The Pt nanowires, produced by Professor James C.M. Li and graduate student Jianglan Shui in the mechanical engineering department, are roughly 10 nm in diameter yet are centimeters long – long enough to create the first self-supporting ‘web’ of pure platinum that can serve as a fuel cell electrode. The work was described in a recent paper in Nano Letters.
Much shorter nanowires have already been used in nanocomputers and nanoscale sensors. The Rochester researchers used electrospinning – a technique used to produce long, ultra-thin solid fibers – to create Pt nanowires thousands of times longer than any previous such wires. ‘Our ultimate purpose is to make free-standing fuel cell catalysts from these nanowires,’ says Li.
Platinum has been the primary material used in making fuel cell catalysts, because of its ability to withstand the harsh acidic environment inside the fuel cell. Its energy efficiency is also substantially greater than that of cheaper metals like nickel. Prior efforts in making catalysts have relied heavily on Pt 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 can touch and merge through surface diffusion, reducing their total surface area and energy. As surface area decreases, so too does the rate of catalysis inside the fuel cell. Secondly, nanoparticles require a carbon support structure. Unfortunately, Pt 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 cm-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.
One of the key challenges Li and Shui overcame was reducing the formation of Pt beads along the nanowires. Without optimal conditions, instead of a relatively smooth wire, you end up with what looks like a series of interspersed beads on a necklace. Such bunching together of Pt particles is another case of unutilized surface area.
Li's aims to further optimize laboratory conditions to obtain fewer beads and even longer, more uniformly thin nanowires. ‘After that, we’re going to make a fuel cell and demonstrate this technology,’ he says.