Ordinary paper can be turned into a battery electrode simply by dipping it into carbon-nanotube inks. The resulting electrodes, which are strong, flexible, and highly conductive, might be used to make cheap energy storage devices to power portable electronics.
It’s now possible to print lightweight circuits and screens for electronics like e-readers, but conventional batteries still weigh these devices down. Carbon nanotubes are a promising material for printing batteries because, in addition to their strength, light weight, and conductivity, they can store a large amount of energy–a quality that helps portable electronics run longer between charges.
Now a group of Stanford University researchers, led by materials science professor Yi Cui, have demonstrated that ordinary office paper soaks up carbon nanotubes like a sponge and can be turned into electrodes for batteries and supercapacitors. The advantage of paper, says Cui, is that it’s cheap and interacts strongly with nanotubes without the need for putting additives in the ink. “We take advantage of the porous structure of paper,” says Cui. “Carbon nanotubes absorb into the paper and stick on really tightly.”
After paper is dipped in the nanotube ink and air-dried, it becomes highly conductive. The Stanford group tested the thin films as electrodes in supercapacitors and found that they could store more total energy, and operate at higher currents, than previous printed nanotube devices. Joel Schindall, professor of electrical engineering and computer science at MIT, says the paper supercapacitors store a surprisingly high amount of charge. The Stanford group also tested the paper electrodes as current collectors in lithium-ion batteries. Their performance matched that of the metal current collectors used in these batteries, even though the metal collectors are much heavier. This work is described this week in the Proceedings of the National Academy of Sciences.
Other groups have worked on using paper as a substrate for making electrodes. However, previous attempts to build nanotube devices on paper were much more complicated, says Cui, and required growing the nanotubes on the paper or using novel paper formulations as a starting point. The dipping method is “simple and nice,” says Nicholas Kotov, professor of chemical engineering at the University of Michigan.
Cui says the paper-nanotube electrodes are robust. Though untreated paper dissolves in water, the nanotube-treated paper won’t, and the nanotubes don’t come off when it’s scratched or rolled up. “The supercapacitor has been tested over 40,000 charge cycles for six months and it’s still working,” says Cui.
The Stanford researchers are now working to improve the performance of their devices and are testing different printing methods and materials. So far Cui has utilized an ink made up of a mixture of semiconducting and metallic nanotubes. Purely metallic inks would likely perform better, but they are more expensive. The group is also experimenting with different ways of bringing nanotubes and paper together, including painting the inks with a pen or brush to make complex patterns.