Carbon Nanotube Based Flexible Supercapacitors

by Christopher M. Anton and Matthew H. Ervin

ARL-TR-5522 April 2011

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ARL-TR-5522 April 2011

Carbon Nanotube Based Flexible Supercapacitors

Christopher M. Anton and Matthew H. Ervin

Sensors and Electron Devices Directorate, ARL

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Carbon Nanotube Based Flexible Supercapacitors

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Christopher M. Anton and Matthew H. Ervin

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14. ABSTRACT

Electrochemical double layer capacitors are fabricated using carbon nanotube (CNT)/paper flexible electrodes. An extensive 

electrode fabrication study was conducted, resulting in single electrode specific capacitances in excess of 100 F/g. Surfactants 

and dispersants in the CNT suspension prior to CNT deposition onto paper substrates was found to be detrimental to electrode 

performance. Following electrode fabrication, three full cell packaged supercapacitor architectures were investigated. One 

architecture used a liquid KOH electrolyte, while the other two architectures used a PVA:KOH gel electrolyte. Devices were 

tested under static bending conditions, and proved to function under bending.

15. SUBJECT TERMS

Carbon nanotube, supercapacitor, electrochemical double layer capacitor

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Matthew H. Ervin

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iii

Contents

List of Figures iv

List of Tables iv

1. Background 1

2. CNT Supercapacitor Electrode Fabrication and Testing 2

2.1 Impact of CNT Solution on Performance........................................................................3

2.2 Impact of Surfactant on Performance..............................................................................5

2.3 CNT Mass Loading Study...............................................................................................6

2.4 MWCNT Electrodes........................................................................................................8

3. CNT Flexible Supercapacitor Device Assembly 9

4. CNT Flexible Supercapacitor Device Testing 11

5. Conclusions 12

6. References 13

List of Symbols, Abbreviations, and Acronyms 14

Distribution List 15
iv

List of Figures

Figure 1. (a) CNT/paper electrode with an attached Ni foil clamp to provide good electrical 

contact to the CNT film. (b) Electrochemical half cell experimental setup. ...........................2

Figure 2. ESEM images of COOH SWCNTs suspended in acetone (a) (sample 1), SWCNTs 

in SDBS/diH2O (b) (sample 4), and COOH SWCNTs in H2O (c) (sample 5), deposited 

on paper substrates. Scale bars = 500 nm. ................................................................................5

Figure 3. CNT film morphology and CV plots of SWCNT supercapacitor electrodes both 

with and without SDBS surfactant. ESEM images show (a) sample 5 with no SDBS, and 

(b) sample 8 with SDBS. CVs were taken at a scan rate of 20 mV/s and a scan range of 

–0.7 V to +0.3 V for both (c) sample 5, and (d) sample 8. ESEM scale bars = 500 nm ..........6

Figure 4. Cyclic voltammograms of sample 5, sample 6, and sample 7 using a 20 mV/s scan 

rate..............................................................................................................................................7

Figure 5. CV (a) and ESEM image (b) of a MWCNT/paper supercapacitor electrode. 

ESEM scale bar = 500 nm .........................................................................................................8

Figure 6. Full cell packaged flexible supercapacitor architectures...............................................10

Figure 7. Cyclic voltammagrams of (a) Liquid KOH electrolyte packaged supercapacitor, 

(b) PVA:KOH gel sandwich packaged supercapacitor, and (c) PVA:KOH cast gel 

packaged supercapacitor under varying states of bending.......................................................11

List of Tables

Table 1. Impact of initial CNT solution on CNT/paper electrode performance.............................4

Table 2. Comparison of electrodes fabricated with and without SDBS surfactant. Sample 8* 

has undergone rinsing and heat treatment to remove unwanted surfactant molecules. .............6

Table 3. Impact of mass loading on CNT/paper electrode performance. .......................................7
1

1. Background

Electrochemical double layer capacitors (Supercapacitors) are expected to play a significant role 

in future hybrid power systems due to their high specific power, cycle life, and tolerance of 

extreme environmental conditions (1). The development of flexible, conformable energy storage 

devices is also of great interest to the Army due to ease of packaging and coupling with flexible 

electronics. In supercapacitors, two electrodes are placed in an electrolyte and a voltage is 

applied between the two electrodes. During the application of voltage, an electrochemical 

double layer is formed on both electrodes when negative ions from the electrolyte are drawn to 

the positive electrode, and positive ions are drawn to the negative electrode. When these ions are 

released by opening a path between the positive and negative electrodes, the energy stored in the 

double layers is discharged through the external circuit.

Because supercapacitors store charge only on the electrode surfaces, maximizing the amount of 

surface area that is accessible to the electrolyte ions is of critical importance. Conventional 

supercapacitors typically use activated carbon electrodes on top of a current collector. Carbon is 

an excellent electrode material due to a variety of favorable chemical properties, including 

chemical stability and large electrochemical windows in a variety of electrolytes (1). The 

advantage of activated carbon is its low cost relative to competing electrode materials. The 

disadvantage is its modest specific capacitance (capacitace per mass), due to the fact that 

electrolyte ions cannot collect between the individual graphene layers of the activated carbon 

electrode.

Carbon nanotube (CNT) electrodes have been thoroughly investigated as supercapacitor 

electrodes over the past decade. CNTs are an appealing electrode material due to their high 

conductivity, and the fact that single walled carbon nanotubes (SWCNTs) are comprised entirely 

of surface atoms that could interact with electrolyte ions. Another advantage of CNT electrodes 

is their ability to bend and flex. Activated carbon electrodes typically require binders and 

conduction additives that do not contribute to overall device capacitance and add weight to the 

system. CNT electrodes do not typically require any type of binder. Several different 

methodologies have immerged in the literature to fabricate flexible CNT based supercapacitor 

electrodes. CNTs have been spin coated (2, 3), filter deposited (4), spray deposited (5), or 

directly grown through chemical vapor deposition (6–8) onto a variety of flexible substrates 

including plastics and carbon cloth. More recently, CNTs suspended with sodium 

dodecylbenzenesulfonate (SDBS) surfactant have been deposited directly onto paper to form a 

porous, conductive CNT electrode (9–10). Hu et al., were able to show a ~3X increase in 

specific capacitance when they compared CNT films deposited onto Xerox paper substrates to 

CNT films deposited onto PET plastic substrates (9). This technique was selected as a starting 
2

point for our supercapacitor electrode fabrication due to its apparent performance advantage 

when compared to other published techniques.

2. CNT Supercapacitor Electrode Fabrication and Testing

In order to realize a packaged flexible supercapacitor, it was necessary to develop an electrode 

fabrication technique. Standard copier paper was chosen as a substrate, and a variety of CNT 

suspensions were deposited onto the substrate by drop casting. A hot plate was used to heat 

1 cm2substrates, which sped up the drop casting process by aiding in solution evaporation. Hot 

plate temperatures ranged from 80 °C to 150 °C depending on the CNT solution being deposited. 

Typically, 25 l of CNT solution was placed on the substrate and allowed to sit until the paper 

dried. The process was repeated until the desired CNT solution volume was deposited onto the 

paper.

CNT/paper electrodes were evaluated both by their CNT film morphology and their 

electrochemical performance as supercapacitor electrodes. CNT film morphology was analyzed 

using an FEI Quanta environmental scanning electron microscope (ESEM). After CNT 

deposition was complete, electrodes were evaluated in the ESEM without further preparation. 

Following ESEM analysis, the electrodes were prepared for electrochemical analysis by 

clamping one edge of the electrode with nickel foil in order to provide an inert connection to the 

measurement apparatus (figure 1). A second piece of Ni foil was used to clamp the electrode to 

an acrylic plate, allowing the entire electrode to be submerged in a liquid electrolyte for testing.

 

Figure 1. (a) CNT/paper electrode with an attached Ni foil clamp to provide good electrical contact 

to the CNT film. (b) Electrochemical half cell experimental setup.

Individual electrodes were tested in a half cell configuration using a Princeton Applied Research 

Versastat 3, in which the CNT/paper electrode was the working electrode and a piece of Ni foil 

was the counter electrode. A Warner Instruments Ag/AgCl pellet electrode was used as the 

a b
3

reference electrode. The three electrodes were submerged in a beaker of 1M KOH solution. Ni 

foil was used in these experiments because it has a useful electrochemical window in KOH. In 

order to obtain capacitance values for each electrode, cyclic voltammetry was performed. The 

measurements were made under realistic conditions so that measured capacitances reflected 

practically usable values. A review of the literature by Istvan has revealed that changing the 

cyclic voltammograms (CV) scan rate from 5 to 20 mV/s, or the galvanostatic charge/discharge 

current from 10 to 200 mA/g, will reduce the measured capacitance by 20% (11). Therefore,

standard test conditions of 20 mV/s and 200 mA/g or higher were used for all experiments. 

Devices were cycled five or more times before analyzing the CV curves in order to exhaust any 

irreversible redox contributions to the capacitance due to impurities that would exaggerate the 

measured capacitance. Electrode capacitance was calculated by dividing one half of the gap 

between the charge current and the discharge current by the scan rate, and dividing this current 

value by the scan rate. This measurement was made at a voltage location where no redox peaks 

were present. We then dvided this number by the total CNT mass deposited onto the electrode in 

grams, yielding specific capacitance. The CNT mass was calculated by multiplying the volume 

of solution deposited and the CNT solution density. 

2.1 Impact of CNT Solution on Performance

We discovered early in this work that the CNT solution used to fabricate the supercapacitor 

electrode played a critical role in its performance. Initial efforts focused on investigating the 

impact that the CNT solution had on the resulting CNT film morphology and electrode 

electrochemical performance. The commercially obtained CNT solutions, except for sample 4 

described in table 1, were drop cast onto 1 cm2paper substrates. Samples 1–3 were fabricated 

using SWCNTs with different functional groups on the CNT surface. These three solutions also 

contained dispersants from the manufacturer that allowed for a greater concentration of CNTs to 

be suspended in the solutions. We made the solution used to fabricate sample 4 by suspending 

15 mg of CVD grown SWCNT powder in a 10 mg/ml solution of SDBS in deionized water. The 

solution was then bath sonicated for 5 min, followed by 30 min of probe sonication to help 

disperse the CNT powder in the surfactant solution. A previous study of CNT/surfactant 

suspensions in water has shown that SDBS is the most effective surfactant for suspending high 

density CNT solutions (12). SDBS was also the solution used by Hu et al., to successfully 

fabricate CNT/paper electrodes with specific capacitance values approaching 200 F/g (9). The 

fifth sample was fabricated using a commercially purchased low density solution of COOH 

functionalized SWCNTs suspended in water without the use of surfactants.
4

Table 1. Impact of initial CNT solution on CNT/paper electrode performance.

Sample CNT type Solution

Volume Deposited/ 

CNT mass Deposited Resistance

Specific 

Capacitance

1 COOH SWCNT acetone 1 ml/0.4 mg 200–2000 Ohm 3.75 F/g

2 Aminopyrene SWCNT acetone 1 ml/0.4 mg Not conductive 0 F/g

3 Pristine SWCNT acetone 1 ml/0.4 mg 80k–400k Ohm 0.25 F/g

4 CVD SWCNT powder SDBS 0.267 ml/0.4 mg 200–1000 Ohm 13.75 F/g

5 COOH SWCNT water 2 ml/0.1 mg 150–200 Ohm 91.25 F/g

The mass of CNTs deposited onto the paper substrate was calculated based on the volume of 

solution drop cast onto the substrate and the CNT density of the various solutions. Initially, the 

paper electrodes were weighed before and after CNT deposition. However, because of variations 

in the water content of the paper substrate, it was impossible to obtain an accurate measurement 

of the deposited CNT mass. The densities of the CNT solutions were confirmed by dispensing a 

known volume of CNT solution onto an aluminum foil sheet and weighing the sheet before and 

after deposition.

The resistance values of the CNT films covering the paper substrates were measured using a 

digital ohmmeter. The paper was probed repeatedly at different locations, which is why a range 

of resistance values appears. This is not intended to be a quantitative measure of the true sheet 

resistance of each electrode, but rather a comparative measure between the five samples above. 

Specific capacitance values were calculated from cyclic voltammetry measurements described 

above. 1M KOH was used as the liquid electrolyte, and scan rates of 20 mV/s were used for all 

five sample measurements.

As can be seen in table 1, the specific capacitance values achieved by the five electrode samples 

vary widely. The surfactant-free COOH SWCNTs in water clearly outperform the other CNT 

solutions, reaching a specific capacitance of 91.25 F/g. This value is on par with the best CNT 

supercapacitor electrodes that do not use pesudocapacitive materials. We believe that the 

surfactants and dispersants that are used to suspend the CNTs in the other solutions are having a 

detrimental effect on the electrode performance. This theory is supported by the CNT film 

morphologies observed under ESEM analysis in figure 2, where CNT bundling and 

contamination are clearly present in samples 1 and 4. CNT bundling was also observed in 

sample 5, but the severity of the bundling was clearly less than the other samples. There were 

also no signs of the additional contamination that was observed in samples 1 and 4. Samples 2 

and 3 (not shown) also contained CNT film contamination similar to samples 1 and 4. It is likely 

that the open pore structure seen in sample 5 allows electrolyte ions to more completely 

penetrate the CNT film, resulting in higher specific capacitance.
5

Figure 2. ESEM images of COOH SWCNTs suspended in acetone (a) (sample 1), SWCNTs in SDBS/diH2O 

(b) (sample 4), and COOH SWCNTs in H2O (c) (sample 5), deposited on paper substrates. Scale bars 

= 500 nm.

2.2 Impact of Surfactant on Performance

The use of surfactants and dispersants in the CNT solutions appear to have a negative impact on 

the overall device performance, resulting in excessive CNT contamination. We were unable to 

achieve the ~200 F/g specific capacitance described by Hu et al., using their SWCNT/SDBS 

solution protocol (9). It is possible that differences in the starting CNT materials resulted in the 

differences in specific capacitance seen between sample 4, 5, and Hu’s work. In order to 

confirm the impact of SDBS surfactant on electrode performance, a new CNT solution was 

made. 10 mg SDBS powder was mixed with 2 ml of the COOH SWCNT in water suspension 

used to fabricate sample 5. This eliminated the variability between nanotubes used, and should 

provide an accurate measure of the impact of having SDBS surfactant present in the CNT 

solution. Sample 8 was made by drop casting 2 ml of the new solution onto a paper substrate. 

CNT film morphology was clearly impacted by the addition of the SDBS surfactant to the 

COOH SWCNT/water solution. The CNT film porosity has been visibly degraded, leading to 

reductions in the discharge current of the CV plots in figure 3. Table 2 shows a 10X reduction in 

specific capacitance due to the surfactant addition, with a large increase in resistance compared 

to sample 5. This indicates that the CNT conducting network is degraded, and the surface area 

available for the electrolyte ions to access has been reduced.

After device testing was complete, sample 8 was soaked in a deionized water bath for 30 min in 

an attempt to wash away unwanted surfactant molecules. We then loaded the sample into a 

furnace and heated to 180 °C for 2.5 hr. Additional device testing was performed, and the results 

are shown in table 2 under sample 8*. The washing and heat treatment did improve the electrode 

performance, most likely by driving off some of the surfactant, but electrode performance was 

still 3X below the sample that did not contain surfactant. Further rinsing and heat treatments did 

not improve device performance. The results of this study indicate that surfactants should be 

avoided in order to preserve CNT film porosity and to maximize electrode conductance and 

capacitance.

a b c
6

Figure 3. CNT film morphology and CV plots of SWCNT supercapacitor electrodes both with and without 

SDBS surfactant. ESEM images show (a) sample 5 with no SDBS, and (b) sample 8 with SDBS. CVs

were taken at a scan rate of 20 mV/s and a scan range of –0.7 V to +0.3 V for both (c) sample 5, and 

(d) sample 8. ESEM scale bars = 500 nm

Table 2. Comparison of electrodes fabricated with and without SDBS surfactant. Sample 8* has undergone 

rinsing and heat treatment to remove unwanted surfactant molecules.

Sample CNT type Solvent

Volume Deposited/ 

CNT mass Deposited Resistance

Specific 

Capacitance

5 COOH SWCNT water 2 ml/0.1 mg 150–200 Ohm 91.25 F/g

8 COOH SWCNT SDBS 2 ml/0.1 mg 50–200 kOhm 10.9 F/g

8* COOH SWCNT SDBS 2 ml/0.1 mg 0.8–1.5 kOhm 28.8 F/g

2.3 CNT Mass Loading Study

In order to ascertain if the entire CNT film was being accessed by the electrolyte, we performed 

a mass loading study. Three electrodes (samples 5–7) were fabricated using COOH SWCNTs in 

water, as described in table 3. The only variable changed in this experiment was the volume of 

CNT solution that was deposited onto the paper substrates. If the electrolyte is not able to 

a b

c d
7

penetrate the CNT film, the specific capacitance would be reduced by roughly 50% for each 

doubling of overall CNT mass because the added mass would not increase the overall 

capacitance of the electrode.

CVs, shown in figure 4, were captured for all three samples under the same experimental 

conditions. Again, 1M KOH was used as the electrolyte, and a scan rate of 20 mV/s was used to 

generate the plots. A scan range of –0.8 V to +0.3 V was used for all three scans. The roughly 

rectangular shape of the CV plots is indicative of capacitive charge storage.

As seen in table 3, no clear trend in specific capacitance was observed when the CNT mass 

loading was increased. The differences in specific capacitance between the three samples may 

have been due to variabilities in the drop casting process, which is done by hand and does not 

ensure consistent CNT film coverage across the entire substrate. However, it is clear than the 

liquid electrolyte is able to penetrate the varying thicknesses of the CNT films of samples 6–8. If 

this was not the case, a clear downward trend in specific capacitance would have been observed.

Figure 4. Cyclic voltammograms of sample 5, sample 6, and sample 7 using a 20 mV/s scan rate. 

Table 3. Impact of mass loading on CNT/paper electrode performance.

Sample CNT type Solvent

Deposited Volume/ 

CNT mass Deposited Resistance

Specific 

Capacitance

6 COOH SWCNT water 1 ml/0.05 mg 100–300 Ohm 101.5 F/g

5 COOH SWCNT water 2 ml/0.1 mg 150–200 Ohm 91.25 F/g

7 COOH SWCNT water 4 ml/0.2 mg 80–160 Ohm 115.6 F/g
8

2.4 MWCNT Electrodes

In addition to SWCNT electrodes, several multi-walled CNT (MWCNT) electrodes were made. 

It is expected that the specific capacitance of MWCNT electrodes would be less than a 

comparable electrode made using SWCNTs because the electrolyte ions cannot access the inner 

shells of the MWCNTs. This means that the inner tubes of a MWCNT would add weight to the 

electrode without providing any additional capacitance. MWCNT electrodes were expected to 

be more conductive due to the fact that MWCNTs are metallic, whereas SWCNT solutions 

contain a mixture of semiconducting and metallic CNTs. A commercially available MWCNT 

suspension in water with no added surfactants or dispersants was used to fabricate the 

MWCNT/paper electrodes. 1 ml of 0.05 mg/ml MWCNT solution was drop cast onto a paper 

substrate that was placed on a 150 °C hot plate. The sample was then imaged, and finally tested 

in the electrochemical half cell measurement setup with the results shown in figure 5.

Figure 5. CV (a) and ESEM image (b) of a MWCNT/paper supercapacitor electrode. 

ESEM scale bar = 500 nm

The electrode CNT morphology appears similar to the SWCNT electrodes. An open pore 

structure is maintained and no obvious signs of contamination are present. The electrode had a 

specific capacitance of 45 F/g, roughly half of the capacitance of a SWCNT electrode. The CV 

plot does show several peaks that are more pronounced than observed in the SWCNT electrodes. 

These may be due to surface groups on the MWCNTs that were not present on the SWCNT 

devices. The specific capacitance of this electrode was evaluated at a potential on the CV plot 

that was free of obvious redox peaks that could confuse the results (–0.15V was used for this 

electrode). We observed a resistance of 50–150 ohms which is lower than the typical SWCNT 

electrode. 

a 45 F/g b
9

3. CNT Flexible Supercapacitor Device Assembly

Following the CNT/paper electrode development work, we considered the issue of electrolyte 

selection and packaging. Liquid electrolytes are advantageous for many systems due to their 

superior ion mobility when compared to solid or gel electrolytes. However, liquid electrolytes 

are more prone to leaking when used in a flexible device. Electrolyte leakage not only 

compromises the functionality of the device, but could cause damage to its surrounding 

environment. In order to address these shortcomings, the performance of liquid KOH 

electrolytes was compared to a PVA:KOH gel electrolyte.

For liquid KOH supercapacitors, a 1M KOH solution was used. The PVA:KOH gel electrolyte 

was prepared in a ratio of 3:1 PVA to KOH (13). To begin, a 70 mg/ml solution of PVA in 

deionized water was made. The solution was heated to a temperature of 130 °C and stirred 

continuously until the liquid turned from opaque to clear. Following the solution clearing, the 

appropriate amount of 1M KOH was added and allowed to mix for 5 min to achieve the proper 

ratio. The PVA:KOH solution was then poured into PTFE dishes and allowed to dry into a moist 

gel. The result is a very robust opaque white elastomeric gel. Care must be taken not to allow 

the gels to dry completely, as they lose flexibility as well as their ability to perform as an 

electrolyte.

The material chosen to encase the electrodes and electrolyte was a flexible polyester film. The 

film was sealed using a thermal sealer to melt the polyester together and form a pouch that 

encapsulates the device. The film did not degrade in the presence of KOH. Assembling full 

devices also required a new method for making electrical contact with the electrodes. To 

accomplish this, the same Ni foil clip was applied to one edge of the electrode. A thin Ni wire 

was then tack welded to the Ni foil clip. This provided good electrical contact, and also allowed 

the polyester pouch to be sealed around the thin wire to limit electrolyte leakage.

Three packaged supercapacitor structures were fabricated and compared, and are represented 

schematically in figure 6. The liquid KOH electrolyte architecture used 1M KOH as the 

electrolyte. Two CNT/paper electrodes were arranged so that the CNT films were facing each 

other. The two electrodes were separated by a third piece of plain copy paper that had been 

soaked in the KOH solution. The stack was assembled in a polyester pouch, which was then 

thermally sealed around the device. The gel electrolyte sandwich architecture used a PVA:KOH 

gel electrolyte. Again, two CNT/paper electrodes were assembled facing each other. A piece of 

PVA:KOH gel was cut to the appropriate size and placed between the two electrodes. This gel 

served as both the electrolyte and the separator that prevented electrode shorting. Again, the 

electrode/electrolyte stack was sealed in a polyester casing. The cast gel electrolyte architecture 

involved casting the electrolyte gel over and around CNT/paper electrodes, encapsulating the 

individual electrodes. This was done by placing each electrode in a small dish, and pouring hot 
10

liquid PVA:KOH solution over the electrodes. The gel was allowed to form, encapsulating the 

electrodes. Two gel encapsulated electrodes were then sandwiched together without any 

separator, and sealed in the package. We hypothesized that the gel casting process would allow 

the gel electrolyte to more completely penetrate the CNT film, proving superior capacitance and 

increased performance stability under bending when compared to the gel sandwich structure.

Figure 6. Full cell packaged flexible supercapacitor architectures.

One additional issue that arose when assembling the packaged supercapacitors was non-flat 

CNT/paper electrodes. During CNT deposition, it was common for the paper to become 

wrinkled due to the repeated wetting and drying that it was subjected to. This became a problem 

when assembling packaged devices. It also made casting electrodes in the gel electrolyte very 

difficult because it was necessary to completely cover the electrodes with the gel in order to 

prevent device shorting. To address these issues, the CNT/paper electrodes were flattened prior 

to being fitted with the Ni clips and assembled. The electrodes were first soaked in deionized 

water for 1 min to wet the substrates. The electrodes were removed from the water bath and 

sandwiched between a piece of absorbent cleanroom paper and a thick glass microscope slide. A 

small amount of weight was placed on top of the glass slide, forcing the paper completely flat. 

The electrodes were left in this configuration for several hours until the substrates were dry. 

This method was successful in producing very flat CNT/paper electrodes, which made device 

assembly much easier.
11

4. CNT Flexible Supercapacitor Device Testing

Device testing began by constructing three packaged devices. One of each device configuration 

described above was constructed using MWCNT/paper electrodes as described in section 2.4. 

Assembled devices were tested in a full cell configuration. All CVs were taken at 20 mV/s scan 

rates. Note that an assembled full cell supercapacitor is actually two capacitors in series, where 

the total capacitance Ct=(C1 X C2)/(C1 + C2). The devices were tested both flat and under static 

bending with the results shown in figure 7. Static bend tests were performed by wrapping the 

packaged devices around mandrels of varying diameters and taking CV measurements under 

each bending condition. We used rubber bands to secure the supercapacitors to the mandrels. A 

pressure test was also conducted on each device, where a small amount of weight was placed on 

top of the supercapacitor package, and CV measurements was taken under this condition.

Figure 7. Cyclic voltammagrams of (a) Liquid KOH electrolyte packaged supercapacitor, (b) PVA:KOH gel 

sandwich packaged supercapacitor, and (c) PVA:KOH cast gel packaged supercapacitor under varying 

states of bending.

a b

c

30 F/g 20 F/g

18 F/g
12

The liquid electrolyte device showed the best energy storage performance, with a specific 

capacitance of 30 F/g. This was expected, as the KOH ions are much more mobile in the liquid 

than they are in the gel matrix. There was some variability as the bending conditions were 

changed, but no appreciable trend was observed. The gel sandwich supercapacitor showed 

greater performance variability due to the loading conditions on the device. The flat or unloaded 

test showed very little capacitance. Applying pressure to the device increased the capacitance, as 

did the bend tests. No appreciable variability was observed between the four different bend 

diameters. It is believed that the pressure applied during the pressure and bending tests forced 

the PVA:KOH gel electrolyte into more intimate contact with the electrodes, improving ion 

intercalation and energy storage. It is also possible that liquid KOH is released from the gel 

during bending, which wet the electrodes and improved device performance. The gel cast 

supercapacitor showed the lowest specific capacitance (18 F/g), but showed no sensitivity to its 

loading conditions. The lower capacitance observed in the gel cast supercapacitor may be due to 

PVA molecules occupying some of the CNT surface area, making them inaccessible to KOH 

ions.

5. Conclusions

Flexible packaged supercapacitor devices have been successfully fabricated and tested. 

Electrode fabrication studies showed a strong correlation between CNT film cleanliness and 

electrode performance. Dispersant agents used to suspend CNTs in solution proved detrimental 

to electrode performance, resulting in low specific capacitance values. Electrodes fabricated 

using commercially available dispersant-free CNT suspensions in water from Brewer Scientific 

proved to produce the best performing CNT/paper electrodes, with specific capacitance values in 

excess of 100 F/g.

Flexible full cell capacitors were produced in three different configurations. Each packaged 

device architecture produced unique device performance. The liquid electrolyte device produced 

the highest specific capacitance, but the possibility of electrolyte leakage is a major disadvantage 

of this device design. The gel sandwich supercapacitor showed the second best device 

performance under bending, but its performance was variable under different bending conditions. 

This variability is undesirable, as it would introduce changes in energy storage capability under 

different system loading conditions. The gel cast supercapacitor had the lowest specific 

capacitance of the three assembled devices. However, the fact that device performance did not 

vary under any of the bending conditions, coupled with the low likelihood of electrolyte leakage 

when compared to a liquid electrolyte device, makes the gel cast device architecture attractive 

despite the performance penalty. 
13

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14

List of Symbols, Abbreviations, and Acronyms

CNT carbon nanotube

CVs cyclic voltammograms

ESEM environmental scanning electron microscope

MWCNT multi-walled CNT

SDBS sodium dodecylbenzenesulfonate

SWCNTs single walled carbon nanotubes
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