If you crack the back off of an iPhone or any other modern portable electronic device, you'll find a big honking battery that takes up a huge amount of space and contributes to a large part of the device's weight. Fuel cells and solar power have both been floated as promising solutions to the battery weight/capacity problem, but new research suggests that carbon nanotubes may eventually provide the best hope of implementing the flexible batteries and supercapacitors needed to shrink our gadgets even more.

Part of the problem with designing flexible batteries and supercapacitors has always been the necessity of layering such devices. Typically, two electrode layers sandwich two charge-holding layers, with an insulating layer in the middle of it all. As the layers build up, flexibility goes out the window.

However, researchers from Rensselaer Polytechnic Institute and MIT have developed a new material that eliminates the need for a multilayer battery. They grew carbon nanotubes on a silicon substrate and impregnated the gaps between the tubes with cellulose—that's right, plain old paper. The cellulose also covered the ends of the nanotubes, but once it had dried, the paper material could be peeled off of the silicon substrate, leaving one end of the carbon nanotubes exposed to form an electrode.





By putting two sheets of paper together with the cellulose side facing inwards (and a drop of electrolyte on the paper), a supercapacitor is formed. These supercapacitors retain the flexibility of normal paper, but they have a rating that is comparable to that of standard commercial hardware—a 100g sheet could replace a 1300mAh battery. Because the medium is flexible, the researchers say you could shape batteries of all sizes for very specific use.

It doesn't stop there, however. By putting a drop of electrolyte on a single sheet and then putting a metal foil consisting of lithium and aluminum on each side, a lithium ion battery is formed. This paper device had a respectable 110mAh/g capacity, and the researchers indicate that small prototypes could already power small mechanical devices like fans. These batteries and supercapacitors are quite stable and have been shown to operate over a wide range of temperatures, with the research showing that they can operate between -78–150°C.

The flexibility (pun intended) of this system shouldn't be understated. Batteries and capacitors can be combined in any way desirable simply by controlling where the electrolyte is placed and where the second sheet of paper is placed. The power density isn't fantastic, but it makes up for that by being able to fit into strange shapes, and it could even be wrapped around the electronics inside a device.

Also noteworthy: bodily fluids can act as the electrolyte, which hints at medical applications. The capacitor would be put into a patient fully charged but dry, and when more power was needed, bodily fluids would be allowed into the device to allow it to discharge.

PNAS, 2007, DOI:10.1073/pnas.0706508104