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University of Southern California Researchers Unveil Supercapacitor That Utilizes Carbon Nanotubes & Metal Nanowires

April 01, 2009 by Jeff Shepard

Researchers from the University of Southern California’s Viterbi School of Engineering have announced that they have developed a completely transparent and flexible energy conversion and storage device that can bend and twist like a poker card. It continues a line of prototype devices that can perform the electronic operations now usually handled by silicon chips using carbon nanotubes and metal nanowires set in indium oxide films, and can potentially do so, according to the researchers, at prices competitive with those of existing technologies.

The device is a supercapacitor, a circuit component that can temporarily store large amounts of electrical energy for release when needed. A team headed by Chongwu Zhou describes it a newly-published paper on "Flexible and Transparent Supercapacitor based on Indium Nanowire / Carbon Nanotube Heterogeneous Films" in the journal Applied Physics Letters. Its creators believe the device points the way to further applications, such as flexible power supply components in "e-paper" displays and conformable products.

The device stores an energy density of 1.29 Watt-hour/kilogram with a specific capacitance of 64 Farad/gram. By contrast, conventional capacitors usually have an energy density of less than 0.1 Wh/kg and a storage capacitance of several tenth millifarads.

Zhou and his group incorporated metal oxide nanowires with carbon nanotubes (CNTs) to form heterogeneous films and further optimized the film thickness attaching on transparent plastic substrates to maintain the mechanical flexibility and optical transparency of the supercapacitors. According to Zhou, the work, based on combing CNTs with metal nanowiers represents an advance on earlier attempts to produce supercapacitors using just CNTs or graphite.

Such efforts resulted in only modest performance compared to those using transition metal oxide materials, including such oxides of iron, manganese and rubidium. Moreover, energy storage devices made by these materials have neither mechanical flexibility nor optical transparency, which have confined their applications in the flexible and transparent electronics.

The critical improvement in performance, according to the research, can be attributed to the incorporation of metal oxide nanowires with CNT films. Indium oxide nanowire, with the properties of wide band gap, high aspect ratio, and short diffusion path length, can be one of the best candidates for transparent electrochemical capacitors. Professor Zhou’s lab has pioneered this material over the past several years.

These new devices, by contrast, "demonstrated enhanced specific capacitance, power density, energy density, and long operation cycles, compared to those supercapacitors made only by CNTs," said the new release.

The researchers not only created metal oxide nanowire / CNT heterogeneous films as active materials and current collecting electrodes for the supercapacitors, but also examined the stability of the transparent and flexible supercapacitors through a large cycle number of charge/discharge measurements.