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PAPER
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Cite this: J. Mater. Chem. A, 2021, 9,
4841
Laser-assisted fabrication of flexible monofilament
fiber supercapacitors†
Phuong Thi Nguyen,a Jina Jang,a Yoonjae Lee,a Seung Tae Choib
and Jung Bin In *ac
Fiber supercapacitors (SCs) are potential and promising candidates for the development of wearable
lightweight energy storage units. Thus, investigations have been conducted on various aspects such as
electrode materials and device configuration to enhance their performance. The representative types of
fiber SC configurations are characterized by certain limitations, including the imprecise assembling
process and increased resistance of the SCs. To address these limitations, herein, we report a laserassisted fabrication method by which active electrodes, current collectors, electrolyte, and a flexible
polymer support can be integrated into a monofilament fiber-type SC. Instead of combining two
individual electrode fibers, two separate microscale current collectors and active electrodes were
implemented on a polymeric monofilament surface by exploiting the micromachining capability of laser
processing. Metallic current collectors were printed on a polyvinylidene fluoride fiber by selective laser
sintering of Ag nanoparticles and nanowires. This highly conductive layer enabled reduction in the
equivalent series resistance, leading to increased specific capacitance. The current collector was coated
with a graphene layer, and a thin layer of pseudocapacitive MnO2 nanostructures was deposited onto the
graphene layer to further improve the specific capacitance of the final fiber SC. The monofilament fiber
SC exhibited a specific capacitance of 24.5 mF cm2 at 0.1 mA cm2 in a PVA–Na2SO4 electrolyte, along
with excellent mechanical flexibility (94.3% capacitance retention in 3000 bending cycles at a bending
radius of 7.5 mm). In particular, this laser-assisted electrode patterning method enabled the fabrication
Received 21st October 2020
Accepted 8th January 2021
of serially connected SCs within a seamless monofilament unit, without post-fabrication assembly
DOI: 10.1039/d0ta10283k
This research suggests the development of an unconventional structure for fiber SCs, which is expected
rsc.li/materials-a
to be highly beneficial for promising flexible/wearable electronics applications.
processing. Thus, the operation of a highly flexible fiber SC was demonstrated in a wide voltage window.
1. Introduction
The rapid growth of the portable and wearable device market
has spurred the development of wearable lightweight energy
storage units, having high electrochemical performance, tiny
volume, and high mechanical exibility. Fiber supercapacitors
(SCs) are promising candidates to fulll this demand owing to
their lamentous structure with diameters generally less than 1
mm. They can be woven into a wearable functional textile to
power the embedded microdevices, such as the internet of
things (IoT) wearable sensors for bio-signals.1,2 In an
a
So Energy Systems and Laser Applications Laboratory, School of Mechanical
Engineering, Chung-Ang University, Seoul, 06974, Republic of Korea. E-mail: jbin@
cau.ac.kr
b
Functional Materials and Applied Mechanics Laboratory, School of Mechanical
Engineering, Chung-Ang University, Seoul, 06974, Republic of Korea
c
Department of Intelligent Energy and Industry, Chung-Ang University, Seoul 06974,
Republic of Korea
† Electronic supplementary
10.1039/d0ta10283k
information
(ESI)
available.
This journal is © The Royal Society of Chemistry 2021
See
DOI:
application stage, this functional textile can be attached onto
a human body tightly but comfortably, thus enabling the user to
take care of his/her health efficiently with remote aid.
Various studies have been conducted to improve the
performance of ber SCs in terms of electrode materials, electrolyte, and device conguration.3 Among them, the development of novel electrode materials, with large surface area and
reversible redox activity, has been mainly pursued. Recently,
however, researchers have also realized the importance of
electrical conductance of the ber SC electrode.4–6 They have
reported that the performance of a ber SC can signicantly
drop with increasing length because of its extremely high aspect
ratio and the resulting large internal resistance.7 This problem
can be mitigated either by developing an active electrode with
high intrinsic conductivity, or by introducing a highly conductive current collector into the ber SC. Related to the former, allcarbon ber electrodes have been developed, consisting of
carbon microlaments,8 carbon nanotubes (CNTs),9 graphene
(Gr),10–12 or carbon-based composites.13,14 The fabricated allcarbon ber SCs exhibited excellent electrochemical
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performance, but their electrical conductivities were not as high
as those of metals. For the latter approach, ultrathin metal
bers15,16 or metal-coated polymer bers17,18 can be adopted as
brous current collectors. The successful incorporation of such
metallic elements into ber SCs and the consequent high-power
performance have recently been reported.6
In parallel with the above material issues, the device
conguration to construct a ber SC is an important factor that
affects not only the electrochemical performance but also the
mechanical properties of the SC.19 Three representative types of
ber SC congurations have been extensively investigated:
parallel structure,15,17,20–25 twisted two-ply structure,8,26–28 and
coaxial structure.29–32 The parallel- or twisted-type ber SC
consists of two separate ber units, which correspond to the
anode and cathode. Although these types are relatively well reported, they require the additional step of combining the
individual anode and cathode bers, which critically complicates the assembling process. Moreover, under mechanical
bending, the two electrodes can get separated due to uneven
strain distribution between them. This can cause serious
problems, such as signicant increase in internal resistance
and even failure of the SC. In this respect, the coaxial structure
is advantageous because in this, unlike the parallel or twisted
type, all the SC components, including a core electrode, a gel
electrolyte, and an outer electrode, are integrated into a single
compact ber. However, its fabrication generally involves layerby-layer coating of the cylindrical ber with different materials,
which is not easily controllable for microscale layers.3
To realize such all-in-one ber SCs in a precise manner,
a laser-based electrode patterning technique can be a plausible
approach. Hu et al. pioneered this method and developed an
unconventional all-in-one ber SC, in which two separated
electrodes are rationally integrated into a single graphene oxide
(GO) ber.33 Reduced graphene oxide (rGO) electrode lines were
patterned on the GO ber surface in the longitudinal direction
by exploiting area-specic reduction of GO by a laser.34 However,
despite their great exibility, these all-in-one ber SCs exhibited
only moderate capacitance (1.2–2.4 mF cm2 at 80 mA cm2),
possibly due to the low electrical conductance of the rGO electrode. As discussed above, this large internal resistance can
cause serious performance deterioration for long-ber SCs.
Fig. 1
Paper
In this study, laser microfabrication has been extensively
exploited to develop exible monolament ber SCs, in which
two separate electrodes are patterned on a microscale. For
active charge-storage materials, thin layers of Gr and MnO2 were
coated on the surface of a core polymer ber. The relatively low
conductivity of the electrode was overcome by introducing
exible metallic current collectors beneath the electrode layer.
This ber SC exhibited signicantly decreased internal resistance, high capacitance, and excellent exibility. Moreover, to
the best of our knowledge, multiple SC units were seamlessly
integrated into a single ber SC for the rst time, with no need
for external terminal connection between individual ber SCs.
This novel architecture enables a compact ber SC to power
a microdevice operated at high voltages.
2.
2.1
Results and discussion
Fabrication of monolament supercapacitors
Fig. 1 depicts the fabrication process of the monolament ber
SC. A polyvinylidene uoride (PVDF) monolament ber
(diameter: 300 mm) was used as the core material. Laserinduced sintering of Ag nanoparticles (AgNPs) was adopted to
produce highly conductive current collectors on the PVDF
surface.35 The ber was coated with AgNPs and Ag nanowires
(AgNWs) and irradiated with a continuous-wave laser beam
along its centerline. The AgNWs were added to enhance the
exibility of the Ag layer.36 This laser irradiation induced siteselective sintering of the AgNPs/AgNWs and increased adhesion of the Ag layer to the underlying ber surface. Thus, when
the ber was rinsed in deionized (DI) water using a bath sonicator, the unaffected AgNPs/AgNWs were washed out to obtain
the line-type AgNP/AgNW current collectors. Based on this
printing method, two symmetric current collectors were
prepared for the cathode and anode. Fig. 2a shows the optical
microscopy image of the Ag current collector line (linewidth:
120 mm), as well as the eld-emission scanning electron
microscopy (FE-SEM) image of the sintered AgNPs/AgNWs,
which exhibits the structural connectivity between the
nanomaterials.
The PVDF ber was selected because of its excellent chemical
resistance, high mechanical exibility, and compatibility with
Schematic of the monofilament fiber SC fabrication process.
4842 | J. Mater. Chem. A, 2021, 9, 4841–4850
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Fig. 2 Optical microscopy image and SEM image of a PVDF fiber with (a) a sintered AgNP/AgNW current collector line and (b) a AgNP/AgNW/Au
line. (c) Cross-sectional TEM images and EDS elemental maps of the AgNP/AgNW/Au layers. (d) Resistance comparison of 1 cm samples with
different current collector compositions (inset: a photograph showing the resistance measurement for the 1 cm-long AgNP/AgNW/Au line on
the PVDF fiber). (e) Cyclic bending test setup. (f) Resistance changes of AgNP and AgNP/AgNW samples (inset: bent state of the sample; the
yellow line represents the current collector line).
the laser-induced sintering process. It especially enabled
excellent adhesion of the sintered Ag layer to its surface. Even
under strong bath sonication (power: 100 W, frequency: 40 kHz,
tank size: 1.95 L), the sintered Ag layer remained almost the
same (veried by electrical measurements and microscopy
images). The laser-induced sintering method was adopted
because PVDF is thermally unstable at the onset temperature
for AgNP/AgNW sintering (150 C). The laser beam was swept
over the ber surface, instantaneously increasing the surface
temperature above the sintering point. However, the heataffected zone was limited near the surface, and thus, thermal
damage to the bulk ber could be avoided. In contrast,
annealing of the PVDF ber using a general electrical heater
resulted in serious deformation (Fig. S1 in the ESI†). Other
polymers can also be used as the core ber material, as
demonstrated in studies on laser-based printed electronics.37–39
In the next step, a thin layer of Au was introduced to cover
the AgNP/AgNW layer via electrodeposition because Ag is
vulnerable to corrosion by water-based electrolytes in the
voltage range for the SC operation, resulting in electrochemical
(EC) instability and rapid deterioration of the SC performance.7,40 The Au deposition was conrmed by the change in
color and micromorphology (Fig. 2b), and also by the energydispersive X-ray spectroscopy (EDS) elemental maps (Fig. S2†).
Fig. 2c shows the cross-sectional transmission electron
microscopy (TEM) images of the coated AgNP/AgNW/Au layers
and the EDS elemental maps of the marked area. From the TEM
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image, the total thickness of the metal layers was measured to
be 595 nm. The EDS maps (Fig. 2c) reveal the Au deposition
thickness to be 40 nm.
The electrical conductance of the current collector was
improved sequentially with addition of the metallic materials.
Fig. 2d compares the electrical resistances of three samples,
each of which contained a single current collector line (length: 1
cm), composed of different composite components: AgNP,
AgNP/AgNW, and AgNP/AgNW/Au. The addition of AgNWs to
the AgNP layer decreased the resistance from 30.1 to 16.0
U cm1, and the following electrodeposition of Au further
reduced the resistance to 10.9 U cm1.
The added AgNWs not only increased the electrical
conductance but also improved the mechanical exibility of the
current collector. This effect was conrmed by the results of
a cyclic bending test. Fig. 2e shows that bending was repeatedly
applied to a 5 cm-long ber with a radius of 7.5 mm for 10 000
cycles, and the change in the electrical resistance of the ber
was measured in a at state to evaluate its structural integrity.
Fig. 2f shows the resistance changes for a AgNP-coated ber and
a AgNP/AgNW-coated ber. The resistance of the sintered AgNP
layer increased rapidly by over 40% in the rst 2000 cycles. Aer
that, the resistance changes asymptotically reached a plateau of
47%. This is attributable to the short-range connection
between the AgNPs, which can result in crack generation when
the AgNP layer is subjected to severe mechanical stress. In our
bending test, the crack was indeed generated, as shown in
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Fig. S3a (ESI†). On the other hand, the sintered AgNP/AgNW
ber exhibited an insignicant change in resistance: only
5% change in the rst 2000 cycles. The SEM image (Fig. S3b in
the ESI†) revealed that the crevice in the AgNP layer was bridged
by the long AgNWs, which provided shorter electrical conduction paths and compensated for conductance loss by crack
formation.
Aer patterning of the metal current collector, the ber was
entirely coated with the Gr dispersion via dip-coating and then
dried at 60 C for 1 h. This Gr layer additionally decreased the
electrical resistance of the ber and widened the electrode area.
To split the carbon electrode into anodes and cathodes, the Grcoated ber was suspended horizontally and scanned with the
nanosecond laser beam along its centerline. Fig. 3a and Movie
S1† show that the high-intensity pulsed laser induced ablation of
the irradiated Gr. For the selected laser scanning speed (0.2 mm
s1) and laser uence (1.3 J cm2), this ablation occurred not only
from the directly irradiated top surface but also from the bottom
surface even by a single sweep with the laser beam, producing
two laser cutting lines simultaneously due to the optical transparency of PVDF with respect to the laser wavelength (532 nm).
The incident laser pulses rst removed the irradiated Gr at the
top surface and then passed through the transparent PVDF
layer,41 ablating the Gr at the bottom surface. Fig. 3b shows that
the kerf widths of the laser cutting were 50 and 75 mm for the
top and bottom surfaces, respectively. The inset of Fig. 3b shows
an SEM image of the coated Gr layer. Notably, a low scanning
speed (0.2 mm s1) was selected because of the low pulse repetition rate (20 Hz) of the nanosecond laser used herein. Thus, if
a high-repetition pulse laser is adopted, the ablation process can
be even faster than that reported here.
Finally, a PVA-based solid-state electrolyte was applied to the
ber via dip-coating to achieve an all-solid-state monolament
ber SC. The cross-sectional SEM image captured at the interface area of the coated layers (Fig. 3c) shows that the thicknesses
of the Gr and the PVA electrolyte layers were estimated to be 5
and 7 mm, respectively. Considering the density of a thick Gr
layer (2.0 g cm3), the mass loading of the thin Gr layer was
estimated to be 82 mg per unit ber length (cm). This ber SC
is hereaer denoted as MG-SC, which is an abbreviation for the
ber SC fabricated with the metallic (AgNP/AgNW/Au) current
collector and Gr layer.
Paper
2.2 Electrochemical performance of the monolament ber
supercapacitor
2.2.1 Effect of metal current collectors. The effect of using
the AgNP/AgNW/Au current collector on the SC performance
was investigated by comparing the performances of the MG-SC
and the SC built with the Gr layer only (denoted as G-SC). Four
SCs with different lengths (1, 2, 3, and 4 cm) were prepared for
each case, and a gel-type PVA–H3PO4 electrolyte was used. The
dimensional characteristics of the Gr layer were the same for all
SCs. Fig. 4a and b show the CV (scan rate: 50 mV s1), CC
(current density: 0.05 mA cm2), and electrochemical impedance spectroscopy (EIS) data for the 1 and 4 cm long MG-SCs
and G-SCs. The data for the other SCs, with lengths of 2 cm
and 3 cm, are provided in Fig. S4 of the ESI.† The results indicate that the AgNP/AgNW/Au current collector effectively
increased the capacitance of the ber SC. For the same length,
MG-SCs exhibited a larger closed-loop area in CV (Fig. 4a) and
slower discharge in CC (Fig. 4b) than G-SCs, suggesting capacitance increase. In particular, the EIS result suggests that the
equivalent series resistance (ESR) dramatically decreased by
almost three orders of magnitude (Fig. 4c).
As the electrochemical contribution of the current collector
itself to the total capacitance was negligible, the increased
capacitance could be attributed to the decreased internal
resistance of the electrode, which was enabled by the AgNP/
AgNW/Au current collector. Fig. 4d shows the change in the
length-specic ESR (ESRL) value of the SCs with the increase in
the SC length. The ESR value was calculated from IRdrop of the
CC curve; it was observed that the ESR of the ber SC increased
with the ber length; however, the ESRL showed different
characteristics depending on the presence of the current
collector. While the ESRL for G-SCs increased with the SC
length, the ESRL for MG-SCs remained almost the same, indicating a linear increase in ESR with the ber length.
The effective role of the printed current collector enabled
consistent performance of the ber SC with respect to the
varying ber lengths. Fig. 4e shows the length-specic capacitances (CL) of the ber SCs that were calculated from their CV
data (scan rate: 50 mV s1). For G-SCs, the CL of G-SCs decreased
signicantly with an increase in the length. For instance, the
4 cm long G-SC (or G-SC-4) retained only 34% of G-SC-1's CL.
(a) Schematic of the laser cutting process. (b) Optical microscopy images of the laser cutting line on the top and bottom surfaces and SEM
image of the Gr layer (inset). (c) Cross-sectional SEM image of the graphene-coated fiber captured at the interface corresponding to the marked
area of the inset schematic.
Fig. 3
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(a) CV, (b) CC, and (c) EIS for G-1, G-4, MG-SC-1, and MG-SC-4 supercapacitors. (d) Length-specific ESR and (e) length-specific
capacitance values of the G-SC and MG-SC samples with the length varying from 1 to 4 cm.
Fig. 4
This performance deterioration with increased length is a critical issue of ber SCs recently reported by researchers.4–6,8
Although a planar SC was used, our group previously demonstrated the mitigation of this problem by incorporating a laserprinted metal grid layer into a Gr electrode.7 A similar approach
was adopted for ber-type SCs in this study, and the result was
in good agreement with the planar SC case. As shown in Fig. 4d,
the CL for MG-SCs was well retained; compared with G-SC-4
(34%), MG-SC-4 maintained 93% CL of MG-SC-1.
2.2.2 Capacitance enhancement by MnO2 deposition.
Despite the improved characteristics of the capacitance, the
specic capacitance values were still low (for MG-SC-1, only 34
mF cm1 or 0.84 mF cm2), possibly due to the limited capacitance of stacked graphene akes.42 Thus, to further increase the
capacitance, a thin layer of manganese oxide (MnO2) nanostructures was coated onto the Gr lm surface via electrodeposition. MnO2 has been extensively studied as a promising
electrode material for pseudocapacitors owing to its nontoxicity, low cost, and excellent pseudocapacitive charge
storage.43,44 The electrodeposition process was added to the
fabrication sequence described in Section 2.1. Between the Gr
coating and the laser ablation (cutting) processes. As the Ag
layer was protected by the Au layer, possible corrosion of Ag
could be avoided during the MnO2 deposition.7 Fig. 5 shows the
SEM image of the deposited MnO2 layer that exhibits sea
urchin-like nanostructures. The energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy spectra of the
MnO2 layer are provided in Fig. S5 of the ESI.† For the MnO2coated ber SC (denoted as MGM-SC), a gel-type PVA–Na2SO4
electrolyte was used.44–46
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A 3.5 cm long MGM-SC was fabricated, and its specic
capacitance was observed to be markedly higher than that of
MG-SCs. Fig. 6a–c show the CV (scan rate: 10–200 mV s1), CC
(current density: 0.1 to 1 mA cm2), and EIS data (frequency: 0.1
to 100 kHz) of the MGM-SC. The potential range was conned
within 0–0.85 V. The CC curves exhibit nearly linear symmetrical shapes with the current density ranging from 0.1 to 1 mA
cm2, indicating good reversibility of the device. The CA of this
ber SC was calculated from the discharge line of the CC curves.
Fig. 5 FE-SEM image of the MnO2 nanostructures deposited on the
graphene layer.
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Paper
Fig. 6 Electrochemical and mechanical characteristics of a 3.5 cm long MGM-SC. (a) CV curves obtained at a range of scan rates (10–200 mV
s1). (b) CC curves obtained at 0.1, 0.2, 0.5, and 1 mA cm2 (c) EIS curve for the frequency range from 0.1 to 100 kHz. (d) CA calculated from the
CC curves at various current densities and cyclic charge/discharge stability at a galvanostatic current density of 2 mA cm2. (e) Capacitance
retention obtained under bending at different curvatures. The photographs show the testing at bending diameters of 1 and 0.5 cm. (f)
Capacitance retention for 3000 bending cycles at a bending radius of 7.5 mm. The inset shows the CV curves (scan rate: 50 mV s1) obtained at
different cycle numbers.
The CA value was 24.5 mF cm2 at 0.1 mA cm2, and it
decreased to 20.8 mF cm2 at 1 mA cm2 (Fig. 6d). When the
MGM-SC was tested as a working electrode in a 3-electrode
conguration with an aqueous Na2SO4 electrolyte (0.5 M) for
comparison, its capacitance was markedly increased to 30.2 mF
cm2 at 0.1 mA cm2 (see Fig. S6†). This capacitance is significantly higher than that (1.2–2.4 mF cm2 at 80 mA cm2) of Hu
et al.‘s all-in-one ber SC that has a similar electrode conguration.33 As the neutral PVA electrolyte was used, the capacitance retention was high (over 95%) during 3000 cycles of
galvanostatic charging/discharging at 2 mA cm2 (Fig. 6d).
However, aer 10 000 cycles, the retention was reduced to
77% (see Fig. S7 in the ESI†).
Owing to the superior exibility of the current collector and
the active electrode nanomaterials, the MGM-SC also exhibited
excellent exibility; this characteristic of the MGM-SC was evaluated based on the capacitance change occurring when the ber
was subjected to various bending conditions. The inset of Fig. 6e
shows the capacitance retention ratio varying with bending
curvature. The capacitance of the MGM-SC was decreased by only
7.5% at the bending curvature of 4 cm1. In addition, cyclic
bending of the ber SC was performed 3000 times at a bending
radius of 0.75 cm, which is identical to the testing conditions
shown in Fig. 2e. The capacitance changes were negligible; as
shown in Fig. 6f, the MGM-SC retained 98% of its initial capacitance aer 1000 bending cycles and 94% aer 3000 cycles.
4846 | J. Mater. Chem. A, 2021, 9, 4841–4850
Due to the lack of a common protocol for bendability evaluation, it is difficult to directly compare bendabilities of
different ber SCs reported in the literature, based on the same
metric. However, although the electrode material was placed on
the outer surface of the ber with a relatively large diameter
(300 mm), the bendability of our ber SC is comparable or
potentially
superior
to
those
of
other
ber
SCs.5,9,12,15–17,20,24,32,33,46–52 For instance, Hu et al. reported the
capacitance decrease of their all-in-one ber SC (diameter: 50
mm) by almost 20% aer only 160 bending cycles at the same
bending radius (0.75 cm).33 Yu et al. demonstrated that the
parallel-type carbon nanotube–graphene ber SC (diameter of
a single electrode ber: 50 mm) supported on a PET lm
retained 97% of its initial capacitance aer 1000 bending
cycles;24 however, the bending angle was only 90 . Many reports
demonstrated bend testing up to 1000 cycles with good retention results (above 90%), but the information of the bending
radius was not specied.5,9,12,20,28,47,48,50 The detailed comparison
of the bendabilities is provided in Table S1 of the ESI.†
The ber SC structure developed in this study presents the
implementation
of
conventionally
planar
microsupercapacitors53–55 on a curved ber platform. For yarn-type
ber SCs and all-carbon ber SCs, the ber core generally
consists of porous active materials; thus, the electrolyte ions can
possibly adsorb to the entire ber core. In contrast, for the ber
SCs developed in our study, the active materials responsible for
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(a) Schematic comparison of the conventional method and the proposed method for serial connection of multiple SCs. (b) Schematic
showing the laser-based fabrication process for MGM-3SC-1 and its electrode structure, and microscopy image captured in the laser ablation
area. (c) CV curves obtained at 50 mV s1 and (d) CC curves obtained at 0.2 mA cm2 for MGM-SC-1 and MGM-3SC-1. (e) Photograph of MGM3SC-1 powering an LED.
Fig. 7
charge storage are concentrated at the outer surface of the core
polymer in a form of thin lm. Nevertheless, the area-specic
energy density (EA) and power density (PA) of the MGM-SC,
which are calculated based on the apparent surface area of
the cylindrical ber, are comparable to those of various bershape SCs (see the Ragone plot of Fig. S8 and Table
S2†).27,47,49,56,57 The EA of the MGM-SC was 0.28–0.63 mW h cm2
in a PA range of 10.6–334.5 mW cm2. However, in terms of
Ragone plot metrics, the performance of the MGM-SC could not
reach those of recently reported high-performance ber
SCs,10,13,15,48 although their fabrication methods did not achieve
the lament-level precise patterning of ber SC electrodes reported in our study. This limited performance can be ascribed
to our distinguished electrode conguration, where the loss in
potential eld can occur via the PVDF domain that spatially
mediates between the anode and the cathode.58
In recent years, ber SCs with pure metallic ber cores have
been developed. Their excellent power performance has been
demonstrated in terms of the very high conductance of the core
wire that is superior to that of carbon cores or our thin metallic
current collector layer.6,15,16 However, our ber SC is distinct
from them in terms of exibility. Due to the low yield strain (3Y
¼ sY/E) of metals (typically 0.1–0.2% for most of metals), general
metal wires with comparable diameters can barely withstand
repeated bending without plastic deformation at a high curvature. In contrast, polymeric bers (such as PVDF ber) can
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deform elastically up to at least several percent of bending
strain (3Y ¼ 3% for our thermally drawn PVDF ber (Fig. S9†)).
The mechanical strengths of metal wires are superior to those of
polymeric bers. However, the promising applications of berSCs are related to wearable devices, whose strengths can match
those of general polymers. Moreover, compared with metals,
ber SCs with a polymer core are light in weight because their
densities are much smaller than those of general metal wires by
about one order of magnitude.59
2.3. Serial integration of monolament supercapacitors
PVA-based solid-state electrolytes are highly useful for ber SCs.
They can conveniently be prepared and applied to ber SCs at
a moderate level of device packaging. Moreover, they are robust
to mechanical deformation. However, most of them have
limited potential windows because of electrochemical instability of the absorbed water over 1 V. Organic electrolytes may
enable a wider potential window up to 3 V;60 however, these
should be tightly sealed for stable operation, which is another
challenge for the ber SCs. Thus, to power a device at a high
voltage, the ber SC with a PVA-based solid state electrolyte
needs to be connected serially.
The technical merit of the proposed laser-patterned ber SC
is highlighted when it comes to the fabrication of the ber SCs
that are seamlessly connected in series. That is, separate individual electrode lines can be precisely patterned in a single
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Journal of Materials Chemistry A
monolament ber, whereas the conventional method requires
a considerable effort to assemble individual ber SCs via
terminal connections (Fig. 7a). Fig. 7b describes the laser-based
fabrication process to implement the serial connection of three
SC units in a single ber. Before electrolyte coating, an MGM-SC
was irradiated with the nanosecond laser for the ablation of the
electrode line at the selected areas. At a laser uence of 1.4 J
cm2, the Ag/Au/Gr/MnO2 layers were completely removed,
exposing the electrical isolating PVDF surface (microscopy
image in Fig. 7b). Consequently, alternating placement of
separate electrode lines was achieved, which is equivalent to 3
SCs that are serially connected. The active length of each SC was
1 cm. This serially integrated ber SC is denoted as MGM-3SC-1.
A single ber SC of 1 cm length (MGM-SC-1) was also prepared
for comparison.
Aer coating with the PVA–Na2SO4 electrolyte, MGM-3SC-1
was characterized via CV and CC in a potential window of
2.55 V, which is three times larger than that of MGM-SC-1.
Fig. 7c and d show the CV (scan rate: 50 mV s1) and CC
(current density: 0.2 mA cm2) curves, respectively, of MGM-SC1 and MGM-3SC-1. Typical features of serially connected SCs
such as reduced total capacitance in the widened V range were
observed.5,13,14,16,24,31,48 Fig. 7e shows a demonstration where
MGM-3SC-1 could indeed turn on the light-emitting diode
(LED) whose minimum operating voltage is 1.7 V.
3.
Conclusions
In conclusion, a monolament ber SC was successfully
developed based on laser processing. The laser microfabrication enabled precise patterning of SC electrodes in
a monolament microber. The addition of AgNP/AgNW/Au
current collectors signicantly enhanced the electrode
conductance. As a result, not only increased capacitance but
also consistent performance with different ber SC lengths was
achieved. The electrochemical deposition of MnO2 nanostructures onto the Gr electrode further improved the capacitance of the ber SC from 0.84 to 24.5 mF cm2. The ber SC
operated reliably under thousands of charge/discharge and
bending cycles, and no signicant change in the EC performance was observed. Moreover, multiple SC units could be
serially integrated into a single monolament ber SC, which
widened the operating voltage. Our fabrication method offers
a highly useful route to implement SCs in series at the lament
level, thereby widening the operating voltage window. The
results of this research put forth the developmental process and
the characteristics of an unconventional structure for ber-type
SCs that are expected to have a high potential in the fabrication
of exible/wearable electronics applications.
4. Experimental section
4.1
Fabrication of PVDF bers
Pellets of the PVDF homopolymer (Kynar 740, Arkema) were
consolidated into a cylindrical preform (diameter: 25 mm,
length: 300 mm) at 200 C. This preform was vertically installed
in a high-temperature furnace (maximum temperature: 230 C)
4848 | J. Mater. Chem. A, 2021, 9, 4841–4850
Paper
and then drawn into a monolament PVDF ber under gravity,
continuously producing a microscale lament (diameter: 300 20 mm) in a starkly long dimension (100 m).51,61 The thermally
drawn monolament ber was annealed at 130 C to improve its
crystallinity.
4.2
Silver current collector patterning
The PVDF ber was ultrasonically cleaned in an ethanol–
acetone mixture (volume ratio: 1 : 1) for 10 min and then rinsed
in DI water. Thin layers of AgNPs (average diameter: 50 nm,
10 wt% in isopropyl alcohol (IPA), Flexio) and AgNWs (average
length: 20 mm, average diameter: 40 nm, 0.5 wt% in IPA, Flexio)
were successively coated on the PVDF ber surface by the
droplet-coating method (for details, see Fig. S10 in the ESI†).
For sintering of the AgNP/AgNW layer, the coated ber was
loaded in an X–Y motorized stage and scanned with a CW laser
(wavelength: 532 nm, MGL-FN-532-400 mW, CNI). The laser
beam was focused via a 2 objective lens (NA: 0.055, PAL-2,
OptoSigma), and the focused beam size (1/e2) was approximately 90 mm in diameter. The laser scanning was performed 5
times at a speed of 40 mm s1 and a laser power of 125 mW.
Aer laser sintering, the unsintered AgNPs/AgNWs were
removed by rinsing in DI water using a bath sonicator.
4.3
Preparation of graphene dispersion
Single-layer graphene powder (ake diameter: 0.4–5 mm, thickness: 0.6–1.2 nm, ACS Material) was dissolved in DI water to
obtain a graphene solution (1.4 wt%). To improve the dispersion
of graphene, a binder mixture (weight ratio ¼ 1 : 1) of carboxymethyl cellulose (CMC) powder (MTI Korea) and styrenebutadiene rubber (SBR) emulsion (EQ-Lib-SBR, MTI Korea) was
added to the aqueous Gr solution.62,63 The weight ratio of the
solid binders to graphene was set to 1 : 9. The solution was then
thoroughly stirred for 8 h at room temperature (20 C).
4.4
MnO2 deposition
The electrodeposition of Au or MnO2 was conducted using
a potentiostat (SP-150, Bio-Logic Science Instruments) in
a three-electrode conguration (reference electrode: Ag/AgCl
(sat. KCl), counter electrode: Pt coil). For Au deposition, the
precursor solution was prepared by dissolving potassium
dicyanoaurate powder (KAu(CN)2, 298115, Sigma-Aldrich) in
a phosphate buffer (0.1 M, pH: 6.8). The amount of KAu(CN)2
was maintained at 8 mg per deposition area (cm2). A constant
potential of 0.5 V was applied to the working electrode (Agpatterned ber), and the deposition was continued until the
delivered charge density reached 4 C cm2. For MnO2 deposition, a constant current density of 250 mA cm2 was applied to
the electrode ber for 15 min in a mixture of 0.02 M manganese
nitrate (Mn(NO3)2$4H2O, 19346, Acros Organics) and 0.1 M
sodium nitrate (NaNO3, Reagent Plus, Sigma-Aldrich).
4.5
Fabrication of a monolament ber SC
A nanosecond laser (wavelength: 532 nm, pulse duration: 6–9
ns, repetition rate: 20 Hz, Nano L200-20, Litron) was employed
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Journal of Materials Chemistry A
to ablate electrode layers. A 10 lens (NA: 0.3, PAL-10-A, OptoSigma) was used to focus the laser, and the beam size was
approximately 26 mm. As an electrical terminal, a strip of Cu
tape was attached to each end of the ber SC. A minute amount
of Ag paste was applied to the Cu–electrode interface to enhance
the electrical contact. Two kinds of PVA-based gel-type electrolytes were used in this study. A PVA–H3PO4 electrolyte was
prepared by mixing 6.91 g of H3PO4 solution (85 wt%, SigmaAldrich), 6 g of PVA powder (molecular weight: 89 000–98 000,
Sigma-Aldrich), and 60 mL of DI water. The mixture was stirred
at 90 C until PVA was completely dissolved and visually
transparent. For a PVA–Na2SO4 electrolyte, the abovementioned process was followed, replacing 0.06 mol of H3PO4
with 6 g of Na2SO4 (powder, 99.9 wt%, Sigma-Aldrich). Before
every EC measurement, the electrolyte was dried under ambient
conditions for 1 h.5
4.6
Characterization
The morphologies of the current collector layers and the active
electrode materials were investigated by FE-SEM (SIGMA, Carl
Zeiss). The thicknesses of the Ag and Au layers were determined
by TEM (JEM-2100F) and EDS. The cross-sectional TEM specimen (thickness: 70–80 nm) was prepared using focused ion
beam milling (FIB, Quanta 3D FEG, FEI). A digital multimeter
(Keysight 34450A) was employed to measure the resistances of
the ber samples. The electrochemical performances of the
ber SCs were investigated by cyclic voltammetry (CV), galvanostatic charge/discharge (CC) analysis, and electrochemical
impedance spectroscopy (EIS), using a potentiostat (SP-150, BioLogic Science Instruments). The cell capacitance (Ccell), lengthspecic capacitance (CL), area-specic capacitance (CA), areaspecic energy density (EA), and area-specic power density
(PA) of the SC were calculated using the following equations:
Ccell ¼
I t
ðFÞ
V IRdrop
CL ¼ 2
Ccell
L
F cm1
CA ¼ 2
Ccell
A
F cm2
1
1
EA ¼ CA V 2 8
3600
PA ¼
EA 3600
t
(1)
(2)
(3)
W h cm2
W cm2
(4)
(5)
where A is the footprint area of a single electrode (cm2), I is the
discharge current (A), t is the discharge time (s), V is the operating voltage (V), IRdrop is the voltage drop at the beginning of
the discharge curve (V), and L is the active length of the ber SC
(cm).10,64
Conflicts of interest
The authors have no competing nancial interest to declare.
This journal is © The Royal Society of Chemistry 2021
Acknowledgements
This research was supported by the Nano$Material Technology
Development Program through the National Research Foundation of Korea (NRF) grant funded by the Korea government
(MSIT) (NRF-2016M3A7B4910532); and also by the Ministry of
Trade, Industry & Energy (MOTIE, Korea) under Industrial
Technology Innovation Program. No. 10062636, ‘Development
of Compact, Lightweight, High-Performance, Highly Durable,
Safe Twisted String Actuation Module Using Reinforced Strings,
Variable Radius Pulleys, and Hybrid Actuation Control’.
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