Chongwu Zhou of USC describes a line of prototype devices built from metal oxide nanowires with carbon nanotubes in a newly published paper titled “Flexible and Transparent Supercapacitor based on Indium Nanowire / Carbon Nanotube Heterogeneous Films” in the journal Applied Physics Letters (Vol.94, Issue 4, Page 043113, 2009).

The research continues a line of prototype devices created at the University of Southern California’s Viterbi School of Engineering 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 at prices competitive with the existing technologies.

The latest device is a supercapacitor or ultracapacitor, a circuit component that can temporarily store large amounts of electrical energy for release when needed, a completely transparent and flexible energy conversion and storage device that you can bend and twist like a poker card. The research team believes the device points the way to further applications, such as flexible power supply components in “e-paper” displays and conformable products.

The performance for the device, storing at an energy density of 1.29 Watt-hour/kilogram with a specific capacitance of 64 Farad/gram is a very good start indeed. The familiar conventional capacitors usually have an energy density of less than 0.1 Wh/kg and a storage capacitance of several tenths of millifarads. The USC device is quite a performer.

Zhou holds the Jack Munushiun Early Career Chair at the USC Ming Hsieh Department of Electrical Engineering led team of USC graduate students Po-Chiang Chen and Sawalok Sukcharoenchoke, and post-doc student Guozhen Shen.

The team incorporated metal oxide nanowires with carbon nanotubes to form up heterogeneous films and then optimized the film thickness by attaching transparent plastic substrates to maintain the mechanical flexibility and optical transparency of the supercapacitors. At the storage performance ratings the desire to be transparent is puzzling. Well, they’re at USC after all.

Zhou describes the work as based on combing carbon nanotubes with metal nanowires as an advance on earlier attempts to produce supercapacitors using just carbon nanotubes or graphite. Those efforts yielded modest performance compared to those using transition metal oxide materials, including oxides such as iron, manganese and rubidium. Those materials used to build capacitors have neither the mechanical flexibility nor optical transparency, which have confined their applications to outside the flexible and transparent electronics markets.

The research showed the key improvement in performance would be attributed to the incorporation of metal oxide nanowires with the carbon nanotube films. Indium oxide nanowire has the properties of wide band gap, high aspect ratio, and short diffusion path length, so it can be one of the best candidates for transparent electrochemical capacitors. Indium oxide nanowire has been pioneered in Professor Zhou’s lab over the past several years.

The quote from the news story at USC says these new devices, “demonstrated enhanced specific capacitance, power density, energy density, and long operation cycles, compared to those supercapacitors made only by carbon nanotubes.”

“Carbon nanotube films were fabricated by the vacuum filtration method. An adhesive and flat polydimethysiloxane stamp was adapted to peel the carbon nanotube film off of the filtration membrane and then released it onto a polyethylene terephtalate substrate.”

Carbon Nanotube and Nanowire Build Up. Click image for more info.

Carbon Nanotube and Nanowire Build Up. Click image for more info.

“Indium2Oxygen3 (indium oxide) nanowires with a diameter of ~ 20 nm and a length of ~ 5 μm were synthesized by a pulsed laser deposition method. The as-grown nanowires were sonicated into IPA solutions and then dispersed upon transferred carbon nanotube films to form the In2O3 nanowire /carbon nanotube heterogeneous film for transparent and flexible supercapacitor study.”

“(By) increasing (the) amount of In2O3 nanowires dispersed upon carbon nanotube films, the specific capacitance of the heterogeneous supercapacitor can be dramatically improved from 25.4 Farad/gram up to 64 Farad/gram. (By) comparison to supercapacitors made by other transition metal oxide nanostructured materials, this observation indicates a good stability of In2O3 nanowire /carbon nanotube heterogeneous films for long-term capacitor applications.”

The team’s research not only created metal oxide nanowire/carbon nanotube heterogeneous films as active materials and current collecting electrodes for the supercapacitors, but also examined the stability of the transparent and flexible supercapacitors over a large number of cycling charges/discharges for measurement.

The over riding point is the performance of the composited material. At 1.29 Watt-hour/kilogram with a specific capacitance of 64 Farad/gram just to start, this is promising technology and perhaps a beginning for larger scale production of the carbon nanotubes driving down the cost. The issues would seem to be the raw cost and processing expense to get the indium oxide nanowires in the sandwich.

Zhou's Transparent Supercapacitor

Zhou's Transparent Supercapacitor

Transparency just doesn’t strike this writer as a significant attribute for electrical storage devices at the moment. With contemporary energy used mainly in portable electrically powered devices and soon transportation by several orders of magnitude over “e-paper displays” and such the positioning of the results of the research seem a little odd. Some things are just more pressing than “transparency” for high performance electron storage capacitors.

One might presuppose that the cost to build these is known to be high by the research team so the driving to a limited market with special needs. Yet the research is on to something that needs further examination. Carbon nanotubes offer a lot of surface area and wiring them with nanowires strikes me as a clever and innovative solution to the matter of getting electrons in and out of the holding medium.

USC’s team has very basic, a bit dreamy, but solid research deserving a further look and development. Professor Zhou’s lab is definitely one to watch.


8 Comments so far

  1. Matt on April 3, 2009 6:38 AM

    It would best be described as a Micro Ultra Capacitor. It’s not going to power a Chevy Volt anytime soon.

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