Many of the great ideas to have shaped the modern world have been communicated on paper. The beauty of this humble medium is that it can record information in such an enduring fashion. Now, paper itself is at the centre of a technological innovation that could play a part in the 21st-century quest for green-energy solutions.

The great recording power of paper, which has been refined over the centuries, results from the interaction of ink with the 3D porous structure of the fibres in the paper. In addition, well-controlled electrical charges and reactive chemicals on the surface help paper to cling onto its ink. Perhaps not surprisingly, researchers are already exploring options for applying paper to bourgeoning research fields such as flexible electronics and microfluidics.

Diverse tubes

New experiments by Yi Cui and colleagues at Stanford University in the US show that paper could also be used as a cheap and effective option for energy storage. They coated an ordinary piece of paper with a mixture of ink, single-walled carbon nanotubes (SWNTs) and silver nanowires before heating to seal the coating.

SWNTs – which can be thought of as rolled-up single sheets of carbon – possess a diverse range of electronic properties that have already been utilized for simple electronic devices such as transistors. Cui and his team realized that, in addition to being highly conductive, SWNTs can also adhere well to the pores in paper due to their small diameters.

In a series of tests, the researchers showed that these two properties combine to give their sheets a resistance as low as 1 Ω for a coating that was just 500 nm thick. Moreover, they demonstrated that the material has a specific capacitance of 200 F per gram, which is maintained over 40,000 charging cycles. These electrical properties as well as the robust nature of their coated paper would make it suitable for improving the performance of lithium-ion rechargeable batteries and supercapacitors, they claim.

Flexible options

The researchers also report that electrical conductivity is maintained at the same level even when the paper is folded or rolled up into a narrow tube – a flexibility that could lead to a diverse range of applications. "If I want to paint my wall with a conducting energy-storage device, I can use a brush," Cui says.

The researchers cite electric and hybrid cars as applications that could benefit from a flexible supercapacitor. They also predict that the biggest impact may be in the electricity grid. Excess electricity generated at night, or by renewables such as wind and solar, could be held in large-scale storage for use during periods of high demand.

This research was published in Proc. Natl Acad. Sci. USA.