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. 
Yarn made of niobium nanowires, seen here in a scanning electron microscope image (background), can be used to make very efficient supercapacitors, MIT researchers have found. Adding a coating of a conductive polymer to the yarn (shown in pink, inset) further increases the capacitor’s charge capacity. Positive and negative ions in the material are depicted as blue and red spheres. (Courtesy of the researchers) 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 ("High-Performance Supercapacitors from Niobium Nanowire Yarns") 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.
read more "Tiny wires could provide a big energy boost"
Yarn made of niobium nanowires, seen here in a scanning electron microscope image (background), can be used to make very efficient supercapacitors, MIT researchers have found. Adding a coating of a conductive polymer to the yarn (shown in pink, inset) further increases the capacitor’s charge capacity. Positive and negative ions in the material are depicted as blue and red spheres. (Courtesy of the researchers) 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 ("High-Performance Supercapacitors from Niobium Nanowire Yarns") 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.
Tetrahedra with a nickel atom at their centre are somewhat elongated due to the Jahn-Teller effect (green), while the tetrahedrons with a copper atom at their centre are compressed (blue). (Image: M. Tovar/HZB) Spinels consist of densely packed, highly symmetrical planes of oxygen atoms (somewhat like a densely packed box of marbles) where different metallic elements are lodged in the spaces between them. A great many different types of compounds arise as a result that are employed in extractive industries and as heat-resistant and magnetic materials. The embedded metal ions in the Ni
These samples with precise proportions of nickel and copper were produced via an improved preparation procedure. (Photo: M. Tovar/HZB) The powders consist of small crystal grains whose diameters are between 30 and 50 microns. The exciting thing about this series of mixed crystals is that nickel or copper atoms sit at what are referred to as tetragonal sites of the crystal structure. Due to their different configurations of electrons, these tetrahedra become elongated along the crystallographic c-axis for nickel, while for copper they are compressed (Jahn-Teller effect). The distortion of the crystal structure can thus be controlled, which in turn has an effect on the magnetic ordering. Phase diagramm between 2 and 900 Kelvin Using neutron scattering experiments at the BER II research reactor, Manfred Reehuis and Michael Tovar were successful in determining the structural and magnetic properties for each of the mixed crystal specimens over quite a wide temperature range, from near the zero point of the Kelvin temperature scale to above 900 K. The two scientists discovered new magnetic ordering and were able for the first time to create a complete phase diagram of the system. This shows that the crystal structure is cubic (three right angles, three equal edges) at high temperatures, since the kinetic energy of the atoms still suppresses the Jahn-Teller effect and magnetic ordering cannot become established. As the temperature declines, the Jahn-Teller effect comes to the fore and causes a reduction of the crystal symmetry initially to tetragonal (three right angles, two equal edges), and finally orthorhombic (three right angles, three unequal edges). New magnetic phases What is interesting is that the magnetic phases only occur in the orthorhombic structure, which lies far below room temperature for pure nickel-spinel as well as for copper-spinel. “We were able for the first time to determine the magnetic characteristics exactly and thereby prove there is a relationship between the conditions for magnetic ordering and the crystal structures. This was a question that physicists have been preoccupied with for more than 50 years”, explains Manfred Reehuis. Peninsula of orthorhombic state At a mixture ratio of 85 % nickel and 15 % copper, the spinel system displays a kind of narrow peninsula of orthorhombic state in the phase diagram where the observed relationship of crystal symmetry and magnetism briefly breaks down. 
Only in the orthorhombic phase (light blue) magnetic ordering occurs, which for most of the crystal mixture ratios lies far below room temperature (293 K). The HZB researchers were able to identify two new states of magnetic ordering (Tc and TM2). At a 15 % proportion of copper, the orthorhombic phase remains stable at temperatures considerably above room temperature (Ts2). (Image: Reehuis/HZB) Contrary to what has been assumed until now, the cause of this is the distortion of the nickel and copper tetrahedrons at 90° to each other rather than in the same direction. This results not in mutual cancellation of the distortions at this mixing ratio, but instead in a maximal distortion of the structure. “Atoms are not just spheres. They do crazy things, especially when they are in a geometrical system like a crystal, rather than in isolation”, says Michael Tovar.
Mapping of the captured magnetisation domains (top, red-blue patterns) in a sample 20 nanometres thick that had been wound in two layers into a tube. The tube has a diameter of 5 microns and a height of 50 microns. (Image: F. Kronast /HZB) Mapping of rolled-up magnetic samples They studied the magnetisation in rolled-up tubular magnetic nanomembranes (nickel or cobalt-palladium) about two layers thick. To obtain a 3D mapping of the magnetisation in the tubes, the samples were illuminated with circularly polarized X-rays. Using the X-ray microscope at the Advanced Light Source and the X-ray Photoemission Electron Microscopy (XPEEM) beamline at BESSY II, the samples were slightly rotated for each new image so that a series of 2D images was created. “The polarised light penetrated the magnetic layers from different angles. Using XPEEM, we were not only able to measure the magnetic features at the surface, but also obtained additional information from the “shadow”, explains Florian Kronast, who is responsible for the XPEEM beamline at HZB. 3D reconstruction of magnetic patterns In the end, the physicists were successful in reconstructing the magnetic features on the computer in three dimensions. “These samples displayed structures not smaller than 75 nanometres. But with this method we should be able to see even smaller structures and obtain a resolution of 20 nanometres”, explains Florian Kronast. However, so far only electron holography could be considered for mapping magnetic domains of three-dimensional objects at the nanometre scale. This required very complicated sample preparation and the magnetisation could only be indirectly determined through the resulting distribution of the magnetic field. “Our process enables you to map the magnetisation in directly in 3D. Knowledge of the magnetisation is prerequisite for improving the sensitivity of magnetic field detectors.” Sensors for weak magnetic fields The new method could be of interest to anyone involved with extremely small magnetic features within small volumes, such as those developing more sensitive devices for medical imaging, for example. Procedures like magnetoencephalography depend on externally detecting very weak magnetic fields created by the electrical activity of individual nerve cells – using appropriately sensitive detectors.