June 18, 1998 A new catalyst dramatically improves the performance of methanol-air fuel cells, which could provide a more practical power source than batteries or the fuel cells powered by hydrogen that are used in space missions, according to an article that will be published in the June 12, 1998, issue of the journal Science.
"The hydrogen fuel cells that have flown on space missions since Gemini are not practical for most applications on Earth because they use catalysts and electrolytes that work only with very pure hydrogen, which is expensive to make and is hard to store and transport," explains Thomas E. Mallouk, professor of chemistry at Penn State and a member of the research team. "Nobody wants to carry a tank of compressed hydrogen with their laptop computer, and they would prefer not to have one in their car. These problems have motivated our research on fuel cells that run on renewable, liquid fuels," Mallouk says.
Along with Mallouk, the research team includes Penn State graduate student Erik Reddington and undergraduate Anthony Sapienza; graduate students Bogdan Gurau and Rameshkrishnan Viswanathan and Associate Professor of Chemical and Environmental Engineering Eugene S. Smotkin, at the Illinois Institute of Technology (IIT); and Dr. S. Sarangapani of ICET, Inc., in Norwood, MA.
Fuel cells derive electrical energy directly from power-packed chemicals such as hydrogen, alcohols, and natural gas. In so doing, they side-step a basic thermodynamic limit on the efficiency of engines that burn fuel to make heat, and then use the heat to generate electricity. "Because fuel cells operate like batteries, they are inherently efficient power conversion devices," says Mallouk.
The new research involves fuel cells that use methyl alcohol, or methanol--a liquid fuel that can be made cheaply from biomass or from fossil reserves such as coal, oil, or natural gas. "Because it is compatible with existing delivery systems for liquid fuels, methanol is used, for example, by race cars at the Indianapolis 500 that already burn methanol in their engines," Mallouk says. For fuel cells, methanol presents a special problem because its oxidation in the cell poisons the catalytic electrode surface. "The platinum catalysts that work so well in hydrogen fuel cells are basically useless for methanol," says Smotkin. "They do not adsorb water, which is needed to oxidize away the carbon monoxide that builds up on the platinum surface. That is why platinum alloys containing elements that bind the oxygen atom in water are much better catalysts." Up to now, a platinum-ruthenium alloy has been the best known catalyst for methanol fuel cells. The new catalyst, a quaternary alloy containing platinum, ruthenium, osmium, and iridium, is between 40% and 100% better, depending on the power demand on the cell and is particularly good under high current/high power conditions, the researchers say.
The idea for looking at these complex catalyst compositions came from a paper published by Smotkin's group in 1995 in the Journal of the Electrochemical Society. "They correlated the ability of an alloying element to bind water with catalyst performance," says Mallouk. "This gave them the idea that three elements might be better than two in a catalyst. Further, they had a good idea of which elements to mix together." However, making and testing these alloys was a time-consuming process, which became worse as more elements were added. "To do a reasonable job at testing combinations of four elements, you would have to look at hundreds of catalysts," says Mallouk. "This could not be done in a reasonable time by making them serially, and further, it would not be much fun for the graduate student who had to do it," he says.
Instead, the Penn State group devised a method for making and testing hundreds of different catalysts at the same time. Using an ink-jet printer, they printed dots of metal-salt mixtures onto a large carbon electrode. Each dot, which was about the size of a lower-case letter "o," contained a slightly different mixture of five elements: platinum, ruthenium, osmium, iridium, and rhodium. All the dots were converted into alloy catalysts by a solution process similar to that used to make bulk fuel cell catalysts. To determine the catalytic activity of each dot, the researchers chemically converted the electrical current to an optical signal. "The good catalysts are lit up, using a trick that is similar to a litmus test," says Mallouk, "Wherever methanol is oxidized on the array electrode, it generates acid. A fluorescent acid-base indicator in the solution above the array pinpoints the most active dots, where the concentration of acid is highest." Using this method, the Penn State group was able to determine quickly where the best catalysts lay in a vast landscape of compositions. They transmitted the compositions to Dr. Sarangapani at ICET, who made larger quantities of the catalysts for testing in methanol-air fuel cells by Smotkin and coworkers at IIT.
While there is still room for improvement in these fuel cells, Mallouk is encouraged by the results. "We now have a method to look far and wide in composition space for new catalysts. By correlating the composition/activity map with a microscopic analysis of catalyst structure, we should be able to learn something about what makes the good catalysts work," he says. "Also, the fact that we found a hot new composition in a rather limited search suggests that there will be other--hopefully better--ones out there."
This research was supported by the Office of Naval Research, the Army Research Office, and the Defense Advanced Research Projects Agency.
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