The damage caused by carbon and sulfur buildup is another source of expense. “Nearly every hydrocarbon fuel that’s available today contains sulfur, and it’s very expensive to take it out,” says Michael Day, director of engineering at NexTech Materials, an Ohio company that’s developing sulfur-tolerant fuel-cell materials. Filtering impurities from the fuel before it’s fed into the cell adds as much as 4 percent to the cost of power generation.
One of the obstacles to improving fuel-cell tolerance has been that researchers are not sure yet what combinations of materials will lead to better performance. Liu stumbled on the poison-tolerant and coking-resistant material while trying to improve the conductivity of the anode. “One day when we tested the cell with a dirty fuel contaminated with hydrogen sulfide, we noticed that the performance didn’t change,” says Liu. “It has a remarkable tolerance to sulfur, from low levels up to 50 parts per million.” Sulfur in the fuel is oxidized and emitted as waste.
The new anode material is a composite of nickel and a ceramic that contains small amounts of two rare-earth metals. Other groups have developed sulfur- and coking-tolerant anodes, but these incorporated expensive materials and degraded cell performance. Replacing the nickel with copper improves a fuel cell’s tolerance, but copper isn’t as good a catalyst. Coating a conventional anode with ruthenium also prevents sulfur and carbon deposition, but this metal is extremely expensive. And all previously developed anodes, no matter how resistant to coking and poisoning, suffered a performance drop when switched to dirty fuels, says Liu. The Georgia Tech anode, he says, “gives the best performance.”
Liu is talking to companies about licensing the anode material. But before it can be brought to market, the new anode will have to be tested over longer periods of time in larger prototypes, he says.