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.