How to construct a wire-shaped micro-supercapacitor from titanium wire sheathed in radially aligned titania nanotubes, wrapped in carbon nanotubes or sheets. (Image courtesy of Elsevier: Energy Storage Materials.)
Traditionally, IoT devices are powered by batteries and this is where the issue lies. These batteries are hard to incorporate into thin wearable fabric and take a considerable time to charge.
To address this, Case Western Reserve University has developed a flexible micro-supercapacitor which can be woven into fabric.
“The area of clothing is fixed,” said Liming Dai, the Kent Hale Smith professor of macromolecular science and engineering. “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. By increasing the surface area of the electrode, you increase the capacitance.”
The capacitor was made by coating a titanium wire with solid electrolytes consisting of polyvinyl alcohol and phosphoric acid. Titanium oxide nanotubes are then added to the capacitor and are used to separate the electrodes. The second electrode is made of either carbon nanotube based sheets or yarn.
The resulting capacitance was shown to increase as more strands of carbon nanotubes were wrapped around the capacitor:
- First carbon nanotube wrapping: 0.57 milliFarads per cubic centimeter
- Second carbon nanotube wrapping: 0.9 milliFarads per cubic centimeter
- Third carbon nanotube wrapping: 1.04 milliFarads per cubic centimeter
Additionally, if a sheet of carbon nanotubes were used to wrap around the capacitors instead of the yarn, the added surface area jumped the capacitance to 1.84 milliFarads per cubic centimeter. This optimal supercapacitor had an energy density of 1.6 x 10-4 miliwatt-hours per cubic centimeter and a power density of 0.01 milliwatts per cubic centimeter.
As a point of comparison, an Energizer CR2032 button battery has an energy density of 653 milliwatt-hours per cubic centimeter. Though the capacitor falls quite short of this benchmark, the researchers note that the capacitors can be hooked up in series or parallel arrangements to meet the needs of wearable IoT devices. As a result, the team is starting to test this procedure.
Though it is beneficial that charging the capacitor will take a matter of minutes, this is a bit of a double edged sword.
The drawback is that the capacitors will also want to discharge quickly. Therefore, the circuit will need to be designed to control the capacitor’s discharge to ensure an IoT device will be powered through each usage cycle.
The researchers noted that the capacitors did show some promise with respect to their lifecycle. The micro-capacitors were able to keep 80 percent of their capacitance after 1000 charge-discharge cycles.
Additionally, the capacitors showed that being bent 180 degrees, 100 times showed little impact on the capacitor’s performance.
“They’re very flexible, so they can be integrated into fabric or textile materials,” said Dai. “They can be a flexible power source for wearable electronics, self-powered biosensors or other biomedical devices, particularly for applications inside the body.”
As a result of the small energy density it is doubtful these capacitors will be ready any time soon. Though the researchers make a point that hooking up the capacitors in series and parallel can make up for these shortcomings, will it be economical? After all, carbon and titanium nanotubes are not cheap and a battery can always be hidden inside a button.
To find more on this discovery read this paper.