1

PAINT ON POWER: ENHANCED DURABILITY AND PROCESSABILITY

Joy Ang Jing Zhi1, Sonia E K Arumuganainar

1, Pang Wan Rion Marion

1and Koh Huan Chin

2

1NUS High School of Mathematics and Science, 20 Clementi Avenue 1, Singapore 129957

2Emerging Systems, DSO National Laboratories, 11 Stockport Road Singapore 117605

ABSTRACT

A novel paint-on fabrication method for electric double-layer capacitors (EDLCs) was

devised, where glue-paint composite electrodes and glue-acid gel electrolytes were applied to

polyethylene substrates. White glue was added to the graphene nanoplatelet paint from XG

Sciences to improve the adhesion and mechanical integrity of the electrode layer. The glue

adheres to the surface of the electrode, lowering surface area and increasing resistance of the

electrodes, thus lowering the capacitance of the device. However, the composite-electrode

devices retained 79% of its capacitance when cut and creased, compared to a previous

prototype with no glue added that only retained 6%, thus demonstrating its durability and

resilience. The traditional poly(vinyl alcohol) (PVA) gel electrolyte of the ultracapacitor was

substituted with a more economical white glue, which is poly(vinyl acetate) (PVAc)

emulsion. The compositions and wet mass loading of both the electrolyte and electrode

mixtures were optimised to give devices with specific capacitances of 620μF/g or 32 μF/cm2.

INTRODUCTION

Background and Motivation

Ultracapacitors are energy sources with high power and energy densities [1,2]

that have been

incorporated into hybrid electric vehicles, back-up power, and portable electronic devices [3]

.

Despite recent improvements [4]

, commercial ultracapacitors are still limited to rigid forms

that can be hard to integrate to curved surfaces. To broaden the applications of

ultracapacitors, there has been an increasing emphasis placed on fabricating flexible

ultracapacitors. Furthermore, many ultracapacitors that utilise carbon nanotubes (CNT) [5]

,

carbon nanoparticles, and graphene are often complex and costly to synthesize [6,7,8]

.

Consequently, there is a high demand [5]

for simple and low-cost methods for ultracapacitor

mass production.

This project introduces a paint-on fabrication that enhances the processability of the device;

the paint-on-devices can potentially power many applications like smart building walls and

flexible electronics. Graphene was chosen as the active material as it is highly electrically-

conductive and possesses a high surface area for ion adsorption that greatly enhances the

device capacitance. A water-based paint of graphene nanoplatelets from XG Sciences was

used as it is environmentally-friendly and is potentially cost-effective compared to other

carbon-based nanomaterials. White glue was added to the graphene paint to serve as an

additional binder to hold the flakes together, thus forming more durable composite electrodes

that would not delaminate. For safety considerations, a solid-state electrolyte was chosen as it

is superior to its liquid counterpart, which requires robust encapsulation to prevent leakage.

Compositional studies were carried out on the device electrode and electrolyte components to

improve their processability, durability and overall device capacitance [5]

.

2

Electric Double-Layer Capacitor Model

With electrodes made of graphene nanoplatelets, the supercapacitors studied in this paper are

symmetrical and most likely pure electric double-layer capacitors (EDLCs) [9]

. As shown in

Figure 1, when the device is subject to a potential difference, charge accumulation occurs as

ions are adsorbed on the surface to form two Stern layers, with some ions polarised in the

diffuse layers [10]

. The magnitude of electric potential decreases from each electrode towards

the centre of the electrolyte, where there is a section of unpolarised bulk electrolyte (at zero

electric potential), assuming that the electrolyte is sufficiently thick.

Figure 1: Theoretical schematic of a symmetrical EDLC when charged, and its equivalent

Randles circuit representation for a symmetric ultracapacitor

The device can also be modelled with an equivalent Randles circuit as illustrated in Figure 1.

The Stern and diffuse layers near each electrode are modelled as a capacitor C, with the

leakage current represented by the parallel resistor r. The effective series resistance (ESR)

can be attributed mostly to the electrode, bulk electrolyte and contact resistances [11]

. A higher

ESR lowers the charging-discharging rate of the device since

, (1)

where is the power density and is the operating voltage window.

3

MATERIALS AND METHODS

Design of Ultracapacitors

Figure 2: Structure of completed device with effective electrode area of 25cm

2 (5cm by 5cm)

The electrodes were fabricated by mixing water-based graphene paint (WG-201) from XG

Sciences and Leeho WG-DM-12 white glue, which was then applied as a thin layer on low

density polyethylene (LDPE) substrates and allowed to dry completely. The electrolyte was

fabricated by mixing phosphoric acid (H3PO4) and a gel polymer that was either LeeHo

WG-DM-12 white glue (PVAc emulsion), Deli No. 7316 clear glue (PVA solution) or PVA

granules (Mw = 72000) dissolved in water in a weight ratio of 1:10. The electrolyte was

spread on one electrode and sandwiched by the second electrode; the structure was then left

to air-dry, thus forming a sandwich structure ultracapacitor as illustrated in Figure 2. The

solidified gel electrolyte functions as both the electrolyte and the ion-porous separator.

Table 1: Tests for optimal component compositions (optimal conditions are bolded)

Component Testing for Fixed Components Independent Variable

Electrode

Optimal Ratio of

White Glue: Graphene

Ink

PVA electrolyte –

H3PO4:PVA:H2O =

4:5:50

1:4, 1:2, 3:4

Electrolyte Optimal Gel Base

Electrolyte –

Gel Base:H3PO4 = 10:1

Electrode –

White Glue:Paint = 1:2

PVA granules

dissolved in water,

white glue (PVAc

emulsion), clear glue

(PVA solution)

Electrolyte Optimal Ratio of

White Glue:H3PO4

Electrode –

White Glue:Paint = 1:2 10:1, 5:1, 10:3, 5:2

The electrode and electrolyte compositions were tested according to Table 1. The optimal

electrode is made of a white glue and graphene ink ratio of 1:2, while the optimal electrolyte

is made of white glue and 85 wt.% H3PO4 ratio of 10:1. These optimized compositions were

used for all devices unless otherwise stated.

4

Testing of Ultracapacitors

The performances of the ultracapacitor devices were measured using Autolab PGSTAT302N

with General Purpose Electrochemical Software (GPES) version 4.9 software in a two-

electrode setup. Cyclic voltammetry (CV) was used to estimate the specific capacitances of

the fabricated ultracapacitors. Due to the 1.23V stability range of water-based electrolytes,

the potential window was set to 0–1V. The CV test was carried out using a scan rate ( )

of 0.1V/s for 10 cycles. To study the effects of scan rate on device performance, the scan

rates of specific samples were varied to 0.05V/s and 0.01V/s in addition to the previous

0.1V/s for one sample. The current values measured were converted to capacitance using

. (2)

With the potential window of 1V, the average capacitance was found via

∫

∫

. (3)

Analysis of Components

Fourier Transform Infrared (FTIR) Spectroscopy was performed on white glue dried on a

potassium bromide (KBr) disk using Shimadzu IR Presige-21 to ascertain the white glue used

was a PVAc emulsion. This was done in transmission mode against a KBr reference. The

morphology and sheet resistance of the composite electrodes with varying white glue content

(glue to paint ratio of 1:4, 1:2, and 3:4) were studied using Zeiss EVO scanning electron

microscope (SEM) and a four-point probe respectively.

Testing of Damage Resistance of Devices

To test the damage resistance of the devices, the devices were then creased 180° vertically

and cut halfway horizontally, after which CV testing was carried out.

Optimisation of Component Masses

To ensure consistency of the fabrication process, the component masses was optimized. The

electrode mixture was applied in varying amounts of 1.0 g, 1.5g and 2.0g while the

electrolyte mixture was applied in varying amounts of 1.0g, 1.5g and 2.0g on a 5cm×5cm

area. The masses of the layers were recorded after drying, and the specific capacitances were

calculated by dividing the measured capacitances by the combined dried electrode masses.

5

RESULTS AND DISCUSSION

Characterisation of White Glue

Figure 3: FTIR transmission spectrum of LeeHo WG-DM-12 white glue (Inset model shows

the chemical structure of PVAc)

Table 2: Analysis of FTIR transmittance nulls of LeeHo WG-DM-12 white glue. [12]

Measured

Wavenumber/c

m-1

Corresponding Functional

Group

Type of

Vibration

Characteristic

absorbance

range/cm-1

1242 C–O (Ester) Stretch 1250-1160

1734 C=O (Ester) Stretch 1750-1735

2936 sp3 C–H Stretch 3100-2900

3800-3000 O–H (with hydrogen

bonding) Stretch 3200-3100

Figure 3 shows the FTIR transmission spectrum of LeeHo white glue. The fingerprint region

(wavenumber ≤ 1500 cm-1

) matches that of FTIR graphs of pure PVAc [13]

. The FTIR

transmission nulls labelled in Figure 3 and tabulated in Table 2 account for the ester

functional group and the C–H bonds in the carbon chain of PVAc, whose chemical structure

is illustrated in the inset of Figure 3.

Figure 3 also shows a strong broad null from 3000-3800cm-1

, indicating the presence of an

–OH group that is not present in pure PVAc. The null width indicates that hydrogen bonding

is present and the hydrogen atom is a labile proton [14,15]

. Thus, it hints towards the presence

of a protic solvent, such as water. The water in the sample may not have been fully

evaporated from the PVAc emulsion. It is also possible that the KBr substrate, being

hygroscopic [16]

, might have absorbed water from the PVAc sample during sample

preparation. The results are still consistent with white glue being PVAc dispersed in water.

Graphene-Glue Composite Electrodes

Based on the devices fabricated with paint-glue composite electrodes of varying ratios, the

capacitance of the device decreases with an increasing proportion of white, as shown in

Figure 4. The graph also shows an increasing sheet resistance with glue content that appears

to follow an exponential relationship. Evidently, a greater proportion of non-conductive glue

1,242 1,734

2,936

0

10

20

30

40

50

60

70

80

5001000150020002500300035004000

Tran

smit

tan

ce/%

Wavenumber/cm-1

6

in the composite results in a more resistive and less conductive layer. The increase in

resistance of electrodes causes the ESR of the device to increase, which leads to a lower

power density as shown in Equation (1). This implies that the charging time would be longer

and less energy would be stored within a fixed short time interval, thus the resultant

capacitance of the device calculated from the CV curves would be smaller.

Figure 4: Graph of graphene paint-glue composite electrode sheet resistance (log scale) and

capacitance (linear scale) of corresponding devices against ratio of glue to graphene paint.

Figure 5: (a) Overview of paint electrode surface showing ridges and protrusions from

graphene flakes. (b) Overview of paint-glue composite electrode surface (1:2 ratio) glue coat.

Figure 6: A close-up view of a protruding stack of graphene flakes on the paint surface.

The SEM image of the paint electrode surface in Figure 5(a) depicts many ridges and

graphene flakes protruding from the surface of the paint. From Figure 6, the stack of

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

1

10

100

1000

10000

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Cap

acit

ance

/F

Re

stiv

ity

/Ω

Ratio of Glue:Paint

Resistivity

Capacitance

(a) (b)

7

graphene flakes have a thicknesses of about 100nm; the thin microstructures implies that the

electrode has a large surface area. In contrast, the paint-glue composite surface in Figure 5(b)

is coated with glue that obscures the graphene surface features. This would increase the

separation distance between the ions in the electrolyte and the electrode, implying a longer

distance and time for ions to diffuse to the active graphene surfaces. Furthermore, it lowers

the active surface area and less ions will be able to adsorb onto the surface to form the Stern

layer. On the whole, the effective surface area of the electrode is reduced, thus lowering the

capacitance and power density of the device.

3.2.1 Damage Resistance of Device

Table 3: Capacitance of different types of devices before and after creasing and cutting

Sample Description

Capacitance

before

Damage/mF

Capacitance

after

damage/mF

% Drop in

Performan

ce

Graphene paint electrodes with

PVA-based electrolyte (previous

prototype)

11.18 0.717 93.6%

Graphene paint-white glue

composite electrodes with white

glue-based electrolyte

0.529 0.416 21.3%

From Table 3, the sample with paint-glue composite electrodes clearly shows a smaller

proportional drop in capacitance after it is subjected to damage as opposed to our previous

prototype with no glue added. Thus, the addition of white glue, to both the electrode and

electrolyte components clearly increases the durability of the overall supercapacitor device.

With a greater proportion of glue, the device has more durable electrodes and performs with a

greater consistency; however, it compromises the power density and capacitance of the

device. To maintain a reasonable device performance while minimising fluctuations, we

chose the electrode composite with white glue and graphene paint in the ratio of 1:2.

Electrolyte

3.3.1 Choice of Electrolyte

Figure 7: Average capacitance of ultracapacitor devices with different gel electrolytes

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Ave

rage

Cap

acti

ance

/mF

White Glue

PVA

Clear Glue

8

Figure 7 presents the performance of the devices with different polymer gel electrolytes.

White glue devices gave the highest average capacitance reading. While the average

capacitance reading for PVA is comparable to that of white glue, the variance in performance

is the highest, indicating poor consistency. Thus, white glue is the optimal polymer gel

electrolyte.

3.3.2 Optimal Ratio for Electrolyte

Figure 8: Capacitance as a function of the H3PO4 to white glue ratio in the electrolyte

Figure 8 shows that when more acid is added to white glue, i.e. the electrolyte concentration

is increased, the capacitance of the device decreases. This is contrary to the typical trend

where capacitance increases with electrolyte concentration due to a greater number of ions

and higher ionic conductivity. The decreasing trend could be attributed to the nature of the

gel electrolyte used in the devices. It is possible that when more acid is added to the white

glue, the viscosity of the mixture is lowered, causing the electrolyte to be spread out easily,

thus decreasing the thickness of the electrolyte layer; this may result in more leakage current

at small thicknesses. Tsay et al. also suggests that as the electrolyte concentration increases,

there are less water molecules available for hydration, reducing ion mobility and lowering

capacitance [9]

.

3Optimisation of Component Masses

Figure 9: Graph of capacitance of devices against the corresponding wet mass of electrolyte

loaded over 25cm2 electrodes with individual wet mass of 2.00g

0

0.5

1

1.5

2

2.5

3

3.5

4

0.00 0.10 0.20 0.30 0.40 0.50

Cap

acit

ance

/mF

Ratio of H3PO4 to white glue in the electrolyte/g

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

0.80 1.30 1.80

Cap

acit

ance

/mF

Amount of 1:10 H3PO4-Glue composite used to fabricate electrolyte/g

9

From Figure 9, the device capacitance initially increases with increased electrolyte loading,

but subsequently decreases. At low electrolyte loading (1.00g), the electrolyte layer is most

likely extremely thin, with little total ions for charge accumulation. The diffuse layer on both

sides would likely overlap [17]

and the limited amount of charges would be shared between

the two regions; it is possible the Stern layer does not form completely. Current may also be

able to flow through the thin electrolyte in the form of self-discharge, as the gel electrolyte

functions poorly as a separator. The overall lower charge accumulation thus leads to a lower

capacitance.

At an electrolyte loading of 1.50g, the increase in electrolyte allows for the formation of a

more complete Stern layer, thus driving up capacitance. However, when excess electrolyte is

loaded (2.00g), the Stern layer is most likely saturated. The bulk electrolyte in between the

two charged regions behaves as an Ohmic resistor [18]

and the additional thickness increases

the ESR of the device, thus the device capacitance decreases as explained previously.

The large variance in the device capacitance with 2.00g of electrolyte could be due to sample

preparation. The electrolyte layer was sandwiched between the two electrode layers while it

was still wet, and any excess electrolyte was not necessarily evenly distributed. The greater

the amount of excess electrolyte present, the more thickness variations in the sample, thus

leading to a spread in the measured capacitances. From Figure 9, it is evident samples

fabricated with 1.50g of electrolyte over an area of 25cm2 had the highest capacitance and the

lowest variation amongst all the samples, thus 0.06g/cm2 of electrolyte was optimal.

3.4.2 Electrode

Figure 10: Graph of specific capacitance of devices against the corresponding wet mass of

both electrodes loaded over 25cm2 (1.50g of electrolyte was used for all samples)

Increasing the electrode loading drives the specific capacitance up, as shown in Figure 10.

For a fixed area, the thickness of the electrode directly relates to the mass loaded. The greater

amount of graphene flakes could imply greater interconnectivity for current to conduct; hence

lowering resistance. However, this effect would eventually become negligible past a certain

thickness, and the specific capacitance would then be expected to increase with a decreasing

gradient and eventually plateau, unlike the trend shown in Figure 10.

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

1.40 1.60 1.80 2.00 2.20 2.40 2.60

Spe

cifi

c C

apac

itan

ce/m

F g-1

Amount of 1:2 Paint-Glue composite used to fabricate electrode/g

10

Another mechanism is needed to account for the increasing gradient. A thicker coat of

graphene paint experiences greater surface stresses as it dries, and the thickest electrode

layers (2.50g load) were observed to curl up. The curled electrodes were then flattened when

the whole device was made into the sandwich structure. This implies that in the device, the

electrode regions nearer to the electrolyte are in tension, while the regions nearer to the

substrate are in compression. The stresses are likely mostly borne by the glue binder, rather

than by the graphene fillers in the electrode. It is possible that due to the stretching of the

electrodes at the surfaces adjacent to the electrolyte, the separation between the graphene

flakes near the surfaces increases, hence exposing more of the graphene surfaces beneath that

are available for charge accumulation. The stretching of the glue binder on the electrode

surface may have also caused it to thin due to a positive Poisson’s ratio. Assuming the glue

coats the graphene surfaces, a thinner glue coating implies easier diffusion and polarization of

the ions in the electrolyte at the electrode-electrolyte interface. The tensile stresses near the

electrode-electrolyte surface could thus result in a higher specific capacitance observed in

Figure 10. An increase in capacitance with tensile stress has been reported by Chen et al. in

their solid-state supercapacitors fabricated with CNT. [19]

However, more studies are required

to confirm this mechanism.

The stresses in the electrode could also introduce fluctuations in our results due to different

amounts of stretching in the samples; this is likely why a greater electrode mass loading

shows greater variation in performance in Figure 10. Using 2.0g electrode load over an area

of 25cm2 achieves a high specific capacitance while maintaining relatively consistent

performance, thus 0.08g/cm2 of electrode composite was found to be optimal. The resultant

specific capacitances with 0.08g/cm2 of electrode and 0.06g/cm

2 of electrolyte are 620μF/g or

32 μF/cm2.

Effect of Scan Rate on Capacitance

Figure 11 (left): Graph of capacitance of an ultracapacitor device against potential

Figure 12 (right): Graph of average capacitance against scan rate

The capacitances shown in Figure 11 and Figure 12 are calculated from I-V curves from the

CV measurement using Equations (2) and (3) respectively, and can be seen to decrease with

increasing scan rates. This trend can be associated with ion transport in the device; at higher

scan rates with the same potential window, the time for the device to charge and discharge is

much shorter. There is thus insufficient time for ion transport in the electrolyte to the

-2.0

-1.0

0.0

1.0

2.0

3.0

0 0.2 0.4 0.6 0.8 1

Cap

acit

ance

(m

F)

Potential (V)

10mV/s

50mV/s

100mV/s

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 20 40 60 80 100

Cap

acit

ance

(m

F)

Scanning Rate (mV/s)

11

electrode layer as the device is charging before the voltage is reversed. This implies that less

ions accumulate at the electrode and that the Stern layers cannot form completely at high scan

rates, leading to a reduction in capacitance and causing the EDLC to function more like a

resistor [20]

. Furthermore, for mesoporous materials, such as the layers of graphene embedded

beneath each other, the effective surface area of the electrode is greatly reduced at high scan

rates [21]

, as there is insufficient time for ions to diffuse and access underlying surfaces,

resulting in the reduced capacitance measured.

CONCLUSION

A paint-on scheme was developed to enhance the processability and flexibility of EDLC.

Polymer composite electrodes were incorporated to enhance the durability of the device, with

some trade-off in capacitance. A mixture of glue and graphene ink in the ratio of 1:2 provided

a relatively high capacitance with low variation. The device with composite electrode

retained much more of its capacitance (79%) when subjected to bending and creasing as

opposed to prior devices with pure graphene paint electrodes (6%).

White glue (PVAc emulsion) was found to provide the highest and most consistent

capacitance. The optimal electrode and electrolyte wet mass loadings were found to be

0.08g/cm2 and 0.06g/cm

2 respectively. The resultant specific capacitances are 620μF/g or 32

μF/cm2.

Our devices can be peeled off from the LDPE substrates, and through the use of adhesives,

these devices could be manufactured in the form of portable stickers, thus expanding the

applications of EDLCs on a multitude of surfaces.

Further analysis of the device architecture could be done in relation to an equivalent circuit

model representative of our device. Self-discharge in the device can also be studied to better

understand the capacitive mechanism in the device and better optimise the components. In

addition, the devices can be further improved by incorporating organic electrolytes that can

function over a much larger potential window, which could help boost the device

performance.

ACKNOWLEDGEMENTS

We are greatly indebted to our research mentor, Ms Koh Huan Chin, for patiently introducing

us to the subject matter and providing insights throughout the course of this project. Our

teacher mentors, Dr Chiam Sher Yi and Mr Poh Boon Hor, helped out tremendously with

administrative matters. We would also like to express our heartfelt gratitude to DSO National

Laboratories for allowing us to use their facilities. Lastly, we would like to thank our family

and friends for their support throughout the course of this project.

12

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14

APPENDIX

A1 Materials and Methods

Composition of Device Components

The optimal electrode composition was found by varying the white glue to paint ratio [1:4,

1:2, 3:4] and studying the resistivity, morphology and mechanical integrity of the electrode

layers; this was done using a four-point probe, scanning electron microscope (SEM) and

visual inspection for flaking on bending. The respective ultracapacitors were then fabricated

with an electrolyte (white glue: H3PO4 = 10:1) and measured using CV.

Electrolyte Composition: Determination of Optimal Material

The electrolyte mixture was made up of varying types of glue, namely PVA, white glue or

clear glue mixed with H3PO4, in the ratio 10:1. The electrode used had a white glue to

graphene paint ratio of 1:2. The CV performance was then measured.

Electrolyte Composition: Determination of Optimal Ratio

The electrode was made up of white glue and graphene paint in the ratio of 1:2. The

electrolyte was made up of white glue and acid is varying ratios of 10:1, 10:2, 10:3, 10:4,

10:5.

Fabrication of Hybrid Samples with Optimal Electrode and Electrolyte Composition

The optimal electrode was made up of graphene paint and white glue in the ratio of 2:1,

while the optimal electrolyte was made up of white glue and phosphoric acid in the ratio of

10:1. These compositions were used for subsequent ultracapacitor prototypes.

A2 Results

Individual Scan Rate CV Curves

The individual CV curves for the 3 scan rates that were compared in 3.5 Effect of Scan Rate

on Capacitance are presented here.

5mV/s 10mV/s

15

Figure 13. CV Curves of sample at different scan rates. 5mV/s (red), 10mV/s (blue), 100mV/s

(green).

100mV/s