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Carbon nanotubes turn paper/ink into batteries

December 8, 2009 - Researchers from Stanford U. have devised a way to turn ordinary paper into a battery: slather it with an inky concoction of carbon nanotubes and silver nanowires, and then cook it.

Coating a sheet of paper with ink containing carbon nanotubes and silver nanowires turns the paper into a "supercapacitor," which holds an electric charge like a battery but for a shorter period of time, and stores/discharges it much more rapidly. The particular version they came up with can last through 40,000 charge-discharge cycles, "at least an order of magnitude" better than conventional lithium-ion batteries, they claim. The thicker the coating, the greater the electrical storage/conductivity.

Yi Cui, assistant professor of materials science and engineering at Stanford, had previously done work on making such capacitive creations using plastics, but found the nanoink adheres better to the paper-based versions and makes them more durable; it can be folded and even soaked in acids or bases and performance does not degrade ("We haven't tested what happens when you burn it," Cui quipped in a statement), and the CNTs resist peeling. No added adhesives also eliminates a factor that would otherwise decrease performance and increase production costs, the researchers note.

From their research, published this week by the Proceedings of the National Academy of Sciences:

Here, we show that commercially available paper can be made highly conductive with a sheet resistance as low as 1 ohm per square (Ω/sq) by using simple solution processes to achieve conformal coating of single-walled carbon nanotube (CNT) and silver nanowire films. [...] When only CNT mass is considered, a specific capacitance of 200 F/g, a specific energy of 30-47 Watt-hour/kilogram (Wh/kg), a specific power of 200,000 W/kg, and a stable cycling life over 40,000 cycles are achieved. These values are much better than those of devices on other flat substrates, such as plastics. Even in a case in which the weight of all of the dead components is considered, a specific energy of 7.5 Wh/kg is achieved.

The main application for this work would be large-scale storage of electricity on the grid, in wind farms and solar energy systems. Other potential applications range from serving as the nonmetallic current collector in Li-ion batteries, to brushing onto a wall to create a conductive energy storage device to which LEDs could be connected, to use in electric or hybrid cars. "Society really needs a low-cost, high-performance energy storage device, such as batteries and simple supercapacitors," Cui said. "I don't think it will be limited to just energy storage devices," noted Peidong Yang, professor of chemistry at the U. of California-Berkeley, quoted by Stanford. "This is potentially a very nice, low-cost, flexible electrode for any electrical device."

SEM images of: (A) interface between carbon nanotubes and silver nanowires on Xerox paper, (B) Ag NW film, and (C) CNT film. (D) The resistance scaling with the Ag NW electrode distance. (Inset) Contact resistance measurement scheme. (Source: PNAS)



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