"What is still needed to complement a further deveopment of printed electronics device technology are truly printable charge storage devices that can be easily fabricated using large-scale, solution-based, roll-to-roll processing, while still displaying good electrochemical performance," Yi Cui tells Nanowerk. "Only fully printable charge storage devices would allow for full integration into the manufacturing process of printed electronics."
Cui, an assistant professor in the Department of Materials Science and Engineering at Stanford University, together with George Grüner's group at UCLA, has been working toward fully printable high-performance supercapacitors based on thin films of single-walled carbon nanotubes (SWCNTs).
One charge storage component that holds great promise for printed electronics is the supercapacitor with its high power density. In their latest work, Grüner and Cui demonstrate that thin film supercapacitors based on SWCNT networks in combination with a printable gel electrolyte have great potential as printed charge storage devices. The paper, first authored by Martti Kaempgen Candace K. Chan, was published in the April 6, 2009 online edition of Nano Letters (Printable Thin Film Supercapacitors Using Single-Walled Carbon Nanotubes).
Cui explains that many high performance supercapacitors use carbon nanotubes because the high surface area can allow for very high capacitances. However, many CNT-based devices are fabricated using methods that cannot be commercially scaled, such as vertically aligned growth onto a metallic substrate.
(a) Scanning electron microscopy image of as-deposited SWCNT networks. (b) Thin film supercapacitor using sprayed SWCNT films on PET as electrodes and a PVA/H3PO4 based polymer electrolyte as both electrolyte and separator. (Reprinted with permission from American Chemical Society)
"Our goal was to make a device that could be fabricated using low-cost methods such a printing without sacrificing performance" he says. "Rather than use an expensive technique to make a few devices, we wanted to take CNT materials that are already being commercialized by the ton and incorporate them into a device using printing techniques that can be used on a large-scale. We have shown that these printed supercapacitors can display good performance while utilizing an inexpensive and scalable manufacturing process."
High surface area, amorphous carbon materials are typically used as the active material in supercapacitors to store charge. However, the poor conductivity of the particle networks makes a metallic current collector a requirement.
In contrast, CNT networks can display very low sheet resistances and can display high porosities and surface areas. These properties make it so that the CNT networks can act as active electrode materials as well as the current collector, combining both functions and simplifying the device architecture. This also makes devices on flexible and light-weight substrates, such as plastic, possible. CNTs can be easily solution processed as an ink.
The team has also explored the use of a gel electrolyte, which can also be printed and can double as a separator between the two electrodes. The end result is that the entire device can be printed.
The device architecture itself is fairly straightforward with two SWCNT thin films serving as both electrodes and charge collectors enclosing the electrolyte. The thin films were fabricated simply by suspending purified SWCNTs in water and then using an air brush pistol to spray it on hot PET substrate. Two of these films were then sandwiched together with the gel electrolyte in the middle.
These printable supercapacitors developed by Grüner's and Cui's labs can be used as an on-chip power source for printed electronics. The flexible and light weight nature of these devices also make them attractive for portable electronics application.
"Even though our devices are not yet optimized in terms of electrical conductivity of the nanotube films and the amount of the various active components, their performance already spans the typical range of conventional supercapacitor devices for all electrolytes used," says Cui. "This can be explained by the increased effective surface area in the thin films maximizing the efficiency of thin film CNT supercapacitors compared to the thick electrodes in regular devices."