A Better Battery from Biology? Osaka University Researchers Publish Promising Results


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Figure 1: Structure of the newly developed ionic crystal. The pathway in which the ions can travel is highlighted in yellow. (Image: Osaka University)

A research team at Osaka University has reported a new advance in the design of materials for use in rechargeable batteries, under high humidity conditions. Using inspiration from living cells that can block smaller particles but let larger particles pass through, the researchers were able to create a material with highly mobile potassium ions that can easily migrate in response to electric fields (Chemical Science“Mobility of hydrated alkali metal ions in metallosupramolecular ionic crystals”).

This work may help make rechargeable batteries safe and inexpensive enough to drastically reduce the cost of electric cars and portable consumer electronics.
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Link to Osaka University’s Joint Research Programs
Rechargeable lithium-ion batteries are widely used in laptops, cell phones, and even electric and hybrid cars. Unfortunately, these batteries are expensive, and have even been known to burst into flames on occasion.
New materials that do not use lithium could reduce the cost and improve the safety of these batteries, and have the potential to greatly accelerate the adoption of energy-efficient electric cars. Both sodium and potassium ions are potential candidates that can be used to replace lithium, as they are cheap and in high supply.
However, sodium and potassium ions are much larger ions than lithium, so they move sluggishly through most materials. These positive ions are further slowed by the strong attractive forces to the negative charges in crystalline materials.
“Potassium ions possess low mobility in the solid state due to their large size, which is a disadvantage for constructing batteries,” explains corresponding author Takumi Konno.
To solve this problem, the researchers used the same mechanism your cells employ to allow the large potassium ions to pass through their membranes while simultaneously keeping out smaller particles. Living systems achieve this seemingly impossible feat by considering not just the ion themselves, but also the surrounding water molecules, called the “hydration layer,” that are attracted to the ion’s positive charge.
In fact, the smaller the ion, the larger and more tightly bound its associated hydration layer will be. Specialized potassium channels in cell membranes are just the right size to allow hydrated potassium ions to pass through, but block the large hydration layers of smaller ions.
The researchers developed an ionic crystal using rhodium, zinc, and oxygen atoms. Just as with the selective biological channels, the mobility of the ions in the crystal was found to be higher for the bigger potassium ions, compared with the smaller lithium ions.
In fact, the potassium ions moved so easily, the crystal was classified as a “superionic conductor.” The researchers found that the current material had the largest hydrated potassium ion mobility ever seen to date.
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Figure 2: Conductivities of lithium (Li , red), sodium (Na , green), and potassium (K , blue) ions inside the crystal at different temperatures. The conductivities increase even as the sizes of the ions increase. (Image: Osaka University)

“Remarkably, the crystal exhibited a particularly high ion conductivity due to the fast migration of hydrated potassium ions in the crystal lattice” lead author Nobuto Yoshinari says. “Such superionic conductivity of hydrated potassium ions in the solid state is unprecedented, and may lead to both safer and cheaper rechargeable batteries.”
Source: Osaka University
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Using one quantum dot to sense changes in another: Applications for developing advanced electronic and photonic devices


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Scanning electron micrograph of InAs self-assembled quantum dot transistor device. Credit: Osaka University

Quantum dots are nanometer-sized boxes that have attracted much scientific interest for use in nanotechnology because their properties obey quantum mechanics and are requisites to developing advanced electronic and photonic devices.

Quantum dots that self-assemble during their formation are particularly attractive as tunable light emitters in nanoelectronic devices and for studying quantum physics because of their quantized transport behavior. It is important to develop a way to measure the charge in a single self-assembled quantum dot to achieve quantum information processing; however, this is difficult because the metal electrodes needed for the measurement can screen out the very small charge of the quantum dot.

Researchers at Osaka University have recently developed the first device based on two self-assembled quantum dots that can measure the single-electron charge of one quantum dot using a second as a sensor.

The device was fabricated using two indium arsenide (InAs)  connected to electrodes that were deliberately narrowed to minimize the undesirable screening effect.

“The two  dots in the device showed significant capacitive coupling,” says Haruki Kiyama. “As a result, the single-electron charging of one dot was detected as a change in the current of the other dot.”

The current response of the sensor quantum dot depended on the number of electrons in the target dot. Hence the device can be used for real-time detection of single-electron tunneling in a quantum dot. The tunneling events of single electrons in and out of the target quantum dot were detected as switching between high and low current states in the sensor quantum dot. Detection of such tunneling events is important for the measurement of single spins towards electron spin qubits.

“Sensing single charges in self-assembled quantum dots is exciting for a number of reasons,” explains Akira Oiwa. “The ability to achieve electrical readout of single electron states can be combined with photonics and used in quantum communications. In addition, our device concept can be extended to different materials and systems to study the physics of self-assembled quantum dots.”

Two quantum dots are better than one: Using one dot to sense changes in another
Real-time traces of the charge sensor quantum dot (QD1) current. Changes in the charge sensor current indicate the increase and decrease of electron number in the adjacent quantum dot (QD2). Credit: Osaka University

An electronic device using self-assembled quantum dots to detect single-electron events is a novel strategy for increasing our understanding of the physics of quantum dots and to aid the development of advanced nanoelectronics and quantum computing.

 Explore further: Simultaneous detection of multiple spin states in a single quantum dot

More information: Haruki Kiyama et al, Single-electron charge sensing in self-assembled quantum dots, Scientific Reports (2018). DOI: 10.1038/s41598-018-31268-x

 

Environmentally friendly photoluminescent nanoparticles for more vivid display colors


IMAGE: THESE ARE STRUCTURES OF SILVER INDIUM SULFIDE/GALLIUM SULFIDE CORE/SHELL QUANTUM DOTS AND PICTURES OF THE CORE/SHELL QUANTUM DOTS UNDER ROOM LIGHT. view more 

CREDIT: OSAKA UNIVERSITY

Osaka, Japan – Most current displays do not always accurately represent the world’s colors as we perceive them by eye, instead only representing roughly 70% of them. To make better displays with true colors commonly available, researchers have focused their efforts on light-emitting nanoparticles.

Such nanoparticles can also be used in medical research to light up and keep track of drugs when developing and testing new medicines in the body. However, the metal these light-emitting nanoparticles are based on, namely cadmium, is highly toxic, which limits its applications in medical research and in consumer products–many countries may soon introduce bans on toxic nanoparticles.

It is therefore vital to create non-toxic versions of these nanoparticles that have similar properties: they must produce very clean colors and must do so in a very energy-efficient way. So far researchers have succeeded in creating non-toxic nanoparticles that emit light in an efficient manner by creating semiconductors with three types of elements in them, for example, silver, indium, and sulfur (in the form of silver indium disulfide (AgInS2)). However, the colors they emit are not pure enough–and many researchers declared that it would be impossible for such nanoparticles to ever emit pure colors.

Now, researchers from Osaka University have proven that it is possible by fabricating semiconductor nanoparticles containing silver indium disulfide and adding a shell around them consisting of a semiconductor material made of two different elements, gallium and sulfur. The team was able to reproducibly create these shell-covered nanoparticles that are both energy efficient and emit vivid, clean colors. The team have recently published their research in the Nature journal NPG Asia Materials.

“We synthesized non-toxic nanoparticles in the normal way: mix all ingredients together and heat them up. The results were not fantastic, but by tweaking the synthesis conditions and modifying the nanoparticle cores and the shells we enclosed them in, we were able to achieve fantastic efficiencies and very pure colors,” study coauthor Susumu Kuwabata says.

Enclosing nanoparticles in semiconductor shells in nothing new, but the shells that are currently used have rigidly arranged atoms inside them, whereas the new particles are made of a more chaotic material without such a rigid structure.

“The silver indium disulfide particles emitted purer colors after the coating with gallium sulfide. On top of that, the shell parts in microscopic images were totally amorphous. We think the less rigid nature of the shell material played an important part in that–it was more adaptable and therefore able to take on more energetically favorable conformations,” first author Taro Uematsu says.

The team’s results demonstrate that it is possible to create cadmium-free, non-toxic nanoparticles with very good color-emitting properties by using amorphous shells around the nanoparticle cores.

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The article, “Narrow band-edge photoluminescence from AgInS2 semiconductor nanoparticles by the formation of amorphous III-VI semiconductor shells” was published in NPG Asia Materials, https://doi.org/10.1038/s41427-018-0067-9.

Electricity from seawater: New method efficiently produces hydrogen peroxide for fuel cells


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Scientists have used sunlight to turn seawater (H2O) into hydrogen peroxide (H2O2), which can then be used in fuel cells to generate electricity. It is the first photocatalytic method of H2O2 production that achieves a high enough efficiency so that the H2O2 can be used in a fuel cell.

The researchers, led by Shunichi Fukuzumi at Osaka University, have published a paper on the new method of the photocatalytic production of in a recent issue of Nature Communications.

“The most earth-abundant resource, seawater, is utilized to produce a solar fuel that is H2O2,” Fukuzumi told Phys.org.

The biggest advantage of using liquid H2O2 instead of gaseous hydrogen (H2), as most fuel cells today use, is that the liquid form is much easier to store at high densities. Typically, H2 gas must be either highly compressed, or in certain cases, cooled to its at cryogenic temperatures. In contrast, liquid H2O2 can be stored and transported at high densities much more easily and safely.

The problem is that that, until now, there has been no efficient photocatalytic method of producing liquid H2O2. (There are ways to produce H2O2 that don’t use sunlight, but they require so much energy that they are not practical for use in a method whose goal is to produce energy.)

In the new study, the researchers developed a new photoelectrochemical cell, which is basically a solar cell that produces H2O2. When sunlight illuminates the photocatalyst, the photocatalyst absorbs photons and uses the energy to initiate chemical reactions (seawater oxidation and the reduction of O2) in a way that ultimately produces H2O2.

After illuminating the cell for 24 hours, the concentration of H2O2 in the seawater reached about 48 mM, which greatly exceeds previous reported values of about 2 mM in pure water. Investigating the reason for this big difference, the researchers found that the negatively charged chlorine in seawater is mainly responsible for enhancing the photocatalytic activity and yielding the higher concentration.

Overall, the system has a total solar-to-electricity of 0.28%. (The photocatalytic production of H2O2 from seawater has an efficiency of 0.55%, and the has an efficiency of 50%.)

Although the total efficiency compares favorably to that of some other solar-to-electricity sources, such as switchgrass (0.2%), it is still much lower than the efficiency of conventional solar . The researchers expect that the efficiency can be improved in the future by using better materials in the photoelectrochemical cell, and they also plan to find methods to reduce the cost of production.

“In the future, we plan to work on developing a method for the low-cost, large-scale production of H2O2 from ,” Fukuzumi said. “This may replace the current high-cost production of H2O2 from H2 (from mainly natural gas) and O2.”

Explore further: How does an enzyme detoxify the cells of living beings?

More information: Kentaro Mase et al. “Seawater usable for production and consumption of hydrogen peroxide as a solar fuel.” Nature Communications. DOI: 10.1038/ncomms11470