Saturday, May 22, 2010

Ultra-thin paper capacitors

If you think that thinner electronic devices are the future, then you are probably right. As time passes by, electronics is becoming smaller, to a scale that is now being referred as nanotechnology. But to power these devices you will probably require a similarly thin power-source. Thanks to Stanford University, researchers there identified that Single Walled Carbon Nanotubes (SWNTs) can be printed on paper, and then treated with polyvinylidene fluoride to create ultra-thin super-capacitors, that will be able to store energy.

The source of energy will also be very stable. “The device also showed an excellent cycling stability, with very little loss of capacitance after 2500 cycles.” In short, these paper-capacitors are the future. We have seen significant developments in nanotechnology in the last ten years. But in order to make ultra-thin computers (a.k.a. computers of the future) a similarly sized power supply has always been necessary. And it sure looks like we are heading to the right direction!

In the paper supercapacitor, all the necessary components are integrated onto a single sheet of paper in the form of single walled carbon nanotubes (SWNTs). High-speed printing could be used to print the SWNTs directly onto a piece of paper - anything from paper to newspaper and even grocery ads will work. At first, the researchers found that the SWNTs were so small that they penetrated the paper through micron-sized pores, which would cause the device to short-circuit. To solve this problem, the researchers first coated both sides of the paper with polyvinylidene fluoride (PVDF), which blocked the pores but still allowed for electrolytes to be transported through the paper. As such, the treated paper could function as an membrane and separator without short-circuiting.

“The key design is that SWNTs stick well on paper and do not penetrate through paper completely to avoid shorting,” Yi Cui of Stanford University

Once the SWNTs were printed onto the treated paper, they experienced strong bonding forces similar to those experienced when writing with a pen or pencil on paper. Even when rubbed or subjected to tape, the SWNTs remained attached to the paper. After printing SWNTs on both sides of single sheets of paper, electrolyte was loaded to form a supercapacitor. The SWNTs served as both the electrodes and current collectors in the supercapacitor, which had a capacitance of about 3 F/g. The device also showed an excellent cycling stability, with very little loss of capacitance after 2500 cycles. The researchers say the same concept could be extended to make batteries, as well.

The fully integrated supercapacitor is based on an earlier version that the researchers made, in which nanomaterials were coated separately onto different anode and cathode substrates and then assembled together with a separator. The advantage of the new integrated structure is that it allows for high-speed printing, which greatly reduces fabrication costs and brings disposable, flexible, and lightweight electronics closer to reality. Cui said that, in the future, the researchers plan “to use this new design for real applications.”

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