Power/Performance Bits: July 3

2D straintronics; longer-lived flow batteries; graphene nanogenerator.


2D straintronics
Researchers at the University of Rochester and Xi’an Jiaotong University dug into how 2D materials behave when stretched to push the boundaries of what they can do.

“We’re opening up a new direction of study,” says Stephen Wu, assistant professor of electrical and computer engineering and physics at Rochester. “There’s a huge number of 2D materials with different properties – and if you stretch them, they will do all sorts of things.”

Using a transistor-scale device platform, the team deposited a small flake of the 2D material molybdenum ditelluride (MoTe2) onto a ferroelectric material. Voltage applied to the ferroelectric, which acts as the transistor’s gate, strains the 2D material by the piezoelectric effect, causing it to stretch. That, in turn, triggers a phase change that can completely change the way the material behaves. When the voltage is turned off the material retains its phase until an opposite polarity voltage is applied, causing the material to revert to its original phase.

“The ultimate goal of two-dimensional straintronics is to take all of the things that you couldn’t control before, like the topological, superconducting, magnetic, and optical properties of these materials, and now be able to control them, just by stretching the material on a chip,” Wu said.

In the case of MoTe2, the material changes from a low conductivity semiconductor material to a highly conductive semimetallic material and back again when stretched and unstretched.

“It operates just like a field effect transistor. You just have to put a voltage on that third terminal, and the MoTe2 will stretch a little bit in one direction and become something that’s conducting. Then you stretch it back in another direction, and all of a sudden you have something that has low conductivity,” Wu says.

The process works at room temperature, he adds, and, remarkably, “requires only a small amount of strain – we’re stretching the MoTe2 by only 0.4 percent to see these changes.”

According to the researchers, the platform has the potential to perform the same functions as a transistor with far less power consumption since power is not needed to retain the conductivity state. It also minimizes the leakage of electrical current due to the steep slope at which the device changes conductivity with applied gate voltage. Additionally, the platform is that it is configured much like a traditional transistor, making it easier to eventually adapt into current electronics.

However, more work is needed before the platform reaches that stage. Currently the device can operate only 70 to 100 times in the lab before device failure. While the endurance of other non-volatile memories, like flash, are much higher they also operate much slower than the ultimate potential of the strain-based devices.

“Do I think it’s a challenge that can be overcome? Absolutely,” said Wu. “It’s a materials engineering problem that we can solve as we move forward in our understanding how this concept works.”

Longer-lived flow batteries
Chemists at Harvard University found a way to make organic aqueous flow batteries last much longer, cutting the capacity fade rate of the battery at least a factor of 40.

Powered by organic anthraquinone molecules, the team’s organic flow batteries are promising for long-term storage of intermittent renewable electricity. The anthraquinone molecule, called DHAQ, is also cheap to produce at scale. However, it was slowly decomposing, reducing the battery’s long-term usefulness.

The researchers thought that the lifetime of the molecules depended on how many times the battery was charged and discharged, like in solid-electrode batteries such as lithium ion. However, in reconciling inconsistent results, the researchers discovered that these anthraquinones are decomposing slowly over the course of time based on age, regardless of how many times the battery has been used.

“We found that these anthraquinone molecules, which have two oxygen atoms built into a carbon ring, have a slight tendency to lose one of their oxygen atoms when they’re charged up, becoming a different molecule,” said Roy Gordon, Professor of Chemistry and Professor of Materials Science at Harvard. “Once that happens, it starts of a chain reaction of events that leads to irreversible loss of energy storage material.”

New flow battery chemistry reduces the capacity fade rate of the battery by a factor of at least 40 while still utilizing only chemicals known to be low-cost at mass-production scale. (Image courtesy of Michael Aziz/Harvard SEAS)

The researchers found two techniques to avoid that chain reaction. The first: expose the molecule to oxygen. The team found that if the molecule is exposed to air at just the right part of its charge-discharge cycle, it grabs the oxygen from the air and turns back into the original anthraquinone molecule. A single experiment recovered 70 percent of the lost capacity this way.

Second, the team found that overcharging the battery creates conditions that accelerate decomposition. Avoiding overcharging extends the lifetime by a factor of 40.

“Low mass-production cost is really important if organic flow batteries are going to gain wide market penetration,” said Michael Aziz, Professor of Materials and Energy Technologies at the Harvard. “So, if we can use these techniques to extend the DHAQ lifetime to decades, then we have a winning chemistry.”

Aziz added, “In future work, we need to determine just how much the combination of these approaches can extend the lifetime of the battery if we engineer them right.”

Graphene nanogenerator
Researchers at Rice University adapted laser-induced graphene (LIG) into small, metal-free devices that generate electricity. LIG takes advantage of the triboelectric effect to generate small amounts of static electricity when put into contact and pulled away from other surfaces.

LIG is a graphene foam produced when chemicals are heated on the surface of a polymer or other material with a laser, leaving only interconnected flakes of two-dimensional carbon.

The lab turned polyimide, cork and other materials into LIG electrodes to see how well they produced energy and stood up to wear and tear. They got the best results from materials on the opposite ends of the triboelectric series, which quantifies their ability to generate static charge by contact electrification.

The team found that a folded strip of LIG connected to a string of LEDs produced enough energy when tapped to make the LEDs flash. A larger piece of LIG embedded within a flip-flop let a wearer generate energy with every step, as the graphene composite’s repeated contact with skin produced a current to charge a small capacitor.

“This could be a way to recharge small devices just by using the excess energy of heel strikes during walking, or swinging arm movements against the torso,” said James Tour, a chemist at Rice.

In the folding configuration, LIG from the tribo-negative polyimide was sprayed with a protecting coating of polyurethane, which also served as a tribo-positive material. When the electrodes were brought together, electrons transferred to the polyimide from the polyurethane. Subsequent contact and separation drove charges that could be stored through an external circuit to rebalance the built-up static charge. The folding LIG generated about 1 kilovolt, and remained stable after 5,000 bending cycles.

The best configuration, with electrodes of the polyimide-LIG composite and aluminum, produced voltages above 3.5 kilovolts with a peak power of more than 8 milliwatts.

“The nanogenerator embedded within a flip-flop was able to store 0.22 millijoules of electrical energy on a capacitor after a 1-kilometer walk,” said Rice postdoctoral researcher Michael Stanford, lead author of the paper. “This rate of energy storage is enough to power wearable sensors and electronics with human movement.”

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