|Nano power: Tin oxide nanoparticles (above} form the foundation of a new catalyst for ethanol fuel cells.|
Portable fuel cells powered directly by ethanol could soon be practical, thanks to a new catalyst that breaks a strong bond at the heart of ethanol molecules, freeing electrons and generating electricity. Such fuel cells could replace batteries in laptops and cell phones, and could eventually be used to power electric vehicles.
Ethanol fuel cells could be far more efficient than conventional ethanol-powered engines. They could also be more practical than hydrogen fuel cells, since ethanol is easier to store and transport than hydrogen. But researchers hadn't been able to create a good catalyst for oxidizing ethanol in order to make such fuel cells possible.
Previous catalysts converted ethanol into acetic acid and acetaldehyde, a process that releases just a couple of electrons per ethanol molecule, hence generates low currents. Breaking down ethanol molecules further to produce carbon dioxide would release far more electrons (a total of 12 per ethanol molecule) and generate higher currents, but that requires breaking a strong bond between two carbon atoms. To break this bond, researchers had to apply high voltages, making the process inefficient: almost all of the voltage produced by oxidizing the ethanol was used to sustain the reaction, reducing net power output to a trickle, says Manos Mavrikakis, professor of chemical and biological engineering at the University of Wisconsin-Madison.
The new catalyst, developed by researchers at Brookhaven National Laboratory, breaks the carbon bonds without high voltages, efficiently releasing enough electrons to produce electrical currents 100 times higher than those produced with other catalysts. The next step is to incorporate the catalyst into a fuel cell, so that its performance can be compared with those of other catalysts in fuel cells, says Brian Pivovar, a scientist at the National Renewable Energy Laboratory, in Golden, CO, who was not involved in the research.
In initial tests outside of a fuel cell, the catalyst efficiently produced currents of 7.5 milliamps per square centimeter. Radoslav Adzic, the senior chemist at Brookhaven National Laboratory who led the work, says that he is "almost positive" that the catalyst, once built into a fuel cell, will produce electrical currents in the range of hundreds of milliamps per square centimeter. Pivovar says that estimate seems reasonable. This level of current, multiplied by the anticipated voltage produced by the cell, would put ethanol fuel cells in the same range as methanol-powered fuel cells, producing enough power for portable electronics. Ethanol is preferable to methanol in several ways: it stores more energy, is less toxic, and is easier to make from renewable sources. For powering vehicles, and competing with the performance of hydrogen fuel cells, the catalyst and fuel cell would need to be improved. The currents would need to be well above 1,000 milliamps per square centimeter, says Andy Herring, a professor of chemical engineering at the Colorado School of Mines, in Golden, CO.
To make the catalyst, Adzic deposited tiny clusters of platinum and rhodium on tin oxide nanoparticles. In earlier studies, rhodium had been shown to break bonds between carbon atoms, but only if vaporized at high temperatures in an ultrahigh vacuum. The combination of rhodium with the tin oxide allowed it to break these bonds as a solid and at the relatively low temperatures needed for portable fuel cells. The platinum plays a key role in producing protons and electrons from hydrogen atoms in ethanol.
Significant challenges remain before the catalyst can be commercialized in ethanol fuel cells. In addition to facing the challenges of incorporating it into fuel cells and engineering these to produce electricity efficiently at high currents, the researchers will need to find ways to reduce costs. Rhodium is the most expensive precious metal--it's even more expensive than platinum--so it will either need to be replaced with another element, or techniques must be developed to reduce the amount of rhodium required.
Still, the new catalyst is a significant improvement over previous attempts. "Breaking the carbon-carbon bond at low temperatures is an extremely hard problem," Herring says. "The fact that [Adzic] is breaking that bond is pretty exciting." But he adds that "it's just one step on the pathway toward this dream of a direct ethanol fuel cell."