Hydrocarbon fuels in solid oxide fuel cells: In situ Raman studies of graphite formation and oxidation

In situ Raman spectroscopy and linear sweep voltammetry were used to characterize graphite formation on Ni/YSZ cermet anodes in solid oxide fuel cells (SOFCs) operating at 715 degrees C. The membrane electrode assemblies were run continuously with Ar-diluted H-2 and exposed to intermittent bursts of hydrocarbons. The appearance and disappearance of carbon deposits was monitored as a function of cell potential and hydrocarbon fuel identity. The hydrocarbon fuels employed in these studies included methane, ethylene, and propylene. Kinetic modeling predicts that of these three fuels, propylene is the most reactive under the conditions of the SOFC experiments. Methane was predicted to be virtually unreactive in the gas phase. Limited exposure of the SOFC anode to methane led to no observable carbon deposits and no appreciable change in SOFC electrochemical performance. Extended exposure to a continuous methane feed resulted in the formation of highly ordered graphite as evidenced by a single feature (assigned as the "G" band) at 1585 cm(-1) in the Raman spectrum. The addition of ethylene to the incident fuel leads initially to the formation of highly ordered graphite as evidenced by the rapid growth of the G band and a small "D" band (at 1365 cm(-1)) in the Raman spectrum. Subsequent additions of ethylene created more disorder and led to deteriorating SOFC performance. Small amounts of propylene added to the fuel feed formed disordered carbon having significant amounts of tetrahedrally coordinated carbon, and SOFC performance suffered reversible degradation. Applying an overpotential to the anode led to the disappearance of carbon deposits with the intensity of the D band diminishing more rapidly than the G band. The disappearance rates depended directly on the anode overpotential.