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Nanowires to bring fuel cells to mass market

Posted: 17 Mar 2009 ?? ?Print Version ?Bookmark and Share

Keywords:fuel cell? nanowire? fuel hydrogen?

Woven nanowire electrodes could provide hydrogen fuel cells the needed boost to make them commercially viable, according to University of Rochester researchers.

Nanowires have aspect ratios in the millions, enabling them to expose thousands of times more surface area than traditional platinum electrodes, thereby potentially boosting efficiency, reducing cost and increasing the longevity of hydrogen fuel cells.

"Our electrodes will be woven from nanowires which are very, very long compared to their width," said professor James "C. M." Li, of the University of Rochester. "Their small size exposes more catalyst, making the fuel cells using them more efficient while reducing their expense since less platinum is required."

Li previously perfected a method of spinning out polymer nanowires with extremely high aspect ratios. He adapted the technique to spinning out individual platinum nanowires that are centimeters long but only nanometers wide. The resulting woven nanowire electrodes require much less platinum than traditional electrodes, potentially extending fuel cells beyond niche markets and into mainstream applications, Li claimed.

The platinum catalyst in a fuel cell is the single most important element since its surface area determines the rate at which the electrons on the hydrogen fuel can be stripped and sent through a circuit to be powered. A second battery chamber recombines the hydrogen ions with oxygen molecules and the electrons that have just passed through the circuit.

The technique, developed with doctoral candidate Jianglan Shui, was shown to be capable of nanometer diameters for wires several centimeters long. The method works by mixing liquid platinum salt with a polymer which is them spun into long, narrow wires. During the process, the platinum metal migrates to the center of the wire. Individual polymer-coated wires are then woven into a braided electrode. Finally, heat is used to burn off the polymer and salts, leaving behind a pure woven platinum surface with vastly more surface area that flat electrodes.

Nanoparticles makers like Quantum Sphere Inc. claim they can achieve the same effect as woven nanowires by coating stainless steel electrodes with platinum nanoparticles. Spheres have 50 percent more surface area than wires, but some must be sacrificed since the bottom side of the nanoparticles is bonded to the steel.

"The Holy Grail for fuel cells is a highly-active catalyst that can maintain a high surface area over time," said Kimberly McGrath, director of fuel cell research at Quantum Sphere. "Only significant extended durability tests will answer the question" of which is better, nanoparticles or nanowires.

Since platinum is conserved in the reaction, the durability of the electrodes to withstand the harsh acidic environment inside the battery without migrating or agglomerating will ultimately determine whether woven nanowire-based electrodes outperform nanosphere-coated electrodes.

So far, the University of Rochester researchers have shown they can use the spinning process to create platinum nanowires with very high aspect ratios. Next, they plan to further reduce wire diameter, which are 10nm compared to 2nm for available nanoparticle coatings.

Within a year, they plan to demonstrate their woven platinum electrodes inside a real fuel cell.

- R. Colin Johnson
EE Times

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