Researchers have shown that arrays of vertically grown carbon nanotubes could be used as the catalyst in fuel cells. The carbon nanotubes, which are doped with nitrogen, would be much cheaper and longer lasting than the expensive platinum catalysts used now.
More than half the cost of fuel-cell stacks comes from platinum, according to the Department of Energy. “Fuel cells haven’t been commercialized for larger-scale applications because platinum is too expensive,” says Liming Dai, a materials-engineering professor at the University of Dayton, in Ohio, who led the work. “For electrodes, you need a cheaper material that still has a high performance.”
Dai and his colleagues make electrodes by depositing the carbon-nanotube arrays on a composite film of polymer and carbon nanotubes. In a Science paper, the researchers show that using the material as a cathode gives four times higher current densities than do conventional platinum-coated electrodes. “There has been very limited success to finding a replacement for platinum, and [carbon nanotubes] could be one,” says Prashant Kamat, a chemistry professor at the University of Notre Dame.
Carbon nanotubes, which are known to be electrically and mechanically robust, could overcome other issues that platinum faces. Carbon monoxide can stick to platinum’s surface and make it less effective, Dai says. Also, platinum is not very durable, and its properties degrade over time. “Carbon nanotubes have long-term operational stability and do not suffer from carbon-monoxide poisoning,” Dai says.
The researchers have shown that the nanotubes work for the chemistry that takes place in a type of highly efficient fuel cells known as alkaline fuel cells. These cells have so far been limited to use for power generation on spacecraft, but the new advance could open the doors for their entry into the vehicle market. The nitrogen-doped carbon nanotubes should also work in the polymer electrolyte membrane (PEM) fuel cells that are being considered for vehicles, Dai says.
Conventional PEM fuel cells for cars contain two platinum-coated electrodes. The catalyst splits hydrogen at the anode into acidic hydrogen ions and electrons. Hydrogen ions flow through an electrolyte membrane toward the cathode, while electrons flow out of the cell to generate an electric current. Alkaline fuel cells also use platinum-coated electrodes, but they use liquid potassium hydroxide as an electrolyte. Hydrogen and oxygen in these cells react to form hydroxyl ions and electrons.
The reaction at the cathode, in which oxygen molecules combine with electrons, limits the efficiency of a fuel cell. The reaction is at least an order of magnitude slower than that at the anode, Kamat says. More-efficient yet cheap catalysts for the cathode would be a game-changing breakthrough, he says.
To make the nanotube electrodes, Dai and his colleagues start with a compound containing carbon, nitrogen, and iron. They place this on a quartz substrate and heat it in the presence of ammonia, resulting in nitrogen-doped carbon nanotubes growing straight up from the surface. Then they oxidize the array to remove residual iron and transfer the array to a polymer film. Immersing the electrode in a potassium hydroxide electrolyte, the researchers found that it speeds up the cathode reaction of oxygen and electrons.
Meanwhile, others are working on different platinum substitutes. Kotaro Sasaki, who does fuel cell catalyst research at Brookhaven National Laboratory and his colleagues at Brookhaven are making atom-thick platinum films, which would use much less of the precious metal. Researchers at Monash University, in Australia, have made cathodes from a polymer called PEDOT. At Argonne National Laboratory, researchers have made nanotube arrays loaded with small quantities of platinum or iron.