Golden Kumar and Miriam Schroers/Yale University Photo
An up-close look at the nanowires.
What do jewelry and batteries have in common?
If a pair of Yale engineers have a say, quite a lot. Jan Schroers and Andre Taylor are using microscopic wires made of “bulk metallic glass” — a platinum alloy — to make fuel cells work better.
Their paper on the fuel cell is on the cover of this month’s ACS Nano, a scientific journal.
The alloy, a combination of platinum and other materials, acts as the catalyst for the fuel cell. While catalysts are traditionally made of carbon black and platinum particles, Schroers said, over time the carbon basically overwhelms the metal. But the nanoparticles in the new wires, he said, have a bigger surface area, and so let the catalyst work longer and more efficiently.
“There’s no platinum that disappears,” Schroers said. “In fact, the opposite happens. You get more platinum on the surface than in the alloy.”
After 1,000 cycles, the catalyst maintains 2.4 times as much performance as traditional catalysts made of carbon and platinum, according to the paper.
Materials made from nano-sized particles, like these, are a promising field of engineering. A nanometer is a billionth of a meter, and some of these super-tiny materials have amazing properties, from making bike frames stronger (carbon nanotubes) to making sunscreen more transparent on the skin (titanium dioxide). Scientists are struggling to figure out just how the properties of these substances change when they’re shrunk to the near-atomic level, and how or whether those changes impact people, animals or the environment.
Substances like the alloy Schroers and Taylor are working with, in which the nanoparticles are effectively encased in a larger structure, are of less concern than applications that allow the nanomaterials to be released more easily, such as sprays or fabric treatments.
The partnership between Schroers, an associate professor of mechanical engineering and materials science, and Taylor, an assistant professor of chemical and environmental engineering, is an example of the cross-disciplinary conversations going on within Yale’s School of Engineering and Applied Science.
Gwyneth K. Shaw Photo
Schroers (pictured, in a Yale lab) is a materials guy, especially interested in metallic compounds, mostly for jewelry making. Taylor is about applications, including fuel cells and sensors.
Together, they’ve made something they’re excited about. The other authors of the paper are Yale postdoctoral research fellows Marcelo Carmo and Golden Kumar, and graduate students Ryan C. Sekol and Shiyan Ding.
Producing the nanowires — “really more of a nanoforest,” according to Schroers — is relatively straightforward. First, the alloy must be created, by heating the metal nuggets in a tube that’s under vacuum until they’re fully blended.
The crucial thing, Schroers said, is keeping the alloy at the right temperature: too cool, and the mixture crystallizes. Get it right, he said, and the metal remains a liquid “that just doesn’t move on our timescale.”
The mixture is then shot into a cold mold, fast enough to avoid the dreaded crystallization. If it’s done right, the material hits what Schroers calls a sweet spot between strength and flexibility, allowing the nanowires to withstand a lot without being brittle.
“You can form them like you form plastic,” Schroers said. But they can take much higher heat.
The molds form the material into a wide variety of shapes. In a petri dish lined with a gray sponge, the molded alloy pieces are miniscule. Some are squiggly, others more like pinheads. Schroers said the material is interesting for a number of applications, from the fuel cells to sensors and watch batteries.
One of the upsides, he said, is that the alloy can be made and molded relatively easily. Lots of other products containing nanomaterials, such as superconductors, have to be manufactured in specialized environments like “clean rooms,” which are both hard to maintain and expensive.
As long as the alloy is created in the right vacuum environment, Schroers said, the metallic glass wires can be made in the same Yale lab where graduate students sip their coffee.
“We can really make these things,” Schroers said. “That’s an aspect that’s very impressive.”