The Race for Wearable Energy
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|Jean-Pierre Joosting||December 21st 2015|
Researchers at Case Western Reserve University (Cleveland, OH) have developed flexible wire-shaped microsupercapacitors that can be woven into clothing, creating a tailored power source for wearable electronics.
By their design or by connecting the capacitors in series or parallel, the devices can be tailored to match the charge storage and delivery needs of electronics donned. While there's been progress in development of those electronics - body cameras, smart glasses, sensors that monitor health, activity trackers and more - one challenge remaining is providing less obtrusive and cumbersome power sources.
"The area of clothing is fixed, so to generate the power density needed in a small area, we grew radially-aligned titanium oxide nanotubes on a titanium wire used as the main electrode," says Liming Dai, the Kent Hale Smith Professor of Macromolecular Science and Engineering. "By increasing the surface area of the electrode, you increase the capacitance."
The microsupercapacitor was based on earlier research on carbon-based supercapacitors. A capacitor is cousin to the battery, but offers the advantage of charging and releasing energy much faster.
In this new supercapacitor, the modified titanium wire is coated with a solid electrolyte made of polyvinyl alcohol and phosphoric acid. The wire is then wrapped with either yarn or a sheet made of aligned carbon nanotubes, which serves as the second electrode. The titanium oxide nanotubes, which are semiconducting, separate the two active portions of the electrodes, preventing a short circuit.
In testing, capacitance - the capability to store charge - increased from 0.57 to 0.9 to 1.04 milliFarads per micrometer as the strands of carbon nanotube yarn were increased from one to two to three, respectively.
When wrapped with a sheet of carbon nanotubes, which increases the effective area of electrode, the microsupercapactitor stored 1.84 milliFarads per micrometer. Energy density was 0.16 x 10-3 milliwatt-hours per cubic centimeter and power density 0.01 milliwatt per cubic centimeter.
Whether wrapped with yarn or a sheet, the microsupercapacitor retained at least 80% of its capacitance after 1,000 charge-discharge cycles. To match various specific power needs of wearable devices, the wire-shaped capacitors can be connected in series or parallel to raise voltage or current, the researchers say.
When bent up to 180 degrees hundreds of times, the capacitors showed no loss of performance. Those wrapped in sheets showed more mechanical strength.
"They're very flexible, so they can be integrated into fabric or textile materials," Dai said. "They can be a wearable, flexible power source for wearable electronics and also for self-powered biosensors or other biomedical devices, particularly for applications inside the body."
Dai's lab is in the process of weaving the wire-like capacitors into fabric and integrating them with a wearable device.