Power/Performance Bits: Oct. 6

Storing energy in waste plastic and red bricks; lowering LED reflections.


Waste plastic supercapacitor
Researchers from the University of California Riverside found a way to recycle waste plastic into energy storage devices. The work focused on polyethylene terephthalate plastic waste, or PET, which is found in soda bottles and many other consumer products.

The researchers first dissolved pieces of PET plastic bottles in a solvent. Using electrospinning, they fabricated microscopic fibers from the polymer and carbonized the plastic threads in a furnace. After mixing with a binder and a conductive agent, the material was then dried and assembled into an electric double-layer supercapacitor within a coin-cell type format.

When tested in the supercapacitor, the material contained the characteristics of both a double-layer capacitor formed by the arrangement of separated ionic and electronic charges, as well as redox reaction pseudo-capacitance that occurs when the ions are electrochemically absorbed onto surfaces of materials.

While it doesn’t store as much energy as a lithium-ion battery, the new supercapacitors charge quickly.

Scanning electron microscope image of a material for energy storage made from upcycled plastic bottles. (Source: Mihri Ozkan & Cengiz Ozkan/UCR)

“At UCR, we have taken the first steps toward recycling plastic waste into a rechargeable energy storage device,” said Arash Mirjalili, a doctoral student at UCR. “We believe that this work has environmental and economic advantages and our approach can present opportunities for future research and development.”

The process is scalable and marketable, according to the researchers. They plan on further work with the technique, including doping the electrospun fibers prior to carbonization with various chemicals and minerals such as boron, nitrogen, and phosphorous to tune the final material to have improved electrical properties.

“The upcycling of PET plastic waste for energy storage applications could be considered the holy grail for green manufacturing of electrode materials from sustainable waste sources,” said Cengiz Ozkan, a mechanical engineering professor at UCR. “This demonstration of a new class of electrodes in the making of supercapacitors will be followed by a new generation of Li-ion batteries in the future, so stay tuned.”

Brick energy storage
Researchers from Washington University in St. Louis developed a method to convert red bricks, the common building material, into energy storage units. In a proof-of-concept, the team powered a green LED directly from a brick converted into a supercapacitor.

“In this work, we have developed a coating of the conducting polymer PEDOT, which is comprised of nanofibers that penetrate the inner porous network of a brick; a polymer coating remains trapped in a brick and serves as an ion sponge that stores and conducts electricity,” said Julio D’Arcy, assistant professor of chemistry at Washington University in St. Louis.

The iron oxide (rust) that gives bricks their red pigment is essential for triggering the polymerisation reaction. The authors’ calculations suggest that walls made of these energy-storing bricks could store a substantial amount of energy.

Red brick device developed by chemists at Washington University in St. Louis lights up a green light-emitting diode. The photo shows the core-shell architecture of a nanofibrillar PEDOT-coated brick electrode. (Source: D’Arcy Laboratory, Department of Chemistry, Washington University in St. Louis)

“PEDOT-coated bricks are ideal building blocks that can provide power to emergency lighting,” D’Arcy said. “We envision that this could be a reality when you connect our bricks with solar cells — this could take 50 bricks in close proximity to the load. These 50 bricks would enable powering emergency lighting for five hours.”

A brick wall comprised of such bricks could be recharged hundreds of thousands of times an hour, D’Arcy noted. Another possible use would be powering sensors, which would take only a few bricks.

“Our method works with regular brick or recycled bricks, and we can make our own bricks as well,” said D’Arcy. “As a matter of fact, the work that we have published in Nature Communications stems from bricks that we bought at Home Depot right here in Brentwood (Missouri); each brick was 65 cents.”

The team coated the bricks with a five-minute epoxy to provide a waterproof, protective casing. A quasi-solid-state electrolyte was used as the ‘mortar’ between bricks. The brick supercapacitor was stable in ambient conditions and underwent 10,000 charge–discharge cycles with ~100% coulombic efficiency and ~90% capacitance retention. A supercapacitor brick module made by connecting three devices in series reached a 3.6 V voltage window.

Lowering LED reflections
Researchers from Imperial College London and the Indian Institute of Technology Guwahati propose a way to improve how much light an LED can produce by placing a single layer of nanoparticles between the LED chip and the transparent casing that protects the chip.

While the casing is necessary to protect the LED chip, it can cause unwanted reflections of the light emitted, so that not all the light escapes. But by adding a two-dimensional layer of sub-wavelength-sized plasmonic nanoparticles, the team predicted through modeling that unwanted reflections could be reduced, allowing up to 20% more light to be emitted.

Reducing reflections also has the added bonus of reducing heat in the device, increasing the lifespan of the LED chip.

“While improvements to the casing have been suggested previously, most make the LED bulkier or more difficult to manufacture, diminishing the economic effect of the improvement,” said Dr Debabrata Sikdar from IIT Guwahati. “We think that our innovation, based on fundamental theory and the detailed, balanced optimization analysis we performed, could be introduced into existing manufacturing processes with little disruption or added bulk.”

“The simplicity of the proposed scheme and the clear physics underpinning it should make it robust and, hopefully, easily adaptable to the existing LED manufacturing process,” asserted Professor Sir John Pendry, from the Department of Physics at Imperial. “It is obvious that with larger light extraction efficiency, LEDs will provide greater energy savings as well as longer lifetime of the devices. This will definitely have a global impact on the versatile LED-based applications and their multi-billion-dollar market worldwide.”

Next, the researchers plant to manufacture a prototype LED device with a nanoparticle layer, testing the best configurations predicted by the theory – including the size, shape, material and spacing of the nanoparticles, and how far the layer should be from the LED chip.

The team thinks that the principles used can work along with other existing schemes implemented for enhancing light extraction efficiency of LEDs. The same scheme could also apply to other optical devices where the transmission of light across interfaces is crucial, such as in solar cells.

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