Journal of Alloys and Compounds 589 (2014) 364–371
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Heterostructured Ni(OH)2–Co(OH)2 composites on 3D ordered Ni–Co
nanoparticles fabricated on microchannel plates for advanced miniature
supercapacitor
Mai Li a, Shaohui Xu a, Yiping Zhu a, Pingxiong Yang a, Lianwei Wang a,⇑, Paul K. Chu b
a
Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electronic Engineering, East China Normal University, 500 Dongchuan Road, Shanghai
200241, PR China
b
Department of Physics and Material Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, PR China
a r t i c l e
i n f o
Article history:
Received 12 October 2013
Received in revised form 28 November 2013
Accepted 30 November 2013
Available online 7 December 2013
Keywords:
Electrochemical capacitors
Electrodeposition
Ni–Co alloys
Ordered three-dimensional microchannel
plates
Nanoparticles
a b s t r a c t
Silicon microchannel plates (Si-MCPs) coated with a layer of nickel cobalt alloy (NCA) constitute an excellent substrate for miniature supercapacitors. Nanoscale Ni(OH)2–Co(OH)2 composite particles serving as
the active materials are electrodeposited on ordered three-dimensional (3D) NCA/Ni/Si-MCPs and the
Ni(OH)2–Co(OH)2 composites have different structures depending on the amount of Co(OH)2 in Ni(OH)2.
The nickel hydroxide synthesized from a water–acetone 0.1 M Ni(NO3)26H2O solvent has a compact
structure, but that from a 0.1 M Ni(NO3)26H2O solvent containing 5% Co(NO3)26H2O is loosely packed
with nanoparticles and that from a 0.1 M Ni(NO3)26H2O solvent containing 10% Co(NO3)26H2O contains
many nanoparticles. Addition of Co(NO3)26H2O results in a smooth morphology. The smooth structure is
also observed from active materials produced in 20% and 30% Co(NO3)26H2O solvents. Five types of electrode materials are investigated from the perspective of electrochemical capacitors by conducting cyclic
voltammogram, galvanostatic charge–discharge measurements, and electrochemical impedance spectroscopy. In this experiment, the highest specific capacitance of 7.8 F cm2 is achieved in the samples prepared in 10% Co(NO3)26H2O at a discharge current density of 20 mA cm2. It is much better than
1.46 F cm2 observed from previous attempts and the materials have excellent capacity retention.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
With potential depletion of fossil fuels and growing concerns
about air pollution and global warming, there are extensive researches on energy storage and alternative energy sources. Pseudocapacitors or electrochemical supercapacitors (ES) have attracted
considerable attention in recent years because they have a high
power density, long cycle life, and fast charge discharge rate compared to batteries in addition to low maintenance cost [1,2]. These
advantages make ES increasingly important in applications such as
memory backup devices, day night storage, and uninterruptible
power supplies for computers [3–5]. In fact, ES has been considered
a key technology in future energy storage systems [6]. Research on
supercapacitors is presently divided into two categories that are
based primarily on the reaction mechanisms, namely nonfaradic
charge separation at the electrode/electrolyte interface as in electrical double layer capacitors (EDLCs) and pseudocapacitors originating from the fast and reversible redox reactions at/near the
surface of the active materials similar to processes occurring in
⇑ Corresponding author. Tel.: +86 21 54345160; fax: +86 21 54345119.
E-mail address: [email protected] (L. Wang).
0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2013.11.230
batteries [7]. The most common EDLC materials are carbon materials which store energy by charge separation and exhibit a very high
degree of reversibility in repetitive charge–discharge cycling for
over 5,00,000 cycles [7–9]. So far, the specific capacitance of carbon
materials is around 200 F g1 [8,9] which is low due to the limited
accessibility of the carbon surface to electrolyte. Pseudocapacitors
generally show relatively less cycling stability than EDLCs because
of the faradic reaction mechanism. However, the specific capacitance of some transition metal oxides or hydroxide-based pseudocapacitive materials such as RuO2, MnO2, Ni(OH)2, and cobalt nickel
layered double hydroxides [10–16] could be 10–100 times higher
than that of EDLCs [15] thereby significantly enhancing the energy
density of supercapacitors. In particular, layered metal hydroxides
such as Co(OH)2 and Ni(OH)2 have drawn immense attention as
alternative capacitor materials to the state-of-the-art amorphous
RuO2 because of the high theoretical capacitance, unique electrochemical properties, low cost, and environmental friendliness
[12,13,16].
The highest specific capacitance achieved on a-Ni(OH)2–nickel
foam composite is 3152 F g1 at a current density of 4 A g1 [12]
but the composite suffers from a significant capacitance loss of
about 50% of the initial capacitance after 300 cycles. In addition,
M. Li et al. / Journal of Alloys and Compounds 589 (2014) 364–371
the specific capacitance decreases to 280 F g1 at a current density
of 16 A g1. In 2007, Gupta and co-workers fabricated nanostructured CoxNi1x layered double hydroxides as electrode materials
for redox-supercapacitors. The capacitive characteristics of the CoxNi1x LDHs in 1 M KOH electrolyte showed that Co0.72Ni0.28 LDHs
had the highest specific capacitance value, 2104 F g1. While they
do not measured the other important parameters e.g. cycle performance [13]. Recently, Martins synthesis a kind of stabilized aNiCo(OH)2 nanomaterials for high performance device application
from sol–gel nickel/cobalt mixed hydroxide nanoparticle precursors which demonstrate that the content of Co(OH)2 in Ni(OH)2
has great effect on the structure of materials [14]. Therefore, further systematic study the properties of these materials on advanced three-dimensional substrate is extremely important.
Porous three-dimensional (3D) structures act as conductive networks enabling access of ions and electrons to the active surfaces
and produce better electrochemical response on the electrodes.
Obviously, the structure and morphology of the electrode materials
are important from the perspective of the limitations imposed by
ionic and electronic transport. The ideal supercapacitors should
possess high power and energy densities, long cycle life, and high
rate capability. However, most of nanostructured materials have
disadvantages mainly in terms of the intrinsic properties associated with the slow kinetics, poor electrical conductivity, and weak
mechanical stability. Apart from the kinetics issue, the surface
shield is essential for maintaining the capacitance during long cycling because side reactions with electrolytes, structure collapse,
and re-agglomeration into large grains may lead to high irreversibility and finally poor cycle life [16,17]. Accordingly, in order to
produce high power and energy density and make the supercapacitors with a long cycle life, nanoscale engineering is necessary
when fabricating the capacitive materials by taking into account
the morphology, pore size and distribution, redox active sites,
and mass transport pathways. Recent reports and reviews have described electrodes consisting of three-dimensional (3D) interpenetrating structures that can provide a good solution to circumvent
the poor ionic and electronic transport in electrode materials
[18,19].
Here, the advantages has been combined by the Ni–Co alloy
nanostructures, 3D ordered porous structure, and controllable
manner of electrodeposition to fabricate capacitor-type electrode
(as the power source) [19,20]. To achieve higher specific capacitance and structural stability, one promising route is to optimize
the 3D nanoarchitecture and hybridizeation of pseudocapacitive
oxides. Ni–Co nano-alloy modified three-dimensional ordered silicon microchannel plates (Si-MCPs) instead of carbon materials or
grapheme are used as the conductive electrodes in the supercapacitors to improve the surface area of the electrodes and cycling characteristics [21]. The nano-alloy contains the same elements as the
active materials (NixCo1x) so that the active materials can be
grown on the electrode surface. At the same time, standard silicon
technology and a 3D template are adopted to fabricate miniature
supercapacitors having specific capacitance of 855.8 F g1
(7.8 F cm2) at 3 A g1. Owing to the large specific surface area of
the 3D structure, the experimental results are better than those obtained from ordinary electrodes such as nickel foam (NF) [22] or
metal deposited structure [23]. In addition, part of Ni(OH)2 be replaced by Co(OH)2 in order to block the transformation of bNi(OH)2 to c-Ni(OH)2 to improve the stability and cycle characteristics of the supercapacitors [13,24].
2. Experimental details
All chemical reagents were AnalaR (AR) grade and used as received without further purification.
365
2.1. Preparation of current collector consisted of 3D ordered low-resistance Ni–Co alloy
(NCA) nanoparticles
Before putting on the Ni–Co films, a nickel layer was electroless-plated on both
the outer surface and inner side walls of the Si-MCPs (Ni/Si-MCPs) to form the current collector as shown in Fig. 1. The Ni/Si-MCPs current collector was prepared
according to the procedures described in previous studied [1,2]. The Ni/Si-MCPs
were cut into small thin foils with an area of 0.8 0.8 cm2 and put in a buffer solution of Triton X-100 for at least 2 min to increase the hydrophilicity. The Ni–Co film
were electrodeposited on the Ni/Si-MCPs (NCA/Ni/Si-MCPs) in an aqueous electrolyte containing 0.06 M NiCl26H2O, 0.04 M CoCl26H2O, and 0.5 M H3BO3 [21]. The
electrolyte pH value was set to 5.7 by addition of NaOH. Electrodeposition was carried out at room temperature in a conventional three-electrode electrochemical
cell. A platinum (Pt) plate was used as the counter electrode and a saturated calomel electrode (SCE) served as the reference to which all the potentials were referred
to. Simultaneous electrodeposition of cobalt and nickel was carried out by imposing
a direct-wave cathodic current. The distance between the two electrodes was 1 cm
and electrodeposition was performed at a constant current of 0.16 A cm2 for 100 s.
In order to determine the electrical properties of the Ni–Co alloy and exclude the
impact of the metal electrode on the active materials, four other electrodes were
fabricated. The Ni/Si-MCPs were replaced with copper and a Ni–Co layer was electrodeposited on plane copper (NCA/Cu) to study the performance of the two-dimensional (2D) electrode. A porous nano-Ni film was electrodeposited on the Ni/SiMCPs (Nano-Ni/Ni/Si-MCPs) in a standard two-electrode glass cell at 23 ± 1 °C containing an electrolyte of 0.06 M NiCl26H2O and 0.5 M H3BO3 with a pH of 5.7. The
other electrical parameters were the same as those used in the fabrication of the
NCA/Ni/Si-MCPs.
2.2. Fabrication of heterostructured Ni(OH)2–Co(OH)2 composites with different
Co(OH)2 ratios on NCA/Ni/Si-MCPs
The NCA/Ni/Si-MCPs were put into a buffer solution of Triton X-100 for at least
2 min to improve the hydrophilicity. Afterwards, the MCPs were dipped in the electroplating solution for the purpose of producing the nano-flakes. In order to investigate the effects of different Co(OH)2 contents in the Ni(OH)2 capacitors on the
active materials morphology and electrical properties, the electrodes were fabricated from electrodeposition electrolyte contained different Co, Ni ion ratios were
100:0, 100:5, 100:10, 100:20 and 100:30 respectively. For pure Ni(OH)2 electrode
[pure-Ni(OH)2], the electrodeposition electrolyte contained 0.1 M Ni(NO3)26H2O,
for 5% Co(OH)2 and 95 Ni(OH)2 [NiCo-20:1] electrode, the electrodeposition electrolyte contained 0.1 M Ni(NO3)26H2O and 0.005 M Co(NO3)26H2O, [NiCo-10:1] 0.1 M
Ni(NO3)26H2O and 0.01 M Co(NO3)26H2O, [NiCo-10:2] 0.1 M Ni(NO3)26H2O and
0.02 M Co(NO3)26H2O, as well as [NiCo-10:3] 0.1 M Ni(NO3)26H2O and 0.03 M
Co(NO3)26H2O. All of the electrodes were fabricated in the electrolyte consisted
1:1 volume ratio water–acetone [25]. The current density was 50 mA cm2 and
the temperature was 23 ± 1 °C. After plating for 6 min, the active materials were
washed with de-ionized water several times. After drying at 60 ± 1 °C, copper wires
were connected to a copper sheet by tin solder and the copper sheet was glued onto
the MCPs by conductive silver paste (DAD-40). The size of the electrode was about
0.5 cm2.
2.3. Characterization
The morphology and microstructure of the nickel and nickel/cobalt hydroxide
thin films were examined by field-emission scanning electron microscopy (FESEM, Hitachi S-4800, Japan) and the crystal structure was detected by X-ray diffraction (XRD, Rigaku, RINT2000, Japan). Electrochemical measurements were performed on a three-electrode electrochemical working station (Shanghai Chenhua
CHI660D) with a saturated calomel electrode and platinum gauze electrode serving
as the reference electrode and counter electrode, respectively. All the measurements were performed at room temperature in a 6 M KOH aqueous electrolyte. In
order to determine the electrochemical properties and specific capacitance of the
electrode samples, CV scans were acquired from 0.4 to 0.6 V (vs. SCE) from both
samples at different scanning rates. Charge–discharge cycle tests were conducted
in the potential range between 0.1 and 0.4 V at different constant current densities.
Electrochemical impedance spectroscopy was performed at the open circuit potential in the frequency range from 10,000 to 0.01 Hz with an excitation signal of 5 mV.
3. Results and discussion
3.1. Characterizations of Ni–Co alloys nanoparticles current collector
This nano-structured thin metal layer which covers the Ni/SiMCPs uniformly plays a crucial role in the performance of the
capacitor. Four samples were studied to determine and understand
the characteristiscs, namely electroless-plated nickel on Si-MCPs
(Ni/Si-MCPs), electrodeposited Ni–Co film on nickel plate (NCA/
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M. Li et al. / Journal of Alloys and Compounds 589 (2014) 364–371
Fig. 1. (a) SEM images acquired from the ordered and large-area-ratio Si-MCPs; (b) electroless-plated nickel on Si-MCPs and the magnified image; (c) electrodeposited Ni film
on Ni/Si-MCPs; (d) cross-sectional SEM morphology of the Nano-Ni/Ni/Si-MCPs; (e) electrodeposited Ni–Co nanoparticle film on Ni/Si-MCPs and the magnified image; (f)
cross-sectional SEM morphology of the NCA/Ni/Si-MCPs; (g) electrodeposited Ni–Co alloy on Ni; and (h) EDS spectrum of the NCA/Ni/Si-MCPs composites structure.
Ni), electrodeposited Ni film on Ni/Si-MCPs (Nano-Ni/Ni/Si-MCPs),
and electrodeposited Ni–Co film on Ni/Si-MCPs (NCA/Ni/Si-MCPs).
The surface morphology of the Si-MCPs and Ni/Si-MCPs before
and after electroless plating is characterized by SEM. As shown in
Fig. 1(a), before electroless plating, the microchannels have a depth
of about 250 lm and size of 5 5 lm yielding an aspect ratio of
about 50. It has a sandwich structure in which the parallel structure becomes a porous lamellar one. Electroless deposition is employed to deposit Ni on the Si-MCPs and Fig. 1(b) depicts the Ni/
Si-MCPs structure after electroless deposition for 30 min revealing
that nickel particles cover the sidewall of the Si-MCPs. The magnified image shows that the nickel layer has a smooth surface which
is not conducive to the growth active substances. Hence, the surface area is increased and the resistance is reduced by electrically
depositing a metal layer on the Ni/Si-MCPs to improve the performance. Fig. 1(d) and (e) shows the morphology of the electroplated
nickel layer on the surface and sidewall. Although the thick Ni
layer has a small resistance of less than 1.5 X, the microstructure
in the nickel increases the surface area of the MCPs. However,
the distribution of the nickel is not uniform particularly on the
sidewall of the microchannel. Hence, the formulation of the plating
solution is changed to prepare the Ni–Co alloy as the modified
layer, as shown in Fig. 1(e) and (f) [21]. Owing to the different sizes
of the nickel particles, a nickel layer with many bumps and pits is
formed and the bumpy surface provides nucleation centers to facilitate deposition of the composites. As shown in the cross-sectional
SEM morphology of the NCA/Ni/Si-MCPs in Fig. 1(f), the alloy is
deposited uniformly onto the sidewall of the microchannel which
has many nanoparticles thereby boding well for further deposition
of active substances on the inner-sidewall of the NCA/Ni/Si-MCPs.
Compared to the nickel plate or nickel thin films (Fig. 1(g)) from
which the active substance can be easily delaminated, the NCA/
Ni/Si-MCPs constitute an ideal substrate to fabricate the active substance firmly. The EDS spectrum in Fig. 1(h) reveals the presence of
Co and Ni in the composites which form the Ni–Co alloy.
Cyclic voltammogram (CV) and chronopotentiometry measurements are conducted to evaluate the specific capacitance and electrochemical properties of the current collector. The total active
mass of the Si-MCPs structure with an area of 0.5 cm2 is around
4 mg as determined by a microbalance with a sensitivity of
0.001 mg. The amounts of metal on Nano-Ni/Ni/Si-MCPs, NCA/Ni/
Si-MCPs, and NCA/Ni are 1.247 mg, 1.372 mg, and 1.154 mg, respectively. Although the quality of the active substance is about the
same, the shape of the CV curves (Fig. 2(a)) and charge and dis-
charge curve (Fig. 2(b)) (charge current 10 mA – discharge current
10 mA) of the samples is different. The enclosed area NCA/Ni/SiMCPs in the CV curve is larger than those of other samples and consistent with the long discharge time. It is speculated that the larger
capacitance of NCA/Ni/Si-MCPs stems from the Faraday capacitance
which is related to the surface of the materials in contact with the
solution [39]. The indirect evidence reveals the large surface area
on the Ni–Co alloy and so subsequent experiments are based on
the NCA/Ni/Si-MCPs electrode.
3.2. Structure characterization of the hybrid nanostructured Ni(OH)2–
Co(OH)2 composite films
The structure of the nano-flaked composites film is detected by
X-ray diffraction and the (XRD) patterns are displayed in Fig. 3. Because of stray signals from other materials, the peaks from the SiMCPs with large intensity are not shown completely here. The XRD
spectrum of the Ni(OH)2/NCA/Ni/Si-MCPs without the deposited
Co(OH)2 is shown and compared to NiCo-20:1, NiCo-10:1, NiCo10:2, and NiCo-10:3, this sample has more diffraction peaks, suggesting successful deposition of Ni(OH)2–Co(OH)2 composites on
the substrate. The XRD pattern of pure-Ni(OH)2 exhibits the characteristics of the a-Ni(OH)2 at 35.86°, 39.35°, 41.78° and 59.64°
according to JCPDS card no. 38-0715 [26].
With the amount of Co(OH)2 is increasing, some of the weak aNi(OH)2 peaks disappear, suggesting that the peaks of a-Ni(OH)2
are influenced by the peaks of Co(OH)2. From the XRD of Co(OH)2,
the diffraction peak of 2h values at 34.1° as well as 60.7°, are characteristic ones belonging to the a-Co(OH)2 phase (PDF, card no. 460605) which is the prominent one [18]. The Ni(OH)2–Co(OH)2 composites consist of Ni(OH)2 and Co(OH)2 phases, but pure-Ni(OH)2
only shows the phase of a-Ni(OH)2. The Ni(OH)2–Co(OH)2 composite with a high density leads to excellent electrochemical performance. The XRD diffraction patterns of NiCo-20:1, NiCo-10:1,
NiCo-10:2, and NiCo-10:3 correspond to both Co(OH)2 and
Ni(OH)2. It is difficult to differentiate between the two phases since
they have similar structures and their diffraction peaks are very
close. Nonetheless, it is observed that the (1 0 0) and (1 1 0) peaks
from the mixed materials broaden thus showing combined effects
of Ni(OH)2 and Co(OH)2 [13].
Fig. 4 shows the morphology of the heterostructured Ni(OH)2–
Co(OH)2 composite thin film on the five samples revealed by
field-emission scanning electron microscopy (FE-SEM). As shown
in Fig. 4(a), the sandwich-like MCPs with a large surface area pro-
M. Li et al. / Journal of Alloys and Compounds 589 (2014) 364–371
367
Fig. 2. (a) CV curves of the four samples at a sweeping rate of 80 mV s1 in 6 M KOH solution and (b) discharge curves of the four samples at a discharge current density of
20 mA cm2 (5 A g1).
Fig. 3. XRD patterns of pure-Ni(OH)2, NiCo-20:1, NiCo-10:1, NiCo-10:2 and NiCo10:3.
vides more nucleation centers and also good support with high
conductivity to decrease the contact resistance between the active
materials. After electrochemical deposition, a smooth and compact
Ni(OH)2 layer more than 300 nm thick is formed and the cracks between active materials resulting from deposition can be observed
in the magnified image in Fig. 4(a). Ni(OH)2 is deposited uniformly
onto the surface and sidewall of the microchannel to enhance the
performance. However, the capacity of pure Ni(OH)2 is smaller
than NiCo-20:1 and NiCo-10:1, even though Ni(OH)2 has a relatively large theoretical value. This is probably due to the smooth
surface which prevents the electrolyte from full contact with the
sample [39].
Fig. 4(b) shows that NiCo-20:1 has a complex microstructure on
the nanometer scale and the network-like structure including
interconnected small nanoparticles exhibits an anisotropic morphology. Fig. 4(c) shows NiCo-10:1 has the same structure but
more nanoparticles on the surface. The unique structure plays a
key role in the electrochemical accessibility of the electrolyte
OH to the active materials and fast diffusion in the redox phase
[40]. It is also believed that this unique structure provides the
important morphological foundation for the extraordinary high
specific capacitance [27]. Fig. 4(b) and (c) discloses the nanoparticle structure. As shown in the cross-sectional SEM morphology of
NiCo-20:1 in Fig. 4(f) and NiCo-10:1 in Fig. 4(g), there are pores
consisted of nanoparticles on the sidewall. After formation of the
nano-rods, they may also be the nucleation centers for growth of
the Ni(OH)2–Co(OH)2 composite pores. It is different from the
nanoparticle structure on the surface. The alloy-coated Si-MCPs
are covered evenly by nanoparticles by electrochemical deposition.
Fig. 4(h) depicts the magnified image of the micro-porous NiCo10:1 and the nanoparticles are around 500 nm in size.
As shown in Fig. 4(d) and (e), on the surface of the Ni–Co film,
there is a dense layer with a smooth surface which has a different
Fig. 4. SEM images of the heterostructures: (a) top view of pure-Ni(OH)2; (b) top view of NiCo-20:1; (c) top view of NiCo-10:1; (d) top view of NiCo-10:2; (e) top view of
NiCo-10:3; (f) cross-sectional view of NiCo-20:1; (g) cross-sectional view of NiCo-10:1; and (h) magnified image of a single micro-porous of NiCo-10:1.
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M. Li et al. / Journal of Alloys and Compounds 589 (2014) 364–371
structure compare with those on NiCo-20:1 and NiCo-10:1. There
are fewer nanoparticles on the surface and the specific surface
areas on NiCo-10:2 and NiCo-10:3 diminish. The morphology is
different from that of the other active materials produced on the
MCPs. The Ni–Co particles on the sidewall of the MCPs provide
nucleation impurities to gather the active materials. The nanostructured materials play important roles in the supercapacitance
while also providing the unique nickel modified template with a
large surface area in a small footprint. The morphology of the sample depends largely on the solvent of the precursor.
3.3. Electrochemical characterization
Typical CV curves obtained at various scan rates of this batch of
samples are displayed in Fig. 5. As the scanning rate increases from
2 to 120 mV s1 [28], the peak current becomes larger and the oxidation and reduction peaks are quite apparent in the CV curves and
the peak shape is similar. However, the peak potential shifts to the
anodic and cathodic directions, respectively, because of an increasing involvement of polarization at high scanning rates [29]. The CV
of the double-layered capacitance normally approaches the ideal
rectangular shape, but the CV obtained from the composite pseu-
do-capacitance is quite different. As shown in Fig. 5, as the scanning rate is increased, the peak currents are proportional to the
square root of the scanning rates, which implies that the electrodes
have good electrochemical performance.
In the reverse scanning direction, the current is almost instantaneous suggesting that in the CV curve, there is a small angle
along the horizontal axis, indicating that the electrodes have smaller impedance. Furthermore, it is apparent that NiCo-10:1 shown
in Fig. 4(c) has a larger area under the same current–potential conditions compared to other samples, suggesting that NiCo-10:1 has
a relative larger specific capacitance (SC) and capacitive behavior.
The nanoparticles in NiCo-10:1 provide the reaction sites based
on the complex nanostructured and network structure. More redox
reactions are expected on NiCo-10:1 because of the larger specific
surface area, more reaction sites, and better electrical conductance
than other samples.
The Ni(OH)2 and CoxNi1x electrodes show very strong redox
peaks due to the following Faradaic reactions of Co(OH)2 and
Ni(OH)2 [30]:
CoðOHÞ2 þ OH () CoOOH þ H2 O þ e ;
ð1Þ
CoOOH þ OH () CoO2 þ H2 O þ e ; and
ð2Þ
Fig. 5. Cyclic voltammetry curves of the Ni(OH)2–Co(OH)2 composites electrode with different Ni–Co ratios at different scanning rates: (a) pure-Ni(OH)2; (b) NiCo-20:1; (c)
NiCo-10:1; (d) NiCo-10:2; (e) NiCo-10:3; and (f) variation of the specific and interfacial capacitances of the Ni(OH)2–Co(OH)2 composite electrodes at different scanning rates.
The concentration of KOH in the electrolyte is 6.0 M.
M. Li et al. / Journal of Alloys and Compounds 589 (2014) 364–371
NiðOHÞ2 þ OH () NiOOH þ H2 O þ e :
ð3Þ
The CV curves show shifts in the redox peaks as the compositions of CoxNi1x are varied, as shown in Fig. 5. The oxidation and
reduction peaks of the pure-Ni(OH)2 are 0.4 V and 0.15 V at a scanning rate 2 mV s1, respectively, whereas for NiCo-10:1, the peaks
are at 0.26 V and 0.1 V, respectively. The two pairs of visible redox
peaks in the CV curves in Fig. 5 confirm the reactions shown, suggesting that instead of a pure electrical double-layer capacitance,
the measured pseudo-capacitance is dominated by a redox mechanism. It should be emphasized that the anodic peak potential, CV
change, and cathodic peak potential shift in the anodic and cathodic directions with increasing sweeping rate and the capacitance
decreases. The observation is consistent with that from chronopotentiometry. By comparing the CV curves, the oxidation peaks of
the samples at sweeping rates of 120 mV s1 and 60 mV s1 are
not obvious probably because the compact structure does not fully
react in the 6 M KOH solution and has a low reaction rate.
The specific capacitance can be calculated from the CV curves
using Eq. (4) [31]:
Cf ¼
Z
i dt ðA DVÞ1 ;
ð4Þ
where Cf is the electrode specific capacitance, i is the instantaneous
current, A is the footprint area of the entire electrode, and DV is potential voltage window. The specific capacitance can be calculated
from the CV curves. The change in the capacitance with scanning
rates is illustrated in Fig. 5(f) which shows that NiCo-10:1 has the
best capacitance characteristics which are consistent with chronopotentiometry. The CV curve shows the pseudocapacitive behavior
with the capacitance obtained for 10% Co(OH)2 in Ni(OH)2 being
slightly higher than the previously obtained value by the same
method (Fig. 5f). The effects of this phenomenon on the capacitance
and the results obtained by electrochemical impedance spectroscopy (EIS) will be described in subsequent sections in this paper.
Fig. 6 displays the discharge curves of the composite electrode
in 6 M KOH at 20 mA cm2 charge–discharge current density in
the potential range between 0.1 and 0.4 V from the five samples.
The shape of the charge–discharge curves shows mainly pseudocapacitance instead of pure double-layer capacitance and the results are consistent with the CV data. Chronopotentiometry is a
recommended method to determine the capacitance of supercapacitors according to Eq. (5) [32]:
C f ¼ ðI DtÞðA DEÞ1 ;
ð5Þ
where I is the constant discharge current, Dt is the discharge time,
DE is the potential drop during the discharge process, and A is the
area of the working electrode immersed in KOH.
369
According to the chronopotentiometry of this batch of samples,
the SC values of pure-Ni(OH)2 is 4.748 F cm2, NiCo-20:1 is
6.16 F cm2, NiCo-10:1 is 7.8 F cm2, NiCo-10:2 is 4.724 F cm2, and
NiCo-10:3 is 4.084 F cm2 for a charging-discharging current density of 20 mA cm2. In our experiments, the capacity of the capacitor changes with the Co(OH)2 content in Ni(OH)2 and 10% Co(OH)2
has the best capacitance of 7.8 F cm2 at a discharge current of
20 mA cm2. A small amount of Co(OH)2 increases the capacitance
but excessively high Co(OH)2 may destroy the capacity of the
capacitor. By co-depositing a certain amount of Co(OH)2 in Ni(OH)2
and activating in KOH, Co(OH)2 is converted into b-Co(OH)2 though
the dissolution–deposition process subsequently depositing on the
Ni(OH)2 particle surface. b-Co(OH)2 is transformed into b-CoOOH
which favors the subsequent charging process. b-CoOOH which
has good electron conductivity increases the depth of discharge
on the electrode as well as the active materials utilization and discharge potential [33].
The total weight of the composite/Ni–Co/Ni/Si-MCPs structure
with an area of 0.5 cm2 is around 4 mg, whereas those of pureNi(OH)2, NiCo-20:1, NiCo-10:1, NiCo-10:2, and NiCo-10:3 are
3.124, 4.152, 4.557, 3.924, and 3.752 mg, respectively. Since the
weight of the nickel coated Si-MCPs is fixed and the electrodeposition time is also the same, the small mass difference can be attributed to the morphology difference. The capacitance values of pureNi(OH)2, NiCo-20:1, NiCo-10:1, NiCo-10:2, and NiCo-10:3 are
759.92 F g1, 741.81 F g1, 782.83 F g1, 601.94 F g1, and
544.24 F g1, respectively at a discharge current density of
20 mA cm2. The converted capacitance per unit mass at low rates
is close to the commonly reported results for Ni(OH)2 as supercapacitors [12–16]. Even the smallest capacitance of NiCo-10:3 obtained at a discharge current density of 10 mA cm2 is about
544.24 F g1 and it is comparable to that of many other pure electrical double layer electrochemical capacitors [34–36].
Since our three dimensional structure electrode is based on the
silicon process, which makes the calibration of the electrode much
more complex than plate structure based on metal. In this content,
the cycle performance of our capacitors were obtained as accurate
as possible. In order to demonstrate the electrochemical stability of
the nano-flaked Co(OH)2 electrode materials, the CV characteristics
are measured in 6 M KOH at a sweeping rate of 120 mV s1. As
shown in Fig. 7, the specific capacitance calculated from Eq. (5) decreases with cycle numbers. In the first 2500 cycles, the calculated
capacitance losses of pure-Ni(OH)2, NiCo-20:1, NiCo-10:1, NiCo10:2, NiCo-10:3 are 5.4%, 3.14%, 5.36%, 1.3%, and 7.5% respectively,
whereas in the last 2500 cycles, they are 6.4%, 3.3%, 8.4%, 7.6%, and
7.7%, respectively, thereby demonstrating good stability in long
charging-discharging cycles. It has been proposed that a smaller
surface area and degradation of the active materials are responsible for the capacitance loss in long charging–discharging [37,38].
The samples after 5000 cycles are carefully weighed and no obvious weight loss is found. The capacitance reduction may originate
from slow oxidation of a-Ni(OH)2 to b-Ni(OH)2 and c-Ni(OH)2 because a-Ni(OH)2 is more stable than b-Ni(OH)2 and c-Ni(OH)2 under alkaline conditions. As shown in Fig. 7, NiCo-10:1 is less stable
than NiCo-20:1 possibly because NiCo-10:1 has more nanoparticles with more active sites which can detach from the substrate
causing larger consumption in the electrochemical reaction. The
electrode with 5% Co(OH)2 has good long-term electrochemical
stability and after repetitive charging it discharging and does not
degrade significantly.
3.4. Evaluation of the overall capacitive performance of hybrid
Ni(OH)2–Co(OH)2 composites structure
Fig. 6. First charge (20 mA cm2)–discharge (20 mA cm2) curves: (a) NiCo-10:1;
(b) NiCo-20:1; (c) pure-Ni(OH)2; (d) NiCo-10:2; and (e) NiCo-10:3.
Another important aspect of a supercapacitor electrode is the
impedance spectra. The measurements are carried out on the com-
370
M. Li et al. / Journal of Alloys and Compounds 589 (2014) 364–371
Table 1
Fitted results of important parameters in the equivalent circuit.
*
Fig. 7. Long cycling performance of the composite Ni–Co/Ni/Si-MCPs at a sweeping
rate of 120 mV s1.
posite electrodes with an excitation signal of 5 mV and the representative results are shown in the dotted line in Fig. 8 where Z0 and
Z00 are the real and imaginary parts of the impedance, respectively.
Since our electrode is a pseudocapacitance structure, the equivalent circuit inset in the upper right corner of Fig. 8 is selected to
fit the impedance spectra by complex nonlinear least square
(CNLS) fitting. The results are shown in the solid line in Fig. 8
[39,40]. The impedance spectra can be fitted well and the parameters are shown in Table 1. In the equivalent circuit, a constant
phase element (CPE) component is introduced [40] and CPE1 and
CPE2 represent the double-layer and Faradaic capacitance that varies with frequencies, respectively. This modification from a pure
capacitance behavior can be explained by distribution effects
[42] and porosity [43] in the samples. Owing to the influence of
the 3D structure of MCPs on mass transport, CPE3 represents the
Warburg impedance.
The complex-plane impedance plots of each sample consist of a
high-frequency component and low-frequency component from
10,000 Hz to 0.01 Hz. The impedance behavior in the high frequency region is characterized of the oxide–electrolyte interface
due to discontinuity in the charge transfer process at the solid
oxide/liquid electrolyte interface. This is a result of the difference
in the conductivity between the solid oxide (electronic conductivity) and aqueous electrolyte phase (ionic conductivity). The impedance behavior in this region also involves resistance from the
Faradaic redox processes associated with the surface phenomena
of the porous composites electrode. Specifically, in the high frequency range and at the point intersecting the real axis, the internal resistance values (which is equal to R1) of the composites
Samples
R1 (X)
R2 (X)
R3 (X)
CPE1n*
CPE2n
CPE3n
Pure-Ni(OH)2
NiCo-20:1
NiCo-10:1
NiCo-10:2
NiCo-10:3
1.253
1.300
1.375
1.125
1.275
2.026
1.643
0.507
0.630
0.797
8.535
3.468
1.055
1.158
2.051
0.185
0.412
1.042
0.322
0.205
0.724
0.602
0.917
0.876
0.312
0.852
0.817
0.899
0.688
0.576
CPE1n represents the exponential parameter of constant phase element.
electrodes are 1.253 X, 1.300 X, 1.375 X, 1.125 X, and 1.275 X
for pure-Ni(OH)2, NiCo-20:1, NiCo-10:1, NiCo-10:2, and NiCo10:3 respectively. R1 of those samples are similar and NiCo-10:1
is a little larger. It can be explained by that as the solvent is changed, the compact nano-flakes reduce the diffusivity of the electrolyte ions in the pores.
A Faradic charge transfer resistance, R2, representing deposition
or desorption of an electroactive species is parallel to the doublelayer capacitance CPE1. The Faradic charge-transfer resistance
(R2) corresponds to the semicircle in the high-frequency range related to the surface properties of the electrode. Table 1 shows that
NiCo-10:1 has a R2 value of 0.507 X which is much smaller than
the R2 values of other samples. As the double-layer capacitance
CPE1, we extract CPE1n as the exponential parameter of CPE1 to assess the capacity of the double layer capacitance [41]. According to
Table 1, the CPE1n values of the composite electrodes are 0.185,
0.412, 1.042, 0.322, and 0.205 for pure-Ni(OH)2, NiCo-20:1, NiCo10:1, NiCo-10:2 and NiCo-10:3 respectively. The rich nanoparticle
structure of NiCo-10:1 has a larger double layer capacitance which
may originate from the porous structure of the sample that can
fully contact the solution [44].
The capacitance is a pseudo capacitance and so R3 and CPE2 are
key performance indicators. The Faradic or chemical resistance, R3,
in the circuit corresponds to the reciprocal of the potential-dependent rate of the process. It enables CPE2 to be omitted or combined
to form an overall product of the reaction [39]. That is to say, R3 reflects the difficulty of the capacitor to carry out chemical reaction
and CPE2n reflects the pseudo-capacitance. According to Table 1,
the R3 values of the composite electrodes are 8.535 X, 3.468 X,
1.055 X, 1.158 X, and 2.051 X for pure-Ni(OH)2, NiCo-20:1,
NiCo-10:1, NiCo-10:2, and NiCo-10:3, respectively, suggesting that
sample (3) has more active materials and can easily react with the
electrolyte. Careful comparison of the index of CPE2n shows that
sample (3) has a CPE2n of 0.917 that is larger than those of
pure-Ni(OH)2 [0.724], NiCo-20:1 [0.602], NiCo-10:2 [0.876] and
NiCo-10:3 [0.312]. It is consistent with the results obtained from
other tests.
On the other hand, the linear parts of the impedance plots at
lower frequencies correspond to the interfacial diffusive resistance.
The process designated as CPE3 in the circuit in Fig. 6(d) describes
the diffusive resistance of OH ions in the composite electrode
pores [40]. As the solvent is changed, there is a gradual change in
the linearity, especially NiCo-10:1. The Nyquist plot manifests as
nearly a vertical line along the imaginary axis. Furthermore, the
slope of the impedance plots of NiCo-10:1 is 0.899 which is larger
than those of pure-Ni(OH)2 [0.852], NiCo-20:1 [0.817], NiCo-10:2
[0.688] and NiCo-10:3 [0.576] at low frequencies. This indicates
that the special complex microstructure with flakes and particles
in NiCo-10:1 enables faster ion diffusion through the channel of
the MCPs and increases the slope [41,44,45].
4. Conclusions
Fig. 8. Nyquist plots of hybrid Ni(OH)2–Co(OH)2 composite electrodes in 6 M KOH
solution of pure-Ni(OH)2, NiCo-20:1, NiCo-10:1, NiCo-10:2, and NiCo-10:3. The
equivalent circuit is used to fit the Nyquist plots.
The effects and mechanism of improved nickel hydroxide
coated NCA/Ni/Si-MCPs fabricated by electrochemical deposition
are investigated. The structure with a nanoparticle Ni(OH)2–
M. Li et al. / Journal of Alloys and Compounds 589 (2014) 364–371
Co(OH)2 composite film exhibits much higher specific capacitance
than the relatively compact composite films. The enhancement can
be attributed to the regular nano-structure consisting of mesopores and faster ion diffusion in the pores. In this experiment, the
samples prepared in 10% Co(NO3)26H2O, the highest specific
capacitance of 7.8 F cm2 is attained at a discharge current density
of 20 mA cm2 and it has the good electrochemical stability up to
5000 cycles. The materials can be upscaled to the mass production
of miniature supercapacitors.
Acknowledgements
This work was jointly supported by Shanghai Natural Sciences
Foundation No. 11ZR1411000, Shanghai Fundamental Key Project
Under Contract Numbers of 11JC1403700 and 10JC1404600,
PCSIRT, and China NSFC Grant Nos. 61176108, 60990312 and
61076060. The work was also supported by Hong Kong Research
Grants Council (RGC) and General Research Funds (GRF) Nos. CityU
112510 and 112212.
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