Yang Shao-Horn says many MIT students are fascinated by a topic very few would have cared about a few short years ago: fuel cells.
These devices — like batteries in that they generate electricity by chemical means, but unlike batteries in needing a constant flow of fuel (hydrogen) — are trendy enough, says Shao-Horn, that she’s even hearing from freshmen. “They want to know if they can use fuel cells in their robots,” she says.
It’s obvious why fuel cells are in the limelight. They’re seen as the key to a “hydrogen future,” notably as an alternative to the internal combustion engine. And given gas prices well over the $2-per-gallon mark, the threat of global warming linked to the use of oil and other fossil fuels, and growing talk oil production may peak in the years ahead, that future has suddenly taken on a shiny new allure.
Shao-Horn, an assistant professor of mechanical engineering and member of the MIT Energy Research Council, says fuel cells have real promise for use in cars and trucks. For one, they meet the size test: unlike a nuclear plant, say, a fuel-cell system small enough to fit comfortably into a car offers all the power such a vehicle needs. “What fuel cells really are,” she says, “are small, mobile power plants.”
But there are big obstacles to a hydrogen-fueled future on our highways, and today’s fuel cells are among them: their cost and durability, especially, are problems. So Shao-Horn and her co-workers have launched a drive to create cells that can last under real-world conditions.
Fascinated by Batteries
Shao-Horn has the background for the challenge. Her Dad was a materials scientist who studied metals and alloys, and as a child she sometimes joined him at the lab.
While an undergraduate at Beijing University of Technology, she began her own materials studies. It was as a Michigan Technological University grad student, though, that her interest in electrochemically-generated power took shape.
“My Ph.D. work was on lithium battery materials,” she notes. “I think I was the only student there working on batteries, but I just found it fascinating.”
(Shao-Horn has more than a theoretical interest in batteries. She and her husband, Quinn Horn, recently bought a used Porsche 914 on e-Bay, and plan to replace its existing propulsion system with a lithium-battery-powered one.)
At MIT, Shao-Horn has expanded her scope to include fuel cells — a move that has taken her on a quest deep into the technology. Every first-year chemistry student knows that catalysts help make chemical reactions happen. That’s true in fuel cells, too. The catalyst, typically platinum, causes the piped-in hydrogen to combine with ambient oxygen. In the process, you get water and an electrical output. But it’s unclear exactly what goes on as the catalyst is working its magic. “The science of electrocatalysis is in its infancy,” says Shao-Horn. “It has been very much a black art.” Yet the process is also critical to a hydrogen future: one big reason fuel-cell power right now costs $275 or more, as measured on a per-kilowatt basis, compared to a gasoline engine’s $10 per kilowatt, is that over time bad things happen to the pricey platinum.
New Fuel Cell
In a new fuel cell, tiny dots of platinum dwell on minuscule carbon spheres that are stuffed into both the cell’s positive and negative poles like packing peanuts in a cardboard box. These dots are an all-but infinitesimal two nanometers across. (How small is that? Cut a human hair in 400 slices lengthwise, and you’ve got the picture.) But they’re great at catalyzing hydrogen-oxygen bonding.
Using advanced electron-microscopy, Shao-Horn has probed what happens as fuel cells run. The platinum seems to clump up, with two-nanometer dots over time turning into, say, irregular dots three to four times that size. Such changes cut the platinum surface area available to catalyze — and hence the system’s efficiency and durability.
Shao-Horn and her co-workers recently became the first to detail precisely how, where, and when such changes happen. As for what to do about it, the group’s starting to explore ideas.
“To be useful in non-experimental vehicles,” notes Shao-Horn, “a fuel cell needs to be able to operate for at least 5,000 hours. Right now, they only last 1,000 to 2,000 hours.”
One possible way to stretch that time span is to use alloys made up of platinum and transition metals like manganese as a catalyst. But while there are theoretical reasons to think it can work, the group has just begun conducting experiments to explore the approach’s limits and feasibility.
Even when that and a lot of related work has been done, of course, fuel-cell-powered cars and the hydrogen infrastructure to supply them may still be years off. But Shao-Horn takes the long view.
“We’re trying to work out the fundamental problems that affect this technology,” she notes. “That way, when the time is right, the technology will be ready.”