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Sustainable Energy

From the Labs: Materials

New publications, experiments and breakthroughs in materials science–and what they mean.

Cheaper Fuel Cells
An inexpensive new catalyst works as fast as platinum.

Catalyst recipe: Ball bearings help pack materials (red, white, and brown) into microscopic pores in carbon black (black).

Source: “Iron-Based Catalysts with Improved Oxygen Reduction Activity in Polymer Electrolyte Fuel Cells”
Jean-Pol Dodelet et al.
Science
324: 71-74

This story is part of our July/August 2009 Issue
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Results: A catalyst made of iron, carbon, and nitrogen works nearly as well as platinum-based catalysts to accelerate the electrochemical reactions inside hydrogen fuel cells. The material produces 35 times as much current as previous catalysts not made of precious metals.

Why it matters: Hydrogen fuel cells for electric cars show promise because they emit no harmful pollutants, but they’ve been far too expensive to be practical. The new catalyst would greatly reduce the need for costly platinum in the fuel cells’ electrodes, making the technology cheaper.

Methods: The researchers improved the performance of a catalyst they had previously developed, in which nitrogen atoms and an iron ion bridge tiny gaps formed in a carbon material to create active sites for catalysis. To increase the number of these active sites, the researchers used a commercially available type of carbon that contains a large number of microscopic pores, which they packed with a material containing nitrogen and iron. When the material is heated under certain conditions, the nitrogen and iron arrange themselves into the catalytic bridges.

Next steps: To be practical, the catalyst needs to become more durable; in the researchers’ experiments, the reaction rates dropped by half after only 100 hours of testing. The reaction rates of the catalyst are also limited by how fast oxygen can move through the material to reach the active sites; this needs to be improved for the catalyst to work in fuel cells.

Bulk Graphene
Slicing carbon nanotubes into ribbons makes speedier transistors.

Source: “Longitudinal unzipping of carbon nanotubes to form graphene nanoribbons”
James M. Tour et al.
Nature
458: 872-876

Results: Researchers at Rice University have developed a simple method for making large numbers of long, narrow ribbons of graphene, a single-atom-thick film of carbon. They chemically sliced open carbon nanotubes, which are essentially rolled-up sheets of graphene.

Why it matters: Graphene conducts electrons faster than silicon, so it could be used to make faster transistors. But it’s been difficult to manufacture the semiconducting type of graphene that’s needed for this application. One way to make semiconducting graphene is to cut the material into narrow nanoscale ribbons, typically a slow process. The new chemical method produces bulk quantities of these ribbons by modifying carbon nanotubes, which are easy to manufacture in large amounts. The approach also solves a problem with carbon nanotubes: their electronic properties can vary widely. Unzipping them to make nano­ribbons makes these properties more uniform.

Methods: The researchers exposed multiwalled and single-­walled carbon nanotubes to sulfuric acid and potassium permanganate, a strong oxidizing agent. The resulting reaction breaks a carbon-carbon bond in each nanotube, and the exposed carbon atoms immediately bind to oxygen atoms, creating a strain on the adjacent carbon-carbon bonds. This strain causes the adjacent bonds to break more easily, and a chain reaction propagates down the length of the tube, cleanly unzipping it into a ribbon. This reaction repeats on each of the nanotubes’ walls, or concentric layers, yielding as many ribbons per tube as there are layers. The graphene nanoribbons must then undergo another reaction to remove the oxygen atoms. Finally, the researchers incorporated the nanoribbons into transistors using previously developed techniques.

Next steps: The researchers are developing thin-film and ink-jet printing methods for depositing nanoribbons, which would speed up the manufacture of graphene-based electronics such as radio-frequency identification tags.

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