This drawing shows the damaged outer wall of a carbon nanotube with nanosized graphene pieces (white patches), which facilitate the formation of catalytic sites made of iron (yellow) and nitrogen (red) atoms. The catalyst reduces oxygen to water. Image: Guosong Hong |
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.
Multiwalled 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 an online edition of 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 multiwalled 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.”
Unzipped
For the study, Li and his coworkers treated multiwalled 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, multiwalled 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.
Multiwalled nanotubes
could also have applications in metal-air batteries made of lithium or zinc.
“Lithium-air
batteries are exciting because of their ultra-high theoretical energy density,
which is more than 10 times higher than today’s best lithium ion
technology,” Dai said. “But one of the stumbling blocks to
development has been the lack of a high-performance, low-cost catalyst. Carbon
nanotubes could be an excellent alternative to the platinum, palladium, and
other precious-metal catalysts now in use.”
Controversial
sites
The Stanford study might also have resolved a long-standing scientific
controversy about the chemical structure of catalytic active sites where oxygen
reactions occur. “One group of scientists believes that iron impurities
are bonded to nitrogen at the active site,” Li said. “Another group
believes that iron contributes virtually nothing, except to promote active
sites made entirely of nitrogen.”
To address the
controversy, the Stanford team enlisted scientists at Oak Ridge National
Laboratory to
conduct atomic-scale imaging and spectroscopy analysis of the nanotubes. The
results showed clear, visual evidence of iron and nitrogen atoms in close
proximity.
“For the first
time, we were able to image individual atoms on this kind of catalyst,”
Dai said. “All of the images showed iron and nitrogen close together,
suggesting that the two elements are bonded. This kind of imaging is possible,
because the graphene pieces are just one atom thick.”
Dai noted that the
iron impurities, which enhanced catalytic activity, actually came from metal
seeds that were used to make the nanotubes and were not intentionally added by
the scientists. The discovery of these accidental yet invaluable bits of iron
offered the researchers an important lesson. “We learned that metal
impurities in nanotubes must not be ignored,” Dai said.
Source: Stanford University