Thursday, May 22, 2008

More-Powerful Fuel Cells

A cheap polymer material increases the power output of methanol fuel cells by 50 percent.

By Katherine Bourzac

Polymer power: An inexpensive polymer thin film increases the power output of methanol fuel cells by 50 percent. Credit: Avni Argun and Nathan Ashcraft, MIT

Multimedia   Watch an MIT engineer work with a new material for fuel cells.

Methanol fuel cells have the potential to replace batteries as a lightweight power source for portable electronic devices. But fuel-cell materials are expensive, and fuel cells that consume methanol are inefficient. In particular, the membranes used in methanol fuel cells are expensive and waste fuel. Now researchers at MIT have developed a cheap membrane material that increases the power output of methanol fuel cells by 50 percent.

The energy density of a methanol fuel cell "compares to the best high-energy-density batteries," says Robert Savinell, a chemical engineer at Case Western Reserve University, in Cleveland, who was not involved in the research. And because they weigh less than batteries, methanol fuel cells are a promising power source for portable electronics. For the military, tanks of methanol for refilling fuel cells would be lighter than extra batteries that would have to be carried on long missions. The energy density of methanol fuel cells could also be an advantage in portable consumer electronic devices such as laptops and iPods. But commercialization of methanol fuel cells has been limited because of their price: they require a thick internal membrane made of an expensive polymer. And even with this expensive material, they use fuel inefficiently.

To overcome these limitations, Paula Hammond, a chemical engineer at MIT, has made a fuel-cell membrane out of layers of polymers whose electrochemical properties can be precisely tuned to prevent fuel waste. The work is described in a recent issue of Advanced Materials. Indeed, says Savinell, Hammond has solved a problem that chemists have been trying to overcome for years.

Methanol fuel cells have two compartments separated by a membrane. On one side, methanol is stripped of protons and electrons. The protons are carried through the membrane to the other compartment, where they are combined with oxygen to form water. The electrons, which can't cross the membrane, are forced into an external current that can be used to power electronic devices.

Because water is being created inside the fuel cell, the membrane is wet. Methanol, which is very soluble in water, is absorbed by conventional fuel-cell membranes and can cross over to the other side. This wastes fuel and makes the cathode, the oxidizing end of the cell, work harder. "Everyone's concerned about methanol crossover," says Merlin Bruening, a chemist at Michigan State University. Researchers have tried many different approaches to improving methanol fuel-cell membranes, but all have entailed trade-offs. "The challenge is to maintain stability and conductivity [to protons]," while decreasing methanol crossover, says Bruening.