The most efficient way to get electricity from hydrocarbon fuels such as natural gas or gasified coal is to oxidize them in a solid-oxide fuel cell. Unlike other fuel cells, solid-oxide cells can run on almost any fuel. But running them efficiently requires high temperatures, which raises prices. Now researchers at Georgia Tech have developed an anode material that resists the buildup of sulfur and carbon that can occur at lower temperatures. With further development, the material might be incorporated into cheaper solid-oxide fuel cells that run efficiently at lower temperatures.
Solid-oxide fuel cells generate an electrical current by pulling oxygen from the air and using it to oxidize fuel at temperatures up to about 1,000 °C. Oxygen comes in through the cathode, fuel enters through the anode, and the two react in the electrolyte to make water and carbon dioxide, which flow out of the cell as waste. Electrons freed during the reaction are pulled into an external circuit. Solid-oxide fuel cells are currently used for stationary applications such as powering building furnaces. They might also be used in power plants to generate electricity from gasified coal, an application the U.S. Department of Energy is pursuing though its Office of Fossil Energy.
The chemical reactions in solid-oxide cells are sped by a catalyst, usually nickel, in the anode. Nickel is cheaper than the platinum catalysts used in other fuel cells, and this cost savings is one of the advantages of solid-oxide fuel cells. But nickel is prone to contamination by sulfur in the fuel, and it can get covered in carbon residue, particularly at low temperatures. Both of these factors tend to clog the cell and reduce performance.
The new anode material, described today in the journal Science, resists sulfur poisoning and carbon coking, even when running at low temperatures, and without compromising performance. Developed by researchers led by Meilin Liu, professor of materials science and engineering and codirector of the Center for Innovative Fuel Cell and Battery Technologies at Georgia Tech, the material has so far been tested over a period of 1,000 hours at temperatures ranging from 500 °C to 700 °C.
Solid-oxide fuel cells on the market today operate at temperatures ranging from about 800 °C to 1,000 °C. In order for them to be more widely adopted, they need to run at lower temperatures, says J. Robert Selman, professor of chemical engineering at Illinois Institute of Technology. High operating temperatures mean using expensive materials to connect the fuel cells in a stack. “If you can run at lower temperatures, you have a greater choice of structural materials to work with,” says Selman. It’s cheaper to connect fuel cells in a stack using metal rather than ceramics, but metal interconnects lose their structural integrity at higher temperatures.
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.