Much shorter nanowires have already been used in a variety of technologies, such as nanocomputers and nanoscale sensors. By a process known as electrospinning—a technique used to produce long, ultra-thin solid fibers—Li and Shui were able to create platinum nanowires that are thousands of times longer than any previous such wires.

Platinum has been the primary material used in making fuel cell catalysts—the material that facilitates splitting hydrogen into electrons and acidic hydrogen ions—because of its ability to withstand the harsh acidic environment inside the fuel cell. Its energy efficiency is also substantially greater than that of less expensive metals such as nickel.

Prior efforts in making catalysts have relied heavily on platinum nanoparticles in order to maximize the exposed surface area of platinum. Li cites two main problems with the nanoparticle approach, both linked to the high cost of platinum.

• First, individual particles, despite being solid, can touch one another and merge through the process of surface diffusion, combining to reduce their total surface area and energy. As surface area decreases, so too does the rate of catalysis inside the fuel cell

• Second, nanoparticles require a carbon support structure to hold them in place. Unfortunately, platinum particles do not attach particularly well to these structures, and carbon is subject to oxidization, and thus degradation. As the carbon oxidizes over time, more and more particles become dislodged and are permanently lost.

Li’s nanowires avoid these problems completely. With platinum arranged into a series of centimeter long, flexible, and uniformly thin wires, the particles comprising them are fixed in place and need no additional support. Platinum will no longer be lost during normal fuel cell operation.

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.

—James Li

One of the key challenges Li and Shui managed to overcome was reducing the formation of platinum beads along the nanowires. Without optimal conditions, instead of a relatively smooth wire, you end up with what looks more like a series of interspersed beads on a necklace. Such bunching together of platinum particles is another case of unutilized surface area.

With platinum being so costly, it’s quite important that none of it goes to waste when making a fuel cell. We studied five variables that affect bead formation and we finally got it—nanowires that are almost bead free.

—James Li

Li plans to further optimize laboratory conditions to obtain fewer beads and even longer, more uniformly thin nanowire, then apply the material in a fuel cell.

Resources

• Jianglan Shui and James C. M. Li (2009) Platinum Nanowires Produced by Electrospinning. Nano Lett., Article ASAP doi: 10.1021/nl802910h