The technique 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 20 nanometers in diameter, with between three and six atom-thin layers of platinum.
With their much higher usage efficiency, the use of these nanocage structures in fuel cell electrodes could potentially change the economic viability of the fuel cells, concludes Younan Xia, a professor in the Department of Biomedical Engineering at Georgia Tech and Emory University. “We can get the catalytic activity we need by using only a small fraction of the platinum that had been required before,” said Xia. “We have made hollow nanocages with walls as thin as a few atomic layers because we don’t want to waste any material in the bulk that does not contribute to the catalytic activity.”
The research – which also involved researchers at the University of Wisconsin-Madison, Oak Ridge National Laboratory, Arizona State University and Xiamen University in China – was reported in the July 24 issue of 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 automobiles and home applications.
According to Xia, the team can control the process well enough to get layer-by-layer deposition, creating one layer, two layers or three layers of platinum. It is also possible to control the arrangement of atoms on the surface so their catalytic activity can be engineered to fit different types of reactions.
Earlier work produced shells with wall thicknesses of approximately five nanometers. The new process can produce shell walls less than one nanometer 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 goal of this research was to reduce the cost of the cathodes in fuel cells designed to power automobiles and homes. The fuel cell’s oxygen-reduction reaction takes place at the cathode, and that requires a substantial amount of platinum. By reducing the amount of platinum by up to a factor of seven, the hollow shells could make automotive and home fuel cells more economically feasible.