Paper electrode: Nanotubes absorb into paper and bind strongly to cellulose fibers, as shown in this scanning-electron micrograph.
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.