One-step synthesis of copper compounds on copper foil and their supercapacitive performance

Article abstract of DOI:10.1039/c5ra04889c

Nanowire-like Cu(OH)₂ arrays, microflower-like CuO standing on Cu(OH)₂ nanowires and hierarchical CuO microflowers are directly synthesized via a simple and cost-effective liquid-solid reaction. The specific capacitance of Cu(OH)₂, CuO/Cu(OH)₂ and CuO are 511.5, 78.44 and 30.36 F g^-1, respectively, at a current density of 5 mA cm^-2. Therefore, the Cu(OH)₂/Cu-foil electrode displays the best supercapacitive performance. The capacitance retention reaches up to 83% after 5000 charge/discharge cycles with the columbic efficiency of ～98%. More importantly, the nanowire Cu(OH)₂ transformed into stable nanosheet CuO after about 600 constant current charge-discharge cycles. Additionally, we fabricate an asymmetric supercapacitor with nanowire Cu(OH)₂/Cu-foil as a positive electrode, activated carbon (AC) as a negative electrode and 6 mol dm^-3 KOH as electrolyte, which exhibits an energy density of 18.3 W h kg^-1 at a power density of 326 W kg^-1.

Full text of DOI:10.1039/c5ra04889c

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One-step synthesis of copper compounds on
copper foil and their supercapacitive performance
Cite this: RSC Adv., 2015, 5, 36656
*
Panpan Xu, Ke Ye, Mengmeng Du, Jijun Liu, Kui Cheng, Jinling Yin, Guiling Wang
*
and Dianxue Cao
Nanowire-like Cu(OH)2 arrays, microflower-like CuO standing on Cu(OH)2 nanowires and hierarchical CuO
microflowers are directly synthesized via a simple and cost-effective liquid–solid reaction. The specific
capacitance of Cu(OH)2, CuO/Cu(OH)2 and CuO are 511.5, 78.44 and 30.36 F gꢀ1, respectively, at a
current density of
5 . Therefore, the Cu(OH)2/Cu-foil electrode displays the best
mA cmꢀ2
supercapacitive performance. The capacitance retention reaches up to 83% after 5000 charge/discharge
cycles with the columbic efficiency of ꢁ98%. More importantly, the nanowire Cu(OH)2 transformed into
stable nanosheet CuO after about 600 constant current charge–discharge cycles. Additionally, we
fabricate an asymmetric supercapacitor with nanowire Cu(OH)2/Cu-foil as a positive electrode, activated
carbon (AC) as a negative electrode and 6 mol dmꢀ3 KOH as electrolyte, which exhibits an energy
Received 19th March 2015
Accepted 15th April 2015
DOI: 10.1039/c5ra04889c
www.rsc.org/advances
density of 18.3 W h kgꢀ1 at a power density of 326 W kgꢀ1
.
reported to be as high as to 1340 F gꢀ1, but RuO2 is very
expensive.11,12 Therefore, cost-effective transition metal oxides
or hydroxides, such as Ni(OH)2,13,14 Co(OH)2,15,16 MnOx,17,18
CuO,19–21 Fe2O3,22–24 V2O5,25,26 and SnO2,27,28 has been widely
investigated. Among the possible materials for pseudocapaci-
tors, cupric oxides have attracted signicant attention in recent
years due to their promising high specic capacitance, envi-
ronmentally friendly nature, and the low cost. Therefore, many
research groups have been considering cupric oxides or
hydroxides as a supercapacitor electrode recently, a brief
summary of which is in Table 1.21,29–32 To our knowledge, studies
on the supercapacitive behavior of Cu(OH)2 is rear and only one
1. Introduction
With the rapid development of the global economy, the
exhaustion of fossil fuels and increasing environmental pollu-
tion, there is an urgent need for efficient, clean, and sustainable
sources of energy, as well as new technologies related to energy
conversion and storage.1–3 In this context, supercapacitors
comprising a characteristic combination of high power and
reasonable energy density with faster response time and near-
innite life cycle can complement other energy storage
devices like conventional capacitors, batteries and fuel cells.4,5
According to the charge storage mechanism, supercapacitors
are classied into two types: (i) electrochemical double layer
capacitors (EDLCs) which stores the energy non-Faradically by
the accumulation of the charges at the electrode–electrolyte
interface and (ii) redox capacitors which stores the energy Far-
adically by battery-type oxidation reduction reactions leading to
the pseudocapacitive behavior.6 The EDLCs usually use carbon-
based materials (activated carbons, carbon nanotubes, tem-
plated carbons, carbide-derived carbons, carbon bers, carbon
aerogels and graphene).7–10 Carbon materials have limited
specic due to their double-layer charge storage mechanism. To
improve the specic capacitance of supercapacitors, lots of
researches have been dedicated to the investigation of metal
oxides or hydroxides pseudocapacitive materials. One prom-
ising candidate is RuO2, which has a specic capacitance
report showed that Cu(OH)2 displayed a specic capacitance of
120 F gꢀ1
.
33
Recent years have witnessed great progress in the design of
additive/binder-free electrode architectures to avoid the “dead
surface” in traditional slurry-derived electrode and allow for
more efficient charge and mass exchange. Different types of
conductive substrates (e.g., Ni foam, Ti foil, stainless-steel foil,
and exible graphite paper) are used as current collector for
electrochemical supercapacitor.34–37 Nanostructured electro-
active materials prepared by in situ oxidation of copper foil
would not only improve the charge transfer but also reduce the
internal resistance and maintain a rapid redox reaction.
Herein, nanowire-like Cu(OH)2 arrays, microowers-like
CuO standing on Cu(OH)2 nanowire and hierarchical CuO
microowers were directly grown onto copper foil via surface
oxidation in alkaline solution to obtain a conducting additive-
free and binderless electrode with high utilization of the
active material. The supercapacitive performance of the
obtained electrode were evaluated. This nanowire structured
Key Laboratory of Superlight Materials and Surface Technology of Ministry of
Education, College of Materials Science and Chemical Engineering, Harbin
Engineering University, Harbin, 150001, P.R. China. E-mail: yeke@hrbeu.edu.cn;
caodianxue@hrbeu.edu.cn
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Table 1 Summery of electrochemical data of CuO and Cu(OH)2 based electrodes
Morphology
Current density or scan rate
Cs (F gꢀ1
)
Stability
Ref.
Nanosheet
Nanosheet
Nanoower
Nanoower
5 mA cmꢀ2
0.41 A gꢀ1
10 mA cmꢀ2
0.41 A gꢀ1
2 mA cmꢀ2
2 mA cmꢀ2
2 mA cmꢀ2
0.41 A gꢀ1
2 mVꢀs1ꢀ1
569
212
134
159
190
278
162
102
88.5
340
130
137
346
120
750
17.5% loss (5000 cycles)
15% loss (800 cycles)
5.2% loss (200 cycles)
5% loss (850 cycles)
33% loss (2000 cycles)
15% loss (800 cycles)
19% loss (2000 cycles)
15% loss (850 cycles)
9.4% loss (500 cycles)
15% loss (2000 cycles)
30% loss (7000 cycles)
12% loss (500 cycles)
18% loss (5000 cycles)
Not available
21
20
43
20
40
29
44
20
45
46
47
48
33
36
Nanoake
Lotus-like CuO/Cu(OH)2
Nanostructure
Cautiower
Nanowire
Nanosheet
Nanobelt
Nanoower
Nanoribbon
Nanograin (Cu(OH)2)
Nanowire (Cu(OH)2)
2 A g
1 A gꢀ1
3 mA cmꢀ2
5 mV sꢀ1
10 mV sꢀ1
2 mA cmꢀ2
17% loss (5000 cycles)
This work
Cu(OH)2 with 6 M KOH electrolyte shows the best super- diffractometer (XRD, Rigaku TTR III) with Ka radiation (l ¼
capacitive performance. Since the cross wire-like architecture 0.1506 nm) with a scan rate of 10ꢃ minꢀ1 at a step width of 0.02ꢃ.
provides good electrical conductivity, effective electrolyte-
accessible channels for ion transportation. Our results
demonstrate that the present cost-effective Cu(OH)2 nanowires
The AC negative electrode was prepared by mixing 80 wt% of
could be a choice for supercapacitor electrode materials char-
acterized by their high specic capacitance and related prop-
erties. Moreover, an asymmetric supercapacitor was obtained by
using the Cu(OH)2/Cu-foil and activated carbon (AC) as the
positive and negative electrodes, respectively.
2.2. Preparation of AC electrode
activated carbon (2000 ꢄ 100 m2 gꢀ1), 10 wt% acetylene black and
10 wt% polyvinylidene diuoride (PVDF) binder in N-methyl-2-
pyrrolidone (NMP). The obtained thick paste was coated onto 1.0
cm ꢂ 1.0 cm carbon ber cloths (CFC) and dried in 100 ꢃC vacuum
oven. Prior to use CFC, the CFC was degreased with acetone for 15
min, rinsed again with water extensively, and dried in air.
2. Experimental
2.3. Electrochemical measurements
2.1. Preparation and characterization of positive electrode
The electrochemical tests were carried in a conventional three-
electrode electrochemical cell using computerized
a
All the chemicals are of analytical grade and were used without
further purication. The schematic illustration of the experi-
mental setup is shown in Fig. 1a, which is conrmed by the extra
experiments below. A fresh Cu foil (0.15 mm, 99.95%, Tianjin
Hengxing Chemical Preparation Co., Ltd China) was cut into 1 cm
ꢂ 1 cm sheet (Fig. 1b). It was cleaned by a consecutive ultra-
sonication in acetone, ethanol, distilled water for 10 min, and
then put it into 1.0 mol dmꢀ3 HCl solution to remove any surface
impurities and oxide layers. The surface of the copper foil became
bright and smooth aer treatment (Fig. 1c). One side of the copper
foil was coated by nail enamel. The pre-cleaned Cu foil was then
immersed in an aqueous solution consisting of 12 mL NaOH (10
mol dmꢀ3), 6 mL (NH4)2S2O8 (1.0 mol dmꢀ3) and 27 mL distilled
water. A few minutes later, the exposed copper foil surface turned
to a faint blue color, and the initial colorless solution became
increasingly blue. The reaction time of copper in the solution were
30 min (Fig. 1d), 60 min (Fig. 1e) and 120 min (Fig. 1f), respec-
tively. The electrodes were all then rinsed with water and ethanol,
and dried in air aer extracted from the solution.
The morphology was examined by scanning electron
microscope (SEM, JEOL JSM-6480) and transmission electron
microscope (TEM, FEI Teccai G2 S-Twin, Philips). The crystal-
Fig. 1 Schematic diagram of Cu(OH)2/Cu-foil electrode (a); photo-
graphs of Cu foil (b); the Cu foil after pretreatment (c); Cu(OH)2/Cu-foil
electrode (d); CuO/Cu(OH)2/Cu-foil electrode (e); CuO/Cu-foil elec-
lographic phases of all the sample were investigated by X-ray trode (f).
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potentiostat (VMP3/Z Bio-Logic) controlled by the EC-lab so-
ware. The as-prepared electrode (1 ꢂ 1 cm2 nominal planar
area) acted as the working electrode, a platinum foil (1 ꢂ 2 cm2)
served as the counter electrode, and a saturated calomel elec-
trode (SCE) was used as the reference electrode. The asymmetric
supercapacitor using nanosheet CuO/Cu-foil as positive and AC
as negative electrode was tested in two-electrode electro-
chemical cell. The cycle life tests were conducted on a Land
battery program-control test system. All electrochemical
measurements were performed in 1.0 mol dmꢀ3 KOH electro-
lyte. The solutions were made with analytical grade chemical
reagents and Milli-Q water (18 MU cm, Millipore). EIS
measurements were performed by applying an alternating
voltage with 5 mV amplitude in a frequency range from 0.01 Hz
to 100 kHz at the open circuit potential.
60 min
2Cu þ 8NaOH þ 2ðNH4Þ S2O8
CuO þ CuðOHÞ
ꢀꢀꢀꢀꢀꢀ!
120 min
2
2
þ 4Na2SO4 þ 4NH3[ þ 5H2O
(2)
(3)
Cu þ 4NaOH þ ðNH4Þ S2O8
CuO
ꢀꢀꢀꢀꢀꢀ!
2
þ 2Na2SO4 þ 2NH3[ þ 3H2O
The surface morphologies of the electrodes were investigated
by SEM. Fig. 3a shows the SEM image of the copper foil aer
pre-treated. At the reaction time of 30 min, the Cu(OH)2 forms a
blue thick lm and covers the entire copper substrate uniformly
and compactly at low magnication (Fig. 3b). The higher
magnication SEM image (Fig. 3c) reveals that the lm is
composed of nanowires, many of them then assemble to form a
cluster which crosses each other. Fig. 3d is the TEM image of a
single Cu(OH)2 nanowire scratched down from the copper foil.
The length of a single nanowire is up to around 4.6 mm with the
width about 330 nm. At the reaction time of 60 min, the hier-
archical structures of CuO appears, as shown in Fig. 3e and f.
The ower-like CuO microspheres are evident with a diameter
of 3–5 mm uniformly (Fig. 3e). The magnied image of a single
3. Results and discussion
3.1. Characterization of electrodes
XRD analysis was rst performed to study the crystal structure
of the electrodes. As shown in Fig. 2, the strong diffraction
peak marked with heart-shape comes from the Cu substrate
(JCPDS le no. 04-0836). The diffraction peaks marked with
cinquefoil and rhombus can be indexed to orthorhombic
Cu(OH)2 (JCPDS le no. 80-056) and monoclinic CuO (JCPDS
le no. 80-1916) perfectly. This indicates that the pure
Cu(OH)2, Cu(OH)2/CuO composites and pure CuO were
successfully synthesized on copper foil at reaction time of 30
min, 60 min and 120 min, respectively. The Cu(OH)2,
Cu(OH)2/CuO and CuO nanostructure could be formed on Cu
substrate by the surface oxidation of copper in alkaline
solution as expressed by the following equation according to
literature.38
30 min
ꢀꢀꢀꢀꢀꢀ!
Cu þ 4NaOH þ ðNH4Þ S2O8
CuðOHÞ
2
2
þ 2Na2SO4 þ 2NH3[ þ 2H2O
(1)
Fig. 3 SEM images and TEM images of pre-cleaned Cu foil (a),
Fig. 2 XRD patterns of the Cu(OH)2/Cu-foil electrode, CuO/Cu(OH)2/ Cu(OH)2/Cu-foil electrode (b) (c) (d); CuO/Cu(OH)2/Cu-foil electrode
Cu-foil electrode and CuO/Cu-foil electrode.
(e) and (f); CuO/Cu-foil electrode (g) and (h).
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microower (Fig. 3f) reveals that the thickness of the ower capacitance, involving a faradic capacitance, which mainly
petals is approximately 50–100 nm. Interestingly, the micro- originates from the redox reactions. The specic capacitance of
owers CuO are standing on Cu(OH)2 nanowire arrays. At the the electrodes can be calculated from the discharge curves
reaction time of 120 min, Fig. 3g and h show that the Cu(OH)2 using the following equation:
nanowires disappear and the CuO presented as the uniform
Id ꢂ Dt
Cm
¼
(5)
microowers covering the Cu substrate. The longer reaction
time did not change the morphology of CuO, but did increase
the amount of microowers.
DV ꢂ m
where Cm (F gꢀ1) is the specic capacitance, Id (mA) is the
discharge current, Dt (s) is the discharge time, DV (V) is the
discharge potential range, and m (mg) is the mass of active
materials. The specic capacitance values of Cu(OH)2/Cu-foil
electrode, CuO/Cu(OH)2/Cu-foil electrode, CuO/Cu-foil elec-
trode are 511.5, 78.44 and 30.36 F gꢀ1 which is in good agree-
ment with the CV curves (Fig. 4a). The potential drop between
the charge and discharge curves (IR drop) is generally caused by
internal resistance and incomplete faradic reaction in the
electrode. The difference in supercapacitive performance of the
three electrodes majorly arise from their distinctive
morphology. On one hand, the cross nano-sized wire-like
Cu(OH)2 provide larger electrode–electrolyte interface for effi-
cient redox reaction than micro-sized ower-like CuO. On the
other hand, since one-dimensional morphologies posses the
property of directional charge transport, the nanowire Cu(OH)2
display lower charge transfer resistance (see Fig. 7) and smaller
IR drop than microower CuO.
Fig. 5a shows the CVs of the Cu(OH)2/Cu-foil electrode
recorded in 6 M KOH solution at various scan rates. The CV
curves show lower slopes for lower scan rates, which indicates
that lower scan rates allow a longer duration for the anions to
access the bulk of the electrode; thereby showing ideal capaci-
tive behavior. However, with increasing scan rate the anodic
peak shis towards positive potentials and the cathodic peak
shis towards negative potentials as has been observed
conventionally. This demonstrates the quasi-reversible nature
3.2. Electrochemical properties of electrodes
Capacitor performance of electrodes could be evaluated with
cycle voltammogram (CV) and galvanostatic charge–discharge
(GCD) measurements. Fig. 4a summarizes the results of the CV
studies of the Cu(OH)2/Cu-foil electrode, CuO/Cu(OH)2/Cu-foil
electrode and CuO/Cu-foil electrode (all have the same
geometrical area of 1 cm2) at a scan rate of 2 mV sꢀ1 in 6.0 M
KOH solution. The CV proles for all the three samples are quite
different from the ideal rectangular shape for double layer
capacitance indicating that the electrodes possess pseudoca-
pacitance properties. The specic capacitance of the samples
could be estimated from the cathodic or anodic part of the CV
data using the equation:
ð
E2
1
Cs ¼
iðEÞdE
(4)
mvðE2 ꢀ E1Þ
E1
where E1 and E2 are the cutoff potentials in the CV curves and
i(E) is the current at each potential, E2 ꢀ E1 is the potential
window, v is the scan rate and m is the mass of the active
materials. In this report, the mass loading of Cu(OH)2,
CuO/Cu(OH)2, CuO on Cu substrate is around 0.7, 1.6 and
2.1 mg cmꢀ2. The specic capacitance for Cu(OH)2/Cu-foil
electrode, CuO/Cu(OH)2/Cu-foil electrode, CuO/Cu-foil elec-
trode based on CV curves are 65.3, 153 and 685 F gꢀ1. Therefore,
the Cu(OH)2/Cu-foil electrode displays the best supercapacitive
property compared with the other two electrodes.
Apparently, when the voltage is beyond 0.4 V, the system
occurs oxygen evolution phenomenon, thus the GCD measure-
ment was tested in the potential window of 0–0.4 V in 6 M KOH
(Fig. 4b) at 5 mA cmꢀ2. The obvious non-linear shape of the
discharge curves further reveal that the capacitance of elec-
trodes are not pure double-layer capacitance, but a pseudo-
Fig. 5 CV curves of Cu(OH)2/Cu-foil electrode in 6 M KOH electrolyte
Fig. 4 CV curves of Cu(OH)2/Cu-foil electrode, CuO/Cu(OH)2/Cu-foil at different scan rates (a); voltammetric current as a function of square
electrode and CuO/Cu-foil electrode in 6 M KOH electrolyte at a scan root of scan rate in 6 M KOH electrolyte (b); discharge curves of
rate of 2 mV sꢀ1 (a); the charge–discharge curves of Cu(OH)2/Cu-foil Cu(OH)2/Cu-foil electrode at different current densities in 6 M KOH
electrode, CuO/Cu(OH)2/Cu-foil electrode and CuO/Cu-foil elec- electrolyte (c); variation of specific capacitance as a function of current
trode in 6 M KOH electrolyte at a current density of 5 mA cmꢀ2 (b).
density in 6 M KOH electrolyte (d).
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surface redox reactions or from bulk diffusion. Fig. 5b shows a
nearly linear relationship, which implies that the electrode
reaction is diffusion-controlled with the electrolyte involved.
Additionally, constant current charge–discharge curves at
various current densities were performed in 6.0 mol dmꢀ3 KOH
solution to further evaluate the electrochemical performance of
the Cu(OH)2/Cu-foil electrode (Fig. 5c). The specic capacitance
values of the Cu(OH)2 evaluated from the discharge curves
according to eqn (5) are 750, 511, 318, 198, and 107 F gꢀ1 at the
current density of 2, 5, 10, 15 and 20 mA cmꢀ2, respectively.
Obviously, compared with the ndings (Table 1), the electro-
spun copper oxide nanowire powder prepared by Vidhyadharan
et al.39 shows the highest specic capacitance (620 F gꢀ1 at the
current density of 2 A gꢀ1). However, the specic capacitance of
our Cu(OH)2 nanowire structure directly grown on copper foil
mentioned here is much higher than that (750 F gꢀ1 at the
current density of 2.86 A gꢀ1). The possible reasons for the
superior capacitance can be attributed to three aspects: (i) the
Cu(OH)2/Cu-foil electrode fabricated by the directly oxidation of
copper foil avoiding using of conductive agent and binder
which reduces the resistance efficiently; (ii) the unique uniform
nanowires interconnected with each other generating abundant
space which enable the electrolyte easier further diffuse to the
nanowire surface and thus reduce the concentration polariza-
tion; (iii) one-dimensional morphologies of nanowire possess
the property of directional charge transport promoting fast
electron transfer, which weaken the electrochemical polariza-
tion. In order to further clarify the effect of Cu-foil substrate on
the capacitance performance, the galvanostatic discharge
curves of the Cu-foil substrate and the Cu(OH)2/Cu-foil elec-
trode with the same geometrical area of 1 cm2 were compared in
inset of Fig. 5c. The discharge time of the Cu-foil substrate is
only 0.36 s, which can be ignored compared to that of the
Cu(OH)2/Cu-foil electrode (105 s). Besides, the surface area of
Cu foil substrate in the Cu(OH)2/Cu-foil electrode was
completely covered by Cu(OH)2. These results (inset of Fig. 5a
and c) displayed that the Cu foil substrate could not bring about
substantial contribution to the specic capacitance of
Cu(OH)2/Cu-foil electrode. The performance of an asymmetric
capacitor employing Cu(OH)2/Cu-foil as one of the electrodes is
not reported herein; the purpose of the present paper is a
thorough understanding of the electrochemical properties of
Cu(OH)2 nanowires.
Fig. 6 Dependences of the discharge specific capacitance and the
columbic efficiency on the charge/discharge cycle numbers at current
density of 5 mA cmꢀ2 in 6 M KOH electrolyte for Cu(OH)2/Cu-foil and
CuO/Cu-foil electrodes(a). The XRD (b) and SEM images for Cu(OH)2/
Cu-foil (c) and CuO/Cu-foil (d) electrodes after cycling.
of redox reaction. From the inset of Fig. 5a, it can be seen that
the capacitive current of Cu-foil substrate is signicantly lower
than that of Cu(OH)2/Cu-foil electrode, suggesting that the
capacitance contribution of the Cu-foil substrate to the
Cu(OH)2/Cu-foil electrode can be neglected. All the plots show
the oxidation (anodic) and reduction (cathodic) events indica-
tive of the pseudocapacitance charge storage mechanism. Based
on literatures reporting about redox of CuO and CuO/Cu(OH)2
composites in aqueous electrolytes,19,32,39–41 within the potential
range of 0–0.6 V, the following reactions between Cu(I) and
Cu(II) species (eqn (2) and (3)) might exist.
2Cu(OH)2 + 2eꢀ 5 Cu2O + 2OHꢀ + H2O
Cu2O ꢀ 2eꢀ + 2OHꢀ 5 2CuO + H2O
(6)
(7)
The anodic peak A appearing can be attributed to the
oxidation of Cu2O to CuO and Cu(OH)2. The cathodic peak C
can be ascribed to the reduction of CuO and Cu(OH)2 to Cu2O.
Previous studies have shown that the reduction of CuO/Cu(OH)2
to Cu and the oxidation of CuO/Cu(OH)2 to Cu(III) species do not
occur in the 0–0.6 V potential range.42,43 It should be pointed out
that the CVs shown in Fig. 5a are the stabilized curves aer
around 5 potential cycles, during which Cu(OH)2 originally
formed on copper foil was partially reduced to Cu2O. The
oxidation of this Cu2O generated the anodic peak (A). Besides,
the Cu(OH)2/Cu-foil electrode did not show supercapacitive
property when tested the CVs in neutral electrolyte, such as
Na2SO4, K2SO4, Li2SO4. This result indicates that the abundant
OHꢀ is necessary when Cu(OH)2 undergoes redox reaction as a
pesudocapacitive material (according to eqn (6) and (7)).
The specic capacitance of Cu(OH)2 is plotted as a function
of the current density in Fig. 5d. The specic capacitance
decreased with increasing current density. When the discharge
current density increases from 2 mA cmꢀ2 (2.86 A gꢀ1) to 20 mA
cmꢀ2 (28.6 A gꢀ1), the specic capacitance of Cu(OH)2/Cu-foil
electrode decreases from 750 F gꢀ1 to 107 F gꢀ1. The note-
worthy decrease in capacitance is possibly the result of the
increased potential drop caused by the electrode resistance and
the relatively in sufficient Faradic redox reaction under high
current densities. This phenomenon can be explained clearly by
Fig. 5b. Generally, the majority of the active surface is utilized by
the ions for charge storage at the lower current density, thereby
resulting in the higher specic capacitance. The ion movement
is limited only to the surfaces of the electrode material at higher
In pseudocapacitive materials, such as Cu(OH)2, the anodic
peak current (ipa) and cathodic peak current (ipc) against n1/2 is
analyzed to determine whether the capacitance originates from
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Table 2 The lattice parameters of microflower CuO and nanosheet
CuO
current density when electrochemical double layer capacity is
the dominant mechanism. Therefore, the contribution from the
bulk diffusion is no longer available and the specic capaci-
tance drops off drastically. Impurity doping or addition of
carbon nanotubes are possible routes to further improve the
rate capability of the Cu(OH)2/Cu-foil electrode.
Lattice parameters
Morphology
Crystal system
a
b
c
Microower CuO
Nanosheet CuO
Monoclinic
Monoclinic
4.70242
4.6883
3.40048
3.4229
5.17389
5.1319
Long cyclic stability is an important requirement for prac-
tical competence of supercapacitor devices. The cyclability of
the Cu(OH)2/Cu-foil electrode was investigated by continuous
charge/discharge measurements over 5000 cycles (Fig. 6a). The
study was performed at a current density of 5 mA cmꢀ2
(7.14 A gꢀ1) in 6.0 M KOH solution within the potential range of
0–0.4 V. The specic capacitance decreased by 17% during the
rst 600 cycles, aer which it was almost constant at approxi-
mately 360 F gꢀ1 for up to 5000 cycles. The possible reason for
the decrease of capacitance may involve microstructure change
and resistance increase (see Fig. 7). The morphology and
structure of the electrode aer 600 cycles were examined by
scanning electron microscopy and X-ray diffraction spectros-
copy. The XRD analysis result (Fig. 6b) shows that all the
diffraction peaks marked with rhombus t well to monoclinic
CuO (JCPDS card no. 45-0937) with different lattice parameters
from the microower CuO prepared in this report (see Table 2).
The diffraction peak at 50.4ꢃ comes from the copper foil (JCPDS
card no. 04-0836). In other words, during charging, the Cu2O
undergoes a oxidation process turning into Cu(OH)2 and CuO
(eqn (6) and (7)). Fig. 6c shows the SEM image of the
Cu(OH)2/Cu-foil electrode aer 600 cycles, which implies that
the original nanowire arrays have turned into nanosheets
completely with a diameter of about 1 mm during the repetitive
charge/discharge cycling. Apparently, since the space between
the nanosheet CuO is much smaller than nanowire Cu(OH)2, it
is difficult for electrolyte to further diffuse. Consequently, the
utilization of nanowire Cu(OH)2 is higher than nanosheet CuO
which causes the electrochemical performance of the Cu(OH)2
is superior than the CuO. Compared with the previous CuO
microower mentioned in this article, on account of the
formation of the former microowers is the result of the
aggregation of nanosheets (see Fig. 3g and h), the charge
transfer and electrolyte diffusion resistance of the microower
CuO is larger (see Fig. 7) which leads to its capacity is smaller
than the present nanosheet CuO. In addition, Fig. 6a shows the
capacitance retention of microower CuO/Cu-foil is nearly
100% during 5000 charge–discharge cycles, which may results
from the unchanged morphology (Fig. 6d). This phenomenon
indicates the morphology of the active materials has great
inuence on the electrochemical property. The columbic effi-
ciency was calculated using the following equation
td
h ¼
(8)
tc
where tc and td represent the time of charge and discharge,
respectively. The columbic efficiency remains above 98% within
5000 cycles. The good contact of the active materials with the
current collector can well explain the phenomenon that the
peeling off of Cu(OH)2 or CuO from copper foil was not observed
during the repetitive charge/discharge cycling. These results
implied that the cheap and available Cu(OH)2/Cu-foil electrode
has potential application in electrochemical supercapacitors.
For pseudocapacitor, it is well accepted that the capacitance
of electroactive material originates from the redox reaction. The
reaction kinetics can be evaluated by EIS test. Fig. 7 shows the
Nyquist plots of the Cu(OH)2/Cu-foil electrode, Cu(OH)2/Cu-foil
electrode aer 5000 charge–discharge cycles, CuO/Cu(OH)2/Cu-
foil electrode and CuO/Cu-foil electrode measured at open
circuit potential in 6.0 mol dmꢀ3 KOH solution. All the plots
consist of a semicircle at high frequency region and a straight
line at low frequency region. A depressed semicircle is observed
in the high frequency region, which results from a parallel
combination of the charge-transfer resistance (Rct) caused by
redox reactions and a constant phase element one (CPE1). The
calculated charge-transfer resistance for Cu(OH)2/Cu-foil elec-
trode is 0.9
5000 charge–discharge cycles is 2 U cmꢀ2; CuO/Cu(OH)2/Cu-foil
electrode is 3.6 U cmꢀ2 and CuO/Cu-foil electrode is 4.6 U cmꢀ2
U
cmꢀ2
;
Cu(OH)2/Cu-foil electrode aer
.
The different resistance is probably owing to the microstructure
effect. In the low frequency, the inclined straight line corre-
sponds to Warburg impedance (W1) related to the diffusion of
electrolyte within the nanorods electroactive materials. The
straight line part leans more toward the imaginary axis, indi-
cates that the electrode has a good capacitive behavior.44 EIS
results show the Cu(OH)2/Cu-foil electrode has a lower charge
transfer resistance and ion diffusion resistance with fast reac-
tion kinetics. It could be considered to be a promising electrode
material for pseudocapacitor applications.
Fig. 7 EIS of the Cu(OH)2/Cu-foil electrode, Cu(OH)2/Cu-foil elec-
trode after 5000 charge–discharge cycles, CuO/Cu(OH)2/Cu-foil
electrode and CuO/Cu-foil electrode measured at open circuit
potential in 6.0 mol dmꢀ3 KOH solution.
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seen, the CV curve of this supercapacitor exhibited a large
current area with broad redox peak, which was the character-
istic of the electric double layer capacitance and Faradaic
pseudocapacitance. With the increase of scan rate from 5 to
40 mV sꢀ1, the shapes of CV curves do not obviously change,
suggesting its good capacitive behavior of the asymmetric
supercapacitor. The galvanostatic charge–discharge curves of
the asymmetric supercapacitor at different current densities are
shown in Fig. 8d. On the basis of the charge–discharge test
results, the energy density (E) and power density (P) was calcu-
lated according to the following equation:
3.3. Electrochemical properties of asymmetric
supercapacitor
To further demonstrate the potential application of Cu(OH)2
nanowire in supercapacitors, an asymmetric supercapacitor was
fabricated by utilising Cu(OH)2/Cu-foil electrode as the positive
electrode and AC as the negative electrode (Fig. 8a). The CV
curves for AC and Cu(OH)2/Cu-foil electrode at a scan rate of 30
mV sꢀ1 in 6 M KOH aqueous solution are shown in Fig. 8b. As
can be seen, the potential windows of the AC and Cu(OH)2/Cu-
foil electrode are ꢀ1.0–0 and 0–0.6 V, respectively. The AC
electrode has a nearly rectangular CV curve, limited by H2
evolution, indicating a typical EDLC behavior since no redox
peaks are observed. On the other hand, the CV shape of the
Cu(OH)2/Cu-foil displays pseudocapacitance behavior, limited
by O2 evolution, based on a redox mechanism. The total cell
voltage can be expressed as the sum of the potential range for
the AC and the Cu(OH)2/Cu-foil electrode. Therefore, the cell
potential can be as high as 1.6 V in 6 M aqueous KOH electro-
lyte, which is higher than that of conventional AC-based
symmetric supercapacitors in aqueous electrolytes (0.8–1.0 V).
Fig. 8c displays the CV curves of the asymmetric super-
capacitor at various scan rates in a potential range of 0–1.6 V. As
Id ꢂ Dt
C ¼
(9)
(10)
(11)
V ꢂ m
1
E ¼ CV2
2
E
P ¼
Dt
where I (mA) is the discharge current density, Dt (s) is the
discharge time, V (V) is the discharge potential range and m
(mg) is the total mass of the active electrode materials. The
Ragone plots of the AC//Cu(OH)2/Cu-foil asymmetric super-
capacitor cell and AC//AC symmetric supercapacitor are shown
in Fig. 8e. The energy density of the AC//Cu(OH)2/Cu-foil
supercapacitor reaches 18.3 W h kgꢀ1 at a power density of
326 W kgꢀ1, and still remains 14.1 W h kgꢀ1 at a power density
of 9.8 kW kgꢀ1. The energy densities reduce slowly with
increasing power densities. Clearly both the energy and power
densities greatly increased compared with the AC//AC
symmetric capacitor. These results render Cu(OH)2 promising
as one of the most attractive candidates for energy storage.
Additionally, the cycling stability of the asymmetric super-
capacitor was performed at a constant current density of 0.5 mA
cmꢀ2 ranging from 0 to 1.5 V for 5000 cycles, as shown in Fig. 8f.
The specic capacitance of the asymmetric supercapacitor
(AC//Cu(OH)2/Cu-foil) decrease at rst and then keep stable.
Aer 5000 cycles, the asymmetric supercapacitor maintains
80% of its original capacitance, which is coincidence with the
single Cu(OH)2/Cu-foil electrode (Fig. 6a). These results render
it promising as one of the most attractive candidates for energy
storage.
4. Conclusions
In conclusion, the Cu(OH)2 nanowire arrays, microower CuO
standing on nanowire Cu(OH)2 and uniform ower-like CuO
were successfully grown onto copper foil by a facile one-step
template-free method to obtain a conducting additive-free and
binderless electrode for supercapacitors. The good contact
Fig. 8 Schematic illustration of the as-fabricated asymmetric super-
capacitor device based on Cu(OH)2/Cu-foil as positive electrode and
activated carbon as negative electrode in 6 mol dmꢀ3 KOH electrolyte
(a); CV curves of Cu(OH)2 and AC electrodes at a scan rate of 10 mV sꢀ1
(b); CV curves of Cu(OH)2/Cu-foil//AC asymmetric supercapacitor at between active materials and the conductive Cu substrate
different scan rates (c); charge–discharge curves of Cu(OH)2/Cu-foil//
AC asymmetric supercapacitor at different current densities (d);
Ragone plot related to energy and power densities of the Cu(OH)2/Cu-
foil//AC asymmetric supercapacitor and AC//AC symmetric super-
capacitors (e); cycle performance of Cu(OH)2/Cu-foil//AC super-
capacitor (f).
enables their use as integrated electrode for supercapacitors
without adding other ancillary materials. The Cu(OH)2/Cu-foil
electrode displays the best electrochemical performance with a
specic capacitance of 750 F gꢀ1 at a current density of 2 mA cmꢀ2
(2.86 A gꢀ1) in 6 M KOH electrolyte. Furthermore, fabricating the
36662 | RSC Adv., 2015, 5, 36656–36664
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asymmetric supercapacitor exhibits a high energy density of 11 C.-C. Hu, W.-C. Chen and K.-H. Chang, How to Achieve
18.3 W h kgꢀ1 at a power density of 326 W kgꢀ1
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Acknowledgements
We gratefully acknowledge the nancial support of this
research by the National Natural Science Foundation of China 13 D.-D. Zhao, S.-J. Bao, W.-J. Zhou and H.-L. Li, Preparation of
(21403044), the China Postdoctoral Science Foundation
(2014M561332), the Heilongjiang Postdoctoral Fund
(LBH-Z13059), the Major Project of Science and Technology of
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application for electrochemical capacitor, Electrochem.
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Heilongjiang Province (GA14A101), the Fundamental Research 14 H. Jiang, T. Zhao, C. Li and J. Ma, Hierarchical self-assembly
Funds for the Central Universities (HEUCF20151004) and the
Project of Research and the Development of Applied Technology
of Harbin (2014DB4AG016).
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Asymmetric
supercapacitors
based
on
b-Ni(OH)2
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