Preparation and application of cobalt oxide nanostructures as electrode materials for electrochemical supercapacitors

Article abstract of DOI:10.1039/c5ra09060a

In a reaction between cobalt(ii) and ammonium oxalate in the presence of CTAB or F-127 as surfactant to control the particle size, a cobalt oxalate complex was formed. The precipitate was calcined and the resulting nano cobalt oxide was characterized by Fourier Transform Infrared spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD) methods. The crystalline pure and nano-sized particles had an average size of less than 40 nm. Electrochemical properties were examined by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy. A maximum specific capacitance of 351 F g^-1 was obtained at a scan rate of 0.85 A g^-1 in 2 M of KOH solution for Co₃O₄@Ni foam electrode (Co₃O₄@NF). Furthermore, the electrode exhibits excellent cycle life stability, and almost 98.6% of its initial specific capacitance was maintained after 1000 cycle tests.

Full text of DOI:10.1039/c5ra09060a

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Preparation and Application of Cobalt Oxide Nanostructures as
Electrode Materials for Electrochemical Supercapacitors
Franak Manteghia,*, Sayed Habib Kazemib,*, Masoud Peyvandipoora and Ahmad Asgharib
In a reaction between cobalt (II) and ammonium oxalate in the presence of CTAB or F-127 as surfactant to control the
particles size, the cobalt oxalate complex was formed. The precipitate was calcined and the resulted nano cobalt oxide was
characterized by Fourier Transform Infrared spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Transmission Electron
Microscopy (TEM) and X-ray Diffraction (XRD) methods. The crystalline pure and nano-sized particles had an average size of
less than 40 nm. Electrochemical properties were examined by cyclic voltammetry, galvanostatic charge/discharge and
electrochemical impedance spectroscopy. A maximum specific capacitance of 351 F g−1 was obtained at a scan rate of 0.85 A
g−1 in 2 M of KOH solution for Co3O4@Ni foam electrode (Co3O4@NF). Furthermore, the electrode exhibits excellent cycle life
stability, and almost 98.6% of its initial specific capacitance was maintained after 1000 cycle tests.
Keywords: Cobalt oxide, Capacitance, Nano-size, Supercapacitor
Introduction
Among the transition metal oxides, Co3O4 is found to be one of the most important materials in a wide
range of applications in various fields such as electrochromic devices, ceramic pigments, heterogeneous
catalysts, solid-state sensors, magnetism, solar energy absorbers, and energy storage, particularly in
supercapacitors, because of its higher surface area, good redox property, controllable size and shape, and
structural characteristics1-4. Electrochemical capacitors are becoming attractive energy storage systems,
because they have higher power density and long cycle life stability compared to common batteries.
Depending on the charge-storage mechanisms, supercapacitors are generally classified into two major
types: carbon based electrical double-layer capacitor (EDLC) and Faradaic capacitor5-12. It was already
reported that the size and morphology of Co3O4 extremely affect its electrochemical performance and
therefore many researchers are attempting fabricate it in newer shapes and morphology in nanoscale
dimensions. Different nanoscale morphologies such as hollow nanospheres, cubic single crystals, fibers,
particles, rods, tubes and films in nano-scale have been reported so far13-19. It is well known that Co3O4
crystallizes in the cubic normal spinel structure with magnetic Co2+ ions in tetrahedral sites and non-
magnetic Co3+ ions in octahedral sites. Several other methods were used to synthesis the spinel Co3O4
nanoparticles such as sol–gel, polyol, solvothermal, polymer assisted methods and thermal
decomposition20-24. Additionally, hydrothermal method has been applied for Co3O4 nanostructures and
cathodic electrodeposition has recently been used for Co3O4 nanostructures5, 23, 25
.
1

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Organic ligands are widely applied in preparation of metallic complexes and in high temperatures, they
burn and eliminate from the coordination sphere, thus remain only oxide material. Considering this issue,
we report the synthesis of Co3O4 nanostructures by thermal decomposition of cobalt oxalate which was
synthesized from co-precipitation of Co(NO3)2·6H2O or CoCl2·6H2O and (NH4)2C2O4 in the presence or
absence of F-127 and CTAB as surfactants, while the molar ratio of ammonium oxalate to metal precursor
was 1:1. The obtained pink precipitate was finally calcined at 400°C.
The structural properties of Co3O4 were investigated by Fourier Transform Infrared spectroscopy (FTIR),
Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD) and Thermal Gravimetric Analysis (TGA).
Experimental results revealed that Co3O4 nanoparticles can successfully form in good crystallinity with an
average size of almost 38 nm. Electrochemical tests confirm that the prepared Co3O4 nanorods maintain
remarkable performance in supercapacitor application.
Experimental
All materials purchased from MERCK were used without any further purification. An aqueous solution of
Co(NO3)2·6H2O or CoCl2·6H2O was rapidly added to (NH4)2C2O4 solution with the molar ratio of 1:1. To
investigate the effect of the temperature and surfactant on morphology and size of Co3O4 particles,
different conditions were applied. The cobalt oxalate precipitate was collected by filtration and washed
with distilled water; finally Co3O4 was obtained by calcining the precipitates in air at 400°C. The
electrochemical properties and capacitance measurement of the supercapacitor electrodes were studied
in a three-electrode system by cyclic voltammetry (CV) and electrochemical impedance spectroscopy
(EIS) using a Compact Stat Galvanostat-Potentiostat (Ivium Technologies, Netherlands). The CV response
of the electrodes was measured at different scan rates varying from 10 to 100 mV s‒1. Voltammetry tests
were carried out at potentials between ‒0.1 and 0.5 V in a 2 M of KOH solution. Impedance spectroscopy
measurements were carried out by applying a sinusoidal signal of 5 mV on the open circuit potential as
DC bias, in the frequency range of 100 kHz to 10 mHz.
Results and Discussion
Characterization of Co3O4 electrode material
The TG/DTG/DSC measurements of cobalt oxalate as precursor were carried out to find the suitable
temperature for calcination. In the TG/DTG/DSC profiles of cobalt oxalate shown in Figures 1a and 1b, a
mass loss rate of 18.47% can be observed from 148 to 200°C on the TG curve, which corresponds to a
broad endothermic peak at around 190°C on the DTG-DSC curve. This is due to the dehydration of
chemically bonded water molecules in the cobalt oxalate complex. The second mass loss and the
2

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corresponding sharp exothermic peak occur at around 293°C, indicating the decomposition and oxidation
reaction of the complex, results in the formation of cobalt oxide, shown in Figure 1a and 1b.
Figure 1. (a) Thermogravimetric analysis (TGA, solid line) and derivative thermogravimetry (DTG, dotted line)
plots and (b) Differential Scanning Calorimetry (DSC) plot of cobalt oxalate in air from 20 to 800°C at a rate of
1
10°C min‒ .
Using the FTIR technique, cobalt oxide and its precursor (cobalt oxalate complex) were characterized
(shown in Figure 2). As seen in Figure 2, a broad band at 3373 cm‒1 is assigned to both symmetric and
asymmetric (O–H) vibrations in crystal water molecules ((O–H)s and (O–H)as). An intense band at 1623
cm‒1 is assigned to (C–O)as vibration, two closely spaced bands at 1359–1317 cm‒1 to (C–O)s vibration
and the band at 825 cm‒1 to the δ(OCO) vibration, indicates the presence of bridging oxalate groups26.
The band at 493 cm‒1 may be attributed to Co–O stretching modes in the complex26, 27. The Co3O4 was
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obtained by calcining the precipitates in air at 400°C. The IR spectrum of cobalt oxide has two major
peaks at 574 and 669 cm‒1 corresponding to metal-oxygen (Co–O) vibrational modes of the spinel
compound27.
100
80
60
40
20
0
3400
2400
1400
400
Wavenumber (cm-1)
Figure 2. FT-IR spectra recorded for the as-prepared a) Co3O4 nanostructures, and b) cobalt oxalate.
311
440
511
220
400
422
111
222
Figure 3. XRD spectrum of pure Co3O4.
Figure 3 shows the representative XRD pattern of prepared Co3O4. All reflection peaks can be readily
indexed as cubic phase of Co3O4 (space group Fd3m) which is consistent with the value given in the
standard card (ASTM No. 01-073-1701) and literature25, 28. No other impurities were detected in the XRD
pattern, indicating the high purity of the product.
Effects of temperature and surfactant on the morphology of Co3O4
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The morphologies of nine prepared Co3O4 nanostructures obtained from cobalt oxalate prepared under
different temperature conditions in the presence or abdsence of surfactants are shown in Figures. 4 and 5.
b
In Figure 4, with nitrate as a counter ion of cobalt, it can be seen that by increasing the temperature from
25 to 40°C, the size of the particles is become smaller but more increasing the temperature up to 80°C
forces the particles to agglomerate. Comparing the presence and absence of CTAB (Figures 4a and 4d) shows that
the surfactant causes the particles to be smaller. In Figure 5, with chloride as the anion of cobalt salt, it can be
observed that by increasing the temperature to 40°C, the particles become finer (Figure 5a,b), but reaching to
80°C causes them to cling together (Figure 5c). Interestingly, the separations between particles can be
seen in Figure 5b. Applying two surfactants, CTAB and F127, causes the particles become much smaller (Figures
5d and 5e). It can be observed in all nine figures that the morphology is not seriously changed, but the sizes of
particles are influenced by temperature and surfactant. Therefore, we conclude that using CTAB and F-127, and
cobalt chloride in preparing cobalt oxalate precursor, force the oxide particles to become smaller
nanostructures, as shown in Figures 5d and 5e. These two structures maintain higher surface area and
shorter diffusion length for electrolyte ions, thus revealed the best specific capacitance in the present
work.
The TEM image given in Figure 6 illustrates the size and morphology of porous Co3O4 nanostructures. It can
be seen in Figures 4-6 that the nanostructures are consisted of nanoparticles smaller than 40 nm.
b
a
200 nm
b
200 nm
Mag=40.00 KX WD=5.0 mm EHT=15.00 kV
Mag=40.00 KX WD=4.9 mm EHT=15.00 kV
c
d
200 nm
200 nm
Mag=20.00 KX WD=4.8 mm EHT=15.00 kV
Mag=20.00 KX WD=5.1 mm EHT=15.00 kV
Figure 4. The SEM images of Co3O4 nanostructures synthesized in a) 25°C b) 40°C c) 80°C, and d) CTAB in 25°C, all
with cobalt nitrate precursor. The scales of the figures are all 200 nm.
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a
b
100 nm
Mag=20.00 KX WD=2.5 mm EHT=15.00 kV
100 nm
Mag=30.00 KX WD=3.9 mm EHT=15.00 kV
c
d
100 nm
100 nm
Mag=20.00 KX WD=4.0 mm EHT=15.00 kV
Mag=50.00 KX WD=3.8 mm EHT=15.00 kV
e
300 nm
Mag=10.00 KX WD=3.9 mm EHT=15.00 kV
Figure 5. The SEM images of Co3O4 with different morphologies prepared in a) 25°C b) 40°C c) 80°C, d) CTAB at
25°C and e) F-127 at 25°C, all produced from cobalt chloride as precursor. The scale bar of the figures is 100 nm,
except figure d which is 300 nm.
(b)
(a)
(c)
(d)
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Figure 6. The TEM images of Co3O4 nanostructure. The scales are a) 250nm, b) 120 nm, c) 90 nm and d) 55 nm.
Moreover, to study the pore structural and the specific surface area of Co3O4 nanostructures fabricated in
this work, Brunauer-Emmett-Teller (BET) gas-sorption measurement is carried out. Nitrogen adsorption-
desorption isotherm of the Co3O4-F127 sample (due to its higher capacitance found in electrochemical
tests) shown in Figure S-1 (provided as supporting information), and the corresponding Barrett-Joyner-
Halenda (BJH) pore size distribution plot is illustrated in Figure S-2 (provided as supporting information).
The N2 adsorption and desorption experiment of the Co3O4-F127 sample shows a typical type IV isotherm,
proposes the microporous structure. When the relative pressure (P/P0) is higher than 0.95, a small
increase appears, suggesting the presence of secondary porous structure formed by aggregation of
nanostructures. The BET specific surface area of the sample is measured to be 24.5 m2 g‒1. Also, the
average pore diameter was 17.22 nm, according to the BJH plot (Figure S-2).
Electrochemical studies of Co3O4@NF electrode
The charge/discharge curves of both electrodes in the potential range of −0.1 to +0.45 V (vs. Ag/AgCl) at a
current density of 1.7 A g−1 are compared in Figure 7. The specific capacitance can be calculated
according to the following equation3, 29
:
ꢁꢂꢃ
ꢀ = ꢄꢂꢅ
(1)
where C (F g−1) is specific capacitance, i (mA) represents discharge current, and m (mg), V (V) and t (s)
refer to mass of active materials, potential drop during discharge and total discharge time, respectively.
The specific capacitance for the Co3O4 electrodes that synthesized from cobalt chloride with F-127 and
CTAB, are 351 F g−1 and 327 F g−1, respectively. Compared to the reported Co3O4 nanorods [24], changing
in the morphology of Co3O4 from nanorod to nanostructure of this paper resulted in the remarkable
increase in specific capacitance. Since the results for Co3O4 synthesized in the presence of F-127 were
better compared to Co3O4–CTAB electrode, Co3O4-F127 electrode was further examined by different
electrochemical methods.
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Figure 7. (a) Discharge curves of cobalt oxide synthesized from cobalt chloride in the presence of F-127
and CTAB surfactants.
The excellent capacitive performance of Co3O4@NF was also verified from its voltammograms at various
scan rates in a 2 M of KOH solution (Figure 8). A pair of redox peaks appeared in the corresponding
voltammograms; indicates the important contribution of pseudocapacitive mechanism in the specific
capacitance of Co3O4@Ni foam. The Faradaic capacitive behavior is originated from the following redox
30, 31
reaction of Co3O4
:
The first redox couple at 0.39 V can be attributed to the conversion between Co3O4 and CoOOH:
Co3O4 + OH- + H2O 3CoOOH + e- (2)
Also, the second redox couple at 0.44 V corresponds to the nearly reversible reaction between CoOOH and CoO2,
represented by the following reaction:
CoOOH + OH-
CoO2 + H2O + e-
(3)
It should be noticed that by increasing the potential scan rate the anodic peaks can overlap and we will see an
overlapped broad peak.
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Figure 8. The CV curves of Co3O4 nanostructures pressed on Ni foam in 2 M KOH at different scan rates.
Figure 9 depicts Galvanostatic discharge curves and specific capacitances against discharge current
densities (Figure 9a and b, respectively). As seen, iR drop value for all curves in Figure 8a is insignificant.
Also, the specific capacitance shows a slight decrease with increasing the current density. Such a slight
decrease may attribute to excellent availability of active sites of electrode material even at high current
densities. It should be noted that there almost 90% of initial capacitance of the electrode (351 F g‒1 at
current density of 0.85 A g‒1) was maintained even at discharge current density as high as 35 A g-1,
indicating the excellent high-rate capability.
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Figure 9. a) Discharge curves at different current density, and b) Specific capacitance versus current
density.
The Nyquist plots of Co3O4@NF electrode is illustrated as Figure 10. As shown, a very low semicircle is
located at high frequency domain; indicating a very low charge transfer resistance associated with Co3O4
redox reaction. The internal resistance can be estimated by extrapolating the plot to infinite frequencies.
Internal resistance includes the ionic resistance of electrolyte, the intrinsic resistance of the active
material, and the contact resistance at the interface of active material and current collector.
Furthermore, the tilted semi-straight line at low frequency domain attributes to the diffusion impedance
32
resulting from the mass transfer resistance in the system3,
. The pattern of the EIS was fitted to an
equivalent circuit shown as scheme 1, in which the charge transfer resistance of Faradaic reaction of cobalt oxide
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(Rct) is in series with diffusion impedance (Warburg impendence, W). Also, the interfacial double layer capacitance
is shown by a constant phase element (CPE). The fitting results were 2.15 Ω, 2.85 Ω, and 0.33 F cm-2 for
electrochemical series resistance (ESR), Rct and double layer capacitance, respectively. Additionally, the
depression factor of CPE was estimated as 0.85. As seen from Figure 9, angle of the straight line at low
frequency domain is more than 70° (71.5° according to the fitting procedure), confirming the relatively ideal
capacitive behavior of the Co3O4@NF electrode material .
Scheme 1. Equivalent circuit used to fitting the EIS experimental results (CPE used instead of Cdl for interfacial
capacitor, also W represents the diffusion impedance or Warburg element).
Figure 10. Nyquist plot of a symmetric Co3O4@NF supercapacitor at open circuit potential.
Figure 11 represents the electrochemical stability of Co3O4@NF examined by charge/discharge cycling at a
current density of 0.85 A g−1. The Co3O4@NF electrode revealed the cycle life stability of more than 98%
of its initial specific capacitance after 1000 successive cycles. Such a remarkable stability suggests the
Co3O4@NF electrode as novel electrode materials for high performance supercapacitors.
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Figure 11. The cycle life stability at a discharge current density of 0.8 A g-1.
It seems that using surfactants in this work was effective in a higher specific capacitance. It can be
compared with the similar works, for example Co3O4 prepared a) with the same salts without template
and b) with CoCl2 and urea without any template having maximum specific capacitance of 202.5 F g‒1 and
280.5 F g‒1, respectively1, 25
.
Conclusion
Nanostructures of Co3O4 in the cubic spinel structure with different sizes were fabricated from cobalt
oxalate at different temperatures in the absence and presence of two surfactants. Electrochemical
measurements revealed that the as-prepared material can deliver a maximum specific capacitance of 351
F g−1 and good stability over 1000 cycles. Furthermore, high specific capacitance was maintained at high
charge/discharge current densities due to the unique structure and high surface area of the Co3O4@NF
electrode. The BET specific surface area of the sample is measured to be 24.5 m2 g‒1 and the average pore
diameter was 17.22 nm, according to the BJH plot. The excellent electrochemical performance coupled
with the low cost and simple preparation process suggests Co3O4 nanostructures as promising material
for practical application in high-rate capable supercapacitors. Of note, the maximum specific capacitance
was obtained for the finest cobalt oxide sample prepared in the presence of surfactant F127 as high as
351 F g‒1.
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Acknowledgment
Financial supports of this work by Institute for Advanced Research in Basic Sciences and Iran University of
Science and Technology are appreciated. S. H. Kazemi also appreciates for the partially support of the
work by Iran National Science Foundation (Grant number: INSF-93012781).
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