Tiny wires set to provide big energy

Wearable electronic devices are a rapidly growing area of consumer electronics; however one of their biggest limitations is the capacity of their tiny batteries to deliver enough power to transmit data. 
Now, researchers say they 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 said.   
“Imagine you’ve got some kind of wearable health-monitoring system,” Hunter said, “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,” said 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,” 
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,” he said. 
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.   
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 said.