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 is electrochemical double-layer capacitors. These supercapacitors have 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. Such supercapacitors will complement batteries and fuel cells to produce hybrid systems with extended capabilities.

The 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.
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. The exceptional properties of carbon nanotubes (CNTs) can produce electrodes with increased accessible surface area and conductivity, which will result in improved energy and power densities.

To fabricate these electrodes, a spray deposition technique called airbrushing was used. A spray deposition technique for preparing CNT films over a large area is ideal due to low costs, ease of use, and manufacturability.

The electrode fabrication process optimizes the CNT electrode capacitance. The main goal was to achieve a specific capacitance in excess of 120 F/g, which is the maximum achievable with activated carbon, the current industry standard. 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.

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 minutes at 20% amplitude, and then centrifuged for 8 minutes 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 to 65 °C for the initial experiments in which acetone-based CNT solutions were used. Higher temperatures (175 to 200 °C) were used 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.

The type of spray nozzle used on the airbrush was determined by the area intended to be covered and the need for low-pressure spraying to prevent splatter. The nozzle produced a very fine mist at a relatively low-pressure range. 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 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 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 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).

Although the SWCNTs are well entangled and interconnected, they are not as dense as initially thought. 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.

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 stable. After switching to Ni current collectors and KOH electrolyte, the 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.

This work was done by Matthew H. Ervin and Benjamin S. Miller of the Army Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Electronics/Computers category. ARL-0119


This Brief includes a Technical Support Package (TSP).
Air Brush Fabricated Carbon Nanotube Supercapacitor Electrodes

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This article first appeared in the February, 2011 issue of Defense Tech Briefs Magazine.

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