Taking the Heat While Measuring Fuel Cells
Raman spectroscopy helps elucidate high-temperature processes.
Solid-oxide fuel cells are getting attention as energy sources, but more information is needed to help optimize their performance. Unfortunately, their working temperatures typically are above 700 °C, making it difficult to get a detailed picture of the molecular processes that occur during operation.
Shown here is a Raman spectroscopic microscope objective near the anode of a solid-oxide fuel cell. The orange glow is from the ~1000 °C furnace (surface temperature ~725 °C). Graphite has begun to build up around the perimeter of the anode.
Now a team from the University of Maryland in College Park and the Naval Research Laboratory in Washington has used Raman spectroscopy to reveal what is going on in these hot fuel cells. Associate chemistry professor and team leader Robert A. Walker — who credits graduate student Michael B. Pomfret with the project’s success — notes that optical methods played a vital role. “Raman spectroscopy is a viable — and likely the only — tool capable of providing the molecularly specific information needed to assess the accuracy of different, often conflicting, kinetic models,” he said.
Solid-oxide fuel cells consist of a cathode, a solid-oxide electrolyte and an anode. Molecular oxygen is reduced at the cathode, with the product then diffusing through the electrolyte to the anode. The researchers wanted to see which chemical intermediaries were present at the anode during normal cell operation.
Although the anode has begun to corrode and flake apart, no graphite is observed except for that which is near the perimeter of the gold lead (top spectrum). The bottom spectrum is taken from a featureless spot on the anode.
Because the optics had to be protected from the heat that was generated during experiments, the researchers blew a steady stream of chilled nitrogen gas onto the optics while suspending them in the furnace. They used a custom-built optical accessory with a right-angle mirror attached to a Raman microscope from Renishaw plc of Wotton-under-Edge in the UK. They used a 488-nm argon-ion laser as the excitation source, thereby avoiding the obscuring glow of the very hot furnace.
The investigators tracked the Raman spectra of fuel cells under various conditions, such as when one was exposed either to an oxidizing atmosphere or to a reducing one. The spectra showed changes, such as the NiO band (at 1072 cm–1) appearing and disappearing. These results were expected, Walker said, although, initially, they were not sure that they would be able to make the measurements at all.
Their first attempts had a signal-to-noise ratio of only 2 and a 2-h spectrum-acquisition time. They eventually upped the signal-to-noise ratio to ~4 to 5 and cut the acquisition time to ~1 min. With that accomplished, they explored various material properties with the cell’s anodes.
The next step in the research is an in situ study of fuel cells that are under electrochemical control. Walker said that they already have made exciting inroads on this project, adding that more findings should be published soon.
Journal of Physical Chemistry B Letters, Sept. 7, 2006, pp. 17305-17308.
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