Fabrication and Characterisation of Energy Storage
Fibres
Ruirong Zhang1, Yanmeng Xu1, David Harrison1, John Fyson1, Fulian Qiu1, Darren Southee2 and Anan Tanwilaisiri1
1
Cleaner electronics, School of Engineering and Design, Brunel University, London, UK
2
Loughborough Design School, Loughborough University, Leicestershire, UK
Abstract—Fibre supercapacitors were designed and
manufactured using a dip coating method. Their
electrochemical properties were characterized using a
VersaSTAT 3 workstation. Chinese ink with a fine
dispersion of carbon and binder was coated as the electrode
material. The specific capacitance per unit length of a
copper fibre supercapacitor with the length of 41 cm
reached 34.5 mF/cm. When this fibre supercapacitor was
bent on rods with a diameter of 10.5 cm, the specific
capacitance per length was 93% of the original value
(without bending). It proved that these fibre supercapacitors
have a good flexibility and energy storage capacity.
Keywords-supercapacitors; chinese ink; energy storage
fibre; gel electrolyte; flexible
I.
INTRODUCTION
Recently, supercapacitors (also named electrochemical
capacitors) have attracted much attention as energy
storage devices. Because of the high power density, long
cycle life and high reversibility [1], supercapacitors have
been considered as a promising high power energy source
for delivering peak power demands in electrical vehicles,
hybrid electric vehicles, portable electronic devices and
emergency power supplies. Electrochemical energy can be
stored in two ways in supercapacitors [1-4]. In electrical
double-layer capacitors (EDLCs), there are no faradic
reactions in the charge storage process, the capacitance
arises
from
the
charge
separation
at
the
electrode/electrolyte interface. Pseudocapacitors are
another kind of supercapacitors, in which, there is a
faradic reaction in the energy storage process.
In recent years, there has been great interest in developing
flexible, lightweight, low-cost and environmentally
friendly energy storage devices. Supercapacitors can
deliver higher power than batteries and conventional
capacitors. However, many existing supercapacitors are
still too bulky and heavy for intended applications. It is a
challenge to develop highly efficient miniaturized flexible
supercapacitors for future energy storage. Recently, some
attempts have been made to manufacture flexible and
weaveable supercapacitors [5-10]. Joonho Bae et al.
developed a kind of novel fibre supercapacitor using ZnO
nano-wires as electrodes, which showed a high specific
capacitance of 2.4 mF/cm2 and 0.2 mF/cm using
PVA/H3PO4 as the gel-electrolyte [6]. Yongping Fu et al.
developed a novel flexible fibre supercapacitor which
consisted of two fibre electrodes using Chinese ink as the
active materials, a helical spacer wire and an electrolyte
showed good capacitance [9].
Coaxial fibre
supercapacitors, using Chinese ink as active materials,
were designed, manufactured and characterised. The
results have been discussed in this study. The particular
novelty or worth reported in this paper is the significant
increase in specific capacitance (from 0.5 mF/cm to 34.5
mF/cm), achieved through increasing the number of
coatings for the carbon electrodes from 4 to 24.
II.
EXPERIMENTAL
A. Materials
Phosphoric acid (H3PO4, dry) and polyvinyl alcohol (PVA,
MW 146,000-186,000, > 99% hydrolyzed) were used
without further purification. The gel electrolyte was made
by dissolving 0.8 g H3PO4 and 1 g PVA in 10 mL
deionized water. Copper fibre (50 µm in diameter) was
used as the core conductor material. Commercial Chinese
ink was used as the active coating material.
B. Design of the structure of the energy storage fibre
Based on the working mechanism [1], fibre
supercapacitors have been designed. As shown in Figure
1, the coaxial single fibre supercapacitors consist of five
layers. The central metal fibre and the outer layer of silver
coating are current collectors. Two active layers made of
Chinese ink are separated by a gel electrolyte layer and
served as electrodes. The energy is stored by the
accumulation of electrical charges at the boundary layers
between the two electrodes and electrolyte.
Figure 1. 3-D schematic of four coating layers on the metal fibre.
C. Manufacturing method
Figure 2 schematically shows the experimental setup of
the coating method for the energy storage fibre. A reel of
metal fibre and two pulleys are fixed horizontally onto a
plate. A weight is clamped to the bottom end of the core
wire to keep the wire straight and in alignment with the
coating vessel. A motor with a two-direction controller
was used to connect the reel to control the coating speed.
When the coating process is carried out, coating liquid is
filled in the coating vessel. During the movement of the
core metal wire, the wire drags the coating liquid with it,
the solvent is vapourised and the coating materials deposit
on the wire.
D. Characterisation of the electrochemical properties
The electrochemical performance of the energy storage
fibre developed was studied by cyclic voltammetry (CV),
galvanostatic charge/discharge and electrochemical
impedance spectroscopy using a VersaSTAT 3
electrochemical workstation. The morphology of the
cross-section of the supercapacitor was studied by optical
microscopy.
is functional, although the capacitance was not ideal. This
was considered to be caused by an uneven structure of the
supercapacitor, which is revealed in Figure 4, so there was
not enough active material deposited.
A typical cross-section of a copper fibre supercapacitor is
shown in Figure 4. The five layers structure can be seen
clearly. The copper fibre core is easy to see. The second
Chinese ink layer was not uniform. Gel electrolyte layer
as the third layer was complete but the thickness was not
uniform. The fourth layer was also a Chinese ink layer and
was also uneven.
0.0006
0.0005
Motor
0.0004
Reel of metal fibre
Coating vessel
Current, A
0.0003
Pulley
0.0002
0.0001
0.0000
-0.0001
-0.0002
0.0
0.2
0.4
0.6
0.8
Potential, V
Figure 3. The Cyclic voltammogram of an initial fibre supercapacitor
sample.
Weight
Figure 2. The schematic of coating method.
A CV test is carried out by applying a positive (charging)
voltage sweeping dV⁄dt (scan rate) in a specific voltage
range and then reversing (discharging) the voltage sweep
polarity immediately after the maximum voltage range is
achieved. The electrochemical behaviour of a
supercapacitor could be evaluated based on the
corresponding current response against the applied voltage
by the equations as:
C=
Qtotal ⁄2
∆V
C L = C ⁄L
3rd Gel electrolyte
2ndChinese ink
layer
5thSilver
paint
(1)
(2)
C is the capacitance, and CL is the specific capacitance per
unit length. Qtotal is the supercapacitor’s charge in
coulombs. The charge is automatically calculated by the
workstation software. L is the length of the fibre
supercapacitors.
III. RESULTS AND DISCUSSION
Following the schematic of fibre supercapacitors (as
shown in Figure 1), the first energy storage fibre was
made using the commercial Chinese ink (coating 4 times)
as the active material.
The CV curve of an initial fibre supercapacitor sample
with the length of 35 cm is illustrated in Figure 3. The
capacitance and specific capacitance per unit length
calculated by Equations (1) and (2) were 18 mF and 0.5
mF/cm. This shows that this kind of energy storage fibre
4thChinese ink layer
1st Copperfibre
Figure 4. Optical image of the cross-section of a fibre supercapacitor.
A new fibre supercapacitor with the length of 41 cm was
made based on the technical manufacturing skills gained
from many times of laboratory trials. The ink layer was
deposited 24 times in this fibre supercapacitor, which was
6 times more than the initial trial sample. The specific
capacitance of the new sample was about 34.5 mF/cm,
which is 69 times that of the trial sample. Generally, when
the number of coatings is increased substantially, the
thickness of ink layer increases dramatically as does the
amount of the active materials in each of the electrodes.
This should enlarge the electrical storage capacity of the
fibre supercapacitor.
The flexibility of the fibre supercapacitors was also
studied. The specific capacitance of the 41 cm long
supercapacitor was examined when the fibre was bent on a
glass rod with different curvatures. The diameters of the
glass rods were 10.5, 3.0 and 1.5 cm. Figure 5 shows the
CV curves of the fibre supercapacitor bent at different
curvatures. It can be seen the CV curves were the same
when the fibre supercapacitor was bent using the glass
rods with the diameters of 1.5 and 3.0 cm. When the fibre
supercapacitor was bent on the glass rod with the diameter
of 10.5 cm, the specific capacitance decreased slightly to
32.2 mF/cm, which was about 93% of the original straight
sample. When the diameter of rods decreased to 3.0 and
1.5 cm, the specific capacitance changed to 26.2 mF/cm
for both cases, which was 76% of the straight sample. The
comparison of the specific capacitance of the fibre
supercapacitor at different bending conditions with the
original sample is shown in Figure 6. These results show
that the fibre supercapacitors are able to work at the severe
bending condition, indicating the fibre supercapacitors are
flexible enough to be woven into other fabrics to make
smart textiles.
0.016
0.014
d
0.012
designed and setup to produce the fibre supercapacitors.
The initial sample made proved the fibre supercapacitor
designed to be functional. When the coating layers
increased substantially, the amount of active materials
deposited on the electrodes increased dramatically. This
improved the electrical storage of the fibre supercapacitor
tens of times. The specific capacitance of the
supercapacitors at various bending conditions was
examined. The results showed that the supercapacitors
manufactured using the present method kept a good
electrochemical performance at the severe bending
conditions, which indicated that the supercapacitors
designed and made in this study have a very good
flexibility. This could be used as a flexible energy storage
and woven into other fabrics to make smart textile
materials.
V.
REFERENCES
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Current, A
0.010
Original
0.008
0.006
10.5 cm
0.004
1.5 cm
0.002
3.0 cm
0.000
-0.002
0.0
0.2
0.4
0.6
0.8
Potential, V
Figure 5. CV curves of the 41 cm long fibre supercapacitor at different
testing conditions (straight and bent with different curvatures).
Specific capacitance per length, mF/cm
40
35
100%
93%
30
76%
76%
25
20
15
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10
5
0
Original
10.5 cm
3.0 cm
1.5 cm
Rolling diameter
Figure 6 Comparison of the specific capacitance per length of the fibre
supercapacitancebeing bent with different curvatures with the original
straight sample.
IV. CONCLUSIONS
The coaxial single fibre supercapacitors were successfully
manufactured and characterised in this study. Chinese ink
was used as electrode materials. A coating method was
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