Power/Performance Bits: Feb. 16

Energy storage on microchips

After more than half a decade of speculation, fabrication, modeling and testing, an international team of researchers from Drexel University in Pennsylvania and Paul Sabatier University in Toulouse, France, confirmed that their process for making carbon films and micro-supercapacitors will allow microchips and their power sources to become one and the same.

Since creating a carbide-derived carbon film material for micro-supercapacitors, their goal has been to create a wafer-scale process for manufacturing strongly adhering carbide-derived carbon films and interdigitated micro-supercapacitors with embedded titanium carbide current collectors.

“This has taken us quite some time, but we set a lofty goal of not just making an energy storage device as small as a microchip — but actually making an energy storage device that is part of the microchip and to do it in a way that is easily integrated into current silicon chip manufacturing processes,” said Patrice Simon, of Paul Sabatier University.

A silicon wafer containing 40 micro-supercapacitors. (Source: C. Lethien/IEMN)

To achieve this, the team deposited a relatively thick layer of titanium carbide (TiC) on top of an oxide-coated Si film. After chlorination, most, but importantly not all, of the TiC was converted into a porous carbon film that could be turned into an electrochemical capacitor. The carbon films were highly flexible, and the residual TiC acted as a stress buffer with the underlying Si film. The films could be separated from the Si to form free-floating films, with the TiC providing a support layer.

The researchers say their method for depositing carbon onto a silicon wafer is consistent with microchip fabrication procedures currently in use, easing the challenges of integration of energy storage devices into electronic device architecture. As part of the research, the group showed how it could deposit the carbon films on silicon wafers in a variety of shapes and configurations to create dozens of supercapacitors on a single silicon wafer.

Sole power

An energy harvesting and storage technology particularly well suited for capturing the energy of human motion was developed by University of Wisconsin-Madison mechanical engineers who aim to reduce mobile devices’ reliance on batteries.

Their target for the technology? Shoes.

“Human walking carries a lot of energy,” said Tom Krupenkin, a professor of mechanical engineering at UW–Madison. “Theoretical estimates show that it can produce up to 10 watts per shoe, and that energy is just wasted as heat. A total of 20 watts from walking is not a small thing, especially compared to the power requirements of the majority of modern mobile devices.”

However, traditional approaches to energy harvesting and conversion don’t work well for the relatively small displacements and large forces of footfalls, according to the researchers.

The researchers’ new energy-harvesting technology takes advantage of “reverse electrowetting,” an approach where, as a conductive liquid interacts with a nanofilm-coated surface, the mechanical energy is directly converted into electrical energy. The reverse electrowetting method can generate usable power, but it requires an energy source with a reasonably high frequency — such as a mechanical source that’s vibrating or rotating quickly.

Upside-down shoe soles with an energy harvester, battery and electronics suite integrated into each sole. The harvester directly powers the electronics suite. (Source: UW–Madison College of Engineering)

To overcome this, the researchers developed what they call the “bubbler” method, which combines reverse electrowetting with bubble growth and collapse. The device consists of two flat plates separated by a small gap filled with a conductive liquid. The bottom plate is covered with tiny holes through which pressurized gas forms bubbles. The bubbles grow until they’re large enough to touch the top plate, which causes the bubble to collapse. The speedy, repetitive growth and collapse of bubbles pushes the conductive fluid back and forth, generating electrical charge.

“The high frequency that you need for efficient energy conversion isn’t coming from your mechanical energy source but instead, it’s an internal property of this bubbler approach,” Krupenkin said.

The proof-of-concept bubbler device generated around 10 watts per square meter in preliminary experiments, and theoretical estimates show that up to 10 kilowatts per square meter might be possible.

Beyond harvesting energy to charge devices, one area the researchers see promise is embedding a shoe with a Wi-Fi hotspot that acts as a middleman between mobile devices and a wireless network to cut battery drain.