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Space-filling model of phenylazomethine dendrimer template for metal cluster assembly. Credit: Nature

Nature Chemistry reported that Japanese researchers have created subnano scale platinum clusters with high catalytic activity for use in fuel cell applications. The tiny catalyst particles - the smallest of which contain just 12 atoms in total - could help to conserve the planet’s limited supply of platinum.

The team found that as they decreased the size of the clusters, their catalytic activity for the reduction of oxygen increased. At 12 atoms, each and every atom was exposed at the surface. The catalytic current produced was 13 times that of commercial platinum nanoparticles, which, by contrast, contain hundreds or even thousands of atoms. According to the researchers, however, the improved performance is probably not due to a simple increase in surface area but to quantum size effects that are not yet fully understood.

Lead researcher, Kimihisa Yamamoto of Keio University in Yokohama, says the fact that their subnano clusters perform so well goes against perceived wisdom within the field. “In the community of catalyst chemistry - especially fuel-cell catalysts - the fact that a platinum nanoparticle around three nanometers exhibits the best performance has become an established theory. However, our findings at least suggest that these subnano clusters made under specific conditions exhibit a high catalytic activity.”

According to Yamamoto, their results will lead to drastic reductions in the amount of platinum needed in fuel cells, with further progress becoming possible through the incorporation of a second metal into the platinum-based clusters. Although decreasing the size of particles is generally thought to decrease the reduction potential, this does not seem to hold true when subnano particles are bimetallic, he notes.

Younan Xia at Washington University, St. Louis, Mo., recently created bimetallic (platinum and palladium) nanoparticles for fuel cell applications, but whereas Yamamoto’s current work focuses on controlling size, Xia’s focused on controlling shape.

“Size and shape are the two most important parameters in determining the activity of a catalyst. Size control is what we would like to achieve too, but it has been difficult using our synthetic method. Interestingly, the method described in this paper cannot control the shape. So it is still a challenge to develop a method capable to controlling both size and shape,” says Xia.

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Liming Dai

A University of Dayton research team - led by Liming Dai, UD’s Wright Brothers Institute endowed chair in nanomaterials - says it has developed a technique that makes carbon nanotubes a cheaper and better fuel cell catalyst than platinum.

The Feb. 6th online edition of Science magazine reports on the team’s findings. Since that announcement, interviews with Dai - a professor of materials engineering in UD’s Department of Chemical and Materials Engineering - also have appeared in online science-community websites and in print trade magazines.

As reported, Dai’s team has shown that vertically-grown arrays of carbon nanotubes act as effectively as platinum in alkaline fuel cells if - and this is key - the carbon nanotubes are doped with nitrogen.

Reporter Stephen Battersby explains the need for nitrogen in NewScientist:

Unaided, this reaction would happen only very slowly, so the cathode has to be formed of a chemical catalyst to speed up the reaction. Traditionally, the only substance that has worked well enough is platinum.

Battersby notes, when carbon nanotubes were used without nitrogen doping in earlier experiments, they catalyzed fuel cell action but “were much less effective than platinum nanoparticles.”

Dai reveals his methodology in a Feb. 5 article in Technology Review. The first step, he says, is starting with a compound comprised of carbon, nitrogen and iron. The next step is placing the compound on a quartz substrate and “heating it in the presence of ammonia, resulting in nitrogen-doped carbon nanotubes growing straight up from the surface.”

Then any latent iron needs to be eliminated by oxidizing the array and moving it to a polymer film. The electrode is then emerged in a potassium hydroxide electrolyte.

It was at this point, Dai says, when his team noticed the technology’s ability to speed up the cathode reaction of oxygen and electrons.

In Dai’s estimation, carbon nanotubes doped with nitrogen “are even better than platinum.” He says they produce four times as much electric current as platinum and “where platinum nanoparticles can lose their effectiveness when they cluster together or become tainted by carbon monoxide, the nanotubes are immune to these degradations.”

Dai believes his team may be able to produce the same results using other forms of nitrogen-doped carbon. “Now we have discovered how this chemistry works,” he points out, “It may not be necessary to use nanotubes.”

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It seems like there is a breakthrough a week in the realm of lower-cost catalysts for producing hydrogen. The latest news comes from Ohio State University, where researchers are using a $1.1 million grant to develop alternatives to pricey rare and precious metallic catalysts like platinum and rhodium. With these metals selling for thousands of dollars per ounce, there is a huge incentive to find a replacement made from materials that are readily available. Umit Ozkan, a professor of chemical and biomolecular engineering who leads the research team, and graduate students Hua Song and Lingzhi Zhang, built the catalysts with calcium and cerium oxide covered with a thin layer of cobalt particles. They then pass a heated mixture of ethanol and water over the catalysts in a converter. Hydrogen and carbon dioxide are then produced. The OSU team imagines a scenario where hydrogen converters could be installed at gas stations, producing the gas on demand, thereby eliminating the expense of transporting or piping the gas. Ozkan claims that their method is 90 percent efficient at 660° F, cooler than other processes.

“Whenever a process works at a lower temperature, that brings energy savings and cost savings,” Ozkan says. “Also, if the catalyst is highly active and can achieve high hydrogen yields, we don’t need as much of it. That will bring down the size of the reactor, and its cost.”

Catalyst researchers often face problems such as “coking,” a situation where carbon accumulates on the catalyst. Ozkan’s team says the cerium oxide and calcium solve that problem buy oxidizing the carbon into carbon dioxide. The process also generates carbon monoxide and methane. These gases, along with the CO₂, are then removed, and the methane is recovered and fed back into the heating system. Ozkan previously has been noted for working on coal degasification techniques and looking for alternatives to iron-chromium catalysts. Recently, she has also been interviewed about the team’s work on Ira Flatow’s popular Science Friday, which is part of NPR.

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In a previous edition of Ceramic Tech Weekly, we posted information and a brief video about MIT researcher Daniel Nocera and his apparently successful efforts to cut down on the use of costly platinum. Now the good folks at Blip.TV have gone out and toured Nocera’s lab and conducted a longer interview with him. Check it out.

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