Fuel cells based on solid acid compounds may offer higher voltages, higher operating temperatures, less system complexity and more fuel options than present-day fuel cells. So report researchers at the California Institute of Technology in this week’s Nature.
Fuel cells convert chemical energy directly to electrical energy, a more efficient process than combustion (see Fill ‘er Up With Hydrogen). The most promising fuel cells use membranes made of polymer electrolytes. They work by passing a fuel, such as hydrogen, across the membrane, which is only permeable to protons. The hydrogen’s electron must go around the membrane, generating the electrical current. On the other side of the membrane, the hydrogen bonds with atmospheric oxygen, so the only byproduct is water.
But polymer electrolyte membranes do have one major drawback: they need humidity to work. That limits operating conditions to below the boiling point of water-a major constraint on where and how fuel cells can be used.
“In order for it to perform, the polymer has to be wet,” says Sossina Haile, assistant professor of materials science at Caltech and lead author of the Nature paper. This means that “there are a lot of water-management issues and temperature management issues. You have to keep it cool but not too cool, wet but not too wet.” (Too much water vapor, and it condenses on the membrane surface and blocks the gas.) All this increases the complexity-and therefore the cost-of fuel cells, she says.
No Water, No Problem
Unlike polymer electrolyte membranes, Haile’s solid acid membranes are anhydrous, which means they operate whether or not water is present. In Nature, Haile reported positive results at temperatures up to 160 °Celsius for membranes made of cesium hydrogen sulfate-a solid acid “about as cheap as table salt,” Haile says.
“A proton-conducting electrolyte free of water is a wonderful thing to have,” says Sekharipuram Narayanan, a fuel-cell researcher at the National Aeronautics and Space Administration’s Jet Propulsion Laboratory who has worked with Haile in the past.
Narayanan’s research focuses on powering fuel cells with methanol-an area that could get a big boost from solid acid membranes. So-called direct methanol fuel cells use methanol instead of hydrogen for fuel. But polymer-based membranes also allow some methanol to pass through without generating power, a problem called methanol crossover. Although it has not been demonstrated, Haile theorizes that solid acids may reduce methanol crossover.
Sulfur, Big Problem?
Major hurdles must be crossed, however, if solid-acid fuel cells are ever to be a viable source of power. Haile’s prototype generated only 50 milliamps of current per square centimeter of membrane, compared to the 1 amp per square centimeter generated by polymer electrolyte membranes.
“There is a very long way to go with her approach before it becomes close to useful,” says Tom Zawodzinski, a fuel cell researcher at Los Alamos National Laboratory, in an e-mail to technologyreview.com.
Another problem: solid electrolytes, such as Haile’s, don’t work at room temperature and must be heated before they begin to generate power. That’s no big deal, says Narayanan, when you’re powering a car, but it makes small applications-such as cell phones-nearly impossible.
Perhaps the greatest obstacle is that Haile’s membrane contains sulfur, which reacts with the hydrogen fuel to gradually reduce the performance of the fuel cell. “The problem is that the fuel cell-the electrolyte-faces an extreme environment,” Haile says. “The membrane has sulfur in it that will react with the hydrogen.” The product of the reaction, hydrogen sulfide, interferes with the chemical reaction that generates electricity.
To work around the problem, Haile’s research group is looking for solid acids that contain no sulfur. “It’s something that could get solved today, or it could get solved never,” she says.