Powering wearables: Are niobium nanowire supercapacitors the answer?

Powering wearables: Are niobium nanowire supercapacitors the answer?

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By eeNews Europe

The key is a new approach to making supercapacitors. The nanowire may also be useful for other applications where high power is needed in small volumes, such as autonomous microrobots.

The new approach uses yarns, made from nanowires of the element niobium, as the electrodes in tiny supercapacitors (which are essentially pairs of electrically
conducting fibers with an insulator between). The concept is described in a paper in the journal ACS Applied Materials and Interfaces by MIT professor of mechanical
engineering Ian W. Hunter, doctoral student Seyed M. Mirvakili, and three others at the University of British Columbia.

Nanotechnology researchers have been working to increase the performance of supercapacitors for the past decade. Among nanomaterials, carbon-based nanoparticles – such as carbon nanotubes and graphene – have shown promising results, but they suffer from relatively low electrical conductivity.

The researchers have shown that desirable characteristics for such devices, such as high power density, are not unique to carbon-based nanoparticles, and that niobium nanowire yarn is a promising an alternative.

“Imagine you’ve got some kind of wearable health-monitoring system and it needs to broadcast data, for example using Wi-Fi, over a long distance.” explained Hunter, the George N. Hatsopoulos Professor in Thermodynamics in MIT’s Department of Mechanical Engineering.

At the moment, the coin-sized batteries used in many small electronic devices have a limited ability to deliver a lot of power at once, which is what such data
transmissions need.

“Long-distance Wi-Fi requires a fair amount of power,” said Hunter “but it may not be needed for very long.” Small batteries are generally poorly suited for such power requirements.

“We know it’s a problem experienced by a number of companies in the health-monitoring or exercise-monitoring space. So an alternative is to go to a combination of a battery and a capacitor,” said Hunter.  The battery for long-term, low-power functions, and the capacitor for short bursts of high power. Such a combination should be able to either increase the range of the device, or – perhaps more important in the marketplace – to significantly reduce size requirements.

The nanowire-based supercapacitor exceeds the performance of existing batteries, while occupying a very small volume. “If you’ve got an Apple Watch and I shave 30
percent off the mass, you may not even notice,” said Hunter. “But if you reduce the volume by 30 percent, that would be a big deal.”

The innovation is especially significant for small devices, said Hunter, because other energy-storage technologies – such as fuel cells, batteries, and flywheels – tend to be less efficient, or simply too complex to be practical when reduced to small sizes.

Ideally, Hunter said, it would be desirable to have a high volumetric power density (the amount of power stored in a given volume) and high volumetric energy density (the amount of energy in a given volume).

“Nobody’s figured out how to do that,” said Hunter. However, with the new device, “We have fairly high volumetric power density, medium energy density, and a low cost."

Niobium is a fairly abundant and widely used material. “The fabrication cost is cheap,” said Mirvakili.

Other groups have made similar supercapacitors using carbon nanotubes or other materials, but the niobium yarns are stronger and 100 times more conductive. Overall, niobium-based supercapacitors can store up to five times as much power in a given volume as carbon nanotube versions.

Niobium also has a high melting point – nearly 2,500 degrees Celsius – so devices made from these nanowires could potentially be suitable for use in high-temperature

In addition, the material is highly flexible and could be woven into fabrics, enabling wearable forms; individual niobium nanowires are 140 nanometers in diameter.

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