Power/Performance Bits: Sept. 5

Energy-harvesting yarn; compound perovskite solar cells; nanodiamonds for safer batteries.


Energy-harvesting yarn
Researchers at the University of Texas at Dallas and Hanyang University in South Korea developed a carbon nanotube yarn that generates electricity when stretched or twisted. Possible applications for the so-called “twistron” yarns include 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 researchers twist-spun carbon nanotubes into high-strength, lightweight yarns. 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 electrolyte, which can be as simple as a mixture of ordinary table salt and water.

“Fundamentally, these yarns are supercapacitors,” said Dr. Na Li, a research scientist at the NanoTech Institute at UT Dallas. “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 a harvester yarn is twisted or stretched, the volume of the carbon nanotube yarn decreases, bringing the electric charges on the yarn closer together and increasing their energy. This increases the voltage associated with the charge stored in the yarn, enabling the harvesting of electricity.

Coiled carbon nanotube yarns, imaged here with a scanning electron microscope, generate electrical energy when stretched or twisted. (Source: UT Dallas)

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. “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.”

To show that twistrons can harvest waste thermal energy from the environment, the team 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. The yarn was also able to use sea water as an electrolyte, generating electricity when stretched by ocean waves.

“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 31 milligrams of carbon nanotube yarn harvester could provide the electrical energy needed to transmit a 2-kilobyte packet of data over a 100-meter radius every 10 seconds for the Internet of Things.”

Compound perovskite solar cells
Researchers at Stanford University developed a new concept for perovskite solar cells, inspired by the eyes of insects, designed to protect the fragile material from deteriorating when exposed to heat, moisture or mechanical stress.

While perovskite solar cells are promising for their efficiency and low cost, they have short life spans, degrading quickly when exposed to real-world conditions. Making them more stable is currently a major focus for perovskite researchers.

Using a fly’s compound eyes as a model, the team created a compound solar cell consisting of a honeycomb of perovskite microcells, each encapsulated in a hexagon-shaped scaffold just 0.02 inches (500 microns) wide.

“The scaffold is made of an inexpensive epoxy resin widely used in the microelectronics industry,” said Nicholas Rolston, a graduate student at Stanford. “It’s resilient to mechanical stresses and thus far more resistant to fracture.”

Scaffolds in a compound solar cell filled with perovskite after fracture testing. (Source: Dauskardt Lab/Stanford University)

Tests conducted during the study revealed that the scaffolding had little effect on the perovskite’s ability to convert light into electricity. “We got nearly the same power-conversion efficiencies out of each little perovskite cell that we would get from a planar solar cell,” said Reinhold Dauskardt, a professor of materials science and engineering at Stanford. “So we achieved a huge increase in fracture resistance with no penalty for efficiency.”

The device is also more resilient than typical perovskite panels. The researchers exposed encapsulated perovskite cells to temperatures of 185 degrees Fahrenheit (85 degrees Celsius) and 85% relative humidity for six weeks. Despite these extreme conditions, the cells continued to generate electricity at relatively high rates of efficiency.

The team has filed a provisional patent for the new technology. To improve efficiency, they are studying new ways to scatter light from the scaffold into the perovskite core of each cell.

Nanodiamonds for safer batteries
Researchers at Drexel University, Tsinghua University, and Hauzhong University of Science and Technology found that using nanodiamonds in the electrolyte of lithium-ion batteries prevents dendrite formation, making the batteries less prone to short circuits and fires.

As ions move between the two electrodes of a li-ion battery, metal whiskers called dendrites can form. After enough charges and discharges, they can reach the point where they push through the separator, a porous polymer film that prevents the positively charged part of a battery from touching the negatively charged part. When the separator is breached, a short circuit can occur, which can also lead to a fire since the electrolyte solution in most lithium-ion batteries is highly flammable.

To avoid dendrite formation and minimize the probability of fire, current battery designs include one electrode made of graphite filled with lithium instead of pure lithium. The use of graphite as the host for lithium prevents the formation of dendrites. But lithium intercalated graphite also stores about 10 times less energy than pure lithium.

If dendrite formation can be eliminated in pure lithium electrodes, a great increase in energy storage is possible.

Nanodiamonds have been used in the electroplating industry for some time as a way of making metal coatings more uniform. When they are deposited, they naturally slide together to form a smooth surface.

The researchers found this property to be useful for eliminating dendrite formation. In the paper, they explain that lithium ions can easily attach to nanodiamonds, so when they are plating the electrode they do so in the same orderly manner as the nanodiamond particles to which they’re linked. They report that mixing nanodiamonds into the electrolyte solution of a lithium ion battery slows dendrite formation to nil through 100 charge-discharge cycles.

Dendrite formation (illustration and microscopy in left column) occurs in lithium-ion batteries over time and can result in hazardous malfunctions, like short-circuits and fires. Adding nanodiamonds to the electrolyte solution inside the battery imposes order on the lithium ion deposition (right column) so dendrites do not form. (Source: Drexel University and Tsinghua University)

Initial results show stable charge-discharge cycling for as long as 200 hours, which is long enough for use in some industrial or military applications, but not nearly adequate for batteries used in laptops or cell phones. Researchers also need to test a large number of battery cells over a long enough period of time under various physical conditions and temperatures to ensure that dendrites will never grow.

“It’s potentially game-changing, but it is difficult to be 100 percent certain that dendrites will never grow,” said Yury Gogotsi, professor of materials science and engineering at Drexel. “We anticipate the first use of our proposed technology will be in less critical applications — not in cell phones or car batteries. To ensure safety, additives to electrolytes, such as nanodiamonds, need to be combined with other precautions, such as using non-flammable electrolytes, safer electrode materials and stronger separators.”