Wearable
electronic devices for health and fitness monitoring are a rapidly
growing area of consumer electronics; one of their biggest limitations
is the capacity of their tiny batteries to deliver enough power to
transmit data. Now, researchers at MIT and in Canada have found a
promising new approach to delivering the short but intense bursts of
power needed by such small devices.
The key is a new approach to making supercapacitors — devices that
can store and release electrical power in such bursts, which are needed
for brief transmissions of data from wearable devices such as heart-rate
monitors, computers, or smartphones, the researchers say. They 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, Mirvakili says.
In this new work, he and his colleagues 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,”
Hunter says, “and it needs to broadcast data, for example using Wi-Fi,
over a long distance.” At the moment, the coin-sized batteries used in
many small electronic devices have very 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,” says Hunter,
the George N. Hatsopoulos Professor in Thermodynamics in MIT’s
Department of Mechanical Engineering, “but it may not be needed for very
long.” Small batteries are generally poorly suited for such power
needs, he adds.
“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,” Hunter says: 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 new 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,” Hunter says. “But if you reduce the volume by 30 percent, that
would be a big deal,” he says: Consumers are very sensitive to the size
of wearable devices.
The innovation is especially significant for small devices, Hunter
says, 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 very small sizes. “We are in a
sweet spot,” he says, with a technology that can deliver big bursts of
power from a very small device.
Ideally, Hunter says, 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,” he says. However, with the new
device, “We have fairly high volumetric power density, medium energy
density, and a low cost,” a combination that could be well suited for
many applications.
Niobium is a fairly abundant and widely used material, Mirvakili
says, so the whole system should be inexpensive and easy to produce.
“The fabrication cost is cheap,” he says. 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 very high melting point — nearly 2,500 degrees
Celsius — so devices made from these nanowires could potentially be
suitable for use in high-temperature applications.
In addition, the material is highly flexible and could be woven into
fabrics, enabling wearable forms; individual niobium nanowires are just
140 nanometers in diameter — 140 billionths of a meter across, or about
one-thousandth the width of a human hair.
So far, the material has been produced only in lab-scale devices. The
next step, already under way, is to figure out how to design a
practical, easily manufactured version, the researchers say.
“The work is very significant in the development of smart fabrics and
future wearable technologies,” says Geoff Spinks, a professor of
engineering at the University of Wollongong, in Australia, who was not
associated with this research. This paper, he adds, “convincingly
demonstrates the impressive performance of niobium-based fiber
supercapacitors.”
The team also included PhD student Mehr Negar Mirvakili and
professors Peter Englezos and John Madden, all from the University of
British Columbia.