Fuel cells, devices that can produce electricity from hydrogen or other fuels
without burning them, are considered a promising new way of powering everything
from homes and cars to portable devices like cellphones and laptop computers.
Their big advantage - the prospect of eliminating emissions of greenhouse
gases and other pollutants - has been outweighed by their very high cost,
and researchers have been trying to find ways to make the devices less expensive.
 | | Associate Professor of Mechanical Engineering and Materials Science and Engineering Yang Shao-Horn. Credit: Donna Coveney |
Now, an MIT team led by Associate Professor of Mechanical Engineering and Materials
Science and Engineering Yang Shao-Horn has found a method that promises to dramatically
increase the efficiency of the electrodes in one type of fuel cell, which uses
methanol instead of hydrogen as its fuel and is considered promising as a replacement
for batteries in portable electronic devices. Since these electrodes are made
of platinum, increasing their efficiency means that much less of the expensive
metal is needed to produce a given amount of power.
The key to the boost in efficiency, the team found, is to change the surface
texture of the material. By creating tiny stair-steps to the surface instead
of leaving it smooth, the electrode's ability to catalyze oxidation of the fuel
and thus produce electric current was approximately doubled in experiments,
and the researchers believe that further development of these surface structures
could end up producing far greater increases, yielding more electric current
for a given amount of platinum.
Their results are reported Oct. 13 in the Journal of the American Chemical
Society. The paper's eight authors include chemical engineering graduate student
Seung Woo Lee and mechanical engineering postdoctoral researcher Shuo Chen,
along with Shao-Horn and other researchers at MIT, the Japan Institute of Science
and Technology, and Brookhaven National Laboratory.
"One of our research focuses is to develop active and stable catalysts,"
Shao-Horn says, and this new work is a significant step toward "figuring
out how the surface atomic structure can enhance the activity of the catalyst"
in direct methanol fuel cells.
Resolving a controversy
In their experiments, the team used platinum nanoparticles deposited on the
surface of multi-wall carbon nanotubes. Lee says that many people have been
experimenting with the use of platinum nanoparticles for fuel cells, but the
results of the particle size effect on the activity so far have been contradictory
and controversial. "Some people see the activity increase, some people
see a decrease" in activity as the particle size decreases. "There
has been a controversy about how size affects activity."
The new work shows that the key factor is not the size of the particles, but
the details of their surface structure. "We show the details of surface
steps presented on nanoparticles, and relate the amount of surface steps to
the activity." Chen says. By producing a surface with multiple steps on
it, the team doubled the activity of the electrode, and the team members are
now working on creating surfaces with even more steps to try to increase the
activity further. Theoretically, they say, it should be possible to enhance
the activity by orders of magnitude.
Shao-Horn suggests that the key factor is the addition of the edges of the
steps, which seem to provide a site where it's easier for atoms to form new
bonds. The addition of steps creates more of those active sites. In addition,
the team has shown that the step structures are stable enough to be maintained
over hundreds of cycles. That stability is key to being able to develop practical
and effective direct methanol fuel cells.
Team members also hope to understand whether the steps enhance the other part
of the process that takes place in a fuel cell. This study looked at the enhancement
of oxidation, but the other side of a fuel cell undergoes oxygen reduction.
Does the addition of steps to the surface also enhance the oxygen reduction?
"We need to find why it does, or why it doesn't," Shao-Horn says.
The researchers expect to have answers to that question in the next few months.
Posted October 14th, 2009
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