Fuel cells use chemicals to create electricity. They are used, for example, to keep the lights on for astronauts in orbiting space stations. They hold promise in a variety of areas, such as fuel-cell cars. But the high price of catalysts used inside the cells has provided a roadblock to widespread use.
Now, nanoscale research at Stanford University has found a way to reduce the cost.
Multi-walled carbon nanotubes riddled with defects and impurities on the outside could eventually replace some of the expensive platinum catalysts used in fuel cells and metal-air batteries, according to Stanford scientists. Their findings are published in the May 27 online edition of the journal Nature Nanotechnology .
"Platinum is very expensive and thus impractical for large-scale commercialization," said Hongjie Dai , a professor of chemistry at Stanford and co-author of the study. "Developing a low-cost alternative has been a major research goal for several decades."
Over the past five years, the price of platinum has ranged from just below $800 to more than $2,200 an ounce. Among the most promising low-cost alternatives to platinum is the carbon nanotube – a rolled-up sheet of pure carbon, called graphene, that’s one atom thick and more than 10,000 times narrower a human hair. Carbon nanotubes and graphene are excellent conductors of electricity and relatively inexpensive to produce.
For the study, the Stanford team used multi-walled carbon nanotubes consisting of two or three concentric tubes nested together. The scientists showed that shredding the outer wall, while leaving the inner walls intact, enhances catalytic activity in nanotubes, yet does not interfere with their ability to conduct electricity.
"A typical carbon nanotube has few defects," said Yanguang Li , a postdoctoral fellow at Stanford and lead author of the study. "But defects are actually important to promote the formation of catalytic sites and to render the nanotube very active for catalytic reactions."
For the study, Li and his co-workers treated multi-walled nanotubes in a chemical solution. Microscopic analysis revealed that the treatment caused the outer nanotube to partially unzip and form nanosized graphene pieces that clung to the inner nanotube, which remained mostly intact.
"We found that adding a few iron and nitrogen impurities made the outer wall very active for catalytic reactions," Dai said. "But the inside maintained its integrity, providing a path for electrons to move around. You want the outside to be very active, but you still want to have good electrical conductivity. If you used a single-wall carbon nanotube you wouldn’t have this advantage, because the damage on the wall would degrade the electrical property."
In fuel cells and metal-air batteries, platinum catalysts play a crucial role in speeding up the chemical reactions that convert hydrogen and oxygen to water. But the partially unzipped, multi-walled nanotubes might work just as well, Li added. "We found that the catalytic activity of the nanotubes is very close to platinum," he said. "This high activity and the stability of the design make them promising candidates for fuel cells."
The researchers recently sent samples of the experimental nanotube catalysts to fuel cell experts for testing. "Our goal is to produce a fuel cell with very high energy density that can last very long," Li said.
Multi-walled nanotubes could also have applications in metal-air batteries made of lithium or zinc.