Mechanical watches and clockwork toys might seem like relics of a bygone age, but scientists in the US and Japan are bringing this old-fashioned form of energy storage into the modern era. By making single-walled carbon nanotubes (SWCNTs) into ropes and twisting them like the string on an overworked yo-yo, Katsumi Kaneko, Sanjeev Kumar Ujjain and colleagues showed that they can store twice as much energy per unit mass as the best commercial lithium-ion batteries. The nanotube ropes are also stable at a wide range of temperatures, and the team say they could be safer than batteries for powering devices such as medical sensors.
SWCNTs are made from sheets of pure carbon just one atom thick that have been rolled into a straw-like tube. They are impressively tough – five times stiffer and 100 times stronger than steel – and earlier theoretical studies by team member David Tománek and others suggested that twisting them could be a viable means of storing large amounts of energy in a compact, lightweight system.
Making and measuring nanotube ropes
To confirm this, the team needed to overcome two challenges. The first was finding the best way of making energy-storing ropes from commercially-available SWCNT materials. After testing various methods, the team settled on a yarn-like rope treated with thermoplastic polyurethane, which accelerates the elastic deformation of individual nanotubes and improves their ability to “share the load” with others.
The second challenge was to measure energy stored in ropes which, at only microns in diameter, are much thinner than a human hair. “This small size made it hard to handle and measure them accurately,” says Kumar Ujjain, an assistant research scientist at the University of Maryland-Baltimore County’s Center for Advanced Sensor Technology (UMBC-CAST) who began the project while working with Kaneko at Shinshu University.
The team’s solution was to develop an instrument that combines a motor for twisting the sample with a laser displacement gauge to measure how much torque the strained rope exerts. By adding a microscope and high-speed camera, the scientists could track how much force and twisting the ropes experienced in real time. “This precise measurement was crucial for determining how much energy the ropes could store,” Kumar Ujjain says.
To measure the stored energy, the scientists added a load to the twisted rope and monitored its rotation as the rope unwound. The maximum gravimetric energy density (that is, the energy available per unit mass) they measured was 2.1 MJ/kg (583 Wh/kg). While this is lower than the most advanced lithium-ion batteries, which last year hit a record of 700 Wh/kg, it is much higher than commercial versions, which top out at around 280 Wh/kg. The SWCNT ropes also maintained their performance over at least 450 twist-release cycles, and Kumar Ujjain says they have other advantages, too.
“Storing energy in mechanically twisted carbon nanotube ropes is generally safer than using chemical energy storage, such as in lithium-ion batteries, which can pose risks like fires or explosions,” he explains. “The energy in these twisted ropes is purely mechanical and doesn’t involve hazardous chemicals.”
Managing and exploiting stored energy
One possible application for a chemically safe, biocompatible energy-storage system would be in medical sensors. The UMBC-CAST team is developing a stretchable, porous CO2 sensor that can be applied directly to a patient’s skin, and Kumar Ujjain says that a micro-generator based on twisted nanotube ropes could be a good way of powering it. Getting to that point will, however, require additional research focused on scaling up the nanotube ropes, integrating them with existing devices, and above all, developing mechanisms for releasing the stored energy in a controlled, predictable way.
“There is a risk if the ropes are twisted too tightly,” Kumar Ujjain explains. “In such cases, the tension could suddenly release, like an over-tightened spring in a clockwork watch, potentially causing damage.” Proper handling and safety measures should, he says, make this risk manageable.
The study is described in Nature Nanotechnology.
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