Cooking up a better cathode
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Steve Simner is like a chef, fine-tuning the ingredients and how they're assembled to find the right recipe for a new cathode material for solid oxide fuel cells (SOFC). As a material scientist at Pacific Northwest National Laboratory, his goal is to find a cathode material that will produce high power in the range of 600 to 800 degrees Celsius--low compared to the more typical SOFC operating temperature of 1,000 degrees Celsius.
"There are two fundamental goals for any cathode. The first is long-term stability and the second is high performance," Simner said. "But without stability, performance is of little consequence."
Simner's research focuses on a lanthanum strontium ferrite material, which has shown good stability in lab tests. Fuel cells using the ferrite cathode are performing two to three times better than more conventional cathode compositions.
Researchers now are trying to manipulate the composition--both physically and chemically--to further enhance its cathodic activity. "Our current composition performs well at 700 to 800 degrees Celsius, but we'd like to drop the operating temperature even further," Simner said. Lower temperatures would make it possible to replace expensive subsidiary ceramic components with more economical metal alloys. The challenge, however, is that the cathode's electrocatalytic activity decreases as temperatures go down.
Researchers are attempting to engineer structured interfaces to increase the effective contact area between the cathode material and the electrolyte. More contact area would make more reaction sites available for the oxygen reaction that takes place on the cathode side of the fuel cell.
Chemically, researchers are making subtle changes to improve material properties at lower operating temperatures. Cathodes need high electronic and ionic conductivity and the interface between the cathode and the electrolyte should readily allow oxygen transfer processes.
"We try to manipulate the chemistry of the cathode and its interfaces to enhance these properties," Simner said. He and his colleagues are adding transition metals such as cobalt to the base composition, which may improve conductivity. They also are exploring materials that may enhance oxygen transfer at the cathode-electrolyte boundary.