Highly Efficient Oxygen Catalyst Found: Rechargeable Batteries and Hydrogen-Fuel Production Could Benefit

Materials Science and Engineering Graduate Student Jin Suntivich (left) and Mechanical Engineering Graduate Student Kevin J. May (right) inspecting the electrochemical cell for oxygen evolution reaction experiment. (Credit: Photo by Jonathon R. Harding)

Science Daily  — A team of researchers at MIT has found one of the most effective catalysts ever discovered for splitting oxygen atoms from water molecules -- a key reaction for advanced energy-storage systems, including electrolyzers, to produce hydrogen fuel and rechargeable batteries. This new catalyst liberates oxygen at more than 10 times the rate of the best previously known catalyst of its type.

The team, which includes materials science and engineering graduate student Jin Suntivich, mechanical engineering graduate student Kevin J. May and professor Yang Shao-Horn, published their results in Science on Oct. 28.The new compound, composed of cobalt, iron and oxygen with other metals, splits oxygen from water (called the Oxygen Evolution Reaction, or OER) at a rate at least an order of magnitude higher than the compound currently considered the gold standard for such reactions, the team says. The compound's high level of activity was predicted from a systematic experimental study that looked at the catalytic activity of 10 known compounds.

The scientists found that reactivity depended on a specific characteristic: the configuration of the outermost electron of transition metal ions. They were able to use this information to predict the high reactivity of the new compound -- which they then confirmed in lab tests.

"We not only identified a fundamental principle" that governs the OER activity of different compounds, "but also we actually found this new compound" based on that principle, says Shao-Horn, the Gail E. Kendall (1978) Associate Professor of Mechanical Engineering and Materials Science and Engineering.

Many other groups have been searching for more efficient catalysts to speed the splitting of water into hydrogen and oxygen. This reaction is key to the production of hydrogen as a fuel to be used in cars; the operation of some rechargeable batteries, including zinc-air batteries; and to generate electricity in devices called fuel cells. Two catalysts are needed for such a reaction -- one that liberates the hydrogen atoms, and another for the oxygen atoms -- but the oxygen reaction has been the limiting factor in such systems.

Other groups, including one led by MIT's Daniel Nocera, have focused on similar catalysts that can operate -- in a so-called "artificial leaf" -- at low cost in ordinary water. But such reactions can occur with higher efficiency in alkaline solutions, which are required for the best previously known catalyst, iridium oxide, as well as for this new compound.

Shao-Horn and her collaborators are now working with Nocera, integrating their catalyst with his artificial leaf to produce a self-contained system to generate hydrogen and oxygen when placed in an alkaline solution. They will also be exploring different configurations of the catalyst material to better understand the mechanisms involved. Their initial tests used a powder form of the catalyst; now they plan to try thin films to better understand the reactions.

In addition, even though they have already found the highest rate of activity yet seen, they plan to continue searching for even more efficient catalyst materials. "It's our belief that there may be others with even higher activity," Shao-Horn says.

Jens Norskov, a professor of chemical engineering at Stanford University and director of the Suncat Center for Interface Science and Catalysis there, who was not involved in this work, says, "I find this an extremely interesting 'rational design' approach to finding new catalysts for a very important and demanding problem."

The research, which was done in collaboration with visiting professor Hubert A. Gasteiger (currently a professor at the Technische Universität München in Germany) and professor John B. Goodenough from the University of Texas at Austin, was supported by the U.S. Department of Energy's Hydrogen Initiative, the National Science Foundation, the Toyota Motor Corporation and the Chesonis Foundation.

Savannas and Forests in a Battle of the Biomes

Princeton researchers report that savanna wildfires — such as this one in South Africa — maintain the balance between forest and savanna, but that these blazes may be susceptible to human activity. The border between the habitats could degenerate into patches of encroachment in some areas. The researchers suggest forests that experience less rainfall due to climate change could become more prone to land-clearing fires. At the same time, the breakup of the savanna through land use, fire-prevention measures and road construction could disrupt the natural path of fires, presenting an opportunity for forests to take root. (Credit: Carla Staver)

Science Daily — Climate change, land use and other human-driven factors could pit savannas and forests against each other by altering the elements found by Princeton University researchers to stabilize the two. Without this harmony, the habitats, or biomes, could increasingly encroach on one other to the detriment of the people and animals that rely on them.

The Princeton researchers reported this month in the journal Science that savanna wildfires, combined with climate conditions, maintain the distinct border between savannas and forests in many tropical and subtropical areas. Savanna fires keep tree cover low and prevent forests from encroaching on the grassland. When tree cover is high, as in a forest, fires cannot spread as easily, halting the savanna's advance into the forest.

But the Princeton team's findings suggest that savanna wildfires could be heavily influenced by factors such as climate change, road construction and fire-prevention measures. Less rainfall can result in an uptick in fires that can transform a forest into a savanna, just as breaking up the landscape through road construction and fire control disrupt natural blazes and allow a forest to sprout where there once was a savanna.

The researchers suggest that because of these factors, large stretches of South American and African forest and savanna could degenerate into chaotic mutual encroachment. The changeover from one biome to the other -- which can happen within several decades -- can be extremely difficult to reverse once it has happened, explained lead author Carla Staver, a doctoral student in the laboratory of co-author Simon Levin, the Moffett Professor of Biology in Princeton's Department of Ecology and Environmental Biology. She and Levin worked with co-author Sally Archibald, a senior research scientist at the Council for Scientific and Industrial Research in South Africa.

Plants and animals that thrive in a forest or savanna often cannot transition from one habitat to the other, Staver said. The Science paper illustrates that the loss of savanna to forest is just as ecologically traumatic -- though less well known -- as deforestation, she said.

"Savanna and forest are definitely not locally compatible," she said. "There is a risk of losing plants and animals endemic to one or the other, which would affect the people who depend on those species.

"Savannas, for instance, are useful to people as cattle rangeland," she said. "When forests encroach, the grass productivity decreases dramatically and the land becomes much less useful. In terms of livelihood, that would have a huge impact."

The team's work provides among the first experimental evidence that fire feedback -- the ecological effect of fires -- is the dominant force in maintaining the division between forests and savannas, and that it can determine where the habitats flourish. The researchers used satellite data of fire distributions -- combined with climate and soil data, as well as satellite data of tree cover -- to survey the tropical and subtropical regions of Africa, Australia and South America.

The researchers found that the frequency of fires determines whether forest or savanna will dominate an area more than other factors such as rainfall, seasons and soil texture, especially in areas with moderate precipitation. Regular fires prevent trees from establishing and savannas from turning into forest. A lack of fires allows a forest to develop, which in turn excludes future fires.

Human alterations to the climate and landscape, however, may disrupt the natural spread of fire in many areas and lead to very rapid changes in biome distribution, the Princeton researchers suggest. Direct actions such as building roads and deploying methods such as controlled burning that prevent the natural spread of wildfires could break up savannas, altering wildfires and allowing forests to take root. At the same time, drier conditions -- particularly in areas now experiencing diminished monsoons -- rob forests of their primary safeguard against fire, rain.

Under these circumstances, a forest can overtake a savanna, or vice versa, in a matter of decades, and a return to the original terrain would prove exceedingly difficult, even if the original climate conditions return, Staver said.

"If a savanna were to turn into a forest, for instance, that change would be quite sudden, much quicker than we might expect, and it would be hard to reverse," Staver said. "You'd cross a threshold where fire cannot spread anymore. Conversely, if a forest dried out and fire started to spread, it could turn into a savanna, maintained by fire. The magnitude of change needed to return a biome to its original state would be much more than it needed to change in the first place."

The Princeton research could be significant in determining the "future trajectory" of global forest cover, and also illustrates the natural obstacles to restoring cleared forests, said Brian Walker, who studies ecological sustainability and resilience as a research fellow at the Commonwealth Scientific and Industrial Research Organization in Australia.

"Savanna systems are very resilient across a range of climatic and herbivore variation, in regard to fire. Forest systems are less so, except under very high rainfall where fire cannot be regular," said Walker, who had no role in the Princeton research but is familiar with it.

"In the case of rainforests, once they are in a state where fire can play a role and therefore keep the system in a savanna state, it is extremely difficult to prevent fires from recurring, and so the chances of a savanna state getting back to rainforest are small. In the original forest state, the amount of dry fuel in the ground layer is insufficient for fire to take hold and 'run.'"

In a broader sense, the Princeton findings stress that encroachment is not a threat unique to forests, Staver said.

"There's a sense among savanna ecologists that the loss of savanna is considered secondary to deforestation as a conservation concern, but it really shouldn't be. The loss of functionality and diversity in the savanna is just as important as in forests.

"At the moment, we can't say that one is winning out over the other," Staver said. "We can come up with examples where savannas are encroaching into forests and forests are encroaching into savannas. Both are happening extensively, and both are really huge issues that are likely to become even more important."

This research, reported Oct. 13 in the journal Science, was supported with funds from the Andrew W. Mellon Foundation.

World's Most Efficient Flexible Organic Light-Emitting Diodes Created On Plastic

Wang and Helander were able to re-construct the high-refractive index property previously limited to heavy metal-doped glass by using a 50-100 nanometre thick layer of tantalum(V) oxide (Ta₂O₅), an advanced optical thin-film coating material. This advanced coating technique, when applied on flexible plastic, allowed the team to build the highest-efficiency OLED device ever reported with a glass-free design. (Credit: © University of Toronto)

Science Daily — Researchers in the University of Toronto's Department of Materials Science & Engineering have developed the world's most efficient organic light-emitting diodes (OLEDs) on plastic. This result enables a flexible form factor, not to mention a less costly, alternative to traditional OLED manufacturing, which currently relies on rigid glass.

The results are reported online in the latest issue of Nature Photonics.

OLEDs provide high-contrast and low-energy displays that are rapidly becoming the dominant technology for advanced electronic screens. They are already used in some cell phone and other smaller-scale applications.

Current state-of-the-art OLEDs are produced using heavy-metal doped glass in order to achieve high efficiency and brightness, which makes them expensive to manufacture, heavy, rigid and fragile.

"For years, the biggest excitement behind OLED technologies has been the potential to effectively produce them on flexible plastic," says Materials Science & Engineering Professor Zheng-Hong Lu, the Canada Research Chair (Tier I) in Organic Optoelectronics.

Using plastic can substantially reduce the cost of production, while providing designers with a more durable and flexible material to use in their products.

The research, which was supervised by Professor Lu and led by PhD Candidates Zhibin Wang and Michael G. Helander, demonstrated the first high-efficiency OLED on plastic. The performance of their device is comparable with the best glass-based OLEDs, while providing the benefits offered by using plastic.

"This discovery, unlocks the full potential of OLEDs, leading the way to energy-efficient, flexible and impact-resistant displays," says Professor Lu.