Tiny hollow porous platinum structures can significantly reduce the amount of the precious metal required to catalyze reactions such as those in fuel cells, report researchers. The ultra-thin-wall nanocages enable catalytic activity on both inside and outside surfaces, increasing the efficiency of platinum in fuel cell electrodes by a factor of as much as seven and allowing use of larger nanocrystals that are less susceptible to sintering, they note.
“We can get the catalytic activity we need using only a small fraction of the platinum that has been required before,” says Younan Xia, a professor in the department of biomedical engineering at Georgia Institute of Technology, Atlanta. The team also included researchers from the University of Wisconsin — Madison; Oak Ridge National Laboratory, Oak Ridge, Tenn.; Arizona State University, Tempe, Ariz.; and Xiamen University in China. “The same approach can be extended to cover other precious metals, not just platinum,” he adds.
The team’s goal was to cut the cost of cathodes in fuel cells for automobiles and homes. However, the development also may offer benefits for industrial reactions like hydrogenations.
Fabrication of the nanocages involves solution-phase deposition of atomic-scale layers of platinum on palladium nanocrystal templates, followed by etching away almost all of the palladium. This retains the structure, leaving behind ultra-smooth platinum surfaces with a relatively low palladium concentration; tests show the selective removal of palladium does not affect the durability of the structure. The nanocages are about 20 nanometers in diameter, with layers of platinum three to six atoms thick. More details appear in a recent article in Science.
“We can control the process so well that we have layer-by-layer deposition, creating one layer, two layers or three layers of platinum,” explains Xia. “We also can control the arrangement of atoms on the surface so their catalytic activity can be engineered to fit different types of reactions.” The choice of palladium nanocrystals used as templates determines whether cubic- or octahedral-shaped nanocages are produced (Figure 1).
Hollow platinum structures have been made before but they have walls about five nanometers thick, note the researchers. In contrast, the new process enables production of walls less than one nanometer thick. Moreover, given the accessibility of both the inner and outer surfaces, up to two thirds of the platinum in a three-layer shell can play a role in catalysis. “This approach creates the highest possible surface area from a given amount of platinum,” says Xia.
He considers use of the nanocages in proton-exchange-membrane fuel cells for transportation applications particularly promising because of the relatively low temperature of the process (60°C). Moreover, the reduction in platinum achievable exceeds what U.S. Dept. of Energy projections indicate is required for economic viability, he notes.
“We are developing methods for scaling up the production of such nanocages and, at the same time, we are working with a company to apply the nanocages as catalysts for fuel cell applications,” Xia adds. “We have been able to test on a small scale in the lab. With the company, we want to perform the test on a 100-times scale and in real fuel cell devices.” A key challenge, he explains, is to “maintain the uniformity and control for the products while we are scaling up the production volume.”
“Ideally, we need to find the optimal shell thickness to achieve the highest utilization efficiency for the platinum atoms without compromising the mechanical property and catalytic durability,” he explains.