MIT research update: New way to store sun's heat

A novel application of carbon nanotubes, developed by MIT researchers, shows promise as an innovative approach to storing solar energy for use whenever it's needed.

Storing the sun's heat in chemical form — rather than converting it to electricity or storing the heat itself in a heavily insulated container — has significant advantages, since in principle the chemical material can be stored for long periods of time without losing any of its stored energy. The problem with that approach has been that until now the chemicals needed to perform this conversion and storage either degraded within a few cycles, or included the element ruthenium, which is rare and expensive.

Last year, MIT associate professor Jeffrey Grossman and four co-authors figured out exactly how fulvalene diruthenium — known to scientists as the best chemical for reversibly storing solar energy, since it did not degrade — was able to accomplish this feat. Grossman said at the time that better understanding this process could make it easier to search for other compounds, made of abundant and inexpensive materials, which could be used in the same way.

Now, he and postdoc Alexie Kolpak have succeeded in doing just that. A paper describing their new findings has just been published in the journal Nano Letters.

The new material found by Grossman and Kolpak is made using carbon nanotubes, tiny tubular structures of pure carbon, in combination with a compound called azobenzene. The resulting molecules, produced using nanoscale templates to shape and constrain their physical structure, gain "new properties that aren't available" in the separate materials, says Grossman, the Carl Richard Soderberg Associate Professor of Power Engineering.

Not only is this new chemical system less expensive than the earlier ruthenium-containing compound, but it also is vastly more efficient at storing energy in a given amount of space — about 10,000 times higher in volumetric energy density, Kolpak says — making its energy density comparable to lithium-ion batteries. By using nanofabrication methods, "you can control [the molecules'] interactions, increasing the amount of energy they can store and the length of time for which they can store it — and most importantly, you can control both independently," she says.

Thermo-chemical storage of solar energy uses a molecule whose structure changes when exposed to sunlight, and can remain stable in that form indefinitely. Then, when nudged by a stimulus — a catalyst, a small temperature change, a flash of light — it can quickly snap back to its other form, releasing its stored energy in a burst of heat. Grossman describes it as creating a rechargeable heat battery with a long shelf life, like a conventional battery.

One of the great advantages of the new approach to harnessing solar energy, Grossman says, is that it simplifies the process by combining energy harvesting and storage into a single step. "You've got a material that both converts and stores energy," he says. "It's robust, it doesn't degrade, and it's cheap." One limitation, however, is that while this process is useful for heating applications, to produce electricity would require another conversion step, using thermoelectric devices or producing steam to run a generator.

While the new work shows the energy-storage capability of a specific type of molecule — azobenzene-functionalized carbon nanotubes — Grossman says the way the material was designed involves "a general concept that can be applied to many new materials." Many of these have already been synthesized by other researchers for different applications, and would simply need to have their properties fine-tuned for solar thermal storage.

The key to controlling solar thermal storage is an energy barrier separating the two stable states the molecule can adopt; the detailed understanding of that barrier was central to Grossman's earlier research on fulvalene dirunthenium, accounting for its long-term stability. Too low a barrier, and the molecule would return too easily to its "uncharged" state, failing to store energy for long periods; if the barrier were too high, it would not be able to easily release its energy when needed. "The barrier has to be optimized," Grossman says.

Already, the team is "very actively looking at a range of new materials," he says. While they have already identified the one very promising material described in this paper, he says, "I see this as the tip of the iceberg. We're pretty jazzed up about it."

Yosuke Kanai, assistant professor of chemistry at the University of North Carolina at Chapel Hill, says "the idea of reversibly storing solar energy in chemical bonds is gaining a lot of attention these days. The novelty of this work is how these authors have shown that the energy density can be significantly increased by using carbon nanotubes as nanoscale templates. This innovative idea also opens up an interesting avenue for tailoring already-known photoactive molecules for solar thermal fuels and storage in general."

New method for making human-based gelatin

Scientists are reporting development of a new approach for producing large quantities of human-derived gelatin that could become a substitute for some of the 300,000 tons of animal-based gelatin produced annually for gelatin-type desserts, marshmallows, candy and innumerable other products. Their study appears in ACS's Journal of Agriculture and Food Chemistry.

Jinchun Chen and colleagues explain that animal-based gelatin, which is made most often from the bones and skin of cows and pigs, may carry a risk of infectious diseases such as "Mad Cow" disease and could provoke immune system responses in some people. Animal-based gelatin has other draw-backs, with variability from batch to batch, for instance, creating difficulties for manufacturers. Scientists thus have sought alternatives, including development of a human-recombinant gelatin for potential use in drug capsules and other medical applications.

To get around these difficulties, the scientists developed and demonstrated a method where human gelatin genes are inserted into a strain of yeast, which can produce gelatin with controllable features. The researchers are still testing the human-yeast gelatin to see how well it compares to other gelatins in terms of its viscosity and other attributes. Chen and colleagues suggest that their method could be scaled up to produce large amounts of gelatin for commercial use.

 

The unsolved mystery of kava toxicity

A major new review of scientific knowledge on kava — a plant used to make dietary supplements and a trendy drink with calming effects — has left unsolved the mystery of why Pacific Island people can consume it safely, while people in the United States, Europe, and other Western cultures sometimes experience toxic effects. The article appears in ACS' journal Chemical Research in Toxicology.

Line Olsen and colleagues point out that for centuries, people of the Pacific Islands have safely consumed a beverage made from crushed kava roots. Kava'scalming effects made it popular in Western cultures in the 1990s, when people also began to use a herbal supplement for the treatment of anxiety, emotional stress and sleep problems. But in 2001, reports of liver damage among Westerners who took kava supplements gained widespread attention. Many Western countries, including the United States, the United Kingdom, and Canada, ban or regulate the sale of kava products. To determine why kava is toxic to some people but not to others, the researchers sifted through the scientific studies published on the topic.

Their review of 85 scientific studies on kava toxicity found no consensus on kava toxicity, despite several theories that have emerged over the years. Culprits include methods for preparing kava, the particular species of kava used, the possible toxicity of substances produced by the body when kava is digested and genetic differences among consumers. "To date, there remains no indisputable reason for the increased prevalence of kava-induced hepatotoxicity in Western countries," the researchers say.

 

New technology allows lenses to change color rapidly

A University of Connecticut scientist has perfected a method for creating quick-changing, variable colors in films and displays, such as sunglasses, that could lead to the next hot fashion accessory.

The new technology also has captured the interest of the U.S. military as a way to assist soldiers who need to be able to see clearly in rapidly changing environments.

The process for creating the lenses, for which a patent is pending, also is less expensive and less wasteful to manufacturers than previous methods. The findings were in the Journal of Materials Chemistry.

"This is the next big thing for transition lenses," says Greg Sotzing, a professor of chemistry in UConn's College of Liberal Arts and Sciences and a member of UConn's Polymer Program.

The typical material behind a transition lens is what's called a photochromic film, or a sheet of polymers that change color when light hits them. Sotzing's new technology does things slightly differently – his electrochromic lenses are controlled by an electric current passing through them when triggered by a stimulus, such as light.

"They're like double pane windows with a gap between them," explains Sotzing. He and his colleagues squirt a mixture of polymers – or as he calls it, "goop" – in between the layers, creating the lens as it hardens. The mixture of polymers used in this lens, says Sotzing, creates less waste and is less expensive to produce than previous mixtures.

"The lifetime of sunglasses is usually very short," says Sotzing, who points out that people often misplace them. So by making the manufacturing less expensive, he says, commercial retailers will be able to produce more of them.

Another benefit of this material is that it can change colors as quickly as electricity passes through it – which is virtually instantaneously. This process could be very useful for the military, Sotzing says. For example, if a person emerges from a dark passageway and into the desert, a lens that would alter its color instantly to complement the surroundings could mean life or death for some soldiers.

"Right now, soldiers have to physically change the lenses in their goggles," Sotzing says. "This will eliminate that need." Sotzing will begin a one-year sabbatical at the Air Force Academy in August, where he hopes to develop some of these ideas.

In November 2010, partially based on work supported by the Center for Science and Technology Commercialization's Prototype Fund, the UConn R&D Corporation started a company, called Alphachromics Inc., with Sotzing and colleague Michael Invernale, now a post-doctoral researcher at MIT, as founders. The university has a patent pending for this new technology, which is currently under option to the company. Alphachromics is also testing applications of these polymer systems for energy-saving windows and custom fabrics.