Power/Performance Bits: May 23

Biosupercapacitor; highly conductive transparent film; electroplating cathodes.

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Biosupercapacitor

Researchers from UCLA and the University of Connecticut designed a biological supercapacitor, a new biofriendly energy storage system which operates using ions from fluids in the human body. The device is harmless to the body’s biological systems, say the researchers, and could lead to longer-lasting cardiac pacemakers and other implantable medical devices.

The supercapacitor charges using electrolytes from biological fluids like blood serum and urine, and it would work with an energy harvester that converts heat and motion into electricity.

Modern pacemakers are typically about 6 to 8 millimeters thick, and about the same diameter as a 50-cent coin; about half of that space is usually occupied by the battery. The new supercapacitor is only 1 micrometer thick. It also can maintain its performance for a long time, bend and twist inside the body without any mechanical damage, and store more charge than the energy lithium film batteries of comparable size that are currently used in pacemakers.

The new biosupercapacitor comprises graphene layered with modified human proteins as an electrode. The new platform could eventually also be used to develop next-generation implantable devices to speed up bone growth, promote healing or stimulate the brain, said Richard Kaner, a distinguished professor of chemistry and biochemistry, and of materials science and engineering at UCLA.

Although supercapacitors have not yet been widely used in medical devices, the study shows that they may be viable for that purpose.

“In order to be effective, battery-free pacemakers must have supercapacitors that can capture, store and transport energy, and commercial supercapacitors are too slow to make it work,” said Maher El-Kady, a UCLA postdoctoral researcher. “Our research focused on custom-designing our supercapacitor to capture energy effectively, and finding a way to make it compatible with the human body.”

El-Kady also noted, “Combining energy harvesters with supercapacitors can provide endless power for lifelong implantable devices that may never need to be replaced.”

Highly conductive transparent film

Researchers at the University of Minnesota, Washington University, and Lawrence Berkeley National Laboratory developed a new nano-scale thin film material with the highest-ever conductivity in its class, as well as a wide bandgap lending it optical transparency.

“The high conductivity and wide bandgap make this an ideal material for making optically transparent conducting films which could be used in a wide variety of electronic devices, including high-power electronics, electronic displays, touch screens and even solar cells in which light needs to pass through the device,” said Bharat Jalan, a University of Minnesota chemical engineering and materials science professor.

Currently, most transparent conductors use indium. The price of indium has generally gone up over the last two decades, adding to the cost of current display technology.

In attempting to find an alternative material, the researchers developed a new transparent conducting thin film using a novel synthesis method, in which they grew a perovskite BaSnO3 thin film (barium, tin and oxygen, called barium stannate), but replaced elemental tin source with a chemical precursor of tin. The chemical precursor of tin has unique, radical properties that enhanced the chemical reactivity and greatly improved the metal oxide formation process. Both barium and tin are significantly cheaper than indium and are abundantly available.

The team said this new process allowed them to create this material with unprecedented control over thickness, composition, and defect concentration and that this process should be highly suitable for a number of other material systems where the element is hard to oxidize. The new process is also reproducible and scalable.

They further added that it was the structurally superior quality with improved defect concentration that allowed them to discover high conductivity in the material.  They said the next step is to continue to reduce the defects at the atomic scale.

“Even though this material has the highest conductivity within the same materials class, there is much room for improvement in addition to the outstanding potential for discovering new physics if we decrease the defects. That’s our next goal,” Jalan said.

Electroplating cathodes

Researchers at the University of Illinois, Xerion Advanced Battery Corporation and Nanjing University in China developed a method for electroplating lithium-ion battery cathodes, yielding high-quality, high-performance battery materials that could also open the door to flexible and solid-state batteries.

Traditional lithium-ion battery cathodes use lithium-containing powders formed at high temperatures. The powder is mixed with gluelike binders and other additives into a slurry, which is spread on a thin sheet of aluminum foil and dried. The slurry layer needs to be thin, so the batteries are limited in how much energy they can store. The glue also limits performance.

The researchers bypassed the powder and glue process altogether by directly electroplating the lithium materials onto the aluminum foil.

The electroplating method could enable flexible, three-dimensional battery designs. This plated aluminum foil rolled up without cracking. (Source: Hailong Ning and Jerome Davis III / Xerion Advanced Battery Corp.)

Since the electroplated cathode doesn’t have any glue taking up space, it packs in 30% more energy than a conventional cathode, according to the researchers. It can charge and discharge faster as well, since the current can pass directly through it and not have to navigate around the inactive glue or through the slurry’s porous structure. It also has the advantage of being more stable.

Additionally, the electroplating process creates pure cathode materials, even from impure starting ingredients. This means that manufacturers can use materials lower in cost and quality and the end product will still be high in performance, eliminating the need to start with expensive materials already brought up to battery grade, said Paul V. Braun, a professor of materials science and engineering and director of the Frederick Seitz Materials Research Lab at Illinois.

The researchers demonstrated the technique on carbon foam, a lightweight, inexpensive material, making cathodes that were much thicker than conventional slurries. They also demonstrated it on foils and surfaces with different textures, shapes and flexibility.

“These designs are impossible to achieve by conventional processes,” Braun said. “But what’s really important is that it’s a high-performance material and that it’s nearly solid. By using a solid electrode rather than a porous one, you can store more energy in a given volume. At the end of the day, people want batteries to store a lot of energy.”