Fuel cells get a boost

A MechE led team discovers the secret to increasing fuel cell efficiency

A high-resolution Transmission Electron Microscopy (TEM) image of platinum nanoparticles

A high-resolution Transmission Electron Microscopy (TEM) image of platinum
nanoparticles on the electrode of a fuel cell reveals surface steps that researchers
say are responsible for dramatically improving efficiency.
Image: Journal of the American Chemical Society, 2009, Vol. 131, NO. 43, 15669-15677

One of the most promising new technologies on the energy frontier is the fuel cell. Fuel cells can produce electricity from hydrogen or other fuels without burning them and have the potential to power everything from homes and cars to portable devices like mobile phones and laptop computers. Their big advantage, eliminating emissions of greenhouse gases and other pollutants, has been outweighed by their high cost, and researchers have been trying to find ways to make the devices less expensive.

Now, an MIT team led by Associate Professor Yang Shao-Horn has found a method that promises to dramatically increase the efficiency of the electrodes in a particular type of cell that uses methanol instead of hydrogen as its fuel. These fuel cells are considered promising as a replacement for batteries in portable electronic devices. Because the 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.

Stepping up the current

The key to boosting efficiency, the team found, was to change the surface texture of the material. Instead of leaving it smooth, the researchers gave it tiny stair steps. This approximately doubled the electrode’s ability to catalyze oxidation of the fuel and produce electric current. 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.

One focus of the research is to develop active and stable catalysts. According to Shao-Horn, the 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.

The results of the team’s research are reported in the October 13 issue of 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.

Doctoral student Yi-Chun Lu

Doctoral student Yi-Chun Lu preparing electrolyte for lithium air battery testing

Resolving a controversy

In their experiments, the team used platinum nanoparticles deposited on the surface of multiwall carbon nanotubes. Lee observes that many researchers 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 it decrease,” Lee explains. “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. Team members are now working on creating surfaces with even more steps to try to increase the activity further. Theoretically, 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. More steps create 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.meche logo

Explore Yang Shao-Horn’s Electrochemical Energy Lab at http://web.mit.edu/eel.