U.S. scientists have engineered a new type of supercapacitor that remains fully functional even when stretched to eight times its original size.
Jointly developed by Duke University and Michigan State University (MSU), the supercapacitor does not exhibit any wear and tear from being stretched repeatedly and loses only a few percentage points of energy performance after 10,000 cycles of charging and discharging.
“Our goal is to develop innovative devices that can survive mechanical deformations like stretching, twisting or bending without losing performance,” says MSU assistant professor Changyong Cao, who co-led the study with Duke University professor Jeff Glass.
“But if the power source of a stretchable electronic device isn’t stretchable, then the entire device system will be constrained to be non-stretchable.”
The scientists have accordingly fabricated a stamp-sized supercapacitor capable of carrying more than two volts.
When connecting four together, as many devices require for AA or AAA batteries, the supercapacitors could power a two-volt Casio watch for an hour and a half.
To make the supercapacitors stretchable, Glass and his team first grew a carbon nanotube forest – a patch of millions of nanotubes just 15 nanometers in diameter and 20-30 micrometers tall – on top of a silicon wafer.
They then coated a thin layer of gold nanofilm on top of the carbon nanotube forest.
The gold layer acts as a sort of electric collector, dropping the resistance of the device an order of magnitude below previous versions, which allows the device to charge and discharge much faster.
From there, Cao and his team took over the engineering process, transferring the carbon nanotube forest to a pre-stretched elastomer substrate with the base gold-side-down.
The gel-filled electrode was then relaxed to allow the pre-strain to release, causing it to shrink to a quarter of its original size.
This process crumples up the thin layer of gold and smashes together the ‘trees’ in the carbon nanotube forest.
The super dense forest is then filled with a gel electrolyte that can trap electrons on the surface of the nanotubes.
When two of these final electrodes are sandwiched close together, an applied voltage loads one side with electrons while the other is drained, creating a charged super-stretchable supercapacitor.
Image and content: Duke University