Power/Performance Bits: Oct. 20

Memristors come in threes; potassium batteries; lighting up leftovers.


Memristors come in threes

The race is on to produce a commercial memristor, and a duo from ETH Zurich may be providing a bit more push.

“Basically, memristors require less energy since they work at lower voltages,” explained Jennifer Rupp, professor in the Department of Materials at ETH Zurich. “They can be made much smaller than today’s memory modules, and therefore offer much greater density. This means they can store more megabytes of information per square millimeter.” But currently memristors are only at the prototype stage.

So, with colleague Markus Kubicek, Rupp built a memristor based on a 5nm slice of perovskite.

The particularly interesting part is that they found the component has three stable resistive states: it can not only store the 0 or 1 of a standard bit, but can also be used for information encoded by three states – the 0, 1 and 2 of a “trit”. “Our component could therefore be useful for a new type of IT that is not based on binary logic, but on a logic that provides for information located ‘between’ the 0 and 1,” said Rupp. “This has interesting implications for what is referred to as fuzzy logic, which seeks to incorporate a form of uncertainty into the processing of digital information. You could describe it as less rigid computing.”

Another potential application is neuromorphic computing, which aims to use electronic components to reproduce the way in which neurons in the brain process information. “The properties of a memristor at a given point in time depend on what has happened before,” says Rupp. “This mimics the behavior of neurons, which only transmit information once a specific activation threshold has been reached.”

Primarily, the researchers characterized the ways in which the component works by conducting electro-chemical studies, which they say will be important in refining the way the storage operates and improving its efficiency.

Potassium batteries

In showing that potassium can work with graphite in a potassium-ion battery, researchers at Oregon State University overturned a long-standing scientific dogma.

“For decades, people have assumed that potassium couldn’t work with graphite or other bulk carbon anodes in a battery,” said Xiulei (David) Ji, lead author of the study and an assistant professor of chemistry at OSU. “That assumption is incorrect. It’s really shocking that no one ever reported on this issue for 83 years.”

The findings are of importance, the researchers say, because they open a new option for batteries that can work with the well-established and inexpensive graphite as the anode, without the high cost of lithium.

“The cost-related problems with lithium are sufficient that you won’t really gain much with economies of scale,” says Ji. “With most products, as you make more of them, the cost goes down. With lithium the reverse may be true in the near future. So we have to find alternatives.”

A graphic outlining the electrical capacity of the newly developed potassium-ion battery. (Source: Oregon State University)

A graphic outlining the electrical capacity of the newly developed potassium-ion battery. (Source: Oregon State University)

The alternative, according to the researchers, may be potassium: an element 880 times more abundant in the Earth’s crust than lithium. Their findings show that it can work effectively with graphite or soft carbon in the anode of an electrochemical battery. However, batteries based on this approach don’t have performance that equals those of lithium-ion batteries, but improvements in technology might narrow the gap.

“It’s safe to say that the energy density of a potassium-ion battery may never exceed that of lithium-ion batteries,” Ji said. “But they may provide a long cycling life, a high power density, a lot lower cost, and be ready to take the advantage of the existing manufacturing processes of carbon anode materials.”

Lighting up leftovers

LEDs can be produced by quantum dots, tiny crystals that have luminescent properties. Quantum dots can be made with numerous materials, some of which are rare and expensive to synthesize, and even potentially harmful to dispose of. As such, some of the research over the past 10 years has focused on using carbon dots (quantum dots made of carbon) to create LEDs instead.

Compared to other types of quantum dots, carbon dots have lower toxicity and better biocompatibility, so they can be used in a broader variety of applications. University of Utah researchers took this idea a step further, and found a way to create LEDs from food and beverage waste.

To synthesize food waste into carbon dots, the team employed a solvothermal synthesis, in which the leftovers – in this case, discarded soft drinks and pieces of bread and tortilla – were placed into a solvent under pressure and high temperature until carbon dots were formed.

“Synthesizing and characterizing carbon dots derived from waste is a very challenging task. We essentially have to determine the size of dots which are only 20 nanometers or smaller in diameter, so we have to run multiple tests to be sure carbon dots are present and to determine what optical properties they possess,” said Prashant Sarswat, metallurgical engineering research assistant professor at the University of Utah.

Left: A small light emitting diode fabricated from carbon dots. Right: The luminescence of carbon dots can be seen when irradiated with UV light. (Source: Prashant Sarswat/University of Utah)

Left: A small light emitting diode fabricated from carbon dots. Right: The luminescence of carbon dots can be seen when irradiated with UV light. (Source: Prashant Sarswat/University of Utah)

The various tests first measured the size of the carbon dots, which correlates with the intensity of the dots’ color and brightness. The tests then determined which carbon source produced the best carbon dots. In this instance, the sucrose and D-fructose dissolved in soft drinks were found to be the most effective sources for production of carbon dots.

The team hopes to continue studying the LEDs produced from food and beverage waste for stability and long term performance. Sarswat’s ultimate goal is “to do this on a mass scale and to use these LEDs in everyday devices. To successfully make use of waste that already exists, that’s the end goal.”