Print-on-Demand Power

It’s now possible to print out components for flexible circuits, resilient displays, and even lightweight x-ray imaging panels. But conventional energy-storage devices still weigh down these printed electronics. Now researchers have made the first printable supercapacitor. This high-performance energy-storage device performs better than conventional supercapacitors currently on the market. The device, which is made up of a gel electrolyte sandwiched between two carbon-nanotube electrodes, could be created using existing ink-jet printing methods.

Capacitors and batteries are both electrical-energy storage devices. These devices have complementary strengths and are often used in tandem in portable electronics. While batteries have a greater total energy-storage capacity, they cannot discharge as rapidly as capacitors. While batteries store energy in chemical reactions, capacitors store it in surface charge, so they can provide rapid bursts of power. In digital cameras, for example, capacitors often provide a rapid pulse of power when the shutter button is depressed.

Electrochemists are working on boosting the storage capacity of capacitors so that they can compete better with batteries, as well as on making batteries charge and discharge as fast as capacitors. But researchers at the University of California, Los Angeles, and at Stanford University decided to focus on developing a simpler manufacturing method. The printing techniques that they used to make the new supercapacitors are “dirt-cheap technologies,” says George Grüner, a professor of physics at UCLA, who led the research with Yi Cui, an assistant professor of materials science and engineering at Stanford.

The new capacitors are made by spraying carbon nanotubes onto two pieces of plastic, then sandwiching a gel electrolyte in between them. In the resulting device, one nanotube network acts as the positive electrode while the other functions as the negative electrode. When a voltage is applied to the electrolyte gel, charges collect on the surfaces of the nanotubes, storing energy. “The performance of the device is comparable to other devices,” says Cui. “The key is that everything is printable.”

The printing process is simple: commercially available carbon nanotubes are suspended in water and then sprayed onto the plastic surface using an air gun similar to the head in an ink-jet printer. As the solution is sprayed, the water evaporates, leaving a randomly entangled layer of nanotubes on top of the plastic substrate. The two layers–each about 0.6 micrometers thick–are brought together to enclose a thin film of polymer gel that acts as the electrolyte. The gel is made by mixing powdered polyvinyl alcohol with acid in a mold. Whereas conventional electrolytes are liquid and therefore difficult to contain, the gel electrolyte won’t spill, which makes it ideal for a flexible device, say the researchers.

“They get good numbers with a simple approach,” says Paula Hammond, a professor of chemical engineering at MIT. She adds that developing cheap, printable energy-storage devices “is something we need to do to make flexible electronics.”

The power density of the printed supercapacitor–how fast it can charge and discharge–compares favorably with existing supercapacitors. At 70 kilowatts per kilogram, Grüner says, it is “significantly higher than that of commercial devices.” Although its low weight and flexibility make it promising for applications in portable electronics, the current prototype doesn’t have enough total energy-storage capacity to operate a device like a cell phone without a battery.

The group is now working on improving the energy density of the capacitors in the hope of eliminating the need for a battery. “We are currently working on capacitors with significantly higher energy density, while preserving the power capability,” says Grüner.