An extraordinary coiled carbon nanotube yarn has been created by researchers at the University of Texas at Dallas that can generate electrical energy when stretched or twisted.
In a study published in the journal Science, co-lead author Shi Hyeong Kim and his fellow researchers were prompted by the demand for “small-scale, portable electronics and wearable devices.” They wanted to find ways to harvest energy from mechanical motion with the hope of providing power -- free from a reliance on batteries and with a small footprint.
The yarns they developed are made of carbon nanotubes, these are hollow tubes of carbon measuring 10,000 times thinner than a human hair but are surprisingly 100 times stronger than steel. Additionally, they are great conductors of electricity. The researchers twist-spun the nanotubes into high-strength lightweight yarns.
“Fundamentally, these yarns are supercapacitors,” said Dr. Na Li, a research scientist at the NanoTech Institute and co-lead author of the study in a press release. “In a normal capacitor, you use energy, like from a battery — to add charges to the capacitor. But in our case, when you insert the carbon nanotube yarn into an electrolyte bath, the yarns are charged by the electrolyte itself. No external battery, or voltage, is needed.”
Such an electrolyte bath can contain ionically conducting materials like salt water. Once submerged all that is needed to increase the voltage is to pull and twist the yarn. Stretching it reduces the yarn’s volume and brings the charges close together, boosting the energy level and then the voltage -- enabling the harvesting of electricity.
“Although numerous alternative harvesters have been investigated for many decades, no other reported harvester provides such high electrical power or energy output per cycle as ours, for stretching rates between a few cycles per second and 600 cycles per second,” said Dr. Ray Baughman, director of the NanoTech Institute and a fellow author of the study.
The researchers then tested a sample of twistron yarn weighing less than a housefly by using a small LED, which lit up every time the yarn was stretched. To show that the yarn can harvest waste thermal energy from the environment, co-author Li also connected a yarn to an artificial polymer muscle that can contract and expand when heated and cooled. The yarn amazingly converted the energy produced by the polymer-tissue into electricity.
“There is a lot of interest in using waste energy to power the Internet of Things, such as arrays of distributed sensors,” Li said. “Twistron technology might be exploited for such applications where changing batteries is impractical.”
They also sewed the Twistron harvesters into a shirt, regular breathing stretched the yarn, generating an electrical signal, showing that it could also work as a potential self-powered respiration sensor.
“Electronic textiles are of major commercial interest, but how are you going to power them?” Baughman said. “Harvesting electrical energy from human motion is one strategy for eliminating the need for batteries.