Coiled carbon nanotube yarns, created at the University of Texas at Dallas and imaged here with a scanning electron microscope, generate electrical energy when stretched or twisted. Image: University of Texas at Dallas.An international research team led by scientists from the University of Texas at Dallas and Hanyang University in South Korea has developed high-tech yarns that generate electricity when they are stretched or twisted.
In a paper published in Science, the scientists describe ‘twistron’ yarns and their possible applications, such as harvesting energy from the motion of ocean waves or from temperature fluctuations. When sewn into a shirt, these yarns served as a self-powered breathing monitor.
"The easiest way to think of twistron harvesters is, you have a piece of yarn, you stretch it and out comes electricity," said Carter Haines, associate research professor in the Alan G. MacDiarmid NanoTech Institute at UT Dallas and co-lead author of the article. The research team also included scientists from Virginia Tech, Wright-Patterson Air Force Base and China.
The scientists constructed the high-strength, lightweight yarns by twist-spinning carbon nanotubes. To make the yarns highly elastic, they introduced so much twist that the yarns coiled like an over-twisted rubber band. In order to generate electricity, the yarns must be either submerged in or coated with an ion-conducting material, or electrolyte, which can be as simple as a mixture of ordinary table salt and water.
"Fundamentally, these yarns are supercapacitors," explained Na Li, a research scientist at the NanoTech Institute and co-lead author of the study. "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."
When the carbon nanotube yarn is twisted or stretched, its volume decreases, bringing the electric charges on the yarn closer together and increasing their energy, Haines said. This increases the voltage associated with the charge stored in the yarn, allowing the harvesting of electricity.
Stretching the coiled twistron yarns 30 times a second generated 250 watts per kilogram of peak electrical power when normalized to the harvester's weight, said Ray Baughman, director of the NanoTech Institute and a corresponding author of the study. "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."
In the lab, the researchers showed that a twistron yarn weighing less than a housefly could power a small LED, which lit up each time the yarn was stretched. To show that twistrons can harvest waste thermal energy from the environment, Li connected a twistron yarn to a polymer artificial muscle that contracts and expands when heated and cooled. The twistron harvester converted the mechanical energy generated by the polymer muscle to electrical energy.
"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."
The researchers also sewed twistron harvesters into a shirt. Normal breathing stretched the yarn and generated an electrical signal, demonstrating its potential as a 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. Our yarns produced over 100 times higher electrical power per weight when stretched compared to other weavable fibers reported in the literature."
"In the lab, we showed that our energy harvesters worked using a solution of table salt as the electrolyte," said Baughman. "But we wanted to show that they would also work in ocean water, which is chemically more complex."
In a proof-of-concept demonstration, co-lead author Shi Hyeong Kim, a postdoctoral researcher at the NanoTech Institute, waded into the frigid surf off the east coast of South Korea to deploy a coiled twistron in the sea. He attached a 10cm-long yarn, weighing only 1mg (about the weight of a mosquito), between a balloon and a sinker that rested on the seabed. Every time an ocean wave arrived, the balloon would rise, stretching the yarn by up to 25% and generating electricity.
Even though the investigators used very small amounts of twistron yarn in the current study, they have shown that the harvester performance is scalable, both by increasing twistron diameter and by operating many yarns in parallel.
"If our twistron harvesters could be made less expensively, they might ultimately be able to harvest the enormous amount of energy available from ocean waves," Baughman said. "However, at present these harvesters are most suitable for powering sensors and sensor communications. Based on demonstrated average power output, just 31mg of carbon nanotube yarn harvester could provide the electrical energy needed to transmit a 2-kilobyte packet of data over a 100m radius every 10 seconds for the Internet of Things."
This story is adapted from material from the University of Texas at Dallas, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.