Like all other batteries, fuel cells too use chemicals to create electricity. However, in contrast to ordinary batteries, the advantage of fuel cells is their very high energy density. The energy they produce is high compared to their weight when compared with other batteries.

The energy density of the fuel cells comes at a high price. The platinum catalysts in the fuel cells are very expensive. Now, scientists at the Stanford University have developed a technique to replace the platinum with carbon nanotubes, which makes for an attractive and low-cost solution.

At Stanford, scientists have used multi-walled carbon nanotubes, which are riddled with impurities and defects on the outside. Such nanotubes may be used to replace the expensive platinum catalysts presently used in metal-air batteries and fuel cells.

Since platinum is very expensive, it is impractical for commercialization on a large scale. Scientists have been researching to find a cheaper alternative for the past several decades. The price of Platinum is anywhere from $800 to $2,200 an ounce. So far, the most promising low-cost alternative has been the carbon-nanotubes.

A carbon nanotube is a rolled-up sheet of pure carbon. This is called grapheme, and only one atom thick. An impression of the thinness can be gaged by the fact that a human hair is more than 10,000 times thicker than a carbon nanotube. Apart from being inexpensive to produce, carbon nanotubes of graphene are excellent conductors of electricity.

For replacing the Platinum catalysts, the Stanford scientists used multi-walled carbon-nanotubes. These had two or more concentric tubes nesting together. The catalytic activity of the nanotubes was enhanced with a shredded outer wall, with the inner walls intact. Moreover, this did not reduce their ability to conduct electricity.

Although typical carbon-nanotubes do not have many defects, to promote the formation of catalytic sites, defects had to be deliberately introduced in the outer walls of the carbon-nanotubes. The net effect of the introduction of the defects was they rendered the nanotubes as very active for catalytic reactions.

If the carbon-nanotubes are thinner than human hair, how did the scientists cause defects in the outer wall, leaving the inner walls intact? They treated the multi-walled nanotubes in a chemical solution. With this treatment, the outer nanotube unzipped partially and formed nano-sized graphene pieces. The inner nanotubes remained mostly intact, and the graphene pieces clung to these inner tubes.

Scientists then added a few impurities such as nitrogen and iron to the outer wall to make it very active for catalytic reactions. The nanotube maintained its integrity because of the inner walls, and the inner walls provided the necessary path for electrons to move. The overall effect was a very active outside wall along with excellent electrical conductivity. This advantage would not have been possible with just a single wall carbon-nanotube, as the damage to the wall would have impaired its electrical property as well.

Metal-air batteries and fuel cells require Platinum catalysts to speed up chemical reactions for converting oxygen and hydrogen to water. The catalytic activity of the partially unzipped, multi-walled nanotubes was very close to Platinum. Scientists are planning to produce fuel cells with very high energy density that can last for a long time.