Air Brush Fabricated Carbon Nanotube 

Supercapacitor Electrodes 

by Matthew H. Ervin and Benjamin S. Miller 

ARL-TR-5368 September 2010 

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Army Research Laboratory 

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ARL-TR-5368 September 2010 

Air Brush Fabricated Carbon Nanotube 

Supercapacitor Electrodes 

Matthew H. Ervin and Benjamin S. Miller 

Sensors and Electron Devices Directorate, ARL 

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Air Brush Fabricated Carbon Nanotube Supercapacitor Electrodes 

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Matthew H. Ervin and Benjamin S. Miller 

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14. ABSTRACT 

Carbon nanotube (CNT) films have a high surface area, as well as outstanding physical and chemical stability. When used in 

creating a supercapacitor electrode, CNTs can result in increased specific capacitance and a lower electrode resistance relative 

to commercial activated carbon electrodes. The result will be a supercapacitor with increased energy and power density, 

making supercapacitors better suited for a variety of Army specific applications. This paper reports on the design, fabrication, 

and testing of supercapacitor electrodes made from single-walled carbon nanotubes (SWCNT), where the CNTs were deposited 

on the substrate using a spray deposition technique. The CNTs were suspended in acetone or water prior to being sprayed onto 

the substrate with an airbrush. Other factors are analyzed in the report, including which electrolyte to use as well as the most 

effective method for spraying the SWCNTs onto a conductive substrate. 

15. SUBJECT TERMS

Carbon nanotube, CNT, supercapacitor, air brush 

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Matthew H. Ervin 

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iii

Contents 

List of Figures iv

List of Tables iv

1. Introduction and Background 1

2. Motivation 1

3. Experiment and Calculations 1

4. Results and Discussion 4

5. Summary and Conclusions 11

6. References 13

List of Symbols, Acronyms, and Abbreviations 14

Distribution List 15 
iv

List of Figures 

Figure 1.  Spray deposition apparatus. The foil substrate is securely placed on a hot plate at a 

constant temperature. A circular stencil is sometimes used to help define the airbrush 

deposit. .......................................................................................................................................2

Figure 2.  The three electrode cell setup used for CV testing. From left to right: A foil 

counter electrode, SWCNT working electrode, and Ag/AgCl reference electrode. ..................3

Figure 3. SWCNT film deposit on Al foil electrode 5. ...................................................................5

Figure 4. SEM images of SWCNT films on Al foil substrate: (a) electrode 3 non-sonicated 

SWCNT film and (b) electrode 5 sonicated and centrifuged SWCNT film. Electrode 3 

shows a more densely packed surface than that on electrode 5 even though electrode 5’s 

film mass was over 50 % more than that of electrode 3’s film mass.........................................6

Figure 5. SEM images of SWCNT films on Al foil substrate: (a) electrode 7 non-sonicated 

SWCNT film zoomed in, (b) electrode 6 sonicated SWCNT film zoomed in, (c) electrode 

7 non-sonicated SWCNT film zoomed out and (d) electrode 6 sonicated SWCNT film 

zoomed out. There is a lot of contamination present. ...............................................................7

Figure 6. ESEM images on electrode 8 after attempted cleaning: (a) 1 micron scale and (b) 

500 nm scale. .............................................................................................................................8

Figure 7. CV readings of the working electrode using the Keithley 4200 or the PAR 

Potentiostat: (a) Electrode 6 & 7 measured on Princeton Potentiostat at a scan rate of 5 

mV/s, (b) Keithley 4200 measurement of electrode 3 at scan rates of 10, 50, and 100 

mV/s, (c) Keithley 4200 measurements of electrode 4 (Al blank) at scan rates of 10, 50, 

and 100 mV/s and (d) Keithley 4200 measurements of electrode 2 at a 10 and 100 mV/s 

scan rate. ....................................................................................................................................8

Figure 8. Similar SEM images of SWCNT films on Ni foil current collectors made from: (a) 

carboxylated CNTs in water with no additives (electrode 9), (b) carboxylated CNTs in 

acetone with dispersant (electrode 10), (c) pristine CNTs in acetone with surfactant 

(electrode 11) and (d) CNTs solubilized in acetone using aminopyrene (electrode 12). .......10

List of Tables 

Table 1. The type of SWCNTs deposited on each electrode as well as the film mass which 

includes the mass of CNTs and any surfactant/dispersants in the deposit. ................................5

Table 2. Capacitance results for NanoLab solutions using CNT solutions in acetone using 

surfactants/dispersants. ..............................................................................................................9

Table 3. Comparison of different CNT solutions for making electrodes. ....................................11
1

1. Introduction and Background 

In order for the batteries used by Soldiers to provide peak power, an increase in battery size and 

weight is necessary, which is becoming a burden. A light, compact alternative to large batteries 

for supplying peak power are electrochemical double layer capacitors. An electrochemical 

double layer capacitor, or supercapacitor, has a much higher energy density than conventional 

capacitors. Supercapacitors also have a higher cycle life, higher efficiencies, and a higher 

specific power than conventional batteries (Pandolfo, 2006). Such supercapacitors will 

complement batteries and fuel cells to produce hybrid systems with extended capabilities. A 

review of the literature has shown that commercially available activated carbon supercapacitors 

using aqueous electrolyte exhibit a specific capacitance of 33 F/g, a specific energy of 13 Wh/kg, 

and a specific power of 0.58 kW/kg (Obreja, 2008). We ultimately aim to improve on these 

values by using the exceptional properties of carbon nanotubes to produce electrodes with 

increased accessible surface area and conductivity, which will result in improved energy and 

power densities. To fabricate these electrodes, we have focused on a spray deposition technique 

called airbrushing (Zhao, 2009; Kaempgen, 2009). A spray deposition technique for preparing 

carbon nanotube (CNT) films over a large area is ideal due to low costs, ease of use, and 

manufacturability. Other ways to prepare CNT electrodes include vacuum filtration, ink jet 

printing, dip coating, and electrostatic spray deposition. (Chou, 2008; Lee, 2009; Kim, 2006; Hu, 

2009) 

2. Motivation 

The purpose of this research is to develop an electrode fabrication process that will optimize the 

CNT electrode capacitance. The main goal is to achieve a specific capacitance in excess of 

120 F/g, which is the maximum achievable with activated carbon, the current industry standard 

(Istvan 2009). This research on CNT supercapacitors will eventually allow Army Soldiers to 

achieve peak power without having to carry as large a battery. 

3. Experiment and Calculations 

The nanotubes used for spraying were either pristine single-walled carbon nanotubes (SWCNTs) 

or carboxylic acid functionalized SWCNTs suspended in acetone or water, with or without the 

aid of surfactants/dispersants. The CNT solutions were commercially obtained from NanoLab, 

Inc. and Brewer Science, Inc.
2

Three different types of SWCNT solution pretreatments were used before the spray deposition. 

Some of the nanotube solutions were sonicated using a probe sonicator for 2 min at 20% 

amplitude and then centrifuged for 8 min at 10,000 RPM; some of the SWCNT solutions were 

only sonicated; and others were used as received. Sonication was used in the hope of debundling 

the CNTs, and centrifugation was used in an attempt to remove remaining CNT bundles so as to 

increase the percentage of single tubes. 

The substrates used for spray deposition were initially strips of aluminum (Al) foil 

approximately 1 × 8.5 cm. The Al foil strip was chosen because of its ease of use and ability to 

withstand high temperatures. Later experiments used nickel (Ni) foil substrates (current 

collectors), as they work well with a potassium hydroxide (KOH) electrolyte. During CNT spray 

deposition, the substrate sat on a hot plate at a temperature of 60–65  ̊C for the initial 

experiments in which we used acetone-based CNT solutions. We used higher temperatures 

(175–200  ̊C) in subsequent experiments to speed the evaporation of acetone- and water-based 

solutions. A circular copper (Cu) weight was placed directly on top of the substrate to prevent 

movement. The SWCNTs were sprayed at a constant pressure of 6–8 psi. Figure 1 shows this 

spray deposition apparatus. 

 

Figure 1. Spray deposition apparatus. The foil substrate is securely placed on a hot plate at a constant 

temperature. A circular stencil is sometimes used to help define the airbrush deposit. 

The type of spray nozzle used on the airbrush was determined by the area we intended to cover 

and the need for low pressure spraying to prevent splatter. The nozzle that we sprayed with 

produced a very fine mist at a relatively low pressure range. 

Air Brush 

Aluminum current 

collector 

Spray mask 
3

A scanning electron microscope (SEM) and an environmental scanning electron microscope 

(ESEM) were used to image the deposited SWCNT films. 

The weight of the deposited SWCNTs was obtained using a microbalance. But for calculating 

the specific capacitances, the estimated weight of CNTs from the solution volumes is used as the 

associated mass of surfactant/dispersant is not contributing to the capacitance. The 

electrochemical, cyclic voltametry, measurements were made using a Keithley 4200 

Semiconductor Characterization System or a Princeton Applied Research (PAR) Potentiostat in a 

three electrode set up. As seen in figure 2, the SWCNT film electrode was used as the working 

electrode, either a Cu, Ni, or Al foil strip was used as the counter electrode, and a silver/silver 

chloride (Ag/AgCl) electrode was used as the reference electrode. The electrolytes used were 4 

M HNO3 (1:3 conc. HNO3:H2O, pH=1), dilute 9E-6 M HNO3 that had a pH=4.5 (a pH at which 

the oxide coating on the aluminum current collector is stable), or 1 M KOH. The latter was used 

with the Ni current collectors. 

Figure 2. The three electrode cell setup used for CV testing. From left to right: A foil counter electrode, 

SWCNT working electrode, and Ag/AgCl reference electrode. 

The cyclic voltamogram (CV) was performed in potential ranges of anywhere between –0.9–0.5 

V at scan rates ranging from 1–100 mV/s. Equation 1 was used to calculate the capacitance of 

the working electrode: 
4

 Capacitance = current / scan rate (1) 

The capacitance is in F, the current is in amps, and the CV scan rate is measured in V/s. The 

current is anywhere from mA to nA measured from the CV. In particular, the current is 

measured from the reductive side of the CV loop at the open circuit potential, where reduction 

and oxidation (redox) reactions, if present, are in equilibrium. This avoids including redox 

reaction contributions in the calculation of the double-layer capacitance. If the deduction side of 

the CV curve is not below zero amps, then half the difference between the oxidative and 

reductive sides of the CV loop at the open circuit potential is measured and used to calculate the 

capacitance. Typically, this approach is not needed when a proper potentiostat is used. The 

specific capacitance is calculated by dividing the capacitance of the working electrode by the 

mass of CNTs in the film, as estimated from the concentration and volume of CNT solution used. 

(These specific capacitances are not directly comparable to those for commercial capacitors, 

since those include the mass of the entire device including the packaging, electrolyte, etc.) 

4. Results and Discussion 

Table 1 shows how the NanoLab SWCNTs solutions were prepared before being deposited onto 

Al foil substrates. Spray deposition of the SWCNT suspensions took 5–10 min and covered a 

~1 cm2 area. A SWCNT film deposit is shown in figure 3. During spraying, the acetone would 

evaporate on contact as long as there wasn’t too much liquid on the substrate. If the liquid began 

to bead on the substrate, the acetone would sit for a few seconds before evaporating. We heated 

the substrate to a higher temperature in subsequent experiments to speed the evaporation. 

Increasing the size of the spray area could be favorable in achieving a more uniform evaporation, 

in that the air brush could be in constant motion, preventing the acetone from beading up as with 

a stationary deposition. By avoiding large liquid accumulation on the surface, it is hoped that 

bundling of the CNTs will be kinetically minimized during the drying process. This may help 

maximize the ultimate surface area and, therefore, capacitance. 
5

Table 1. The type of SWCNTs deposited on each electrode as well as the film mass which includes the mass of 

CNTs and any surfactant/dispersants in the deposit. 

Electrode 

CNT 

functionalization Sonicated Centrifuged 

CNT film 

Mass (mg) 

Volume 

Sprayed (mL) 

1 Pristine Yes Yes 1.4 4 

2 Pristine Yes No 1.06 1 

3 Pristine No No 1.16 1 

4 None-blank No No 0 0 

5 Pristine Yes Yes 1.92 3 

6 Pristine Yes No 1.16 1 

7 Pristine No No 0.74 1 

8 Carboxylic No No 0.35 1 

Figure 3. SWCNT film deposit on Al foil electrode 5. 

Figure 4 shows a comparison of a sonicated and centrifuged (#5) SEM image to a non-sonicated 

(#3) SEM image. The sonicated and centrifuged SWCNT film had the higher mass even though 

there seemed to be less SWCNTs on the substrate shown in SEM images, as well as capacitance 

readings. Because there is more mass present in the centrifuged electrode film, but not as many 
6

SWCNTs, it leads to the conclusion that there is a lot more contamination deposited on this film 

than expected. Presumably, the centrifugation removed the CNTs (CNT bundles) more 

effectively than the contaminants (surfactants/dispersants). 

The surface morphology of the spray-deposited CNT films can be seen in figures 4 and 5. 

Although the SWCNTs are well-entangled and interconnected, they are not as dense as we had 

initially hoped for them to be. There also does not seem to be a difference between the sonicated 

and non-sonicated electrodes in SEM images. There is a lot of contamination present that seems 

to be clogging the pores expected in these films, decreasing capacitance. The solutions that these 

CNT films were made from used dispersants and surfactants to aid the solubilization of the 

CNTs, but which contaminate the final films. 

 

Figure 4. SEM images of SWCNT films on Al foil substrate: (a) electrode 3 non-sonicated SWCNT film and 

(b) electrode 5 sonicated and centrifuged SWCNT film. Electrode 3 shows a more densely packed 

surface than that on electrode 5 even though electrode 5’s film mass was over 50 % more than that of 

electrode 3’s film mass. 

a b
7

 

 

Figure 5. SEM images of SWCNT films on Al foil substrate: (a) electrode 7 non-sonicated SWCNT film zoomed 

in, (b) electrode 6 sonicated SWCNT film zoomed in, (c) electrode 7 non-sonicated SWCNT film 

zoomed out and (d) electrode 6 sonicated SWCNT film zoomed out. There is a lot of contamination 

present. 

A few methods were used to clean the electrodes, but none worked well. Soaking electrode 8 in 

toluene, acetone, and isopropyl alcohol (IPA) did not seem to get rid of the contamination. 

ESEM images taken before and after the cleaning were nearly identical. Annealing the electrode 

to approximately 300 °C in hydrogen also did not clean the film. A post-clean attempt on 

electrode 8 can be seen in figure 6. There seems to be a film covering all of the SWCNTs. 

a b

c d
8

 

Figure 6. ESEM images on electrode 8 after attempted cleaning: (a) 1 micron scale and (b) 500 nm scale. 

The CVs of some of the SWCNT electrodes are shown in figure 7. For all of the films, the 

specific capacitance will decrease as the scan rate increases because of kinetic limitations of the 

ion diffusion into the pores of the entangled SWCNTs. There were two different types of HNO3

electrolytes used during the CV tests in figure 7 and table 2. One of the electrolytes was diluted 

1:3 with de-ionized water, while the other was diluted 1:450,000 with de-ionized water. 

CNT Electrode CV

-300

-200

-100

0

100

200

300

-1000 -900 -800 -700 -600 -500 -400 -300

Voltage (mV)

Current (nA)

Electrode 6 Electrode 7

 

 

Figure 7. CV readings of the working electrode using the Keithley 4200 or the PAR Potentiostat: (a) Electrode 6 

& 7 measured on Princeton Potentiostat at a scan rate of 5 mV/s, (b) Keithley 4200 measurement of 

electrode 3 at scan rates of 10, 50, and 100 mV/s, (c) Keithley 4200 measurements of electrode 4 (Al 

blank) at scan rates of 10, 50, and 100 mV/s and (d) Keithley 4200 measurements of electrode 2 at a 10 

and 100 mV/s scan rate. 

a b

a b

c d
9

Table 2. Capacitance results for NanoLab solutions using CNT solutions in acetone using 

surfactants/dispersants. 

Electrode Scan Rate (mV/s) Electrolyte Capacitance 

Specific 

Capacitance (F/g) 

6 5 Dilute .014 

7 5 Dilute .016 

3 10 Concentrated 30mF 22 

3 50 Concentrated 17 

3 100 Concentrated 13 

4 10 Concentrated 25mF N/A (no SWCNT) 

4 50 Concentrated 32mF N/A (no SWCNT) 

4 100 Concentrated 23mF N/A (no SWCNT) 

2 10 Concentrated 20mF 19 

2 100 Concentrated 6 

We found that when making CV tests of the SWCNT electrodes in a higher concentration HNO3

electrolyte, the film will eventually slip off of the substrate, ruining the electrode, which 

happened with electrode #8. The high concentration of HNO3 was etching the aluminum oxide 

under the SWCNT film, causing it to detach. The SWCNTs, though, did not seem to be 

detaching from each other. The shape of the CNT film remained intact when floating in the 

HNO3. When the electrodes were left overnight in the dilute electrolyte solution, we found that 

the SWCNT film remained intact on the Al foil substrate. 

Figure 7 (a) contains CV curves of electrodes #6 and 7 taken on the Princeton Potentiostat in the 

dilute HNO3 electrolyte. The specific capacitance found while performing CV curves in the 

dilute electrolyte was over 1000 times less than the specific capacitance found while testing in 

the concentrated electrolyte (e.g., figure 7b). This indicates that the dilute electrolyte is not 

suitable for making these measurements. As a result, more experiments were performed using a 

1 M KOH electrolyte and Ni current collectors, which are more typical materials for such 

experiments. Electrode 3, shown in figure 7b, was tested using the Keithley 4200 with a much 

more concentrated solution, and showed significantly higher capacitance. However, when a 

blank current collector is tested, it can be seen to have similar capacitance (figure 7c), so that the 

CNT film in electrode #3 does not appear to be contributing much if any capacitance. Similarly, 

the capacitive current measured for electrode #2 (figure 7d) is also similar to the blank. We 

believe that the poor capacitance of these CNT films is due to the contamination in these CNT 

films. 
10

In an attempt to improve the CNT electrode capacitance above that of a blank current collector, 

we purchased a CNT solution that does not include surfactants or dispersants from Brewer 

Science, Inc. This solution was tested along side of the NanoLab CNT in acetone with surfactant 

solutions used previously. Figure 8 shows SEM images of electrodes 9 and 11–13. In figure 8a, 

the “clean” Brewer Science solution is shown to produce a film with small pores and small CNT 

bundles. The carboxylated CNT solution that includes dispersant produces a porous electrode, as 

well, although the CNT bundles look larger due to associated contamination. The remaining two 

images show that the electrodes made with surfactants/dispersant or aminopyrene are so 

contaminated with these additives that the films are not porous and appear to charge in the SEM. 

These additives may be reducing capacitance by reducing the electrode surface area (no pores) 

by increasing the distance of approach between the electrolyte ions and the CNTs, as well as by 

increasing the electrode resistance by preventing the CNTs from making good electrical contact 

between themselves and the current collector. 

Figure 8. Similar SEM images of SWCNT films on Ni foil current collectors made from: (a) carboxylated 

CNTs in water with no additives (electrode 9), (b) carboxylated CNTs in acetone with dispersant 

(electrode 10), (c) pristine CNTs in acetone with surfactant (electrode 11) and (d) CNTs solubilized 

in acetone using aminopyrene (electrode 12). 
11

CV testing of electrodes 9–14 yields the capacitance results shown in table 3. Electrode 14 is a 

Ni current collector without any CNTs, which is used as a blank sample to measure the 

capacitance due to the Ni current collector. This capacitance is subtracted from each electrode’s 

measured total capacitance to obtain a capacitance representative of the CNT’s contribution 

alone. These corrected capacitances are divided by the estimated CNT masses, resultant from 

the CNT concentrations of the solutions, to obtain the reported specific capacitances. Not 

surprisingly, electrodes 12 and 13, which appeared nonporous and somewhat electrically 

insulating in the SEM, yielded little or no increase in capacitance due to the CNT deposits. 

Electrodes 10 and 11 made with the dispersant containing CNT solution had somewhat better 

capacitance, presumably due to the increased surface area caused by the porosity of the deposit. 

The clean deposit obtained using the surfactant/dispersant-free solution yielded the best specific 

capacitance of all. This could be due to a range of factors including the porosity, small CNT 

bundles, and the cleanliness of the CNTs in the electrode, which allows the electrolyte ions to 

approach close to the CNTs. In a traditional capacitor, the capacitance is proportional to the 

overlapping area of the electrodes and inversely proportional to the gap between the electrodes 

(the dielectric thickness). A properly made supercapacitor is able to store so much energy in part 

because the ions can adsorb within a few atomic distances of the CNT electrode. 

Table 3. Comparison of different CNT solutions for making electrodes. 

Electrode CNT solution 

Surfactant 

Or 

Dispersant? 

CNT 

solution 

volume 

(ml) 

CNT 

mass 

(μg) 

Total 

Capacitance 

(mF) 

Specific 

Capacitance 

(F/g) 

9 

Brewer Sci –

carboxylic N 1 50 2.9 53 

10 

NanoLab - 

carboxylic Y 0.5 200 0.55 1.5 

11 

NanoLab - 

carboxylic Y 0.25 100 0.8 5.5 

12 

NanoLab - 

pristine Y 0.25 100 0.25 — 

13 

NanoLab - 

aminopyrene Y 0.25 100 0.19 — 

14 None (blank) N/A 0 0 0.25 — 

5. Summary and Conclusions 

Part of this work consisted of developing the electrode fabrication and testing protocols for 

investigating CNT-based supercapacitor electrodes. Our initial results showed that using Al 

current collectors with a nitric acid electrolyte was not a viable system. While the Al current 

collector electrodes were stable in dilute nitric acid, the measured capacitances were low, 

presumably due to electrolyte resistance. At higher concentration, the electrodes were no longer 
12

stable. After switching to Ni current collectors and KOH electrolyte, our electrodes were stable 

and yielded useful capacitance data. A comparison of electrodes formed by airbrush deposition 

of various commercial CNT solutions yielded a wide range of specific capacitances. In 

particular, solutions that used surfactants and/or dispersants produced poor specific capacitances 

due to contamination of the CNTs, yielding poor porosity and conductivity. Electrodes made a 

solution of carboxylated CNTs and water only produced the cleanest looking electrodes and the 

largest specific capacitance of 53 F/g. Further experiments are underway to determine what role 

the fabrication process, itself, has on the specific capacitance achieved. 
13

6. References 

Pandolfo, A. G.; Hollenkamp. A. F. J. Power Sources 2006, 157, 11–27. 

Zhao, X.; et al. Nanotech 2009, 20, 065605. 

Kaempgen, M.; Chan, C. K.; Ma, J.; Cui, Y.; Gruner, G. Nano Lett 2009, 9, 1872–1876. 

Lee, S. W.; Kim, B.-S.; Chen, S.; Shao-Horn, Y.; Hammond, P. T. J. Amer. Chem. Soc. 2009, 

131, 671–679 (2009). 

Chou, S.-L.; Wanga, J.-Z.; Chew, S.-Y.; Lui, H.-K.; Dou, S.-X. Electrochem. Comm. 2008, 10, 

1724–1727. 

Kim, J. H.; Nam, K.-W.; Ma, S. B.; Kim, K. B. Carbon 2006, 44, 1963–1968. 

Hu, L.; Choi, J. W.; Yang, Y.; Jeong, S.; La Mantia, F.; Cui, L.-F.; Cui, Y. Proc. Nat. Acad. Sci. 

2009, 106, 21490–21494. 

Isvan, R. The 19th Int’l Seminar on Double Layer Capacitors and Hybrid Energy Storage 

Devices, Deerfield Beach, FL 7–9 Dec 2009. 

Obreja, V.V.N., Physica E. 2008, 40, 2596–2605. 
14

List of Symbols, Acronyms, and Abbreviations 

AL aluminum 

CNT carbon nanotube 

Cu copper 

CV cyclic voltamogram 

ESEM environmental scanning electron microscope 

F/g Farads per gram 

IPA isopropyl alcohol 

KOH potassium hydroxide 

mF milliFarad 

mg milligram 

ml milliliter 

Ni nickel 

PAR Princeton Applied Research 

SEM scanning electron microscope 

SWCNT single wall carbon nanotube 
15

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