"We have a paper battery, supercapacitor and battery-supercapacitor hybrid device that could be used in a variety of energy storage applications,” Linhardt told nanotechweb.org. "These devices are lightweight and flexible and composed of cellulose (paper) – an environmentally friendly and biocompatible material. They also operate over a wide range of temperatures, from –73 to 176 °C, making them useful in a freezer or oven."

Batteries are used to chemically store electrical energy. They are charged by applying electricity to electrodes to separate out positive and negative ions (the electrolyte) across a membrane (the separator). Discharging occurs by attaching a "load", such as a light or a motor, to the same electrodes, which causes the positive and negative ions to move back together.

A charged battery gives up its electricity slowly once a load has been applied. It can then be recharged (if it is a rechargeable battery) and used again and again. A charged supercapacitor – a capacitor that can store a lot of electrical energy – discharges quickly and is used in applications where a great deal of power is needed in a short time. This includes starting motors in hybrid vehicles, for example. Supercapacitors can also be used over and over again.

Linhardt's team has now made batteries and supercapacitors using cellulose paper as the separator, carbon nanotubes as the electrodes and liquid salt as the electrolyte. The researchers made the devices by dissolving the cellulose in an ionic liquid and casting it onto a collection of oriented nanotubes. The cellulose is gelled by cooling, the excess ionic liquid washed away and the resulting membrane peeled off the surface.

The device looks like a piece of paper that is black on one side, from the embedded carbon nanotubes, and white on the other from the exposed cellulose. "Just by folding the paper in half, with the black side out, we have a supercapacitor," explains Linhardt. "If we coat the white side with lithium, we have a battery."

The nanotubes on which the cellulose is cast contact the paper at the molecular level with a huge surface area, allowing the device to efficiently store and release power. "In our supercapacitor we use carbon nanotubes as both electrodes but in our battery we use the nanotubes as one electrode and lithium as the other," says Linhardt. "We can also make a hybrid device that can act as both a supercapacitor and battery."

The paper devices work even if rolled up, bent or twisted and function with a variety of electrolytes, including biological liquids, such as blood and sweat. This means they could be used for implantable medical applications, including pacemakers or drug-delivery devices such as insulin pumps. "They could also be easily carried into the desert," suggests Linhardt. "After using the battery to remove sweat, it could be connected to a lightweight solar cell for charging."

The team now plans to optimise the energy storage in the devices, decrease their resistance and make larger devices. It will also learn how to mass-produce them and test for reliability and safety. A patent on the invention has already been filed.

The work was published in Proc. Natl. Acad. Sci..