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'Nanocages' could reduce platinum use in fuel cell electrodes

25 July 2015

A new fabrication technique that produces platinum hollow 'nanocages' with ultra-thin walls could dramatically reduce the amount needed of the costly metal.

Representation of platinum hollow nanocages. The yellow spheres correspond to platinum atoms while the green spheres indicate a small fraction of palladium atoms remaining in the lattice of platinum atoms (image: Xia Laboratory)

The technique, developed by researchers at Georgia Tech and the University of Wisconsin-Madison, Oak Ridge National Laboratory, Arizona State University and Xiamen University in China, uses a solution-based method for producing atomic-scale layers of platinum to create hollow, porous structures that can generate catalytic activity both inside and outside the nanocages.

The layers are grown on palladium nanocrystal templates, and then the palladium is etched away to leave behind nanocages approximately 20nm in diameter, with between three and six atom-thin layers of platinum.

Use of these nanocage structures in fuel cell electrodes could increase the utilisation efficiency of the platinum by a factor of as much as seven, potentially changing the economic viability of the fuel cells.

A report on the work is published in the journal Science.

Platinum is in high demand as a catalyst for a wide range of industrial and consumer applications. The high cost of platinum needed for the catalysts deposited on electrodes has limited the ability to use low-temperature fuel cells in vehicles and home applications.

In catalytic applications, only the surface layers of platinum contribute to the chemical reaction, leading researchers to develop new structures designed to maximise the amount of platinum exposed to reactants. The hollowing out process reduces the amount of the precious metal not contributing to the reaction, and allows the use of larger nanocrystals that are less susceptible to sintering, an aggregation phenomenon which reduces catalyst surface area.

Hollow platinum structures have been made before, but not with walls this thin, say the researchers. Earlier work produced shells with wall thicknesses of approximately 5nm. The new process can produce shell walls less than one nanometre thick. With both the inner layer and outer layer of the porous nanocages contributing to the catalytic activity, the new structures can use up to two-thirds of the platinum atoms in an ultra-thin three-layer shell. Some palladium remains mixed with the platinum in the structures.

The nanocages can be made in either cubic or octahedral shapes, depending on the palladium nanocrystals used as templates. The shape controls the surface structure, thus engineering the catalytic activity.


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