High-temperature fuel cells promise clean, efficient energy in quantities large enough to power cities. But, so far, they’ve been too expensive for widespread use. One major problem is the sulfur in fossil fuels, such as coal, oil, and natural gas, which contaminates the hydrogen gas that runs the cells. The sulfur attacks and degrades a part of the fuel cell called the anode, reducing power production – and eventually shutting down the cell.
Now chemical engineers at Tufts University in Medford MA, led by Maria Flytzani-Stephanopoulos, have found a way to continuously remove sulfur from incoming hydrogen before it feeds these cells. The work, published in the June 9 issue of Science (abstract), could be a significant step in making high-temperature fuel cells practical.
[For images of this new fuel-cell technology, click here.]
Low-temperature fuel cells have already found uses in laptops and buses, for example. But these fuel cells produce relatively little power. In contrast, high-temperature solid oxide fuel cells (SOFCs) could generate enough power to supply cities. And their heat can be channeled into other uses: for heating buildings or turning steam turbines to produce more power.
Lanny Schmidt, professor of chemical engineering and materials science at the University of Minnesota, says many operational issues have kept more powerful fuel cells off the market, including long startup times and parts wearing out under high heat. But, he says, sulfur is “one of the major problems.” Schmidt predicts that researchers will overcome these obstacles in the next few years, and, if successful, SOFCs “may become the fuel cell of choice.” He says that Flytzani-Stephanopoulos has “an innovative, clever new way to remove sulfur.”
For low-temperature fuel cells (such as proton-exchange membranes), engineers have addressed the sulfur problem using a series of processing steps. They remove most of the sulfur from fossil fuels by refining the liquid fuel, and then use a reformer and materials called “sorbents.” In the reformer, the fossil fuel is heated with air and water to make a hydrogen-rich gas. The sorbents then soak up hydrogen sulfide, so that the gas reaching the fuel cell is sulfur-free. But common sorbents, such as zinc oxide, would degrade in high-temperature fuel cells, which operate at 600 to 1,000 degrees Celsius.
The Tufts group has designed the first sorbent system for high-temperature fuel cells. First, they use new materials: rare earth oxides, known to be stable and able to absorb hydrogen sulfide at high temperatures. And, instead of filtering gas through a thick sorbent bed, they pass it over the surface of a thin sorbent layer. Flytzani-Stephanopoulos calls the new design a “simple” solution to the sulfur problem.
Rare earth oxides are inexpensive and easy to obtain. The system could be added to a SOFC using two small boxes – one for fresh sorbents, the other for spent ones. Sulfur-free gases generated by the fuel cell would sweep the spent sorbents clean, allowing the same sorbents to be used over and over. “You don’t need valves or pumps,” she says, because all gases would diffuse naturally through the system. She adds that her sorbents could also outperform those used for in low-temperature fuel cells.
The Tufts research is funded by the Army Research Laboratory, which wants to use SOFCs as backup power for tanks and trucks. Since these vehicles run on fuel oil that’s rich in sulfur, they would need effective sorbents.