
ð4Þ
ð5Þ
C ¼
1
2
de ¼ CðDVÞ
2
Figure 5. Slow-charge and fast-discharge characterization. a) Cyclic
chronopotentiometric curves with a density of charge currents of
1 Agꢀ1 and various densities of discharge currents (ratio of the charge
to thꢀe1discharge current density: solid lines in red 1/3 Agꢀ1, in blue 1/
6 Ag , and in black 1/18 Agꢀ1; dotted lines in red 3/3 Agꢀ1, in blue 6/
6 Agꢀ1, and in gray 18/18 Agꢀ1). b) Ragon plot (power density vs.
energy density) derived from the discharge portion of the cyclic
chronopotentiometric curves (* charge with 1 Agꢀ1; & ratio of the
charge to the discharge current density with the same current density).
de
dp ¼
Dt
where I is the charge/discharge current, Dt is the time for a full charge
or discharge, m is the mass of the active material, and DV is the
voltage change after a full charge or discharge.
Received: February 13, 2011
Published online: May 30, 2011
Angew. Chem. Int. Ed. 2011, 50, 6847 –6850
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Communications
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Keywords: electrochemistry · metal oxides · nanoparticles ·
supercapacitors
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