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Fiber-Based Supercapacitors
www.advenergymat.de
Utilizing Waste Cable Wires for High-Performance
Fiber-Based Hybrid Supercapacitors: An Effective
Approach to Electronic-Waste Management
Goli Nagaraju, S. Chandra Sekhar, and Jae Su Yu*
nature.[3] Unlike from the planar SCs, fibershaped SCs (FSCs) have fascinated more
and more research attention in use as a
new class of power sources for the design
innovation of lightweight and reconfigurable electronic products.[4] Benefited from
the yarn-shaped device structure, FSCs can
be easily embroidered into human cloths,
which open up a new avenue in the development of electronic textiles.[5] However,
apart from the merits of tiny size as well
as, more flexible and wearable abilities,
FSCs still suffer from much lower energy
density compared with batteries due to
the narrow potential window and low
capacitance.[6] Therefore, it is important to
investigate the high energy storage FSCs
to meet the practical applicability in wearable electronics. Technically, enhancement
of energy density in FSCs can be expected
via the fabrication of asymmetric/hybrid
device configuration.[7] This effective
approach obviously extends the potential window and capacitance, and thus
increases the energy density by assembly
of intrinsic charge-storage capabilities
of positive electrode (pseudocapacitive/battery-type materials)
and porous negative electrode (electric double layer capacitive
materials (EDLCs, carbon-based materials)) for FSCs.[8] Furthermore, the high potential window of asymmetric/hybrid
FSCs with high energy density also decreases the number of
devices in series to reach the expected output voltage. For positive electrodes, the battery-type materials (Co3O4, Ni3Se2, NiO,
etc.) exhibited higher charge-storage properties compared to the
pseudocapacitive materials (such as RuO2) due to their excellent electrochemical activity and ability to perform more redox
centers with electrolyte ions.[9] Accordingly, significant achievements have been made through the development of battery-type
materials with versatile morphologies for positive electrode in
hybrid devices.[10] Especially, the materials with multicomponent combination of core–shell-like nanoarchitectures have
been suggested as a promising concept to enrich the electrochemical properties due to the advantage of synergistic effect.[11]
Apart from the incessant research efforts to improve the electrochemical performance of hybrid FSCs, the investigation of lowcost current collectors and novel electroactive materials is also
important because the major contribution of fabrication cost is
mainly based on both of these components.[12]
In recent years, electronic waste (e-waste) such as old cable wires, fans,
circuit boards, etc., can be often seen in large piles of leftover in dumping
yards. Employing these e-waste sources for energy storage devices not only
increases the economic value but also decreases the reliance on fossil fuels.
In this context, waste cable wires are utilized to obtain precious copper (Cu)
fibers and used as a cost-effective current collector for the fabrication of
fiber-based hybrid supercapacitor (FHSC). With the braided Cu fibers, forestlike nickel oxide nanosheet grafted carbon nanotube coupled copper oxide
nanowire arrays (NiO NSs@CNTs@CuO NWAs/Cu fibers) are designed via
simple wet-chemical approaches. As a battery-type material, the forest-like
NiO NSs@CNTs@CuO NWAs/Cu fiber electrode shows superior electrochemical properties including high specific capacity (230.48 mA h g−1)
and cycling stability (82.72%) in aqueous alkaline electrolyte. Moreover, a
solid-state FHSC is also fabricated using forest-like NiO NSs@CNTs@CuO
NWAs/Cu fibers as a positive electrode and activated carbon coated carbon
fibers as a negative electrode with a gel electrolyte, which also shows a higher
energy and power densities of 26.32 W h kg−1 and 1218.33 W kg−1, respectively. The flexible FHSC is further employed as an energy source for various
electronic gadgets, demonstrating its suitability for wearable applications.
1. Introduction
The portable and wearable electronic designs have attracted
a great deal of scientific interest and are becoming crucial in
our daily life.[1] This innovate technology enables the creative approach to develop highly flexible and lightweight energy
storage devices to use as a power supply.[2] Among the various
flexible energy storage devices, supercapacitors (SCs) have
recently been considered as a promising power source because
of their versatile advantages including high power density, quick
charging–discharging ability, long cycling life, and eco-friendly
Dr. G. Nagaraju, S. C. Sekhar, Prof. J. S. Yu
Department of Electronic Engineering
Institute for Wearable Convergence Electronics
Kyung Hee University
1 Seocheon-dong, Giheung-gu, Yongin-si
Gyeonggi-do 446-701, Republic of Korea
E-mail: jsyu@khu.ac.kr
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/aenm.201702201.
DOI: 10.1002/aenm.201702201
Adv. Energy Mater. 2017, 1702201
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© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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On the other hand, electronic waste (e-waste) is becoming a
major concern and one of the fastest-growing waste streams in
the world.[13] Some common e-waste sources such as expired
household appliances (electrical cable wires, fans, televisions,
heaters, etc.) and information technology electronic equipments (computers, mobile phones, circuit boards, etc.) can be
often seen in large piles of leftover in dumping grounds/store
rooms.[14] These wastes are environmentally harmful on one
hand and precious on the other hand. They contain substantial amounts of valuable metals.[15] Among the various e-waste
sources, electrical cable wires are largely composed of metallic
fibers such as copper and aluminum which have high utilization value.[16] Considering economic and environmental safety,
reusing these metal fibers from e-waste rather than dumping
them is highly desirable to carry out metal recovery for various
electronic and energy-related applications. Especially, utilizing
these old cable wires to fiber-type energy storage devices not
only decreases the fabrication cost of SCs but also reduces the
depletion of fossil fuel usage, which provides new ideas for
real-life applications along with eco-friendliness.
Inspiring from recycling approach of e-waste, we utilized Cu
fibers (which are peeled out from leftover/waste electric cable
wires) as a cost-effective current collector and facilely synthesized
core–shell-like composite material over them for high-performance fiber-based hybrid SC (FHSC). The Cu fibers also served
as a sacrificial template for the growth of one-dimensional (1D)
copper hydroxide nanowire arrays (Cu(OH)2 NWAs/Cu fibers)
via an alkaline oxidative etchant solution-based dipping method.
Compared to the other forms of nanostructures, the vertically
aligned and binder-free growth of 1D NWAs exhibits relatively
large surface area and provides efficient channels to induce the
dense growth of shell-like electroactive species for enhanced
electrochemical properties.[17] In addition to being inexpensive,
carbon nanotubes (CNTs) have been further wrapped over 1D
Cu(OH)2 NWAs/Cu fibers (i.e., CNTs@Cu(OH)2 NWAs/Cu
fibers) to improve the electron transfer kinetics and electrochemical conductivity of the electrode during the electro­
chemical measurements. Meanwhile, as an efficient batterytype material, nickel oxide (NiO) has attracted widespread
attention for positive electrode in SCs, because of its salient
features including high theoretical capacity, low cost, and environmental friendliness.[18] However, rationally designed core–
shell-like structure offers more electrochemical sites to enhance
the redox reaction rate rather than solitary components, which
indeed delivers superior energy storage properties.[11c,19] Therefore, it is auspicious to prepare a smart integration of artificial
forest-like and/or core–shell-like nanoarchitecture for positive
electrode in FHSC. By a facile wet-chemical approach in nickel
salt and urea-based growth solution, nickel hydroxide carbonate
precursor with nanosheet-like network was formed on CNTs@
Cu(OH)2 NWAs/Cu fibers. After post-annealing treatment,
nickel hydroxide carbonate and Cu(OH)2 were converted into
NiO nanosheets (NSs) and copper oxide (CuO), which intends
to form artificial forest-like NiO NSs@CNTs@CuO NWAs/
Cu fiber electrode. Using this hierarchical electrode, we fabricated the flexible solid-state FHSC, which exhibits a wide potential window of 1.55 V, high energy density of 26.32 W h kg−1,
and high power density of 1218.33 W kg−1 along with excellent
cycling stability.
Adv. Energy Mater. 2017, 1702201
2. Results and Discussion
The designed wisdom and preparation process flow of forestlike NiO NSs@CNTs@CuO NWAs/Cu fibers using facile and
green wet-chemical approaches are schematically illustrated in
Figure 1. Electric cable wires are mainly composed of metallic
conductive fibers wrapped in a plastic insulation. These cable
wires are basically used for the distribution of household or
industrial electricity (Figure 1a). The waste/scrap cable wires
which are virtually useless for electricity were thrown into store
rooms or landfills in our daily life. In such a way, the waste
cable wires are considered as one of the abundantly available
e-waste. Utilizing these waste cable wires for energy storage
fiber-based SC would certainly increase the economic value
and reduce the reliance on fossil fuels. Accordingly, herein, we
proposed to use waste cable wires to recover the valuable metal
(i.e., Cu) fibers and employed these Cu fibers for flexible current collector in SC. Also, the recovered Cu fibers act as a main
source to grow the vertically aligned Cu(OH)2 NWAs without
the use of any Cu-based chemical salts. Prior to the growth,
three of the Cu fibers were braided into the form of human
hair and acid-treated with aqueous HCl solution to remove the
surface adhered native oxide layer, as shown in Figure 1b. Such
braiding process could be performed to improve the mass
loading of the active material and expected to enhance the specific capacity values of the material, as compared to the single
Cu fiber electrode. The braided Cu fibers were then immersed
vertically into the alkaline oxidative etchant solution (which
was prepared by mixing aqueous NaOH and (NH4)2S2O8 solutions) for 20 min under ambient condition for the development of 1D Cu(OH)2 NWAs, as shown in Figure 1c. Here,
the surface of Cu fibers was oxidized with oxidative species
of peroxodisulfate (S2O82−) ions, which leads to the formation of copper (Cu2+) ion on the surface of Cu fibers. Simultaneously, the Cu2+ ions interact with OH− ligands under the
strong alkaline environment to enable the growth of Cu(OH)2
NWAs along the (100) direction on Cu fibers (designated as
Cu(OH)2 NWAs/Cu fibers), which is due to the hydrogen
bond linkages and smaller interplanar spacing of (100). The
as-grown 1D Cu(OH)2 NWAs/Cu fibers serve as a backbone
for the subsequent growth of CNTs and NiO NSs. For the decoration of CNTs, Cu(OH)2 NWAs/Cu fibers were dipped into an
aqueous CNT dispersion, thus leading to the uniform coating
of CNTs on Cu(OH)2 NWAs/Cu fibers, as shown in Figure 1d.
During this dipping process, it is important to wait for at least
about 5 min to penetrate CNTs sufficiently into interior parts
of Cu(OH)2 NWAs, which would also benefit the twisting/covering of CNTs over the entire parts of Cu(OH)2 NWAs. Subsequently, the hierarchical Ni(OH)2CO3·nH2O NS precursor
was synthesized on CNTs@Cu(OH)2 NWAs/Cu fibers with a
growth solution containing NiSO4·6H2O and NH2CONH2 via a
simple oil bath method. After post-annealing treatment, forestlike NiO NSs@CNTs@CuO NWAs/Cu fibers were obtained,
as displayed in Figure 1e. The formation of such hierarchical
NiO NSs@CNTs@CuO NWAs/Cu fibers can be understood
by the chemical equations given in Section S(1) in the Supporting Information. During the growth process, the dissolved
NH2CONH2 in the growth solution was easily hydrolyzed
to form the NH3 and CO2, which were then converted into
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Figure 1. Schematic illustration showing the fabrication process of forest-like NiO NSs@CNTs@CuO NWAs/Cu fibers using waste cable wires.
CO32− and OH− ions, as shown in Equations (S5)–(S7) in the
Supporting Information. Afterward, the Ni(OH)2CO3·nH2O
precursor was formed on CNTs@Cu(OH)2 NWAs/Cu fibers
through the reaction of Ni2+ ions with CO32− and OH− ions,
respectively. Eventually, the thermal analysis was subjected to
convert the Ni(OH)2CO3·nH2O precursor and Cu(OH)2 into
the form of hierarchical NiO NSs@CNTs@CuO NWAs/Cu
fibers, based on Equations (S9) and (S10) in the Supporting
Information. In this hierarchical nanoarchitecture, CNTs and
CuO NWAs play a vital role in the enhancement of energy
storage. The vertically aligned 1D CuO NWAs provide the large
surface area as well as a strong support for the growth of NiO
NSs with high mass loading. Owing to the good conductivity,
the decorated CNTs on CuO NWAs/Cu fibers provide the rapid
electron conduction pathways throughout the NWAs, which
would facilitate the complete utilization of NiO NSs during the
electrochemical measurements.
The surface morphology and structure of the as-prepared
1D Cu(OH)2 NWAs/Cu fibers, CNTs@Cu(OH)2 NWAs/
Cu fibers, and forest-like NiO NSs@CNTs@CuO NWAs/
Cu fibers are shown in Figure 2. The microscopic and fieldemission scanning electron microscope (FE-SEM) images of
pristine Cu fibers peeled out from the waste cable wires are
shown in Figure S1 (Supporting Information), from which
we can see that these pristine Cu fibers were braided into the
form of human hair and each of these fibers had a diameter of
≈170 µm with a smooth surface. After immersing the braided
Cu fibers into the alkaline oxidative etchant solution, the surface of these fibers became rough, which is due to the coverage
of Cu(OH)2, as shown in Figure 2a. With the increased magnification as presented in Figure 2b, the Cu(OH)2 NWAs were
vertically aligned on the surface of Cu fibers with a diameter
Adv. Energy Mater. 2017, 1702201
of 100–120 nm. Figure 2c shows the FE-SEM images of CNTs
decorated Cu(OH)2 NWAs/Cu fibers, which indicates that the
network-like CNTs were uniformly covered over the surface of
NWAs. Herein, the as-decorated CNTs can play a key role in
increasing the electrical conductivity during the electrochemical
measurements. Figure 2d displays the cross-sectional FE-SEM
images of the NiO NSs@CNTs@CuO NWAs/Cu fibers. In
the low-magnification FE-SEM image in Figure 2d(i), it can be
clearly seen that the Cu fiber core was coated with a shell-like
NiO NSs@CNTs@CuO NWAs layer. From the magnified part
of FE-SEM images in Figure 2d(ii) and (iii), it is observable that
the NiO NSs@CNTs@CuO NWAs were grown radially on Cu
fibers with a length of 10–12 µm. Throughout this hierarchical
structure, CNTs were unambiguously evidenced, as shown
in Figure 2d(iv). More clearly, the top-view FE-SEM image in
Figure 2e reveals that the hierarchical and interconnected NiO
NSs were abundantly coated onto the CNTs@CuO NWAs
core with high uniformity in thickness and homogeneously
distributed along the CNTs@CuO NWAs, forming core–shelllike architectures with high interspaces. Such hierarchical and
crossed-sheet network of NiO NSs on CNTs@CuO NWAs enables a large accessible surface area for electrolyte penetration,
initiates the rapid electrochemical reactions with electrolyte
ions, and enhances their specific capacity and rate capability.
The appearance of these NiO NSs@CNTs@CuO NWAs on
Cu fibers resembles with natural trees in wild forest, as shown
in the inset of Figure 2d(i). Moreover, each of these NSs has a
thickness of 15–25 nm (Figure 2g). The photographic images
of the corresponding sample (inset of Figure 2d(iii)) demonstrate that the samples have high flexibility under flexed condition without any cracks, indicative of the applicability for flexible and wearable SCs. The individual NiO NSs@CNTs@CuO
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Figure 2. Low- and high-magnification view FE-SEM images of a) Cu(OH)2 NWAs/Cu fibers, b) CNT@Cu(OH)2 NWAs/Cu fibers, and d–f) forest-like
NiO NSs@CNTs@CuO NWAs/Cu fibers. g) FE-TEM images and h-i) HR-Raman images of NiO NSs@CNTs@CuO NWAs which are peeled out using
scalpel. The insets of (d-i), (d-iii), and (g-i) show the natural forest, photographic image, and SAED pattern of the corresponding sample.
NWAs composite was also revealed by transmission electron
microscope (TEM) analysis, as displayed in Figure 2g. Figure
2g(i),(ii) shows that the CNTs were covered on CuO NWAs and
can still be seen after the growth of NiO, which was present as
ultrathin NSs attached on CNTs@CuO NWAs. In addition, the
high-resolution TEM (HR-TEM) images taken on the surface
of both NiO NSs and CuO NWAs (indicated with green and
sky blue boxes) revealed a lattice fringe spacing of about 0.24
and 0.22 nm, which correspond to the (111) and (200) planes
of both materials, respectively (Figure 2h(i),(ii)). Moreover, the
ring pattern spots observed in the selective area electron diffraction pattern (SAED) confirm polycrystalline nature of the prepared material (inset of Figure 2g).
Adv. Energy Mater. 2017, 1702201
The crystal structure of the as-prepared samples was examined
by X-ray diffraction (XRD) analysis. Figure 3a shows the XRD
patterns of (i) pristine Cu fibers and (ii) NiO NSs@CNTs@CuO
NWAs/Cu fibers, respectively. The strong diffraction peaks
observed at the 2θ of 43.4, 50.5, and 74.3° belong to Cu fibers
(JCPDS cards# 85–1326). Meanwhile, the low intense peaks
observed at 32.4, 35.6, and 61.5° correspond to (110), (111), and
(113) planes of CuO (JCPDS cards# 45–0937) and the characteristic crystal planes of (111) at 37.5°, (200) at 43.2°, and (220)
at 62.8°, respectively are the typical diffraction peaks of NiO
(JCPDS cards# 47–1049), indicating the successful growth of
the material. The Raman spectrum of NiO NSs@CNTs@CuO
NWAs/Cu fibers given in Figure 3b revealed the several low
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Figure 3. a) XRD patterns, b) HR-Raman spectrum, c) EDX spectrum, d) elemental mapping images of the NiO NSs@CNTs@CuO NWAs/Cu fibers,
and e) XPS analysis of the same sample.
and strong intense peaks of the prepared materials. Especially,
the small broad peaks present at 1353 and 1575 cm−1 correspond to the D and G bands of carbon, indicating the presence
of CNTs in the prepared sample. In addition to the CNTs peaks,
the Raman bands observed at about 258.1, 366, and 521 cm−1
are associated with the shaking peaks of metal oxide nanostructures (i.e., CuO and NiO). Energy dispersive X-ray (EDX)
spectrum and elemental analysis of the NiO NSs@CNTs@
CuO NWAs/Cu fiber are shown in Figure 3c,d. From the EDX
spectrum in Figure 3c, several elements such as Cu, Ni, O, and
C appeared in the prepared sample without any impurities.
Moreover, the elemental mapping images further manifest the
homogeneous distribution and coexistence of Cu, Ni, O, and C
elements in the NiO NSs@CNTs@CuO NWAs/Cu fiber material (Figure 3d). Such EDX results unambiguously reveal that
the NiO, CNTs, and CuO were instantaneously grown onto the
Cu fibers to form hierarchical NiO NSs@CNTs@CuO NWAs/
Cu fiber electrode. The surface chemical compositions and
surface electronic states of the prepared materials were also
examined using X-ray photoelectron spectroscopy (XPS) analysis, as shown in Figure 3e. The high-resolution XPS spectra
Adv. Energy Mater. 2017, 1702201
of Ni 2p (Figure 3e(i)) exhibited Ni 2p3/2 at 854.4 eV and Ni
2p1/2 at 873.07 eV along with their satellite peaks (represented
by Ni 2p3/2 (s) and Ni 2p1/2 (s)) at 861.14 and 879.24 eV, indicating that the Ni species in NiO have divalent oxidation state.
As presented in Figure 3e(ii), the deconvoluted spectra of Cu
2p region had Cu 2p3/2 peaks at 933.2 and 935.1 eV and Cu
2p1/2 peaks at 952.9.16 and 955.2 eV, respectively. These energy
level peaks along with shake-up satellites in the Cu 2p spectra
confirm the presence of divalent and monovalent Cu species
in the prepared sample. The deconvoluted O 1s spectra in
Figure 3e(iii) exhibited two peaks; one peak at a binding energy
value of 529.3 eV is attributed to the metal–oxygen bonds and
another peak at 531.14 eV is assigned to the O2− bond from
CuO and mixture of CuO and NiO. Meanwhile, the deconvoluted C 1s spectra of Figure 3e(iv) revealed three fitted peaks
at 284.6, 286.5, and 289.1 eV, which correspond to CC, CO,
and OCO (carboxyl or ester groups) of the dip-coated CNTs,
respectively. These results clearly confirm that the forest-like
NiO NSs@CNTs@CuO NWAs/Cu fibers were successfully
grown and they can be used as an efficient positive electrode
for FHSC.
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Figure 4. a) CV curves, b) GCD curves, and c) calculated specific capacity values (with respect to mass loading) of the prepared (i) Cu(OH)2 NWAs,
(ii) CNTs@Cu(OH)2 NWAs, (iii) NiO NSs, (iv) NiO NSs@CNTs@CuO NPs, (v) NiO NSs@ CuO NWAs, and (vi) NiO NSs@CNTs@CuO NWAs/Cu
fiber samples at constant scan rate and applied current, respectively. d–h) CV curves, linear relationship between the square root of scan rate and peak
current, GCD curves, calculated specific capacity values, and cycling stability of NiO NSs@CNTs@CuO NWAs/Cu fibers. i) Schematic representation
showing the merits of forest-like composite structure during electrochemical measurements.
The electrochemical properties of forest-like NiO NSs@
CNTs@CuO NWAs/Cu fibers and other samples were first
evaluated in three-electrode system using 1 m KOH electrolyte
solution at room temperature (RT). Figure 4a shows the comparative cyclic voltammetry (CV) curves of (i) Cu(OH)2 NWAs/
Cu fibers, (ii) CNTs@Cu(OH)2 NWAs/Cu fibers, (iii) NiO NSs/
Cu fibers, (iv) NiO NSs@CNTs@CuO NPs/Cu fibers, (v) NiO
NSs@CuO NWAs/Cu fibers, and (vi) NiO NSs@CNTs@CuO
NWAs/Cu fibers at a constant scan rate of 20 mV s−1 with a
potential window of 0–0.55 V, respectively. From the CV curves,
it is evident that the pair of redox peaks can be observed in all
the samples, which mainly originated from the Faradaic reactions of samples in KOH solution. Meanwhile, it is noticeable
that the CV curves of (i) Cu(OH)2 NWAs/Cu fibers and
(ii) CNTs@Cu(OH)2 NWAs/Cu fibers exhibited the relatively
low current response/CV integral area compared to the one
grown with NiO NSs. This indicates that the NiO NSs are more
electrochemically active than the CuO NWAs. In addition, the
Adv. Energy Mater. 2017, 1702201
(iii) NiO NSs/Cu fibers (i.e., sample prepared without CNTs
and CuO NWAs) exhibited clear redox peaks (originated by Faradaic redox chemistry of Ni2+ ↔ Ni3+ in KOH electrolyte), but
the peak current response and CV enclosed area of the sample
were much lower than those of the (v) NiO NSs coated CuO
NWAs/Cu fibers and (vi) NiO NSs@CNTs@CuO NWAs/Cu
fibers. This is attributed to the compact layer of material coated
on pristine Cu fibers (see FE-SEM images of Figure S2, Supporting Information). The compact layer of the material is
unfavorable for fast diffusion of electrolyte ions and electron
transportation during electrochemical reactions, which leads to
the lower electrochemical capacity. In the same time, when we
see the redox behavior and/or CV integral area of (iv) NiO
NSs@CNTs@CuO NPs/Cu fibers, (v) NiO NSs@CuO NWAs/
Cu fibers, and (vi) NiO NSs@CNTs@CuO NWAs/Cu fibers,
the forest-like NiO NSs@CNTs@CuO NWAs/Cu fibers exhibited relatively higher response, demonstrating the substantial
enhancement of electrochemical capacity for the corresponding
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sample. This higher redox chemistry of the forest-like electrode
is mainly due to the vertically aligned CuO NWAs and CNTs,
which are helpful for increasing the electrochemical conductivity throughout the structure and favorable to coat high mass
for the NiO NSs, and provide large accessible area for electrolyte penetration. This would ultimately enhance the redox reactions of electrochemically active NiO NSs material in aqueous
electrolytes for higher energy storage performance. Besides, the
galvanic charge–discharge (GCD) analysis was also tested to
confirm the high capacity performance of forest-like NiO NSs@
CNTs@CuO NWAs/Cu fibers in comparison with other samples. Figure 4b shows the GCD curves of the (i) Cu(OH)2
NWAs/Cu fibers, (ii) CNTs@Cu(OH)2 NWAs/Cu fibers, (iii)
NiO NSs/Cu fibers, (iv) NiO NSs@CNTs@CuO NPs/Cu fibers,
(v) NiO NSs@CuO NWAs/Cu fibers, and (vi) NiO NSs@
CNTs@CuO NWAs/Cu fibers at a constant charge–discharge
current of 0.96 mA. All the GCD curves exhibited a typical battery-type behavior, which is distinct from that of EDLC materials characterized by inverted “V” shape of GCD plots. From
the GCD curves, it is clear that the charge–discharge times of
forest-like NiO NSs@CNTs@CuO NWAs/Cu fiber electrode
were evidently greater than other samples, which is consistent
with the results in CV curves of Figure 4a. Based on loading
mass of active materials, the corresponding gravimetric specific
capacity values at a discharge current of 0.96 mA were estimated to be 33.52, 39.23, 61.42, 94.93, 175.01, and
230.48 mA h g−1 for the (i) Cu(OH)2 NWAs/Cu fibers,
(ii) CNTs@Cu(OH)2 NWAs/Cu fibers, (iii) NiO NSs/Cu fibers,
(iv) NiO NSs@CNTs@CuO NPs/Cu fibers, (v) NiO NSs@CuO
NWAs/Cu fibers, and (vi) NiO NSs@CNTs@CuO NWAs/Cu
fibers, respectively. Obviously, the forest-like nanoarchitectures
of NiO NSs@CNTs@CuO NWAs/Cu fibers with ≈0.48 mg
mass loading exhibited the highest capacity (Figure 4c). Thus, it
is worth mentioning that the loading mass and electrochemical
accessible surface area of hierarchical electrode are favorable to
enrich the specific capacity. More detailed electrochemical properties of forest-like NiO NSs@CNTs@CuO NWAs/Cu fibers
were measured with CV and GCD measurements to understand the electrochemical activity, rate capability, and Faradaic
efficiency. Figure 4d shows the CV curves of the forest-like NiO
NSs@CNTs@CuO NWAs/Cu fibers at different scan rates of
5–50 mV s−1. With the increase of scan rate, a pair of redox
peaks were observed with ascended peak currents values, suggesting the good reversibility and rapid reversible electrochemical reactions of the hierarchical material. Meanwhile, the
anodic and cathodic peak potentials shifted in a more anodic
and cathodic direction with the increase of scan rate. This indicates the good ion diffusion rate and lower resistance of the
material during the electrochemical redox reaction. Figure 4e
shows the linear relationship of the square root of the scan rate
and redox peak current of the hierarchical electrode, verifying
that the oxidation and redox reaction at the electrode–electrolyte
interface is initiated by a quasireversible and diffusion-controlled process. The forest-like NiO NSs@CNTs@CuO NWAs/
Cu fiber sample was further tested with GCD curves measured
at various discharge currents of 0.96–12 mA in 1 m KOH electrolyte solution. From the GCD plateaus, as displayed in
Figure 4f, the consistent battery-type redox behavior and symmetric charge–discharge time further confirm the good
Adv. Energy Mater. 2017, 1702201
reversibility and Faradaic efficiency of the material. The calculated specific capacity values as a function of discharge current
are plotted in Figure 4g. The maximum gravimetric specific
capacity of the corresponding material can reach up to
230.48 mA h g−1 at a discharge current of 0.96 mA and it was
retained up to 196.11 mA h g−1 at 4.8 mA with a capacity retention of 82.08%. To our surprise, the 76.83% of capacity was
retained at a high discharge current of 12 mA, signifying that
the coaxial electrode exhibited the superior rate capability even
at high discharge currents. The specific capacity values of NiO
NSs/Cu fibers (61.42 and 42.26 mA h g−1 at the discharge currents of 0.96 and 12 mA), NiO NSs@CNTs@CuO NPs/Cu
fibers (94.93 and 68.95 mA h g−1 at the discharge currents of
0.96 and 12 mA), NiO NSs@CuO NWAs/Cu fibers (175.01 and
141.66 mA h g−1 at the discharge currents of 0.96 and 12 mA),
and the electrochemical data of the corresponding materials are
also presented in Figures S3–S5 (Supporting Information),
respectively. As explained in Figure 1, the braiding process of
Cu fibers allows for the forest-like NiO NSs@CNTs@CuO
NWAs with high mass loading compared to the single Cu fiber
electrode and helps to improve the electrochemical performance. The comparative electrochemical properties of forestlike composite material grown on braided and solitary Cu fiber
electrodes were included in Figure S6 (Supporting Information), which clearly demonstrates that the braiding process of
Cu fibers is beneficial to improve the loading mass of active
material as well as the electrochemical properties. Furthermore,
long-term cycling performance is also another key factor in
determining the practical applicability of forest-like NiO NSs@
CNTs@CuO NWAs/Cu fiber electrode for FHSC. Accordingly,
the cycling stability of the corresponding sample was evaluated
by repeating GCD measurements in three-electrode system at a
constant charge–discharge current of 4.8 mA, as shown in
Figure 4h. After 2000 cycles, the forest-like NiO NSs@CNTs@
CuO NWAs/Cu fiber sample maintained the capacity retention
of 82.72% with a good Faradaic efficiency (98.2%), indicating an
excellent electrochemical stability of the material. Furthermore,
electrochemical impedance spectroscopy (EIS) analysis of the
forest-like NiO NSs@CNTs@CuO NWAs/Cu fibers before and
after cycling test was also measured in a frequency range of
100 kHz to 0.01 Hz at the alternating current voltage amplitude
of 5 mV (vs open circuit potential), as shown in the inset of
Figure 4h. The Nyquist plots exhibited a sloped line in the low
frequency region corresponding to the ion diffusion resistance
and a small circle in the high frequency region related to the
charge-transfer resistance (Rct) induced by the redox reactions
at the interface between the electrode and electrolyte. The Rct
values before and after cycling performance of the material
were obtained as 0.41 and 0.67 Ω, respectively, which once
again demonstrates a stable electrochemical stability. The
higher specific capacity, rate capability, and cycling stability of
the forest-like NiO NSs@CNTs@CuO NWAs/Cu fiber electrode during the electrochemical measurements are mainly
attributed to its synergic features that result from the hierarchical structure, as schematically annotated in Figure 4i. At
first, the abundantly available and low-cost Cu fibers provide
the high conductivity and flexibility, which is beneficial to reduce
the cost of electrodes. Second, the lack of binders and freestanding growth of NiO NSs@CNTs@CuO NWAs forest-like
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structures on Cu fibers significantly reduce the “dead volume”
and improve the electrochemical activity in the material. Third,
the vertically aligned CuO NWAs which are surrounded with
CNTs improve the mass loading and the conductivity of material and act as the electron “superhighways” to promote the
ion/electron transfer rate. Fourth, the interconnected arrangement of NiO NSs adhered on CNTs@CuO NWAs/Cu fibers
with high specific surface provides numerous pathways for
electrolyte penetration. As a result, most of the active material
would be available for rapid redox reactions with electrolyte
ions, leading to the enhancement of energy storage properties.
Apart from the synergistic effect of hierarchical structure for
improvement of energy storage, the abundantly available electrodes and methods to prepare the rationally designed nanoarchitecture are also facile and eco-friendly, and can be used in
various flexible/wearable energy storage applications.
Although the forest-like NiO NSs@CNTs@CuO NWAs/Cu
fiber electrode was observed to exhibit high specific capacity
and rate capability, the small potential window (0–0.55 V) of the
material limits its practicable applicability as an electrochemical
device for high energy and power density. Therefore, we fabricated hybrid FHSC with AC@CF as the negative electrode
(-1.0 to 0 V) and forest-like NiO NSs@CNTs@CuO NWAs/
Cu fibers as the positive electrode (0–0.55 V) to enlarge the
potential window (fairly about 1.55 V), as shown in Figure S7
(Supporting Information). The electrochemical properties of
the AC@CF sample measured in three-electrode system are
included in Figure S8 (Supporting Information), where nonFaradaic behavior was clearly evidenced. The collective combination of battery-type material (as the energy source) and EDLC
material (as the power source) could be expected to increase the
energy and power density owing to their high potential window
and capacitance. Meanwhile, the FHSC could also provide
several advantages rather than the planar SC, such as small
volume, high flexibility, and possibility for use in wearable
applications. Herein, the FHSC was assembled by balancing
the electrode mass to ensure the maximum energy storage performance of the FHSC based on the following relationship[11b]
−
m + CSC
× ∆V −
=
−
Q+
m
(1)
where m+ is the mass and Q+ is the specific capacity of the
−
positive electrode, and ∆V −, m−, and CSC are the working
voltage, mass, and specific capacitance of the negative electrode, respectively. After mass balancing, the estimated mass
loading ratio of positive to negative electrodes is optimized to
be 0.282. Figure 5a shows the schematic diagram of the fabrication process of solid-state HFSC, using the forest-like NiO
NSs@CNTs@CuO NWAs/Cu fibers and AC@CF electrodes
with a polyvinyl alcohol (PVA)-KOH gel electrolyte. To avoid the
absorbing moisture and contact of gel electrolyte with human
skin, the device was inserted into the heat shrinkable plastic
pipe. Herein, 7 cm in length of the solid-state FHSC was fabricated for electrochemical measurements. To validate the working
potential of the HFSC, the CV and GCD curves operated at different working potentials with a constant scan rate and charge–
discharge current, respectively (Figure 5b,c). When the potential
is extended from 0 to 0.8 V and 0 to 1.55 V, interestingly, there
Adv. Energy Mater. 2017, 1702201
is no obvious of H2/O2 evolution in the CV plots and no overcharging region in the GCD plots was noticed, signifying the
good electrochemical operation stability and maximum potential
window of the device (1.55 V). Such high potential window of
this solid-state FHSC is also wider/comparable to the previous
reports based on wire/fiber-type hybrid SCs, such as Ni3S2/Ni //
pen ink-based wire-type SC (1.4 V),[20] fiber-based β-Co(OH)2//
CoFe2O4@rGO@PANI solid-state SC (1.5 V),[11a] and fiber-based
Co3O4//graphene SC (1.5 V),[21] respectively. Figure 5d shows the
CV curves of the FHSC tested at various scan rates of 5–70 mV
s−1 with a stable potential window of 0–1.55 V. Distinctive from
the solid redox peaks observed in three-electrode system, the CV
curves of the fabricated FHSC exhibited the excellent capacitive
behavior due to the inclusion of EDLC material. With increasing
the scan rate, as shown in CV curves, the current response also
distinctively increased, signifying the good I–V response of the
device. The GCD analysis of the solid-state FHSC was also carried to estimate the maximum capacitance, energy density, and
power density values. Figure 5e displays the GCD curves of the
FHSC at different charge–discharge currents of 0.7–9 mA. It
is noticeable that all the GCD curves exhibited the symmetric
charge–discharge times, further indicating the good rate capability and columbic efficiency of the device. The calculated specific capacitance and length capacitance of the FHSC are plotted
in Figure 5f. The corresponding specific capacitance and length
capacitance of the FHSC as of function of discharge current
were obtained to be 93.42, 91.18, 87.2, 83.57, 77.37, 71.15, and
64.63 F g−1 and 29.09, 28.39, 27.16, 26.02, 24.09, 22.33,
and 20.13 mF cm−1 at a discharge current of 0.7, 1, 1.5, 2, 3, 4,
and 5 mA, respectively. Moreover, the energy density and power
density resulting from the GCD curves were plotted in Ragone
diagram (Figure 5g). At a discharge current of 0.7 mA, the device
exhibited a maximum energy density of 26.32 W h kg−1 with a
power density of 219.03 W kg−1. The obtained energy density of
our device is higher or comparable with those of the previously
reported fiber-based solid-state SCs, such as NiCo2O4//PC (6.61
W h kg−1),[22] Ni3S2/Ni//pen ink (8.2 W h kg−1), β-Ni(OH)2//AC
(9.8 W h kg−1),[23] NiCo2O4//pen ink (7.66 W h kg−1),[24] meshtype CuO@MnO2//activated graphene (29.9 W h kg−1),[25] and
MnO2//graphene (27.2 W h kg−1),[26] respectively and some
planar SCs, such as Cu2O/CuO/Co3O4//activated graphene
(12 W h kg−1),[27] CuO//AC (19.7 W h kg−1),[28] and core–shell
Ni3S2@CoS//AC (23.69 W h kg−1),[29] and MnFe2O4//LiMn2O4
(5.5 W h kg−1),[30] respectively. Furthermore, as the discharge
current increased from 0.7 to 5 mA, the device showed a maximum power density of 1218.33 W kg−1 with the energy density
of 11.05 W h kg−1, respectively. The cycling performance of the
solid-state FHSC was measured through a GCD process at 3 mA
up to 2000 cycles. As shown in Figure 5h, the capacitance retention of the device still remained about 83.6% to the initial cycle
capacitance after 2000 cycles, suggesting the excellent energy
storage capability of the solid-state device.
The photographic image of the fabricated solid-state FHSC
protected with heat shrinkable tube is shown in Figure 6a. In
view of wearable and portable electronic applications, highly
flexible energy storage devices, which are working under various bending positions, are crucial. In order to verify the flexibility of the fabricated solid-state FHSC, a series of CV analyses
under different bending positions are performed with a
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Figure 5. a) Schematic diagram showing the fabrication process of PVA-KOH gel electrolyte-coated electrodes and assembly of FHSC. b) CV and
c) GCD curves of the FHSC operated at different potentials with a constant scan rate and current, respectively. d) CV curves, e) GCD curves, and
f) specific/length capacitance values. g) Ragone plot and h) cycling stability of the NiO NSs@CNTs@CuO NWAs/AC hybrid device.
constant scan rate of 30 mV s−1 (Figure 6b). The CV shapes were
almost the same and no noticeable deterioration under normal,
flexed (i–iv), and recovered conditions was observed, indicating
the stable performance of the device. After flexed (in the similar
way like photographic images in the inset of Figure 6c) at various positions, the recovered device still exhibited a capacitance
retention of 98.9% (Figure 6c). Besides, the effect of bending
times on the capacitance performance of FHSC is presented
in Figure S9 (Supporting Information). Due to the flexibility of
all the components in FHSC, the device retained its maximum
capacitance of 94.5% after continues bending cycles (up to 200
times). These results prove that the good mechanical stability
can be possible to be applied in wearable electronic devices.
Accordingly, two serially connected solid-state FHSCs were
easily woven into a human shirt to operate the light-emitting
diode (LED) and electronic display, as shown in Figure 6d,g.
The as-serially connected two FHSCs were charged at a current of 2 mA up to 3.1 V and the stored energy is utilized to
glow the red LED, as shown in the photographic images of
Adv. Energy Mater. 2017, 1702201
Figure 6e,f, and it can still give the light continuously for 5 min
(see Movie S1, Supporting Information). More importantly,
the two serially connected FHSCs woven into the human cloth
can also drive the multifunction electronic display which has
digital humidity and temperature monitor with alarm clock
(as presented in Figure 6h,i for about 20 min constantly (see
Movie S2, Supporting Information). These results collectively
suggest that the high energy storage and good mechanical stability of the fabricated solid-state FHSC with forest-like NiO
NSs@CNTs@CuO NWAs/Cu fibers and AC@CF electrodes
could be used as an effective power source in various wearable
electronic applications.
3. Conclusion
In summary, we obtained Cu fibers, which are peeled out from
waste cable wires for low-cost current collector in FHSC. The
Cu fibers also act as a sacrificial template for the growth of
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Figure 6. a) Photographic image of FHSC, b) CV curves, and c) capacitance retention values of the FHSC under different bending conditions measured at a constant scan rate. d–i) Two serially connected FHSC powering LED and electronic display, showing its potential applicability for wearable
applications.
Cu(OH)2 NWAs without use of any Cu-based chemical salts.
After coating of CNTs, the electrical conductivity of CNTs@
Cu(OH)2 NWAs/Cu fibers is increased, which provides the
rapid pathways for electron transfer and increases the active
material loading. Then, the forest-like NiO NSs@CNTs@CuO
NWAs/Cu fibers were grown by a simple wet-chemical method
followed by calcination treatment. As a battery-type electrode
material, the rationally designed forest-like NiO NSs@CNTs@
CuO NWAs/Cu fibers enabled a maximum specific capacity of
230.48 mA h g−1 at a discharge current of 0.96 mA and it was
retained up to 196.11 mA h g−1 at 4.8 mA with a capacity retention of 82.08% in aqueous electrolyte. To our surprise, 76.83%
of capacity still remained at a high discharge current of 12 mA,
signifying that the coaxial electrode exhibited the superior rate
capability even at high discharge currents. Furthermore, twoelectrode system-based fiber-type hybrid SC was assembled
using the forest-like NiO NSs@CNTs@CuO NWAs/Cu fibers
as a positive electrode and AC@CF as a negative electrode with
Adv. Energy Mater. 2017, 1702201
a PVA-KOH gel electrolyte. The device exhibited the superior
electrochemical performance, including high capacitance, high
energy, and high power density. With the advantage of flexibility and high energy storage performance, the solid-state
fibrous device designed in this work is expected to be suitable
for integration into human cloths, providing a route for wearable energy storage systems.
4. Experimental Section
Materials and Chemicals: Ammonium persulfate ((NH4)2S2O8),
nickel sulfate hexahydrate (NiSO4·6H2O), and polyvinyl alcohol (PVA,
C4H6O2)n) were purchased from Sigma Aldrich, Corp, South Korea. Urea
(NH2CONH2), sodium hydroxide (NaOH), and potassium hydroxide
(KOH) were received from DaeJung Chemicals, Ltd, South Korea. CNTs
water dispersion (OD: 5–15 nm, length: 50 µm) was purchased from US
Research Nanomaterials, Inc., USA. Carbon fibers were received from
Cetech Co., Ltd. South Korea. Waste cable wires used in this experiment
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were collected from our laboratory store room. All the chemicals were
of analytical grade purity and were used as received without any further
purification.
Synthesis of Cu(OH)2 NWAs/Cu Fibers: Vertically aligned and 1D
Cu(OH)2 NWAs were synthesized on Cu fibers at RT via an alkaline
oxidative etchant solution-based dipping method. Prior to the growth
process, the collected waste cable wires were stripped properly by using
a wire stripper to remove the outer layer of plastic/insulating cover. The
as-obtained Cu fibers (three) were braided into the form of human hair
and were cleaned using 1 m HCl solution followed by de-ionized (DI)
water to remove native oxide layer. The braided Cu fibers were then
prepared with a length of 8 cm, where the 7 cm length was used for
the active material growth and another 1 cm was covered with a plastic
tape for electrode contact, respectively. Consequently, these fibers were
dipped into the freshly prepared alkaline oxidative etchant solution
containing 20 ml of 2.5 m NaOH and 20 mL of 0.1 m (NH4)2S2O8 at
ambient temperature for about 20 min. During this immersing process,
the surface color of the braided Cu wires was turned gradually to blue
from brown (indicative of the growth of Cu(OH)2 NWAs). Subsequently,
the blue colored samples were taken out from the solution bath and
gently rinsed with DI water to remove the residual chemicals, followed
by drying with a flow of nitrogen (N2) gas at RT.
Fabrication of Cu(OH)2 NWAs@CNTs/Cu Fibers: The decoration of
CNT network on Cu(OH)2 NWAs/Cu fibers was conducted by a simple
dip coating approach. At first, the water diluted CNT dispersion was
ultrasonicated for 30 min to separate the congregated CNTs. Then, the
Cu(OH)2 NWAs/Cu fibers were dipped into the CNT suspension and
kept in shaking water bath (Jeio Tech Co. Ltd.) for about 5 min to entirely
coat it over Cu(OH)2 NWAs. Eventually, the obtained Cu(OH)2 NWAs@
CNTs/Cu fibers were rinsed with DI water and dried at 60 °C.
Preparation of Forest-Like NiO NSs@CNTs@CuO NWAs/Cu Fibers:
Hierarchically structured NiO NSs were facilely decorated on CNTs@
CuO NWAs/Cu fibers via a low-temperature-based oil bath method and
pyrolysis process, respectively. In a typical synthesis, the growth solution
was prepared by mixing 0.07 m of NiSO4·6H2O and 0.35 m of NH2CONH2
in 40 mL of DI water under stirring at RT for 10 min. Afterward, the
growth solution was transferred into a Pyrex screw cap beaker with
the CNTs@Cu(OH)2 NWAs/Cu fibers and kept in oil bath at 90 °C for
50 min, followed by cooling down to RT. After the growth process, the
samples were taken out, washed several times with DI water, and dried
under the flow of N2 gas. Finally, the dried samples were calcined at
300 °C for 2 h to obtain the forest-like NiO NSs@CNTs@CuO NWAs/
Cu fibers, which were then used as a positive electrode in fiber-type SCs.
The mass loading of the active material (i.e., NiO NSs@CNTs@CuO
NWAs) on Cu fibers was obtained to be 0.48 mg (≈0.0686 mg cm−1). To
decipher the importance of CuO NWAs and CNTs in the improvement
of electrochemical performance, the growth of NiO NSs on pristine
Cu fibers (NiO NSs/Cu fibers) and NiO NSs@CuO NWAs/Cu fibers
(without CNTs) was also performed. Moreover, we also synthesized CuO
nanoparticles (NPs) on Cu fibers via thermal annealing (300 °C for 2 h),
and then CNTs and NiO NSs over CuO NPs/Cu fibers were grown
using the similar procedure as mentioned above. The corresponding
sample was designated as film-like NiO NSs@CNTs@CuO NPs/Cu
fibers. In such a way, the importance of vertically aligned CuO NWAs for
electrochemical performance was elaborated clearly.
Preparation of Gel Electrolyte and Fabrication of FHSC: PVA-KOH
gel electrolyte was prepared to fabricate the solid-state FHSC. For the
gel electrolyte, 2 g PVA was dissolved in 20 mL of DI water at 80 °C
with continuous stirring until a clear viscous solution was obtained.
Subsequently, 2 g of concentrated KOH solution was added dropwise
to the above viscous solution under stirring. The mixture solution was
then kept at the same temperature until a clear gel-like solution was
obtained. After cooling to RT, both the electrodes, including NiO NSs@
CNTs@CuO NWAs/Cu fibers as a positive electrode and AC@CF as a
negative electrode (the preparation process of AC@CF was mentioned
in Section S(2) in the Supporting Information) were immersed into the
PVA-KOH gel electrolyte for 10 min to diffuse the gel electrolyte into the
interior parts of active materials and hung on hanger for 20 min to dry
Adv. Energy Mater. 2017, 1702201
the electrodes. This immersing process was repeated two or three times
to completely cover the gel electrolyte over the surface of electrodes.
After that, the two electrodes were placed together, dipped into the
PVA-KOH gel electrolyte, and then solidified at RT overnight to remove
the excess water in the electrolyte. Finally, the device was inserted into
the heat shrinkable pipe and the tube was shrunken with hot air sealing
to protest the device with dust particles and/or to avoid the PVA-KOH
contact with human skin and cloths. The working length of FHSC was
about 7 cm, respectively.
Characterizations: The elemental analysis and morphology of the
prepared samples were observed by using a FE-SEM (Carl Zeiss,
LEOSUPRA 55) equipped with the energy dispersive X-ray spectrometer
and a TEM (JEM 200CX, JEOL). The crystalline properties of the samples
were measured by XRD analysis (M18XHF-SRA, Mac Science) with Cu
Kα radiation (λ = 0.154 nm) at an angular speed (2θ) of 5° min−1. Raman
analysis of the sample was carried on inVia HR-Raman spectrometer,
with 514 nm laser excitation. The chemical compositions and valence
states of the prepared sample were examined by XPS (Thermo Electron
MultiLab2000) with Al Kα radiation.
Electrochemical Measurements: Electrochemical properties, including
CV, GCD, and EIS were measured by using a computerized
electrochemical workstation (Iviumstat, The Netherlands) at RT. For
three-electrode system, the electrochemical properties were investigated
using the synthesized samples as a working electrode and Pt wire as
a counter electrode and Ag/AgCl electrode as a reference electrode,
respectively in a beaker-type cell consisting of aqueous 1 m KOH
electrolyte. For the two-electrode system, the as-fabricated FHSC with
solid-state electrolyte was utilized for electrochemical measurements.
The specific capacity (QSC, mA h g−1), specific capacitance (CSC,
F g−1), length capacitance (CLC, mF cm−1), energy density (E, W h kg−1),
and its corresponding power density (P, W kg−1) for three-electrode
system and FHSC were calculated using the following formulae given
below[4b,11b,22]
Q SC =
I × ∆t
m × 3.6 C SC =
I × ∆t
m × ∆V
(2)
and C LC =
I × ∆t
L × ∆V (3)
E=
C SC × ( ∆V )2
7.2
(4)
P=
E
× 3600
∆t
(5)
Here, I (A) is the discharge current, ∆t (s) is the discharge time, m (g) is
the mass of the active material, ∆V (V) is the potential, and L (cm) is the
length of the electrode/device, respectively.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
This work was supported by the National Research Foundation of Korea
(NRF) grant funded by the Korea government (MSIP) (Grant Nos.
2017R1A2B4011998 and 2017H1D8A2031138).
Conflict of Interest
The authors declare no conflict of interest.
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Keywords
copper fibers, energy density, fiber-based supercapacitors, forest-like
composite materials, waste cable wires
Received: August 10, 2017
Revised: September 4, 2017
Published online:
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