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March 25th, 2009

Fuel cells: Will a nanowire net cut catalyst costs?

Published on March 25th, 2009 | By: pwray@ceramics.org
Platinum nanowires with (left) and without problematic "beads." Credit: Univ. of Rochester

Platinum nanowire net with (left) and without problematic "beads." Credit: Univ. of Rochester

One of the big divides the world of proton exchange fuel cell research is between those who are looking for an alternative to platinum (such as the University of Dayton’s Liming Dai) and those who are sticking with a platinum catalyst.

The pro-platinum group, populated by realists, are quick to acknowledge that ordinary catalyst systems are prohibitively expensive because the cost of the precious metal makes fuel cells containing them unaffordable except for military uses, space applications and specialized research centers. For them, the trick now is to find a way to use the least amount of platinum possible without reducing a fuel cell’s power output. Not surprisingly, they think the platinum Holy Grail can be found in nanotechnology.

Along these lines, one research team from the University of Rochester thinks they may have found the solution: long platinum nanowires.

According to a paper in Nano Letters, the concept is to use wires only 10 nanometers wide but several centimeters long to create a catalytic web of platinum. Lead author James C. M. Li, a professor of mechanical engineering at the university, and graduate student Jianglan Shui says they learned how to produce the long wires using electrospinning techniques.

The platinum nanowires produced by Li are roughly ten nanometers in diameter and also centimeters in length-long enough to create the first self-supporting “web” of pure platinum that can serve as an electrode in a fuel cell.

Much shorter nanowires have already been used in a variety of technologies, such as nanocomputers and nanoscale sensors. But the duo turned to a process known as electrospinning, a relatively old, noninvasive technique that uses an electrical charge to draw very fine (typically on the micro or nano scale) fibers from a liquid or molten material.

It’s easy to understand the attractiveness of electrospinning in this application. The technique is known for producing high surface-to-volume ratios, strong (approaching theoretical maximum strength) and defect-free structures. Li and Shui apparently are able to use this method to create platinum nanowires that are thousands of times longer than any previous such wires.

The electrospinning wasn’t without problems. Initial attempts at it left Li and Shui with platinum beads projecting the platinum nanowires. The beads block the surface of the wires, and if enough are present, large amounts of catalytic surface are effectively inaccessible. “With platinum being so costly, it’s quite important that none of it goes to waste when making a fuel cell,” says Li. “We studied five variables that affect bead formation and we finally got it – nanowires that are almost bead free.

Li and Shui say their approach avoids some of the pitfalls that other researchers have run into when using nanoscale amounts of platinum, such as the tendency of nanoparticles of the metal to merge through surface diffusion, and to become dislodged by oxidation of the support material.

Li says he understands why few have used his long-wire method. “The reason people have not come to nanowires before is that it’s very hard to make them. The parameters affecting the morphology of the wires are complex. And when they are not sufficiently long, they behave the same as nanoparticles,” says Li.
Li and Shui are now working on methods to make the wires longer, more uniform and with even fewer beads. “After that, we’re going to make a fuel cell and demonstrate this technology,” says Li.

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