Batteries made from plain copier paper could make for future energy storage that is truly paper thin. The approach relies on the use of carbon nanotubes - tiny cylinders of carbon - to collect electric charge. While small-scale nanotube batteries have been demonstrated before, the plain paper approach lends itself to making larger devices more cheaply. The work, published recently in Proceedings of the National Academy of Sciences, could lead to ‘paintable’ energy storage. Because of its structure of millions of tiny, interconnected fibres, paper is a good candidate to hold on to carbon nanotubes, providing a scaffold on which to build devices. However, paper is

also mechanically tough, and can be bent, curled or folded, more than the metal or plastic surfaces that are currently used or under development. A team of researchers at Stanford University started with off-the-shelf copier paper, painting it with an 'ink' made of carbon nanotubes. The coated paper is then dipped in lithium-containing solutions and an electrolyte to provide the chemical reaction that generates a battery's electric current. The paper effectively collects the electric charge from the reaction. Using paper in this way could reduce the weight of batteries, typically made with metal current collectors, by 20%. The team's batteries are also capable of releasing their stored energy quickly. This is a valuable characteristic for applications which need quick bursts of energy, such as electric vehicles - although the team has no immediate plans to develop vehicle batteries. Liangbing Hu, lead author on the research, said the most important aspect of the demonstration was that paper is an inexpensive and well-understood material - making wider usage of the technology more likely.