Paper Battery : Power Source of Future
An energy source made out of paper impregnated with carbon nanotubes has been demonstrated by US scientists. The researchers say the paper, which can be bent, twisted and folded, could be used in flexible electronic devices.
The energy source – a hybrid battery and supercapacitor – was developed by researchers at Rensselaer Polytechnic Institute in Troy, New York, US. A capacitor is like a battery except that, rather than relying on a chemical reaction to store and release energy, it allows electrical charge to build up on a series of conducting plates separated by an insulator.
Conventional capacitors cannot store much energy, but researchers have begun to experiment with “supercapacitors” created using carbon nanotubes as electrodes. The high surface area of these nanotubes can potentially store a relatively large amount of charge.
To make carbon nanotube supercapacitors flexible, Victor Pushparaj and colleagues first grew nanotubes on top of a silicon substrate using standard chemical vapour-deposition.
They then dissolved a mixture of plant cellulose and chloride and spread this among the nanotubes. After peeling this off the silicon substrate they were left with a piece of paper a few tens of micrometres thick, with carbon nanotubes sticking out from one side.
To create the supercapacitor, two sheets of this paper were stuck back-to-back, nanotube side out, with aluminium foil coating both sides as current collectors. When a current was applied, the paper stored the charge, with the carbon nanotubes serving as electrodes and the cellulose as an insulator.
The researchers then used a similar method to create a flexible battery. They used a piece of paper containing carbon nanotubes as a cathode and evaporated a layer of lithium onto the other side to serve as an anode. Once again they sandwiched it between sheets of aluminium foil, which served as current collectors.
Finally, by layering supercapacitors and lithium-ion batteries, they created a hybrid device in which the batteries could be used to charge the supercapacitors.
Pushparaj says that the flexible, energy-storing paper could be ideal for powering devices with flexible plastic screens.
The supercapacitors and batteries in the experiment are not yet energy-dense enough to compete with conventional batteries, however. Pushparaj says the next step will be to develop different formulations of cellulose and electrolyte that will increase their storage capacity.
Joel Schindall, an electrical engineer at the Massachusetts Institute of Technology in Boston, US, calls the new paper a “promising development”. But he says cost and reliability issues will have to be overcome.
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