A method for smaller and more efficient intermetallic nanoparticles for fuel cells has been discovered by researchers at Ames Laboratory and Iowa State University. This new method also uses less platinum, which is an expensive precious metal.

There have been technical challenges in the fabrication of platinum-zinc nanoparticles that have an ordered lattice structure. Before the new discovery, this product functioned best in small sizes, when the chemically reactive surface area is highest in proportion to the particle volume.

"That surface-to-volume ratio is important in getting the most out of an intermetallic nanoparticle," said Wenyu Huang, Ames Laboratory scientist and assistant professor of Chemistry at Iowa State University. "The smaller the particle, the more surface there is, and more surface area increases the catalytic activity."

The high temperatures of the annealing process were necessary to form intermetallic nanoparticle, but they often defeated the goal of achieving small size.

This high-resolution image shows the distribution of platinum and zinc atoms in a PtZn intermetallic nanoparticle. (Ames Laboratory)

"High-temperature annealing can cause the particles to aggregate or clump and produces larger sizes of particles that have less available surface and aren't as reactive. So, just the steps necessary to produce them can defeat their ultimate chemical performance," said Huang.

Huang’s research group worked hard at preventing aggregation from happening during the heating process. First, they used carbon nanotubes as support for the PtZn nanoparticles, then coated them with a sacrificial mesoporous silica shell for the high-temperature annealing to form the intermetallic structures. They then used a chemical etching process for removing the silica shell afterward.

This discovery was made possible in part by the capabilities of the new Titian scanning electron microscope located at Ames Laboratory’s Sensitive Instrument Facility.

"Being able to see the distributions of the material at the atomic level with our new microscope has made an enormous positive impact on the Laboratory's capabilities to fine-tune materials," said Lin Zhou, associate scientist and instrument lead for the Sensitive Instrument Facility. "It's a much more immediate process, being able to collaborate directly with the fabrication scientists in-house. Based on the results and suggestions we provide, they can improve the material, we can characterize it yet again, and the discovery cycle is much faster."

This research was funded by the National Science Foundation, Iowa State University, Ames Laboratory Directed Research and Development (LDRD) funds, and the U.S. Department of Energy's Office of Science. It was published in the Journal of American Chemical Society.