4 April 2005— Fuel cells, the clean-burning engines of the envisioned hydrogen economy, offer the promise of less pollution and lower fuel consumption. Until there are hydrogen fueling stations along all our roads, however, researchers are trying to make fuel cell advances for what we already have—thousands and thousands of gasoline stations.
Researchers at Northwestern University in Evanston, Ill., have now come up with a new, compact fuel-cell design that will take a type of gasoline, turn it into hydrogen, and convert it to electricity at a fuel efficiency higher than that of commercial gas/electric hybrids.
They started with a solid-oxide fuel cell (SOFC), which can reach 800 degrees Celsius in operation. At that temperature these cells can convert some fossil fuels such as natural gas and propane to hydrogen and then to electricity. But for gasoline and diesel, the hydrogen extraction has to be done in a separate system, since feeding the fuel directly to the fuel cell causes carbon buildup, which makes the cell fizzle out.
In the Northwestern University design, which was published in the online version of the journal Science on 31 March, the researchers use a special catalyst to re-form the liquid fuel inside the fuel cell without any carbon deposits. This eliminates the need and cost of an external re-former system, an especially useful trick when designing fuel-cell-powered cars. "We've taken a re-forming plant and a fuel cell and put them right together," says Scott Barnett, a professor in Northwestern's Materials Science and Engineering department and the lead researcher of this project.
Standard SOFCs have a layer of electrolyte sandwiched between two electrodes. A hydrocarbon fuel is fed into the nickel-based anode, where it is oxidized, resulting in water, carbon dioxide, heat, and free electrons. The electrons run through an external circuit providing electricity. At the cathode, which is fed with air, the electrons combine with oxygen to create oxygen ions that diffuse through the electrolyte to the anode. Feeding iso-octane, a smooth burning version of gasoline, to such a fuel cell, encrusts the anode in carbon and causes the cell voltage to drop off after a few hours.
To avoid carbon deposits, the Northwestern team added a catalyst layer to the anode. The catalyst, made of cerium oxide mixed with ruthenium, re-formed the iso-octane before it reached the anode, resulting in a clean anode and a fuel cell that held its voltage for 50 hours.
"[This re-forming] is an endothermic reaction, which means it sucks up heat to run," Barnett says. "But the fuel cell is giving off a lot of excess heat, which is generally lost. That excess heat can be used by the catalyst." Using thermodynamic calculations, the researchers show that this heat increases the overall gasoline-to-electricity efficiency of the fuel cell to about 50 percent. Barnett compares this with external re-forming of natural gas—the efficiency of getting hydrogen from natural gas is 60 percent, and a typical fuel cell converts this hydrogen to electricity with 50 percent efficiency, giving an overall estimate of 30 percent. Gas/electric hybrid cars currently have a slightly higher fuel efficiency of 32 percent.
Using the cell's own heat for re-forming is certainly a good development, says Romesh Kumar, manager of the fuel-cell department at Argonne National Laboratory in Argonne, Ill. But he believes that the system would still need some external re-forming to feed hydrogen into the fuel cell when it starts up—before it has had a chance to get very hot. "Otherwise the endothermic re-forming will tend to quench the fuel cell so that the temp would drop," he says. "I think most systems that use internal re-forming do end up having to do some pre-re-forming."
There are many other design issues to be worked out before this technique can be used in fuel cells in cars and trucks. One drawback of the catalyst layer is that it reduces the rate at which the fuel reaches the anode, decreasing power density over the long run. Also, the cell has been demonstrated to work only with iso-octane, an uncommonly clean fuel, which doesn't have any additives or sulfur contaminants that could muck up the cell. And finally, just like any other precious metal used in catalysts, ruthenium is expensive; though the catalyst layer can be redesigned to use less of the metal, says Barnett.
The next step for Barnett is to extend the fuel cell to use different kinds of fuels. Once he has tackled that, he will have to expand the cells' power-generating capabilities. "Before you can really use these you have to scale them," he says, "make larger fuel cells and stack them together to get a reasonable amount of power out for them to be useful."
Even then, it will be a while before your car runs entirely on energy from solid-oxide fuel cells. For now these cells are a top candidate for use in an auxiliary power unit (APU) that would meet all the electrical needs in cars, trucks, and even aircraft. "One of the main reasons for the interest in an APU onboard automobiles is that more and more of the automotive components are being electrified," says Kumar. "There's a lot more electronics, like DVD players and navigation systems."
In a gasoline-powered vehicle, the engine provides energy to rotate a shaft. This powers the alternator, which converts mechanical energy into electricity. But today's alternators could not be made very efficient without making them very expensive, Kumar says. They have an overall efficiency of converting gasoline into electricity of about 15 percent. A fuel cell using Barnett's design could do the same job with 50 percent efficiency. "So in terms of fuel economy and consumption there's definitely an advantage," Kumar says.
"If [the Northwestern discovery] is borne out at a larger scale and the catalysts are determined to be, or can be made durable for an automotive application...it could be a pretty interesting development," says Kumar.