Multilayer supercapacitor threads for woven flexible circuitsFulian Qiu1, David Harrison1

1College of Engineering, Design and Physical Sciences, Brunel University, Uxbridge, UB8 3PH, UK

AbstractPurpose – Wearable electronic devices have emerged which require compact, flexible power storage devices

such as batteries and supercapacitors. Recently energy storage devices have been developed based on

supercapacitor threads. However, current supercapacitor energy storage threads which use electrolytes based on

aqueous gels have a 1V potential window. This is much lower than the voltage required by most electronic

devices. This current contribution presents an approach for fabricating a multilayer supercapacitor working as a

circuit unit , in which series combinations of the multiple layer structures can achieve a higher potential

window, which can better meet the needs of wearable electronic devices.

Design/methodology/approach – Two-capacitive layer thread supercapacitors were fabricated using a semi-

automatic dip coating method by coating two capacitive layers sequentially on a 50 m stainless steel core wire,

each capacitive layer includes ink, aqueous based gel electrolyte and silver conductive paint layers.

Findings – Two capacitive layers of the single thread supercapacitor can work independently, or as combination

circuits – parallel and series. Cyclic voltammograms showed all flexible circuits have high electrochemical

stability. For the case of series circuit configuration, with H3PO4/PVA (Polyvinyl alcohol) gel electrolyte, a

working potential window of 2 V was achieved.

Originality/value - A flexible single thread supercapacitor of multilayer structure, with working voltage above

1 V in H3PO4 / PVA gel electrolyte, has not been reported before. A semi-automatic dip coating setup used to

process the thread supercapacitor has high potential for transfer to an industrial environment for mass

production.

Keywords Flexible circuits, Multilayer supercapacitor, dip coating method, energy storage device,

supercapacitor thread

Paper type Technical paper

Introduction

Developments and applications of portable electronic devices (De Rossi et al., 2005), wearable gadgets (Salvo

et al., 2010), bio/chemical sensors (Shim et al., 2008) and flexible solar cells (Zou et al., 2010) require

comparable energy storage devices such as batteries (NishideOyaizu., 2008) and supercapacitors

(electrochemical double-layer capacitor) (Kang et al., 2012a). Supercapacitors have many advantages over Li

ion batteries with high power density, fast charge-discharge process, easy fabrication, low cost, long life time

and a good safety record. Miniaturised, flexible, light-weight and weaveable thread supercapacitors are of

interest because they may be fully integrated into fabrics. Xu et al reported all-solid-state flexible

supercapacitors based on a carbon/MnO2 (C/M) core–shell fibre structure, with a high electrochemical

performance, including a high rate capability with a scan rate up to 20 V s–1, and high volume capacitance of

2.5 F cm–3. Recently extensive research has focused on 1D fibre supercapacitors as structured in figure 1 /(a, b

& c), In the case of (a), Chinese ink coated nickel wires (Fu et al., 2012) were arranged in parallel, sealed in

PVA gel electrolyte, and the capacitance did not vary with bending angles, which demonstrated the high

flexibility of the device. Electrochemical deposited 3D graphene microfiber (Meng et al., 2013) were twisted in

PVA gel electrolyte to form a stretchable supercapacitor, the device showed high electrochemical stability and

flexibility during spring shape compress- stretch test. Parallel two-fibre supercapacitor integrating with solar

cell fibre has been reported (Fu et al., 2013), the stainless wires coated with polyaniline electrochemically

served as energy storage electrodes. Coaxial fibre supercapacitor made from all-carbon materials (Le et al.,

2013) and Chinese ink coated onto a conducting core wire(Harrison et al., 2013) were examples for the case of

(c), where the PVA gel electrolyte served as a separator between two concentric active layers. These

supercapacitors have a simple structure. For all solid-state 1D supercapacitors (a, b and c), as PVA-H3PO4 was

employed as electrolyte, their applications have been hindered by an intrinsic potential window of 1V, as most

electronic devices operate at above 1V. As Zhao describes in his review of electrolytes, (Zhao et al. 2015),

aqueous electrolytes such as these have the advantages of high-ionic conductivity, low cost, non-flammability,

non-corrosiveness, safety, and convenient assembly in air, compared to organic electrolytes, which are believed

to be less conductive, expensive, usually flammable, and have higher toxicity. (However, adopting an organic

electrolyte, such as propylene carbonate with an organic salt, can give a working voltage of up to 2.5 volts).

Combination circuits of parallel and series supercapacitors (stacked) have also been produced to increase the

working voltage for LED lighting tests (Yuan et al., 2011) as LEDs are operated above 1V.

So far, almost all reported 1D supercapacitors have been made manually, and their length uniformity has been

limited. 1D fibre supercapacitors have a low volume and high flexibility, and a few woven supercapacitors in

fabric structures have been reported (Dalton et al., 2004).

Figure 1 Schematic of fibre supercapacitors (a) two-fibre parallel, (b) two-fibre twisted and (c) one-fibre

coaxial

(a)

(b)

(c)

Here, we report a coaxial multilayer thread supercapacitor. Although a bipolar flat supercapacitor has been

reported (Ng et al., 2009), no coaxial structure multilayer fibre supercapacitor has been attempted. A two-

capacitive-layer single thread supercapacitor was fabricated and characterised, and its capacitances and

electrochemical stability were studied. A semi-automatic dip coating method was employed for the fabrication.

Multilayer thread supercapacitor design and fabrication

Figure 2a shows the schematic of a multilayer thread supercapacitor. It could be composed of many capacitive

layers on a core conductor; each capacitive layer consisting of ink-gel-ink-conductive paint layers. The ink used

was a suspension of carbon in an aqueous solution. The function of each ink layer was to act as an electrode in

the supercapacitor structure. When a potential window of V Volt is assumed for a single capacitive layer thread

supercapacitor, for a “n” capacitive layers thread supercapacitor as one unit, its potential window would be nV

Volt. For this study, a 50 m stainless steel microwire was employed as the core conductor.

A series of capacitive layers were coated onto the stainless steel microwire sequentially using a semi-

automatic dip coating setup. This setup consists of a multi-speed controlled motor and Perspex discs of a radius

of 1.5 cm or 1 cm diameter with PTFE pipette reservoirs attached. In the centre of the discs and the reservoirs,

eight different sizes of sub-millimetre holes were machined using a laser cutting setup, which allowed the core

wire through, and facilitated the dip coating processes. Holes of different diameters were created for different

coating layers.

A pre-wired bobbin was fixed onto the motor; a small weight was clamped to the bottom end of the core wire,

which kept the wire straight in an up-down alignment. The motor has a two-direction controller which allows

PVA gel electrolyte

Active fibre

PVA Gel electrolyte

Active fibre

Silver paint

Active layer Active fibre

PVA gel electrolyte

the load to move up or down. When the coating process was performed, a drop of coating liquid (slurry) was

applied to the centre of the disc such that the wire moves through it. During the movement of the core wire it

dragged the liquid with it, the solvent vaporised, and a coating layer was formed on the wire. Different coating

layers were coated onto the core microwire sequentially. The thread was pulled first through the disc with the

smallest diameter, to make the first coated layer. Following drying, the thread must be unclamped, and the next

smallest diameter disc used for the subsequent layer. The thickness of each coating layer can be adjusted by the

coating time controlled by motor speed; a motor speed of 0.5 m/minute was used throughout experiment. The

time interval was approximately 5 minutes for the ink and gel electrolyte coatings. This approach currently

results in a relatively lengthy fabrication time of up to 40 minutes for the complete structure. Work is

proceeding with industrial partners to adapt the coating techniques to produce an efficient continuous process.

Multiple coating passes were performed for each of the three active layers and the silver paint layer. Ink, 10 wt

% H3PO4 / 8.3 wt% PVA gel electrolyte, and silver paint were used throughout the experiment for each layer

coating respectively. In order to coat the gel electrolyte uniformly, the first gel electrolyte coating was

conducted using 5 wt% PVA gel.

Figure 2b shows the basic coating process for a two-layer thread supercapacitor starting from bare conducting

core microwire (50 m diameter stainless steel) using the semi-automatic dip coating setup. All coatings were

conducted at room temperature. Eight coating layers were performed. Figure 2c shows the schematic of the

coaxial two capacitive layers thread supercapacitor and electrical circuit connections. It consists of two ink-gel-

ink-silver paint capacitive layers. The core stainless steel wire and two silver paint layers serve as current

collectors. Four different electrical circuits can be constructed when different leads are connected together in

different combinations. The two capacitive layers can serve as two independent single supercapacitors.

Connecting to leads 1 and 2 gives connection to one independent single supercapacitor, which we call C12.

Connecting to leads 2 and 3 gives connection to the other independent single supercapacitor, which we call

C23. Connecting to leads 1 and 3 gives connection to the series combination of the two independent

supercapacitors, which we call C13, and gives the highest potential window of 2 volts. Making one connection

to lead 2, and one to both leads 1 and 3 connected together, gives connection to a parallel circuit combination of

the two independent supercapacitors, which we call C2-13. For the case of C13, the middle silver paint layer

serves as a bipolar electrode.

Figure 2 Schematic of a section of a coaxial multi layer single thread supercapacitor (a), coating process flow

(b) and Structure of unit circuit (c)

(a)

(b)

Apply silver paint

Apply ink

Apply gel electrolyte

Apply ink

Apply silver paint

Apply ink

Apply PVA gel electrolyte

Apply ink

(c)

Optical microstructure of a two-layer thread supercapacitor

Figure 3a shows the photo of a 16 cm long coaxial two capacitive layers single thread supercapacitor. It has one

stainless steel core wire and two copper wires attached to two silver paint layers as outside connectors. Four

different electrical circuits could be formed between the three wires. The diameter of the supercapacitor is 300

m. Owing to its coaxial, thin and compact structure, the thread supercapacitor has high flexibility. An optical

microscope (Olympus BHM, Trinocular MTV-3 with Nikon Coolpix 990 3.34MP Digital Camera, Japan) was

used to examine the surface morphologies of the cross-section structure of the device. The sample was prepared

by sealing the thread in Araldite® and leaving it overnight for solidification. It is then polished using sandpaper

(P1250, P2500 and P4000) and gamma alumina powder of size from 1m down to 0.025 m, rinsed using

deionised water and methanol, then dried in air. Figure 3b shows a cross section image of the device. As

carbon layers are soft, and the gel electrolyte layer is flexible, the cross section was slightly distorted after

polishing. The coating layers are relatively uniform as shown in figure 3b. Average thicknesses are measured as

18 m, 15 m and 25 m for the ink, gel electrolyte and ink in the inner capacitive layer respectively, and 18

m, 10 m and 20 m for the ink, gel electrolyte and ink in the outer capacitive layer respectively.

Figure 3 Photo of a 16 cm long two capacitive layer thread supercapacitor (a) and Optical image of the cross

section of the thread supercapacitor (b)

16 cm thread length

(a)

(b)

Electrochemical characterisations

Electrochemical measurements including cyclic voltammetry, galvanostatic charge-discharge and

electrochemical impedance spectroscopy were conducted with a two-electrode setup using an electrochemical

workstation – VersaStat 3.0 (Princeton Applied Research). For electrochemical impedance measurements, a

frequency range of 100 kHz – 0.005 Hz with a 5 mV bias was employed. A 4.3 cm length of fibre was used to

make these initial characterisation measurements. Figure 4 shows typical cyclic voltammograms recorded at a

scan rate of 50 mV/s for four different circuits C12, C23, C2-13 and C13. It can be seen from figure 4a that

cyclic voltammograms were distorted from ideal square-box shape for all cases; this may result from ions’ slow

diffusion in the porous carbon structure and high series resistance. The parallel circuit (C2-13) displayed larger

capacitance CV circled area than those of two single capacitors. The series circuit shows a larger potential

window of 2V. The capacitance can be estimated using the following equation (1) from a cyclic voltammogram

(CV)

C¿ACV

2× v ×V(1)

Where C is the capacitance; ACV, the CV circled area; v, the scan rate and V, the potential window.

Approximated capacitances for four circuits are 0.48, 0.66, 1.22 and 0.54 mF for circuit C12, C 23, C2-13 and

C13 respectively. No faradaic process was observed for all cases of electrical circuits indicating high

electrochemical stability of the device. The stored energy (E) can be calculated using equation (2)

E=0.5CV2 (2)

0.24, 0.33, 0.63 and 1.0 mJ were obtained for C12, C23, C2-13 and C13 respectively, which demonstrated that

the coaxial two capacitive layers thread supercapacitor stores more energy than single capacitors due to a large

potential window. For the C13, as a single thread supercapacitor unit, it demonstrated that, in PVA-H3PO4 gel

electrolyte, the device can be operated within 2 V potential. If compared to a single capacitive layer device of

the same capacitance, its stored energy would be four times bigger based on the equation (2). The energy

density of the series combination capacitor C13 was approximately 0.5 mJ.mm-3

Figure 4b shows typical cyclic voltammograms recorded at six different scan rates (0.01, 0.05, 0.1 0.2, 0.5 and

1.0 V/s) between 0 V and 2 V. It can be seen that the charge current increases with increasing scan rate as

expected. The capacitive rectangular CV shape is distorted, and the distortion becomes more pronounced when

scan rate is increased. This may be explained by the series resistances including that of the gel electrolyte, and

by the diffusion processes in the porous carbon nanostructure which will be more marked at higher scan rates as

the ions cannot move quick enough through the viscous medium of the PVA. Other factors could include some

electrical leakage and self-discharge due to some shorting occurring between the carbon layers. There will also

be some effect due to the electrical resistance of the porous carbon network. No faradaic processes were noted,

which showed good stabilities of the stainless steel microwire and the Ag coating layer current collectors.

Calculated capacitance as a function of scan rate using equation (1) is shown in Figure 4c. It can be seen that

the capacitance decreases sharply before the scan rate reaches 0.05 V/s; and then the capacitance decreases

gradually with increasing scan rate further. A range of 0.62 mF to 0.03 mF was obtained for the scan rate

varying from 0.01 to 1.0 V s-1. This deceasing trend of the capacitance with increasing scan rate was attributed

to the slow diffusion of the ions in gelled electrolyte during the charge-discharge process.

Figure 4d shows electrochemical impedance spectroscopy performed at 5 mV AC perturbation for a frequency

range of 100 kHz down to 0.01 Hz for this 4.3 cm thread supercapacitor. A pure capacitance trend was noted as

expected from a transmission line model; no semicircle shape at high frequency ranges was observed, which

would have resulted from interface processes of active layers and the current collectors and porous carbon

electronic structure. A series resistance of 234 Ω was obtained; this higher resistance was possiblly due to the

water loss in gel electrolyte, which leads to high diffusion resistance in gelled electrolyte. A similar trend of

Nyquist plots for gelled electrolyte systems has been reported (Kang et al., 2012b) .

Figure 4 Cyclic voltammograms recorded at 50mV/s for four electrical circuits for a 4.3cm coaxial two

capacitive layers thread supercapacitor (a), cyclic voltammograms at different scan rates (0.01, 0.05, 0.1, 0.2,

0.5 and 1.0 V/s) for the C13 (b); capacitance as a function of scan rates (c) and Niquist plot recorded for the C13

at open circuit potential using a 5 mV AC modulation for a frequency ranged from 100 kHz down to 0.005 Hz

(d)

(a)

(b)

(c)

(d)

Figure 5 shows typical Galvanostatic charge-discharge curves between 0 V and 1V for four circuits (a) C12 at

charge-discharge 50 A, (b) C23 at 80 A, (c) C2-13 at 150 and C13 at 50 Repetitive charge-

discharge curves were observed for all cases. Discrepancies for the 1st couple of cycles were possibly due to no

zero open circuit potentials resulting from different sizes of electrode and materials of current collectors. This

could be improved in the future by coating a thin conductive silver paint layer onto the stainless steel wire first.

The electrochemical stabilities are noted.

The capacitance can be calculated using the equation (3) for discharge branches,

(3)

Where i is the charge-discharge current; Δt, the discharge time; V, the potential window, and R the internal

resistance of the capacitor. The value of C was obtained as 1.26, 0.88, 1.66 and 0.75 mF calculated using the

10th discharge curves for C12, C23, C2-13 and C13 respectively.

Figure 5 Galvanostatic charge-discharge curves recorded for (a) C12 at 50 A, (b) C23 at 80A, (c) C2-13 at

150 A and (d) C13 at 50A

(a)

(b)

(c)

(d)

To study the fabrication procedure’s robustness and the device’s functionality further, a 16 cm long two

capacitive layers thread supercapacitor was produced. Figure 6 shows cyclic voltammograms at 100 mV/s for

C12, C23, C2-13 and C13 (a) and at different scan rates for C13 between 0 V and 2 V (b). Capacitances were

calculated using equation (1) as 3.2, 1.5, 4.1 and 2.1 mF respectively. Stored energies were calculated as 1.6,

0.75, 2 and 4.2 mJ for C12, C23, C2-13 and C13 respectively, C13 held the highest energy as expected. As

shown in figure 6b, for all scan rates, the device was stable within the 2 V potential window. The capacitance

(4.1, 2.8 and 2.1mF) decreases with increasing scan rate.

Figure 6 Cyclic voltammograms for a 16 cm two-layer thread supercapacitor (a) at 100 mV/s for four different

electrical circuits, and (b) at different scan rates for the C13

(a)

(b)

Conclusions and outlook

We have developed a novel coaxial two capacitive layers supercapacitor with a large potential window and a

high energy storage capacity. Two concentric capacitive layers were coated on a single stainless steel wire using

a newly designed semi-automatic dip coating setup, sequentially. The fabrication procedures are robust, and

have great potential for scale-up. Owing to its coaxial structure, it is compact, has high energy density, and a

wide operating potential window. This device as a flexible energy storage unit is ready to integrate with other

wearable electronics.

Operating as a single integral thread but with a series connection between the two internal capacitors can

provide a driving voltage of 2 volts, sufficient for simple applications like lighting an LED. The multilayer

threads themselves can be further combined in series and parallel to match the power requirement for other

loads. For example, three thread structures in series could be used to drive a 6 volt motor. The flexible thread

combinations have the possibility to eventually be used in wearable applications such as providing 5 volts to

recharge a mobile phone or other wearable electronic devices.

We are aware that there are some limitations with charging and discharging at high scan rates. This could be in

part due to leakage and self-discharge, but we believe this is also due to the poor diffusion of ions to the

electrode surface, as the ions have to move through a viscous medium. If the viscosity were to be reduced, this

might reduce some of the effects seen.

The concept of the device and fabrication procedure are not only applicable to two capacitive layers and carbon-

carbon symmetric configurations but also many capacitive layers and asymmetric supercapacitors. The

electrical properties are easy to tune by varying the number of capacitive layers and the active layers’ thickness,

to satisfy the needs of electronic devices, and these will be explored in the future.

Acknowledgment

This work was funded by the EU FP7 programme.

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