Journal of Electroanalytical Chemistry 778 (2016) 110–115
Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry
journal homepage: www.elsevier.com/locate/jelechem
Synthesis of Co(OH)2/Ni(OH)2 nanomaterials with excellent
pseudocapacitive behavior and high cycling stability for supercapacitors
Limin Chang ⁎, Fang Ren, Cuimei Zhao, Xiangxin Xue
Key Laboratory of Preparation and Applications of Environmental Friendly Materials, Jilin Normal University, Ministry of Education, Changchun 130103, China
a r t i c l e
i n f o
Article history:
Received 12 May 2016
Received in revised form 15 June 2016
Accepted 14 August 2016
Available online 16 August 2016
Keywords:
Cobalt hydroxide
Nanomaterial
Specific capacitance
Cycling stability
Supercapacitor
a b s t r a c t
Sheet-like Co(OH)2 or Co(OH)2/Ni(OH)2 nanomaterials has been synthesized on conducting carbon fiber paper
(CFP) via a simple electrochemical deposition. The microstructural, chemical bonding and surface morphology
of the nanomaterials were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS)
and scanning electron microscopy (SEM). And the electrochemical properties of nanomaterials were investigated
by cyclic voltammetry and galvanostatic charge-discharge. Compared to Co(OH)2, Co(OH)2/Ni(OH)2
nanomaterials exhibits not only a higher specific capacitance (1498 Fg−1 than Co(OH)2 (709 Fg−1) at a current
density 2 Ag−1) but also a higher cycling stability (92.4% specific capacitance remain over 20,000 chargedischarge cycles than Co(OH)2 (72.3% remains) at a high current density of 32 Ag−1). The introduction of
Ni(OH)2 brings a synergistic effect for the whole material.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
The development of high-performance energy storage device has
become an urgent requirement in recent years due to the high-speed
development of mobile technology [1–3]. Supercapacitors, also known
as electrochemical capacitors, have attracted tremendous attention
over the past decade because of their drive to meet the demand of
low-cost, high specific capacitance and clean energy conversion/storage
systems [4,5].
The supercapacitors electrode material can store charge in two
ways: one is the double electric layer capacitance electrode material,
such as carbon particles, carbon nanotube, graphene, in which no electrochemical reaction occurs on the electrode material during the charging and discharging processes but pure physical charge accumulation at
the electrode/electrolyte interface [6]. The other type is the faradaic
pseudocapacitance, the electrode material is electrochemically active,
e.g. metal oxides, which can directly store charges during the
oxidation-reduction processes [7,8].
Although hydrated ruthenium oxide is an optimal material with remarkable high specific capacitance. The high cost of material becomes a
major barrier for its commercial applications. Inexpensive candidates
with good pseudocapacitive properties have attracted many efforts,
such as transition metal (Ni, Mn, Co, etc.) oxides and hydroxides, that
⁎ Corresponding author at: Key Laboratory of Preparation and Applications of
Environmental Friendly Materials, Ministry of Education, Jilin Normal University, Siping,
Jilin 136000, China.
E-mail address: [email protected] (L. Chang).
http://dx.doi.org/10.1016/j.jelechem.2016.08.020
1572-6657/© 2016 Elsevier B.V. All rights reserved.
have been explored as a promising candidates for supercapacitors
[9–11].
Cobalt oxide/hydroxide is considered as a promising electrode material for supercapacitors due to its excellent electrochemical properties
and high theoretical specific capacitance (3460 Fg−1) [12]. Mondal
et al. [13]showed that the specific capacitance of flower-like Co(OH)2
is about 416 Fg−1 and 93% specific capacitance is retained after 500 cycles. Kong et al. [14] reported a very high specific capacitance
(1473 Fg−1) of porous α-Co(OH)2 thin film electrochemically deposited
on nickel foam, after 1000 cycles at current density of 2 Ag−1, 88% of initial capacitance maintains. However, the specific capacitances are still
much lower than the corresponding theoretical values. That shows the
electrochemical utilization of cobalt oxide/hydroxide is limited. The improvement of specific capacitance and cycle stability has become a
challenge.
Recently, the growth of double hydroxides on various substrates has
received considerable attention. Double hydroxides can play a synergistic effect of various materials, increase the specific surface area, promote
the electronic transmission and improve the cycle stability [15,16]. For
example, Huang et al. [17] have reported a facile hydrothermal synthesis method to fabricate Co\\Al layered double hydroxide in a water system, and the specific capacitance of the nanomaterial increased as 25%
and 34% compared with the pure component. Among the
pseudocapacitive electrode materials, nickel-based oxides/hydroxides
have attracted severe attention due to their various morphologies, excellent redox activity, high theoretical specific capacitance, and lowcost [15,18–20].
In this paper, via a green and low-cost electrodeposition method,
Co(OH)2/Ni(OH)2 nanomaterials has been successfully synthesized on
L. Chang et al. / Journal of Electroanalytical Chemistry 778 (2016) 110–115
a conducting carbon fiber paper (CFP). We aim at developing a new
electrode active material with excellent pseudocapacitive behavior
and high cycling stability that could be used for hybrid supercapacitors.
The experiment investigates the electrochemical properties of the different composite ratios of Co(OH)2 and Ni(OH)2, in which best synergistic effect for high pseudocapacitance and high cycling stability.
111
2.2. Physical characterization
2. Experimental
The surface morphologies and micro structures of the thin film electrodes were observed by scanning electron microscopy (SEM, SIRION100, FEI Co. Ltd.) and X-ray diffraction (XRD, Rigaku Ultima IV) patterns
recorded with Cu Kα (λ = 1.5418 Å) irradiation. X-ray photoelectron
spectroscopy (XPS, PHI 5000C ESCA System) measurements were applied to investigate the compositions of the products.
2.1. Preparation of materials
2.3. Electrochemical tests
A piece of commercial CFP of 1 cm2 in area, was ultrasonically
cleaned with acetone, ethanol, and distilled water for 15 min respectively and dried in a vacuum oven at 40 °C for 12 h. Co(OH)2/Ni(OH)2
nanomaterials were electrodeposited on CFP in an aqueous solution
containing 0.02 M Co(NO3)2 and 0.08 M Ni(NO3)2. The deposition process was performed at 45 °C in a conventional three-electrode system
consisting of the prepared working electrode (1 cm2 in area), a platinum
plate counter electrode (1 cm2 in area) and a saturated calomel electrode (SCE) reference electrode. The deposition potential was controlled
at −0.9 V. The electrochemical deposition process was expressed as follows [21,22]:
The electrochemical measurements were carried out on a CHI660D
electrochemical workstation (Shanghai CH Instrument Company,
China) in an aqueous 1.0 M KOH electrolyte with a three-electrode
cell. The as-prepared Co(OH)2 or Co(OH)2/Ni(OH)2 nanomaterial on
CFP, a platinum foil, and SCE were used as the working electrode, counter electrode, and reference electrode, respectively. Cyclic voltammetry
(CV) experiments were performed at various scan rates of 2, 5, 10, 25,
50 mV s− 1. And the galvanostatic charge-discharge tests were performed at various current densities of 2, 4, 8, 16, 32 Ag−1.
3. Results and discussion
NO3 − þ 7H2 O þ 8e− →NH4 þ þ 10OH−
ð1Þ
3.1. Microstructure
þ 2OH →CoðOHÞ2
ð2Þ
Ni2þ þ 2OH− →NiðOHÞ2
ð3Þ
Fig. 1(a) and (b) exhibit the SEM images for Co(OH)2 and Co(OH)2/
Ni(OH)2 nanomaterials (before 10,000 discharge-charge cycles), respectively. From Fig. 1(a), Co(OH)2 interlaced sheets with channels
and pores can be formed on carbon fiber supported. From Fig. 1(b),
compared with Co(OH)2, smaller and finer layer structure for
Co(OH)2/Ni(OH)2 can be seen. Since nickel ion has a higher electrical
conductivity than cobalt ion, during electrochemical deposition of
Co(OH)2, nickel ion will contribute more electrons to form double
metal hydroxide with cobalt hydroxide than single hydroxide electrode
material in the same deposition time [23]. Hence if the content for
Co
2þ
−
After deposition, the Co(OH)2/Ni(OH)2/CFP electrode was rinsed
consecutively with double-distilled water and dried in a vacuum oven
at 40 °C for 12 h. The mass of the deposited Co(OH)2/Ni(OH)2
nanomaterials was measured from the weight difference before and
after electrochemical deposition by means of a micro-balance with an
accuracy of 0.01 mg.
Fig. 1. SEM images for Co(OH)2 (a) and Co(OH)2/Ni(OH)2 nanomaterials (b) before discharge-charge cycles. Co(OH)2 (c) and Co(OH)2/Ni(OH)2 nanomaterials (d) after 10,000 dischargecharge cycles.
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Fig. 2. X-ray diffraction patterns for Co(OH)2 (a) and Co(OH)2/Ni(OH)2 nanomaterials
(b) on CFP.
Co(OH)2 and Co(OH)2/Ni(OH)2 is kept the same, Ni(OH)2 will provide
more nucleation sites (in a unit volume) for Co(OH)2 than single
Co(OH)2 electrode, and thus the Co(OH)2/Ni(OH)2 nanosheets are
small and prosperous [24]. Fig. 1(c) and (d) shows the morphology of
Co(OH)2 and Co(OH)2/Ni(OH)2 nanomaterials after 10,000 chargedischarge cycles, respectively. It can be found that after 10,000 cycles,
single Co(OH)2 nanosheets electrode turn incomplete and their surfaces
become coarse, indicating the pulverization of the nanosheets during
cycling [25,26]. And the initial morphology and structural integrity of
the Co(OH)2/Ni(OH)2 nanomaterials are still well-maintained. The
formation of double metal oxide or hydroxide can confine the dissolution and improve the stability for composite electrode in the chargedischarge process. Furthermore, nickel ion with a high conductivity
can improve electrical properties, thus a high rate capability could be
expected [27].
The XRD patterns for Co(OH)2 and Co(OH)2/Ni(OH)2 nanomaterials
in Fig. 2(a) and (b) are nearly the same, wherein the peaks at 2θ values
of 10.46°, 33.74° and 59.08° are indexed to α-Co(OH)2 (001), (100) and
(110) (PDF, card no 46–0605), respectively. And the peaks at 2θ values
of 34.17° and 59.98° are indexed to Ni(OH)2 (101) and (110) (PDF, card
no 38–0715), respectively. And the reflection peaks at 26.38°, 43.92°
and 54.20° are the characteristic of CFP used as current collector (PDF,
card no. 41–1478, card no. 19–0268, card no. 26–1077). Compared
with the pattern for Co(OH)2, the Co(OH)2/Ni(OH)2 nanomaterials
shows a weaker Co(OH)2 (001) peak, implying a (001) preferential orientation growth [28].
Fig. 3(a) and (b) shows the Co 2p3/2 XPS spectra for Co(OH)2 and
Co(OH)2/Ni(OH)2 nanomaterials (before cycling), respectively. The
main peaks centered around 781.3 and 783.2 eV correspond to
Co(OH)2, confirming the existence of Co(OH)2 predominantly covered
on the surface, which is consistent with the XRD results shown in
Fig. 2. After 10,000 charge-discharge cycles, the Co 2p3/2 XPS spectra
for Co(OH)2 and Co(OH)2/Ni(OH)2 nanomaterials in Fig. 3(c) and
(d) show that the peak at 779.6, 781.5, 782.6 and 789.9 eV can be
assigned to CoOOH. As the deposited Co(OH)2 cannot be fully recovered
after being anodized, CoOOH will become the dominant phase in the
electrode that undergoes the repeating redox cycles [29].
Fig. 3(c) exhibits that the peak intensity of Co 2p3/2 for Co(OH)2 after
10,000 cycles is much lower than that before cycling (Fig. 3(a)), indicating that during cycling the dissolution of the active material (Co(OH)2 or
CoOOH) occurs. Contrarily, the peak intensity of the Co 2p3/2 for
Co(OH)2/Ni(OH)2 nanomaterials has no any significant change before
Fig. 3. Co 2p3/2 XPS spectra for Co(OH)2 (a) and Co(OH)2/Ni(OH)2 nanomaterials (b) before discharge-charge cycles; Co(OH)2 (c) and Co(OH)2/Ni(OH)2 nanomaterials (d) after 10,000
discharge-charge cycles.
L. Chang et al. / Journal of Electroanalytical Chemistry 778 (2016) 110–115
113
Fig. 4. Cyclic voltammograms for Co(OH)2 (a) and Co(OH)2/Ni(OH)2 nanomaterials (b) at a scan rate of 50 mV s−1; Co(OH)2 (c) and Co(OH)2/Ni(OH)2 nanomaterials (d) at different scan
rates.
(Fig. 3(b)) and after cycling (Fig. 3(d)), which implies that the dissolution of active material (Co(OH)2 or CoOOH) upon cycling is significantly
suppressed due to the existence of Ni(OH)2 [15,20,30–31].
3.2. Electrochemical properties
As shown in Fig. 4(a) and (b), the shape of the CV curves tested in
1 M KOH solution at a scan rate of 50 mV s−1 for either Co(OH)2 or
Co(OH)2/Ni(OH)2 nanomaterials. For the Co(OH)2 and Co(OH)2/
Ni(OH)2 nanomaterials, the oxidation and reduction reactions could
be as follows [32,33]:
CoðOHÞ2 þ OH− ↔CoOOH þ H2 O þ e−
ð4Þ
NiðOHÞ2 þ OH− ↔NiOOH þ H2 O þ e−
ð5Þ
According to the CV curves for Co(OH)2 in Fig. 4(a), a pair of redox
peaks are observed, which is consistent with the reaction process mentioned in Eq. (4). The anodic peak is due to the oxidation of Co(OH)2 to
CoOOH, while the cathodic peak corresponds to the reverse process.
Fig. 4(b) shows the CV curves of Co(OH)2/Ni(OH)2 nanomaterials with
a longer the potential scanning range (− 0.1–0.6 V) than single
Co(OH)2 electrode (−0.3–0.3 V). The redox peaks are related to faradaic
reactions of Co(OH)2/Ni(OH)2 nanomaterials described by reversible
pseudocapacitive reactions of Eqs. (4) and (5) [34]. Both Co(OH)2 and
Co(OH)2/Ni(OH)2 nanomaterials possess a redox peaks with very
large current, and Co(OH)2/Ni(OH)2 nanomaterials have wider potential range than Co(OH)2. The result indicates that Co(OH)2/Ni(OH)2
nanomaterials have a better pseudocapacitance behavior.
Fig. 4(c) and (d) shows the CV curves for Co(OH)2 and Co(OH)2/
Ni(OH)2 nanomaterials on CFP at different scan rates (2, 5, 10, 25 and
50 mV s−1), respectively, wherein the peak current of anodic oxidation
basically equals to that of the cathodic reduction for each curve, and the
change of peak potential separation is very small with increasing scan
rate. And the characteristic CV shapes for both Co(OH)2 and Co(OH)2/
Ni(OH)2 nanomaterials almost do not change with increasing potential
scan rates, indicating that these two systems have desirable fast chargedischarge properties, which is generally required by power devices.
A galvanostatic charge-discharge experiment was conducted to obtain the specific capacitance of Co(OH)2 and Co(OH)2/Ni(OH)2
Fig. 5. Galvanostatic charge-discharge curves at different charge-discharge current densities for Co(OH)2 (a) and Co(OH)2/Ni(OH)2 nanomaterials (b).
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L. Chang et al. / Journal of Electroanalytical Chemistry 778 (2016) 110–115
nanomaterials, respectively, at various current densities of 2, 4, 8, 16 and
32 Ag−1. The charge-discharge behavior is shown in Fig. 5(a) and (b).
Both of Co(OH)2 and Co(OH)2/Ni(OH)2 nanomaterials shape of the
charge-discharge curves show the characteristic of pseudocapacitance,
in agreement with the results from CV test. The specific capacitance of
an electrode during galvanostatic charge-discharge can be calculated
by the following equation [35]:
C ¼ I△t=m△V
where m is the mass of hydroxide (g), △V is the potential window (V),
and I is the discharge current (A) applied for time △t (s).The capacitance of the Co(OH)2/Ni(OH)2 nanomaterials at various current densities is 1498, 1395, 1225, 1003 and 569 Fg−1 at 2, 4, 8, 16 and 32 Ag−1,
respectively, higher than Co(OH)2 (709, 677, 516, 487 and 462 Fg−1).
The decrease in capacitance with increase in discharge current suggests
an increasing participate in polarization, a phenomenon that results in a
low utilization of the active materials at higher charge-discharge currents, in agreement with other studies [36,37].
Coulombic efficiency can be defined as the value of discharge capacity than charge capacity [38]. The Coulombic efficiency of the Co(OH)2/
Ni(OH)2 nanomaterials at various current densities is 99.7%, 99.3%,
98.7%, 98.3% and 97.9% at 2, 4, 8, 16 and 32 Ag−1, respectively, higher
than Co(OH)2 (94.2%, 92.4%, 90.5%, 88.3% and 85.4%). The result indicates that Co(OH)2/Ni(OH)2 nanomaterials has a relatively strong
charge storage capacity.
The cycling stability for either Co(OH)2 or Co(OH)2/Ni(OH)2
nanomaterials is examined by continuous charge-discharge experiments for 20,000 cycles at high current density of 32 Ag−1, shown in
Fig. 6. The Co(OH)2/Ni(OH)2 nanomaterials exhibit a higher cycling stability (92.4% of the initial specific capacitance remains) than Co(OH)2
electrode (72.3% is maintained). It is indicated that the prepared
Co(OH)2/Ni(OH)2 nanomaterials has a long-term electrochemical stability and high degree of charge-discharge reversibility. The excellent
pseudocapacitive behavior and high cycling stability may be mainly
originate from Co(OH)2/Ni(OH)2 nanosheets with excellent electrochemical activity and reversibility are directly grown on CFP. With
short fiber random arrangement, CFP is more advantageous in high uniformity requirements. And the synergistic effects between Co(OH)2 and
Ni(OH)2 can significantly suppress the dissolution of active material, favoring the electrochemical stability [39].
Fig. 7 shows a typical shape of an electrochemical impedance spectroscopy (EIS) (in the form of a Nyquist plot) for Co(OH)2 and
Co(OH)2/Ni(OH)2 nanomaterials operated at 0.1 V, in which the Nyquist
plots are composed of approximate semi-circles at high frequencies and
Fig. 6. Cycling performance of Co(OH)2 and Co(OH)2/Ni(OH)2 nanomaterials in a 1 M KOH
electrolyte measured using the galvanostatic charge-discharge technique with a current
density of 32 Ag−1.
Fig. 7. Electrochemical impedance spectra (EIS) obtained from Co(OH)2 and Co(OH)2/
Ni(OH)2 nanomaterials.
a slope along the imaginary axis at low frequencies. As shown in Fig. 7,
the value of the intersecting point with the real axis of Co(OH)2/Ni(OH)2
nanomaterials are smaller than that of Co(OH)2, revealing that
Co(OH)2/Ni(OH)2 nanomaterials has a relatively lower inner resistance
(compared to Co(OH)2). As a result, Co(OH)2/Ni(OH)2 nanomaterials facilitates to transfer more smooth charges (electrons), leading to a high
cycling stability. The diameter of the semi-circles and the slope of the
straight line at low frequency for Co(OH)2/Ni(OH)2 nanomaterials is
steeper than Co(OH)2, indicating that Co(OH)2/Ni(OH)2 nanomaterials
has a smaller interfacial charge-transfer resistance and lower diffusion
resistances that correspond to a high specific capacitance, compared
to Co(OH)2 [39,40].
To further demonstrate the capacitive performance of the Co(OH)2/
Ni(OH)2 nanomaterials, the aqueous asymmetric supercapacitor (ASC)
has been assembled using the Co(OH)2/Ni(OH)2 nanomaterials as the
positive electrode and AC as the negative electrode. The electrochemical
properties of Co(OH)2/Ni(OH)2 and AC electrodes were tested using CV
at 50 mV s−1 in a three-electrode cell with 1 M KOH electrolyte and SCE
reference electrode, as shown in Fig. 8, which shows the potential windows of the Co(OH)2/Ni(OH)2 nanomaterials and AC electrodes are
complementary. Therefore, the Co(OH)2/Ni(OH)2 nanomaterials and
AC electrodes are good candidates for the ASC device.
Fig. 8. CV of the Co(OH)2/Ni(OH)2 nanomaterials and AC electrodes collected at 50 mV s−1
in a three-electrode system.
L. Chang et al. / Journal of Electroanalytical Chemistry 778 (2016) 110–115
115
Fig. 9. Electrochemical performances of the Co(OH)2/Ni(OH)2//AC ASC: CV at different scan rates (a); galvanostatic charge-discharge curves at different current densities (b).
Fig. 9a shows the CV of the Co(OH)2/Ni(OH)2//AC ASC at various
scan rates of capacitive behavior with the appearances of roughly
rectangular-like shapes. However, there is no obvious distortion in the
CVs even at a high scan rate of 100 mV s−1, indicating good fast
charge-discharge properties of the device [41,42]. As illustrated in
Fig. 9b, the shapes of the charge-discharge curves for the Co(OH)2/
Ni(OH)2//AC ASC at different current densities tend towards
triangular-shaped curves, and the discharge curves are nearly linear
and symmetric with the corresponding charge counterparts, suggesting
a rapid I–V response and good electrochemical reversibility [42,43].
4. Conclusion
Co(OH)2/Ni(OH)2 pseudocapacitive active materials have been successfully prepared on CFP via electrodeposition method with no binder.
Compared with Co(OH)2, Co(OH)2/Ni(OH)2 nanomaterials shows a
high specific capacitance (1498 Fg−1 at 2 Ag−1) and a high cycling stability (92.4% of initial capacitance retains after 20,000 charge-discharge
cycles at 32 Ag− 1). Ni(OH)2 have great influences on the
pseudocapacitive properties of Co(OH)2. Nickel-cobalt double metal hydroxide can improve the surface area and conductivity of composites.
Acknowledgments
Financial support from Science and Technology Development Project of Jilin Province (No. 20140101160JC). This work was also supported by the projects of Jilin Province Department of Education (No.
0520385 and 0520386).
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