|Polymer power: An inexpensive polymer thin film increases the power output of methanol fuel cells by 50 percent. Credit: Avni Argun and Nathan Ashcroft, MIT.|
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.
Hammond synthesizes fuel-cell membranes using a technique called layer-by-layer assembly. She starts with a very thin membrane of the polymer used in conventional fuel cells. She dips it into a water solution of a positively charged polymer, then into one of a negatively charged polymer; the process is repeated until many layers are built up. The result, explains Hammond, is "a polymer backbone that resists the permeation of methanol" while still conducting protons.
The resulting 100-nanometer-thick membrane conducts two orders of magnitude less methanol than conventional, 50-micrometer-thick membranes do. And fuel cells incorporating it have a greater power output.
Hammond says that methanol is a better candidate to power portable fuel cells than hydrogen because it's a liquid and not nearly as flammable. "It's a dense power source that's safe to carry around," she says.
Savinell says that Hammond's work could have applications beyond methanol fuel cells. By picking the right polymers and varying assembly conditions including pH, says Savinell, "you can customize and optimize [the films] for any application." Layer-by-layer films might be used to improve the conductivity of hydrogen fuel-cell membranes and to increase the efficiency of ethanol fuel cells. Ethanol is safer than methanol but has similar drawbacks as a feedstock for fuel cells: ethanol seeps across the polymer membranes.
"The real promise is the power of the technology to make new materials," says Savinell. Hammond is now working on new fuel-cell membranes that contain none of the expensive conventional polymer.