Power/Performance Bits: May 30

Flexible nanogenerator acts as loudspeaker, microphone

Engineers at Michigan State University developed a paper-thin, flexible ferroelectret nanogenerator, or FENG, that can both generate energy from human motion and act as a loudspeaker and microphone.

“This is the first transducer that is ultrathin, flexible, scalable and bidirectional, meaning it can convert mechanical energy to electrical energy and electrical energy to mechanical energy,” said Nelson Sepulveda, MSU associate professor of electrical and computer engineering.

The FENG starts with a silicone wafer, which is then fabricated with several layers of environmentally friendly substances including silver, polyimide and polypropylene ferroelectret. Ions are added so that each layer in the device contains charged particles.

In 2016, the team demonstrated the device by using it to power a keyboard, LED lights and an LCD touch-screen. That process worked with a finger swipe or a light pressing motion to activate the devices – converting mechanical energy to electrical energy.

Recently, the researchers discovered the high-tech material can act as a microphone (by capturing the vibrations from sound, or mechanical energy, and converting it to electrical energy) as well as a loudspeaker (by operating the opposite way: converting electrical energy to mechanical energy).

The FENG developed by MSU can transmit sound and be embedded into a flag or other fabric. (Source: G.L. Kohuth/MSU)

To demonstrate the microphone effect, the researchers developed a FENG security patch that uses voice recognition to access a computer. The patch was successful in protecting an individual’s computer from outside users. “The device is so sensitive to the vibrations that it catches the frequency components of your voice,” Sepulveda said.

To demonstrate the loudspeaker effect, the FENG fabric was embedded into an MSU Spartan flag. Music was piped from an iPad through an amplifier and into the flag, which then reproduced the sound flawlessly. “The flag itself became the loudspeaker,” Sepulveda said. “So we could use it in the future by taking traditional speakers, which are big, bulky and use a lot of power, and replacing them with this very flexible, thin, small device.”

Wei Li, an MSU engineering researcher, said other potential applications of the FENG include noise-cancelling sheeting and a health-monitoring wristband that is voice-protected.

“Many people are focusing on the sight and touch aspects of flexible electronics,” Li said, “but we’re also focusing on the speaking and listening aspects of the technology.”

Anode for li-metal batteries

Scientists at Rice University created a rechargeable lithium metal battery they say has three times the capacity of commercial lithium-ion batteries. The battery stores lithium in a unique anode, a seamless hybrid of graphene and carbon nanotubes.

The anode itself approaches the theoretical maximum for storage of lithium metal while resisting the formation of damaging dendrites, the lithium deposits that grow into the battery’s electrolyte. If they bridge the anode and cathode and create a short circuit, the battery may fail, catch fire or even explode.

The researchers found that when the new batteries are charged, lithium metal evenly coats the highly conductive carbon hybrid in which nanotubes are covalently bonded to the graphene surface.

According to Rice chemist James Tour, the new anode’s nanotube forest, with its low density and high surface area, has plenty of space for lithium particles to slip in and out as the battery charges and discharges. The lithium is evenly distributed, spreading out the current carried by ions in the electrolyte and suppressing the growth of dendrites.

Lithium metal coats the hybrid graphene and carbon nanotube anode in a battery created at Rice University. (Source: The Tour Group/Rice University)

Though the prototype battery’s capacity is limited by the cathode, the anode material achieves a lithium storage capacity of 3,351 milliamp hours per gram, close to the theoretical maximum and 10 times that of lithium-ion batteries, Tour said. Because of the low density of the nanotube carpet, the ability of lithium to coat all the way down to the substrate ensures maximum use of the available volume.

To test the anode, the team built full quarter-sized batteries with sulfur-based cathodes that retained 80% capacity after more than 500 charge-discharge cycles, approximately two years’ worth of use for a normal cellphone user, Tour said. Electron microscope images of the anodes after testing showed no sign of dendrites or the moss-like structures that have been observed on flat anodes.

“We had to develop a commensurate cathode technology based upon sulfur to accommodate these ultrahigh-capacity lithium anodes in first-generation systems,” said Tour. “We’re producing these full batteries, cathode plus anode, on a pilot scale, and they’re being tested.”