Microscopic observation of SOFC anodes under operation with hydrocarbon fuels

Carbon deposition on a SOFC anode was investigated under direct hydrocarbon fueling condition. Microscopic behaviors were observed with a newly designed sample holder that enabled in-situ observation of an electrode in operation under controlled atmosphere at elevated temperatures. The preferential carbon deposition site, the structure of the deposited carbon, and the chemical or electrochemical re-oxidation processes were investigated with an optical microscope combined with a laser Raman microscope. Color and morphology change of the electrode surface was recorded with a CCD camera, and the deposited materials were identified with a laser Raman microscope. A nickel micro grid was used as a model electrode of a well-defined microstructure. When the cell was kept at an open circuit condition in methane, carbon started to deposit on the surface of Ni grid electrode. The deposition of carbon was clearly observed as the change in the reflection on the nickel surface by optical microscope as well as by the appearance of the specific peak at ∼1560 cm^-1 in Raman spectroscopy. The deposited carbon was in the form of graphite on the nickel grid electrode. The carbon coverage on the surface was not uniform but varied from grain to grain. When a certain anodic overpotential (e.g. 200 mV) was applied to the electrode, the carbon disappeared gradually from the edge of the electrode i.e. from the electrode/electrolyte boundary where oxygen was supplied electrochemically. It is the first in-situ observation of the electrochemical carbon oxidation in a real operation condition.