7 murugan pccp 2017

4396 | Phys. Chem. Chem. Phys., 2017, 19, 4396--4404 This journal is ©the Owner Societies 2017
Cite this: Phys.Chem.Chem.Phys.,
2017, 19, 4396
Ni–CeO2 spherical nanostructures for magnetic
and electrochemical supercapacitor applications
Ramachandran Murugan,a
Ganesan Ravi,*a
Gandhi Vijayaprasath,a
Somasundharam Rajendran,a
Mahalingam Thaiyan,a
Maheswari Nallappan,b
Muralidharan Gopalanb
and Yasuhiro Hayakawac
The synthesis of nanoparticles has great control over the structural and functional characteristics of
materials. In this study, CeO2 and Ni–CeO2 spherical nanoparticles were prepared using a microwave-
assisted method. The prepared nanoparticles were characterized via thermogravimetry, X-ray diffraction
(XRD), Raman, FTIR, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS),
vibrating sample magnetometry (VSM) and cyclic voltammetry (CV). The pure CeO2 sample exhibited a
flake-like morphology, whereas Ni-doped CeO2 showed spherical morphology with uniform shapes.
Spherical morphologies for the Ni-doped samples were further confirmed via TEM micrographs.
Thermogravimetric analyses revealed that decomposition varies with Ni-doping in CeO2. XRD revealed
that the peak shifts towards lower angles for the Ni-doped samples. Furthermore, a diamagnetic to
ferromagnetic transition was observed in Ni-doped CeO2. The ferromagnetic property was attributed to
the introduction of oxygen vacancies in the CeO2 lattice upon doping with Ni, which were confirmed by
Raman and XPS. The pseudo-capacitive properties of pure and Ni-doped CeO2 samples were evaluated
via cyclic voltammetry and galvanostatic charge–discharge studies, wherein 1 M KOH was used as the
electrolyte. The specific capacitances were 235, 351, 382, 577 and 417 F gÀ1
corresponding to the pure
1%, 3%, 5% and 7% of Ni doped samples at the current density of 2 A gÀ1
, respectively. The 5% Ni-doped
sample showed an excellent cyclic stability and maintained 94% of its maximum specific capacitance
after 1000 cycles.
1. Introduction
Energy conversion and storage is one of the main challenges
in modern society, and in order to overcome this problem,
portable energy storage devices are required. Nowadays, nano-
materials attract much interest due to their potential use in
portable device applications such as supercapacitors, dilute
magnetic oxides (spintronics devices), batteries, sensors, solar
cells and biomedical applications for antibacterial and anticancer
treatments.1–6
Among energy storage technologies, supercapacitors
have received special attention, as they bridge the gap between
batteries and conventional capacitors, and result in specific capa-
citances of six to nine orders of magnitude larger, higher energy
densities, higher power densities, low equivalent series resistance
and a long charge–discharge cycle life, compared to those of
conventional capacitors.7
Nanoelectrodes are used in electro-
chemical technologies because of their high charge–discharge
rates, which result from a high surface to volume ratio and
short path length for electron and ion transport. Due to their
high charge/discharge rates and long life, supercapacitors are
used for practical applications in electric vehicles, laptops, cell
phones, flashlights, and memory cards.8–10
In general, depending
on the nature of the interfacial processes, supercapacitors can be
classified as electrochemical double layer capacitors (EDLCs) and
pseudocapacitors. EDLCs are charged by the reversible adsorption
of ions on the electrolyte–electrode interface of carbon-based
materials, such as activated carbon, carbon nanotubes, aerogels
and graphene, with high surface areas. On the other hand,
pseudocapacitors are charged by redox reactions taking place
on the surface of electrodes made of metal oxides or conducting
polymers.
Various materials, such as transition metal oxides, carbon-
aceous materials and conducting polymers, have been investi-
gated for supercapacitor applications.11–17
Among these materials,
RuO2 and IrO2 have been studied extensively due to their high
intrinsic capacitance. However, their high cost, toxic nature and
poor abundance limit their commercialization. To overcome these
a
Department of Physics, Alagappa University, Karaikudi-630 003, Tamil Nadu,
India. E-mail: gravicrc@gmail.com; Fax: +91 4565-225202; Tel: +91 4565-230251
b
Department of Physics, Gandhigram Rural Institute, Gandhigram-Deemed
University, Tamilnadu, India
c
Research Institute of Electronics, Shizuoka University, Hamamatsu-432-8011,
Japan
Received 4th December 2016,
Accepted 9th January 2017
DOI: 10.1039/c6cp08281e
rsc.li/pccp
PCCP
PAPER

This journal is ©the Owner Societies 2017 Phys. Chem. Chem. Phys., 2017, 19, 4396--4404 | 4397
problems, cheaper and better materials are needed to replace
RuO2. Nowadays, much research is being devoted to carbon-
based materials and transition metal oxides for supercapacitor
applications.11
There is vast literature available on high-
performance transition metal-based supercapacitors. NiO nano-
particles have been successfully synthesized by a single-source
precursor method, which is very important for improving the
manufacturing process of energy storage devices. Despite that,
only few reports have been published on the effect of calcination
on the structural and electrical properties of NiO nanoparticles.18,19
Likewise, only a limited amount of research work has been
published in the field of rare earth oxide-based materials for energy
storage applications. Consequently, there is a newly found interest
in developing rare earth oxide materials with different morpholo-
gies for supercapacitor applications. There have been few very
interesting reports on these recently developed class of nanomater-
ials, whose unique photophysical properties are helping to create a
new generation of devices in the field of photonics and
microelectronics.20–22
Due to their incomplete 4f shell, the trivalent
and divalent rare earth ions provide very interesting optical and
magnetic properties. Different valence states exist in rare earth
oxides. It has been reported that rare earth ions enhance the
specific capacitance and cyclic stability. Particularly, CeO2 is a
promising rare earth oxide material that is widely used in several
technological applications.8,23
Defects, such as oxygen vacancies in
CeO2 play an important role in catalytic, magnetic and electro-
chemical properties, and the oxygen vacancies, can be easily
formed and removed. Oxygen vacancies in CeO2 are highly reactive
with transition metal, graphene–CeO2 nanocomposite, which
could enhance the photocatalytic, magnetic and electrochemical
properties. Despite this, transition metal-doped rare earth-based
nanoparticles have rarely been investigated for their magnetic and
electrochemical properties.
In the present study, Ni–CeO2 nanoparticles were synthesized
with different weight percentages of Ni. The prepared nano-
particles were characterized via various analytical techniques to
determine their structural, optical, compositional, morphological
and magnetic properties. The electrochemical performance of the
Ni–CeO2 nanoparticles was evaluated by cyclic voltammetry and
galvanostatic charge–discharge tests, and the results are discussed
in detail.
2. Experimental
In a typical synthesis of CeO2 and Ni–CeO2 nanoparticles, 0.2 M
of cerium nitrate hexahydrate was dissolved in 40 ml of distilled
water and stirred for 5 min at room temperature. Subsequently,
0.1 M of citric acid were added as a capping agent. Simulta-
neously, in order to keep the pH around 10, 3 ml of ammonia
was added dropwise to this aqueous solution under vigorous
stirring. Then, the solution was transferred to a polypropylene-
lined autoclave bottle and irradiated by microwaves with a
power of 300 W for 10 min in a microwave oven (Samsung
CE1031LFB), which works at 2.45 GHz and a maximum power of
900 W. The system was then allowed to cool to room temperature.
The resultant products, at the bottom of the vessel, were collected
via centrifugation and the precipitates were rinsed several times
with distilled water and finally with methanol to remove soluble
ions. Product was subsequently dried at 80 1C in a hot air oven for
12 h and then calcined at 500 1C for 3 h. To obtain Ni-doped CeO2
nanoparticles of different compositions, the same procedure was
followed with different concentrations of Ni (1, 3, 5 and 7 wt%),
and products obtained were named as CeO2 N1, CeO2 N3, CeO2
N5 and CeO2 N7, respectively.
Thermogravimetric analyses were conducted with a Perkin
Elmer Pyris Diamond analyzer. The structural properties of
nanoparticles were studied via X-ray diffraction (XRD) with a
Cu-Ka (l = 0.154 nm) radiation source (X’ pert Pro PANalytical)
over a 2y scan range of 10–701. Raman spectra were recorded
using a micro-Raman spectrometer (Acton Spectra Pro 2500i,
Princeton Instruments, Acton Optics & Coatings). The functional
characterization was performed via Fourier transform infrared
(FT-IR) spectroscopy using a Thermo Nicolet 380 by the KBr pellet
method at room temperature in the range of 4000–400 cmÀ1
. The
morphologies of the prepared samples were studied using SEM
(FEIQUANDA 200F Field Emission SEM (FESEM) operating at
30 kV) and TEM (JEOL JEM 2100 (200 kV) system). XPS analyses
were performed under ultra high vacuum with an Al Ka
(1486.6 eV) radiation source on a Carl Zeiss instrument. The
magnetic properties of the nanoparticles were studied by
Vibrating Sample Magnetometry (Lake Shore, Model: 7404).
The electrochemical properties of Ni–CeO2 nanoparticles were
studied with a using CHI-660D electrochemical workstation.
The working electrode was prepared by separately mixing 80%
of the nanoparticles (pure and doped with different percentages
of Ni), as the active material, with a 15% of activated carbon,
a 5% of polyvinylidene fluoride, as the binder, and a few drops
of ethanol as the solvent. A slurry was formed and deposited
(3 mg of active material) onto a nickel foam electrode (1 cm Â 1 cm).
The prepared electrodes were dried at 80 1C for 10 h. The electrodes
composed of either non-doped CeO2 or Ni-doped CeO2 with
different percentages of Ni were employed as the working
electrode, a saturated calomel electrode was used as the reference
electrode and a platinum wire was used as the counter electrode.
As the electrolyte, 1 M aqueous KOH solution was used.
3. Results and discussion
Thermo gravimetric analyses (TGA) were conducted for pure
and Ni-doped CeO2 nanoparticles to study the material behavior
during annealing, and the results are shown in Fig. 1a. The first
weight loss, of approximately 9.15%, occurs before 216 1C and
corresponds to the loss of physically adsorbed water.24
The second
weight loss, of 28.32%, in the range of 216–330 1C, is related to the
loss of structural water molecules.25,26
The last weight loss occurs
from 330 1C to 400 1C and is ascribed to the decomposition
of metal nitrate groups in the prepared nanoparticles.27
There is
no major weight loss observed at higher temperatures, which
indicates the absence of additional phases or other structural
changes. Compared with the pure CeO2, Ni–CeO2 shows a
Paper PCCP

considerable weight loss up to 234 1C, which is characteristic
of the loss of weakly bonded water on Ni–CeO2. Then, a
gradual weight loss was observed up to 446 1C. Thus, TGA
results confirm that the thermal stability of CeO2 is greatly
enhanced by the presence of Ni.
Fig. 1b shows the XRD spectra of pure CeO2 and Ni-doped
CeO2 (with different percentages of Ni) nanoparticles. The
diffraction peaks at the 2y values of 28.61, 33.11, 47.41, 56.51,
59.11, 69.61, 76.81 and 79.11 are ascribed to the crystal planes of
(111), (200), (220), (311), (222), (400), (331) and (420), respectively,
with a space group of Fm%3m (225). It is observed that there are no
diffraction peaks corresponding to the doped material. A peak
shift towards lower angle side was observed with the increase of
Ni content in doped CeO2. The peak broadening is due to the
presence of smaller crystallite size of CeO2 nanoparticles. As the
Ni content increases, the intensity of the XRD peaks decreases,
showing the decrease in crystallinity due to the generation
of crystal defects around the dopants, resulting in a charge
imbalance arises.28
The crystallite size, microstrain and dis-
location densities of pure and Ni-doped CeO2 nanoparticles
were estimated based on relevant relations.29
It is clear from
the calculated parameters that CeO2 crystallite size decreases
from 18 to 9 nm when Ni content increases from 0% to 5% and
then increases for 7% of Ni. The data shows that the presence
of Ni ions in CeO2 hinders the growth of crystal grains and
slows the motion of grain boundaries due to the interruption by
Zener pinning.30
When the moving boundaries are pinned by
the interstitial cerium atoms and dopant Ni ions, they develop a
retarding force on the boundaries. If the generated retarding
force is more than the driving force for grain growth, then the
particles cannot grow further. For 7% Ni-doped CeO2 nano-
particles, the crystallite size increases, which may be due to the
possible presence of some Ni atoms in the cubic interstitial
sites, which in turn is due to the strong preference of Ni for
cubic coordination with oxygen.
The phase purity of the synthesized samples was further
examined via Raman spectroscopy. This is an effective technique
to detect the possible secondary phases, which cannot be
observed below the detection limits of XRD. Furthermore, it
gives information about structural defects caused by the
dopants, as defects lead to a shift in Raman peaks. Fig. 1c
displays the Raman spectra of non-doped and Ni-doped CeO2
nanoparticles. A well defined Raman band is observed at
447 cmÀ1
, which corresponds to the first order F2g mode of
the cubic fluorite structure of CeO2. The observed Raman band
has a large shift compared to bulk CeO2 (the F2g mode for bulk
CeO2 is at 465 cmÀ1
). The F2g mode is very sensitive to any
disorder/defects in the oxygen sublattice.31
Hence, a large shift
in the F2g mode may be linked to the presence of Ce3+
ions and
oxygen vacancies in the samples.32–34
The only differences
between the Raman spectra of pure and Ni-doped CeO2 is the
reduction in the peak intensity and the shift towards higher
wavenumber for Ni-doped CeO2. The reduction in peak intensity
is associated with lattice defects such as oxygen vacancies.35
For
Ni–O, Raman peaks appear in the range of 570–645 cmÀ1
, which
are displayed in the inset of Fig. 1c.
Fig. 1d shows the FTIR spectra of pure and Ni-doped CeO2
nanoparticles calcined at 500 1C. Two broad absorption bands
positioned at 3418 cmÀ1
and 1532–1630 cmÀ1
are associated to
the symmetrical stretching (n OH) and bending modes (d OH)
Fig. 1 (a) Thermo gravimetric analyses for non-doped and Ni-doped CeO2; (b) powder X-ray diffraction patterns for non-doped and Ni-doped CeO2
(with different percentages of Ni) nanoparticles; (c) Raman spectra for non-doped and Ni-doped CeO2 (with different percentages of Ni) nanoparticles
(the inset shows an enlarged portion of defects); (d) FTIR spectra for pure and Ni-doped CeO2 (with different percentages of Ni).
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of internally bonded water molecules, respectively. A band at
2363 cmÀ1
is observed for the Ni-doped nanoparticles, which
confirms additional CO2 absorption at the surface. Furthermore,
the O–C–O stretching band is also observed in the 1335 cmÀ1
region. The bending mode of Ce–O–C (d) is observed well
below 650 cmÀ1
, confirming the formation of CeO2 without
impurities.36,37
Finally, the peak positioned at 431 cmÀ1
is
attributed to the O–Ce–O stretching mode of vibration.38
The morphology of pure and Ni-doped CeO2 samples was
investigated via FESEM. Fig. 2(a–e) shows the FESEM images of
CeO2, CeO2 N1, CeO2 N3, CeO2 N5 and CeO2 N7, respectively.
The growth mechanism of pure and Ni-doped CeO2 nano-
particles is represented in Fig. 3. It can be clearly seen from
Fig. 2 and 3 that non-doped CeO2 nanoparticles are evenly
distributed and have a flake-like morphology. For 1% of Ni
doped sample, nanoflakes are aggregated and do not exhibit
any particular shape. Upon further increasing the dopant
percentage to 3%, 5% and 7%, the agglomerated mountains
split to yield well-defined spherically-shaped particles that
cover the entire surface. It can be clearly observed in Fig. 2
that the agglomeration is almost restricted to Ni-doped CeO2
particles, which exhibit a uniform shape and narrow size
distribution. Moreover, some porosity is found in the back-
ground portion. The presence of larger particles for the samples
with higher percentages of Ni might be attributed to the
aggregation or overlapping of smaller particles.
Fig. 4(b–d) shows the TEM images of CeO2 N5 nanoparticles
with different magnifications and the selected area energy
diffraction (SAED) pattern of the TEM micrograph is presented
in the inset of Fig. 4d. Most grains exhibit sizes between 5–8 nm
and the average size is close to 6 nm. The polycrystalline nature
of the prepared nanoparticles is further confirmed from the
SAED pattern. These results agree well with the XRD data.
The elemental composition and oxidation states in CeO2 N5
nanoparticles were examined using X-ray photoelectron spectro-
scopy (XPS) technique. The survey spectrum of CeO2 N5 nano-
particles shows (Fig. 5) sharp peaks corresponding to C 1s, O 1s,
Ce 3d and Ni 2p. The Ce 3d, Ni 2p and O 1s spectra were fitted
using the Gaussian fitting. Eight peaks were observed for the Ce
3d spectrum and these peaks correspond to diﬀerent oxidation
states of Ce3+
and Ce4+
. The characteristic signals of Ce3+
due to
the 3d5/2 and 3d3/2 electron states are observed at 878 and 896 eV
(Fig. 6a), respectively, with a separation of 18 eV. The additional
satellite signal observed at 919 eV is due to the 3d3/2 electron
state of Ce4+
. These peaks are denoted as v, v0
, v00
and v0 00
for Ce
3d5/2 and u, u0
, u00
and u0 00
for the Ce 3d3/2 spin orbit states. The
main peaks observed at 878 and 919 eV represent the relative
amounts of Ce3+
and Ce4+
in the sample, respectively. From the
area of the respective peaks, it is clear that the concentration
of Ce3+
is relatively higher in the samples, which confirms the
Fig. 2 FESEM images of (a) pure, (b) CeO2 N1, (c) CeO2 N3, (d) CeO2 N5
and, (e) CeO2 N7 nanoparticles.
Fig. 3 Growth mechanism of pure and Ni-doped CeO2 nanoparticles.
Fig. 4 (a) FESEM image of CeO2 N5 nanoparticles; (b–d) TEM images of
CeO2 N5 nanoparticles at different magnifications. Inset of Fig. 4d shows
the SAED pattern for CeO2 N5 nanoparticles.
Paper PCCP

presence of oxygen vacancies in CeO2.39
The Ni 2p spectrum
shows the two characteristic spin–orbit doublets of Ni3+
and
Ni2+
, with two shake-up satellites (Fig. 6b). On deconvolution,
the O 1s spectrum shows three components at 531.19, 531.95
and 533.34 eV, corresponding to metal–oxygen bonds, surface
hydroxyl or oxyhydroxide functional groups, and adsorbed metal
atoms at the surface, respectively (Fig. 6c). The binding energy
values of Ce, Ni and O atoms are in good agreement with those
of previous reports.40,41
The prepared non-doped and Ni-doped CeO2 nanoparticles
were characterized for their magnetic properties using vibrating
sample magnetometry. Ni-doped CeO2 nanoparticles are ferro-
magnetic, whereas non-doped CeO2 exhibits a diamagnetic
character. Semiconducting systems develop ferromagnetic
behaviour when doped with 3d transition metal ions. For Ni,
which has 3d8
+ 4s2
valence electrons, the low spin state of Ni2+
shows that 3d8
electrons are not compensated, resulting in a
spin state of S = 1/2. Hence, Ni is often used for synthesizing
magnetic semiconductors. Ferromagnetism is due to the exchange
interaction between unpaired electron spins arising from oxygen
vacancies formed at the surface of the nanoparticles.42
As the
Ni concentration increases, the probability of forming oxygen
vacancies on the surface of the nanoparticles also increases.
The increased number of oxygen vacancies is also observed in
the PL and Raman spectra. The uncompensated spins on the
surface of the nanoparticles induce ferromagnetism, while the
compensated spins at the inner core of the particle still exhibit
the diamagnetic behavior.42
The surface to volume ratio of the
nanoparticles system plays an important role in determining
the magnetic properties of the prepared nanoparticles, such
that there is a competition between diamagnetic and ferro-
magnetic behavior. Enhanced surface to volume ratio is used to
the increase magnetization under high magnetic fields for the
as-prepared nanoparticles. Fig. 6d shows the ferromagnetic
properties of the as-prepared Ni-doped CeO2 nanoparticles.
The increase in the dopant concentration of Ni2+
ions in the
host increases the ferromagnetic property. Saturation magne-
tization considerably increases up to 5 wt% Ni and slightly
decreases for 7 wt% doped samples, which can be attributed
to the variation in crystallite size, as calculated from the XRD
results.
The capacitance of pure and Ni-doped CeO2 with various
percentage of nickel was studied via cyclic voltammetry (CV) at
diﬀerent scan rates. Fig. 7 shows the CV curves of (a) pure CeO2,
(b) CeO2 N1, (c) CeO2 N3, (d) CeO2 N5 and (e) CeO2 N7 in 1 M
KOH solution, as the electrolyte. The electrodes coated with
non-doped and Ni-doped CeO2 nanoparticles store charges at
the electrode/electrolyte interfaces. As it can be seen from the
CV curves, redox peaks are present for all electrodes at current
densities of 2 A gÀ1
. Furthermore, the current densities of the
Ni-doped CeO2 electrodes indicate an increased capacitance
when compared to that of the pure CeO2 electrodes. These curves
Fig. 5 XPS wide survey of the CeO2 N5 nanoparticles.
Fig. 6 (a) Ce 3d, (b) Ni 2p and (c) O 1s deconvoluted XPS spectra of CeO2
N5; (d) VSM spectra for non-doped and Ni-doped CeO2 (with diﬀerent
percentages of Ni).
Fig. 7 CV curves of the Ni foam electrode coated with the prepared
nanoparticles: (a) pure, (b) CeO2 N1, (c) CeO2 N3, (d) CeO2 N5, and
(e) CeO2 N7 nanoparticles at scan rates from 2 to 100 mV sÀ1
.
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are diﬀerent from the rectangular CV curves of traditional electric
double layer capacitors. CeO2 exhibits prominent redox peaks,
indicating that the sample has typical faradic pseudocapacitance
characteristics, attributed to the redox reaction between Ce3+
and
Ce4+
.43,44
Compared with the other electrodes, the CeO2 N5
electrode shows a higher current response in the potential range
of 0 to 0.45 V, as shown in Fig. 7.
The cyclic behavior and stability were studied, and slow-charge
and fast-discharge tests were carried out using the chrono-
potentiometry technique.45
The charge–discharge measurements
were carried out in 1 M KOH between 0 to 0.45 V at different
current densities from 2 to 20 A gÀ1
. Fig. 8(a–f) shows the charge–
discharge curves of (a) non-doped CeO2, (b) CeO2 N1, (c) CeO2 N3,
(d) CeO2 N5 and (e) CeO2 N7, respectively. The specific capaci-
tance (Cs) was calculated using the following formula:46
Cs ¼
IDt
mDv
where I is the discharge current (mA), Dt is the discharging time
(s), m is the mass of active material (g), and Dv is the potential
window (V).
The Cs values of pure CeO2, CeO2 N1, CeO2 N3, CeO2 N5 and
CeO2 N7 samples are 235, 351, 382, 577 and 417 F gÀ1
,
respectively at a current density of 2 A gÀ1
. As the Ni content
increases (up to 5%), as shown in the figure the specific
capacitance also increases. However, with excessive Ni content,
as in CeO2 N7, the specific capacitance decreases due to
structural disintegration and self-aggregation of nickel ions.
The excessive self-aggregation of nickel can produce higher
solution resistance (Rs), which reduces the capacitance output.
The variation in specific capacitance of non-doped and Ni-doped
CeO2 at diﬀerent current densities are presented in Fig. 8d.
Among all samples, CeO2 N5 exhibits the highest specific capaci-
tance. A Cs value of 167 F gÀ1
has been reported by Padmanathan
et al.47
for the NiO–CeO2 binary oxide, calcined at 700 1C. Rudra
Kumar et al.48
reported a Cs value of 78 F gÀ1
for CeO2 at a current
density of 2 A gÀ1
by the galvanostatic charge–discharge method.
Among these reported values, CeO2 N5 exhibits the highest
specific capacitance of 577 F gÀ1
at a current density of 2 A gÀ1
.
The charge storing mechanism in Ni-doped CeO2 can be
represented using the following equation:
Ce IV
O2 + K+
+ eÀ
2 Ce III
OÁOK
In order to explore the lifetime of the capacitor for practical
applications, the stability of the electrode was tested by cyclic
charge/discharge tests. Fig. 9 shows the specific capacitance for
CeO2 N5 as a function of the number of cycles. The capacitance
retention after the first and last few cycles is shown in the upper
part of Fig. 9. There is gradual degradation observed throughout
1000 cycles. Furthermore, we observed that 94% of the maximum
capacity was retained even after 1000 cycles. Recently, Dongyang
Deng et al.49
achieved specific capacitance retention of 86%
for graphene decorated with cerium oxide nanoparticles after
500 cycles. Zhang et al.41
reported a capacitance retention
of 93.9% after 1000 charge/discharge cycles for MnO2/CeO2
composites. Vijayakumar et al.50
observed a capacitance retention
of 88% after 1000 cycles for a NiO electrode. Compared to these
results, our studies demonstrate increased capacitance retention
even after 1000 cycles.
As supercapacitors are power devices in an electrode, for
supercapacitor applications, it is preferred to have a low
electrochemical resistance. The impedance plots of the prepared
CeO2, CeO2 N1, CeO2 N3, CeO2 N5 and CeO2 N7 electrodes in the
frequency range from 0.01 to 100 kHz at the bias potential of
0.4 V are shown in Fig. 10. The inset of Fig. 10 shows the
equivalent fitting circuit with pure and Co-doped electrodes. Rs
is the solution resistance, Rct is the charge transfer resistance,
Cdl is the double layer capacitance and W is the Warburg impedance.
Fig. 8 Charge–discharge curves of the Ni foam electrode coated with
the prepared nanoparticles: (a) pure, (b) CeO2 N1, (c) CeO2 N3, (d) CeO2
N5, (e) CeO2 N7 nanoparticles at different current densities; (f) variation in
the specific capacitance for non-doped and Ni-doped CeO2 (with different
percentages of Ni) nanoparticles at different current densities.
Fig. 9 Charge/discharge curves indicating the cyclic stability of the CeO2
N5 electrode at 20 A gÀ1
.
Paper PCCP

The semicircle portion represents the charge transfer resistance,
which is 9.52, 7.12, 5.78, 4.21 and 6.34 O for the CeO2, CeO2 N1,
CeO2 N3, CeO2 N5 and CeO2 N7 electrodes, respectively. The
CeO2 N5 electrode gives the lower Rct value, which is due to the
enhanced diffusivity of the OHÀ
ions at the porous electrode.
The diffusive resistance of the OHÀ
ions within the CeO2 N5
electrodes (known as the Warburg resistance), is represented by
the straight line situated in the lower frequency range.
4. Conclusion
In summary, pure and Ni-doped CeO2 nanoparticles were
prepared using a microwave-assisted method. The prepared
nanoparticles were less than 20 nm in size, as confirmed from
our XRD and TEM analyses. Oxygen vacancies were present in
Ni-doped CeO2 samples, which was confirmed through Raman
and XPS analyses. Furthermore, oxygen vacancies lead to a
ferromagnetic behavior in the samples, as confirmed through
VSM analysis. Electrochemical analyses showed that Ni-doped
CeO2 electrodes exhibit significantly higher specific capaci-
tances than that of the non-doped counterpart. A specific
capacitance of 577 F gÀ1
was achieved for the CeO2 N5 electrode.
A capacitance retention of 94% was observed after 1000 cycles.
These studies demonstrate that microwaves are suitable for
elemental doping and that elemental doping is an effective way
to improve the performance of pseudo-capacitive metal oxides,
thus enabling the fabrication of active electrode materials for
supercapacitor applications.
Acknowledgements
The authors R. Murugan and S. Rajendran gratefully acknow-
ledge UGC, New Delhi for awarding UGC-BSR (Grant No. F.25-1/
2013-14 (BSR)/7-14/2007 (BSR)/30th May 2014.) and Emeritus
fellowship respectively. G. Ravi acknowledge Japan Society for
the Promotion of Science (JSPS), Japan, for the financial support
under Invitation fellowship programme (Award Ref.: JSPS/236 ID
No. S15167).
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